Prepared for: Prepared by: The Municipal Subdistrict of the AECOM Northern Colorado Water Conservancy Denver, Colorado District and WGFP Participants March 2014

Windy Gap Firming Project

Chimney Hollow Dam and Reservoir Preliminary Design Report

Main Report

Prepared for: Prepared by: The Municipal Subdistrict of the AECOM Northern Colorado Water Conservancy Denver, Colorado District and WGFP Participants March 2014

Windy Gap Firming Project

Chimney Hollow Dam and Reservoir Preliminary Design Report

Main Report

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Contents 1.0 Overview ..................................................................................................................... 1-1 2.0 Site Description ........................................................................................................... 2-1

2.1 Introduction ...................................................................................................2-1 2.2 Geology ........................................................................................................2-6 2.3 Seismicity ................................................................................................... 2-15 2.4 Climate and Evaporation ............................................................................. 2-20

3.0 Field Exploration and Laboratory Testing Program ....................................................... 3-1 3.1 General.........................................................................................................3-1 3.2 Site Exploration .............................................................................................3-1 3.3 Offsite Borrow Sources ..................................................................................3-7 3.4 Laboratory Testing ........................................................................................3-8

4.0 Hydrology.................................................................................................................... 4-1 4.1 Reservoir Area and Capacity .........................................................................4-1 4.2 Precipitation and Reservoir Evaporation .........................................................4-1 4.3 Inflow Design Flood (IDF) ..............................................................................4-3 4.4 Surface Water ...............................................................................................4-4 4.5 Groundwater .................................................................................................4-5 4.6 Runoff during Construction ............................................................................4-7 4.7 Sediment Loads ............................................................................................4-8

5.0 Dam Type Selection .................................................................................................... 5-1 5.1 General.........................................................................................................5-1 5.2 Foundation ....................................................................................................5-1 5.3 Borrow Materials ...........................................................................................5-4 5.4 Dam Type Evaluation ....................................................................................5-8 5.5 Recommended Dam Types ......................................................................... 5-20 5.6 Staged Construction .................................................................................... 5-21

6.0 Dam Design Considerations ........................................................................................ 6-1 6.1 General.........................................................................................................6-1 6.2 Foundation Preparation and Treatment ..........................................................6-1 6.3 Zoned Earthfill/Rockfill Embankment ..............................................................6-3 6.4 Concrete Face Rockfill Embankment .............................................................6-8 6.5 Asphaltic Concrete Core Rockfill Embankment ............................................. 6-10 6.6 Embankment Instrumentation ...................................................................... 6-12 6.7 Conclusions ................................................................................................ 6-13

7.0 Saddle Dam Design Considerations ............................................................................ 7-1 7.1 General.........................................................................................................7-1 7.2 Foundation Preparation .................................................................................7-1 7.3 Embankment Dam ........................................................................................7-3 7.4 Embankment Instrumentation ........................................................................7-6 7.5 Conclusions ..................................................................................................7-6

8.0 Water Conveyance Appurtenances ............................................................................. 8-1 8.1 General.........................................................................................................8-1

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8.2 Spillway ........................................................................................................8-3 8.3 Diversion and Inlet/Outlet Facilities ................................................................8-8 8.4 Penstock Connection .................................................................................. 8-15

9.0 Site Design Considerations ......................................................................................... 9-1 9.1 General Site Layouts and Access ..................................................................9-1 9.2 Facility Relocations .......................................................................................9-3

10.0 Construction Program, Cost, and Project Schedule .................................................... 10-1 10.1 Construction Program.................................................................................. 10-1 10.2 Opinion of Probable Cost ............................................................................. 10-3 10.3 Project Schedule ......................................................................................... 10-7 10.3.1 Design, Agency Review, and Bidding Schedule Considerations .................... 10-8 10.3.2 Construction Sequence and Schedule Considerations .................................. 10-9

11.0 References ............................................................................................................... 11-1

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List of Appendices Appendix A - Photos

Appendix B - Seismic Analysis

Appendix C - Field Exploration Program

Appendix C1 - Phase I Exploration Logs and Water Pressure Test Data

Appendix C2 - Phase I Lab Test Data

Appendix C3 - Phase II Exploration Logs

Appendix C4 - Phase II Lab Test Data

Appendix C5 - Exploration Logs and Lab Test Data from GEI Study

Appendix C6 - Triaxial Test Results

Appendix C7 - Geophysical Data

Appendix D - Calculations

Appendix E - Drawings

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List of Tables Table 2.1 - Climate Summary ........................................................................................... 2-20 Table 2.2 Free Water Surface Evaporation Summary ..................................................... 2-21 Table 3.1 - Summary of Phase I Field Exploration Lab Testing for Chimney Hollow Dam ... 3-16 Table 3.2 - Summary of Phase II Field Exploration Lab Testing for Chimney Hollow Dam .. 3-17 Table 4.1 Reservoir Area and Capacity ............................................................................ 4-1 Table 4.2 PMP Precipitation Estimates ............................................................................ 4-2 Table 4.3 25-year Storm Precipitation Estimates .............................................................. 4-3 Table 4.4 Chimney Hollow Basin Characteristics .............................................................. 4-3 Table 4.5 PMF Peak Flows and Total Storm Volumes ...................................................... 4-4 Table 5.1 Dam Type Comparison .................................................................................. 5-10 Table 5.2 Estimated Material Volumes for Zoned Earthfill/Rockfill Dam .......................... 5-12 Table 5.3 Estimated Material Volumes for Concrete Face Rockfill Dam .......................... 5-14 Table 5.4 Estimated Material Volumes for Asphaltic Concrete Core Rockfill Dam ........... 5-16 Table 6.1 Permeability Values for Zoned Earthfill/Rockfill Dam ......................................... 6-5 Table 6.2 Stability Analysis Results for Zoned Earthfill/Rockfill Dam ................................. 6-7 Table 6.3 Stability Analysis Results for Concrete Face Rockfill Dam ................................. 6-9 Table 6.4 Stability Analysis Results for Asphaltic Concrete Core Rockfill Dam ............... 6-12 Table 7.1 Permeability Values for Zoned Earthfill Saddle Dam ......................................... 7-4 Table 7.2 Stability Analysis Results for Zoned Earthfill Saddle Dam.................................. 7-5 Table 8.1 Wave Height and Run-up Results ..................................................................... 8-4 Table 8.2 - Spillway Configuration Options .......................................................................... 8-6 Table 8.3 Preliminary Diversion and Inlet/Outlet Design Considerations ........................... 8-9 Table 8.4 Reclamation Reservoir Evacuation Criteria ..................................................... 8-11 Table 8.5 - Inlet/Outlet Location Alternatives ..................................................................... 8-13 Table 8.6 - Outlet Energy Dissipation Alternatives ............................................................. 8-14 Table 10.1 Temporary Construction Facilities and Staging Areas ................................... 10-1 Table 10.2 Primary Cost Differences Among Dam Variations ......................................... 10-6 Table 10.3 Summary of Opinions of Probable Cost for Dam Variations ........................... 10-7 Table 10.4 Concrete Face Rockfill Dam Opinion of Probable Construction Cost ....... 10-13 Table 10.5 Asphaltic Concrete Core Dam Opinion of Probable Construction Cost...... 10-14 Table 10.6 Zoned Earthfill/Rockfill Dam Opinion of Probable Construction Cost ........ 10-15

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List of Figures Figure 2.1 Project Site Geologic Map ............................................................................. 2-11 Figure 2.2 Geologic Section A A1 ................................................................................ 2-12 Figure 2.3 Legend for Geologic Map and Section ........................................................... 2-13 Figure 2.4 Geology in Vicinity of Proposed Chimney Hollow Reservoir ........................... 2-14 Figure 4.1 IDF Inflow Hydrograph (80-percent General PMF) ........................................... 4-4 Figure 4.2 25-year Storm Inflow Hydrograph .................................................................... 4-7 Figure 8.1 IDF Inflow and Outflow Hydrographs ............................................................... 8-5 Figure 10.1 Preliminary Project Schedule Dam Type Selected Prior to Final Design .... 10-8 Figure 10.2 Preliminary Project Schedule Dam Type not Selected Prior to Final Design10-8

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1.0 Overview

This Preliminary Design Report (PDR) for Chimney Hollow Dam and Reservoir supplements previous information developed for this potential water storage component of the Windy Gap Firming Project (WGFP) as provided in the Alternative Plan Formulation Report dated February 2003. Technical analyses presented herein incorporate the results of additional geotechnical field investigations conducted in the summers of 2003 and 2005 and additional operational modeling of the Windy Gap and Colorado-Big Thompson (C-BT) projects done in relation to the WGFP Environmental Impact Statement (EIS).

The primary purpose of this PDR is to provide useful information upon which to narrow the overall project configuration and alternative design features and to serve as the basis to formulate final design activities.

The main feature of this potential component of the Windy Gap Firming Project is a dam to impound up to 90,000 acre feet of water delivered via C-BT facilities in the vicinity of the Flatiron power plant and Carter Lake. Based on topographic mapping prepared in the summer of 2003 and shifting the dam axis a short distance upstream of the axis shown in the previous Alternative Plan Formulation Report, it is currently estimated that a dam with an approximate crest elevation of 5876 feet msl will provide the required active reservoir storage capacity and freeboard above the normal maximum reservoir pool. The maximum normal reservoir pool elevation would be approximately 5866 feet msl. The dam would rise approximately 360 feet above the existing streambed at the dam axis.

Alternative methods of constructing the dam embankment are presented and evaluated herein. It is concluded that three variations of rockfill dam types should be carried forward for consideration in future design refinements. Two types of dams, zoned earthfill and roller compacted concrete (RCC), are not recommended for further consideration at the Chimney Hollow site.

Primary appurtenances required for Chimney Hollow Dam and Reservoir include:

A combined reservoir inlet and outlet consisting of a submerged single-level inlet at elevation 5600 feet msl, a tunnel under the right (east) abutment of the dam with a nine-foot-diameter steel-lined, concrete-encased conduit. Alternative methods to control reservoir releases and to connect the inlet/outlet to existing C-BT facilities were considered. One alternative is

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presented herein based on preliminary reviews by NW and Reclamation incorporated in this PDR.

A spillway to convey a peak discharge of about 750 cfs after the reservoir attenuates the peak inflow to the reservoir of about 15,300 cfs resulting from the inflow design flood (resulting from 80-percent of the general storm Probable Maximum Precipitation).

A 36- foot- high saddle dam is required to close the southern end of the reservoir. It could be constructed using zoned earthfill or any of the three rockfill alternatives still under consideration for the main dam.

The original calculations in this report were based on dam safety rules and regulations published by the Office of the Colorado State Engineer (SEO) in 1988 [SEO, 1988]. The SEO published new dam safety rules and regulations [SEO, 2007] after these calculations were performed and prior to finalizing this report. Generally, the revised rules and regulations did not affect certain calculations (e.g. slope stability factors of safety did not change); however, other calculations were updated to comply with the revised rules and regulations (e.g. flood hydrology).

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2.0 Site Description

2.1 Introduction

The proposed site for the Chimney Hollow Dam and Reservoir Project is located in Larimer County, Colorado, approximately eight miles southwest of Loveland, Colorado and a short distance west of Carter Lake Reservoir. It is primarily within Section 33, Township 5 North, Range 70 West and Sections 4 and 9, Township 4 North, Range 70 West. Existing access into the site is by a gated entrance on Pole Hill Road (County Road 18E) located a short distance west of the entrance to the Flatiron Hydroelectric Facility owned by the U.S. Department of the Interior, Bureau of Reclamation (Reclamation). The dam and reservoir will be in a south to north trending valley identified as Chimney Hollow. The crest of the eastern valley slope is the drainage divide between Chimney Hollow and Carter Lake. The Reclamation Flatiron Hydroelectric facility and reservoir are located in the mouth of the Chimney Hollow valley. Access through the site is by a dirt road that runs along the west side of Chimney Hollow Creek. A few 4-wheel drive trails spur off of the main access road to the west. A vicinity map of the Chimney Hollow Project site is shown on Drawing 100. Site photographs are located in Appendix A.

2.1.1 Site Topography

The Chimney Hollow site is a long narrow valley that parallels the base of the foothills of the Rocky Mountains, and is bounded by moderate and steep slopes on the west and east sides, respectively. The valley floor slopes down from south to north at about a 2.5 percent grade from the upstream (south) drainage divide and the location of the proposed saddle dam down to the proposed dam site. The approximate ground elevations at the upstream drainage divide (vicinity of saddle dam) and at the centerline of the proposed main dam are 5840 and 5530 respectively.

The catchment area and valley shape, in cross section from west to east, is topographically asymmetrical. Moderate slopes (primarily of granite and shallow colluvium) generally extend from the western drainage divide eastward down into the valley area. The western drainage divide runs from about elevation 7880 at the south down to about elevation 7100 above the main dam location. The toe area of the western slopes is slightly less steep as the near surface bedrock is of the Fountain Formation and the surface tends to mirror the plane of the uplifted easterly dipping strata. The elevation at the dams downstream toe is approximately 5,800.

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The eastern slope of the valley is steep (almost canyon-like) and is underlain entirely by the sedimentary units of the Fountain Formation except for the uppermost capping beds of Ingleside Formation along the crest of the slope. The crest elevation of the eastern slope is about 6,200 along the center portion with a peak elevation at about 6,220. This central crest area is flanked by crest lines at about 6,100 to the south and 6050 to the north. The south and north areas peak at about 6,180 and 6,150, respectively. The central area crest is separated from the north and south areas by saddle sections to either side with both having an elevation of about 6000 to 6010. The base of the eastern slope is moderately steep as a result of build-up of colluvial debris falling to the toe of the slope from the upper portion of the slope. The valley bottom is topographically asymmetrical with gentle slopes up to the bases of the steeper eastern and western slopes as a result of valley infilling with fine-grained alluvial deposits.

Vegetation over the western slope (within the proposed reservoir area) is primarily sparse to moderate tree cover with a ground cover of native grasses. Some scattered shrubbery is present among the trees, especially within the drainages. The valley area and moderate slopes along the eastern side are generally grass covered with moderate to dense shrubbery growing in the drainages and intermittently along Chimney Hollow Creek. Trees are also found along the banks of the creek. In areas of near-surface and outcropping bedrock in the valley floor, the grasses are replaced with stands of trees (this is only along the western side of the valley floor area). Trees are more prevalent in the valley area and away from the banks of the creek in the lower (northern) end of the valley. The eastern slopes above the limits of the colluvial deposits are primarily covered with sparse, low-lying shrubbery and grasses rooted in the ledges and drainage features of the slope. Trees are sparse on the eastern slope and are generally located within tributary drainages and on the colluvial slopes.

The valley is drained by the intermittent north-flowing Chimney Hollow Creek located along the base of the eastern slope of the valley. It is an intermittent creek fed predominantly by the catchment area to the west of the creek. Six significant drainages from the western catchment area of the site join the north-flowing Chimney Hollow Creek. These tributary creeks are characterized as intermittent with flow derived from snowmelt and precipitation. At the completion of the field explorations in 2003, flow from the drainages into the valley area of the site had declined to where noticeable surface water in the drainages was limited to isolated pools in low areas of the drainage channels. Drainage from the eastern side of the valley is limited to direct runoff from snowmelt and precipitation.

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Surface runoff patterns down to the valley are characteristic of sheet flow and short drainage channels incised in the slopes. There were no noticeable surface flows into Chimney Hollow Creek from the east side of the valley during the 2003 field explorations. The USGS topographic map for this area also identifies Chimney Hollow Creek and its major tributaries as intermittent drainages. No explorations were conducted to determine the presence or extent of any base flows within the alluvium (below the streambed) of Chimney Hollow Creek.

2.1.2 Existing Facilities

Historically, the Chimney Hollow valley was a working cattle ranch and more recently, leased pastureland for seasonal pasturing of cattle. Existing structures and facilities within the site reflect the historical and recent use of the valley. Ranch-related structures within the project area include an uninhabited main ranch house and associated out buildings near the northern portion of the project area and within the limits of the footprint of the proposed main dam. Upslope to the west of the house is a small breached dam and a circuit of shallow-buried 1-inch diameter polyethylene pipe that leads to a buried cistern or spring house. An abandoned vehicle, rubble and debris, and old machinery are scattered throughout the area of the ranch house and adjacent facilities. A second abandoned house, out buildings, and corrals are located approximately one mile south and up-valley a short distance west of Chimney Hollow Creek. There were no feed lot conditions or manure piles observed in the areas of the ranch houses or out-lying facilities.

A remnant of a low embankment dam, estimated on the order of 10 to 20 feet in height, is located on Chimney Hollow Creek near the upper abandoned house. The dam has been breached and a flow channel eroded down to the streambed so that no water is impounded behind the dam.

In the upstream and southern portion of the project area, there are a series of low embankment structures in the bottom of the valley and above the creek channel in the lower end of the drainages from the west. These structures are small and impound only a limited amount of water. The most significant one is in the vicinity of the proposed saddle dam. It is a low; off-stream embankment dam located in the west side of the valley and has an estimated maximum height of approximately 15 feet and a crest length on the order of 300 feet. At the time of the site investigation, no water was ponded behind this structure and there was evidence of temporary pooling of runoff and direct precipitation and subsequent dissipation through evaporation and seepage.

In the upper portion of the site (in the vicinity of the second abandoned house and above) there is evidence of an irrigation system or water diversion in the

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form of ditches and low rock walls. Fence lines are common in the western portion of the proposed reservoir area and above. Three noticeable rubble dumps (abandoned car bodies, discarded appliances, and non-perishable debris) were present between the two abandoned houses. These dump areas were located in the creek bank and along the banks of two of the drainages entering the creek from the west. No garbage dumps were found within the project area during the site reconnaissance or field explorations; however, their non-existence and actual contents of the rubble sites is not conclusive based on the level of site work completed for this phase of the project.

Public utilities observed at the project area appear to be limited to a buried natural gas line and overhead electrical power that service or used to service the main (northern most) ranch house. These utilities enter the site from the north and do not appear to extend into the site than the main ranch house area. It is assumed that no municipal water line serviced the ranch house or out buildings and water was supplied from the cistern noted above. Furthermore, it is likely wastewater from the house fed into a leach system and that at some time an out house also existed in the ranch house area. Neither of these facilities was located in regards to site characterization, nor would such facilities significantly impact site preparations. Two vertical plastic pipes, approximately 3-inches in diameter, were observed a few hundred feet east-northeast of the main ranch home. The purpose of these pipes is unknown.

A water pipeline of unknown size extends down the left (west) abutment area of the main dam and into the valley bottom several hundred feet upstream (south) of the proposed dam axis. There are a series of manholes that mark the pipeline down the left abutment area of the dam and into the proposed impoundment area where it terminates in the valley bottom. It is not known if any extensions or secondary distribution lines were completed from the pipeline.

An overhead 115 kv power transmission line is present along the eastern side of the valley. The transmission line runs from the Flatiron facilities into the valley along the access road, and then traverses up the eastern reservoir slope to the south end of the Chimney Hollow site. It ascends out of the reservoir limits about 2000 feet north of the saddle dam and passes through the eastern ridge line approximately one-quarter mile north of the proposed saddle dam area.

2.1.3 Existing Flatiron Hydroelectric Facility

As previously mentioned, the existing Flatiron Hydroelectric Facility is located a short distance north of the proposed Chimney Hollow Project and near the

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mouth of Chimney Hollow valley. The Flatiron Hydroelectric Facility is part of the Colorado-Big Thompson Project and was completed in the 1950s. The facilitys main structures include two six-foot diameter penstocks, which convey water to the facility, a powerhouse, the powerhouse afterbay, the Flatiron Dam and Reservoir, and a pressure tunnel connecting to Carter Lake Reservoir. Additionally, a bypass with a fixed sleeve valve is located in the powerhouse to allow flow to be diverted around the hydropower units. fixed sleeve valve Water from the Flatiron Facility is diverted either north to Horsetooth Reservoir by means of a supply canal or pumped via a pressure tunnel to Carter Lake Reservoir located in the adjacent drainage east of the project area.

Both the penstock alignment and the pressure tunnel location influence the proposed dam location for this project. The penstocks (two parallel 6-ft diameter steel pipes) cross over the Chimney Hollow western drainage divide just above the left abutment of the proposed dam alignment and are routed directly down the slope to the power plant. The penstocks are above ground structures their entire length in the Chimney Hollow valley until they reach the existing access road just before entering the power plant. The dam centerline was located to avoid encroachment onto the penstock alignment by the dam toe and spillway chute. The pressure tunnel from Carter Lake Reservoir to the powerhouse is an 8-ft diameter (inside) concrete and steel structure with a total length of about 6,750 ft. The western portion consists of about 1,090 ft of 8-ft diameter steel reinforced concrete pipe installed by cut and cover construction methods from the powerhouse to the eastern valley slope. The remainder of the system consists of a tunnel constructed through the east slope of Chimney Hollow Valley and the west slope of Carter Lake Reservoir. The tunnel section is about 5,660 ft in length and lined with cast-in-place reinforced concrete. The tunnel was excavated by drill and blast methods and reinforced with steel sets. The pipeline and tunnel slopes upward from the powerhouse to Carter Lake Reservoir at a slope of 2.18 percent [Reclamation, 1949; Reclamation, 1953; Reclamation, 1954]. Flow through the tunnel can be in either direction. The western portion of the Carter Lake Pressure Tunnel is nearly parallel to the downstream dam right abutment contact and is offset downstream at least 100 ft in plan. The surface expression (portal area) of the transition from cut and cover to tunnel is about 300 ft to 400 ft downstream of the dam toe area, depending on the rockfill dam variation, and at a surface elevation of approximately 5,515. The tunnel invert elevation at this location is about 5,494 [Reclamation, 1954].

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2.1.4 Prepositioning with Chimney Hollow

The option for constructing Chimney Hollow reservoir was reviewed with two other alternatives in the Windy Gap Firming Project Modeling Report [Boyle, 2003]. It was decided the Chimney Hollow Reservoir alternative was the alternative that would be further developed.

This alternative includes prepositioning, which is a method of operation intended to facilitate delivery of Windy Gap water to the East Slope during times when C-BT system capacities are less limiting. Prepositioning involves use of available Adams Tunnel capacity to deliver C-BT water from Lake Granby into Chimney Hollow to occupy storage space that is not occupied by Windy Gap water. Delivery of C-BT water to Chimney Hollow in this manner maintains Chimney Hollow essentially full at all times. The delivery of C-BT water from Lake Granby into Chimney Hollow creates space for Windy Gap water in Lake Granby. When Windy Gap water is diverted into Lake Granby, the C-BT water in Chimney Hollow is exchanged for a like amount of Windy Gap water in Lake Granby. This operation relieves the need to physically deliver Windy Gap water through Adams Tunnel to Chimney Hollow during the diversion season because this operation is accomplished through the exchange of water instead.

2.2 Geology

The project site geology was evaluated by reviewing available published data, performing reconnaissance-level geologic mapping, reviewing aerial photographs, and completing preliminary design level geotechnical/geological field explorations.

2.2.1 Regional Geology

The project is located in the Front Range area of the Southern Rocky Mountains in the north central portion of Colorado. In the project region, the Southern Rocky Mountains can be physiographically subdivided into two subsections, the Lower Mountain Subsection and the Hogback Subsection. The Lower Mountain Subsection is generally located west of the project area and does include the western approximately one-third to one-half of the project area. The Lower Mountain Subsection is characterized by mountain peaks, slopes and valleys generally ranging in elevation between 9,400 down to about 5,400. Bald Mountain, located about one mile west of the project area, has a maximum elevation of about 7,050 and Blue Mountain, located about two miles west, has a maximum elevation of about 7,890.

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This physiographic subsection is generally underlain by a complex series of Precambrian-age metasedimentary and metavolcanic rocks. These rocks were subsequently intruded by younger granites, pegmatites, and dikes approximately 1,400 million years ago [Braddock, et al., 1988]. Northwest of the project area, the bedrock is comprised of a quartzofeldspathic mica schist while west and southwest of the project area, including the western portion of the project area, the rocks consist of the younger Silver Plume Granite. The area between these two major rock units consists primarily of gneiss and amphibolites in a zone referred to as the Moose Mountain Shear Zone. This shear zone trends generally east-west and extends into the northwestern portion of the project area [Braddock, et al., 1970 and 1988, Punongbayan, et al, 1989].

The surface of these older Precambrian rocks generally slopes downward to the east beneath the Pennsylvanian-age sedimentary bedrock that generally comprises the Hogback Subsection. The Hogback Subsection is characterized by a series of approximately north south trending ridges and valleys. The ridges are generally sharp-crested and asymmetrical in cross-section profile. The ridges consist of tilted resistant bedrock, generally sandstone and, locally, limestone. The lower slopes and valleys consist of less resistant bedrock, generally siltstone and shale with the lower portions of the slopes generally covered by a mantle of colluvium [Crosby, 1978a]. The Hogback Subsection is underlain by a series of sedimentary bedrock units ranging from lower Permian through upper and middle Pennsylvanian age and younger. These sedimentary rocks dip toward the east and as exposed at the ground surface, generally grade younger in age in an easterly direction.

The processes that have contributed to the current geologic setting began during the Laramide Orogeny 55 to 60 million years ago when the mountains toward the west were uplifted resulting in a north-south trending sag or trough along the eastern boundary of the mountains. As the mountains were uplifted and subjected to erosion, the resulting sediments were deposited in the basin trough. These general geologic conditions extend from about Pueblo, Colorado northward into Wyoming. The basin trough is referred to as the Denver Basin. In its deepest part in the vicinity of the City of Denver, the basin was filled with approximately 13,000 ft of sediments.

In the project region, the sedimentary bedrock consists of the lower Permian- and upper and middle Pennsylvanian-age Fountain Formation with a thickness on the order of 1,100 ft. Other younger and overlying bedrock consist of the lower Permian-age Ingleside, Owl Canyon, and Lyons Sandstone, and the lower Triassic and upper Permian-age Lykins Formations. These units range in thickness from about 50 to 200 feet. These

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younger units are located east of the project area and none of these units are present in the project area.

Approximately 28 million years ago, after a long period of relative stability, the entire region began a slow uplift. As a result of this uplift, several thousands of feet of sediments were removed creating a broad gentle surface sloping downward from the mountains towards the east that is referred to as the Colorado Piedmont. During this period and subsequent time, this region has been subjected primarily to the forces of erosion resulting in the present landforms. The published geology in the vicinity of the proposed project is presented in plan on Figure 2.1 and in section on Figure 2.2 (the accompanying key for the geologic map and section are shown on Figure 2.1C; all from Braddock, et al., 1988).

2.2.2 Site Geology

Bedrock. The upper portion of the western slope of the project site is underlain by a series of Precambrian-age metamorphic bedrock units in the vicinity of the proposed main dam site and by granitic rocks of Precambrian age along the west reservoir rim. The lower western slope, valley floor and the eastern slope of the site are underlain by the lower Permian- and upper and middle Pennsylvanian-age sedimentary rocks of the Fountain Formation. Younger rocks, the lower Permian-age Ingleside and Owl Canyon Formations, are located east and beyond the project limits and form the top of the ridge east and above the right (east) abutment area and reservoir rim. These formations consist primarily of moderately- to well-cemented sandstone and siltstone that are more resistant to erosion than the underlying Fountain Formation. The general geology in the vicinity of the project area, including the reservoir and saddle dam, is presented in plan on Figure 2.1 and in section on Figure 2.2. The geology in the immediate vicinity of the proposed main Chimney Hollow dam is presented on Figure 2.4. A geologic interpretation of subsurface conditions in the vicinity of the proposed Chimney Hollow dam is presented in cross-section on Drawing 130. This interpretation is based on the results of the field investigation program.

The surface of the metamorphic and granitic rocks dips easterly beneath the Fountain Formation sedimentary rocks. These sedimentary rocks were deposited directly on the surface of the older metamorphic and granitic rocks. The absence of any older sediment between these two rock units is due either to non-deposition or to erosion of any older materials prior to the deposition of the Fountain Formation. The Fountain Formation is comprised primarily of sandstone, siltstone, and shale. In surface exposures and drill holes, these sedimentary rocks are poorly to moderately cemented and soft to

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moderately hard. These sedimentary rocks are sometimes referred to as red beds based on their color. The poor sorting, lithology, and the lenticular nature of the rock types comprising the Fountain Formation indicate that the material was supplied by sediments derived from a large area of high relief to the west. Rapid deposition and burial is evident from the excellent state of preservation of the feldspars within the sediments [Bradley, 1951].

There are extensive exposures of the Fountain Formation on both the left and right abutments. In the left abutment, these exposures consist solely of sandstone and exposed surfaces are interpreted to represent dip slopes. These exposures all dip towards the east at slopes of about 15 to 18 degrees. The exposed sandstone bedrock is generally characterized by relatively wide-spaced fractures/joints. Joint attitudes obtained in these major sandstone exposures indicate that the joint set attitudes are similar within each exposure but vary widely between individual exposures. All of the observed joints/fractures are generally steeply plunging. There are several other exposures of the sedimentary bedrock within the proposed footprint of the dam structure. The extreme right abutment shown as the Fountain Formation on Figure 2.3 consists primarily of surface bedrock exposures.

The geologic mapping completed in the Carter Lake tunnel excavation described the Fountain Formation as:

torrential cross-bedding, discontinuous layers, buried channels, and the comparative freshness of much of the feldspar suggest that the Fountain Formation resulted from the coalescence of rapidly-formed alluvial fans. In general, the materials are poorly cemented and, therefore, friable and soft. Even the thickest sandstone beds rarely persist for more than a few hundred feet laterally. Perhaps because of the heterogeneity of the formation, it exhibits no regular pattern of jointing. Many of the joints make low angles with the bedding planes. [Reclamation, 1954].

The metamorphic bedrock is intermittently exposed at the surface or is anticipated to be near the surface in the northwestern portion of the project area. Exposures are generally limited to the lower portions of the drainages. The metamorphic bedrock consists primarily of a complex of hornblende gneiss and amphibolites. These rocks have a schistosity (i.e., banded layering due to alignment of crystal grains) and are characterized by a joint set that trends approximately east-west and appears to some degree to control the orientation of the drainages in the upper portion of the left abutment area. The surface of the metamorphic rocks slopes downward towards the east. Between drill holes, from west to east, DH-111, DH-103, DH-104, and DH-105, the surface of these Precambrian rocks is relatively smoothly dipping towards the east at about 4H: 1V (horizontal to vertical) or

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about 14 degrees. The dip of this surface apparently steepens east of drill hole DH-105, as these types of rocks were not encountered in drill Hole DH-101 at a depth of 200 feet. Alternatively, the rocks may be locally offset by a small bedrock fault. The Silver Plume Granite present along the upper west slope of the project site intruded the metamorphic bedrock in the left dam abutment area and likely underlies the metamorphic bedrock at depth. Some stringers of granite were encountered in drill hole DH-104 between the depths of 868 and 1474 and below 1474 to the bottom of the drill hole at depth 1709. Metamorphic bedrock was encountered in drill hole DH-107 beneath a series of sedimentary bedrock units at a depth of 4011and between the depth of 514 and the bottom of the drill hole at a depth of 736.

The Silver Plume Granite is present on the slopes above the western portion of the proposed reservoir. The granite is intermittently exposed throughout the area and generally in the lower portions of drainages originating in the western site area. The granite is generally hard, medium- to coarse-grained, with moderate to widely spaced joints and/or fractures.

Surficial Deposits. The bedrock is generally mantled with overburden deposits of varying thickness. The overburden deposits include colluvium and alluvium. Colluvium comprised primarily of silt, sand and rock fragments mantles most of the upland surfaces with the exception of the extreme upper portions of the right (east) abutment area. These deposits are assumed to be thickest in the lower elevations. Exposures of these deposits are very limited. They are at least in part assumed to be residual material, the result of weathering of the underlying bedrock material that have been transported relatively short distances downslope under gravity and rainfall/snowmelt runoff. On the moderately steep slopes in the eastern portion of the site, the colluvial deposits contain rock fragments ranging up to boulder size.

Alluvial deposits are located in the lower portion of the valley and in the general vicinity of Chimney Hollow Creek. Based on the explorations to date, these deposits vary in composition. In the areas where small streams flow onto the valley floor, the alluvial deposits tend to be coarser with increasing percentages of sand and gravel and with occasional cobbles and boulders. These streams are primarily present in the western portion of the project area. Between these drainages and in the central and upper portion of the valley, the alluvial deposits sampled and tested were typically finer grained materials exhibiting significant silt and clay content.

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Geologic Hazards. No geologic hazards such as landslides or areas of instability have previously been mapped in the project area [Braddock, et al., 1988 and Crosby, 1978b], nor were any observed during the site reconnaissance or field explorations.

Slickensides were observed along bedding planes in the finer-grained portion of the bedrock in drill core and in some of the test pits. Similar features were observed in the geologic mapping of the same material types in the excavation of the Carter Lake tunnel and in excavations for the construction of the Flatiron powerhouse. These features have been observed to function as slip surfaces when exposed during construction where the bedrock dips into the excavations. Some instability was experienced during the excavations for the nearby Flatiron powerhouse.

Filling of the reservoir will result in wetting of the reservoir slopes. This wetting, in conjunction with wave action and seepage induced by raising and lowering the reservoir level, will likely result in minor instability around the reservoir rim. It is anticipated that this will be limited to surface erosion and related, shallow slope movement. These processes are not likely to pose a threat to the safety of the dam, but they will contribute sediment to the reservoir and may locally affect shoreline uses if not controlled.

2.3 Seismicity

A detailed, site-specific probabilistic seismic hazard analysis (PSHA) was completed to develop recommended seismic loadings for the proposed Chimney Hollow and Jasper North dam sites. The complete report of that assessment is presented in Appendix B. This chapter includes a summary of the PSHA for the Chimney Hollow Dam site.

2.3.1 Seismotectonic Setting

The Chimney Hollow Dam site lies within the Southern Rocky Mountains (subdivided into Eastern and Western Mountains for purposes of the PSHA), bordered on the west by the Rio Grande rift and on the east by the Great Plains Province. The Great Plains Province comprises part of the stable continental interior of the U.S., and tectonism associated with plate boundary interactions probably ceased about 100 Ma (million years ago) in most of this province. For this reason, zones of major deformation are rare in this province, but occasional, late Quaternary faults and dispersed, diffuse historical seismicity are observed near the western boundary.

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The Southern Rocky Mountains province extends from northern New Mexico to south-central Wyoming and is characterized by large, asymmetrical uplifts of Precambrian rock and adjacent deep basins. Most of this crustal deformation likely occurred during the Laramide orogeny (late Cretaceous to Eocene), and has since been modified by regional extension, fluvial incision, and denudation. During the late Neogene and Quaternary, the locus of extension was the Rio Grande Rift, which consists of a series of north-south-trending grabens that form a nearly continuous, interconnected tectonic depression from Wyoming to the Mexican border. In general, the most active late Quaternary faults in Colorado are associated with the portion of the Rio Grande Rift extending through western Colorado.

In the vicinity of the dam site, the eastern margin of the Southern Rocky Mountains Province is defined by the northern Front Range of the Rocky Mountains. The Front Range has several known or suspected north- to northwest-striking late Cenozoic normal faults that are typically found within the hanging walls of Laramide thrust faults. Defining the late Quaternary activity of such faults is difficult, because of the scarcity of deposits of that age. Consequently, very few, if any, late Quaternary faults have been documented in the Front Range. According to available literature, this portion of the Southern Rocky Mountains and Great Plains Provinces is generally characterized by ENE-WSW extensional crustal stresses, normal faulting, and low to moderate levels of seismicity.

2.3.2 Historical Seismicity

A historical catalog containing 1,110 events was compiled for the study region. Primary data sources used in the compilation include: the Decade of North American Geology (DNAG) catalog; the Stover, Reagor, and Algermission U.S. historical catalog; the National Earthquake Information Centers Preliminary Determination of Epicenters; and the Microgeophysics Corporation Front Range Network catalogs (1983-1993).

The concentrated seismicity along the Front Range south of Boulder is a product of the Front Range network that operated from 1983 to 1993. Earthquakes as small as about ML (local magnitude) 1 were recorded by this network, in contrast to other parts of the region where the location threshold is probably ML 3.

The most significant earthquake in Colorados history is the 1882 event. The available information on that event is discussed in detail in Appendix B. There is significant uncertainty regarding the location, magnitude, and source of the 1882 event. The best estimate for the magnitude of the event is M 6.6. Its source remains unknown to this day. Based on review of available

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information, a location at latitude 40.5 N. and longitude 105.5 W. was adopted for this study. This places the earthquake about 25 mi (40 km) from the proposed Chimney Hollow dam site. The location is probably accurate, however, to only within 0.5 latitude and longitude, or roughly 30 mi (50 km). Regardless of the actual location of the 1882 earthquake, it is difficult to argue against the possible occurrence of similar-sized events anywhere in the Rocky Mountain portion of Colorado. Therefore, in the PSHA, it was assumed that background earthquakes in the Colorado Rocky Mountains can be at least as large as M (magnitude) 6-3/4 +0.3.

2.3.3 Probabilistic Seismic Hazard Analysis (PSHA)

The intent of the PSHA was to complete a state-of-the-practice probabilistic analysis of the seismic hazards posed by all known seismic sources that could potentially generate strong ground shaking at the site. Seismic source characterization is concerned with three fundamental elements: (1) the identification of significant sources of earthquakes; (2) estimation of the maximum size of these earthquakes; and (3) estimation of the rates at which they occur.

Seven identified faults and fault groups and two background source zones were considered in the PSHA. The specific faults and fault zones considered were a) the Frontal fault, b) the Golden fault, c) the Mosquito faults, d) unnamed faults in Granby basin, e) unnamed faults in Williams Fork Valley, f) the Valmont fault, and g) the Williams Fork Mountains fault. Available data were used to characterize these faults and fault zones for input into the PSHA model.

The two background source zones considered in the model were the Great Plains and the Eastern Mountains zones. Available data, including the available earthquake catalog, were used to characterize these source zones for input into the PSHA model.

Details of the characterizations of the faults and fault groups and the background source zones are discussed in Appendix B.

Using the source zones noted above, a probabilistic seismic hazard analysis [Cornell, 1968] was performed using the computer program HAZ-32. Uncertainties in the input parameters were characterized as detailed in Appendix B.

To calculate ground motions in probabilistic analyses, empirical crustal attenuation relationships are generally used. However, no empirical attenuation relationships have been developed for central Colorado because

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no strong motion records exist for the state. To address the uncertainty regarding the attenuation relationships, five relationships for both the western United States and the Midcontinent were considered, with all of the relationships weighted equally.

2.3.4 PSHA Results

The results of the probabilistic seismic hazard analysis are presented in terms of ground motion as a function of annual exceedance probability. This probability is the reciprocal of the average return period. The mean, median (50th percentile), 5th, 15th, 85th, and 95th percentile hazard curves for peak horizontal acceleration are included in Appendix B. These fractiles indicate the range of uncertainties about the mean hazard. The mean 10,000-year return period (annual exceedance probability of 0.0001) peak horizontal acceleration is 0.21 g for the Chimney Hollow Dam site. The seismic hazard curve for the 1.0 sec horizontal spectral acceleration and the 10,000-year return period Uniform Hazard Spectrum (UHS) are presented in Appendix B. Evaluation of the PSHA results presented in Appendix B indicates that the Eastern Mountains background zone dominates the seismic hazard for all return periods for the Chimney Hollow Dam site. The estimated maximum magnitude earthquake associated with the Eastern Mountain zone in the model is an M 6-3/4 event.

2.3.5 Comparison with Other Studies

Two other probabilistic seismic hazard analyses have been performed recently that are relevant to the results of this PSHA.

The U.S. Bureau of Reclamation (Reclamation) recently completed a site-specific probabilistic seismic hazard analysis for Carter Lake Dam, as part of their Comprehensive Facility Review process. Based on discussions with Reclamation personnel, it is understood that Reclamations analysis resulted in an estimated 10,000-year return period peak horizontal acceleration of 0.28 g. Carter Lake Dam is located about 1 mi (1.5 km) east of the proposed Chimney Hollow Dam site. A peak horizontal acceleration of 0.28 g with a return period of 10,000 years corresponds to slightly less than the 85th percentile in the PSHA completed for the Chimney Hollow site. The U.S. Geological Surveys (USGS) National Hazard Maps, which are the basis for the current U.S. building codes, provide estimated probabilistic ground motions for the U.S. for an annual exceedance probability of 2 percent in 50 years (2,500-year return period). In the 1996 maps, the 2,500-year return period peak horizontal acceleration for the Chimney Hollow Dam site is 0.11. In the updated 2002 maps, the 2,500-year return period peak horizontal

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acceleration for the Chimney Hollow Dam site is 0.10. The site-specific mean 2,500-year return period peak horizontal acceleration from the PSHA completed for the Chimney Hollow Dam site is 0.10 g, which agrees well with the USGS values.

2.3.6 Recommended Ground Motions

Traditionally, seismic design ground motions for dams have been developed using a deterministic approach, often using the concept of a Maximum Credible Earthquake. Probabilistic seismic hazard analysis has become increasingly used in dam design and dam safety analyses and is now the accepted approach for Reclamation. The U.S. Army Corps of Engineers uses both approaches. Both the U.S. Society on Dams, USSD (previously the U.S. Committee on Large Dams [USCOLD]) and the International Congress on Large Dams (ICOLD) endorse a probabilistic approach to developing a Maximum Design Earthquake (MDE). USSD and ICOLD recommend an annual probability of recurrence in the range of 1/3,000 to 1/10,000 for determining an MCE.

Because the proposed Chimney Hollow Dam will be classified as a Colorado Class I high hazard dam, it is recommended that the seismic design ground motions be based on the mean values for a return period of 10,000 years. The site-specific PSHA reported herein indicates a mean 10,000-year return period peak horizontal acceleration of 0.21 g for the Chimney Hollow Dam site. For comparison, a mean estimated peak acceleration of 0.17 g with a return period of 10,000 years was accepted by the State of Colorado for the enlargement of the Green Ridge Glade Reservoir, located about seven (7) miles to north of the Chimney Hollow site. Reclamations most recent published PSHAs for the Horsetooth Reservoir Dams and Carter Lake Dam, both located near the Chimney Hollow site, indicate mean 10,000-year return period peak ground accelerations of 0.17 g and 0.36 g, respectively. However, based on discussions and email correspondence with Dr. Jon Ake of Reclamations Seismotectonic Group, it appears that Reclamation will likely reduce the estimated mean 10,000-year peak ground acceleration for Carter Lake to about 0.28 g.1 Because the Chimney Hollow Dam will be located immediately upstream of Reclamations Flatiron Dam and Power plant, it is appropriate to consider the ground motion values that Reclamation is using for these nearby structures. Therefore, we recommend a peak horizontal acceleration of 0.28 g for use in the preliminary design of the dam.

1 Contact with Dr. Ake occurred in 2003.

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The scope of the PSHA completed for this study was limited to consideration of peak ground accelerations and response spectra. Recommended earthquake time histories were not considered. Peak ground accelerations are sufficient for preliminary design, but time histories will be required for final design.

The recommended value is conservative based on state regulations that have been published since the PSHA was completed. The Colorado State Engineers Office (SEO) Criteria [SEO, 2007] published in 2007 requires that High Hazard Dams be designed for either the MCE or for an earthquake with a minimum 5,000-year return frequency. Since the recommended value is based on an earthquake with a 10,000-year return frequency, the value meets the SEO Criteria. It is possible to use a smaller value for peak horizontal acceleration based on the new SEO criteria [SEO, 2007] for final design.

2.4 Climate and Evaporation

2.4.1 Climate

The Chimney Hollow Dam site is located in the foothills of the Rocky Mountains, one hogback to the west of the Front Range and about 600 feet higher in elevation. The climate at the site can be expected to be similar to that of the Front Range, between Longmont and Fort Collins, with slightly greater precipitation and slightly lower average temperatures. Table 2.1 summarizes the average rainfall and maximum and minimum temperatures expected at the site each month based on historical data. Average precipitation is defined as the mean monthly precipitation, including rain, snow, hail, etc. Average maximum and minimum temperatures are defined as the monthly mean of the maximum and minimum daily temperatures, respectively.

Table 2.1 - Climate Summary

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year

Ave. Precipitation

(inches) 0.4 0.5 1.3 2.0 2.8 1.9 1.6 1.4 1.5 1.1 0.6 0.5 15.6

Ave. Daily High Temp.

( F) 41.9 45.0 50.5 59.9 69.1 79.2 86.2 83.7 74.5 64.2 50.9 43.0 62.2

Ave. Daily Low Temp.

( F) 12.9 16.7 23.4 31.6 40.5 48.4 54.1 52.7 44.1 34.0 23.4 15.1 33.1

Source: www.worldclimate.com

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2.4.2 Evaporation

Evaporation potential is limited at Chimney Hollow because the proposed reservoir is relatively deep compared with the surface area. The surface areas for varying water surface elevations can be viewed on the area-capacity curve in Chapter 4. Evaporation at the site is expected to total about two feet per year [Boyle, 2003]. A monthly breakdown of estimated evaporation based on a water year is presented in Table 2.2.

Table 2.2 Free Water Surface Evaporation Summary

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Year

Evaporation (ft)

0.14 0.07 0.06 0.06 0.08 0.07 0.16 0.16 0.29 0.35 0.33 0.23 2.00

.

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3.0 Field Exploration and Laboratory Testing Program

3.1 General

The purpose of the field exploration and laboratory testing program was to provide a level of information suitable to complete the preliminary design of the proposed project. The locations and depths of the exploration holes were generally set to comply with dam design and exploration requirements set in the State Engineers guidelines for dams founded on rock.

The field exploration program consisted of the drilling and sampling of a series of continuously cored holes in the area of the proposed main dam, saddle dam, and reservoir. In two separate efforts, auger holes and test pits were excavated in the reservoir area for the purpose of reservoir and borrow exploration. In addition, geophysical seismic refraction surveys were completed along the main dam alignment and in the northern portion of a proposed rock borrow area.

A smaller dam and reservoir project in the Chimney Hollow Valley was previously considered in 1997. The boundary limits of the smaller dam and reservoir project lie within the limits of the currently proposed dam and reservoir project. The previously considered dam alignment (with a crest elevation of 5850) is located approximately 1200 ft upstream (south) of the proposed main dam site for this project. The geotechnical exploration for the smaller facility included the completion of two drill holes, GB-101 and GB-102, along the proposed dam axis, and 20 auger holes in the proposed reservoir area for borrow exploration [GEI, 1997]. The locations of these explorations are shown on Drawing 120.

3.2 Site Exploration

Two separate explorations were completed for the proposed Chimney Hollow Dam site as part of this preliminary design work by AECOM. Phase I of the exploration was completed in 2003 and consisted of drill holes, auger holes, and test pits. Phase II of exploration was completed in 2005 to supplement soil borrow material data collected in 2003 and to better quantify the amount of suitable material for use in a clay core for an earthfill/rockfill dam. The drill logs for the Phase I exploration are located in Appendix C1, and the Phase II logs are located in Appendix C3.

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The Phase I geotechnical exploration for the proposed Chimney Hollow Dam and Reservoir Project consisted of ten drill holes in the vicinity of the proposed main dam, one drill hole in the area of the upstream saddle dam, and one drill hole in the right side of the proposed reservoir. A proposed rock quarry area was explored with one core hole and three pneumatic hammer holes. All cored drill holes were completed during the Phase I exploration.

During Phase I exploration, four auger holes were completed along the general proposed alignment of a downstream outlet conduit structure. In addition, 12 auger holes and 14 test pits were completed within the proposed reservoir area for borrow exploration. The location of these explorations is shown on Drawing 120.

Phase II exploration was based on the data collected during the Phase I borrow area exploration. The exploration program was limited to the area of likely suitable low permeability core material within the reservoir footprint as indicated by previous field work, and consisted of drilling 25 auger holes followed by excavating 36 test pits. The resulting information from the field and laboratory work was used to estimate the potential volume of material suitable for a clay core dam. The locations of these explorations are also shown on Drawing 120.

3.2.1 Reconnaissance

Reconnaissance level geologic mapping was completed during the 2003 field exploration. The primary purpose of the mapping was to augment and confirm the published geologic mapping. Site Geology is described in Section 2.2.2. An additional reconnaissance study of the surface geology along the potential penstock alignments was also performed during the 2005 field exploration program.

3.2.2 Drilling/Sampling

Drilling and sampling the exploration holes was accomplished by auger and SPT/split spoon sampling through the unconsolidated material and continuous core drilling in the bedrock materials. All of the exploration holes were drilled vertically. Core orientation was based on the general strike of structures observed in the surface geology and the dip of planes/features in the core measured from horizontal. Water pressure tests were completed at adjoining approximately 10-foot long intervals once bedrock was intercepted. The groundwater level in each of the boreholes was recorded based on the depth at which free water was encountered during drilling. No piezometers were installed in the boreholes for long term water level monitoring.

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Main Dam

Geologic surface mapping and a series of ten drill holes were completed to explore geologic and geotechnical conditions in the foundation of the proposed main dam site. One of the drill holes (DH-102) is located in the ridge immediately downstream of, and adjacent to, the left (west) abutment. This area was originally selected as the left abutment for the dam. Shortly following the start of the field explorations, the dam alignment was moved upstream on the left side to the next ridge to avoid conflicts with existing Reclamation facilities and to take advantage of more favorable topography. All remaining drilling was completed along the revised dam alignment. Drill hole exploration in the right (east) abutment of the dam was limited due to the steep terrain and limited access into the area. The drill hole locations are shown on Drawing 120 and the logs are presented in Appendices C1 and C3.

Reservoir

Exploration in the reservoir area with respect to foundation conditions and potential seepage was limited to a single drill hole (DH-110). This drill hole was located in the middle area of the reservoir and towards the eastern side (see Drawing 120). Originally, three drill holes were planned for exploration along the eastern side of the reservoir. However, two of the reservoir holes were relocated based on dam height revisions and findings in the left abutment area. The proposed height of the dam was raised to increase storage capacity in the Chimney Hollow Reservoir as a result of updated water model runs and detailed site topography. This increased the height of the saddle dam such that it was determined necessary to drill at least one exploration hole along centerline in the maximum section of the saddle dam. The upstream reservoir hole was relocated to this area. Also, based on findings in DH-103, a second drill hole (DH-111) in this area of the left abutment was believed to be advantageous. The downstream reservoir hole was moved to this location. Based on the geology along the eastern side of the reservoir, sufficient data regarding the reservoir foundation and potential reservoir seepage were available by considering the data collected from the right abutment drill holes, the upstream toe drill hole and the saddle dam drill hole in conjunction with DH-110.

Saddle Dam

Subsurface conditions in the vicinity of the proposed saddle dam site were explored by drill hole DH-109. This hole augmented data from a series of three auger holes completed during the 1997 exploration for the previously proposed Chimney Hollow dam and reservoir project.

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Drill hole DH-109 is located near the maximum section of the proposed saddle dam, as shown on Drawing 120. The unconsolidated materials encountered in the drill hole were sampled using the Standard Penetration Test (SPT). The bedrock portion of the drill hole was continuously sampled by coring. Water pressure tests were completed in the bedrock portion of the drill hole.

Two test pits, TP-232 and TP-201 were also completed in the vicinity of the proposed saddle dam to explore for borrow material. The test pits were completed during Phase II of exploration. The test pits were excavated to a maximum of 15 to 16 feet or to bedrock/refusal.

The three auger holes from the 1997 site explorations were identified as GB-G1, GB-G2, and GB-G3. Auger hole GB-G1 was located in the vicinity of the right (west) abutment area of the saddle dam, auger hole GB-G2 was located in the vicinity of drill hole DH-109 and in the vicinity of the maximum saddle dam section, and auger hole GB-G3 was located in the vicinity of the left (east) abutment area. These auger holes were completed for borrow exploration and did not penetrate into the foundation bedrock beyond about five feet based on the 1997 report.

Inlet/Outlet System

A low level inlet/outlet conduit encased in reinforced concrete and located just right of the maximum section was considered the primary option for the outlet works at the onset of the exploration program. No foundation exploration within the dam footprint was performed specific to the outlet works. It was assumed that the dam foundation exploration would provide sufficient data for this preliminary phase of design. Four auger holes were drilled along a potential conveyance alignment from the downstream toe of the dam to the Flatiron Reservoir to characterize foundation conditions for excavation and burial of the pipeline. The drill hole locations (CB-101 through CB-104) are shown on Drawing 120 and the logs are presented in Appendix C1. Based on the field exploration along the alignment and observations of the site conditions, it appears that the lower portion of Chimney Hollow Creek has been realigned as a result of the construction for the Flatiron Hydroelectric Facility. The area west of Chimney Hollow Creek and immediately west of the proposed outlet conduit alignment appears to consist of excess earth material from construction of the Flatiron Hydroelectric Facility. Additional exploration is recommended when the final pipeline alignment is established to determine subsurface conditions along the alignment.

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Subsequent to the Phase I field explorations, tunnel diversions in either abutment were identified as the preferred option for the low level inlet/outlet. A reconnaissance survey was conducted for this option during Phase II exploration.

Geologic conditions for potential alignments were based on the survey, and on data from the dam foundation exploration program. Geologic data for the right abutment alignment was supplemented with published data from the Carter Lake pressure tunnel construction. For the penstock connection along and above the left dam abutment, a hand-held Global Position System (GPS) unit was used to locate rock outcroppings and other features during the reconnaissance survey (designated as G1 through G12 on Drawing 120).

Borrow Areas

Two potential sources for borrow material were explored under this program. One was a rock borrow area on the west side of the reservoir and the other was the earthfill borrow area in the valley bottom (see Drawing 110). Exploration in the borrow areas consisted of core and percussion drilling, a geophysical survey line, and a series of test pits and auger holes.

The purpose of the test pits and auger holes was to supplement the 1997 exploration data and to collect bulk soil samples for laboratory testing. The locations of these explorations are shown on Drawing 120 and the logs are presented in Appendices C1 and C3.

Rock Borrow. The potential rock borrow area was explored with drill hole DH-113, three percussion hammer holes, HH-1 through HH-3, and two auger holes, BB-101 and BB-102. In addition, a seismic refraction geophysical survey was completed in the northern portion of the proposed rock borrow area. The bedrock portion of the drill hole was continuously sampled by coring from a depth of nine feet to the bottom of the drill hole at a depth of 150 feet. No water pressure tests were completed in the bedrock portion of drill hole DH-113. The hammer holes were completed using a downhole pneumatic hammer with air to clear cuttings from the holes. The depth of the hammer holes ranged between 138 and 150 feet. Cuttings from the hammer holes were collected at approximate five-foot intervals and logged for lithology (i.e., rock type). In addition, drilling characteristics and groundwater conditions were noted and recorded, as encountered, and drilling rates (time per 5-ft interval) were logged as an indication of relative hardness and to assess consistency in the rock among the holes. Groundwater was encountered in all of the holes, which resulted in difficulty blowing cuttings to the surface and generally increasing the drilling time. It is inferred that the groundwater represents water present in joints and/or fractures. Two auger holes, BB-101

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and BB-102, were completed to refusal in areas of low ground relief and in areas between bedrock exposures to explore the near-surface conditions. No coring was completed in these auger holes. Locations of the exploration holes are shown on Drawing 120 and the logs are located in Appendix C1.

A seismic refraction survey was completed in the northern portion of the proposed rock borrow area. The purpose of the survey line was to evaluate the hardness and consistency of the bedrock in the proposed rock borrow area. The seismic line was approximately 460 feet long and was completed at an approximate right angle to, and across, a northeast-southwest trending drainage that is mapped as a shear contact between the main body of the Silver Plume Granite and a metamorphosed portion of the unit mapped within the Moose Mountain Shear Zone. The location of the seismic line is shown on Drawing 120. The results of the seismic refraction survey are presented in Appendix C7.

Soil Borrow. The Phase I exploration of soil borrow in the valley area consisted of the completion of 12 auger holes, BB-103 through BB-114, and the excavation of 14 test pits, TP-1 through TP-14. The auger holes were located in the lower elevations of the Chimney Hollow Creek drainage and in relatively close proximity to the creek. The holes were completed to depths ranging between about 19 and 26 feet. The unconsolidated materials in the auger holes were sampled by performing a Standard Penetration Test (SPT) with a standard split-spoon sampler. No coring was completed in the bedrock portions of the auger holes. The test pits were excavated to refusal or in a few cases to the limits of the backhoe (approximately 12 to 15 feet). Bulk samples were collected from selected test pits for laboratory testing (see Section 3.4). The Phase I drill hole and test pit locations are shown on Drawing 120 and the logs are presented in Appendix C1.

The Phase II exploration of soil borrow in the valley area consisted of the completion of 25 auger holes, BB-201 through BB-226, and the excavation of 36 test pits, TP-201 through TP-236. The auger holes were completed to depths ranging between about 6 and 40 feet. The sampling process consisted of the collection of small, disturbed samples obtained from the flights of the augers. The general procedures for the auger holes consisted of drilling the initial approximately 10 feet of the hole, withdrawing the auger, and logging and sampling the materials on the augers. The auger was then reinserted into the drill hole and advanced and the procedure repeated until the auger hole was terminated by either refusal in advancing the auger or interception of bedrock.

The unconsolidated materials in the auger holes were sampled by performing a Standard Penetration Test (SPT) using a standard split-spoon sampler. No

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coring was completed in the bedrock portions of the auger holes. The test pits were excavated to refusal or in a few cases to the limits of the backhoe (approximately 12 to 15 feet). Bulk samples were collected from selected test pits for laboratory testing (see Section 3.4). The Phase II drill hole and test pit locations are shown on Drawing 120 and the logs are presented in Appendix C3.

As stated above, the Phase I and Phase II test pit and auger hole explorations were completed to supplement the 1997 borrow exploration data. Therefore the locations of the auger holes and test pits were selected to fill in areas not explored in 1997 to improve the estimate of potential borrow volume available within the valley.

3.3 Offsite Borrow Sources

The need for offsite borrow sources will be a function of the type of dam to be constructed and the quality of sands and gravels that can be produced from the proposed rock borrow. Based on the dam types presented in Chapter 5, the primary offsite borrow sources required for dam construction will be cement and aggregate for concrete production, and bituminous materials for an asphaltic concrete core. For the purposes of this report it is assumed that sand and gravel for filter and transition zones will be produced from the proposed rock borrow source. Aggregate and cement for concrete will be hauled to the site in bulk to supply an onsite batch plant. The concrete aggregate may be obtained from the rock borrow source with further testing to determine its suitability. It is estimated that about 71,000 cubic yards of concrete could be required for the Chimney Hollow concrete face rockfill dam (CFRD) alternative, including the spillway and inlet/outlet works. Depending upon the actual concrete mix; the cement, sand, and gravel quantities will be on the order of 15,000 cubic yards, 25,000 cubic yards, and 45,000 cubic yards, respectively. Based on review of aggregate and ready-mix concrete suppliers north of Denver, Colorado there are sufficient commercial resources available to the project for the supply of these materials. At this time, no single commercial resource was identified as a primary supplier.

Bitumen materials for the asphaltic core are also available from area commercial suppliers. It is estimated that about 95,000 cubic yards of asphaltic concrete core material would be required for this alternative dam type. No specific commercial resource was identified as a preferred supplier at the time of this report. As in the case of the concrete materials, aggregates required for the mix design may be produced from the rock borrow source. Therefore, only the bitumen component of the mix may be required from a commercial resource.

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3.4 Laboratory Testing

The laboratory testing program was developed to estimate the range of various geotechnical characteristics of the foundation and borrow materials sampled in the field exploration program. The primary concern with the foundation material was the estimated range of bedrock strength for a dam approximately 360 feet in structural height (above the excavated foundation level). The concern for the borrow materials included suitability of the fine grained material for use as a low permeability core material and of the bedrock within the impoundment as a rock fill for the shells of the dam. It was assumed in the testing program that distinctions between materials for use in the upstream saddle dam and the main dam was not necessary as material suitable for the main dam would also be suitable for the upstream saddle dam. Also, for this level of study, it was assumed that the foundation materials of the saddle dam would be sufficient in regards to strength characteristics and stability of the dam based on observation of the core samples and review of the drill log for the boring. Therefore, no core samples from the upstream saddle dam foundation exploration boring were selected for testing under this laboratory program. Lab test data from Phase I and Phase II testing is summarized in this chapter, and the detailed results from the two phases are presented in Appendices C2 and C4, respectively.

3.4.1 Foundation Material

Unconfined compressive strength tests (with Youngs modulus determination) were performed on selected core samples of the foundation material. The selection of core samples was based on review of the core logs and observation of core samples to obtain samples representative of the main types of materials encountered in the exploration holes. Below is a summary description of the samples selected. The test results for these samples are presented in Table 3.1. The samples described below are referred to by a Sample number to facilitate the following discussion; note that these sample numbers are not used on the field core boring logs or laboratory test sheets.

Samples 1 and 2 Borehole DH-101 at about 25.1 ft and 75.5 ft respectively: These samples were chosen to represent the sandstone encountered in the area of the maximum section of the proposed dam. The upper sample was selected as representative of the near surface bedrock in the valley area of the dam and towards the right (east) abutment. The lower sample was selected as typical bedrock and representative of the general foundation bedrock in the valley area and right abutment.

Samples 3 and 4 Borehole DH-103 at about 49.7 ft and borehole DH-104 at about 52.7 ft, respectively: These samples were selected as

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representative of the gneiss bedrock encountered in the left abutment of the dam. Sample No. 3 (borehole DH-103 at 49.7) was selected as representative of the weathered gneiss samples recovered from which a test sample could be obtained. Sample No. 4 was selected as representative of typical, less weathered or altered, gneiss bedrock encountered in the boreholes.

Samples 5 and 6 Borehole DH-105 at about 75.9 ft and 77.5 ft, respectively: Samples from this borehole were selected as representative of the sedimentary bedrock units in the left (west) abutment that overlie the gneiss. The sedimentary units encountered in the left abutment consisted of more fine-grained sandstone, siltstone, and claystone than in the valley and right abutment areas. Although the samples selected are relatively close together in elevation, they represent two different types of material encountered in the borehole. The upper sample selected was considered representative of the interbedded fine-grained sandstone, siltstone, and claystone. The lower sample was selected as representative of poorly to moderately cemented sandstone encountered in the sedimentary units in the left abutment.

The test results show the variability in the strengths of the bedrock present in the dam foundation. However, the data also show that the bedrock is sufficiently competent to support an earthen or rockfill dam. The test values, however, indicate that the foundation characteristics (strength and modulus) may not be suitable for dam types that result in high or concentrated foundation loading conditions (i.e. conventional concrete or RCC gravity dams) at the estimated heights of the Chimney Hollow Dam. The data also indicate that foundation response to the dam loads (i.e., elastic compression/deformation) will likely not be linear (directly proportional to the load) along the dam centerline or in the transverse (i.e., upstream-downstream) direction in the left abutment. Along the dam centerline and on transverse sections in the valley and right abutment areas, foundation response to the dam loading may be more linear since the geotechnical/geological conditions (rock type and physical properties) are expected to be more consistent in these areas.

Samples 1 and 2 were selected to represent the predominant rock type in the valley and right abutment reaches of the main dam foundation. As anticipated, these samples exhibited stress-strain behavior and failure modes characteristic of a homogeneous rock mass. As previously discussed, however, there are interbeds and zones of weaker material within the predominant sandstone rock mass in these areas. The shear strength of these zones and/or structural discontinuities (e.g., bedding planes and

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joints/fractures) in the otherwise predominantly sandstone rock mass may control the stability of the foundation and thus the overlying slope rather than the intact strength of the more competent sandstone. Samples of these weaker zones and discontinuities should be sampled and tested (either in situ or in the laboratory as appropriate) during final design if they are located and oriented in a manner that could reduce stability of the dam and foundation.

Test results of Samples 5 and 6 indicate that the sedimentary units overlying the gneiss in the left abutment are notably weaker than the sandstone sampled from the valley area. The failure mode of Sample 5 was controlled by structural discontinuity while Sample 6 failed in shear characteristic of a more homogeneous rock mass, as did Samples 1 and 2. The unconfined compressive strengths of the two samples were similar while the Youngs modulus value for Sample 6 was significantly lower than for Sample 5 indicating a softer, more deformable rock under load. Based on review of the logs and observation of the core, it was found that many zones of soft rock and soil-like material are present in the upper portions of the boreholes. A zone of more competent sandstone was encountered about 10 feet below Sample 6 extending to just above the contact of the gneiss in borehole DH-105. This material was not sampled and tested as it represented a more competent, but less typical foundation material than the material higher in the borehole (i.e., near ground surface). In addition, layers of noticeably softer rock (weathered and/or altered zones) were observed in the core samples but were not tested at this level of study. Such zones will require special sampling and handling during future field and laboratory investigations to support final design of the dam.

Included among the softer intervals encountered in the core samples were shales (interbedded with sandstone) that exhibited clayey, slickenedsided surfaces and degradation by spalling along bedding planes after a short period of exposure to air (i.e., air slaking). Direct shear or other tests were not run on such samples under this preliminary laboratory program. Their potential impact on the dam foundation was instead evaluated by estimating strength values for the material based on experience with similar materials in the area.

Test results for the gneiss samples (Samples 3 and 4) show that there is a significant range of material properties depending upon the degree of weathering or alteration. This agrees with the variances noted in the core logs and observed in the core samples. Based on the test values, the gneiss in the left abutment is, however, judged sufficiently strong overall to support the proposed dam. Failure modes of the samples indicate that the degree of weathering/alteration, structure and fracture patterns control the strength of the material. Sampling and testing of the more weathered, altered and/or

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structurally disturbed material may be required in final design to confirm the stability of the dam as impacted by the foundation, and the extent of foundation preparations and possibly treatment and/or improvement required in the left abutment. The primary focus of future testing will be highly weathered/altered material near surface and zones of the weaker material and structural discontinuities that may impact dam and foundation stability in the sedimentary units.

3.4.2 Phase I Borrow Material

Potential borrow material was sampled and tested to determine the suitability of the material for use in the proposed dam. Two primary borrow sources were identified in the field explorations and sampled for testing. These were the granite along the west-central rim of the reservoir area for use as rockfill in the dam shells of a zoned earthfill/rockfill dam or CFRD, and the fine-grained alluvial/colluvial deposits over the floor and lower slopes of the reservoir area for use as low permeability fill in the core of the dam. Below are discussions of the laboratory testing program and results for each of the two borrow types. A summary of the Phase I test results is presented in Table 3.1.

Rockfill Material

The suitability of the rockfill was based on review of the seismic refraction survey results, the logs of core hole DH-113 and the hammer and auger holes described previously, and observation of the core samples from DH-113. A section of core sample (borehole DH-113 @ 47.5 ft to 55 ft) was selected for abrasion testing as an initial indicator of the competency of the rock relative to breakdown under handling and environmental exposure (including freeze-thaw cycles). The core sample was crushed to minus 2-inch size material and evaluated for loss in the Los Angeles Abrasion test. Test results show about 35 percent wear following completion of the test. This was judged as an initial indication that the granitic rock would likely be suitable for use as rockfill in either a zoned earthfill/rockfill dam CFRD or asphaltic concrete core rockfill dam.

Core Material

Index tests (Atterberg Limits and Grain Size Distribution) were run on bulk test pit samples and auger hole samples collected from the reservoir area. This was do