GEOTECHNICAL EVALUATION - Redmond

GEOTECHNICAL EVALUATION TO#17-02 Redwood Road Slump 10100 Redmond-Woodinville Road NE Redmond, Washington

Prepared for: City of Redmond

Project No. 160422-01 February 1, 2018

V:\160422 On-Call Geotech for City of Redmond\Deliverables\Redwood Road Geotechnical\Redwood Road-Geotechnical Report_Final_20180201.docx

GEOTECHNICAL EVALUATION TO#17-02 Redwood Road Slump

10100 Redmond-Woodinville Road NE Redmond, Washington Prepared for: City of Redmond

Project No. 160422-01 February 1, 2018

Aspect Consulting, LLC

Mark Swank, LG, LEG Senior Engineering Geologist [email protected]

Jesse Favia, LG Senior Staff Geologist [email protected]

Erik Andersen, PE Senior Associate Geotechnical Engineer [email protected]

e a r t h + w a t e r Aspect Consulting, LLC 401 2nd Avenue S. Suite 201 Seattle, WA 98104 206.328.7443 www.aspectconsulting.com

mailto:[email protected]

mailto:[email protected]

ASPECT CONSULTING

PROJECT NO. 160422-01 FEBRUARY 1, 2018 i

Contents

1 Introduction ................................................................................................. 1 1.1 Scope of Services ..................................................................................... 1 1.2 Project Description ................................................................................... 1 1.3 Summary of Findings and Recommendations ....................................... 1

2 Site Conditions ............................................................................................ 3 2.1 Surface Conditions ................................................................................... 3

2.1.1 Drainage .............................................................................................. 3 2.2 Geologic Setting ....................................................................................... 4

2.2.1 Geology ............................................................................................... 4 2.2.2 Faults and Seismicity .......................................................................... 4

2.3 Soil and Groundwater Conditions ........................................................... 5 2.3.1 Pavement and Roadway Fill ............................................................... 5 2.3.2 Alluvium .............................................................................................. 6 2.3.3 Landslide Debris ................................................................................. 6 2.3.4 Advance Outwash ............................................................................... 6 2.3.5 Pre-Fraser Non-Glacial Deposits ........................................................ 6 2.3.6 Groundwater ....................................................................................... 7

3 Conclusions and Recommendations ......................................................... 8 3.1 Slope Stability Analyses ........................................................................... 8 3.2 Conceptual Stabilization Alternatives Recommendations ..................... 9

3.2.1 Horizontal Drains .............................................................................. 10 3.2.2 Cantilevered Soldier Pile Wall .......................................................... 10

3.3 Stormwater Improvements .................................................................... 11

4 Future Work ............................................................................................... 12

5 References .................................................................................................. 13

6 Limitations ................................................................................................. 14

ASPECT CONSULTING

PROJECT NO. 160422-01 FEBRUARY 1, 2018 ii

List of Tables 1 Summary of Slope Stability Analyses Results

List of Figures 1 Site Location Map

2 Site and Exploration Map

3 Geologic Cross Section A-A’ and Slope Stability Model Setup

4 Geologic Cross Section B-B’

List of Appendices A Soil Boring Logs

B Slope Stability Analyses

C Site Photograph

ASPECT CONSULTING

PROJECT NO. 160422-01 FEBRUARY 1, 2018 1

1 Introduction

This geotechnical evaluation summarizes Aspect Consulting, LLC’s (Aspect) observations, conclusions, and recommendations made for the City of Redmond’s (City, Client) improvements along Redmond-Woodinville Road NE in Redmond, Washington (Site). The Site location is shown on Figure 1, Site Location Map.

1.1 Scope of Services Our scope of services included a literature review, Site reconnaissance, subsurface explorations, and geotechnical engineering analyses. This report includes:

Existing literature review findings;

Descriptions of Site conditions including geologic setting, surface and landslide features, reconnaissance map annotated with reconnaissance observations and photographs;

Locations, logs, and data from soil borings and open-hole piezometer;

A summary of geologic and geotechnical subsurface conditions and design parameters;

Conclusions of probable mechanism(s) and causation(s) of slumping movement;

Seismic design parameters;

Quantitative, limit equilibrium slope stability analyses of existing landslide conditions and results; and

Conceptual landslide mitigation alternatives.

1.2 Project Description Based on the surficial pavement cracks that parallel the embankment slope, we understand an approximately 100-foot-long section in the southbound lane of Redmond-Woodinville Road NE (the road) is unstable. The purpose of this study is to assist the City with geotechnical engineering evaluations of the roadway embankment and shoulder slumping/failure, surface water drainage control, and the formation of a sinkhole on the west side of the guard rail.

1.3 Summary of Findings and Recommendations Our studies indicate the three feet adjacent to the embankment of Redmond-Woodinville Road NE and the downslope embankment to the west are composed of landslide debris. The movement appears to be restricted to the portions of the embankment that extend east of a concrete slab below the road. Surface cracks, the leaning gabion wall, and landslide debris in borings indicate that the failure extends from about 15 feet north of the Site stormwater culvert and at least 80 feet south of it. Our understanding of the southern

ASPECT CONSULTING


extents of the failures are limited to the surface expressions and additional subsurface explorations may be needed to more accurately identify the southern extent.

In addition, the groundwater beneath Redmond-Woodinville Road NE is confined under artesian conditions. The combination of loose soils and high groundwater pore pressures are destabilizing the embankment slope along at least 100 linear feet of the road.

We evaluated alternatives to stabilize the failing section of roadway in conjunction with stormwater improvements that included a cantilevered soldier pile wall and lowering the groundwater table using horizontal drains. A cantilevered soldier pile wall will stabilize to the west edge of Redmond-Woodinville Road NE, and can be installed with minimal disturbance to the private property to the west. The wall could be installed entirely within City right-of-way.

Horizontal drains will improve stability somewhat, but their installation will be disruptive to the private property to the west, and will significantly change the flow regime of surface water in the channel extending west. Because horizontal drains would extend west of the right-of-way onto the adjacent private property, a permanent easement agreement for access and maintenance would be required.

Due to the presence of artesian groundwater conditions beneath the road, we conclude that controlling stormwater runoff and making localized drainage improvements alone, will not be sufficient to stabilize the road. For these reasons, a cantilevered soldier pile wall is the recommended stabilization alternative.

ASPECT CONSULTING


2 Site Conditions

2.1 Surface Conditions Redmond-Woodinville Road NE traverses generally north to south through the Site with one travel lane each way. The road surface slopes gently down to the south along the direction of travel between Elevation (EL) 154 feet (NAVD88) and EL 144 feet. Prominent west-facing cracks in the asphalt roadway were observed sub-parallel to the fog line along the western side of the southbound lane of the road and have up to one inch of vertical separation. Cracks were observed along the western edge of the road surface in an area about 70 feet long and 4 feet wide. The crack nearest to the roadway center was relatively straight while the cracks nearer to the pavement edge were typically arcuate-shaped.

On the eastern side of Redmond-Woodinville Road NE, the ground slopes up to the east towards a drainage and residential properties. These slopes are vegetated with mature deciduous and coniferous trees and thick underbrush. At the base of the slopes, a City vault collects stormwater that a culvert beneath Redmond-Woodinville Road conveys into a channel on its west side.

A residential parcel is immediately west of Redmond-Woodinville Road NE right-of-way (ROW). The ground slopes down to the west and is between EL 154 feet and EL 144 feet along Redmond-Woodinville Road NE and down to approximately EL 130 feet on the residential parcel. Between top of road and the ROW, the embankment slope is approximately 2H:1V (horizontal:vertical) and contains a gabion wall that is leaning downslope in several places. Further west of the ROW, the slope flattens progressively to about 8H:1V.

The gabion basket wall partially supports the soils in the upper portion of the slopes to the west of the road. The wall is about 75 feet long and extends from 15 feet north of the stormwater culvert crossing to about the sinkhole as indicated on Figure 2. The wall is about 4 feet high, is exposed up to 2 feet above the ground surface in places, and entirely buried in some places. Around the stormwater culvert outlet, the base of the wall is undermined, causing it to sag and lean to the west (See Appendix Figure C-1).

2.1.1 Drainage The surface and near surface water gradient flows from east and north of the Site. A portion of this water is collected in a stormwater system on the east side of the road and conveyed beneath the road via a culvert and discharged at the ROW on the west of the road (refer to Figure 2). During our Site visits, we did not see any indications of sheet flow or surface water flow to the east of the road.

West of the road, a small sinkhole and minor rills indicate surface water flow along the road edge and related erosion. Two incised channels extend from near the west edge of the road onto the private parcel to the west. One channel extends from the stormwater culvert outlet and was conveying water to the west during our visits. The other channel initiates about 20 feet south of boring B-03 and travels northwest before meeting the

ASPECT CONSULTING


northern channel 85 feet west of the road; this channel was not conveying water at the time of our visit.

The sinkhole on the slope west of the cracks in the roadway is indicated on Figure 2. The sinkhole is on the mid-to upper portion of the slope and directly below small erosional gullies indicating surface water flow is directed from the roadway to the sinkhole. The sinkhole was about 1.5-feet in diameter and indicated by subdued topography. At the base of the slope we saw two holes downslope of the sinkhole where water flows from the interior of the slope. The neighboring resident has seen rabbits burrowing in the holes on the slope associated with surface water piping. The surface water has a larger effect on slope stability than the rabbit burrowing and resolving uncontrolled surface water may in turn resolve the problem of rabbit burrows.

2.2 Geologic Setting

2.2.1 Geology The geology map (Minard, 1983) shows the Site vicinity is underlain by Fraser glacial advance outwash (Qva) overlying Fraser to Pre-Fraser transitional beds (Qtb). These units are mantled by Fraser glacial recessional outwash (Qvr) in the western part of the Site.

The Qtb are composed of massive to bedded gray clay, silt, and fine sand with occasional peat layers. The Qva is composed of massive gravelly sand with grading to predominantly gravel in the upper portions of the units. The Qvr is composed of stratified sand and gravel with minor silt and clay layers. The Qva was deposited over the pre-existing Qtb during the Vashon stade of the Fraser glaciation by the Cordilleran ice sheet. The Cordilleran ice-sheet was up to a mile-thick in places and compacted the Qva and Qtb creating very dense or hard soil deposits. The retreating Cordilleran ice-sheet deposited the Qvr in massive alluvial channels. After the retreat of the ice-sheet, surface soils have been subject to disturbance or alteration by natural weathering and vegetation and, more recently, by Site grading activities.

2.2.2 Faults and Seismicity The Site area is located in an area of active seismicity that is subject to earthquakes on shallow crustal faults and deeper subduction zone earthquakes. The Site area lies about 1.6-miles southeast of splays of the Southern Whidbey Island fault zone (Sherrod et al., 2008). The Southern Whidbey Island fault zone consists of shallow crustal tectonic structures that are considered active (evidence for movement within the Holocene [since about 15,000 years ago]). The recurrence interval of earthquakes on this fault zone is believed to be on the order of 500 years or more. Based on paleoseismologic investigations (Sherrod et al., 2008), the Southern Whidbey Island fault had at least four earthquakes since deglaciation. There are also several other shallow crustal faults in the region capable of producing earthquakes and strong ground shaking.

The Site area also lies within the zone of strong ground shaking from earthquakes associated with the Cascadia Subduction Zone (CSZ). Subduction zone earthquakes occur due to rupture between the subducting oceanic plate and the overlying continental plate. The CSZ can produce earthquakes up to magnitude 9.3, and the recurrence interval

ASPECT CONSULTING


is thought to be on the order of about 500 years. A recent study estimates the most recent subduction zone earthquake occurred around 300 years ago in the early-1700s (Atwater et al., 2015).

Deep intra-slab earthquakes, which occur from tensional rupture of the sinking oceanic plate, are also associated with the CSZ. An example of this type of seismicity is the 2001 Nisqually earthquake. Deep intra-slab earthquakes typically are magnitude 7.5 or less and occur approximately every 10 to 30 years.

2.3 Soil and Groundwater Conditions Subsurface conditions at the Site were inferred from our field reconnaissance, subsurface explorations, review of applicable geologic literature, and experience with the local geology. Five exploratory borings, designated B-01 through B-05 at the approximate locations shown on Figure 2, were advanced between August 21 and 23, 2017, and between September 18 and 19, 2017. Borings B-01 and B-02 were drilled along Redmond-Woodinville Road NE east of cracks in the pavement and borings B-03 through B-05 were drilled on the embankment slope west of the road and near the toe of the gabion basket wall. The embankment borings were accessed through the private property.

Field exploration methods for the borings are included in Appendix A. Detailed descriptions of the subsurface conditions encountered in our explorations, as well as the depths where characteristics of the soils changed, are indicated on the geologic cross sections in Figures 3 and 4 and on the soil boring logs presented in Appendix A. The depths indicated on the log where conditions changed may represent gradational variations between soil types.

Soils were classified per the Unified Soil Classification System (USCS) in general accordance with the American Society for Testing and Materials (ASTM) D2488, Standard Practice for Description and Identification of Soils (Visual and Manual Procedure). A key to the symbols and terms used on the logs is provided in Figure A-1.

Our subsurface explorations encountered a stratigraphy generally conforming to the mapped conditions. Beneath Redmond-Woodinville Road NE, our explorations encountered fill over alluvium from the ground surface down to about EL 130 feet. To the west of Redmond-Woodinville Road NE, our explorations encountered landslide debris down to about EL 130 feet. Below the alluvium or landslide debris our explorations encountered Qva over Qvr deposits. The Pre-Fraser non-glacial deposits have been correlated to the Qtb in newer geological mapping.

2.3.1 Pavement and Roadway Fill We advanced two borings, B-01 and B-02 in the southbound lane of Redmond-Woodinville Road NE. To avoid buried water and gas lines below the southbound fog line, these borings were shifted east of the areas of pavement settlement and distress, in the approximate center of the southbound lane.

We encountered 11 inches of hot mix asphalt over a 7-inch-thick concrete slab in both borings. The concrete slab was not anticipated below the asphalt. Below the concrete

ASPECT CONSULTING


slab, loose fill extended about 7 to 7½ feet below the pavement surface and was moist, brown gravelly, silty sand (SM) or gray silt (ML).

The fill was very loose to loose, with Standard Penetration Test (SPT) N-values ranging from 3 to 8 blows per foot (bpf). Please refer to Appendix A for a description of SPT sampling.

2.3.2 Alluvium Below the fill, alluvium was encountered in borings B-01 and B-02 and extending to a depth of about 17.5 feet below the pavement surface. The soils were wet, gray, gravelly, slightly silty SAND (SP-SM); silty SAND (SM), SAND (SP); or brown, organic silt (OL) with trace to numerous organics. The coarse-grained deposits were medium dense to dense, with N-values between 6 and 35 bpf.

2.3.3 Landslide Debris Landslide debris includes soils deposited by the downslope movement of in situ Site soils and were encountered in borings B-03 and B-05 from the ground surface to 13 feet below ground surface (bgs). Landslide debris includes native and fill soils above the shear zone that have been displaced by landslide movement. The landslide debris was very moist to wet, brown and gray gravelly, slightly silty to silty sand (SP-SM, SM), gray silt (ML), or a mix of the two containing trace organics.

The landslide debris was medium dense to dense, with N-values between 13 and 52 bpf. The landslide debris also contained fragments of silt blocks and cobbles that elevated the SPT blow counts.

2.3.4 Advance Outwash Advance outwash was encountered in the road at about 15 feet bgs and in the embankment borings between about 5 feet and 12 feet bgs. These depths correlate to elevations between EL 130 feet and EL 140 feet. The advance outwash was wet, gray gravelly, sand (SP); gravelly, slightly silty sand (SP-SM); and gravelly, silty sand (SM), with scattered silt (ML, OL) pockets that are rip-up clasts of the underlying geologic unit.

The N-values in the advance outwash ranged from 9 to greater than 50 bpf. Blow counts in saturated advance outwash were reduced due to sand heave from below and SPT blow counts in the advance outwash were typically between 31 and greater than 50 bpf.

Boring B-04 was terminated in advance outwash at 16.4 feet below ground surface, or approximate Elevation 128.5 feet.

2.3.5 Pre-Fraser Non-Glacial Deposits Pre-Fraser non-glacial deposits includes soils deposited in an interglacial environment and were subsequently overridden during at least one continental glaciation. We encountered Pre-Fraser non-glacial deposits in borings B-01 through B-03 and B-05 from below Elevation 128 to the end of each boring. Pre-Fraser non-glacial deposits were very moist to wet, brown organic silt (OL), peat (PT), or gray slightly silty to silty sand (SP-SM, SM) and contained trace organics.

The SPT data in the Pre-Fraser non-glacial deposits ranged from 23 to greater than 50 blows per foot, indicating a medium dense to very dense or very stiff to hard consistency.

ASPECT CONSULTING


The Pre-Fraser non-glacial deposits possesses low compressibility and high shear strength characteristics.

2.3.6 Groundwater Groundwater was encountered in the borings between 6 and 8 feet bgs. We noted artesian groundwater conditions in boring B-01 during well construction and later measured groundwater in the boring at 3.1 feet above the ground surface (Elevation 154) on August 28, 2017 by attaching a riser to the well casing. During drilling, we measured the groundwater level in B-01 at 7 feet bgs, which was similar to the static groundwater in Boring B-02 was measured at 7 feet bgs (Elevation 146). The at time of drilling (ATD) groundwater conditions are likely representative of groundwater confined beneath the fill and separated from the surface water, with a water head of up to 3 feet above the road surface. Downslope of the roadway in borings B-03, B-04, and B-05 ATD groundwater levels were at 6.5 feet bgs in each boring (between Elevation 136 and 138). Groundwater elevations will fluctuate during the year due to changes in precipitation and local water use.

ASPECT CONSULTING


3 Conclusions and Recommendations

Based on the findings from our subsurface explorations and Site reconnaissance, the combination of loose soils and high groundwater pore pressures are destabilizing the embankment slope along at least 100 linear feet along Redmond-Woodinville Road NE. We did not see evidence of landslide deposits or surficial indications of failure to the north near Section B-B’ (refer to Figures 2 and 4). From field observations of the failing gabion wall, we estimate fill or landslide debris extends at least 15 feet north of the stormwater culvert. From cracks visible in the pavement we estimate the failure extends at least 80 feet south of the culvert as shown in red on Figure 2. The southern extent is poorly defined and estimated only by the cracks in the road.

The existing road and embankment have evidence of incipient slope failure that includes tension cracks and ground settlement at the pavement surface between the inferred edge of the concrete slab below the road and the toe of the embankment. Pavement distresses or other indicators of slope movement were not observed in the northbound lane where the road is underlain by a concrete slab. The most-eastern pavement crack is in a straight-line, likely forming along the edge of the slab. The concrete is providing resistance to the surficial rupture of the slope instability.

Because the surface expressions of the failure are seen to the west of the concrete slab beneath the road base, we conclude the slab is currently providing resistance to failure further east. We have not reviewed the roadway construction history, but it is likely the roadway was widened at some point beyond the existing edge of concrete and the gabion wall may have been constructed at that time to retain the new embankment fill along the ROW.

Artesian groundwater conditions can reduce soil shear strengths, resulting in slope failures. The groundwater conditions below the roadway were measured to be under artesian conditions. The groundwater levels confined beneath the road surface are up to 20 feet higher than groundwater levels at the toe of the slope about 50 feet to the west.

In addition, surface water is also decreasing slope stability, indicated by erosion along the west edge of the roadway and the sinkhole on the embankment slope. Failure or removal of the surface soils can accelerate the failure process.

If nothing is done, the soils along the west side of the road will continue to move downslope. Mitigation of the landslide will be required to stabilize the road for the long-term. It may be helpful to review the history of this roadway, if records are available, to assess past construction details.

3.1 Slope Stability Analyses To understand the extent of the landslide at the Site, and as a means to evaluate the effect of slope stabilization alternatives, we conducted a stability analysis of a critical section transecting the steep slope (Section A-A’; refer to Figures 2 and 3).

We modeled the existing conditions and evaluated two roadway stabilization measures (1) lowering the water table using horizontal drains and (2) a cantilevered soldier pile

ASPECT CONSULTING


wall. We modeled each alternative using the critical section (A-A’) and SLIDE. The results of our analysis are presented in Table 1. Model details are provided in Appendix B and on Figures B-1 through B-6.

Table 1. Summary of Slope Stability Analyses Results

Scenario Static FS1 at West Edge of Concrete Slab

Seismic FS1 at West Edge of Concrete Slab

Do Nothing (Existing Conditions) 1.0 0.58

Horizontal Drains 1.28 0.78

Soldier Pile Wall >3.0 1.28

Notes: 1) Factor of Safety—The minimum FS at the location indicated found using

Spencer’s method in computer program SLIDE.

The results of our analyses indicate surfaces with factors of safety less than 1.0 extend between the edge of the concrete slab and the gabion wall. These surfaces penetrate through the landslide debris and exit below the gabion wall and at the base of the slope west of the road on the residential parcel. Where it exists, the concrete slab beneath Redmond-Woodinville Road NE appears to restrict surfaces from penetrating through the road.

The summary results of our slope stability analyses indicate:

1) If nothing is done, failures will continue to affect the roadway embankment and roadway.

2) Lowering the groundwater with horizontal drains will increase static stability, but will not provide adequate protection during an earthquake.

3) A cantilevered soldier pile wall would adequately stabilize the roadway under static and seismic loading conditions.

Details of the stabilization alternatives are discussed in more detail below.

3.2 Conceptual Stabilization Alternatives Recommendations We have considered a variety of stabilization techniques. Based on the Site conditions and restraints, we identified two alternatives for stabilizing the embankment and the failing section of the road; these included lowering the water table using horizontal drains and installing a cantilevered soldier pile wall. Both alternatives may be combined with stormwater improvements that are designed to better manage surface water runoff.

Based on the current Site conditions, rates of movement, and results of our analyses, ongoing maintenance—such as planting of slope stabilizing vegetation, surface repair of roadway cracks, addition of quarry spalls to the slope, or stormwater repairs—is not a

ASPECT CONSULTING


sustainable or acceptable long-term approach to managing the roadway and should be considered for short-term improvements until a long-term stabilization is installed.

Under current conditions, slope instability will be limited to the area west of the concrete slab. However, over the longer-term (i.e. the next decade or two), if nothing were done to stabilize the Site, as existing material continues to erode and/or move progressively downslope and to the west, the subgrade beneath the old concrete slab will potentially mobilize. Voids under the slab combined with heavy traffic loading, would ultimately cause the slab to crack and break up.

3.2.1 Horizontal Drains Horizontal drains are used to lower the groundwater table in the area of the unstable soils, which increases the shear strength of the soils and resisting forces of the slope. Horizontal drains are installed on a spacing based on the conductivity of the surrounding soils from the base of the slope by drilling horizontally into the slope and installing slotted drain pipes. Water collected in the horizontal drain pipes is typically collected in a series of dewatering pits at the outlet of a group of drain pipes and then directed from the downslope to a suitable outlet location. In this case, the water would be directed into the existing ditch/stream extending west along the private property.

We evaluated lowering the groundwater table beneath the road and embankment to about EL 138 feet to increase slope stability. Lowering the water table to EL 138 feet would require installing the horizontal drains and locating the dewatering pits on the private parcel west of the roadway ROW.

Based on our recent experience with design and construction management for similar slope stabilization, we anticipate a construction cost of approximately $45 to $90 per lineal foot of horizontal drain. We estimate 600 feet of drain will be necessary to stabilize a 150-foot-long section of the embankment—between $27,000 and $55,000 depending on materials and drilling conditions. Excavation and installation of dewatering pits and the collection system will cost an estimate additional $15,000. For a total cost of between $42,000 and $70,000. This cost does not include any permits necessary for water discharge, storm water upgrades, and other site restoration items.

Our analyses indicate that horizontal drains will increase stability adequately under static conditions. However, the analyses indicate the slope would potentially fail during a design earthquake event (refer Table 1 above). In addition, since the horizontal drains will require construction access and permanent installation to the private property west of the ROW, horizontal drains are not considered the preferred alternative.

3.2.2 Cantilevered Soldier Pile Wall A cantilevered soldier pile wall is composed of wide flange or HP steel sections set vertically into drilled shafts that are filled with concrete, which extend into competent soil. The soldier piles are typically installed on a 6 to 8 foot horizontal spacing. Timber beams or concrete panels are used as lagging, to span between the exposed portions of the solder piles and to retain soils.

The wall gains support to retain upper loose soils from the portions of the wall embedded into the underlying competent and stable soil. A cantilevered soldier pile wall can retain

ASPECT CONSULTING


about 12 feet of loose consolidated soils without additional support from tie-back anchors. When the retained height exceeds about 12 feet, then tieback anchors are typically used to provide additional lateral support to the soldier piles. Given the soil conditions at the Site, we anticipate a cantilevered wall is feasible.

We evaluated stability with piles on an 8-foot horizontal spacing and 75,000 pounds effective shear strength. At this Site, the cantilevered soldier pile wall would have an initial exposed height of 4 feet or less but would be designed for an ultimate exposed height of about 10 feet.

Our analyses indicate a cantilevered soldier pile wall will adequately stabilize the slope under existing static and design seismic conditions. The soldier pile wall could be installed in front of the existing gabion wall, or the gabion wall could be demolished and the soldier pile wall could be installed to replace the gabion basket wall. It would be more economical to install the soldier pile wall in front of (i.e., west of) the gabion basket wall, but this would encroach the adjacent private property somewhat. The gabion basket wall could be left in place if the solider pile wall were installed in front of the gabion basket wall.

Soldier pile wall construction could be constructed with drilling equipment operating from the temporarily closed southbound lane. It would also be possible to complete the work from the private property below, and maintain the southbound lane open to traffic except for limited temporary closure periods.

The soldier pile wall can be designed and constructed to accommodate the existing culvert which under-crosses Redmond-Woodinville Road NE.

The soldier pile wall could be designed to allow stormwater improvements for roadway runoff. Based on all of these considerations, we consider a cantilevered soldier pile wall to be the preferred stabilization alternative.

Based on our recent experience with design and construction management for a very similar slope stabilization, we anticipate a construction cost of approximately $1,000 per lineal foot of soldier pile wall— approximately $150,000 for a 150-foot long wall. This cost does not include pavement restoration, storm water upgrades, and other site restoration items.

3.3 Stormwater Improvements The detrimental effects of uncontrolled stormwater runoff are indicated by erosion features such as sinkholes and voids. Capturing and diverting stormwater away from the west side of the road will reduce: the amount of water available to saturate the surface soils on the slope; stormwater infiltration into the slope; and surficial slope failures due to saturated soils.

However, due to the presence of artesian groundwater seepage conditions, likely related to a confined aquifer separate from surface, such surface water upgrades will not significantly reduce the groundwater levels in the confined aquifer. Our slope stability models (see Appendix B) show the groundwater from the confined aquifer is causing failures beneath the shallow surface soils. If no additional geotechnical improvements are

ASPECT CONSULTING


made, our analyses indicate these slope failures will continue even with improved stormwater facilities.

4 Future Work

This report provides our preliminary geotechnical recommendation and conceptual alternatives to stabilize the road embankment. After the City confirms and selects the preferred alternative, the next step will be to finalize the geotechnical design; our final design-level geotechnical engineering report can be completed within about 4 weeks. Aspect is also available to provide design drawings and specifications for the recommended soldier pile wall. We are available to discuss these additional services with the City of Redmond.

ASPECT CONSULTING


5 References

AASHTO, 2012, American Association of State Highway and Transportation Officials LRFD Bridge Design Specifications.

Atwater, B.F., S. Musumi-Rokkaku, K. Satake, Y. Tsuji, K. Ueda, and D.K. Yamaguci, 2015, The orphan tsunami of 1700—Japanese clues to a parent earthquake in North America, US Geological Survey, Professional Paper 1707.

Minard, J.P., 1983, Geologic Map of the Kirkland Quadrangle, U.S. Geological Survey, Miscellaneous Field Studies Map, MF-1543.

Palmer, S.P., et al., 2004, Liquefaction Susceptibility Map of King County, Washington, Washington Division of Geology and Earth Resources, Open File Report 2004-20, Map 17a.

Rocscience, 2017, Slide 7.027 Analysis Program. Build date August 15, 2017

Sherrod et al., 2008, Finding concealed active faults: Extending the southern Whidbey Fault across the Puget Lowland, Washington, Journal of Geophysical Research, Vol. 113.

U.S. Geological Survey (USGS), 2014, U.S. Seismic Design Maps, Website, Accessed on October 03, 2017, https://earthquake.usgs.gov/designmaps/us/application.php

Washington State Department of Transportation (WSDOT), 2015, Geotechnical Design Manual, M 46-3.11, May 2015.

ASPECT CONSULTING


6 Limitations

Work for this project was performed for the City of Redmond (Client) for specific application to the proposed project as described herein, and this report was prepared in accordance with generally accepted professional practices for the nature and conditions of work completed in the same or similar localities, at the time the work was performed. This report may be used only by the Client and for the purposes stated, within a reasonable time from its issuance. This report does not represent a legal opinion. No other warranty, expressed or implied, is made.

This report is issued with the understanding that the information and recommendations contained herein will be brought to the attention of the appropriate design team personnel and incorporated into the project plans and specifications, and that the necessary steps will be taken to verify that the contractor and subcontractors carry out such recommendations in the field. We do not direct the contractor’s operations, and we cannot be responsible for the safety of personnel other than our own on the Site; the safety of others is the responsibility of the contractor. The contractor should notify the property owner if he considers any of the recommended actions presented herein unsafe.

All reports prepared by Aspect Consulting for the Client apply only to the services described in the Agreement(s) with the Client. Any use or reuse by any party other than the Client is at the sole risk of that party, and without liability to Aspect Consulting. Aspect Consulting’s original files/reports shall govern in the event of any dispute regarding the content of electronic documents furnished to others.

Recommendations presented herein are based on data that we acquired, our geotechnical engineering calculations, and judgment in accordance with our mutually agreed-upon scope of work. Variations may exist between soil and groundwater conditions reported, and those actually underlying the Site. The nature and extent of such soil variations may change over time and will not be evident before construction begins. If any soil conditions are encountered at the Site that are different from those described in this report, we should be notified immediately to review the applicability of our recommendations.

The existing gabion wall will need to be partially demolished to facilitate the construction described in this report. Aspect did not evaluate the as-constructed details, nor the stability of the existing gabion wall, and we assume no liability for the existing wall.

It is the Client's responsibility to see that all parties to this project, including the designer, contractor, subcontractors, etc., are made aware of this report in its entirety. The use of information contained in this report for bidding purposes should be done at the contractor's option and risk.

Our scope of our work does not include services related to construction safety precautions. Our recommendations are not intended to direct the contractors' methods, techniques, sequences or procedures. Our scope of our work also excludes the assessment of environmental characteristics, particularly those involving potentially hazardous substances in soil or groundwater.

FIGURES

^

GIS Path: T:\projects_8\Woodinville_RedmondRd_160422\Delivered\01 Site Location Map.mxd || Coordinate System: NAD 1983 StatePlane Washington North FIPS 4601 Feet || Date Saved: 11/15/2017 || User: kschrup || Print Date: 11/15/2017

Site Location MapPreliminary Geotechnical Evaluation

TO#17-02 Redwood Road SlumpRedmond, Washington

C O N SU LTI N G

FIGURE NO.

1NOV-2017PROJECT NO.

160422-01

BY:MWS / KES

REVISED BY:- - -

0 2,000 4,000

Feet

!

!

!

#!

!

!

!

!(

W A S H I N G T O N

Bellingham

Olympia

Port Angeles

SeattleSpokane

Tacoma Wenatchee

Yakima

!

!

!

!

!

!

!

!

!!

!

#

!!

!(

Lake S amma

m

i sh

Lake

Washi

ngton

ElliottBay

P u g e tS ou n d

Bellevue

Bothell

Carnation

Issaquah

Kenmore

Kirkland

Lynnwood

Mercer Island

Mill Creek Monroe

SammamishSeattle

ShorelineWoodinville

Basemap Layer Credits || Sources: Esri, HERE, DeLorme, Intermap, increment P Corp., GEBCO, USGS, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, Ordnance Survey, Esri Japan, METI, Esri China (Hong Kong), swisstopo,MapmyIndia, © OpenStreetMap contributors, and the GIS User CommunityCopyright:© 2014 Esri

SITE LOCATION

SITELOCATION

SITELOCATION

B-01

B-02 B-03

B-04

B-05

WOODINVILLE-REDMOND RD. NE

CAD

Pat

h: Q

:\_G

eoTe

ch\1

6042

2 Ci

ty o

f Red

mon

d\20

17-1

1 Re

dwoo

d Ro

ad S

lum

p\20

17-1

1 G

eote

chni

cal I

nves

tigat

ion\

1604

22-0

2.dw

g 11

x17

Land

scap

e |

| D

ate

Save

d: J

an 0

5, 2

018

10:4

9am

|

| U

ser:

cvan

slyk

e

Redwood Road Slump10100 Redmond-Woodinville Road

Redmond, Washington

2BY:

JGF/CMV

Site and Exploration Map

Jan-2018REVISED BY:

-PROJECT NO.

160422

FIGURE NO.

Boring Location

Monitoring Well Location

Right-of-Way

Existing Storm Drain

Existing Overhead Power

Source: Base map provided by Reid Middleton, November2017. Imagery by Bing OpenStreet Maps, 2017.

Legend

Feet

0 30 60

SINKHOLE

A

A'

B

B'

Estimated Extent of Proposed Soldier Pile Wall

Existing Guard Rail

Existing Gabion Wall

Existing Pavement Cracks

Section LocationC C'

AutoCAD SHX Text

S

AutoCAD SHX Text

129

AutoCAD SHX Text

130

AutoCAD SHX Text

135

AutoCAD SHX Text

133

AutoCAD SHX Text

132

AutoCAD SHX Text

131

AutoCAD SHX Text

134

AutoCAD SHX Text

140

AutoCAD SHX Text

141

AutoCAD SHX Text

139

AutoCAD SHX Text

138

AutoCAD SHX Text

137

AutoCAD SHX Text

136

AutoCAD SHX Text

142

AutoCAD SHX Text

143

AutoCAD SHX Text

144

AutoCAD SHX Text

146

AutoCAD SHX Text

147

AutoCAD SHX Text

149

AutoCAD SHX Text

150

AutoCAD SHX Text

151

AutoCAD SHX Text

153

AutoCAD SHX Text

132

AutoCAD SHX Text

135

AutoCAD SHX Text

140

AutoCAD SHX Text

138

AutoCAD SHX Text

137

AutoCAD SHX Text

136

AutoCAD SHX Text

142

AutoCAD SHX Text

145

AutoCAD SHX Text

152

AutoCAD SHX Text

141

AutoCAD SHX Text

142

AutoCAD SHX Text

141

AutoCAD SHX Text

140

AutoCAD SHX Text

135

AutoCAD SHX Text

132

AutoCAD SHX Text

138

AutoCAD SHX Text

136

AutoCAD SHX Text

143

AutoCAD SHX Text

145

AutoCAD SHX Text

154

W

W 250.00 lbs/ft2

B-01

Right of Way

B-03

Material Name Color Unit Weight(lbs/ 3) Strength Type Cohesion

(psf)Phi

(deg) Water Surface

Concrete 150 Mohr‐Coulomb 1000 50 None

Gabion Wall 135 Mohr‐Coulomb 0 50 None

Fill 120 Mohr‐Coulomb 0 30 Water Surface

Landslide Debris 120 Mohr‐Coulomb 0 22 Water Surface

Alluvium 125 Mohr‐Coulomb 0 33 Water Surface

Advance Outwash 130 Mohr‐Coulomb 0 42 Water Surface

Pre‐Fraser Deposits 125 Mohr‐Coulomb 0 38 None

Redwood Road

Groundwater under Artesian Conditions

200

180

160

140

120

100

80

-20 0 20 40 60 80 100 120 140

APPENDIX:

3REVISED BY:

MWS

BY:

JGFPROJECT NO.

160422-01

11/30/2017

Geologic Cross Section and Slope Stability Model Setup

Redwood Road Slump10100 Redmond-Woodinville Road, Redmond, Washington.

C:\Users\jfavia\Desktop\Slide Projects\Redwood Road.slmd

SCALE: 1:240

SLIDE 7.029

Section A-A'

W

W

B-02B-04

Right of Way

Redwood Road


(psf)Phi

(deg)

Concrete 150 Mohr‐Coulomb 1000 50

Fill 120 Mohr‐Coulomb 0 30

Alluvium 125 Mohr‐Coulomb 0 33

Advance Outwash 130 Mohr‐Coulomb 0 42

Pre‐Fraser Deposits 125 Mohr‐Coulomb 0 38

200

180

160

140

120

100

-40 -20 0 20 40 60 80 100 120 140

APPENDIX:

4REVISED BY:

MWS

BY:

JGFPROJECT NO.

160422-01

12/28/2017


Slope Stability Analysis

S:\City of Redmond\2017_2018 On Call Geotech\TO1 RedWood RoadSlump\Data\Analyses\Slope Stability\Redwood Road - Section B-B'.slmd

SCALE: 1:240

SLIDE 7.029

Section B-B'Geological Cross Section

APPENDIX A

Subsurface Explorations

ASPECT CONSULTING

PROJECT NO. 160422-01 FEBRUARY 1, 2018 A-1

A. Subsurface Exploration Program Geotechnical borings B-01 and B-02 were advanced between August 17 and 19, 2017 by Gregory Drilling with hollow-stem auger methods using a track-mounted CME-55. Borings B-03 through B-05 were advanced on September 20, 2017 by Boretec1, Inc. using a portable Acker drill rig. Samples were obtained every 2.5 or 5 feet using the Standard Penetration Test (SPT) in general accordance with ASTM Method D1586.

The SPT method involves driving a 2-inch-outside-diameter split-barrel sampler a distance of 18 inches into the soil with a 140-pound hammer free falling from a distance of 30 inches. The number of blows for each 6-inch interval is recorded and the number of blows required to drive the sampler the final 12 inches is known as the Standard Penetration Resistance (“N”) or blow count. The resistance, or N-value, provides a measure of the relative density of granular soils or the relative consistency of cohesive soils.

The exploration locations are shown on Figure 2 and were estimated in the field by measuring from existing Site features and later surveyed. The machine drilled borings were backfilled with bentonite chips and capped with about 1 foot of excavated soils or concrete.

Classifications of soils in this report are based on visual field and/or laboratory observations, which include density/consistency, moisture condition, grain size, and

plasticity estimates and should not be construed to imply field or laboratory testing unless presented herein. Visual-manual and/or laboratory classification

methods of ASTM D-2487 and D-2488 were used as an identification guide for the Unified Soil Classification System.

Terms Describing Relative Density and Consistency

Estimated Percentage

Symbols

Moisture ContentPercentage

by Weight

SamplerType

Sampler Type

Description

Blows/6" orportion of 6"

Component DefinitionsSize Range and Sieve Number

Larger than 12"Descriptive Term

Smaller than No. 200 (0.075 mm)

3" to 12"

Coarse-Grained Soils

Fine-

Grained Soils

DensityVery LooseLooseMedium DenseDenseVery Dense

SPT blows/foot

0 to 44 to 1010 to 3030 to 50>50

(2)

0 to 22 to 44 to 88 to 1515 to 30>30

Consistency

Very SoftSoftMedium StiffStiffVery StiffHard

SPT blows/foot(2)

2.0" OD Split-Spoon Sampler(SPT) Continuous Push

Non-Standard SamplerBulk sample

3.0" OD Thin-Wall Tube Sampler (including Shelby tube)

Grab Sample

Portion not recovered

(1)

ATD = At time of drillingStatic water level (date)

Percentage by dry weight(SPT) Standard Penetration Test (ASTM D-1586)In General Accordance withStandard Practice for Description and Identification of Soils (ASTM D-2488)

Test Symbols

Depth of groundwater(4)

(1)

(2)

(3)

Cement grout surface seal

Groutseal

End cap

Filter pack with blank casing section

Boulders

Silt and Clay

Gravel Coarse Gravel Fine Gravel

Cobbles

Sand Coarse Sand Medium Sand Fine Sand

Dry - Absence of moisture, dusty, dry to the touch

Slightly Moist - Perceptible moisture

Moist - Damp but no visible water

Very Moist - Water visible but not free draining

Wet - Visible free water, usually from below water table

Hig

hly

Org

anic

Soils

Fin

e-G

rain

ed S

oils

- 5

0%

or

More

Passes N

o.

200 S

ieve

(1)

Coars

e-G

rain

ed S

oils

- M

ore

than 5

0%

Reta

ined o

n N

o.

200 S

ieve

Gra

vels

- M

ore

than 5

0%

of

Coars

e F

raction

Re

tain

ed

on

No

. 4

Sie

ve

15%

Fin

es

5%

Fin

es

Sands -

50%

or

More

of

Coars

e F

raction

Pa

sse

s N

o. 4

Sie

ve

Silt

s a

nd

Cla

ys

Liq

uid

Lim

it L

ess t

han 5

0

Silt

s a

nd

Cla

ys

Liq

uid

Lim

it 5

0 o

r M

ore

(5) Combined USCS symbols used for fines between 5% and 15% as estimated in General Accordance with Standard Practice for Description and Identification of Soils (ASTM D-2488)

(1)

(1)

15%

Fin

es

5%

Fin

es

(5)

(5)

(5)

(5)

FC = Fines ContentG = Grain SizeM = Moisture Content A = Atterberg Limits C = ConsolidationDD = Dry DensityK = PermeabilityStr = Shear StrengthEnv = EnvironmentalPiD = Photoionization

No. 4 (4.75 mm) to No. 10 (2.00 mm)No. 10 (2.00 mm) to No. 40 (0.425 mm)No. 40 (0.425 mm) to No. 200 (0.075 mm)

3" to No. 4 (4.75 mm)3" to 3/4"3/4" to No. 4 (4.75 mm)

No. 4 (4.75 mm) to No. 200 (0.075 mm)

Well-graded gravel and

gravel with sand, little to

no fines

Poorly-graded gravel and gravel with sand, little to no fines

Silty gravel and silty gravel with sand

Clayey gravel and clayey gravel with sand

Well-graded sand and sand with gravel, little to no fines

Poorly-graded sand and sand with gravel, little to no fines

Silty sand and silty sand with gravel

Clayey sand and clayey sand with gravel

Silt, sandy silt, gravelly silt, silt with sand or gravel

Clay of low to medium plasticity; silty, sandy, or gravelly clay, lean clay

Organic clay or silt of low plasticity

Elastic silt, clayey silt, silt with micaceous or diato-maceous fine sand or silt

Clay of high plasticity, sandy or gravelly clay, fat clay with sand or gravel

Organic clay or silt of medium to high plasticity

Peat, muck and other highly organic soils

GW

GP

GM

GC

SW

SP

SM

SC

ML

CL

OL

MH

CH

OH

PT

Trace

Slightly (sandy, silty,clayey, gravelly)Sandy, silty, clayey,gravelly)Very (sandy, silty,clayey, gravelly)

Modifier

<5

5 to 15

15 to 30

30 to 49

Screened casing or Hydrotip with filter pack

Bentonitechips

FIGURE NO.

PROJECT NO.DATE:

REVISED BY:

DRAWN BY:

DESIGNED BY:

www.aspectconsulting.com

earth + water Exploration Log Key

A-1

Q:\_A

CA

D S

tandard

s\S

tandard

Deta

ils\E

xplo

ration L

og K

ey A

1.d

wg

Detector

Grouted Transducer

BGS = below ground surface

9/12/2017 Artesian

8/23/2017 Artesian

8/21/2017

8-inch flushmonument in concreteExpansion plug

2-inch Sch 40 PVCriser 0.3 to 10 feet

Bentonite chips 1.5 to8 feet

2-inch Sch 40 PVC0.010-inch-slot screen10 to 30 feet

10x20 sand 8 to 32feet

2

1

2

2

2

1

4

7

5

2

4

5

3

2

4

7

6

5

17

25

27

20

27

21

11 inches Hot Mix Asphalt

7 inches Concrete

BASE COURSEVery loose, slighty moist, light brown, gravelly, silty SAND(SM) with cobbles; fine to coarse sand, fine and coarserounded to subangular gravel, base course.

FILLVery loose, slightly moist, light brown, silty SAND (SM);trace gravel, fine sand, fine subrounded to subangulargravel.

Becomes slightly gravelly at 5 feet bgs.

ALLUVIUMMedium dense, wet, light brown, silty SAND (SM); tracegravel, fine sand, fine subrounded to subangular gravel.Becomes gray at 8 feet bgs.

Loose, wet, gray, very silty SAND (SM); fine sand.

Loose, wet, gray, gravelly, SAND (SP); fine to coarsesand, fine and coarse subrounded to subangular gravel.

Loose, wet, gray, slightly silty SAND (SP-SM) interbeddedwith silty SAND (SM); fine to coarse sand.

Medium dense, wet, gray, very gravelly, silty SAND (SM);fine to coarse sand, fine and coarse subrounded tosubangular gravel.

ADVANCE OUTWASHVery dense, wet, gray, slightly gravelly, SAND (SP); tracesilt, fine to coarse sand, fine subrounded gravel.

Dense, wet, gray, slightly gravelly, slightly silty SAND(SP-SM); fine to coarse sand, fine subrounded gravel.

Hard, moist, brown, slightly sandy, SILT (ML); non-plastic,fine sand.

S1

S2

S3a

S3b

S4

S5

S6

S7

S8

Depth(feet)

MaterialType

Ground Surface (GS) Elev. (NAVD88)

Josh

Ecology Well Tag No.BJY-953

No Soil Sample Recovery

Wat

erLe

vel

B-01

Sheet 1 of 2

Depth(ft)

Sampling Method

8/21/2017 to 8/22/2017

Project Address & Site Specific Location

Liquid LimitSee Exploration Log Key for explanationof symbols

Exploration Completionand Notes

Static Water Level

AS

PE

CT

ST

AN

DA

RD

EX

PLO

RA

TIO

N L

OG

TE

MP

LAT

E

P:\

GIN

TW

\PR

OJE

CT

S\C

ITY

OF

RE

DM

ON

D_R

ED

WO

OD

RO

AD

-160

422-

01.G

PJ

Nov

embe

r 30

, 20

17

SampleType/ID

Elev.(feet)

Operator Work Start/Completion Dates Top of Casing Elev. (NAVD88)

Blows/6"

5

10

15

20

Tests

Autohammer; 140 lb hammer; 30" dropTrack-Mounted CME-55

Rotary Drill

Hollow-Stem Auger

Gregory Drilling, Inc.

Exploration Method(s) Depth to Water (Below GS)

-3.1' (Static)

Exploration Number

Sam

ple

Met

hod

Description

Equipment

Legend

Contractor

150

145

140

135

130

B-01

Coordinates (Lat,Lon WGS84)

Plastic Limit

Blows/footWater Content (%)

5

10

15

20

Geotechnical Exploration Log

Water Level ATDLogged by: JGFApproved by: MWS

NA

10100 Redmond-Woodinville Road, Redmond, Washington.

ExplorationLog

150.8'(est)

254420, 1321140 (est)

Split Barrel 2" X 1.375" (SPT)

TO#17-02 Redwood Road Slump - 160422-01

10 20 30 400 50

Threaded cap

Bentonite chips 32 to40 feet

4

11

15

2

9

22

9

17

28

8

18

23

Medium dense, wet, gray, SAND (SP); fine to mediumsand.Reduced blow counts due to sand heave or slough fromabove.

PRE-FRASER NON-GLACIAL DEPOSITSVery stiff to hard, wet, gray and brown, SILT (ML)interbedded with medium dense, silty SAND (SM);non-plastic, fine sand, very thinly laminated.

Hard, moist, brown, organic SILT (OL); low plasticity, verythinly laminated.

Hard, moist, brown, PEAT (PT); glacially consolidatedpeat.

Dense, wet, gray, slightly silty SAND (SP-SM); fine sand,scattered very thin laminations of low plasticity SILT (ML).

Bottom of exploration at 41.5 ft. bgs.

S9

S10

S11

S12

Depth(feet)

MaterialType


Josh

Ecology Well Tag No.BJY-953


Wat

erLe

vel

B-01

Sheet 2 of 2

Depth(ft)

Sampling Method

8/21/2017 to 8/22/2017




Static Water Level

AS

PE

CT

ST

AN

DA

RD

EX

PLO

RA

TIO

N L

OG

TE

MP

LAT

E

P:\

GIN

TW

\PR

OJE

CT

S\C

ITY

OF

RE

DM

ON

D_R

ED

WO

OD

RO

AD

-160

422-

01.G

PJ

Nov

embe

r 30

, 20

17

SampleType/ID

Elev.(feet)


Blows/6"

30

35

40

45

Tests


Rotary Drill

Hollow-Stem Auger



-3.1' (Static)

Exploration Number

Sam

ple

Met

hod

Description

Equipment

Legend

Contractor

125

120

115

110

105

B-01


Plastic Limit


30

35

40

45


Water Level ATDLogged by: JGFApproved by: MWS

NA


ExplorationLog

150.8'(est)

254420, 1321140 (est)



10 20 30 400 50

8/23/2017

Exploration backfilledwith bentonite groutand capped withconcrete.

2

3

3

4

4

4

15

11

8

6

9

15

9

16

19

13

17

15

4

4

5

4

4

5

11 inches Hot Mix Asphalt

7 inches Concrete

BASE COURSELoose, slightly moist, light brown, gravelly, silty SAND(SM) with cobbles; fine to coarse sand, fine and coarserounded and subangular gravel, base course.

FILLLoose, slightly moist, light brown, gravelly, silty SAND(SM); trace gravel, fine to medium sand, fine subroundedgravel.

Stiff, moist to very moist, gray with orange mottles, sandySILT (ML); trace fine subangular gravel, non-plastic, fineto medium sand.

ALLUVIUMMedium dense, wet, gray, gravelly, slightly silty SAND(SP-SM); fine to coase sand, fine and coarse subroundedto subangular gravel.

Very stiff, moist, brown, organic SILT (OL); low plasticity,numerous wood fragments.

Dense, wet, gray, slightly silty GRAVEL (GP-GM) withcobbles; fine to coarse gravel.Increased blow counts due to over-sized gravel andcobbles.

ADVANCE OUTWASHDense, wet, green-gray, slightly gravelly SAND (SP); tracesilt, fine to medium sand, fine subrounded gravel.

Loose, wet, green-gray, slightly gravelly, slightly silty SAND(SP-SM); fine to medium sand, fine and coarsesubrounded to angular gravel.Reduced blow counts due to sand heave from below.

Hole collapsed between 20 and 25 feet after sampling at25 feet bgs.

S1

S2

S3

S4

S5

S6

S7

S8

Depth(feet)

MaterialType


Josh


Wat

erLe

vel

B-02

Sheet 1 of 2

Depth(ft)

Sampling Method

8/23/2017




AS

PE

CT

ST

AN

DA

RD

EX

PLO

RA

TIO

N L

OG

TE

MP

LAT

E

P:\

GIN

TW

\PR

OJE

CT

S\C

ITY

OF

RE

DM

ON

D_R

ED

WO

OD

RO

AD

-160

422-

01.G

PJ

Nov

embe

r 30

, 20

17

SampleType/ID

Elev.(feet)


Blows/6"

5

10

15

20

Tests


Rotary Drill

Hollow-Stem Auger andMud Rotary



7' (ATD)

Exploration Number

Sam

ple

Met

hod

Description

Equipment

Legend

Contractor

150

145

140

135

130

B-02


Plastic Limit


5

10

15

20


Water Level ATD

Logged by: JGFApproved by: MWS

NA


ExplorationLog

152.9'(est)

254510, 1321120 (est)



10 20 30 400 50

15

17

16

8

11

12

PRE-FRASER NON-GLACIAL DEPOSITSDense, wet, gray, slightly silty SAND (SP-SM); tracegravel, fine to medium sand, fine subrounded tosubangular gravel.

Very stiff, moist, brown, organic SILT (OL) interbedded with glacially consolidated PEAT (PT); low plasticity.


S9

S10

Depth(feet)

MaterialType


Josh


Wat

erLe

vel

B-02

Sheet 2 of 2

Depth(ft)

Sampling Method

8/23/2017




AS

PE

CT

ST

AN

DA

RD

EX

PLO

RA

TIO

N L

OG

TE

MP

LAT

E

P:\

GIN

TW

\PR

OJE

CT

S\C

ITY

OF

RE

DM

ON

D_R

ED

WO

OD

RO

AD

-160

422-

01.G

PJ

Nov

embe

r 30

, 20

17

SampleType/ID

Elev.(feet)


Blows/6"

30

35

40

45

Tests


Rotary Drill

Hollow-Stem Auger andMud Rotary



7' (ATD)

Exploration Number

Sam

ple

Met

hod

Description

Equipment

Legend

Contractor

125

120

115

110

105

B-02


Plastic Limit


30

35

40

45


Water Level ATD


NA


ExplorationLog

152.9'(est)

254510, 1321120 (est)



10 20 30 400 50

9/18/2017

Exploration backfilledwith 3/8 inch bentonitechips and capped withexcavated soil.

4

3

7

6

7

5

14

38

14

9

8

5

17

20

24

22

42

50/4"

27

38

50/3.5"

LANDSLIDE DEBRISLoose, moist, brown, slightly gravelly, very silty SAND(SM); fine to medium sand, fine subrounded to angulargravel.

Medium dense, very moist, brown, gravelly, silty SAND(SM); fine to medium sand, fine rounded to subangulargravel.Stiff, very moist, brown and gray, sandy SILT (ML);non-plastic, fine sand, very thinly laminated, very strongiron-oxide alteration from 6.2 to 6.3 feet bgs.

Becomes gravelly at 7.5 feet bgs, blow counts elevateddue to gravel.

Very slow drilling; cobble from 8.5 to 9.5 feet bgs.

Stiff, wet, gray, sandy SILT (ML); trace gravel, mediumplasticity, fine to medium sand, fine gravel.

ADVANCE OUTWASHDense to very dense, wet, gray, silty SAND (SM); tracegravel, fine to medium sand, fine rounded gravel.

PRE-FRASER NON-GLACIAL DEPOSITSHard, moist, brown, organic SILT (OL); low plasticity,scattered SAND (SP) interbeds.


S1

S2a

S2b

S3

S4

S5

S6

S7

Depth(feet)

MaterialType


J. Carlos


Wat

erLe

vel

B-03

Sheet 1 of 1

Depth(ft)

Sampling Method

9/18/2017




AS

PE

CT

ST

AN

DA

RD

EX

PLO

RA

TIO

N L

OG

TE

MP

LAT

E

P:\

GIN

TW

\PR

OJE

CT

S\C

ITY

OF

RE

DM

ON

D_R

ED

WO

OD

RO

AD

-160

422-

01.G

PJ

Nov

embe

r 30

, 20

17

SampleType/ID

Elev.(feet)


Blows/6"

5

10

15

20

Tests

Rope & catheadAcker Drill

Hollow-Stem Auger

Bortec1


6.3' (ATD)

Exploration Number

Sam

ple

Met

hod

Description

Equipment

Legend

Contractor

140

135

130

125

120

B-03


Plastic Limit


5

10

15

20


Water Level ATD


NA


ExplorationLog

143.7'(est)

254410, 1321120 (est)



10 20 30 400 50

9/18/2017


28

15

19

12

16

15

29

30

52

18

39

31

27

40

50/3.5"

23

45

50/4"

FILLHard, slightly moist, light brown and orange, sandy SILT(ML); non-plastic, fine to coarse sand, moderate iron-oxidealteration.

ADVANCE OUTWASHDense, wet, gray, gravelly very silty SAND (SM); fine tomedium sand, fine rounded to angular gravel.

Very dense, wet, gray, gravelly, slightly silty SAND(SP-SM); fine to coarse sand, find and coarse rounded tosubrounded gravel.

Very dense, wet, gray, slightly silty SAND (SP-SM); tracegravel, fine to coarse sand, fine rounded gravel.


Note: Refusal due to heave at 16.4 feet bgs.

S1

S2

S3

S4

S5

S6

Depth(feet)

MaterialType


J. Carlos


Wat

erLe

vel

B-04

Sheet 1 of 1

Depth(ft)

Sampling Method

9/18/2017




AS

PE

CT

ST

AN

DA

RD

EX

PLO

RA

TIO

N L

OG

TE

MP

LAT

E

P:\

GIN

TW

\PR

OJE

CT

S\C

ITY

OF

RE

DM

ON

D_R

ED

WO

OD

RO

AD

-160

422-

01.G

PJ

Nov

embe

r 30

, 20

17

SampleType/ID

Elev.(feet)


Blows/6"

5

10

15

20

Tests


Hollow-Stem Auger

Bortec1


6.5' (ATD)

Exploration Number

Sam

ple

Met

hod

Description

Equipment

Legend

Contractor

145

140

135

130

125

B-04


Plastic Limit


5

10

15

20


Water Level ATD


NA


ExplorationLog

145.2'(est)

254500, 1321100 (est)



10 20 30 400 50

9/19/2017


5

5

8

5

3

11

14

10

8

7

13

12

7

50/5"

38

50/5"

50/3"

25

46

50/5.5"

LANDSLIDE DEBRISMedium dense, moist, brown and dark brown, slightlygravelly, very silty SAND (SM); fine to mediumn sand, finerounded to subrounded gravel, trace organic fragments,rare orange mottles.

Becomes gray with strong iron-oxide alteration.

Medium dense, wet, gray, gravelly, slightly silty SAND(SP-SM); fine to coarse sand, fine subrounded gravel.

Very stiff, wet, gray, slightly sandy SILT (ML); low tomedium plasticity, fine to coarse sand.

Medium dense, wet, gray, slightly gravelly, silty SAND(SM); fine to medium sand, fine subrounded gravel.

ADVANCE OUTWASHHard, very moist, brown and gray, gravelly, organic SILT (OL) interbedded with very dense, wet, gray, gravelly, slightly silty SAND (SP-SM); low plasticity, fine to coarse sand, fine and coarse subrounded gravel.Gravelly drilling from 13 to 15 feet bgs.Very dense, wet, gray, gravelly, slightly silty SAND(SP-SM); fine to medium sand, fine and coarse subrounded gravel, scattered brown, gravelly, organic SILT (OL) beds.Blow counts elevated due to heave from below.

PRE-FRASER NON-GLACIAL DEPOSITSHard, very moist, brown, organic SILT (OL); non-plastic,trace fine sand, scattered PEAT (PT) beds.


S1

S2

S3a

S3b

S4

S5

S6

S7

S8

Depth(feet)

MaterialType


Maclen


Wat

erLe

vel

B-05

Sheet 1 of 1

Depth(ft)

Sampling Method

9/19/2017




AS

PE

CT

ST

AN

DA

RD

EX

PLO

RA

TIO

N L

OG

TE

MP

LAT

E

P:\

GIN

TW

\PR

OJE

CT

S\C

ITY

OF

RE

DM

ON

D_R

ED

WO

OD

RO

AD

-160

422-

01.G

PJ

Nov

embe

r 30

, 20

17

SampleType/ID

Elev.(feet)


Blows/6"

5

10

15

20

25

Tests


Hollow-Stem Auger

Bortec1


6.2' (ATD)

Exploration Number

Sam

ple

Met

hod

Description

Equipment

Legend

Contractor

140

135

130

125

120

B-05


Plastic Limit


5

10

15

20

25


Water Level ATD


NA


ExplorationLog

143.1'(est)

254440, 1321120 (est)



10 20 30 400 50

APPENDIX B

Slope Stability Analysis

ASPECT CONSULTING

PROJECT NO. 160422-01 FEBRUARY 1, 2018 B-1

B. Slope Stability Analysis

B1 Slope Stability Parameters We utilized the computer program SLIDE (Rocscience, 2017) to evaluate this critical section. Soil engineering properties used in our analyses are summarized in Table B1.

Table B1. Summary of Soil Engineering Properties Used in Slope Stability Analyses

Geologic Unit Unit Weight (pcf)(1)

Strength Parameters

Friction Angle (deg.)(1)

Cohesion (psf)(1)

Concrete 150 50 1000 Gabion Wall 135 50 0

Fill 120 30 0 Landslide Debris 120 22 0

Alluvium 125 33 0 Advance Outwash 130 42 0

Pre-Fraser Non-Glacial Deposits 125 38 0

Notes: 1) pcf = pounds per cubic foot; psf = pounds per square foot; and deg = degrees

The SLIDE program performs two-dimensional limit equilibrium slope stability computations based on the modeled slope conditions. SLIDE calculates a factor of safety against slope failure, which is defined as the ratio of resisting forces to driving forces. A factor of safety of 1.0 indicates a “just-stable” condition, and a factor of safety less than one would indicate unstable conditions. We selected Spencer’s method of slices and searched for rotational failure surfaces in our analyses.

The engineering properties summarized in Table B1 and used in our analyses were selected based on our experience in similar geologic settings with similar geologic materials and in situ relative densities determined by SPT measurements. Engineering properties of landslide debris were determined by back-calculating to account for the “just-stable” existing conditions. A conservative aerial load of 250 psf was applied to the slope stability model across the width of Redmond-Woodinville Road NE. The groundwater table depth was modeled based on water levels measured in our subsurface explorations.

B2 Seismic Design Criteria This seismic parameters and hazard evaluations as they relate to the road embankment stabilization concepts were based on methodologies presented in the WSDOT’s GDM

ASPECT CONSULTING

B-2 PROJECT NO. 160422-01 FEBRUARY 1, 2018

(WSDOT, 2015) and the soil boring data. We recommend classifying the Site as seismic site class “D.”

The WSDOT GDM describes seismicity to be represented by the acceleration coefficient, As, which represents the peak ground acceleration (PGA) adjusted for seismic site class. Determination of the PGA is based on U.S. Geologic Survey (USGS) regional probabilistic ground motion studies for various recurrence intervals equating to 7 percent occurrence in 75 years (approximately a 975-year return period event) (USGS, 2008).

Based on this, we compute the acceleration coefficient, As, used in seismic hazard evaluations as 0.44 g (where g is gravity constant). We applied a pseudo-static horizontal acceleration (Kh) of one-half the peak ground acceleration to model the slope during the design-level earthquake. Use of this kh value with a target factor or safety against slope instability of 1.05 to 1.1 assumes that limited deformation (on the order of 1 to 2 inches) of the slope is acceptable during earthquake shaking (WSDOT, 2015).

1.00

250.00 lbs/ft2

1.00

W

W

Results:

Spencer's Method

Search Method:Auto Refine Search Divisions along slope:10

Circles per division:10 Number of iterations:10

Divisions to use in next iteration:50% Number of vertices per surface:12

Minimum Elevation:Not Defined Minimum Depth:Not Defined

Minimum Area:Not Defined Minimum Weight:Not Defined

Surfaces with a factor of safety below 1.000


(psf)Phi

(deg)


Gabion Wall 135 Mohr‐Coulomb 0 50


Landslide Debris 120 Mohr‐Coulomb 0 22




Right of Way RedWood Road

Groundwater under artesian conditions

B-01

B-03

Safety Factor0.000.130.250.380.500.630.750.881.001.131.251.381.501.631.751.882.002.132.252.382.502.632.752.883.00+

200

180

160

140

120

100

-40 -20 0 20 40 60 80 100 120 140 160

APPENDIX:

B-1REVISED BY:

MWS

BY:

JGFPROJECT NO.

160422-01

11/30/2017


Slope Stability


SCALE: 1:240

SLIDEINTERPRET 7.029

Section A-A' - ExistingStatic Conditions

1.28

250.00 lbs/ft2

1.28

W

W


(psf)Phi

(deg)








Right of WayResults:

Spencer's Method







RedWood RoadRedWood Road

Dewatering pit location

Safety Factor0.000.130.250.380.500.630.750.881.001.131.251.381.501.631.751.882.002.132.252.382.502.632.752.883.00+

200

180

160

140

120

100

-40 -20 0 20 40 60 80 100 120 140

APPENDIX:

B-2REVISED BY:

MWS

BY:

JGFPROJECT NO.

160422-01

11/30/2017


Slope Stability


SCALE: 1:240


Section A-A' - Proposed Horizontal DrainsStatic Conditions

250.00 lbs/ft2

W

W

Material Name Color Unit Weight(lbs/ 3)

Strength Type Cohesion(psf)

Phi(deg)

Water Surface Hu Type

Concrete 150 Mohr‐Coulomb 1000 50 None

Gabion Wall 135 Mohr‐Coulomb 0 50 None

Fill 120 Mohr‐Coulomb 0 30 Water Surface Automa cally Calculated

Landslide Debris 120 Mohr‐Coulomb 0 22 Water Surface Automa cally Calculated

Alluvium 125 Mohr‐Coulomb 0 33 Water Surface Automa cally Calculated

Advance Outwash 130 Mohr‐Coulomb 0 42 Water Surface Automa cally Calculated

Pre‐Fraser Deposits 125 Mohr‐Coulomb 0 38 None

Results:

Spencer's Method







RedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood RoadRedWood Road

Right of Way


Support Name Color Type Force Applica on Out‐Of‐PlaneSpacing ( )

Pile ShearStrength

(lbs)Force Direc on

Soldier Pile MicroPile Ac ve (Method A) 8 75000 Perpendicular to pile

Safety Factor0.000.130.250.380.500.630.750.881.001.131.251.381.501.631.751.882.002.132.252.382.502.632.752.883.00+

200

180

160

140

120

100

-40 -20 0 20 40 60 80 100 120 140 160

APPENDIX:

B-3REVISED BY:

MWS

BY:

JGFPROJECT NO.

160422-01

11/30/2017


Slope Stability


SCALE: 1:240


Section A - A' - Proposed WallStatic Conditions

0.57

W

W 250.00 lbs/ft2

0.57

Results:

Spencer's Method








(psf)Phi

(deg)








Right of Way

Redwood Road

0.22


Safety Factor0.000.250.500.751.001.251.501.752.002.252.502.753.003.253.503.754.004.254.504.755.005.255.505.756.00+

200

180

160

140

120

100

-40 -20 0 20 40 60 80 100 120 140

APPENDIX:

B-4REVISED BY:

MWS

BY:

JGFPROJECT NO.

160422-01

11/30/2017


Slope Stability


SCALE: 1:240


Section A-A' - ExistingPseudostatic Conditions

0.78

250.00 lbs/ft2

0.78

W

W

Results:

Spencer's Method







Right of Way

RedWood Road

Dewatering pit location

1.06


(psf)Phi

(deg)








0.22

Safety Factor0.000.130.250.380.500.630.750.881.001.131.251.381.501.631.751.882.002.132.252.382.502.632.752.883.00+

200

180

160

140

120

100

-40 -20 0 20 40 60 80 100 120 140

APPENDIX:

B-5REVISED BY:

MWS

BY:

JGFPROJECT NO.

160422-01

11/30/2017


Slope Stability


SCALE: 1:240


Section A-A' - Proposed Horizontal DrainsPseudostatic Conditions

1.28

250.00 lbs/ft2

1.28

W

W


(psf)Phi

(deg)








Results:

Spencer's Method







Right of WayRight of WayRight of WayRight of WayRight of Way

RedWood Road


Support Name Color Type Force Applica on Out‐Of‐PlaneSpacing ( )

Pile ShearStrength

(lbs)Force Direc on

Soldier Pile MicroPile Ac ve (Method A) 8 75000 Perpendicular to pile

0.22

Safety Factor0.000.130.250.380.500.630.750.881.001.131.251.381.501.631.751.882.002.132.252.382.502.632.752.883.00+

200

180

160

140

120

100

-40 -20 0 20 40 60 80 100 120 140 16

APPENDIX:

B-6REVISED BY:

MWS

BY:

JGFPROJECT NO.

160422-01

11/30/2017


Slope Stability


SCALE: 1:240


Section A - A' - Proposed WallPseudostatic Conditions

i

APPENDIX C

Site Photo

February 2018 JGF BY:

160422PROJECT NO.

FIGURE NO.

C-1

Site Photograph

Redwood Road Slump Redmond, Washington

PHOTO 1: Culvert exiting from beneath Redwood Road and the failing gabion wall (looking west).

Documents

GEOTECHNICAL EVALUATION - Redmond