Nicholson Construction Company 12 McClane Street Cuddy, PA 15031

Telephone: 412-221-4500 Facsimile: 412-221-3127

Installation of Drilled Case Micropiles using Low Mobility Grout

by

Curt Fitzgerald Nicholson Construction Company, Kalamazoo, Michigan

Dwayne Lewis Nicholson Construction Company, Kalamazoo, Michigan

Presented at:

12th Annual Great Lakes Geotechnical/Geoenvironmental Engineering Conference Akron, Ohio May 7, 2004

04-06-143

1

INSTALLATION OF DRILLED CASED MICROPILES USING LOW MOBILITY GROUT

Curt Fitzgerald1 and Dwayne Lewis2

Abstract

Occasionally, projects come out to bid that the designer welcomes the innovative contributions from the contractor. This is usually presented in such a way that welcomes alternates to what is shown in the bid documents. In situations where time constraints and/or cost are of significant impact, the alternates may be presented to the contractors as part of the bid documents. Such was the case at the Grand Rapids Convention Center Project.

The site geology is comprised of miscellaneous fill consisting of

various sand, gravel, and clay layers. Cobbles and boulders were present throughout the site. The underlying bedrock consisted of alternating layers of moderately weathered shale, limestone, and gypsum. Voids were evident in most of the exploratory borings within the limestone and gypsum layers as the result of karstic weathering. The anticipated column loads for the new building varied from 300 to 800 kips. Taking into account the existing geology and column loading, it was determined that a deep foundation system would be required.

The designer’s primary system used small diameter friction caissons. Large diameter end bearing caissons and groups of drilled cased micropiles were also presented as alternates by the

1Project Engineer, Nicholson Construction Company, 5945 W. Main Street, Suite 102, Kalamazoo, MI 49009 2 Project Manager, Nicholson Construction Company, 12 McClane Street, Cuddy, PA 15031

2

engineers. All three options specified that a grouting program using low mobility (LMG) grout to fill the suspected voids would be required.

The selection of the micropile alternative was based on many

factors. The most important factor was the ability to penetrate the various man-made and natural obstructions using drilling techniques compared to excavation operations. Secondly, the grouting requirement would be satisfied by using low mobility grout in the installation of the piles. A third advantage was that the micropile operation utilized smaller equipment, greatly reducing the congestion on an already busy site.

The paper discusses many of the considerations taken in the use of

LMG as it pertains to the installation of the micropiles on this project. It also addresses sequencing issues regarding grout placement as it relates to drilling and adjacent grouting operations for the installation of 352 micropiles, totaling 18,656 linear feet (lf), during the third phase of work. Introduction

The Grand Rapids-Kent County Convention/Arena Authority desired to renovate and expand the existing convention facilities as part of an ongoing development of Grand Rapids which is the metropolitan focal point of West Michigan. The new facility consists of a 160,000 square foot (sf), column-free exhibit hall, with a 54,000 sf grand gallery (public/welcome area and meeting rooms), and a 50,000 sf ballroom area. The project was a multi-phase project and this paper discusses the foundation work completed for the third of three phases. The foundation work was identified as part of Phase 3C.

The Phase 3 work for the project involved removal of the existing Welsh Auditorium and renovation of the existing Grand Center area converting the space into support facilities including the addition of new stairwells and an elevator shaft within the existing structure and construction of the ballroom area of the new structure. Along with foundations for the new construction, foundations for the new stairwells and elevator shaft, and to accommodate increased loads at existing foundations were required.

Column loads for Phase 3C varied from 300 to 800 kips. These

loads combined with the existing geology and site history created the need for a unique foundation system. Following is a brief description of the site geology and history, along with a summary of foundation work completed during the first two phases. The first two phases of foundation work were considered in the selection of the Phase 3C foundations.

3

Site Geology/History

The Grand River Valley is a glacial outwash area and soils are primarily granular with numerous cobbles and boulders deposited by glacial water flows. An 1837 map of the area shows the east bank of the Grand River as a “black ash swamp” in the vicinity of the project site. It is presumed that the area was filled with miscellaneous material above the river bank level to the current grade at an approximate elevation of 611 feet National Geodetic Vertical Datum (NGVD) when the existing flood wall was constructed. Test borings found the post-glacial alluvium ranged from poorly graded gravel with sand, to lean clay. Cobbles and boulders were encountered in both the fill and alluvium.

The bedrock formation consists of a limestone and sandstone cap of the Bayport formation overlying weathered shale and gypsum of the Michigan Formation. The Map of Bedrock Formations of the Southern Peninsula of Michigan and test borings of the site revealed the limestone/sandstone cap is only present in the northern two-thirds of the site. The top of the limestone/sandstone cap is relatively shallow with top of rock elevations ranging from 587 to 599 feet and the formation was found to be slightly weathered but heavily fractured. Test boring logs indicated that circulation of drilling fluids was frequently lost within the limestone/sandstone cap and grout quantities required to backfill the boreholes were typically 10 to 15 times the theoretical volume. Test boring logs also indicated voids ranging from 4 inches to more than 2 feet were encountered within the bedrock. The voids encountered and excessive grout takes within the bedrock are indicative of a karstic formation.

The project site is located along the east bank of the Grand River as it passes through downtown Grand Rapids. Figure 1 shows the site location. The site housed industrial facilities during the late 1800s and early 1900s, prior to construction of the existing Welsh Auditorium in the 1930s. The existing convention center and performance hall facilities were constructed during the early 1980s. A power canal and tailrace for a water wheel that served the industrial facilities traversed the middle of the site in a north-south direction. This canal was converted to a 6.5-foot by 6-foot trunk sewer in the northern portion of the site and routed to the east of the Welsh structure around the time of its construction.

4

Fig. 1 Topographic Map of Grand Rapids, MI

Settlement of the existing structures was detected in 1992 and subsequently efforts to determine the cause of failure, stabilize the subgrade, and raise the subsided structures were completed. Investigation and remedial efforts included injection of approximately 1,700 cubic yards (cy) of LMG under pressures of up to 600 pounds per square inch (psi) into voids within the bedrock formation, repair of two broken sanitary sewer lines, and mechanically raising the subsided structures. The epicenter of the subsidence was at a location were the Welsh Auditorium, the Grand Center, and the DeVos Performance Hall join. This spot corresponds with the power canal tailrace location just south of where the trunk sewer was routed to the east to remain outside of the Welsh Auditorium. Confirmatory borings revealed a grout-filled void 5 to 8 feet in height, beneath 20 to 25 feet of fractured limestone cap. Other borings revealed several soft clay seams within the limestone, indicative of

Convention Center Construction Site

Grand River

5

sediment-filled voids. A settlement study report submitted to the City of Grand Rapids Engineer’s Office concluded that the cause of the destabilization was “a vertical migration of soils, through natural or man-made passages, to unknown destinations.” Project History

Phase 1 was a lobby addition for the existing DeVos Performance Hall with a limited work area. Given the limited space the foundation was designed for 25 and 75-ton micropiles with a specified pile tip elevation below the zone where voids had been encountered during the site investigation. The designers anticipated the use of LMG with the material specified as a sanded grout “suitable for pile construction and void filling purposes” in order to accomplish “injection of a low-slump, specially formulated, cementitious, cohesive grout under pressure to fill existing voids. The method is not intended to fill small fractures and seams within the bedrock formations.” The slump of the grout was not specified and an 8 to 10-inch slump grout was selected for the Phase 1 micropiles. Excessive grout takes occurred in areas.

The Phase 2 foundation work was awarded as a combination of

deep, small-diameter, friction caissons and shallow end bearing caissons with grout points “for void filling purposes” at each foundation element through the middle portion of the project site where the limestone/sandstone cap tapers off. A total of 22 micropiles were installed at locations adjacent to the existing structures with limited access where caissons could not be installed. These micropiles were installed with the same tip elevations and grout material specifications as the Phase 1 piles. However, a 6-inch slump was specified to reduce grout takes. The grout injection for the caisson locations was through 4-inch casing to a depth of approximately 55 feet below grade. The average grout takes were approximately 10 cy, 50 times the theoretical volume per injection point. Several locations took 100 to 250 times the theoretical volume. One location took 500 times the theoretical volume at just over 100 cy.

During installation of the Phase 2 caissons, obstructions became a significant contract issue as nestled boulders and old foundations from the industrial facilities were frequently encountered, particularly in the southwest portion of the Phase 2 area, just north of the Phase 3 work area. The contractor performing the LMG work and installation of the 100-ton micropiles successfully used low-slump grout under high pressures to grout the bedrock. For the Phase 3C work, the construction manager recognized that LMG could be successfully used at the site, but it was important to use a pre-qualified micropile contractor experienced with

6

LMG grouting and micropile installation in karstic or otherwise fractured and voided rock. Selection of the Foundation System

The bid package for the Phase 3C work contained a base bid foundation system along with two alternate foundation designs. All three options specified a grout program using LMG to fill the voids within the bedrock formation as in the previous phases. The designer’s primary system consisted of approximately 100 small-diameter, deep-friction caissons with 54 micropiles at locations adjacent to and within the existing structures. Large-diameter, end-bearing caissons and groups of drilled cased micropiles were presented as alternates by the engineers. The micropile work consisted of 352 piles with an average depth 52 feet.

Many factors made the micropile alternate the most favorable of the three options. One significant factor was the ability to penetrate the various man-made and natural obstructions using smaller diameter drilling techniques compared to the excavation operations and large diameter drilling procedures associated with caisson installation. Significant obstructions had been encountered during the Phase 2B work, particularly in the area just north of the Phase 3C work creating significant cost overruns. Because smaller-diameter drilling techniques penetrate obstructions without much impact to production rates, obstructions are not typically a line item cost for micropile installation work.

Satisfying the grouting requirement using LMG in the installation of

the piles as a single operation was another significant factor. For both of the caisson designs, the required grout program would be a second operation separate from the caisson work. Although the number of caissons was significantly less than the number of micropiles required, having a second operation for the grout program required schedule time and also limited the access to the area by other trades.

A third advantage of the micropile alternate was the use of smaller equipment which greatly reduced congestion on the busy site and presented an opportunity for accelerated scheduling of subsequent work. Along with installation as a single operation, the equipment and tooling for micropile installation is significantly smaller than equipment typically used for caisson installation and the volume of spoils generated is significantly less. The reduced volume of spoils was particularly significant for this project since disposal costs were higher due to the presence of contaminated soils in areas.

7

Pile Design Requirements

Phase 3C project documents called for 100-ton drilled, steel cased, pressure-grouted micropiles with a minimum diameter of 8 inches. The pile tip elevations were to be set at 545 feet or below, and the casing tip elevations were to be set at 585 feet or below. Full-depth, no. 18 bar with centralizers were required and the pile cap connection was a bent no. 9 deformed bar extended 4 feet into the micropile. The specification called for a base bid volume of 10 cy of grout at each pile location with the material specified as in Phase 2. The final structural design of the micropiles was the contractor’s responsibility. This approach ensured the drilling would reach a minimum depth for grouting of the potential voids and allowed the contractors to be flexible with the internal structural design. Two load tests, at different ends of the work area, were specified to confirm the suitability of the pile design and installation methods.

The internal structural design submitted for the project was determined by analyzing the micropiles for axial load using allowable stress percentages from the Federal Highway Administration Micropile Implementation Manual (FHWA-SA-97-070). This analysis resulted in the pile configuration shown in Figure 2, with 7-inch by 0.5-inch N80 Casing and a no. 18 (2.25-inch diameter), grade 75 threadbar with 4,000 psi grout. A 9-inch by 9-inch by 1-inch bearing plate and nut configuration was proposed and accepted in lieu of the bent no. 9 bar detailed in the project documents for the pile cap connection.

8

Fig. 2 Typical Pile Detail

Load Testing

The micropile test requirements were for two load tests to twice the allowable design load in accordance with the quick load procedures of

Grout fc’ 4,000 psi

9

American Society for Testing and Materials (ASTM) D1143 with the total load maintained for 24 hours. Acceptance criteria were established as net settlement not to exceed 0.01 inch per ton of test load or 0.5 inch total. With a maximum test load of 200 tons, the 0.5 inch total net settlement was the controlling criteria. Due to voids within the bedrock formation the grout volumes at the two piles were significantly different at 24.2 cy and 12.4 cy at the first (northeast) and second (southwest) locations, respectively. Four hours into the 24 hour load hold of the first test, the deflection readings had stabilized with total deflection at 0.40 inches and remained stable for the remainder of the test. Approximately 15 hours into the load hold period crews for the other trades began working the morning shift, and vibrations of the reference beam along with erratic deflection readings were noted as equipment passed near the test location. Performance of the test pile was discussed with the project team and the load test was terminated early since deflection at full test load was below the allowable net settlement acceptance criteria. Net settlement after unloading the test pile was 0.102 inches, 20 percent of the allowable deflection. Initially the design team was concerned that the large grout volume had influenced the results. However, performance of the second load test was consistent with the first and deflections were less even though the grout take was half of that placed for the first test pile. Deflection readings stabilized at 0.23 inches by the fourth hour of the load hold and net settlement was only five percent of the allowable deflection at 0.024 inches. Installation Methods

Micropile locations were set with a total station and referenced to control points provided by the construction manager. Casing was advanced to the plan tip of pile elevation using rotary percussive drilling techniques with a down the hole hammer (DTHH) and eccentric bit to penetrate the cobbles, old foundations, and bedrock formations. High volume, high pressure air flush with water injection was used to clear the cuttings as the drilling advanced. An underhead swivel and discharge hoses were used to control and contain the flush return in small pits excavated and maintained by the excavation contractor.

Once the casing was advanced to depth, the core steel with centralizer was placed into the casing prior to grouting. With an overall depth of 55 to 60 feet, the core steel was placed in sections using couplers with set-screws to prevent unscrewing of the couplers as the casing was turned during the extraction process. Grout was delivered from a ready-mix supplier and pumped at a 4 to 6-inch slump by a truck mounted concrete pump through an underhead swivel into the casing as

10

the drill rig withdrew the casing to the plan tip of casing elevation. Pressure at the grout header (underhead swivel) was maintained at a minimum of 150 psi with the actual pressure and volume recorded as each 10 foot section of casing was removed. The minimum pressure was specified to ensure any voids intersecting the pile location were filled. Occasionally distinct voids could be identified while drilling by a lack of resistance to casing advancement, however most of the time the volume of grout required could not be predicted based on the drilling conditions. A total of 352 micropiles, totaling 18,656 linear feet (lf), were installed. Scheduling/Sequencing Issues

Several factors impacted the scheduling and sequencing of the micropile installation including distance restrictions in the project specifications, an accelerated schedule developed by the construction management team, drilling and grouting production rates, and availability of different work areas at different times.

Concerns about communication between locations disturbing fresh

grout and compromising the integrity of completed piles prompted the design team to place distance restrictions in the project specification. Micropile installation activities were not allowed to occur within 20 feet of a previously installed micropile within 24 hours of the completion of the adjacent pile.

Attempts were made to conduct the drilling operation 20 feet from fresh grout by keeping the drill several locations ahead of the grouting operation to avoid drilling near locations grouted each day. However, it became apparent that meeting the distance requirement for the grouting operation required skipping pile caps at 15 foot spacing. As the sequence moved along wall lines, only two locations per day could be completed within a stair tower supported on 4 pile caps with 5 micropiles each. This restriction doubled the number of times the equipment had to pass along the wall line and doubled the amount of time required for completing the stair tower as the original plan was to complete one location at each of the four pile caps each day.

Ultimately, it was determined that the best approach was to use

one rig drilling with a second rig grouting. This approach would allow the drill crew to work ahead of the grouting operation, placing casing at pile caps that were spaced 15 feet apart, keeping the air flush well away from the grouting operation.

11

Increased production was important as completion of the exterior wall and stair tower were on the critical path of an accelerated schedule the construction management team had developed. The construction manager developed the accelerated schedule to allow erection of the structural steel framing eight weeks ahead of the original date specified in the bid documents. Acceleration of the schedule was made possible with the grouting and pile installation as a single operation and the smaller equipment allowing access for pile cap construction and below grade mechanical, electrical, and plumbing work while the micropile installation was ongoing.

An average of 12.5 cy of grout were placed at the first micropile installed at each pile cap. Subsequent micropiles had significantly lower grout takes (with an average of 3.6 cy) indicating major voids within the bedrock formation were substantially filled. Based on the difference between grout takes at initial and successive micropile locations for individual pile caps, it was apparent that the micropile sequencing was appropriate. The micropile sequencing and production met the schedule needs of the construction team and provided the design team with a level of confidence that the integrity of the installed micropiles were not compromised. As the installation work advanced into new areas of the site the drilling operation would get ahead of the grouting which would then catch up as each area was being completed due to the differential in grout volumes between installation of initial and successive micropiles at each pile cap. Balancing the drilling and grouting production rates to maintain the distance requirements through the center area with pile caps spaced at 30-foot centers worked very well. However, installation of micropiles at four other areas around the existing Grand Center structure, along the north side of the remaining lobby, and the ramp area in the southwest corner, required extraordinary coordination and cooperation by all parties involved. This cooperation enabled the schedule requirements to be met without violating without violating the distance requirements. Summary and Conclusions

The Grand Rapids Convention Center (now called DeVos Place) project’s geological conditions, site history, and design loads presented many challenges for foundation construction. There were several factors that contributed to the success of this project, beginning with a site investigation and geotechnical report that clearly identified the challenges. The geotechnical report provided recommendations for several foundation systems, allowing the construction management team to obtain bids for

12

alternate systems and select the most cost effective system. In this case, the drilled cased micropiles provided a cost effective solution with high level of quality. The micropile drilling techniques used, with the right tooling, were able to quickly penetrate the various man-made and natural obstructions without additional cost to the owner. Grouting of the micropiles with LMG enabled the filling of the voids within the bedrock formation. Confirmation was secured through the monitoring of volumes and pressures. Since the design team had included an appropriate base bid volume for the micropiles, the installation was completed without additional costs to the owner for the grout volumes. Completion of pile load tests and maintenance of a high level of quality control provided confidence regarding the performance of the installed micropile.

Along with preventing additional costs to the owner, the selection of the LMG micropiles with smaller equipment and single operation installation, allowed the construction management team to develop an accelerated schedule for the trades that followed. The execution of the accelerated schedule depended on the ability to efficiently sequence the work and coordinate the parties involved. It must be recognized that effective communication and mutual respect between the specialty contractor, construction manager, architectural/structural engineers, inspectors, and geotechnical engineer was critical to the success of this project. Acknowledgements

The authors wish to acknowledge Steven M. Elliott, PE of Materials Testing Consultants, John Bassett, AIA of Progressive AE, and Don Van Beek and Howard Oosterink of Erhardt Construction for sharing the breadth of their knowledge regarding the site history and conditions encountered during remediation work completed in the 1990s. The authors also thank Brian O’Gara and Marty Taube of Nicholson Construction Company for their assistance in preparing the paper. References Materials Testing Consultants, Inc., (December 7, 2000). “Report of Geotechnical Investigation, Grand Rapids Convention Center, Grand Rapids, Michigan.” Map of Bedrock Formations of the Southern Peninsula of Michigan