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Micropiles and Soil Nails: True Supporters of the Arts

by Antonio Marinucci, P.E., University of Texas at Austin, Andrew C. Baxter, P. G., Schnabel Engineering, and Paul Gintonio, Foundation Specialties, Inc. IFSI GEOCON

The following article was written by Antonio Marinucci, PE., with the assistance of Andrew Baxter; PG., Schnabel Engineering and Paul Gintonio, President, Foundation Specialties, Inc. Tony is in the final stages of completing his Doctorate Degree at the University of Texas, Austin. He is a recipient of an ADSCIIAF Civil Engineering Graduate Study Scholarship. The idea for the article came from Schnabel’s Director of Geotechnical Engineering and ADSC Board Member Allen Cadden. Allen told me about the project when it was ·in its formative stage. I immediately suggested that it had the potential to be a Cover Feature for Foundation Drilling magazine. As chance would have it, Tony was planning a trip to Arkansas to visit a colleague of his, Brady Cox, who is a member of the Civil Engineering faculty at the University of Arkansas. Brady happened to have been an attendee at the ADSC’s Foundation Engineering Faculty Workshop held in Chattanooga, Tennessee in June, 2008. This synergistic connection caused me to suggest to Tony that perhaps he could visit the project site and write an article for Foundation Drilling. Tony has been busy completing the research that will be the basis of his doctoral dissertation and finishing course work for his degree. I prevailed upon him to “work us in” as his schedule would permit. This article is the culmination of these conversations, travels, and collaborative effort between Tony and the good folks at Schnabel Engineering and Foundation Specialties, Springdale, Arkansas, the ADSC Contractor Member company that was installing the micropiles.

Many thanks to all of these folks for putting together the excellent article that follows. (Editor)

Through a very generous land donation by Alice Walton and the Walton Foundation, a 100 acre area in Bentonville in northwest Arkansas will soon house the new home for the Crystal Bridges Museum of American Art. According to their website ( www. crystal-bridges.org), the museum, as the name implies, will be dedicated to promoting to more than 250,000 expected annual visitors American art and the artists who create it. Upon completion, the museum will have a footprint in excess of 100,000 square feet and will showcase various American artworks spanning from the 1700s to the present-day in a gallery space totaling more than 25,000 square feet. The Museum was designed by Moshe Safdie and Associates, a world-renowned architectural firm, and will include the gallery space(s), a 250-seat theater space, two pond spaces on the site created by using dams and bridges, a library, a museum store and dining facilities. Additional details and design features can be found on the museum website. The unique and challenging building excavations and foundation requirements discussed below were met by ADSC members Schnabel Engineering Geostructural Group (Schnabel and Foundation Specialties, Inc. (FSI).

Site Layout
The museum is being constructed in a densely forested ravine with a small creek bisecting the site. Extensive clearing and grubbing was required prior to the start of construction to create suitable work areas (Figure 1). In addition, the water flow from within the creek had to be diverted from the center of the project site prior to construction, but will be returned to its original alignment upon completion of construction. The proposed museum structure is separated into ten areas, as shown in Figure 2, according to future usage of the spaces. Given the nature of the original landscape, the elevations of adjacent sections of the building (e.g. Area PIB and Area A-l/A-2) can be quite different, as seen in Figure 3 which is shown on the cover of this issue, making construction operations and ‘site maneuverability more cumbersome and complicated.

Subsurface Conditions
The geotechnical field testing and sampling program, performed during site development, used to characterize this site consisted of 47 borings and ten test pits. Standard penetration testing (SPT) blow counts (N-values) and disturbed samples of the overburden soils were obtained using a driven 2-inch split-spoon sampler. Core samples of the underlying rock were obtained using rotary drilling and a diamond bit core barrel.

According to the geotechnical engineering report, the generalized subsurface profile at this site consists of interbedded, variable thickness overburden soils (Figure 4) overlying weathered rock (Figure 5) and competent rock (Mississippian-aged Boone Formation, 359 to 318 million years ago). The overburden soils are predominantly comprised of stiff-to-hard, lean-to-fat clay with varying amounts of sand, silt and gravel: residual soil resulting from the weathering of the underlying Boone formation. The Boone Formation consists of limestone with chert (a hard, brittle sedimentary rock comprised of quartz). Typical characteristics of this Formation include pinnacled bedrock and horizontal and vertical weathering.

The presence of water was noted in three of the soil borings during the drilling and sampling, but was not present immediately thereafter. Water was encountered in only one of the ten test pits during and immediately following excavation, but decreased in elevation with time. Though the potential for seepage from rainfall and perched water conditions existed, construction ground water control systems were not used in the majority of the site except in the area of the deep bridge abutments.

Geotechnical design parameters used in the design of foundation and structural elements were determined based on empirical correlations, laboratory testing, and an extensive field testing program by Schnabel Engineering and FSI. As determined from laboratory testing, values of unconfined compressive strength (UCS) for the limestone and chert bedrock ranged from 2,000 psi to 10,000 psi. In general, the (Bieniawski) Engineering Rock Mass Rating was II or Good Rock. The high variability in unconfined compressive strength between the chert and limestone, however, resulted in a large range of design values for bearing and shear friction.

As discovered during the site investigation and during construction, the building site is highly variable with respect to solution features. Most of the east side of the site is not significantly affected by solution features, but some have been found east of the Bl gridline. Major solution features, however, have been encountered on the west side of the project site. As a result, significant consolidation grouting with both low mobility (lM) grout and self-consolidating concrete (SCC) has been performed.

Complex Structure Warrants Multiple Foundation Solutions
Given the complexity of the museum structure and the highly variable geology of the project site, multiple foundation and support systems were required to satisfy the intended needs of this structure. Drilled shafts will support the maximum anticipated column loading of approximately 1200 kips. For the abutment areas adjacent to the future suspension roofs, rock anchor groups will provide uplift resistance to support the anticipated 2,500 to 4,200 kip loading. Building A-6, a three story clear span on the west side of the site, warranted permanent earth retention system to support a maximum cut height of about 80 feet. Strip footings have been designed for bearing pressures between 15 and 40 ksf. Depending upon spatial location, the footings will be founded on either rock or variable thickness overburden soils. Micropiles will be used to adequately carry the vertical and lateral loading down to competent rock.

Overburden Stabilization and Protection
Given the abundance of weathered rock and clay overburden at this site, soil nail walls were selected for the excavation support system. A permanent soil nail wall was constructed on the west side of the project site. In the northeast corner, a temporary soil nail wall was constructed to stabilize the residual soils above a stable rock excavation. In areas where excavation limits were not constrained, the overburden soil was sloped and protected from erosion, and the rock was open cut, monitored for breaks, and draped with chain link fencing (to catch falling rock and/or debris). A third soil nail wall, however, was used for a slightly different purpose.

As difficult sites challenge designers and contractors, existing technologies and techniques are frequently used in new ways. On this project, a soil nail wall was used on the southeastern portion of the jobsite in the vicinity of Area C to protect two longstanding inhabitants of the site. This pair of guarded residents, affectionately referred to as “Thelma and Louise” (Figure 6), are Tulip Poplar trees and currently stand at approximately sixty feet tall. Not wanting to uproot the mature trees, the poplars were incorporated into the final design and layout of the museum.

The soil nail wall was approximately 140 ft long, ranged from 18 to 26 ft high, and formed a half circle around the two trees. To avoid potential critical tap roots, the soil nails were splayed horizontally to maximum of 25 degrees (from perpendicular to the wall) and crisscrossed in front of the trees. Depending on potential intersection with previously installed soil nails, the inclination of soil nails on lower rows varied from the typical 15 degrees. The typical spacing pattern of the soil nails was 5 ft by 5 ft, but the horizontal spacing was increased to a maximum of 17.5 feet in localized areas to avoid encountering potential tree roots.

On the west side of the project site, along the AI-A6-A5 line, a permanent soil nail wall was designed for a maximum support height of 47 feet. A bench, used for a micropile supported foundation, will separate the upper and lower portions of the wall. Resulting in a total of about 24,200 linear feet of drilling, the soil nails, ranging from 23 to 43 feet in length, are No.’ ll, zinc-coated solid bars and were placed at approximately five feet on center in both the horizontal and vertical directions. The drilling of the soil nail bars was accomplished using an excavator mounted TEl Rock Drills* Models RDS350 rotary drill and HEM 300 feed system. The design minimum thickness of the shotcrete facing (compressive strength of 4000 psi) was eight inches, and was placed in a single shotcrete lift. Ultimately, the completed surface area of the soil nail wall was about 18,100 square feet with 643 soil nails and 84 rock bolts. Competent rock encountered in the bottom 15 feet of this wall permitted the redesign of portions of the soil nail wall with rock bolts, which resulted in a cost reduction and an increased production rate. In all, 36 soil nail proof tests were performed to confirm design assumptions (design parameters and computed capacities) and production methods.

For the majority of the soil nail wall construction, the shotcrete facing was applied after the installation of the soil nails. However, in areas where the soil mass was unstable and eroding prior to drilling or where grouting of the voids and redrilling was required, the shotcrete facing was applied prior to the installation of the soil nails. At times during the drilling of the soil nails, numerous voids, very soft saturated clay and loose gravel were encountered behind the wall, requiring significant pump volumes ranging from a few cubic yards to volumes in excess of twenty cubic yards. As the drilling progressed, FSI’s soil nail installation crew adjusted drilling and grouting methods as each of the aforementioned conditions was encountered. Large amounts of over-grouting with various viscosity grouts and redrilling efforts were performed in parallel with and ahead of nail installation. The total length of soil nails and rock bolts installed was approximately 24,200 linear feet, although about 31,200 linear feet of drilling was performed. A photograph showing the drilling for the lower portion of the soil nail wall is shown in Figure 7.

In the northeast corner of the project site (Area A-3/A-4), a temporary soil nail wall was used to stabilize the sloping ground above an exposed-face rock cut (Figure 8). The soil nails were installed in a square pattern at approximately five feet on center, though tighter spacing was used when conditions warranted. For the construction of the temporary soil nail walls, a hollow core bar with a sacrificial drill bit installation method was used, while the permanent soil nails were installed in an open hole drilled using a downhole hammer. In total, for the temporary and permanent soil nail excavation support systems, more than 1000 soil nails and 150 rock bolts will be installed.

Micropiles for Grade Beam Support
4, Band P The result of a strong cooperative design and construction relationship between Cristobal Correa, Winchester Falbe, Michael Rysdorp, and Denise Richards of Buro Happold Inc., and Schnabel’s geostructural design and field staff was an effective and versatile foundation system that met the projects needs. The design loads on the grade beams are comprised of a longitudinal load up to 65 kiplft, a transverse load up to 35 kip/ft, and a vertical load on the grade beam/pile group up to 85 kip/ft. The resulting loads on individual micropiles ranged from 65 to 310 kips in compression and 95 to 130 kips in tension. In order to resist the large lateral forces and avoid adjacent elements, the micropiles were battered at 0, 5, 15 or 30 degrees. Depending upon the grade beam location, the micropile foundation system is comprised of either vertical elements (Figure 9) or a combination of varied inclined elements (Figure 10). Open hole drilling in rock and duplex drilling in soil was accomplished using Beretta T41 and T59 track drills*. To facilitate the placement of rebar and formwork required for the grade beams, the mud slab was installed by the concrete contractor prior to drilling the micropiles resulting in a better working surface. The subgrade became extremely soft and messy when exposed to rain/water.

Due to the variability in the thickness of the overburden soils and the quality of the rock, Schnabel formulated two design alternatives for the micropile foundation system. For each option, the micropile element was comprised of a No. 20, all-thread Grade 75 bar centered in a API N-80 steel casing (Table 1). The total bond length for each micropile is 14 feet (developed in suitable rock) with the casing extending 3.5 feet and 6 feet into the bond zone for the vertical and battered micropiles, respectively. The top of each micropile extends a minimum of 12 inches into the grade beam/pile cap. The corrosion protection consists of sacrificial steel thickness and grout cover. The 28-day compressive strength of the grout was specified to be a minimum of 5000 psi.

A comprehensive testing program, which included tension, compression and lateral loading tests, was implemented. The tested micropiles were sacrificial and were installed/tested prior to the start of production. The distribution of the testing was based on the building layout and the observed geologic conditions encountered in the excavations. Micropiles were loaded in tension (l0 each) and compression (4 each) to a minimum of 200% of the design load. In addition, three lateral load tests were performed, loading the micropiles between 133% and 150% of the design load. Many of the micropile load tests were instrumented with vibrating wire strain gages. Five micropiles were instrumented with vibrating wire strain gages, which were used in determining ultimate bond stresses when full pile failure was not achieved. Based on the results of the testing on the piles that were instrumented with strain gages, the average mobilized bond stress was found to be about 300 psi. A photograph of the setup and tension testing of a micropile is presented in Figure 11.

For the micropiles acting as shear pins in rock, a total of 12 piles were tested to determine their load-deflection behavior. The test setup consisted of laterally loading two adjacent pile caps, each comprised of two test micropiles. A schematic and photograph of the test setup are presented in Figure 12 and 13, respectively. To provide meaningful and usable results, the test pile cap dimensions and reinforcement were selected to be representative of the grade beams that would be used for the production micropiles. Adjacent pile caps were jacked against each other during the load test, thereby allowing testing of four micropiles Simultaneously. The lateral deflection at the top of the micropiles at the maximum lateral test load ranged from 0.1 to 0.25 inches. Lateral load test results for micropiles TP-A2-1AD, TP-B-OA-D and TP-A4-2A-D are presented in Figure 14.

In 2008, about 1000 out of the 1250 on the project to resist uplift design loads ranging from 75 to 240 kips. The rock anchors are comprised of 1-3/4 inch and 1-3/8 inch diameter, 150 ksi all-thread bar installed in a 7 inch diameter nominal drill hole. Five pre-production rock anchor tests were performed to evaluate both the pressure testing requirements and the grout-rock ultimate bond stress. In the limestone bedrock, the ultimate bond stress ranged from 200 to 350 psi. However, in the area where the rock anchors were installed, softer portions of the bedrock were encountered, consequently reducing the ultimate bond strength by nearly 140 psi.

The 72 anchors in the abutments , which Schnabel designed as groups of threaded bar anchors, are tensioned at the mat footing, with extensions that will be post tensioned at the roof level. Tight tolerances on anchor location and verticality were met by the crew and equipment of FSI. Anchors at C building maintain the roof in tension allowing the open and free span. In addition, the Weir wall footings are also tied down with anchors. In total, over 3,600 LF of anchors will be installed below the suspension roof abutments, weir wall and retaining walls.

Closing
To date, about three-quarters of the specialty construction work (micropiles, anchors, drilled shafts and soil nail walls) has been completed, and the foundation work is still ongoing. As a result of the sequencing of construction activities, the balance of the foundation construction should be completed in 2009. Construction activities for the structural portions of the museum are well under way. The project site is a literal fury of activity. An aerial view of the site taken in March 2009, as presented in Figure 15, provides a glimpse of the various activities being simultaneously orchestrated…proving American Art is more than just show pieces for gallery viewing!

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