UNDERPINNING OF NEW STUDENT HOUSING BUILDING USING MICROPILES, NORTH CAROLINA USA

Transcription

1 UNDERPINNING OF NEW STUDENT HOUSING BUILDING USING MICROPILES, NORTH CAROLINA USA John R. Wolosick, P.E.,D.GE 1, Michael W. Terry, P.E. 2, W. David Kirschner 3 and Robert F. Scott Jr. P.E. 4 SYNOPSIS In late 2011, the general contractor was erecting a U-shaped building for a planned 10 storey new student housing building in North Carolina USA using tunnelform concrete construction methods. In September, the general contractor had reached the 8 th level of the East Wing of the structure and had completed foundations and begun placing slabs-on-grade for the West Wing. When settlement of more than 100 mm (4 inches) was observed in the East tower, concrete construction was halted. Micropiles were chosen to stabilize the rapidly settling foundation. Low-headroom installation was required for the East Wing while open headroom conditions were available at the West Wing. Due to a schedule requirement of opening the student housing in August 2012, a very rapid construction schedule for the underpinning was required. Health and safety issues were of paramount concern due to the intense schedule, heavy resource load, and a requirement to work adjacent to interior and utility trade contractors. Approximately 1,035 micropiles, drilled to bedrock, were installed in a 10 week schedule using up to 8 drill rigs and 2 work shifts without a safety incident. An artist s rendition of the proposed building is shown on Figure 1. Figure 1. Artist s rendition of student housing building 1 Director of Engineering, Hayward Baker Inc., 515 Nine North Court, Alpharetta, GA, 30004,USA , jrwolosick@haywardbaker.com 2 Senior Vice President, Hayward Baker Inc., 515 Nine North Court, Alpharetta, GA, 30004,USA , mwterry@haywardbaker.com 3 Area Manager - Carolinas, Hayward Baker Inc., 208 Little Santee Road, Colfax, NC, 27235,USA , wdkirschner@haywardbaker.com 4 Senior Engineer, Hayward Baker Inc., 4979 Lebanon Pike, Old Hickory, TN, 37138,USA , rscott@haywardbaker.com 1

2 PROJECT BACKGROUND In September, 2011, Hayward Baker was requested to make a site visit to discuss possible remedial deep foundation techniques to address the differential settlements observed at the site. During the walk through, the general contractor stated that they had not noticed the settlement until a thunderstorm had flooded the first floor. Ponded water was observed on the slabs up to 100 mm (4 inches) deep at the midpoint of the East Wing (near points 5 and 6 shown on Figure 3). The tunnel-formed construction used for the structure seemed to accommodate the settlements and showed little to no cracking at the time. A settlement survey was performed and measured 25 to 50 mm (1 to 2 inches) at each end of the East Wing and 100 mm (4 inches) at the midpoint of the East Wing. However, the survey was repeated on a regular basis and showed a rapid rate of settlement. A system of micropile underpinning was selected for the foundation repair and the load test program began immediately while the underpinning plans were prepared by the structural engineer. GEOLOGIC SETTING The project is located in the Piedmont physiographic province. The Piedmont is characterized by residual soils weathered from parent bedrock. The parent rocks are typically gneisses and schists with some granitic intrusions. The rocks typically lie in northeast trending belts and have a thick residual soil cover known as saprolite. The residual soils have formed as the result of chemical weathering of the parent rock. The degree of weathering varies based on mineral composition and the degree of natural fracturing and jointing. Therefore the soils are typically highly variable in density and consistency and often contain areas of partially weathered rock (PWR) at variable depths and thickness. The predominant soil type is either a sandy silt or silty sand, typically containing mica, with some clay fraction closer to the ground surface. GEOTECHNICAL INVESTIGATION Fourteen borings were drilled to initially investigate the subsurface conditions in An additional fourteen borings were also drilled during the remedial work at the site. The remedial borings showed similar conditions to the initial investigation. The groundwater table was at a depth of about 3.6 m (12 feet). The original geotechnical report identified a deeply weathered and soft soil profile over much of the site. Depth to PWR averaged about 65 feet but was measured as deep as 90 feet. Up to 11 feet of PWR was encountered by the borings. See Figure 2 for a boring log (Boring T-12) from the initial investigation near the center of the site. 2

3 ORIGINAL FOUNDATION CONSTRUCTION The original geotechnical report, recognizing potential excessive settlement of shallow foundations, recommended driven 305 mm (12 inch) square concrete piles (citing high steel prices in 2007) or auger-cast piles. Aggregate piers were recommended by the geotechnical engineering report only for the adjacent 7 storey parking garage. For construction, however, aggregate pier foundations were selected for both structures, the student housing structure and the parking garage. The authors are not aware of any foundation issues at the parking garage. However, after the installation of the aggregate piers, the student housing building suffered significant settlements during construction, prior to completion of the building. When the settlements exceeded 114 mm (4 ½ inches), concrete construction was halted and a remedial design for micropiles was initiated. Figure 2. Boring T-12 from the center of the site 3

4 STUDENT HOUSING BUILDING DETAILS The site for the student housing building was gently sloping and required cuts of about 1.8 m (6 feet) and fills of about 3.6 m (12 feet). The structure was U-shaped as shown on Figure 5. The building occupied approximately one square city block. Tunnel-form construction, which allows the casting of the reinforced concrete walls and slabs simultaneously, was used. Only wall footings were utilized, with no individual column footings due to this design. The concrete tunnel-form frame for the East Wing of the structure had been completed to the 8 th level out of the planned 11 storeys before concrete construction was halted due to the observed settlements. An extensive settlement monitoring program was set up for the structure in early September, 2011, after significant settlement were observed. The locations of the observation points for the East Wing and the results from the monitoring are shown on Figures 3 and 4. The settlement monitoring indicated that the structure was settling rapidly and did not indicate slowing of the rate of settlement. Figure 3. Settlement point monitoring locations for the East Wing 4

5 Figure 4. Settlement monitoring of East Wing starting in early September The West Wing slab foundation had been placed, but elevated work had not yet begun. Therefore, the micropile installation for the East Wing had to be constructed in limited headroom and access conditions as shown on Figures 8 and 9. The West Wing micropile installation was performed in open headroom (Figure 10). ORIGINAL FOUNDATION INSTALLATION DETAILS The aggregate pier design for the student housing building included approximately 1,050 piers, 0.77 m (30 inch) in diameter (not installed by the authors company). The aggregate piers varied in length from m (12-28 feet). The piers were spaced as shown on Figure 5. Spread footing foundations rested on top of the aggregate piers. The footings were apparently designed for allowable bearing pressures of 287 and 335 kpa (6,000 and 7,000 psf). 5

6 Figure 5. Initial foundation treatment with aggregate piers PROJECT HEALTH AND SAFETY All projects require a strong commitment to health and safety. This project specifically presented several unique safety challenges. Implementing an effective Health and Safety Plan (HSP) was critical due to the intense and quick build up of resources on the project, and around the clock operations. Senior management (offsite) was required to perform independent safety inspections of the site on a weekly basis. Additionally, the crew was incentivized to report safety concerns and near miss incidents which helped foster the existing company culture that shows how safety ownership belongs to everyone. The overall Health and Safety Plan included both daily and weekly reviews, safety inspections and toolbox talks. A daily task analysis was completed each morning and required the crew to actively engage in discussing their work task, hazards present and safe work procedures to mitigate these hazards. The general contractor assisted in communication and logistical planning through trade meetings to keep safety as the first priority throughout the project. Policies adopted to promote safety on the project involved: mandatory spotters for all equipment movement, signage and barricades to 6

7 inform other trades of ongoing activities and active monitoring of all fall protection and open hole issues. The total safety effort yielded a project which was completed without any OSHA recordable incidents. MICROPILE UNDERPINNING Small diameter, 100 mm (4 inch) maximum outside diameter, 9 12 mm ( inch) thick wall steel pipe micropiles were used for the underpinning of the student housing building. Design loads for the piles were 445 and 667 kn (50 and 75 tons). The piles were drilled using water flush into the underlying PWR or to refusal using disposable drill bits and then fully grouted from the bottom-up. About 20 percent of the micropiles encountered the aggregate piers during drilling, which presented caving conditions in the borehole. The piles were advanced rapidly through the piers into the residual soils, which then tended to stabilize the hole by caking the aggregates with sandy silt. Typical socket depths into the PWR were 3 to 4.5 m (10 to 15 feet). The piles drilled for the East Wing were installed in limited access and headroom. A typical micropile layout for the southeast side of the East Wing is shown on Figure 6. Figure 6. Micropile locations in southeast corner of East Wing 7

8 Other micropile layouts throughout the building were similar. The micropiles were installed through holes cored or drilled through the existing spread footing foundations. The micropiles were attached to the existing footings by non-shrink grout and weld beads were added onto the micropile pipe where the micropiles extended through the roughened holes made in the footings. MICROPILE LOAD TESTING Two compression micropile load tests were conducted, one at the East Wing and another at the West Wing. Although the production piles were designed for two different loads, only testing of the higher loading condition was performed. The higher loading condition was a design load of 667 kn, (75 tons, 150 kips) and a test load of 1,334 kn (150 tons, 300 kips) as shown. The pile design required a minimum socket into PWR of about 4.5 m (15 feet) or refusal on bedrock. As shown on Figure 7, the test piles performed well, with deflections of 11.5 mm (0.45 inches) measured at design load. The piles exhibited some lateral movements as the load was applied, which slightly skews the deflection results. The East Wing pile test was discontinued slightly early due to safety concerns about the leaning test pile. Figure 7. Micropile compression load test results 8

9 Figure 8. East Wing restricted access conditions Figure 9. East Wing micropile drilling in low headroom 9

10 Figure 10. West Wing micropile drilling in open headroom FOOTING CONNECTION DETAILS AND LOAD TESTING Eight inch diameter holes were cored or percussion drilled through the existing spread footing foundations to accommodate the micropiles. Rotary coring utilized diamond coring bits and percussion drilling used down-the-hole hammers. The diamond cored holes were roughened using a scabbling apparatus. A photo of a typical roughened core hole is shown on Figure 11. Initially in the design, it was understood that a 900 mm (3 feet) thick footing was the thinnest available for bonding of the micropiles. A working bond stress of 1,140 kpa (165 psi) was therefore initially selected for design. However, later it was determined that some footings were only 600 mm (2 feet) thick. Since the connection detail was key to the design, it was decided to increase the working bond value to 1,725 kpa (250 psi) using non-shrink grout and to prove this value with testing on site. Tension load tests were conducted on non-production pieces of micropile pipe, including weld beads around the perimeter of the pipe, bonded to the footing with non-shrink grout for both cored/roughened and percussion drilled holes. The tests were done at an adjacent tower crane foundation at the West Wing. Figure 12 illustrates the connection test setup. Results from the coring and drilling tests were almost identical and are shown on Figure 13. The connection tests netted a maximum bond stress of 3,450 kpa (500 psi) versus the required bond stress of 1,725 kpa (250 psi). 10

11 Figure 11. Photo of roughened core hole through existing footing Figure 12. Micropile pipe to footing connection test set-up 11

12 Connection Test Percussion Drill Load Test Load (Kips) Elongation (Inches) Design Load = 150 Kips Average Deflection Rebound Curve Design Load Ave Defl. Minus Elastic Rebound Curve Minus Elastic Figure 13. Percussion drill connection load test results RESULTS As shown on Figure 14, the settlement was arrested successfully after micropile installation as load was transferred to the piles by the structure. The foundation remediation project was completed in only 10 weeks, with approximately 1,035 micropiles installed. Construction of the student housing building was then resumed. A recent photo of the construction is shown on Figure 15. CONCLUSIONS After an aggregate pier foundation installation failed to perform adequately, drilled and grouted micropiles were successfully installed to underpin a large structure that had settled severely. This intense, resource loaded and aggressive schedule project was completed without a safety incident. Drilled footing connections using nonshrink grout worked very well to attach the building to the micropiles with minimal disruption to the existing structure. 12

13 Figure 14. East Wing settlement data indicating no further settlement of the building after underpinning Figure 15. Recent photo of the student housing building construction 13

14 ACKNOWLEDGEMENTS The original geotechnical report was written by GeoTechnologies. Ed Hearn, P.E. was the on-site geotechnical engineer for GeoTechnologies during the remediation work. The general contractor was a Joint Venture of Brasfield & Gorrie General Contractors / Clancy & Theys Construction Company. Jarrett Frazier was the project manager for the general contractor. Tom Finn and Stephen Edwards were the superintendents for the micropile/underpinning work for Hayward Baker. Satoshi Mitsumori was the field engineer for Hayward Baker. Martin Cuadra, P.E. with Uzun & Case was the structural engineer for the remedial work. Sandy Triplett helped prepare this paper. REFERENCES 1. Sowers, G.F. and Richardson, T.L., Residual Soils of the Piedmont and Blue Ridge, Transportation Research Record No. 919, National Academy Press, Washington, DC, 1985, pp Preliminary Report of Subsurface Investigation, Ten Storey Building and Parking Deck, North Carolina, GeoTechnologies, Inc., November 30, 2007, 53 pages. 14