Additional geotechnical investigation and settlement modeling predicted that augercast pile settlements could exceed 12 in (305 mm), rendering that option untenable. Drilled shafts were also proposed and considered. Due to cost, schedule advantage and technical merit, a micropile option proposed by Hayward Baker was selected. A particular design consideration was elastic deflection. With a 300 kip (1,334 kN) working load and anticipated micropile depths that might exceed 235 ft (72 m), elastic deflections of 2 in (51 mm) were predicted. These anticipated deflec- tions were modeled and accounted for by the structural engineer in the final design. The structural engineering design for the IMP project, by Ludwig Structural Consultants, Seattle, Wash., required approximately 700 micropiles. Using a highly-efficient design, compression loads were resisted using 300 kip (1,334 kN) and 200 kip (890 kN) micropile elements. The micropile foundation elements consisted of flush-joint threaded 7-5/8 in (194 mm) and 5-1/2 in (140 mm) OD steel pipes, grade 80 ksi (551 MPa), with all- thread bar reinforced bond zones. The bond zone sockets were founded in dense soil or rock located beneath the lagoonal deposits. The bond zones were reinforced with 2-1/4 in (57 mm) or 2-1/2 in (64 mm) diameter all-thread bars, grade 150 ksi (1,034 MPa). The larger piles had nominal working load capacities of 300 kips (1,334 kN) in compression and 154 kips (685 kN) or 200 kips (890 kN) (where necessary) in tension and also 10 kips (44.5 kN) of lateral resistance. The smaller piles had working load capacities of 200 kips (890 kN) in compression and 75 kips (334 kN) (where necessary) in tension and 15 kips (67 kN) of lateral resistance. Micropiles with pipe joints in the upper 8 ft (2.4 m) of the pile required special reinforcement. Tension piles required full-length all-thread bars. A 1/8 in (3 mm) corrosion reduction in the steel pipe thickness was included in the micropile design as agreed with by the owner and structural engineer. Although no particularly aggressive soils were noted, the site was very close to the beach and salt water. A working bond strength of 55 psi (380 kPa), with a factor of safety of two, was selected for the design. This resulted in a nominal bond zone length of 25 ft (7.6 m) for both types of piles; note that the smaller piles were installed in smaller drill holes. This bond length was derived from ’hard drilling’ soils and rocks. It was noted that there were often softer zones intermixed in the profile. Hayward Baker used a proprietary Data Acquisition System (DAQ) to develop a ‘Drilling Index,’ which was used to identify and quantify the denser soils, capable of achieving the required design bond stress. Two drill rigs were outfitted with this equipment, and this information was discussed and shared with all of the drillers on the project. This in- situ testing resulted in real-time engineering decisions to extend many of the bond zones through soft soil pockets in the highly variable soil profile, resulting in some deeper and longer bond zones. Aerial view of IMP site Load Testing Program Due to the difficult drilling conditions and unknown nature of the cement grout bonding characteristics expected for the complex subsurface strata, an initial load testing program was included in the scope of work. The load tests were performed prior to full mobilization and were primarily located on the northern side of the site to coincide with the demolition and hazardous materials abatement schedule. Initially, six tension load tests were performed on isolated bond zones to confirm the design bond strength followed by two full-scale compression verification loads tests. The lengths of the tested piles ranged from 84 ft (25 m) to 204 ft (62 m) and they were positioned to analyze data from both within and outside the alluvial valley. Tension testing was selected for the majority of the initial test program and for the proof tests on production piles due to ease of test pile set up and reduced cost. DAQ equipment was used during the installation of test program piles. The DAQ system was used for real-time monitoring to measure drilling parameters including depth, rotation speed, crowd pressure and flow rate. These parameters were then processed to create a depth-based drilling index and drill logs as a means to directly measure the quality of the bonding strata by drawing comparisons to test results and the adjacent boring logs. The test program yielded good results that indicated ultimate bond strength capacities just in excess of design requirements. Some of the test piles showed acceptable but larger than expected deflections up to 3-1/4 in (83 mm) at design load. Five percent of the production micropiles required proof testing according to the specification. The proof test piles were identified by the owner’s representative. Full-length bars were installed at proof test pile locations during construction to allow for tension testing. Thirty-five proof test piles, tested to 1.33 over the design load, were all successfully verified by tension testing over the course of the project, in various areas of the site. Twenty-seven of DEEP FOUNDATIONS • SEPT/OCT 2016 • 15
