reasonably uniform within the western two-thirds of the site at about 20 to 50 ft (6 to 15.2 m), with a few exceptions. In the eastern third of the site, the rock drops off dramatically to the north and east. In one mega column cap, there was almost 80 ft (24.3 m) in elevation difference. In two borings in the cap that were 20 ft (6.1 m) apart, the top of rock varied in elevation by 34 ft (10.3 m). The quality of the rock as defined by Recovery and Rock Quality Designation (RQD) also varied significantly. Recovery and RQD values both varied from 0 to 100%. To evaluate the compressive strength of the rock, five compression tests were performed on intact rock core samples. The strength values ranged from 6,000 to over 27,000 psi (41.4 to 186.2 MPa) with an average value of 14,372 psi (99.1 MPa). Drilled Pier Design The preliminary foundation design, upon which the project was bid, consisted of one hundred ten 6 ft (1.8 m) diameter piers and sixteen 5.5 ft (1.7 m) diameter piers, all with 10 ft (3 m) embedment into bedrock. As the design progressed, 6.5 ft (1.9 m) diameter piers were introduced. Also, during construction there were areas of shallow rock where drilled piers were added in lieu of augercast piles. The final total was thirteen 5.5 ft (1.7 m) diameter piers, eighty-seven 6 ft (1.8 m) diameter piers and forty-one 6.5 ft (1.9 m) diameter piers. To achieve the axial capacities in the piers, end bearing and skin friction in rock were considered. Preliminary design information provided by the geotechnical engineer recommended 4,000 to 5,000 kips (1,814 to 2,268 tonnes) allowable compressive capacity for 5.5 ft (1.7 m) and 6 ft (1.8 m) diameter piers installed 10 to 15 ft (3 to 4.6 m) into competent bedrock. Modifications to the preliminary recom- mendations were made based on the results of the load test program. Load Testing and Non-destructive Testing A preproduction load test program was completed to confirm the drilled pier design parameters and construction techniques. The test program was planned to account for the variability of the subsurface conditions and design requirements. One axial Statnamic test and two Osterberg (O-Cell) load tests were performed, as well as two lateral load tests. The Statnamic test was performed in an area of shallow rock, and was chosen because adequate embedment to provide a balanced O-Cell test was not available. The two O-Cell tests were performed in areas of deeper rock and tested the poorer quality rock and the higher quality rock, respectively. The lateral load tests were performed on a shallow pier and a deep pier. Drilled Pier Construction Based on the preliminary foundation design and the available subsurface information at the time of bidding, the intent was to install all of the piers using the temporary cased-hole method. Because the original design included only 6 ft (1.8 m) and 5.5 ft (1.7 m) diameter piers, 6 ft (1.8 m) nominal diameter, double wall segmental casing was planned for all of the piers. The quantity of rock excavation was a major consideration in planning the drilled pier installation for this project. After considering cluster drills Statnamic load test and conventional rotary tools, the decision was made to use double-walled core barrels equipped with roller bits as the primary rock cutting tool. To be able to excavate hard rock with this type of tooling, drill rigs with relatively high down force and torque are required, so 400 kNm (295,000 ft-lb) class drill rigs were selected. Specifically, a Bauer BG 40 and Liebherr LB 36 were used for the majority of rock drilling. Near the end of the project, a Bauer BG 42 was also used and a BG 24 was used throughout the project, mainly for piers without rock excavation. Three drill rigs worked on site from start to finish, with a maximum of five rigs working at one time. As the project moved forward, additional subsurface information was obtained. In the area of one of the mega column caps, borings indicated that the elevation of the bedrock surface sloped off steeply within the cap. The depth to rock varied from about 22 to 101 ft (6.7 to 30.8 m) from one side of the cap to the other. For the deeper piers in this cap, segmental casing could not be extracted without using an DEEP FOUNDATIONS • JULY/AUG 2016 • 13
