Caltrans recently completed a Pile Demonstration Installation Project (PIDP) for the new San Francisco-Oakland Bay Bridge (SFOBB). The work entailed driving three 2.4 m diameter steel pipe piles at two different locations adjacent to the existing eastern span of the SFOBB. Each pile was composed of four sections and had a total completed length of approximately 107 meters. Pile wall thickness varied from 40 mm to 70 mm. The individual pile sections were connected using complete joint penetration welds, and Ultrasonic testing (UT) was performed on 100% of the weld volume.
The primary objectives of the PIDP included the following:
The first three sections of each pile were driven with a 500 kJ hydraulic hammer, and the final section was driven with a 1700 kJ hydraulic hammer. Pile run, the penetration caused by the weight of the pile and the hammer, ranged from 6 to 15 meters. Estimated soil resistance to driving (SRD) was generally within the range predicted with a maximum SRD between 50 and 75 MN for piles 1 and 2 at a penetration of 90 m. Soil set up occurred rapidly and resulted in a significant SRD increase. During the three to five day welding delays, SRD increased by a factor of 2 to 3. Restrikes performed on piles one and two also confirmed the rapid increase in soil resistance. Blow counts per quarter meter (bpqm) for the restrikes on pile 1 increased from 54 after 2 days to 144 after 9 days. Restrikes were performed using the 1700 kJ hammer at full energy after the pile had reached specified tip elevation.
The Contractor was able to place and drive the piles within the tolerances specified. The batter angle of 1:6 was maintained as well. It should be noted, however, that a significant amount of effort and expense went into the construction of a template strong enough to withstand the forces involved. The 500 kJ hammer was able to drive the piles to a final penetration of 70 m, and was able to restart a pile at a depth of 45 meters after a 5 day delay. Specified tip elevations were achieved using the 1700 kJ hammer with bpqm less than 55.
Quality control and quality assurance protocols developed to assure the structural integrity of the welded pile joints were stringent, and the contractor experienced some difficulty qualifying UT personnel due to an inability of several technicians to pass a test, developed by Caltrans, based upon AWS D1.5, Section 6. Estimated times to complete the weld process, particularly for the thicker sections, were also found to be overly optimistic. The contractor used a FCAWG system with two welding machines on a track to perform the 7.7 m long weld. The 70 mm joint took the contractor approximately 40 hrs to weld, with UT and weld repairs consuming as much as another 24 hrs depending on the initial weld quality.
As part of the environmental permitting process, Caltrans agreed to monitor the marine mammal response to pile driving, and to investigate two techniques to attenuate sound pressure in the water. Seals located at the haul out areas on Yerba Buena island were essentially unaffected by the PIDP, and the few seals and sea lions which wandered into the project limits prior to driving would leave upon commencement of driving without any noticeable signs of distress. Two types of bubble curtains were employed during driving operations to attenuate sound pressure. Both curtains employed compressed air delivered to pipes at the mud line. Pile one was driven without sound attenuation. Pile two was driven with an air only bubble curtain, and pile three was driven using a fabric encased bubble curtain. Both devices were shown to provide some sound attenuation.
The PIDP demonstrated the feasibility of using the piles presently being considered for the new SFOBB. It also provided Caltrans with a wealth of information that will be used to ensure the substructure of the new bridge is built in a cost effective and structurally sound manner while mitigating any negative environmental effects.