San Rafael Bridge

AGRA Foundations Inc., one of North America’s leading foundations contractors and Fugro Seacore Limited, one of the world’s leading specialist marine and offshore construction and exploration drilling contractors, have combined their considerable expertise to successfully tackle a technically challenging major bridge foundation reinforcement project in San Francisco Bay, California.

The two contractors have been installing large diameter, rock socketed, cast-in drilled hole (CIDH) reinforced concrete piles. The huge reinforced piles are adjacent to, and incorporated, in, the existing bell shaped pier foundations of the Richmond-San Rafael Bridge, which is a vital transportation link carrying the Interstate Highway 580 across the northern end of the bay.

The 3.8m (150 inch) diameter, 69m long CIDH piles are a key part of an extensive strengthening programme of the bridge’s foundations to protect against catastrophic failure of the structure in the event of a major earthquake. The seismic retrofit of the Richmond-San Rafael Bridge has been instigated by the California Department of Transportation and is part of the multi-billion dollar toll bridge seismic retrofit programme affecting five of the seven state-owned bay area toll bridges. Finance for the vast retrofit programme will come from state and federal funds, together with a US$1 “seismic surcharge”, which Caltrans has been collecting since 1998 on all toll-paying vehicles crossing the state-owned toll bridges in the bay area.

The 8.8km long cantilever and truss Richmond-San Rafael Bridge, with its 327m main navigation span and 56.4m high shipping clearance, was one of the longest in the world when it opened in the autumn of 1956. It is supported by bell shaped concrete substructures consisting of two or four concrete shafts interconnected by concrete spandrel beams and diaphragm walls and founded on concentric rings of raking steel H beams.

The bridge is in a precarious location just 16km west of the San Andreas Fault and 6km east of the Hayward Fault and without strengthening is vulnerable to a major seismic event.

The bridge’s existing foundations could not absorb the excessive horizontal movement generated by a major earthquake. Bridge owner Caltrans commissioned consulting engineer joint venture Ben C Gerwick, DMJM Harris and Jacobs Engineering to produce a design to reinforce the structure. Part of the retrofit programme involves strengthening some of the key pier foundations with the 3.8m diameter CIDH concrete piles. These huge piles will be incorporated with special pile caps, acting as shear diaphragms to limit the lateral movement of the existing piers, and will take the shear forces transferred between them and existing foundations. This concept of reducing the horizontal loading demand on the original H piles is aimed at preventing them from buckling, while maintaining their original capability to cater for overturning forces and axial loads.

Caltrans awarded AGRA a US$40M foundations strengthening contract, which involved installing about 640 piles of varying sizes and depths. For the 11 CIDH piles AGRA approached Seacore and the two companies came up with a novel rig share agreement. This involved sharing the costs of designing, building, owning and operating a new Teredo T40-4 reverse circulation drill rig and specially shrouded full face drill bit for the demanding project. This is believed to be the biggest pile top drill rig of its type in the world with the capacity to drill up to 6m diameter. In its present configuration, as built for this project, the T40-4, together with its in-hole equipment, can readily drill at diameters from 2m to 4.4m. Seacore, drawing on its vast knowledge of large diameter reverse circulation drilling, designed and built the drilling equipment at its headquarters in Cornwall, England and provided the experienced crew and technical support team to operate and drill the challenging marine shafts. AGRA, with its main office in Seattle, Washington, supplied other essential complementary support services, including the special closed loop drilling mud slurry system for the environmental management of the spoil.

Seabed positioning templates for the CIDH piles were provided with special precast concrete pile caps, which were accurately placed centrally between the bell shaped foundations and supported on the seabed by steel pin piles. The pile caps, which are anchored together with tremie concrete, have preformed large diameter pilot guide holes to accommodate permanent 3.8m diameter steel casings. Working from barges in up to 18m of water, the casings, up to 60m long, were driven by a hydraulic hammer through the pile caps and into the seabed overburden to found on the underlying bedrock and top level with the pile cap. A secondary larger diameter temporary conductor extension casing was then installed over the permanent one. This temporary casing formed a seal and extended above water level prior to mucking out the internal spoil to just above rock head and thoroughly cleaning the interior wall of the permanent shell.

The water was pumped from the two part casing and replaced with a synthetic drilling mud. Seacore placed the T40-4 drill on top of the temporary casing ready for drilling the 3.35m diameter rock sockets extending for a depth of 9m from the toe of the permanent steel shell into the underlying rock. During full face rotary drilling the spoil slurry mixture was airlifted up the hollow drill string, using the reverse circulation technique, and discharged into an adjacent barge with settling tanks and slurry cleaning system for recycling back to the shaft. This closed loop system was essential to meet the stringent Californian environmental restrictions, which prevented any spoil being discharged into the bay.

Ground conditions were complex and included new bay mud, overlying old bay mud, overlying a Cretaceous/ Jurassic Franciscan bedrock formation. This is a mixture of shale, greywacke, chert and serpentine with a complex stratigraphy made even more complex by the extensive structural faulting and micro fissure fracturing from extensive seismic activity over past geological periods. Strength of the heavily fractured rock was about 20Mpa. Seacore was aware that other contractors had had problems drilling rock sockets in the intensely fractured material at much smaller diameters and anticipated the possibility of collapse during drilling “To counteract this possibility we designed and built our own completely shrouded bottom hole assembly to match the depth of the socket. This equipment resembled a vertical tunnel boring machine, and together with our special reverse circulation drilling techniques and careful practices, aimed to minimise ground collapse during socket drilling,” says Seacore divisional manager Jason Clark. Despite these precautions two of the completed sockets caved in after pull out.

Seacore and AGRA devised a remedial works stabilisation plan for the collapsed sockets, involving drilling in incrementally larger diameters through the cave in to produce an oversized hole. The modular design of the Seacore drill bit and in hole equipment allowed for easy diametric adjustments without the need for new drill bits for the diameters required. The bigger hole was filled with a weak fibrous tremie concrete mix, which was subsequently drilled back to the original socket diameter of 3.35m to leave a 150mm thick supporting concrete collar lining in the open blind hole. As with the other sockets a reinforcement cage, complete with seismic monitoring equipment, was lowered through the deck of the drilling platform and down the slurry filled casing and into the socket. An average 460m3 of concrete was then tremied in to fill the permanent casing and complete the pile, with the displaced synthetic slurry being collected as it overflowed from the shaft for re-use on other locations. The management and containment of the drilling slurry was a challenging exercise, which AGRA and Seacore successfully solved. These were the largest single concrete pours AGRA has ever performed. On completion of a pile the drilling platform was removed and temporary conductor extension casing lifted away ready for the cycle to be repeated on the next rock socketed pile.

All the 3.8m diameter piles were successfully completed and AGRA and Seacore anticipate the specialist experience gained on the Richmond-San Rafael Bridge will also be considered for the new San Francisco Oakland Bay Bridge.