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San Rafael Bridge

San Rafael Bridge
Date: 2005

The 8.8 km Richmond San-Rafael Bridge, which crosses San Francisco Bay, was originally opened in 1956. One of the longest bridges at the time it was built, the structure had to undergo extensive seismic retrofitting between 2001 and 2005. Seismic activity not being uncommon in the area, the US $540-million project enabled the structure to 7.4 magnitude earthquake on the Hayward Fault (located 6 km to the bridge’s west) and an 8.3 magnitude quake on the San Andreas Fault (located 16 km to the bridge’s east), both susceptible to producing violent earthquakes.
Bridge owner Caltrans awarded AGRA Foundations Limited a US $40-million foundations strengthening project that included the installation of about 640 piles of varying sizes and depths into marine grounds covered by up to 18 m of water. The ground conditions on site were complex and included new and old bay mud, as well as existing concrete bell foundations overlying a Cretaceous/ Jurassic Franciscan bedrock formation. Past seismic activity in the area had caused substantial structural faulting and micro fissure fracturing to the bed rock, making the ground susceptible to collapse during socket drillings.

Solution:

AGRA used four reverse-circulation rigs to drill through existing concrete bell foundations to construct deep rock sockets and pressure grouted in two stages. The crew further designed rock bond length for marine micropiles and conducted pile load tests to 1,200 kips, proof testing each pier. A T-40 pile top drill rig was used for the construction of 3.8 m diameter cast-in-drilled hole piles. Marine micropile templates of weighing 175,000 lbs were placed only inches away from existing H-piles and cored through pier concrete elements, demanding a high amount of precision. AGRA’s special closed loop drilling mud slurry system prevented any spoil from being discharged into the bay, allowing to satisfy Californian environmental restrictions.