Micropile Application for Seismic Retrofit of the Richmond-San Rafael Toll Bridge

The Richmond-San Rafael Toll Bridge, spanning the northern end of the San Francisco Bay on Interstate Freeway 580, is undergoing extensive seismic retrofit, which will prevent it from collapsing during a major earthquake. To increase the axial foundation capacities in compression and tension, micropiles are installed in between the existing H-piles. The installation is carried out by placing a micropile template on top of the conical section of the bell foundation and coring 355 mm holes with grooved surface through the existing bell. Then a 324 mm diameter permanent steel casing is advanced through each of the cored and grooved holes. A 292 mm diameter rock socket is then drilled from the bottom of the permanent casing into bedrock. While the tip of permanent casing is defined, the length of rock socket varies depending upon rock quality, load demands, and other design constraints, including a designated minimum bedrock embedment length. Grout is tremied after the reinforcement of 210 mm diameter steel pipe is placed inside the casing and rock socket. Acceptance criteria for micropiles are based on specific tension load demand at each support location and the maximum allowable vertical movement under that load. Two tension load tests are performed at each pier location to confirm that the micropiles meet the acceptance criteria.