<p><p><h2>Project Summary/Scope:</h2><div>After the 1989 Loma Prieta Earthquake, sand boils and cracking were observed north and south of the east abutment of the Laurel Street Bridge in Santa Cruz, California. The bridge was designed and built in 1967 prior to when there was a good understanding for seismic design. Compaction grouting was chosen to densify the soils thus increasing the Cyclic Shear Resistance (CSR). The area improved was 37.5 by 26 meters extending to the depth of the liquefiable material ranging from 21 to 24 meters.</div>Subsurface Conditions: Alluvium from the Holocene era over bedrock from the Purisima Formation. The top 27 meters consisted of interbedded gravelly sand, sand, and silty sand. Gravels were present from 27 to 43 meters.</p><p>PGA was estimated using a magnitude 8.0 earthquake located on the San Andreas Fault and was found to be 0.5 g. The computer program Liquefy2 was used to calculate the required SPT N-values to have a FS against liquefaction above 1.1. This data set was used to set guidelines for required SPT N-values. This corresponded to a relative density of 80%. Limits of the area to be improved were determined by drawing a line between 30 and 45 degrees from vertical at the extents of the existing foundations. A granite sand, cement, and water mix was used for the grout. The sand used was a well-graded silty fine sand with greater than 20% non-plastic fines by weight. Type II Portland cement was used at between 4 and 7% by weight. The grout has a target unconfined compressive strength of 0.35 Mpa. The rate of pumping was 0.53 m³/min.<br><h2>Alternative Technologies:</h2><div>Drilled piers also were considered but the installation locations necessary to avoid existing battered piles made them unfeasible.</div><h2>Performance Monitoring:</h2><div>Replacement ratios of 3.5 and 9.5% were achieved at piers 3 and 2 respectively. Post verification testing showed a large increase in density of granular soils and a moderate increase for fine-grained soils.</div><h2>Project Technical Paper:</h2><div>Ivanetich, K., Gularte, F. and Dees, B. (2000). “Compaction Grout: A Case History of Seismic Retrofit,” Advances in Grouting and Ground Modification, ASCE, GSP 104, pp. 83-93.</div><h2>Date Case History Prepared:</h2><div>November 2012</div></p></p>