Contra Costa Transportation Authority (CCTA) and Caltrans proposed to improve the intersection of Interstate 680 (I-680) and State Route 4 (SR-4) in Contra Costa County.
The initial phase of the project widened the SR-4 median in both directions of about four miles, including the widening of five structures and replacement of one at Grayson Creek. The Grayson Creek Bridge was raised to pass the 100-year flood with new embankments and retaining walls. The improvements also included replacement of several overhead sign structures.
- PARIKH performed “Surface Fault Rupture Displacement Hazard (SFRDH) Analysis” for the Walnut Creek Bridge and Solano Way Bridge because these structures are within the Alquist-Priolo Earthquake Fault Zone (EFZ). Our engineering geologist communicated with the Caltrans geologist to calculate the deterministic and probabilistic displacements in both horizontal and vertical directions per Caltrans Memo-To-Designer 20-10 and 20-8.
- Ground settlement under the new embankment was a major concern at Grayson Creek Bridge because the site is underlain by approximately 40 feet of very soft to medium stiff fat clay. The embankment would have impacted the existing Stage 1 structure and an existing 90-inch storm sewer on the west side of the creek. The embankment design adopted “load-balancing” concept by using lightweight Cellular Concrete (LCC) fill as the embankment fill with over-excavation to minimize the potential settlement.