WHAT HAVE BEEN ACCOMPLISHED:

In the preliminary engineering phase of the I-680 NB Express Lane project, a seismic evaluation and retrofit design was performed to evaluate impacts on the existing I-680 due to retrofit construction of the 6-span 860-ft long BART Overcrossing over I680 in Walnut Creek.

Seismic performance evaluation of the existing 6 span steel plate girder bridge supported on reinforced concrete columns and spread footings has indicated that the bridge cannot satisfy the design requirements of the current BART Facilities Standard (BFS), where foundation rocking is not allowed. Retrofit alternatives for the existing foundations include enlarging spread footings or adding perimeter CIDH piles have been considered, but it was not adopted due to traffic impacts on I-680. Subsequent freeway improvements were focused on improving traffic flow of freeway segments on both north and south of the BART Overcrossing, and no improvements have been planned for the BART Overcrossing.

In the ED/PA phase, JMEC designed 5 MSE walls with height up to 34' to accommodate an elevated roadway of the new braided ramp that carries both the off-ramp traffic to North Main Street and Treat Boulevard. The existing North Main Street off-ramp is modified and an elevated roadway section, making up of two new bridges and 5 MSE walls, is built to carry the Treat Boulevard off-ramp traffic over the existing Lawrence Way NB I-680 On-Ramp and the Contra Costa Canal. In addition, JMEC designed 2 retaining walls on the east side of existing NB I-680 near SR242 to widen the roadway from 6 lanes to 7 lanes.  One retaining wall is replacing an existing sound wall supported on a retaining wall. Another retaining wall, consisting of soil nail wall with height up to 15-ft, is built to accommodate the widened roadway.

Schedule:  ED/PA was completed in 2022