Proposed Changes to AASHTO LRFD Bridge Design Specifications for the Seismic Design of Retaining Walls

An AASHTO-sponsored project, NCHRP 12-70, was conducted to address current limitations and deficiencies in the methods being used within the AASHTO LRFD Bridge Design Specifications for the seismic design of retaining walls. The existing AASHTO Specifications use the Mononobe-Okabe (M-O) equations. Under certain combinations of ground motions and wall geometry, unrealistically high seismic active earth pressures are estimated. NCHRP 12-70 Project was conducted to address this and other limitations of the M-O equations for estimating seismic earth pressures. The project also considered seismic design requirements for slopes and embankments, and for buried structures. The products of the project include (1) recommendations on ground motions to use, (2) an updated method for estimating permanent ground movement, (3) charts for including the effects of soil cohesion in seismic earth pressure calculations, and (4) a generalized limit-equilibrium approach for evaluating seismic active earth pressures. This paper provides a summary of the NCHRP 12-70 work with a focus on seismic ground motions and earth pressure evaluations.