CCEER-91-3: Seismic Lateral and Rotational Pile Foundation Stiffnesses at Cypress

Abstract

The Cypress Viaduct, located some 60 miles from the earthquake epicenter, suffered catastrophic structural damage during the 5 to 10 seconds of strong shaking of the Loma Prieta earthquake of October 17, 1989. While much has been made of the structural details that ultimately led to the viaduct's failure, less has been presented relative to the possible contributing effect of the soil and foundations. This paper discusses the difference in the soil and foundations (spread footings and short end bearing piles in Merritt sand, abruptly changing to long friction piles in Bay mud) along the length of the viaduct and the possible difference in ground surface motions over the northern (Bay mud) versus the southern (Merritt sand) sections. Likewise, given soil borings at Bents 61 and 97, the nonlinear variations in both the rotational and lateral pile group stiffnesses are assessed and presented for consideration. The lateral response is compared with measured response from Caltrans' lateral pile group load tests. It is argued that there is such difference in the lateral and rotational stiffnesses of pile groups in the Merritt sand versus the Bay mud that given the abrupt change in soil and foundation conditions between Bents 71 and 72, a dynamic analysis intending to show the progress and arrest of collapse along the length of the viaduct would need to take this into consideration. These same foundation stiffness calculations are what are currently needed to assure a meaningful linear structural dynamic analysis in the assessment of the seismic behavior of a highway bridge. In regard to the stiffness evaluations, the authors consider the effect of developing pore water pressure in the Merritt sand and the choice of free-field versus near-field (or inertial interaction) strain for the evaluation of soil modulus values for stiffness calculations (Abstract by authors).