RESIDUAL STRENGTH AND LARGE-DEFORMATION POTENTIAL OF LOOSE SILTY SANDS

Laboratory and field evidence on residual shear strength, density, and penetration resistance of very loose, water-deposited silty sands and sandy silts capable of liquefying and developing large shear deformations is summarized. A program of laboratory monotonic and cyclic undrained tests is conducted on reconstituted, water-deposited, layered triaxial specimens of silty sand retrieved from the Lower San Fernando Dam. The results indicate that the soil behaves contractively in shear, and both its density and steady-state strength increase rapidly with consolidation pressure. These trends are verified for field conditions by comparison with available case histories of earthquake-induced flow failure and large lateral deformation of embankments, slopes, and mildly sloping ground. The 1971 flow slide in the Lower San Fernando Dam is examined in detail. The normalized Standard Penetration Index (N-1)(60) used to develop engineering charts is developed from the case histories. These charts can be used to evaluate residual shear strength and to indicate if a saturated silt-sand slope or site is capable of developing flow or lateral deformations greater than 1-3 ft during earthquakes of magnitude M(W) less than or similar to 8.