Pedro de Alba

Impacts of Unsaturated Zone Soil Moisture and Groundwater Table on Slope Instability

The combined effect of soil moisture in unsaturated soil layers and pore-water pressure in saturated soil layers is critical to predict landslides. An improved infinite slope stability model, that directly includes unsaturated zone soil moisture and groundwater, is derived and used to analyze the factor of safety’s sensitivity to unsaturated zone soil moisture. This sensitivity, the change in the factor of safety with respect to variable unsaturated zone soil moisture, was studied at local and regional scales using an active landslide region as a case study. Factors of safety have the greatest sensitivity to unsaturated zone soil moisture dynamics for shallow soil layers ( <2 m ) and comparatively deep groundwater tables (1 m). For an identical groundwater table, the factor of safety for a 1 m thick soil mantle was four times more sensitive to soil moisture changes than a 3-m thick soil. At a regional scale, the number of unstable areas increases nonlinearly with increasing unsaturated zone soil moisture ...

Residual and Postliquefaction Strength of a Liquefiable Sand

AbstractSeismic design of geotechnical structures often requires estimates of how shearing resistance of liquefiable soil is reduced to its minimum value (residual strength, sr) as pore pressures build up and is subsequently regained as pore pressures dissipate. It was envisioned that the shear strength of liquefying sands could be measured in-flight in a seismic geotechnical centrifuge model by pulling thin coupons (plates) horizontally through the sand models before, during, and after shaking to simulate the large strains and strain rates associated with liquefaction flow failures. This paper presents the results of seismic centrifuge tests that were used to make such measurements. Companion ring shear (RS) tests also are described. Although centrifuge and RS residual strengths were generally similar and increased with relative density, the centrifuge sr and sr/σvo′ (sr divided by preshaking effective vertical stress) increased significantly with small changes in relative density, while the RS test sr a...

Measurement of res idual strength of liquefied soil in centrifuge models

A technique for measuring the evolution of shear strength of liquefiable soils in physical models as earthquake-induced pore pressures change is desirable. It was envisioned that the shear strength of liquefying sands can be measured in-flight in a seismic geotechnical centrifuge model by pulling thin coupons (plates) horizontally through the soil models. The large strains and strain rates associated with liquefaction flow failures would thus be simulated by moving the coupon relative to the sand before, during, and after shaking. This paper presents the experimental details and results of seismic centrifuge tests on models of saturated F-75 sand that were used to make such measurements. It is shown that by measuring the drag force on the coupon, it is possible to observe the evolution of the soil shear strength as it decreases to a minimum (residual strength) and subsequently increases as excess pore pressures

Cyclic Simple Shear Apparatus for Low-Strain Soil Tests

A testing system has been developed to study the behavior of saturated sand under low-level cyclic shearing strains. The system has been used to determine threshold shear strain levels for fabric destruction in sand aged for different time periods. The system includes a special soil chamber and a direct simple shear (DSS) machine. To impose very small shearing strains, the DSS machine was designed to apply and measure horizontal deformations as small as 0.0005 mm (2 × 10−5 inches). Data obtained to date support the results of previous investigators who performed triaxial tests on freshly deposited samples, indicating a threshold cyclic shear strain level of approximately 0.01 percent. At strains in excess of those levels, destruction of the sand fabric occurred, as evidenced by a reduction in shear modulus at low strain levels. Subsequent modest increases in shear modulus were observed after the specimens were allowed to recover for 24 hours and then tested again. During the recovery period, drainage valv...