Stephen G. Wright

Improved Undrained Strength Interpolation Scheme for Rapid Drawdown

AbstractLinear interpolation between isotropically consolidated undrained strength envelopes and fully drained (effective stress) strength envelopes is commonly used to calculate undrained strengths for slope stability analyses of rapid drawdown. However, the current linear interpolation method presents numerical problems at low effective normal stresses with soils that possess an effective stress cohesion intercept. The resulting undrained shear strength envelopes are unrealistic at low normal stresses. An improved interpolation method has been developed that interpolates a single undrained strength envelope for the entire slip surface. The improved interpolation method produces identical results to the current method for cases where c′ equals zero.

Risk Analysis for Hurricane-Wave Induced Submarine Mudslides

Hurricane waves in the Gulf of Mexico can trigger mudslides in the relatively weak clays in the offshore Mississippi Delta region. Mudslides can cause extensive damage to the infrastructure that supports offshore oil and gas production, including platforms and pipelines. This paper presents a method for assessing the probability and risk for future mudslides in the Delta as a function of the possible sizes of waves, bathymetry, and shear strengths of sea floor soils. The method has been calibrated to the extent possible using historical information. The method is transparent and can readily include either generic or site-specific information about soil properties and wave characteristics. The method is applied regionally to develop maps depicting the risk to locations and to pipelines. Return periods for mudslides vary significantly across the Delta, with values less than 30 years in the shallowest water and values greater than 1,000 years in the deepest water. Wave period, in addition to wave height, is an important factor in mudslide vulnerability. Mudslides are localized features, on the order of several thousand feet in lateral extent and about 50 to 150 feet deep. The areal extent and depth of mudslides are related to the lengths and widths of the storm waves that cause them.

Influence of Higher Modes And Test Geometry On SASW Measurements Underwater

Experimental and analytical investigations are being conducted to develop the Spectral-Analysis-of-SurfaceWaves (SASW) method for use underwater. The SASW method is a site characterization technique that was originally developed for use on terrestrial geotechnical sites. The studies described in this abstract have established that a simple plane-wave approach to interpretation of an underwater SASW measurement will provide satisfactory results for sites that are normally dispersive (stiffness increasing with depth) and do not contain sharp boundaries with large stiffness contrasts. A more complex axisymmetric analysis that permits multiple-mode contributions and explicit modelling of test geometry may be required for other cases. An analytical approach incorporating a cylindrical wavefront and utilizing the free-field numerical technique was developed and applied to the more complex field conditions, with measured success. These analytical approaches are applied to SASW measurements in a near-shore underwater setting.