Volume 16, Issue 3 (September 1993)
The Multidirectional Direct Simple Shear Apparatus
The paper describes a new simple shear testing device, the multidirectional direct simple shear (MDSS) apparatus, for testing soil specimens under conditions that simulate, at the element level, the state of stress acting within the foundation soil of an offshore Arctic gravity structure. The MDSS uses a circular specimen that is consolidated under both a vertical effective stress (σ′vc) and a horizontal shear stress (τ1). The specimen is subsequently sheared undrained by applying a second independent horizontal shear stress (τ2) at an angle ϑ relative to the horizontal consolidation shear stress τ1. Evaluation of the MDSS first compares conventional K0-consolidated undrained direct simple shear (CK0UDSS) test data (τ1 = 0) on normally consolidated Boston blue clay (BBC) with results obtained in the Geonor DSS device. The MDSS gives lower secant Young's modulus values and on average 8% lower strengths, but produces remarkably less scatter in the test results than the Geonor DSS. Kinematic proof tests with an elastic material (rubber) confirm that the setup procedure, application of forces, and strain measurement systems in the MDSS work properly and produce repeatable results.
Results from a MDSS test program on BBC wherein specimens were first normally consolidated with σ′vc and τ1 = 0.2σ′vc and then sheared undrained at ϑ varing in 30° increments from zero (shear in same direction) to 150° show dramatic differences in the response of the soil as a function of θ. The peak undrained strength varies almost twofold from θ = 0 to 120°, while the deformation behavior varies from very brittle at low θ angles to becoming ductile at higher angles. The experimental results indicate that dramatic changes in foundation response at different θ angles will be an important design issue.