Peak and Post-Peak Shear Strength of Cement-Bentonite-Article

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The study evaluated the peak and post-peak shear strength of cement-bentonite (c-b) slurry walls built to stabilize the downstream slope of Tuttle Creek Dam in Kansas. The unreinforced walls were intended to protect pressure-relief wells from earthquake-induced slope movement and liquefaction. Because seismic shaking could crack the relatively brittle walls, their large-strain strength was important for estimating stability and permanent deformation after cracking.<br /><br />Laboratory testing included isotropically consolidated undrained triaxial compression (R-bar) tests and drained direct shear tests on recovered wall cores and wet-grab samples. The mixes had cement-to-water ratios of 0.3, 0.4, or 0.5 and contained 5% bentonite. Testing focused on identifying strength and stiffness values for limit-equilibrium and numerical deformation analyses.<br /><br />Results for the production mix (c/w = 0.5) showed that its measured post-peak strength exceeded the design requirements. Strength did not show a clear relationship with sample depth or void ratio. Direct shear testing indicated some post-peak cohesion, suggesting the imposed displacement had not reached residual strength. The authors relied more heavily on R-bar results because direct shear imposed a fixed failure plane and had limited displacement.<br /><br />The deformation analyses used an average peak total-stress strength of 655 kPa cohesion and a 24° friction angle, post-peak effective friction angle of 46°, and tangent Young’s modulus of 496 MPa. Peak strength was assigned at 0.8% axial strain, with post-peak behavior beginning at 1.6%. The measured material response was generally stronger than the model assumptions, suggesting predicted post-cracking deformations were conservative. The authors conclude that the 0.5 cement-to-water mix with 5% bentonite met or exceeded the required peak and post-peak strength and may inform similar seismic retrofit projects.

Keywords

cement-bentonite slurry wall

Tuttle Creek Dam

post-peak shear strength

peak shear strength

seismic retrofit

liquefaction resistance

triaxial compression test

direct shear test

bentonite stabilization

slope stability