Testing the Shear Strength of Mass Concrete Lift Lines: A Comparison of Procedures

Accurately evaluating the break bond and sliding shear strength of mass concrete lift lines is critical for any structural analysis of a dam. Of paramount importance and difficulty is the determination of break bond strength and of realistic peak and residual sliding shear strength parameters, in order to develop the anticipated strength degradation with shear displacement. Traditional multistage direct shear testing repeatedly shears the same specimen surface under increasing normal loads. The first sliding stage post-break bond has the freshest shear surface, which then degrades with each subsequent sliding stage, resulting in an artificially lower sliding friction angle and higher apparent cohesion due to accumulated damage on the shear surface. A novel approach has been proposed that, when a group of specimens are assumed to have similar characteristics, utilizes a matrix-based variable normal loading schedule that develops unique insight into shear strength degradation with sliding displacement. To eliminate the uncertainty as to which approach should be used and when, this paper documents a unique laboratory testing program where two different direct shear procedures were used for two differently sized cores obtained from the Thief Valley Dam. The two procedures were: (1) a state-of-the-art matrix-oriented approach which varies the order of the normal loads applied to develop an understanding of the shear strength degradation with sliding displacement, and (2) the typical direct shear procedure outlined in ASTM D5607, where normal loads are applied in an increasing order. This paper presents the results from: (1) the two different direct shear testing procedures and (2) the obtained strength parameters of the different core sizes.

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