CS25F - Concurrent Session 25F: How a Barton-Choubey Evaluation can be Used to Better Assess the Effective Friction Angle at Your Dam Foundation’s Rock Interface

Abstract Description: For many years, the dam safety industry has been moving towards a “no cohesion” approach when evaluating stability analyses for dams, particularly when laboratory or testing data is not available. Historically, sliding stability analyses have often relied upon cohesion to achieve recommended minimum factors of safety. Without cohesion, in some cases, target factors of safety may not be achieved. Gravity dam foundation-bedrock interface friction angles have historically been classified based on bedrock type or historic references covering basic friction angle, without considering site-specific factors. If dams do not meet recommended minimum factors of safety during stability evaluations, costly modifications to the structure may be deemed necessary.

Barton-Choubey evaluations provide site-specific assessments of the effective friction angle at the foundation-bedrock interface and can result in significant changes to the assumed friction angle. In many cases, this site-specific approach can lead to increased friction angles and acceptable sliding factors of safety. For new structures, a more accurate effective friction angle can reduce foundation size, concrete mass, excavation quantities or the need for stabilizing measures such as buttressing or rock anchors. For existing structures, Barton-Choubey evaluations have eliminated or reduced the scope of additional required stabilization, resulting in significant owner cost savings.

The primary inputs for a Barton-Choubey evaluation include the basic friction angle, the joint roughness coefficient, and the joint compressive strength. These inputs can be determined using available site documentation and reference materials for a desktop study, through non-invasive onsite tests, and/or through obtaining samples from the site for laboratory testing. More data for evaluation inputs improves the accuracy of the resulting effective friction factors.

Kleinschmidt has performed Barton-Choubey evaluations for dam owners looking to update existing stability analyses, satisfy regulatory comments, and inform new design. Barton-Choubey evaluations are accepted by many regulators and in some cases have eliminated the need for expensive modifications. For a fraction of the cost of capital improvements, a defendable Barton-Choubey evaluation can significantly impact a dam’s sliding factor of safety. The Barton-Choubey evaluation should be considered a valuable option for assessing the stability of existing and proposed structures.