Centrifuge Model Design for Axially Loaded Structures under Large Ground Movements

Buried pipelines need to withstand soil friction forces generated during relative movement between the soil and structure. The forces that develop along pipelines with diameter inconsistencies (i.e., with enlarged connections) are severely underestimated or ignored by current design practices. The movement of these enlarged connections can mobilize soil resistance similar to the response of vertical anchor plates. Previous centrifuge tests of vertical anchor plates have helped determine the soil’s breakout capacity and mapped the failure planes for various anchor conditions. Although centrifuge testing allows large-scale geotechnical problems to be modeled and studied at a reduced-scale, practical limitations exist. In addition, rigid container boundaries are not present in situ and limit the soil’s ability to develop its entire failure plane. Hence, experiments should be designed to reduce such boundary effects. An ideal test box is large enough to fully develop key failure planes in every direction. This paper determines the minimum boundary spacing necessary to adequately develop failure surfaces around a buried pipe. Centrifuge tests were conducted in samples of dense sand to determine how the distance to rigid container boundaries influences the force required to axially displace a pipe joint. Boundary influence was evident through a work-hardening response after the peak force, in which proximity to the boundary increased lateral force because of restricted volume expansion at the soil-container interface. The most influential parameter is experimentally shown as the distance from the pipe joint to the boundary parallel to the joint face in the direction of axial movement. The least significant parameter is the distance from the pipe to the rigid floor because a negligible quantity of soil is mobilized below the joint providing resistance. The results also provide insight into how rigid container boundaries are expected to impact the force-displacement response when minimum required dimensions are not achieved.