Use of null gauges to monitor soil stresses during excavation in a centrifuge

Abstract

A centrifuge test is presented that tracks the earth pressures around a stiff retaining wall, propped at the top, during in-flight excavation in dry sand. The test featured the first use in a centrifuge of a new earth pressure cell, the null gauge, which is designed to avoid measurement errors due to soil arching around flexible cells. Any tendency for membrane deflection is detected and quickly countered by applying a measured air pressure inside the cell. A sequence of centrifugal loading, unloading and reloading was conducted prior to any excavation. Null gauge measurements of (analogue) vertical stress showed some hysteresis from which the effects of boundary friction were assessed. Small corrections could be made using silo theory. Measurements of initial horizontal earth pressures conformed to conventional empirical estimates. The process of in-flight excavation using a scraper was monitored, and found to generate lateral pressure spikes owing to scraper penetration. These were analysed and found to fit an estimate based on soil compaction effects. Finally, the horizontal pressures during excavation are presented. Mobilised passive pressures are found to be non-linear with depth beneath the excavation, confirming the importance of excavating soil in flight rather than simulating the process using a heavy fluid.