Cut Slope Design and Pressuremeter Testing in Weakly Cemented, Unsaturated Soil

In the arid regions of the southwest United States, engineers have taken advantage of the strength characteristics of unsaturated, weakly cemented soils for many years. Undisturbed soil is difficult to sample and test in the laboratory. In the past, in situ methods typically consisted of down-hole seismic. The US 70 Hondo Valley Project in New Mexico presented a unique opportunity to compare the results of triaxial tests on unsaturated soil with the results of in situ testing via pressuremeter testing (PMT) in these deposits. Using a numerical analysis approach, we were able to estimate the effective friction angle and cohesive strength of unsaturated, weakly cemented soil via PMT. We completed the design of excavations within the unsaturated soils based on observations of the existing cuts, the results of triaxial tests on unsaturated samples, and in situ PMT testing. We found what engineers have intuitively known for years: that the design of new excavations based on the past performance of cuts in the same soils is a valid design approach. Reliable safety factors for new cuts are difficult to estimate without the benefit of lab testing on undisturbed samples, in situ testing, or prior slope failures. Moreover, the PMT numerical analysis approach for estimating both the frictional and cohesion component of the strength of unsaturated soil compares well with the strength estimates on samples tested with triaxial loading under unsaturated conditions.