Interpretation of In Situ Tests as Affected by Soil Suction / Gerald A. Miller, Rodney W. Collins, Kanthasamy K. Muraleetharan, Amy B. Cerato, Roy Doumet.

Abstract

In situ testing of soil with invasive methods such as the Cone Penetration Test (CPT) are increasingly used in geotechnical engineering practice. However, there has been very little work to develop methods for interpreting results of these tests when performed in unsaturated soil. It is important to develop such methods because the in situ test results in unsaturated soil will depend on the moisture conditions at the time of testing and may not reflect the soil behavior corresponding to the moisture conditions during the life of supported structures. Research described in this report involved conducting in situ tests at two test sites, containing lean to fat clayey soils, at various times of the year. The purpose was to investigate the influence of changes in moisture conditions and soil suction on the test response. In situ testing included the Cone Penetration Test (CPT), Standard Penetration Test (SPT) and Pre-bored Pressuremeter Test (PMT). Test sites were characterized by sampling and laboratory testing to determine important soil properties and moisture content profiles. Additionally, test sites were instrumented with weather monitoring equipment and sensors to measure temporal variations in soil moisture with depth. Another goal was to evaluate two commercially available computer programs to evaluate their predictive ability with respect to soil moisture change. The PMT, CPT and SPT parameters determined from standard interpretation of the results were compared to total suction measured at corresponding depths obtained from measurements on samples obtained in the field using a chilled mirror hygrometer. At both sites, the suction had a profound impact on the in situ test parameters, especially at total suction in excess of approximately 150 psi, which generally corresponded to shallow test depths. At both sites, for the shallow depths there was a noticeable trend of increasing magnitude of in situ test parameters with increasing suction, as expected. Plots of normalized in situ test parameters against suction show expected, albeit weak, correlations for the two test sites. These correlations give a sense of the variation of in situ parameters that might be expected for similar soils under changing suction conditions. It was demonstrated how correlations for PMT, CPT and SPT parameters with suction can be used to roughly predict possible changes in these parameters as a result of suction changes. Unsaturated seepage modeling was found to provide reasonable comparisons to measured soil moisture profiles; however, significant effort was required to define the various input parameters through calibration procedures.