Assessing Soil Strength From Geophysical Surveys

A study is presented to develop framework for correlating sand strength (friction angle) to geophysical measurements, primarily shear wave and body wave velocities. Triaxial tests accompanied by wave velocity measurements was performed to generate a matrix relating bulk modulus and shear modulus to soil strength over a range of soil types, stress levels, stress history and soil density.

Two types of sands- Mystic White (angular) and Ottawa (rounded) were tested at four different confining pressures (5kPa, 15kPa, 30kPa and 60kPa) and for each sand type at a given stress level, two types of samples, dense and loose, were prepared, resulting in 16 tests overall. The sands were selected so as to match the requirements of maximum and minimum void ratios and angularity. Separate samples were prepared for each test and duplicate testing was conducted for at least one stress level of each material type.

The GEOTAC TruePath Automated Stress Path system was employed to carry out anisotropically consolidated tests. This system also includes pore pressure and cell pressure flow pumps allowing automatic feedback control of the cell pressure and back pressure through the software. Flow pumps were needed primarily to measure volume changes.

The caps in the triaxial apparatus were equipped with piezoelectric transducers, manufactured by GCTS Testing Systems and fit the GEOTAC setup. One set of caps had bender elements (protruding from the caps) and p-crystals installed while the other set had both p and s-crystals (mounted under the surface of the cap) installed. The system is completed by a Tectronix arbitrary function generator AFG320, a Tectronix oscilloscope TD3014B and TDS3GV and a Piezosystems PiezoLinear amplifier.

A qualitative correlation between sediment strength normalized with respect to confinement and stiffness is obtained and the conclusion drawn that further tests are required for a quantitative correlation.