In Situ Stress Determination by Iowa Stepped Blade

Lateral soil pressure is a vital element in soil mechanics theory and practice, but is very sensitive to disturbance and, therefore, difficult to measure. A step-tapered blade was developed to compensate for disturbance by measuring soil pressures on three thicknesses of blades, and extrapolating for the hypothetical pressure at zero blade thickness. The nature of the extrapolation was found to be exponential, with the slope theoretically inverse to the soil compression index Cc. In situ soil stresses measured in fine-grained soils generally agree with stresses from overburden pressures, elastic theory, and pressuremeters. Measured Ko's in natural soil deposits are generally within acceptable ranges, 0.5-1.5 depending on the consolidation state of the soil, and up to 4.2 in expansive clay. About 8-10 tests may be performed per hour; the test may not be reliable in dense sands. Directional horizontal stresses may be obtained with a three-bladed stepped vane.