Penetration Type Field Velocity Probe for Soft Soils

Abstract

The assessment of the shear stiffness of dredged soft ground and soft clay is extremely difficult due to soil disturbances caused during sampling and field access. Several in situ methods such as spectral analysis of surface waves, multichannel analysis of surface wave, cross hole, and downhole methods have been developed to measure the shear-wave velocity, but a few disadvantages hinder the adoption of existing methods to soft ground. This study presents a new apparatus, the penetration type field velocity probe (FVP), which overcomes several of the limitations of commonly used shear-wave measurement methods in the field. Design concerns of the FVP include the tip shape, soil disturbance, transducers, self acoustic insulation, protectors, and the electromagnetic coupling from transducer-to-transducer and cable-to-cable. The crosstalk between cables is eliminated by grouping and extra grounding of the cables. The shear-wave velocity of the FVP is directly calculated, without any inversion process, by using the travel distance and the travel time. After calibration tests are carried out in the laboratory, application tests in the field are conducted up to 29 m in depth. Calibration results show the velocity profiles obtained by the FVP and by the rods fitted with transducers are similar to each other. The experimental results obtained in the field show that the FVP can produce reasonable and detailed shear-wave velocity profiles in soft clay. This study suggests that the FVP may be an effective technique for measuring the shear-wave velocity in soft ground.