National Cooperative Highway Research Program (NCHRP)
Jeramy Ashlockjashlock@iastate.edu email >
Faculty Affiliate, InTrans
About the research
Static soil shear strength parameters in the form of friction angle and cohesion are required inputs for the safe design of foundations and earth retaining structures for virtually all transportation infrastructure including bridges, buildings, railways, wharves, piers, ports, tunnels, and pavements. Additionally, measuring the dynamic and cyclic behavior of soil in terms of stress-strain hysteresis loops as well as the associated evolution of pore water pressure is important for obtaining modulus and damping parameters for seismic design, determining post-cyclic strength, and liquefaction susceptibility analysis. These soil parameters are typically obtained by retrieving soil samples and testing them in the laboratory, which is time-consuming, expensive, and the results are sensitive to sample disturbance. Alternatively, the shear strength parameters may be estimated using empirical correlations to in situ penetration tests such as the Standard Penetration Test (SPT) or Cone Penetration Test (CPT). However, neither of these tests directly measure the shear strength of soil and instead rely upon empirical correlations that can be imprecise due to large statistical variability. Furthermore, the SPT and CPT do not subject the soil to repeated continuous cyclic loading conditions like those imposed by earthquakes or vibration sources. The goal of this project was to develop a new in situ testing device that could measure static and dynamic soil properties in the soil’s natural setting, with less sample disturbance and requiring less time than laboratory tests.
In this project, a Cyclic Borehole Shear Test (CBST) device was developed to enable the rapid in situ measurement of cyclic behavior and monotonic shear strength properties of soil. Based on the results of several field testing trials, numerous refinements and modifications were made to the system including the physical testing apparatus inserted into the borehole, the electronic and pneumatic measurement and control system, and the software control program. Comparisons of field test results to those of conventional laboratory tests demonstrated that the device can measure meaningful cyclic behavior of soil in situ. Further research will be pursued to more rigorously relate the measured displacements from the device to shear strains in the soil surrounding the borehole, and to study applications of the device to in situ measurement of the liquefaction behavior of soils. With further research, the device thus has the potential to fundamentally transform the presently empirical techniques used in practice for assessment of soil liquefaction resistance into a more mechanistic physics-based framework.