New Tool to Improve Geotechnical Site Investigation
The INDOT Office of Geotechnical Services hosted a workshop and demonstration led by research project leaders Rodrigo Salgado and Monica Prezzi, Purdue University on May 5, 2014. The workshop focused on the use of in situ test-based correlations in the design of geotechnical structures. A demonstration on the use of the SPT torque equipment took place after the lecture, with results shared immediately after.
The SPT is a common in situ testing method. Geotechnical site investigations in the United States rely heavily on the SPT. The test is used to determine the geotechnical engineering properties of subsurface soils. The SPT has served as a simple test to estimate the relative density of soils and approximate shear strength parameters for engineers planning geotechnical structures such as foundation design and slope stability checks. The test has remained essentially the same since the 1960s.
One of the advantages of the newly designed SPT-T is that it takes only a few minutes to measure the torque and the conventional SPT procedure remains the same. This provides a reduced amount of effort for the user learning the new equipment. The test measures the torque required to rotate the sampler in the ground at constant rate; the torque is then related to the shear strength in these soils.
This tool has the potential to provide a substantial reduction in cost overruns and change orders caused by inadequate site investigations. “This two-in-one tool provides in-situ strength parameters in the field for design purposes as well as to obtain samples in one operation at the same time for visual inspection without mobilizing another piece of equipment,” says Athar Khan, Manager of INDOT Office of Geotechnical Engineering. “Further, it enables the test to be performed during subsurface investigation, reducing transportation delay and sample disturbance. This will enable us [INDOT] to reduce the time and cost of laboratory testing on a project,” adds Khan. The better and more reliable SPT data provides more confidence in estimation of soil properties for design, which would result in savings of up to 20 percent in geotechnical engineering project costs, given the large factors of safety currently used or implied in geotechnical design based on SPT results.