InTrans / Apr 11, 2023

New soil testing device developed at InTrans

CBST device in borehole ready for test in Ames, Iowa

A newly developed soil testing device will allow users to measure the cyclic behavior of soils in the ground rather than in a laboratory, saving time and money and delivering more accurate measurements.

The Cyclic Borehole Shear Test (CBST) device was developed as part of an Institute for Transportation (InTrans) project sponsored by the National Cooperative Highway Research Program (NCHRP) Innovations Deserving Exploratory Analysis (IDEA) program. The project developed a prototype device and software control program, and a future research project is expected to further develop and commercialize the CBST product.

“The device has the potential to help advance the safety and sustainability of transportation infrastructure by improving the speed, reliability, and accuracy with which daily foundation design inputs and liquefaction susceptibility of soils are assessed,” said Jeramy Ashlock, principal investigator on the project and InTrans faculty affiliate.

The CBST is unique in its ability to measure the parameters in the soil’s natural setting, under cyclic loading, in a matter of minutes. The current most commonly used tests can require up to several weeks and occur in a laboratory setting for which the collection of samples requires extraction, transportation, preparation, and trimming of soil specimens.

“By testing the soil in situ, the device will not only save time but also reduce or avoid effects of soil sample disturbance, which can significantly affect laboratory test results,” said Ashlock, who is also an associate professor at Iowa State University’s Department of Civil, Construction, and Environmental Engineering.

The results from the test device are typical required inputs for the safe design of foundations and earth retaining structures for virtually all transportation infrastructure. The measurements are also important for infrastructure design in areas where natural disasters such as earthquakes and landslides are more likely to occur.

Field tests in Fort Dodge Iowa: CBST control and data acquisition equipment, left, and downhole device after test in sandy lean clay fill, right

The CBST will enable engineers to measure the in situ soil response under cyclic loading as occurs in earthquakes and soil residual strengths that occur at large deformations in landslides. It may also prove useful for other geotechnical problems such as pavement subgrades and foundations of offshore structures, wind turbines, bridges, and marine retaining walls, among others.

“With further research, the device also 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,” said Ashlock. “This is because the device directly or indirectly measures the actual physical mechanisms responsible for liquefaction; namely stress, strain, and pore water pressure.”

The three-year project developed and further refined the device through field tests that occurred in five states that included a variety of soil types: Iowa, South Carolina, Maryland, Virginia, and Oregon. The field test results were compared to the typical conventional laboratory tests, which demonstrated that the CBST can measure meaningful cyclic behavior of soil in situ.

The CBST employs two serrated shear plates to engage the soil in shearing just beyond the borehole wall. The device first applies a normal stress to the borehole wall, and then after a consolidation delay time, applies a cyclic vertical shear stress to the borehole wall. It requires only a single borehole and can be performed at specific targeted depths with controlled stresses and measurements of pore pressure and shear displacement.

More information about the research project and the developed CBST device can be found at this link, which will also include a link to future phases of the project once they are underway.