Shaun Malin showed that it is possible to estimate the in situ stress in unsaturated soils using hydraulic fractures. His thesis research began with a series of lab tests where he created hydraulic fractures by injecting dyed water into samples of remolded clayey silt. He used the lab tests to show that it was possible to interpret pressure logs from the period when a hydraulic fracture is closing to estimate in situ stress. He did this by evaluating many different data transforms.

The lab experiments were also used to refine field equipment. The design of the bottom of the casing proved to be particularly tricky because many of the configurations produced fractures that curved and twisted into peculiar forms. Shaun finally figured out that a beveled-shape point with an underlying slot would reliably create fractures in the plane of the initial notch. This was important for developing a reliable field method.

Shaun took his insights from the lab experiments and applied them to the field in a series of tests at our facility near Pendelton, SC. The tests were conducted at depths of a few meters in a massive B soil horizon. The results indicated that the material is overconsolidated, with the lateral stress several times greater than the vertical stress. Billy Camp at SM&E conducted tests with a flat-blade dilatometer and those results indicated that the lateral stress was slightly less than the results from Shaun's technique. This was an encouraging verification of the technique.

The in situ state of stress is particularly important if you want to predict the propagation path of hydraulic fractures. This is a long-term goal of our research group, and Shaun's investigation makes an important contribution to this goal.

Last Updated: August 28, 2007 -- Questions or comments, contact Larry Murdoch.
School of Environment, Department of Geological Sciences
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