Experimental Evaluation of Rapid Flat Jack Testing with Various Shaped Saw-Cut Slots

The flat jack test is one of the oldest and simplest in situ stress measurement techniques that has generally fallen out of favour in current rock engineering practice. Although there are a number of limitations with the test, the direct pressure measurement obtained from the test, without need of stiffness parameters, and significant volume of rock involved in the test, compared to other tests, is an asset. The proliferation of relatively easy to use three-dimensional numerical modelling software codes can help to overcome some of the theoretical and practical limitations of the test. In addition, modelling also presents opportunities to utilize various shaped bladders within simple saw-cut slots, as opposed to the conventional lined-drilled slot cut with grouting of the flat jack in place. The use of a simple rock saw to cut a narrow slot for simple insertion of an ungrouted bladder allows for more rapid and economical testing. In this study, the use of rapid, saw-cut slots in combination with various bladder shapes (e.g., semi-circular or rectangular) was investigated experimentally in the laboratory. An experimental program was developed to conduct flat jack testing on large concrete blocks in the laboratory under known, controlled stress conditions. The findings were used to develop correction factors to account for various shapes of saw-cut slots in combination with various shapes of bladders. Three-dimensional finite element modelling was used to further verify and examine the experimental findings.