Prediction of unconfined compressive strength and deformation modulus of weak argillaceous rocks based on the standard penetration test

Standard penetration test is often used as an index for prediction of geomaterial elastic and strength properties. It consists of releasing a 63.5 kg hammer from a specified distance of 762 mm on a series of connected drill rods. The N-value is defined after a total 457 mm of penetration is achieved by counting the below counts for the last 300 mm of penetration. This required penetration is not often achieved in weak rocks due to their relatively large compressive strength. To eliminate the need for achieving a total required 457 mm of penetration, a novel approach is introduced where the rate of penetration of the split spoon sampler is used for the characterization of the engineering properties of the weak argillaceous rocks. The rate of penetration is defined as the slope of the final segment of the blow count versus penetration plot. The test results show that the average penetration rate at each test depth approaches a relatively constant value as the penetration increases. It is found that penetration rate is affected by the unconfined compressive strength, deformation modulus, and water content of the rock mass as well as the mean effective stress at the elevation of each test. Using in situ tests from 21 weak rock sites in Illinois, United States, models are proposed for the prediction of deformation modulus and unconfined compressive strength based on the penetration rate. The models account for the influence of mean effective stresses and the in situ water content of rock.