Soil strength estimation using free falling penetrometers

Obtaining soil strength parameters can be costly and time consuming for sites with low accessibility such as those offshore and at remote inland locations. A free falling penetrometer can be a cost effective solution for such sites and this has led to the development of numerous penetrometer systems. These penetrometers (retrievable or expendable) can be deployed from small vessels and be allowed to free-fall into the seabed. However, free falling penetrometer application is not widespread due in part to difficulties in data interpretation. There is a lack of appropriate theory and well controlled experimental data to estimate the rapid penetration resistance of the soil. This study has investigated the interpretation of free falling penetrometer through performing a series of laboratory model tests. A penetrometer of various tip diameters and masses has been dropped from different fall heights into kaolin with a range of undrained shear strengths. The effects of the influencing variables on the penetrometer performances are evaluated. Empirical equations are formulated to estimate the undrained shear strength of kaolin based on the measured dynamic penetration resistance and final penetration depth. The equations are successful in estimating the undrained shear strength in the model tests with reasonable accuracy. Good agreement has also been obtained in validating the empirical equations using a published case study.