Modified Standard Penetration Test for Illinois Weak Fine-Grained Rocks

Drilled shafts socketed in weak fine-grained rocks, such as, weak shales, mudstones, and silt-stones, i.e., unconfined compressive strength (UCS) of 0.48 to 4.8 MPa (10 ksf to 100 ksf), are increasingly used to support structural loads from bridges and other transportation structures. The design of a drilled shaft requires knowledge of strength and compressibility parameters of the weak fine-grained rocks. However, the weathered nature and fragility of weak fine-grained rock limits recovery of intact core samples for laboratory triaxial compression testing. As a result, in situ tests are preferred to measure design parameters for weak fine-grained rocks. The standard penetration test (SPT) was modified and calibrated to assess the in situ UCSUCS and Young’s modulus of weak fine-grained rocks for design. This new procedure is termed the Modified SPT (MSPT) and uses the penetration rate (cm or inches per blow), not the sum of blow counts for the last 30 cm (12 inches) of penetration, to estimate UCS. A correlation between penetration rate and UCS was developed based on a subsurface investigation at twenty-one (21) weak fine-grained rock sites throughout Illinois. The proposed correlation can be used to foundation design parameters weak fine-grained rocks.