Cone penetration testing in stiff glacial soils using a downhole cone penetrometer

Abstract

Cone penetration testing (CPT) in Canada is usually performed using locally available drilling rigs. The limited pushing capacity of most drilling rigs coupled with the risk of damage to expensive cone penetrometers has tended to restrict the CPT to generally loose or soft soils. Therefore, in regions dominated by stiff glacial soils the more rugged standard penetration test (SPT) is still the most commonly used in situ test during geotechnical investigations. However, there are many limitations with the SPT with respect to interpretation and repeatability, especially the uncertainty with the energy delivered from various SPT hammer anvil systems. A downhole cone penetration test (DCPT) has been developed by modifying the equipment and procedure of the standard electric CPT. The DCPT consists of a simple, inexpensive electric cone penetrometer attached to a 1.5 m (5 ft) length of AW drill rod. The test is performed by pushing the cone 1.5 m into the base of an open borehole to produce a continuous profile of penetration resistance Qc, over the 1.5-m interval or whatever interval penetration is possible. The test incorporates the simplicity, ruggedness, and depth capability of the SPT but is able to define a near-continuous, accurate, and repeatable cone penetration resistance profile. The equipment and procedure of the DCPT is described in detail, and results from a near-continuous DCPT and an adjacent continuous CPT are presented and compared with the results obtained from an adjacent borehole with SPT. Excellent agreement was found between the results of the DCPT and the CPT. Key words : in situ, cone penetration testing, stiff soils.