Abstract: As one of the primary insitu test application in the geotechnical investigation, the cone penetration testing (CPT) has the advantages of continuous probing, high efficient and relatively reliable test data. In the marine investigation, an offshore platform is usually used to provide stable base for the CPT probing. During the investigation, breakage of the drilling rods is frequently encountered due to the excessive length to rod diameter and the alternating deposition of the soft and hard soil layer below the seabed. Based on the CPT practice on the platform, this paper proposes a new testing method of applying a follow up casing behind the CPT cone tip. This new method was applied in the 10 site investigation projects of wind turbine installation and no CPT rod breakage occurred. At the same time, the construction time of each hole is reduced to 1 day. It is estimated that every time the rod breaking accident is reduced, the cost can be saved about 220000 yuan. This case history indicate that the new testing method can minimize the rod breakage, improve the efficient of penetration and reduce the CPT investigation cost.

Key words: cone penetration test, offshore platform, length-diameter ratio, follow up casing