A 40-Foot Static Cone Penetrometer

Abstract The Navy needs a lightweight device for testing seafloor soils to sub bottom depths of 12 meters in water depths to 60 meters. To meet this need a quasistatic cone penetration device that uses water jetting to reduce friction on the cone rod has been developed. This device is called the XSP-40. The 5-ton XSP-40 stands 15 meters tall and pushes a standard 5-ton cone into the seafloor. It is remotely controlled with an electronic unit on the deck of the support vessel. All cone outputs are recorded directly as a function of penetration depth with a strip chart recorder. A full suite of gauges is provided. on the electronic unit for monitoring the XSP-40's performance during a test .. About 40 penetration tests have been performed with very good success. The XSP-40 was field tested in Norton Sound, off the west coast of Alaska. The general objective, in addition to evaluation of the device, was to gather geotechnical information on sediments that may be involved in processes potentially hazardous to offshore development. Four example penetration records are presented from gas charged sediment zones and areas near the Yukon River delta. In general it was determined that soil classification from cone data agreed well with classifications from core samples. Relative densities of the silt-sand to sandy-silt soils were usually very high. The significance of these results are discussed with respect to storm wave, liquefaction. It is concluded that the XSP-40 is a durable and reliable piece of equipment capable of achieving penetration beyond that possible when not using the water jet system.. Introduction Cone penetration testing is a widely accepted means of testing soils. The results can be used directly for design purposes and for estimating soil strength parameters At-sea cone testing is usually done with wire-line techniques down a drill string or with heavy (20 ton) bottom resting equipment9 When drilling is not possible or too costly, and heavy lifts cannot be made, lightweight (5 ton) cone devices can be used. A disadvantage of a lightweight device is that penetration is limited by the weight of the device and often does not exceed 3 meters in firm seafloors. The Navy needs to test soils to depths of 12 meters in shallow water (60 meters) for design of propellant embedded anchors and lightweight structures founded on piles. A complication is that drilling and heavy-lift equipment are usually not available. To meet both the exploration depth required and the limitations of support equipment it was decided to build a cone penetration device that would use water jetting to minimize friction on the cone rod. In this way penetration could be maximized for a given reaction weight. This paper presents a description of the device, an overview of operational procedures, and a performance evaluation. Some results from a series of tests performed in Norton Sound are presented and discussed. The device is called the XSP-40 to designate that it is an experimental static electrometer with a 40-foot e12-meter) depth capability. The development was funded by the Naval Facilities Engineering Command. A conceptual design was done by Woodward-Clyde Consultants.