Pullout Characteristics of A Suction Anchor In Sand

ABSTRACT A lightweight .temporary anchoring system is needed to secure equipment to the seafloor for measuring in situ properties of marine soils. Existing measuring systems require large deadweights to provide the necessary reaction forces, particularly on dense sandy soils. The high reaction force to weight ratio of the suction anchor makes it a promising solution to handling problems associated with large deadweight anchors. The objectives of this investigation were twofold: to observe directly the failure mode of the anchor and to provide additional test data on suction anchors for design purposes. Twelve laboratory tests of a 400-mm-diameter suction anchor were made in sand having grain size characteristics similar to the Atlantic continental shelf sands. The data obtained shows a linear correlation of .77_ between the flow rate of water through the anchor and the maximum pullout force of the anchor. The observed failure mode generally consisted of a shear failure of the sand below the anchor. Through the use of the test results and the Hohr-Coulomb failure criteria predictions of the pullout force agreed with the measured values within 13%. Preliminary calculations indicate that three anchors 2 m in diameter could produce the 200 kN (20 metric ton) reaction force needed to push a cone penetrometer about 20 m into dense sandy materials without further anchor design optimization. INTRODUCTION Measurements of seafloor engineering properties in situ often require instruments to be pushed deep into the soils. There are two broad classes of in situ testing equipment currently recognized. The first of these - includes down hole or wire-line devices that are associated with drilling; the second class consists of bottom-supported devices. This second category conventionally utilizes either mechanical. or deadweight anchors to provide the reaction forces necessary for their operation. Mechanical anchors are not easily deployed in or removed from the soil, making their use time-consuming and difficult. Deadweight anchors may be so large and cumbersome that deployment from specially equipped ships may be required. A need exists for an easily-embedded reusable anchor having a high reaction force to weight ratio for use on the sands of the continental shelf and possibly in deeper water. Suction anchors have the potential of meeting these requirements. Their reaction force results from a difference between the larger ambient pressure adjacent to the anchor and the smaller pressure within the anchor chamber produced by a suction pump. In theory, the maximum reaction force, or pullout force, at a given pressure difference is dependent primarily upon the anchor cross-sectional area. Anchor disembedment is accomplished by pumping water into the anchor chamber, which dislodges it from the soil. The principle of using hydrostatic pressure to perform work in marine applications is not new (1), but only recently has it been considered for anchoring purposes. Mackereth (2) was a pioneer in successfully using a suction anchor to fasten a piston corer during the sampling operation in. the soft soils flooring a British lake.