The Vertical Holding Capacity Of Marine Anchor Flukes Subjected To Static And Cyclic Loading

Abstract

ABSTRACT Laboratory tests were performed using the Naval Civil Engineering Laboratory shape fluke embedded in saturated sand, silty sand and clay soils. The major conclusions are that there is a characteristic relative embedment depth for each soil condition below which static holding capacity factors are approximately constant, that suction is a significant holding component in clay and, that cyclic loading always causes increasing upward displacements. Design figures for embedded marine anchor flukes are presented. INTRODUCTION One of the problems confronting many endeavors in the ocean is that of providing reliable anchors. Of the three major types of anchors, i.e., dead weight anchors, drag anchors, and embedment anchors, the embedment anchor offers the largest ratio of the holding capacity to the weight of the anchor. The hording capacity of an embedment anchor depends primarily upon the complex interaction between the anchor and soil. The major design requirements for an embedded anchor are:the ability to develop sufficient resistance to either static or oscillatory loadings or a combination thereof, andthe ability to develop this resistance with movements which are compatible with the design criteria of that particular structure. The origin of these loadings may be the result of buoyant ocean floor installations or wave induced forces transmitted down a mooring line from ships of buoys on the ocean surface. Several theories have been advanced which provide a prediction of the vertical holding capacity of embedded anchors subjected to static loading. These theories are based on various assumed mechanisms of failure; some are based on theoretical considerations, some on empirical considerations, and some on combinations thereof. A comparison of holding capacity, in terms of a dimensionless breakout factor, N, for anchors in a cohesionless soil with a friction angle equal to 30° is presented by Figure 1. Similarly, a comparison of the holding capacity, in terms of a dimensionless breakout factor, Nu' for anchors in a cohesive soil is presented by Figure 2. The range of the holding capacity is widespread. An extensive review of the theories is presented by Kupferman (2). This work is an extension of that previously reported by Bemben, et al (1). The new work includes laboratory tests with longer intervals of cyclic loading than previously reported and additional constant velocity pullout tests. SOIL PROPERTIES AND TESTING EQUIPMENT Three soils were used in this investigation, a medium to fine sand, known as Sunderland sand; a silty sand, known as BBY sand; and a clay, known as Panther Creek Bentonite. The engineering properties of these soils are presented in Table 1. The Sunderland sand and Panther Creek Bentonite are the same soils used by Bemben, et al (1).