Two types of scour can be identified at piles supporting offshore structures:erosion of the sea bottom (sea-bottom scour) at obstructions due to oscillatory waves and unidirectional currents, andthe loss of soil around a pile due to the cyclic deflection of the pile under wave forces. The second type (pile scour) is associated with cohesive soils and is the subject of this study. Observations during onshore testing have revealed that the loss of lateral capacity of a pile in cohesive soils is due principally to the creation of a gap at the pile-soil interface and to the subsequent ejection of water from the gap at the space opens and closes during the cyclic loading. The paper describes the phenomenon of pile scour, presents the details of a laboratory test to investigate the scour potential of various soils, discusses the significant findings of a research study, and indicates the influence of pile scour on the design of laterally loaded piles for offshore structures. INTRODUCTION A 24-in (610 mm) 0.D. pipe pile was driven into overconsolidated clay at a site near Manor, Texas and tested under static- and cyclic-lateral loading (1). The test was conducted in a shallow pit and water a few inches deep was kept in the pit at all times. The following statement is abstracted from the report on the, testing program. "During cyclic loading, considerable scouring action was observed around the pile. The cyclic loading caused a pumping action in the water in the gap between the pile and the soil. This pumping action eroded the soil and widened the gap between the pile and soil; a hole to the side and slightly behind the pile developed during cyclic loading; the hole at the side was approximately 4 in. (100 mm) in diameter and 2 ft (600 mm) deep." A test of a 3 by 3 group of piles, each 10.75 in. (273 mm) in diameter and on a center-to-center spacing of 3 diameters, was conducted at a site on the campus of the University of Houston (2). The surface soils were overconsolidated clays that were jointed and slickensided. The investigators reported that grayish silty clay, existing below a depth of 4 ft (1.2 m), was pumped up to the ground during cyclic loading. The objectives of the study described herein were to present a qualitative description of the mechanism of scour or soil erosion at the interface of pile and clay (pile scour) and to provide a means of identifying clays that are susceptible to such scour. The mechanism of scour at the pile-soil interface is described in general terms, and details are presented of a laboratory test for investigating the scour potential of various soils. In the laboratory tests, soil samples with different characteristics were selected and tested. Studies (4) were made of natural soils from the Sabine, Texas site (5), the Manor, Texas site (1), and from a North Sea site.

New concepts for platform design have been developed recently to allow for the production of oil and gas in deep water, The foundation for each leg of the platform is usually a group of piles that are subjected to both uplift and lateral loads. A conventional approach to the design of the piles is to neglect the interaction between these two types of loading, but evidence indicates that the tensile capacity and the lateral capacity can both be affected because of the combined loading. The paper describes an approach to a rational solution of the interaction problem. Both analytical and experimental studies were conducted and the influence of cyclic loading is considered. INTRODUCTION Piles that are used for foundations of offshore platforms in deep water are frequently subjected to both uplift and lateral loading. The conventional approach in design is to assume no interaction between the two types of loading. Analysis shows that soils near the mudline are most important with regard to lateral loading, and in most instances are relatively unimportant with respect to axial loads. While this assumption has apparently been satisfactory for lateral and compressive loading, some special consideration is warranted for cases where the axial loading is tensile. There are two important differences in behavior of piles in compression and in tension that might decrease the capacity of the latter type:piles loaded in compression are more apt to exhibit an increased resistance with increase in settlement, partly due to end bearing; andthe loads in skin friction along the upper portion of a pile act against a free surface in the case of uplift, but against supporting layers in the case of compression. These two handicaps for a pile in tension may be of minor importance, but could be critical in cases where the load transfer in side resistance is decreased due to deflection-softening. A consideration of decreased load transfer with relative movement between the pile and the soil indicates the need to consider the possibility of progressive failure in uplift, especially for large offshore structures. This paper considers briefly the concept that coupling of uplift and tensile loading results in the need to consider reduced load transfer in lateral resistance and in side resistance. Models for the uncoupled method are given, results from small-scale studies are presented, a model for the coupled method is shown, and design recommendations are suggested. MODELS FOR THE UNCOUPLED PROBLEM According to methods presented by the American Petroleum Institute in RP 2A (1987), the computation of axial and lateral load employs the following equations. For axial loading:(mathematical equation)(available in full paper) where:Qd = total load,Qf = skin friction resistance,Qp = total end bearing, zero for uplift resistance,f = unit skin friction capacity,As = side surface area of pile,q = unit and end bearing capacity, zero for uplift resistance, andAp = gross end area of pile. With regard to the computation of the axial movement of a pile, RP 2A implies the use of the t-z method (Coyle and Reese, 1966), where t is the load transfer and z is the relative movement of the pile.