Scour From Cyclic Lateral Loading of Piles

Abstract

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.

Foundation Scour as a Geohazard

“…It is possible that over time this cyclic loading may trigger a Foundation scour as a geohazard John M. Harris, Richard J.S. Whitehouse, Nicholas S. Tavouktsoglou and Pedro M. Godinho response in the local seabed soils although very few studies have been published on this potential mechanism (Reese et al, 1989;Al-Hammadi and Simons, 2016). The corresponding movement of the pile in response to loading being dependent on the resultant soil reaction.…”

Scour Development around Large-Diameter Monopiles in Cohesive Soils: Evidence from the Field

“…There have been few studies published into the effect of scour development due to cyclic loading. Reese et al (1989) reported on experiments carried out to assess the impact on soil loss due to cyclic deflection of a pile under wave forces.…”

Influence of scour depth and type on p–y curves for monopiles in sand under monotonic lateral loading in a geotechnical centrifuge

“…Several researchers have previously investigated the effect of scour formation on the lateral response characteristics of piles (Li et al, 2018;Prendergast et al, 2013). Reese et al (1989) presented an overview of the scour formation process in cohesive soils, focusing on the gap created at the soil-pile interface due to water injection induced by the cyclic loading. They concluded that this gap increases with the number of cycles, leading to a considerable decrease in lateral capacity.…”