Field Tests Of The Lateral-Load Behavior Of Pile Groups In Soft Clay

Abstract

Abstract Field experiments with laterally-loaded pile groups have been conducted in a very soft clay at Harvey, Louisiana. Six series of loadings were performed; both static and cyclic tests were done with five- and ten-pile circular groups of six-inch diameter pipe piles, and corresponding single-pile tests were performed for comparison. Deflections were enforced at two elevations by a special loading device to simulate pile-head restraints typical of offshore structures. The measurements taken to record the behavior of the piles throughout the history of loading included total load and deflection of the group, plus individual pile shears and bending moments. Center-to center spacing in the 5-pile groups was 3.4 pile diameters and 1.8 diameters in the 10-pile group. The designer is often concerned with the magnitude of maximum bending moment produced by a given lateral load or shear. The variations in shear and bending moment among the piles in each group were found to be surprisingly small. Moderate increases in bending moment were seen with reduced pile spacing in the static tests. For a given lateral load per pile, the lateral deflections also increased with decreased pile spacing. In cyclic testing, the 10-pile group showed substantially increased deflections and bending moments over those of the single pile. However, the cyclic behavior of each pile in the 5-pile group was almost identical to that of the corresponding single pile test. For all cyclic tests, the soil resistance degradation was observed to be initiated at approximately the same small deflection regardless of the number of piles or spacing. The results emphasize the highly inelastic response of the soft clay soil and, for such soils, give little encouragement to current concepts of elastic interaction among piles in a group. However, a well-documented basis is provided for evaluation of various methods of analysis for this important problem. Pertinent data on the soil at the site are included. Introduction In recent years, considerable knowledge of the behavior and analysis of individual piles laterally loaded in soft clays has been developed and used to improve design procedures for foundations of offshore platforms. As deeper water and higher platforms have been considered, there has been a trend toward structural simplification and the use of fewer but larger vertical legs. This has led to the use of groups of closely-spaced piles in each leg. In 1966, at the outset of the research project upon which this paper is based, there was little experimental evidence upon which to base the lateral load design of pile groups. There has been relatively little change in, the situation to the present date (Refs 1 and 2). In the project presently reported, six tests were performed, one static and one cyclic for each of three cases: a single pile, a 5-pile group, and a 10-pile group. The groups were circular and of the same gross diameter, with center-to-center pile spacings of 3.4 and 1.8 diameters, respectively.