Numerical analysis of passive pile groups in offshore soft deposits

Abstract

The performance of pile foundations in offshore soft deposits is frequently dominated by passive loading from approach embankments for reclaimed land in Korea. This is mainly because of a Korean offshore soil profile, involving an over 30 m normally consolidated soil, with a strength that varies linearly with depth. This paper presents the results of an investigation on short- and long-term passive loading on the piles using two dimensional elastoplastic-consolidation coupled FEM analyses with large strain mode. This numerical scheme is essentially based on an updated Lagrangian formulation, which is favorably validated in cases of both centrifuge tests and field measurements. Although a parametric study is limited by some of the application, it is shown that passive pile loading is primarily affected by soil profile, pile head boundary condition, magnitude of embankment load, and average degree of consolidation. Simultaneously, time-dependent effect of shear transfer at the pile head is explicitly identified and a useful starting point in design is recommended for passive pile loading in construction (short-term) and consolidation (long-term) phases.

Introduction

Passive pile loading is generally triggered by pile displacement less than surrounding soil displacement and is a common geo-frontier for pile foundations, as well as active pile loading which is related to external loads (e.g., wind, waves, current, ice, traffic, ship impact, and mooring forces). In Korea, its evaluation is significantly associated with general problems in the planning, design, construction, and maintenance of pile foundations constructed in offshore soft deposits. Simultaneously, these could lead to more significant remedial costs than onshore pile foundations. As Korean government’s land development has subsequently impacted on offshore reclamation since the 1990s, more consideration has been given to soil–structure interaction problems associated with passive pile loading. It is mainly attributed to Korean representative offshore deposits which are composed of an over 30 m normally consolidated soft clay, with a strength that varies linearly with depth.

Much work has been carried out on group piles under active loading [10], [11], [18], [27]. A relatively less work has been done on the passive pile loading, with the initial works [14], [22], [31] focusing on the performance of in-situ pile structures which is limited to site-specific characteristics. Due to the introduction of sophisticated technology, the focus of many works has tended to be on both centrifuge and numerical modeling since the Springman’s work [29], [30]. Numerical studies into the response of passive pile loading fall into two categories, based on a 2-D plain strain FE approach which are corresponded to a series of centrifuge tests [4], [12], [13], [30], [31] or field tests [15] and a 3-D FE approach with focus on piles within the slope [8], [9] or piles subjected to lateral soil movements [5], [23]. Most of the studies mentioned above have been carried out on the assumption of small strain theory. It is fortunate that the results of 3-D FE analysis on single pile was reported by Miao et al. [21], however, in view of excessive pile–soil displacements, little has been done on the basis of large strain theory, which is taken into account in the paper.

Also, no attempt has been made to investigate the behavior of two-row pile groups with fixed pile tips in soft clay deposits with up to 30 m depth, which are typical types frequently encountered in the Korean offshore environment. Focusing on this type of pile structure, accordingly, this paper describes the results of numerical investigation into short- and long-term passive pile loadings. For this work, 2-D elastoplastic-consolidation coupled FEM analyses were conducted on the basis of large strain mode using an updated Lagrangian formulation, which is favorably verified in cases of both centrifuge tests and field tests. From a parametric study with some of the idealizations, it is shown that passive pile loading is mainly influenced by soil profile, pile head boundary condition, magnitude of surcharge load, and average degree of consolidation. From the viewpoint of the passive pile loading in construction (short-term) and consolidation (long-term) phases, simple table is presented as an initial framework for more detailed pile design.