Settlement of jacked piles in clay: Theoretical analysis considering soil aging

Abstract

This paper develops an analytical approach to estimate the long-term load-displacement behaviors of jacked piles in clayey soils. Apart from considering the effects of pile installation, subsequent re-consolidation and loading, the proposed approach properly considers the effects of soil aging on the long-term behavior of the surrounding soil so as to determine the load-displacement response of the jacked pile in the long run. An elastic-viscoplastic soil model is incorporated into the load transfer method to predict long-term soil properties. The comparison between the predictions from the presented approach and the data measured from the field pile loading test shows the validity of the proposed method. Parametric studies are conducted to illustrate the effects of the in-situ overconsolidation ratio, internal friction angle and the secondary consolidation coefficient on the long-term load-displacement behaviors of old piles. Results show that the proposed approach not only is capable of predicting the short-term set-up of jacked piles, but also could yield reasonable long-term load carrying behavior. The three parameters pertaining to the strength and stiffness of soil have significant impacts on the long-term load-displacement curve of the pile.

Introduction

The set-up of displacement piles in clay after pile installation has been one of the most popular topics in foundation engineering [1], [2], [3], [4], [5], [6]. However, the total settlement and ultimate bearing capacity of jacked piles depend on not only the changes caused by re-consolidation of the surrounding soil, but also the increment developed during subsequent soil aging. It has been well recognized that the stiffness and load carrying capacity of piles still increase after consolidation of the disturbed soil around the piles [7], [8]. For the sake of the reuse of old piles, it is of great significance to predict the long-term load-displacement behaviors of old piles after the surrounding soil undergoes many years of aging [9].

Extensive research efforts, either theoretical or experimental, have been dedicated to exploring the installation effects and subsequent set-up of jacked piles. Some of the representative publications are summarized and listed in Table 1. In terms of theoretical studies, Soderberg [10] applied Terzaghi’s consolidation theory to interpret the increase of the bearing capacity of a friction pile in clays and silts, which, however, did not consider the effects of soil aging on the long-term load carrying behaviour of the pile. Randolph and Wroth [11] regarded pile installation process as undrained expansion of a cylindrical cavity and proposed analytical solutions to jacked pile installation and subsequent dissipation of the excess pore pressure. Although they interpreted the setup mechanism of jacked piles, they did not investigate the ageing of the soil and the long-term load-carrying behaviour of the pile. Roy et al. [12] conducted full scale pile installation and loading tests in the soft Champlain clay deposit in St-Alban, west of Quebec City. By instrumenting the pile and the surrounding soil, the excess pore water pressure around the pile was recorded during the pile installation process and subsequent consolidation until the excess pore pressure fully dissipated. The loading tests conducted at different time after pile installation revealed that the load-carrying capacity of the pile increased substantially with dissipation of the excess pore water pressure after pile installation. Subsequently, Konrad and Roy [13] examined the load carrying capacity of the piles two years later. They found that the load-carrying capacity of the jacked piles kept increasing after primary consolidation of the surrounding soil. Azzouz and Morrison [14] also experimentally investigated the change of the excess pore water pressure and the horizontal effective stress of the soil around the pile during installation and subsequent consolidation through field tests on instrumented piles in clay deposits. The filed results indicated that the horizontal stress decreases during pile installation but increase significantly after installation, which well explained the setup mechanism of the jacked piles in clayey soils. Yang and Liang [15] collected a great deal of pile testing data regarding the pile setup, and statistically investigated the increase in pile axial load carrying capacity after driven pile installation. Based on the database, they proposed a reliability-based empirical formulate to predict pile setup after installation so as to incorporate the setup effects in driven pile design. However, the reliability approach they developed largely depended on the database, and hence it still needs further calibration and validation before its application to different pile types or soil geology conditions. Recently, Li et al. [16], [17] developed analytical and semi-analytical approaches to evaluate time-dependent load-carrying capacity of jacked piles in clay. Since they employed an anisotropic Cam-clay model the soil behaviour, the change of the soil properties due to soil ageing cannot be considered in their studies. Therefore, the methods are only capable of predicting the short-term setup of the jacked piles. Apart from theoretical and experimental studies, Zhang et al. [18] and Goh et al. [19] performed finite element analyses to explore the pile wall responses induced by deep excavations and groundwater drawdown. Although the work provided significant guidance for evaluation of the pile wall deformation due to deep excavation, it did not involve the effects of soil ageing. From above literature review, it can be found that most available studies mainly focus on the time-dependent performance of piles during the primary consolidation stage, which neglect the increase of load carrying capacity of piles due to creep effects of soil. The clay has been proved to display different stress-strain relationships at different time during soil aging [20], [21], [22]. Some rheological component models for fluids have been used to describe the aging properties of soft clay. Li et al. [23] adopted a nonlinear three-component model to simulate the viscosity of soft clay and compared the results with data measured from one-dimensional compression tests and drained triaxial compression tests. It is not hard to find that the performance of the rheological component models greatly depends on the test results and cannot take the stress history and strain rate of soil into account. Due to the shortages of the fluid rheological component models, the elastic-viscoplastic soil models have been used more and more in theoretical research on aging of clayey soils [21], [24], [25], [26], [27], [28]. Subsequently, Cui et al. [9] theoretically investigated the long-term load carrying capacity of jacked piles with the nonstationary flow surface theory following the same framework of Li et al. [16], [17]. However, it should be noted that the work of Cui [9] only involved the increase of the load-carrying capacity due to soil aging, while ignored the effects of primary consolidation, which, however, was considered as the primary cause for the setup of the jacked piles. Furthermore, they used a simple isotropic viscoplastic model to simulate the ageing of the surrounding soil, which could only approximately reflect the ageing of the soil, as the model parameters were determined empirically. Therefore, it is obvious that scarce research effort has been devoted to adopting the advanced elastic-viscoplastic constitutive models to investigate the long-term pile performance during the process of soil aging.

This paper aims to develop a theoretical approach based on the elastic-viscoplastic soil model to predict the long-term load-displacement behaviors of jacked piles in natural clayey soils. Different from the previous studies of the authors [9], [16], [17], the present study takes the effects of both primary consolidation and soil ageing into account so as to determine the long-term load carrying capacity of the pile. The expression of the quasi overconsolidation ratio with respect to time is derived and used to determine the relationship between soil strength and stiffness with aging time. A nonlinear load-transfer method is employed to predict the long-term vertical load-displacement behavior of the jacked pile, which is used to evaluate performance of old piles and new piles, respectively, in an engineering project in Shanghai soft soil. The predictions show good agreements with the measured data. Parametric studies are conducted to investigate the effects of the in-situ overconsolidation ratio, internal friction angle, and the secondary consolidation coefficient on load-displacement behaviors of piles. It is expected that the proposed theoretical approach can be used to estimate the long-term pile performance with sufficient accuracy and can serve as the theoretical base for reuse of old piles.