Improved p-y curve models for large diameter and super-long cast-in-place piles using piezocone penetration test data

Abstract

Due to rapid population growth and infrastructure demand in China, more effective use of existing land is required. Infrastructure construction in coastal regions has driven rapid economic development. With increasing numbers of port facilities, cross-sea bridges and offshore engineering projects, large diameter and super-long cast-in-place piles are increasingly needed in these regions. The p-y curves of piles are key to evaluating their horizontal bearing characteristics, which is critical to the risk evaluation, design and construction of large infrastructural supports. In the present study, a p-y curve analysis method based on piezocone penetration test (CPTu) data is proposed, which uses the Matlock and hyperbolic p-y models. The relationship between CPTu cone tip parameters (q_t, u₂ and q_e) and p-y curve parameters (p_u and y₅₀) is established. Moreover, the deformation and mechanical characteristics of large diameter and super-long cast-in-place piles in the Yangtze River floodplain are analysed thoroughly. The feasibility and reasonability of the proposed p-y curve method based on CPTu test data are verified through numerical simulation and horizontal load field tests. A hyperbolic p-y model based on CPTu test data is applicable to sandy soils, while a Matlock p-y curve model based on CPTu test data is applicable to cohesive soils.

Introduction

In recent years, there have been increasing numbers of port facilities, high-rise buildings, cross-sea bridges, offshore engineering projects (Ullah and Hu, 2020), and underground structures (Li et al., 2019a, Li et al., 2019b, Wan et al., 2019, Zhang et al., 2020a, Zhang et al., 2020b). The horizontal loads on their upper structures, including wind loads, impact forces of ships and braking forces of vehicles, mean that their pile foundations require higher horizontal bearing capacity (Ng et al., 2016, Tran and Kim, 2017, Xie et al., 2017). Large diameter and super-long cast-in-place piles (LSCPs) have been widely used in large infrastructure built in coastal soft-soil regions (Feng et al., 2016, Liu et al., 2017, Xiao et al., 2019). They have a high bearing capacity per pile, good anti-seismic performance and strong adaptability to geological conditions. However, existing studies on these piles have mainly focused on mechanisms of vertical load transmission and bearing characteristics (Feng et al., 2016, Xiao et al., 2019). Therefore, research attention of foundation bearing capacity of LSCPs has shifted from vertical-loading piles to horizontal-loading piles and composite-loading piles dominated by horizontal loads. The bearing characteristics and calculation methods of pile foundations under horizontal loads have become key areas of research (André de Almeida et al., 2011, Ebrahimian et al., 2015, Suryasentana and Lehane, 2016).

Currently, determination of the horizontal bearing capacity of pile foundations is mainly done via methods involving elastic theory (Basu and Salgado, 2008, Douglas and Davis, 1964, Fan and Long, 2005, Guo and Lee, 2001), subgrade reaction coefficients (Shen and Teh, 2004, Yang and Liang, 2006), p-y curves (Dash et al., 2017, Kim et al., 2009, Tak Kim et al., 2004, Zhang, 2009, Zhang et al., 2005), numerical simulation (Brown and Shie, 1991, Fan and Long, 2005, Shen and Teh, 2004, Yang and Liang, 2006, Yang and Jeremić, 2002, Yang and Jeremić, 2005), model tests (Barden and Monckton, 1970, Lee et al., 2011, Lin et al., 2015, Zhang et al., 2005), and in-situ tests (André de Almeida et al., 2011, Faro et al., 2015, Lee et al., 2010, Lee and Randolph, 2011, Li et al., 2014, Suryasentana and Lehane, 2014, Suryasentana and Lehane, 2016, Titi et al., 2000). The p-y curve method is an internationally-used mainstream method of determining the horizontal bearing capacity of pile foundations and is also the most effective method (Suryasentana and Lehane, 2016). The p-y curve method is important in the risk evaluation, design and construction of pile foundations. However, traditional p-y curves are mainly acquired from horizontal loading tests conducted in the field or laboratory (Li et al., 2014). Existing methods of determining the horizontal bearing capacity of pile foundations have certain shortcomings. Field-based horizontal load testing is expensive and consumes much labour and materials. Laboratory-based model tests cause great disturbance to the soil, while centrifuge tests are energy-consuming and unreliable. Due to the low amount of discontinuous data obtained per test, Pressuremeter test (PMT) and Standard penetration test (SPT) methods require many tests in complex soil layers. DMT tests cause low lateral deformation and their predictive accuracy decreases when the load level is relatively high. Numerical methods are restricted by parameter selection, and parameter accuracy depends completely on the accuracy of test results. Therefore, methods of determining the horizontal bearing capacity of pile foundations based on in-situ tests need to be developed to solve these problems (Li et al., 2014, Suryasentana and Lehane, 2016).

A new in-situ testing technology, piezocone penetration tests (CPTu), provide much data and are continuous, fast, economical and highly accurate (Cai et al., 2012, Ching et al., 2014). They do not require sampling, so cause little disturbance to the soil. CPTu are highly reliable in predicting the physical-mechanical properties of in-situ soils. Based on these advantages, CPTu has been widely applied, especially in the vertical bearing capacity design of pile foundations (Cai et al., 2012, Fateh et al., 2017, Niazi and Mayne, 2016). Nevertheless, few scholars have studied the horizontal bearing capacity of pile foundations using CPTu technology. Schnaid and Houlsby (1991) pointed out that the tip resistance from CPTu is controlled by the horizontal effective stress of the soil rather than the vertical effective stress. Lee et al. (2012) pointed out that the resistance to the tip insertion of CPTu is closely related to the horizontal bearing responses of pile foundations. Compared to vertical bearing parameters, tip resistance can reflect horizontal stress parameters more accurately. Therefore, a correlation between CPTu test parameters and horizontal pile bearing design can be constructed to determine the horizontal bearing capacity of pile foundations. This approach can consider the continuous soil layer parameters around the piles and the heterogeneity and stratification of the soil.

In floodplain areas of the Yangtze River, the horizontal bearing characteristics of LSCPs (diameter = 1.0 m, length = 55 m) were analyzed by numerical simulation combined with CPTu technology. According to the Matlock and hyperbolic p-y curve models, a p-y curve analysis method based on CPTu tests is proposed. The accuracy of the proposed method is verified by numerical results and horizontal load test field data. The results provide a reference for the development of engineering design and horizontal bearing theory for LSCPs. They also extend the application of CPTu technology in pile foundation engineering.