Analysis of non-uniform piles driven into cohesive soils

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Highlights

  • A method based on the CEM has been developed for tapered pile driving into cohesive soils.

  • With tapering pile, top displacement and velocities of piles increase.

  • If drivability of piles is critical, tapered piles are preferable than cylindrical piles of the same length and volume.

Abstract

In this paper, a theoretical method based on cavity expansion method (CEM) in conjunction with wave equation theory is used for analysis of driven tapered piles into cohesive soils. The results show that tapered piles are driven easier than cylindrical piles of the same length and volume. Also, with increasing the pile taper angle, the permanent pile displacement increases, which is significant in pile driving.

Introduction

It is desirable to drive piles into the ground with sufficient safety, least hammer blows, and minimum driving stresses in the pile. Issacs [1] and discovered that upon the hammer impact, a longitudinal wave propagates along the pile length. Smith [2] developed first one-dimensional numerical solution for pile driving based on wave equation theory.

Coyle and Gibson [3] proposed a non-linear expression for dynamic soil resistance during driving using triaxial impact test on soil specimens. Chow [4] and Smith and Chow [5] presented finite element solution for pile driving. Lee et al. [6] also presented a one-dimensional model based on the wave equation theory for pile driving analysis. Mabsout et al. [7] presented an axisymmetric finite element analysis for pile driving. Ghazavi et al. [8] performed a numerical 1-D finite element analysis for driven tapered piles using the wave equation theory in conjunction with elasto-dynamic theory of Novak et al. [9]. Sakr et al. [10] performed laboratory tests on tapered piles in a large pressurized soil chamber to study the behavior of FRP reinforced concrete driven piles with different taper angles. Ghazavi and Tavasoli [11] performed a three-dimensional numerical finite difference analysis on cylindrical, tapered, and partly tapered piles of the same volume material and lengths.

In this study, a wave equation based theory is presented for analysis of tapered piles during driving into cohesive soils, using the CEM.

Section snippets

Analytical method

The relationship between the pile displacement due to hammer impact and the mobilized soil resistance along the pile shaft is derived based on Smith [2] and Lee et al. [6]. For tapered piles subjected to static loading, this relationship was proposed in three phase, by Kodikara and Moore [12]. In the first phase, the pile and the ground are fully connected with sufficient bonding in the elastic range and thus they deform together. In the second phase, the ground still deforms elastically but

Solution scheme

In this research, 1-D finite difference solution of is used for pile-soil-hammer system in which the pile is divided into small segments to consider the wave nature of pile driving loading. Also, a numerical algorithm is used to solve the wave equation governing the pile driving procedure. The velocity of any particular segment in Fig. 1 produces a displacement in the next time interval given by:Wm(T+1)=Wm(T)+(Vm(T)*t)where t is the time interval. Wm(T + 1) is the displacement of segment “m” at

Verification and results

For validation of the presented method, numerical results reported by Mabsout and Tassoulas [19] and Ghazavi and Tavasoli [11] are used. Mabsout and Tassoulas [19] used finite element method and bounding surface plasticity model for isotropic cohesive soil. They studied a cylindrical concrete pile driven into cohesive saturated clay. The pile had 50 cm diameter and 20 m length. They also used a force function introduced by Goble and Hery [20].

Ghazavi and Tavasoli [11] performed finite

Discussion

The main difference between the proposed method and those proposed by Smith [2] and Lee et al. [6] lies on the way of estimating the side resistance of the pile. For cylindrical piles, after mobilizing the entire side resistance, it will remain constant. However, for tapered piles, the side resistance of the pile will increase continuously with further axial deformation [12]. The methods proposed by smith [2] and Lee et al. [7] are not able to consider this ongoing increasing of the side

Conclusions

An analytical method based on the CEM and wave equation theory is presented for predicting the drivability of tapered piles into cohesive soils. The method simulating the behavior of tapered piles during driving shows that with tapering the pile, top displacements and velocities of piles increase compared with cylindrical piles.

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    The one-dimensional wave theory is simple and prevailing theoretical framework for this type of analysis (Choe and Juvkam-Wold, 2002; Rausche et al., 2009; Han et al., 2013; Tavasoli and Ghazavi, 2020a, 2020b; Vaidya et al., 2020). Although simple, the accuracy of wave theory predictions depends significantly on the input parameters, which relay on users' experiences, especially for the damping properties of the soil and pile (So and Ng, 2011; Ghazavi and Tavasoli, 2012; De Chaunac and Holeyman, 2014; Sormeie and Ghazavi, 2018). Various empirical driving equations were proposed for calculating the soil resistance during driving (SRD) (Smith, 2012), including the Gates (1957), modified Engineering News Record (ENR, 1965), Danish (Olson and Flaate, 1967), Janbu (Olson and Flaate, 1967), and Pacific Coast Uniform Building Code (PCUBC) (Bowles, 1996) equations.

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    Tavasoli and Ghazavi (2016) performed experimental and numerical analyses on the drivability of hollow piles with different geometries and proved that driving hollow tapered piles was easier due to better drivability performance and lower energy consumption than concrete filled piles of the same volume and length. By the application of the theoretical method based on cavity expansion method (CEM) in conjunction with the wave equation, Sormeie and Ghazavi (2018) evaluated the behavior of driven tapered piles into cohesive soils indicating that tapered piles were driven easier than cylindrical piles of the same length and volume. Additionally, Tavasoli and Ghazavi (2018) investigated the wave propagation and ground vibrations due to the drivability of non-uniform cross-section piles using field testing and numerical analysis and concluded that the application of such piles decreases the noise pollution, applying of tapered piles considered in practice from the viewpoint of allowable ground vibrations.

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    They reported that in closed or open-ended pile cases, the tapered pile had a final penetration more than a cylindrical pile, and when the end of the pile is open, the pile efficiency is greater than the closed-ended piles. The behavior of driven tapered piles into cohesive soils using the theoretical method based on cavity expansion method (CEM) was assessed by Sormeie and Ghazavi [19] in conjunction with wave equation, indicating that tapered piles are driven easier than cylindrical piles of the same length and volume. Comprehensive studies have been carried out about waves propagation caused by uniform-section piles during driving by the use of laboratory, numerical and analytical methods.

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