X-section Cast-in-place Concrete Research Articles

Numerical modelling of the long-term effects of XCC piling in fine-grained soil

Although the development and utilization of X-section cast-in-place concrete (XCC) pile have been reported for some time, the long-term effects of XCC piling in fine-grained soil, in particular the set-up effects, have received little attention. This paper reports a coupled effective analysis of XCC piling using the dual-stage Eulerian-Lagrangian (DSEL) technique. The pile installation and subsequent soil consolidation was explicitly and consecutively modelled. The generation of excess pore pressure and alteration of stress states during the pile installation were explored. The distribution and magnitude of effective normal stress at pile/soil interface, in particular its evolution with time, were investigated. The influence of set-up effects on the XCC pile shaft resistance was assessed and quantified. It was found out, although the shaft resistances of both XCC and circular pile develops substantially with time, the consolidation in the wake of XCC pile installation can bring in more capacity enhancements and therefore practical benefits. The ultimate shaft resistance of XCC pile is 45% higher than that of the circular pile of the same cross-sectional area. Additionally, practical advice was given on how to optimize of the cross-sectional shape of XCC piles to take full advantage of set-up effects and achieve economical designs.

Numerical analysis of XCC pile group effect and bearing capacity of piled raft foundation

A series of three-dimensional numerical models were established based on the field test to study the interaction of pile-raft-soil system in X-section cast-in-place concrete (XCC) piled raft foundation. The analysis of influence factors of XCC pile group effect was carried out by introducing the interaction factor of piles, and the bearing capacity of XCC piled raft foundation was discussed through comparison with XCC elevated pile foundation and single XCC pile foundation. The computed results shown that the pile location, pile number, pile and soil modulus ratio and pile spacing had a certain effect on the XCC pile group effect. The raft had a negative effect on the bearing capacity of XCC piled raft foundation compared with the foundation without rafts. The ultimate bearing capacity of XCC piled raft foundation was larger than that of XCC elevated pile foundation because the raft and pile of XCC piled raft foundation could provide stronger constraints on the soil between piles and strengthened it.

Experimental Assessment of Performance of XCC Pile in Sand

XCC (X-Section Cast in place Concrete) pile is new type of pile developed on the basis of cast-in-place pile from the conventional circular pile and capable of resisting displacement. In this study, an attempt is made to investigate the performance of XCC Pile under different loading conditions viz., vertical loading, lateral loading and uplift loading. Experimental investigation is carried out on small scale model piles embedded in sand, by changing type of loading and distance between arc to diameter ratio of the pile. The relative density of soil, type of soil and spacing between the piles are kept constant during investigations. Ultimate capacities of piles are compared with those of conventional circular pile with same diameter and length. The results show that XCC pile with arc distance to diameter ratio equal to 0.3 provides higher vertical and lateral capacity to the extent of 45 % and 39 % respectively compared to that of conventional pile. XCC Pile with arc distance to diameter ratio equal to 0.4 provides higher uplift load capacity to the extent 29 % compared to conventional circular pile.

Dynamic Response of Ballastless Track XCC Pile-Raft Foundation under Train Axle Loads

In order to study the dynamic response of ballastless track X-section cast-in-place concrete (XCC) pile-raft foundations under different axle loads, this article presents an experimental study focusing on the dynamical soil stresses and velocity response of a ballastless track XCC pile-raft foundation under vertical cyclic loading. It was found that the dynamic soil stresses on the subgrade surface and the subsoil surface exhibit “ω” and “U” shapes, respectively. The gravel cushion has given an invaluable damping effect to the soil in transmitting velocity to the soil foundation.

Limit lateral resistance of XCC pile group in undrained soil

This paper investigates the issue of laterally loaded X-section cast-in-place concrete (XCC) pile group in undrained soil and, in particular, evaluates limit lateral resistance (LLR) through finite element limit analysis. Four types of XCC pile groups that are commonly used in practice are selected for discussion here. Both the upper- and lower-bound numerical results for the LLR of the XCC pile groups are presented. The influence of loading direction, pile group shape, and XCC pile cross-sectional shape is evaluated with a series of parametric studies. Design equations are subsequently proposed for computing the LLR of four types of XCC pile groups in undrained soil.

Numerical simulation of XCC pile penetration in undrained clay

This paper presents large deformation analysis of X-section Cast-in place Concrete (XCC) pile (a type of non-cylindrical pile) penetration in undrained clay using the Coupled Eulerian-Lagrangian (CEL) numerical technique. The main objective of this study was to investigate the shape effect of XCC pile cross-section on the penetration mechanism, such as the plastic zone around pile shaft and stress developed in the soil. The discrepancy of the stress mechanism between the circular and XCC piles are captured. The results show that the shape effect has insignificant influence on the plastic zone surrounding penetrated XCC pile. The shape effect only influences of the radial stress distribution around a rough XCC pile, while it can be neglected for smooth interface case. In addition, the radial stress, radial stress gradient and vertical stress around the smooth XCC pile shaft in θ=0∘ profile is larger than the one in θ= 45∘ profile. Moreover, the radial stress near the ground surface in θ= 45∘ profile is nearly equal to zero, while the radial stress near the ground surface in θ=0∘ profile sharply increases with the increasing of soil depth. The presented analysis provided a basis for developing design method for XCC pile in the future.

Analytical model for evaluating XCC pile shaft capacity in soft soil by incorporating penetration effects

The derivation of a new analytical model for predicting the soil displacement of X-section cast-in-place concrete (XCC) piles installed vertically into soft clay as well as for predicting limit shaft resistance during loading phases is presented in this paper. The analytical model is formulated by assuming that the XCC pile penetration process is an X-shaped cylindrical cavity expansion process. Based on the theoretical framework of the Strain Path Method (SPM), the strain, displacement and stress induced by X-shaped cavity expansion can be obtained. The proposed analytical model is validated by comparing the degenerate solution of this study with that of conventional circular (cylindrical) cavity expansion theory. Analytical model-based design methods are then proposed for evaluating soil displacement and XCC pile shaft capacity. Theoretical predictions are compared with field test measurements to verify the suitability of the proposed design method. The proposed new analytical model reveals the fundamental penetration mechanics of XCC piles and gives improved design methods for determining XCC pile shaft capacities.

Finite element limit analysis of ultimate lateral pressure of XCC pile in undrained clay

X-section Cast in place Concrete (XCC) pile is a non-cylindrical pile that the cross section not obeying plane rotationally symmetric. In this case, the geometry of cross section as well as the lateral load direction may have influence on the ultimate lateral pressure of XCC pile. This paper presents rigorous upper and lower bound solutions for the ultimate lateral pressure of X-section Cast in place Concrete (XCC) pile in clay through two dimensional plane strain finite element limit analysis (FELA). The geometric effect, lateral load direction and the adhesion factor at the pile-soil interface are selected for parametric study. Subsequently, an empirical closed-form equation of the ultimate lateral pressure factor is proposed for predicting the lateral force acting on a XCC pile in undrained clay. It is found that the XCC pile with relatively small ratio of b/a has larger ultimate lateral pressure than the conventional circular pile under the condition of the same area of cross section.

Vibration velocity of X-section cast-in-place concrete (XCC) pile–raft foundation model for a ballastless track

This paper presents two case studies of the dynamic response of a ballastless track, X-section cast-in-place concrete (XCC) pile–raft (referred to as BTXPR) foundation embedded in sand subsoil. Model tests were conducted at a scale of 1/5 using a 7 m deep box with cross-sectional dimensions of 5 m × 4 m. In one case the box was filled with subsoil consisting of air-dried sand, whereas in the other case the box was filled with saturated sand. The tests involved measurement and analysis of the response in velocity under different applied cyclic load frequencies. It has been shown that the magnitude and variation of vibration velocity in the BTXPR foundation are closely related to the degree of saturation of the subsoil. Due to the existence of pore water in the saturated sand subsoil, the first natural frequency of the BTXPR foundation embedded in saturated sand is 5 Hz lower than that in air-dried sand. In addition, the amplitude of vibration velocity of the BTXPR foundation embedded in the saturated sand is about 9%∼36% higher than that of the air-dried sand. The research results provide an insight into BTXPR foundations with respect to theoretical analysis and calculation.

Complex variable solution for boundary value problem with X-shaped cavity in plane elasticity and its application

A new type of displacement pile, the X-section cast-in-place concrete (XCC) pile, has recently been developed in China. Extensive field tests and laboratory experiments are undertaken to evaluate its performance and quantify the non-uniform deformation effect (NUDE) of the X-shaped cross section during installation. This paper develops a simplified theoretical model that attempts to capture the NUDE. Based on the theory of complex variable plane elasticity, closed-form solutions of the stress and displacement for the X-shaped cavity boundary value problem are given. Subsequently, the analytical solution is used to evaluate the NUDE, the concrete filling index (CFI), and the perimeter reduction coefficient of the XCC pile cross section. The computed results are compared with field test results, showing reasonable agreement. The present simplified theoretical model reveals the deformation mechanism of the X-shaped cavity and facilitates application of the newly developed XCC pile technique in geotechnical engineering.

Experimental and analytical study of X-section cast-in-place concrete pile installation effect

A new type of displacement pile, the X-section cast-in-place concrete (XCC) pile, has been developed recently in China. Extensive field and laboratory experiments are being undertaken in order to evaluate the soil displacements during pile installation. The paper describes laboratory arrangements and results of transparent soil model tests aimed at capturing the non-axisymmetric soil displacement field. Results from model tests using the XCC pile are presented to reveal the complete soil displacement patterns (a) in the horizontal plane, and (b) in vertical planes of symmetry. Based on results from the transparent soil tests, a modified cavity expansion model (MCEM) is proposed to predict the radial displacements in any vertical plane induced by XCC pile installation. The radial displacements predicted by the MCEM are compared with data from the transparent soil test and the field experiment. The results show reasonable agreement for all cases, with the proposed MCEM proving more suitable than the conventional (axisymmetric) CEM for predicting the radial displacements induced by XCC pile installation. It may therefore provide a basis in the future for developing predictions of stress changes and, ultimately, shaft capacity of the XCC pile in comparison to displacement piles of circular cross-section.

An analytical solution for load transfer mechanism of XCC pile foundations

X-section cast-in-place concrete (XCC) piles have been studied and implemented since 2008. To advance this technique, primarily for the design of XCC piles, it is important to describe the load transfer mechanism of XCC pile foundations precluding any straightforward constitutive model. In this work, an analytical solution dependent on the cross-sectional geometry and the vertical shearing mechanism is derived by means of equilibrium analysis to calculate the effective vertical stress in the surrounding soil, the positive/negative skin friction, and the axial force/dragload of an XCC pile. Additionally, the soil stress distribution due to the skin friction and the end bearing of the XCC piles is proposed. These stresses are calibrated using a previously reported large-scale model test and the original Geddes solution, which validates the derivation procedures and formulae obtained.

XCC pile installation effect in soft soil ground: A simplified analytical model

This paper presents a simplified analytical model: the X-shaped cylindrical cavity expansion method (XCEM), which can be used to interpret and predict the displacement, stress, and excess pore pressure caused by the X-section cast in place concrete (XCC) pile installation in soft soil. Analytical solutions for the displacement and strain field are obtained with a streamlined solution, base on the strain path method (SPM). The stress and excess pore pressure can then be subsequently determined with the assumption of the elastic-perfectly plastic soil behavior. A positive agreement was found when the theoretical prediction of the displacement, stress, and excess pores pressure was compared against the field measurements. The proposed XCEM improves the conventional cavity expansion method (CEM) and offers a framework for understanding the non-circular cross-section penetrator problem, which is different from the conventional circular penetrator problem.

Three-dimensional numerical analysis of the stress transfer mechanism of XCC piled raft foundation

An X-section cast-in-place concrete (XCC) piled raft is a new type of foundation that uses piles with X-shaped cross-sections. Compared to a traditional circular cast-in-place concrete (CCC) pile, an XCC pile of the same cross-sectional area has a larger side resistance due to its larger cross-sectional perimeter. Although the pile capacity and load transfer mechanism of the XCC piled raft have been studied, the influence of this X-shaped geometry on the stress transfer between pile and surrounding soil is still not fully understood. To investigate the effects of the cross-sectional geometry, three-dimensional numerical analyses are conducted on an XCC and a traditional CCC piled rafts using the finite element method. The numerical results are verified by a field test of an XCC piled raft. Computed results reveal that lateral soil arching develops to a distance of approximately twice pile diameter surrounding the XCC pile, which can be demonstrated by the rotation of the principal stresses, leading to a non-uniform effective normal stress across a given cross-section and along the pile depth. The magnitude of the unit side resistance that acts on the concave surfaces of the XCC pile is up to twice the magnitude of the unit side resistance that acts on the flat surfaces. For a given applied load, the total side resistance mobilised on the XCC pile is usually larger than the total side resistance of the CCC pile, by a factor that ranges from 0.5 to 10, depending on the pile depth. Therefore, approximately 66% and 46% of the applied load is carried by the XCC and CCC piles respectively, and simultaneously, approximately 45% and 24% of the applied load is taken by the side resistance of the XCC and CCC piles, respectively. The larger effect of the XCC piled raft contributes to the cross-sectional geometry, which results in a larger perimeter and arching effects.

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