Xsection castinplace (referred to as XCC) pile, which is one of new pile types developed by Hohai University, is widely used for pile foundation and pilesupported embankment over soft ground in China. However, little research has been carried out on this new type pile, especially the surrounding soil disturbance under XCC pile installation. This paper presents an analytical solution for estimating the horizontal stress and displacement of surrounding soil of XCC pile after XCC pile installation. The reliability and accuracy of the present solution are verified by comparing them with the field test results. Then, parametric studies, such as outsourcing diameter
Driven castinplace pile, which belongs to displacement pile type, is widely used in China [
As a new immersed tube pile, XCC pile is developed by Hohai University, China [
Figure
Mechanics model: (a) passive limit balance state after the pile mold installation; (b) XCC pile crosssection.
According to the elasticity [
The three stress components in Cartesian coordinates system,
According to the complex variable elasticity [
For obtaining the solution to calculate the stress and displacement distributions, a conformal mapping function is provided to map the outside of the Xshaped cavity in the
(a)
Substituting the conformal mapping function (
The two stress functions
By the complex elasticity [
From the mechanics model in Figure
Additionally, the two stress functions
Thus (
Then, (
Equations (
Equations (
According to the principle of the series expansion, the terms of
Equation (
After the coefficients
Similarly, the coefficient
Normally, the surrounding soil will enter into plastic stage after XCC pile installation and lead to a formation of a plastic zone around the Xshaped cavity wall. Therefore an elastoplastic analysis is necessary. The nonaxisymmetric problem in the original plane can be transformed into axisymmetric problem in the phase plane by the conformal mapping technique. Thus, it can be easily processed in the phase plane for the axisymmetric characteristics. Considering an element at a radial distance
Note that the Tresca yield criteria has the following form
The stress boundary conditions in the phase plane are
Combining (
In the elastic zone, the stress can be written as
At the EP boundary, the stress in the elastic zone should also obey the Tresca yield criteria. Thus, the stress redistribution coefficient in the elastic zone can be expressed as follows by substituting (
At the EP boundary, the stress in the plastic zone should be equal to that in the elastic zone. Therefore, combining (
Substituting the limit cavity pressure
The radius of the plastic zone in the physical plane can be obtained by combining (
According to the above analysis, EP boundary is circle curve with radius equal to
After the soil around the cavity wall enters yield state, the stress in the elastic zone has a redistribution effect and the stress in the elastic zone cannot be calculated by the elastic analysis directly. However, the stress redistribution effect can be considered by introducing a coefficient
Under the undrained condition, the volume change of the Xshaped cavity induced by the XCC pile installation is equivalent to the change in position of the EP boundary. The mathematical relation can be expressed as follows:
The
The stress redistribution factor
The Fourth Yangtze River Bridge’s northline soft soil treatment field is located in Nanjing, China. The total length of the soft ground improvement engineering is 29.0 km. Physicalmechanical properties of soils on site are shown in Table
Physicalmechanical properties of soils on site.
Soil name 









Silt clay  2.00  30.5  18.50  0.3  5.49  0.913  26.4  15.1 
Silt clay  4.60  41.4  17.90  0.3  2.97  1.159  10.8  3.4 
Silt soil  3.40  30.3  18.60  0.3  11.68  0.897  12.7  26.4 
Silt clay  3.70  41.4  17.90  0.3  2.97  1.159  10.8  3.4 
Silt soil  2.00  30.3  18.60  0.3  11.68  0.897  12.7  26.4 
Silt clay  1.60  32.9  18.80  0.3  4.41  0.915  26.4  16.1 
Silt soil  0.30  30.3  18.60  0.3  11.68  0.897  12.7  26.4 
Notes:
Geometry of XCC pile crosssection.
The arrangement of the test equipment and measuring points are shown in Figure
Equipment arrangement and measuring points on site.
The radial stress and displacement at the depth of 3 meters are selected for comparison, which are shown in Figure
Comparison between this study and measured data: (a) radial displacement, (b) radial stress.
Figure
In order to provide engineers and researchers with calculation charts and tables for estimating horizontal stress, displacements, and the radius of the plastic zone induced by XCC pile installation, a parametric study is carried out. The stress, displacement, and the radius of the plastic zone have many influence factors. This paper focus on the factors of outsourcing diameter, open arc distance, open arc angle, the pile hole pressure
From (
The plastic zone radius
Variation of the plastic radius
Based on the three geometric parameters of the XCC pile crosssection and the undrained strength
As shown in Figure
Radial stress and displacement distribution of different outsourcing diameter
Radial stress and displacement distribution of different open arc angle
Radial stress and displacement distribution of different open arc angle
Figure
Radial stress and displacement distribution of different
An analytical solution considering the pile crosssection shape for the horizontal stress and displacement of the soil around the XCC pile after installation is presented in this study. An elastoplastic model for calculating the horizontal stress and displacement is established by complex variables. Some main results can be concluded as follows.
Compared with the data of the field test, it can be seen that the elastoplastic model calculation results on the horizontal stress and displacement of the soil around the XCC pile after installation are in agreement with those of field results. A theoretical method for studying the special shaped piles installation is provided in this paper.
The radius of the plastic zone caused by the XCC pile installation can be calculated conveniently by this study. The
The radial displacement increases with the increasing of outsourcing diameter
The authors wish to thank the National Science Foundation of China (nos. 51278170 and U1134207), Program for Changjiang Scholars and Innovative Research Team in Hohai University (no. IRT1125), and 111 Project (no. B13024) for financial support.