Statistical Analysis on the Static Characteristics of the Geosynthetic Encased Stone Column

The geosynthetic encased stone column is made of stone column encased with geosynthetic encasement. The geosynthetic encased stone column is often used for foundation treatment of roadbeds, dams, buildings and other structures. At present, a series of new developments have been made in the researches of bearing capacity, stress concentration ratio and deformation of the geosynthetic encased stone column. This paper statistically analyzes the three important static characteristics of the geosynthetic encased stone column. and large ultimate bearing capacity ratio both occur in the condition of small shear strength of soil. allowable bearing capacity of ESCs is 1.5 to 8.5 times of the stone columns (SCs), and the ultimate bearing capacity of the ESCs is 1.0 9.4 times different conditions different experiments.


Introduction
Geosynthetic encased stone column (ESC) is made by enfolding the stone column (SC) with a geosynthetic encasement. The ESC includes fully encased stone column and partially encased stone column, and the former one with encasement length equaling to the column length, and the latter one with encasement length less than the column length, as shown in Fig. 1. Geosynthetic or geotextile are often chosen to be the material of the encasement.
The encasement could constrain the column deformation, and prevent the gravel from leaking into the surrounding soil. The stiffness of ESC is higher than SC, which induces a larger stress concentration ratio. The smaller deformation and larger stiffness contribute to higher bearing capacity for the ESC.
Currently, the main and effective approach to study the static characteristics of ESCs is to use experiments, including laboratory model tests or in-situ fullscale tests. Experiments in the references from 2007 to 2021 were collected. In the paper, the bearing capacity, stress concentration ratio, and pile deformation of ESCs in these experiments were analyzed.

Static Characteristics
Bearing capacity Allowable bearing capacity and ultimate bearing capacity of the geosynthetic encased stone columns (ESCs) are collected from different model tests and listed in Table 1. The allowable bearing capacity is obtained at the pressure of elastic state tending to plastic state of ground. The ultimate bearing capacity is obtained at the pressure in a failure state of the ground. Table 1 shows that the allowable bearing capacity ratios of the ESCs to unreinforced ground are between 2.5 and 15.11. The ultimate bearing capacity ratios of the ESCs to unreinforced ground are between 1.1 and 34.6. The large allowable bearing capacity ratio and large ultimate bearing capacity ratio both occur in the condition of small shear strength of soil. The allowable bearing capacity of ESCs is 1.5 to 8.5 times that of the stone columns (SCs), and the ultimate bearing capacity of the ESCs is 1.0 to 9.4 times that of the SCs under different conditions for different experiments.  (2017) found that the optimal mat thickness is 0.15 to 0.2 times diameter of foundation, in which the composite ground shows the greatest bearing capacity.  Fig. 2. This indicates that the encasement modulus has important in uence on the limit bearing capacity of ground. Fig. 2 also shows that the bearing capacity differences induced by mat thickness for the larger encasement modulus case are much larger than those induced by embankment load for the smaller encasement modulus case, which suggests that the large modulus may amplify the effects of the other parameters.

Stress concentration ratio
Stress concentration ratio is one of the important factors in the composite ground design, which shows the load shared by piles and soil. The stress concentration ratio (n) is calculated by Equation 1. Mat didn't use in the model tests before 2015, and thus the stress concentration ratio wasn't considered in these references. The stress concentration ratio is studied in experiments with embankment load or mat in recent years. Stress concentration ratio of different geosynthetic encased stone columns (ESCs) composite grounds is shown in Table 2. As only a few experiments considered stress concentration ratio, some FEM results are also involved in Table 2. The stress concentration ratio is obtained at a stable state or at an allowable load. From the table, it can be found that the stress concentration ratio is between 0.48 and 25 for the ESCs composite ground. The larger stress concentration ratio occurs at the smaller shear strength of the surrounding soil. The stress concentration ratio also increases with an increase in encasement modulus. The table also shows that the stress concentration ratio increases with the increase of the embankment load, and the stress concentration ratio is larger for the end bearing pile case compared to the oating pile case.   (1) where, σ vc = load shared by columns; σ vs = load shared by soil.

Radial deformation of piles
Few tests have explored pile deformation due to the di culty of measuring it in the experiment. Pile deformation data of model tests from references are collected and listed in Table 3. The radial strain of the column is calculated from Equation (2).
Where, d = pile diameter; ∆d = variation of pile diameter.
From Table 3, it can be found that the maximum deformation occurs at 1.5 to 4 times diameter depth for the fully encased stone column. As the modulus of the encasement increases, the depth of the maximum deformation downward. For the partially encased stone column, the maximum deformation just occurs under the encasement. In-situ te d=80cm l esc =3d Where, l presents pile length; d presents pile diameter; l esc presents encasement length; s presents pile to pile space; and H d presents embankment height. maybe achieve 4.7 times the latter one. The deformation of the fully encased stone column is much smaller than that of the SC at limit load if the encasement strain difference at the pile failure and at the encasement failure is much large. That is to say, if the encasement still has much strength to develop after the pile fails, the deformation of the ESC is much smaller than the SC. The ESC may fail from upper punching or bending with high stiffness of encasement, at which case the ESC behaves as a rigid pile. When the encasement stiffness is low, the ESC may fail from bulging. However, the increase of the encasement stiffness induces more cost. This suggests that in ESCs composite ground design, the balance between the pile stiffness and cost should be given attention.
The partially encased stone column may fail from bulging under encasement.

Conclusions And Discussion
This paper studies the static characteristics of ESCs, including bearing capacity and deformation. The main conclusions and suggestions are made, as follows, The bearing capacity of ESCs composite ground mainly in uenced by encasement modulus, encasement length, ratio of pile length to diameter, area replacement, soil shear strength and mat thickness. The allowable bearing capacity of the ESCs composite ground is 1.5 to 8.5 times that of the SCs. The limited bearing capacity of the ESCs composite ground is 1.1 to 9.4 times that of the SCs.
The stress concentration ratio of the ESCs composite ground is in uenced by pile stiffness, shear strength of surrounding soil, area replacement ratio and length to diameter ratio. The stress concentration ratio is between 0.48 and 31 for the ESCs composite ground.
If the encasement still has much strength to develop in the state of pile failure, the ESC behaves as rigid pile, which may fail from upper punching or bending. When the encasement stiffness is low, the ESC may fail from bulging. Figure 1 ESC including partially encased stone column and fully encased stone column