Deformation behaviour of granular column reinforced by geosynthetic encasement

Use your smartphone to scan this QR code and download this article ABSTRACT In Vietnam, the overpopulation and strong economic development require the synchronous development of infrastructure such as roads, urban areas, industrial parks, export processing zones, etc. With such requirements, the development of land fund for infrastructure construction is an indispensable need. Meanwhile, the appropriate land fund is very limited. Therefore, the land fund must be developed for areas with little value for agriculture, such as swamps, estuaries, and coastal areas, etc. These areas often have weak geological conditions; hence, to meet the requirements of infrastructure construction on the soft ground, it is necessary to carry out soil improvement to ensure load bearing capacity, total settlement, and consolidation settlement but still ensuring economic effectiveness. Beside several conventionalmethodswidely used for soft soil improvement in order to increase bearing capacity and accelerate consolidation settlement of the ground, geosynthetic reinforced granular column is one of the new methods that has been applied to improving soft ground in designing practice in the recent years due to the many advantages of this method compared with other methods. In this paper, based on the unit cell model, the authors research on deformationbehavior of granular column reinforcedbygeosynthetic encasement through the analytical analysis by varying external loadings corresponding to column diameter, stiffness of geosynthetic encasement. The settlements of a single geosynthetic encased granular column and load bearing capacity of the composite foundation are calculated on geological conditions of Ash Pond Area of Song Hau 1 Thermal Power Plant located in Hau Giang Province. The relationship between settlement and load bearing capacity with external loadings for different column diameters and geosynthetic stiffnesses are shown schematically. Other considerations related to factor of safety are also presented. The future researches are also proposed.


INTRODUCTION
There are many methods that have being used to improve soft soil such as granular column, vertical drain, vacuum preloading, limestone column, soil cement column, concrete column, etc. Recently, scientists have proposed a method for soft soil improvement by geosynthetic encased granular column. This is an extension of the stone column method. This method has advantages over granular column due to geosynthetic granular column derives its loading capacity through two factors including i) passive pressure from the surrounding soft soil owing to bulging of granular column ii) additional lateral confinement by considering the hoop tensile force in the geosynthetic encasement. Furthermore, geosynthetic prevents clogging of the granular aggregate by surrounding soft soil, preserves the drainage and frictional properties of the granular soils as described by Raithel  In this study, the authors use a simple analytical method developed to estimate the settlement and bearing capacity of an individual geosynthetic encased granular column and its composite foundation due to the possible bulging failure mechanism proposed by Murugesan and Rajagopal 2010 7 . The authors investigated the importance of column diameter, tensile stiffness of geosynthetic to the settlement and bearing capacity. The settlement and bearing capacity are calculated based on geological conditions of Ash Pond Area of Song Hau 1 Thermal Power Plant located in Hau Giang province.

METHODOLOGY
In this paper, the authors apply the unit cell model introduced by Raithel and Kempfert 2000 8 . The model is used to calculate settlement and bearing capacity of geosynthetic encased granular column by using analytical solution. The problem is tested in a real project in Viet Nam. The results then are shown and discussed.

ANALYTICAL SOLUTION Settlement
The proposed analytical method by Raithel and Kempfert 2000 8 , for the settlement of a geosynthetic encased granular column reinforced foundation is based on a unit cell model as present in Figure 1. This model considers the contribution of geosynthetic encasement by providing additional confinement to the column 9 . The radial stresses in soil and column are given by: △σ r,s = △σ s K a,s + σ z0,s K a,s The radial stress on the geosynthetic encasement is given by: The radial stress difference between the column and the soil which represents the partial mobilization of the passive earth pressure in the surrounding soft soil is computed by Jie-Han 2015 9 : The radial displacement, ∆rc is computed by: The settlement of the soft soil, Ssl is computed by: The settlement of the column, Scl is computed by: Here: σ z0,c = overburden stress of the column σ z0,s = overburden stress of the soil σ r,c = radial stress in the column by the overburden stress of the column σ r,s = radial stress in the soil by the overburden stress of the column σ r,g = radial stress on the geosynthetic encasement (additional confining stress) K a,c = active earth pressure coefficient in the column K 0,s = at-rest earth pressure coefficient in the soil J = tensile stiffness of geosynthetic encasement h = thickness of the soil or length of the column r g = radius of the geosynthetic encasement r c = radius of the column E*= derive from elastic modulus of the soil The details analytical solution for settlement of the column can be found in Jie-Han 2015 9 .

Bearing capacity
Murugesan and Rajagopal 2010 7 considered an additional confinement by geosynthetic encasement for an ultimate bearing capacity of an individual column due to possible bulging failure with a bulging length of four times the column diameter as follow 9 : And the ultimate bearing capacity of the composite foundation based on the ultimate bearing capacities of the soil and the individual column, and the area replacement ratio using equation is given as follows 9 : The factor of safety can be computed by: Here: σ r,0 = radial soil stress induced by the overburden stress at the middle point of the column bulging length. K p = coefficient of passive earth pressure of the column. c u = undrained shear strength of the soil. q ult,s = ultimate bearing capacity of the surrounding soil. = external applied load a s = area replacement ratio. The details analytical solution for bearing capacity can be found in Jie-Han 2015 9 .

Geological Conditions
A series of calculation was carried out based on material parameters of column presented in Table 1 and soil parameters presented in Table 2 10,11 for Ash Pond Area.

Effect of diameter of column
It can be seen in Figure 2 the stress -settlement response of 0.6 m diameter. The settlement is found to be increasing corresponding to an increase of vertical stress. The settlement at stress of 100 kN/m 2 is found lower than 2 to 5 times correspondingly to vertical stress at 200, 300, 400 kN/m 2 .  Figure 3 shows the relationship between the vertical stresses -factor of safety against bearing failure of the composite foundation of 0.6 m diameter. The factor of safety decreases with increasing the vertical stress, respectively. The factor of safety reduces correspondingly to an increase of the settlement of the column and vice versa. It is found that the factor of safety at 100 kN/m 2 higher than 2, 3, 4 times of factor of safety corresponding to stresses at 200, 300, 400 kN/m 2 . It can be seen in Figure 3 that vertical stress is found lower than 230 kN/m 2 leading to factor of safety higher than 2 times and meeting requirements of the project. As indicated in Figure 4, the settlement of 0.6 m diameter column is found higher than settlement of 0.8 m diameter column from 2 down to 1.6 times and higher than settlement of 1.0 m diameter column from 6.6 down to 3 times. The settlement of 0.8 m diameter column is higher than settlement of 1.0 m diameter column from 3.2 down to 1.9 times.      Figure 6 shows vertical stress -settlement responses of the column for different values of tensile stiffness of 1 m column diameter. As can be seen, stress on column increases with an increase of tensile stiffness of geosynthetic. The hoop stress in geosynthetic increases, it leads to increasing in confining pressures in the column as described by Murugesan and Rajagopal 2006 12 . Hence, the column with higher tensile stiffness will induce larger confining pressures, leading to a stiffer and stronger response of the column 5 .

Effect of tensile stiffness of geosynthetic
As indicated in Figure 6, the settlement of column at 500 kN/m tensile stiffness is higher than that at 1000 kN/m tensile stiffness from 1.16 to 1.2 times and higher at 1500 kN/m tensile stiffness from 1.35 to 1.4 times. The settlement of column at 1000 kN/m tensile stiffness is higher than that at 1500 kN/m tensile stiffness from 1.16 to 1.4 times. The variation of settlement slightly increases with an increase of vertical tress.  Figure 7 shows the relationship between the vertical stresses -factor of safety against bearing failure of the composite foundation for different tensile stiffness.
The factor of safety also shows the similar trend with an increase of tensile stiffness of the column and vice versa. It is shown that the geosynthetic with higher stiffness has a higher bearing capacity of composite foundation. The factor of safety in case of 500 kN/m tensile stiffness is found lower than that of 1000 kN/m and 1500 kN/m tensile stiffness about 1.8 times and 2.6 times, respectively, corresponding to stresses at 100, 200, 300, 400 kN/m 2 . The factor of safety of 1000 kN/m tensile stiffness is found lower than that of 1500 kN/m tensile stiffness about 1.45 times corresponding to stresses of 100, 200, 300, 400 kN/m 2 . It is shown that the column with higher tensile stiffness of geosynthetic will increase the bearing capacity of composite foundation.

CONCLUSIONS
From the analytical results, some main conclusions can be taken: • The settlement increases from 2 to 5 times while factor of safety decreases from 2 to 4 times correspondingly with an increase of vertical stress varying from 200 to 400 kN/m 2 in case of 0.6 m diammeter. • In case of diameter varying from 0.6m up to 1.2m, the settlement of column decreases continuously from maximum of 6.6 times down to 1.6 times, respectively; factor of safety increases from 1.3 to 1.7 times correspondingly with increasing diameter of column. • The variation of settlement slightly decreases from 1.4 to 1.16 times while factor of safety increases from 1.8 to 2.6 times for different values of tensile stiffness from 500 to 1500 kN/m.
• Through this investigation, the importance role of diameter and tensile stiffness of granular column reinforcement by geosynthetic are shown.

FUTURE WORK
• Study on deformation behavior of GEC by analytical method and numerical method. • Study on deformation behavior of GEC by considering the impact of interface area between geosynthetic and soil, geosynthetic and column. • Study on deformation behavior of GEC by considering the effect of geosynthetic reinforcement length. • Study on the effect of internal friction angle and cohesive of column materials to its bearing capacity.