Combined functioning of geotextile as barrier and drainage material in unsaturated. earth retaining structures. Amalesh Jana 1 and Arindam Dey 2

Transcription

1 1 2 3 Combined functioning of geotextile as barrier and drainage material in unsaturated earth retaining structures Amalesh Jana 1 and Arindam Dey Post graduate student, Department of Civil Engineering, Indian Institute of Technology Guwahati, India, -janaaamalesh@gmail.com Assistant Professor, Department of Civil Engineering, Indian Institute of Technology Guwahati, India, -arindamdeyiitg16@gmail.com (Corresponding author) 1

2 Abstract This article presents an insight into the effective functioning of geotextile as a barrier and drainage material when used in earth retaining structures under unsaturated conditions. A numerical model of the infiltration test on a one-dimensional soil column has been simulated and a finite element transient seepage analysis is conducted to determine the progression of saturated wetting front through the clay-geotextile interfaces. The barrier mechanism of a geotextile layer was highlighted in terms of the water accumulation over the soil-geotextile interface, and retardation of wetting front migration beneath the geotextile layer. It was observed that the barrier breakthrough time is substantially affected by the initial volumetric water content of the surrounding soil and the hydraulic characteristics of the geotextile. Rainwater infiltration analysis of a geotextile revealed that the barrier effect assures the geotextile to be functioning as horizontal drainage layer after exceeding the breakthrough suction of the geotextile. Provision of a facing drain channels the surface runoff, as well as the water emanated from the geotextile layers, to the toe drain. Such provisions resulting in an effective drainage system substantially lowers the lateral thrust and improves the stability of the soil wall, which inadvertently enhances the sustainability of the reinforced soil walls with marginal soil backfills Keywords: Geotextile; Reinforced soil wall; Infiltration; Soil column; Suction; Barrier breakthrough; Drainage behavior, Sustainability. 2

3 Introduction In the present era of industrialization and urbanization, sustainable solution for infrastructure development is the primary endeavor of geotechnical researchers. Geosynthetics have proved to provide suitable and cost effective solutions for many geotechnical and geoenvironmental applications such as reinforcement in earth retaining structures, barrier in the waste containment systems, as well as separation and drainage materials in pavement construction. Applications of geosynthetics as reinforcements in earthwork construction lead to improved mechanical properties of soil, thus enhancing the efficacy and life-period of such systems, making them more sustainable. In most cases, cohesionless granular sandy soil is used as backfill material because of their good drainage capacity, high drained frictional strength, and low creep potential. However, the local unavailability of the fill material substantially increases the cost of construction, thus leading to the application of marginal soil as backfill material. The major problem with the marginal soil is its poor drainage behavior, resulting in the generation of excess pore-water pressure within the backfill, when affected by external load or rainwater infiltration. Such circumstances jeopardize the stability of the retention system. In this regard, several studies have reported about the suitability of locally available marginal soils as an effective replacement of the granular backfill. Ingold and Miller [1] performed undrained triaxial test on Kaolin clay samples reinforced by aluminum plates. Excess pore-water pressure generation was observed at the soil-reinforcement interface, leading to the reduction of the interface bond strength. Fabian and Fourie [2] conducted unconsolidated undrained triaxial tests on specimens of fine soil, reinforced by inclusions having varying permeability. Apart from enhancing the strength of the soil, the permeable inclusions minimized the generation of excess pore-water pressure at the soil-reinforcement interface, thus improving the interface bond strength. Geotextiles in earth retention systems serve the dual purpose of reinforcement (by stabilizing the marginal soil backfill) and a 3

4 drainage material (by dissipating the excess pore-water pressure built up in the backfill due to rainfall infiltration) [3-12]. Based on the investigation related to the movement of the saturation front in a prototype geotextile reinforced clay wall, it was reported that geotextile acts as a drainage material only after the occurrence of capillary break, otherwise manifesting the functionality of a barrier [11]. Dual functionality of geotextile as reinforcement and drainage material in earth retaining structures, using locally available marginal soil, reduces the overall cost of backfilling and construction cost of such structures, thus leading to the development and application of sustainable retention systems. The hydraulic characteristics of geotextiles are commonly investigated by including them in a soil medium with prevailing unsaturated condition. Consequently, it becomes important to understand the unsaturated characteristics of the geotextile. Soil-geotextile interface acts as a capillary barrier in an unsaturated condition [13], resulting in additional moisture retention above the geotextile layer [14]. Based on the experimental investigation of water infiltration through an unsaturated one-dimensional sand column having horizontal geotextile inclusion, it was observed that the geotextile forms an efficient drainage mechanism [15], thus delaying the advancement of the wetting front [17]. Small-scale tests on rainwater infiltration through a geotextile-reinforced embankment exhibited water accumulation at the soil-geotextile interface, leading to local failure [16]. Migration of saturated wetting front through a geotextile inclusion was observed upon the occurrence of capillary break at the unsaturated soil-geosynthetic interface, upon attaining a near-saturation stage [18, 19]. The relationship between the volumetric water content and negative porewater pressure (suction) for an unsaturated soil is defined by the soil-water characteristic curve (SWCC). Similarly, geotextile-water characteristic curve (GWCC) defined the relationship for an unsaturated geotextile. It has been observed that the hydraulic conductivity of unsaturated soil decreases with increase in suction. The function of nonwoven geotextile, 4

5 be it barrier or drainage, depends on the magnitude of suction of the soil at the soil-geotextile interface. Depending upon the degree of saturation during progressive infiltration, the geotextile layers might not exhibit their intended function as a drainage material. Geotextile-reinforced earth retention systems should be thoroughly investigated for their performance when subjected to the rainfall infiltration. In this regard, this article reports the extent of performance and functionality of a geotextile layer as a barrier or a drainage material, when used in unsaturated backfill of reinforced soil wall. Finite element numerical model for transient seepage has been developed, using the SEEP/W module of GeoStudio [20], to provide an insight into the water flow characteristics and distribution of pore-water pressure throughout the soil-geotextile system. The barrier effect of the geotextile has been studied by investigating a numerical model representing a one-dimensional cylindrical soil column. Further, the effectiveness of geotextile as drainage material was investigated for a reinforced soil wall under unsaturated conditions. Stability of the soil wall is also examined using limit equilibrium method adopting the Slope/W module of GeoStudio [30] TRANSIENT SEEPAGE THROUGH A ONE-DIMENSIONAL SOIL COLUMN Transient seepage through a one-dimensional soil column was analyzed using GeoStudio SEEP/W [20], a finite element CAD program used for the mathematical simulation of the physical process involved with groundwater seepage, pore-water dissipation, steady-state confined and unconfined flow, transient flow, 2D flow in cross-section or in plan, and 3D axisymmetric flow through porous medium, such as soil or rock. In addition to the traditional saturated analysis, the saturated/unsaturated modeling allows analyzing seepage as a function of time and considers the processes such as the infiltration of precipitation. In SEEP/W, the basic stages of analysis comprises of discretization of the soil domain into finite elements, assigning the corresponding material properties and specifying proper boundary conditions. 5

6 The soil properties are mainly provided in terms of the hydraulic conductivity as a function of suction in the unsaturated regions; the rate of change of water content is dependent on the rate of change of pore-water pressure during transient processes. Various types of boundary conditions can be provided in the form of total head, pressure head, flux specified as constant or a function of time, and as transient flux as a function of the computed head. SEEP/W simulates the flow of water that follows Darcy s Law. The primary objective of finite element formulation in SEEP/W is to compute total head at each node of the soil domain. In SEEP/W, an incremental time sequence is used to perform transient seepage analysis, where each time step analysis represents localized steady-state analysis. The adaptive time stepping procedure ensures the use of optimal time steps during the transient analysis and sudden change in boundary conditions. The nonlinear finite element equations are handled by convergent radial search iterative scheme. Once the analysis is complete, various types of output can be visualized in the form of contours of head, pressure, gradient, velocity and conductivity. Velocity vectors can show the flow direction and rate. It is also possible to exhibit the transient condition by fluctuating water table over time. Complete information at any desired location can be obtained from the nodes, Gauss points or flux sections Numerical modeling The transient water flow in a soil-geotextile medium was numerically simulated to check the effectiveness of geotextile as the barrier material. Laboratory investigation of the onedimensional flow of water in a cylindrical tank is commonly conducted to measure the breakthrough time in a soil-geotextile medium [19, 21, 22]. However, such experiments are extremely rigorous and require substantial instrumentation (sensors and pore-pressure transducers) to get a complete picture of the wetting front movement through the material. For geotextiles placed within very fine soil, the breakthrough time increases significantly, 6

7 requiring several days (or months) to complete the wetting front progression. In this regard, a well-calibrated numerical model is suitable in providing the insight to the behavior of water flow through an unsaturated soil-geotextile medium. The capillary behavior of geotextile was investigated by modeling the onedimensional flow of water through a homogeneous clay column of height 45 cm and width 20 cm (Fig. 1). The soil domain was discretized by four-noded quadrilateral element, forming a mesh grid with an average element size of m. To maintain the one-dimensional flow of water, the lateral boundaries of the column was modeled to be impermeable. The base of the soil column was also modeled as an impermeable boundary, allowing accumulation of seepage water. A second soil column model consisted of a geotextile inserted at a depth of 30 cm (Fig. 1b), modeled as an interface element of thickness 3 mm, in accordance with the published data [9-11] Material Properties In unsaturated condition, suction governs the water flow characteristics in the system. Matric suction is defined as the difference between the atmospheric-air and the pore-water pressures. The volume of water stored within the voids of soil depends on the matric suction. The relationship between suction and water content of materials (soil or geotextile) is an important parameter necessary to conduct a transient seepage analyses. In saturated condition, all the voids in the soil skeleton remain filled with water, which, upon drying, is gradually replaced by air. Such a phenomenon results in a reduction of the hydraulic conductivity of the geomaterials. In this study, water characteristics curves were generated using the van Genuchten model (Eq. 1) [23], where the various model-fitting parameters were considered from the available literature. The hydraulic properties of soil and geotextile are shown in Table 1, and 7

8 the water characteristics curves are shown in Fig. 2a. The hydraulic conductivity curves for soil and geotextile is shown in Fig. 2b. From the water characteristics curves, it can be seen that the water-entry value of geotextile is quite less as compared to that of the soil material. The water-entry suction value of geotextile is approximately 1 (one) kpa. The hydraulic conductivity curves of the soil and geotextile indicate that the geotextile will remain impermeable as long as the suction value at the interface of the geotextile does not reach 1 kpa. The hydraulic conductivity of the geotextile is less than the fine clay material for suction values more than 1 kpa. The van Genuchten model [23] is given by 170 s r [1 ( ) ] w r n m (1) 171 k w ( n 1) n m 2 [1 ( ) {1 ( ) } ] k n m/2 (2) s [1 ( ) ] n 1 m (3) 173 where, ks is the saturated hydraulic conductivity, kw is the hydraulic conductivity at a particular suction value, w is the volumetric water content, s is saturated water content, r is residual water content, is negative pore water pressure, α, n and m are the model fitting parameters Boundary Conditions The topmost free boundary of the soil column was modeled using very small thickness (4 mm) surface elements, which was subjected to constant rate of water infiltration (unit flux 2.06x10-7 m/hr), the flow rate being less than the saturated hydraulic conductivity of the soil. Ponding of surface water was not allowed on top of the soil column. When the pore-water pressure at the top of soil column exceeded 0 (zero) kpa (i.e. at the commencement of developing positive pore water pressure), the boundary condition was reset to the elevation 8

9 head, for the subsequent estimation of the flux. The excess flux was lost as runoff to simulate the removal of accumulated excess water from the top of the soil surface, as observed in the real physical model of one-dimensional soil column test. The initial water content of the soil material significantly affects the unsaturated flow of water. The initial volumetric water content of clay was considered 12%. The geotextile was considered to be in an initial dry state. In the numerical model, an initial suction of 120 kpa (corresponding to 12% volumetric water content) (Fig. 2a) was chosen for clay soil before providing the water infiltration. The residual water content of geotextile is extremely low at the residual suction considered in the range of kpa [21]. The water content of geotextile is even lower when the suction is more than 10 kpa [21, 31, 32]. Hence, in this study, the initial suction of the geotextile was assumed 10 kpa, indicating a state of suction having water content lower than its residual state (Fig. 2a) Methodology The governing Richards equation, derived from Darcy s law of flow and continuity, is utilized to simulate transient water flow within the unsaturated soil [24]. This is expressed as h h w h kx k 2 y m 2 w w x y t t [4] 202 where h is the total hydraulic head, k x and k y are the hydraulic conductivities in unsaturated 203 condition, mw is the coefficient of water volume change, and w is the volumetric water content. The parameters k and are dependent on suction, obtained from the water characteristic curve (Fig. 2). The finite element seepage analysis is numerically solved in SEEP/W, where seepage follows Darcy s equation, and is represented in a matrix form as k H Q [5] 9

10 where, [ k] is a matrix of the hydraulic conductivity which is determined from the water characteristics curve (Fig. 2), { H } is then vector of the total hydraulic heads at the nodes represented by the intersecting flow and equipotential lines, and { Q} is the vector of the flow quantities at the node. The total hydraulic head at each node in a seepage analysis is computed relative to the specified values of H at some nodes, and/or based on specified values of Q at the other appropriate nodes Results and Inferences from Transient Seepage Analysis Finite element based transient seepage analysis was conducted for 200 hours. The time step of each analysis was considered 15 min. The resulting flow of water through the soil medium (at 200 hr) is depicted in Fig. 3. From Fig. 3a (soil column without any geotextile), it can be observed that percolating water crossed the 30 cm depth without any alteration to the flow characteristics. On the other hand, for the same material characteristics and inflow boundary conditions, Fig. 3b (soil column with geotextile) reveals that the progress of the wetting front is curbed at the soil-geotextile interface, leading to the accumulation of water over the geotextile layer. For both the models, it can be observed that there was no flow through the lateral and bottom boundaries, thus verifying the one-dimensional flow of water, resembling the physical movement as observed in experimental investigations. Four time-history nodes were selected at specific locations of the model in order to determine the variation in water content and pore-water pressure (Fig. 3a). The initial volumetric water content of the soil medium was maintained at a low value of 0.12, so that the breakthrough phenomenon in the geotextile is avoided, and the soil below the geotextile layer remains unsaturated all the time. The variation of volumetric water content with respect to time is depicted in Fig. 4. It can be observed that the wetting front migrated downwards with the progress in infiltration time. The migration front first reaches to Node 1, and 10

11 subsequently to the other nodes located at larger depths, exhibited by the progressive increase in the water content at each of the nodes. It can be observed that the presence of geotextile hinders the water movement at Node 4 (13 cm from bottom), exhibited by the unchanged volumetric water content (Fig. 4b). It can be noticed that the increase in water content at the nodes located at lesser depths is different for the two cases, exhibiting gradual accumulation of water above the geotextile layer for the latter. Moreover, in the presence of the geotextile layer, the time taken to achieve full saturation at Nodes 1, 2, and 3 is lesser than the former case (soil column without the geotextile). The reason is attributed to the fact that in the latter case (soil column with geotextile), the water accumulation nitiates from the geotextile layer, while in the former case (soil column without geotextile), the same is initiated from the bottom boundary of the soil model. The variation of water content was also determined along depth of the soil column, as shown in Fig. 5. Figure 5a shows that at each depth, the migration of the wetting front leads to the rise of water content with time. At 144 hr, the wetting front just reaches the bottom of the soil column, while at 168 hr, water accumulates in the entire soil column, exhibited by the constant volumetric water content through the height of the soil column. At 192 hr, the excess water forms surface runoff leading to a slight reduction in the water content at the extreme upper regions of the soil column. Fig. 5b clearly indicates that the water movement is hindered below the geotextile layer, marked by unaltered volumetric water content throughout the infiltration process. The variation of pore water pressure (Fig. 6) exhibits gradual rise of pore-pressure and progressive decrease in suction at different sections of the soil column. In presence of the geotextile layer, the suction value beneath it remains unaltered at the initial suction of 120 kpa (Fig. 6b). The saturated hydraulic conductivity of the geotextile is greater than the surrounding soil (Table 1). However, under unsaturated conditions, increase in suction value beyond 1 kpa leads to lowering of the hydraulic 11

12 conductivity of the geotextile, significantly lesser than the overlying soil medium, thus hindering the migration of the wetting front. These observations portray the barrier action of the geotextile layer in an unsaturated soil medium. Natural soils may consist of high initial volumetric water content. Figure 7 highlights the effect of high initial volumetric water content (23%) of the clay soil on the hydraulic response of the soil-geotextile system. At higher water content, the suction value of the soil is lesser, thus portraying high hydraulic conductivity. Compared to Fig. 4a, it can be noted from Fig. 7a that the rate of progression of the saturated wetting front through the unreinforced soil column is faster when the initial water content of the soil is high. Compared to Fig. 4b, Fig. 7b illustrates that the hydraulic behavior of the geotextile-reinforced soil column is different. As earlier, it can be observed that the geotextile layer hindered the movement of the wetting front up to a specific time instant indicated by the rise in water content at Node 3 due to water accumulation over the geotextile layer. However, with the passage of time, as the suction value at the interface attains the water-entry value of the geotextile, barrier breakthrough takes place leading to ingress of water from the overlying soil to and through the geotextile layer. As a result, the water content at Node 4 suddenly increases with the simultaneous reduction in the water content at the other nodes located at lesser depths. The barrier action of the geotextile is diminished after breakthrough. With the passage of time, the water content at all the locations progressively increases owing to the water accumulation at the impermeable bottom of the soil column. This observation indicates that the barrier action of geotextile is significantly diminished if the overlying soil possesses sufficiently high water content. It is also important to investigate the effect of type of geotextiles on the hydraulic response of the soil-geotextile system. Six different types of geotextiles are considered for the present study. Table 2 lists the water characteristics parameters, while the hydraulic properties of all the six geotextiles are shown in Fig. 8 (developed based on van Genuchten 12

13 model [23]). The initial volumetric water content of the soil was considered 23%. Figure 9 depicts the variation of the volumetric water content as an outcome of the usage of six different varieties of geotextiles. The geotextile layer is found to act as a barrier for all the cases, although with different characteristics. Barrier breakthrough was not observed to occur for GT-2 and GT-5 due to their significantly low water entry value (Fig. 8). On the other hand, GT-1, GT-3, and GT-4 distinctly hindered the water movement before the barrier breakthrough mechanism became predominant. As the water entry suction for GT-6 is significantly high (Fig. 8a), the breakthrough occurred rapidly at a very early stage. Hence, from this analysis, it can be stated that the efficacy of barrier action of the geotextile primarily depends on its water-entry suction value GEOTEXTILE AS DRAINAGE MATERIAL IN EARTH RETAINING WALLS The unsaturated flow characteristics of geotextile revealed that it acts as a drainage material if the suction value of the surrounding soil reaches the water entry value of the geotextile. Due to their porous structural formation, the saturated hydraulic conductivity of the geotextile is sufficiently high to facilitate drainage. In this study, transient seepage analysis has been conducted for a reinforced soil wall with sandy clay loam (marginal soil backfill), subjected to rainfall infiltration, is investigated to check the effective performance of geotextile as a drainage material Seepage Analysis Reinforced soil wall of height 4 m comprises of a marginal soil backfill having initial volumetric water content (Fig. 10). For the transient seepage analysis, rainwater precipitation was provided at the top free surface in the form of the constant uniform flux of 12.7 mm/hr. The wall facing comprises of a vertical drain to dissipate the excess runoff (generated from 13

14 the inclined top free surface), as well as provide facing connection to the horizontally placed reinforcing GT1 geotextiles. A toe drain is used at the bottom of the model to establish the hydraulic gradient required for draining generated from the top free boundary. The unsaturated hydraulic parameters of the marginal backfill soil and vertical facing drain are given in Table 3, and the water characteristics curves of the materials, as per [28], are depicted in Fig Results and Inferences from Seepage Analysis As an outcome of the seepage analysis, Fig. 12 illustrates the pore-water pressure distribution and wetting front movement within the reinforced soil wall. It can be seen that water starts emanating from the toe drain at the 10 th hour, thus indicating proper functionality of the 319 facing drain. At the 33 rd hour, the water-flow vectors through the first geotextile layer indicated the commencement of its functioning as drainage material. It can be seen that the soil overlying the first layer of geotextile is saturated, while the underlying soil remained unsaturated. The wetting front could not migrate in the downward vertical direction since the suction of the soil beneath the geotextile was significantly high, indicating low hydraulic conductivity of the underlying soil. As a result, the first layer of geotextile acted as the horizontal drainage medium, transmitting the accumulated water to the facing drain. The high hydraulic conductivity of the facing drain allow easy passage of the accumulated water to the toe drain. Figure 13 depicts four time-history nodes, selected near the soil-geotextile interface, to monitor the performance of the geotextile as drainage material. The water content and pore water pressure variation at these four nodes are depicted in Fig. 14. The water content of the soil overlying the first geotextile layer gradually increased from the 30 th hour. When the wetting front reaches the topmost geotextile layer, its relative impermeability resulted in the 14

15 rise in the water content at the nodes located above it. Once the suction attains the water entry value, the geotextile functioned as drainage medium, resulting in the decrease in the volumetric water content at Nodes A and B. The water content and suction value beneath the first layer of geotextile continued to remain the same as in its initial unsaturated state. Thus, the geotextile layer inhibited the moisture movement in the vertically downward direction, while facilitating the water to flow in the horizontal direction towards the facing drain. Such mechanism helped the soil wall to preserve a higher suction, thus maintaining a higher strength. The seepage analysis was also conducted for unreinforced and reinforced soil wall considering higher initial volumetric water content of 0.23 for the marginal soil backfill. Figure 15 exhibits the migration of saturated front due to rainwater infiltration through the unreinforced soil wall. It is noted that the wetting front progresses uniformly downward with the passage of time, clearly demarcating the saturated and unsaturated zones within the backfill of the wall. It is observed that it takes more than 200 hours to attain complete saturation of the entire soil wall. Figure 16 reveals the movement of saturated wetting front through the geotextile-reinforced soil wall. It can be observed that the presence of multiple geotextile layers, acting as horizontal drains, substantially alters the wetting front migration, and results in the significant of the pore-pressure within the reinforced backfill by facilitating the water into the vertical drain. It can be observed that in comparison to unreinforced backfill, a larger volume of the reinforced backfill remain unsaturated. Fig. 16(e) shows that complete saturation of the reinforced soil wall takes place in significantly long time (more than 350 h), thus exhibiting the efficacy of the draining geotextile layer in prolonging the stability and sustainability of the reinforced soil wall. The water content and pore-water pressure variation at the top and bottom of the topmost geotextile is shown in Fig. 17. The sharp rise in the volumetric water content at nodes A and B indicates initial accumulation of 15

16 water above the geotextile, while the nodes below (C and D) remained unsaturated. With further time, the geotextile acted as a drainage material, leading to the reduction of water content at nodes A and B, which eventually becomes constant with time. Due to high initial volumetric content of the soil, beyond a specific time, barrier breakthrough occurred in the geotextile layer when the soil suction reached its water-entry value (Fig. 17b), thus leading to the increase in water content at nodes C and D (Fig. 17a). Overall, the migration of saturation front through the reinforced soil wall is progressive. As the first geotextile layer prevented the migration of saturated wetting front towards the underlying soil through its barrier action, water accumulated over the same. During this time, the geotextile drained the water in the horizontal direction to the facing drain. If the reduced suction, due to water accumulation, reaches the water-entry value, the first geotextile layer might undergo the barrier breakthrough mechanism, allowing further migration of the saturated front. In that case, the second geotextile layer begins to exhibit its functioning as a barrier towards moisture movement, while the first geotextile layer continues to act as a drainage layer. In this manner, the downward progression of the saturated wetting front takes place through the geotextile layers, when each layer of geotextile sequentially acts as a barrier and a drainage material. This dual function of geotextile leads to the enhanced sustainability of reinforced soil walls. Moreover, the high hydraulic conductivity of the facing drain prevented water accumulation, thus ascertaining no backflow of the water from the face of the wall into the marginal soil backfill. Such backflow of water into the geotextilereinforced soil wall from the saturated facing is reported in various literatures [25, 29]. In the absence of any facing drain, the geotextile layer facilitates the water to ingress from saturated facing to the backfill. Hence, provision of a facing drain further enhances the sustainability of the reinforced soil wall

17 Mechanical Behavior of Reinforced Soil Wall The stability of the geotextile-reinforced soil wall is investigated considering transient infiltration of the rainwater and functioning of the geotextile as a drainage material. Infiltration induced pore-water pressure is used in GeoStudio SLOPE/W module [30], utilizing limit equilibrium method to investigate the effective transient stability of the reinforced soil wall. The factor of safety has been determined using the Morgenstern-Price method [33], which satisfies both the force and moment as well as force equilibrium of the slices with a half-sine interslice force function [34]. The unsaturated strength of the soil is obtained using Eq. 6 proposed by Fredlund et al. [35]. 392 ( ) tan ( ) tan b (6) ' ' c ua ua uw where, is the shear strength; is the total normal stress, ' c is the effective cohesion, ua is ' the pore air pressure, u w is the pore water pressure, is the friction angle which depends on the change of ( u a ) when ( ua uw) is constant, and on the change of ( u u ) when ( ) is constant. The value of a w u a b is the friction angle which depends b is equal to ' tan [36], where, is the normalized water content which is given by van Genuchten model [23] as w r (7) s r In absence of specific shear strength parameter reported for the soil, representative values for sandy clay loam soil is taken from published literature [37, 38]. The considered value of average frictional angle of the soil is 30, and the effective cohesion is 15 kpa. The unit weight of the soil is taken to be 18 kn/m 3. In SLOPE/W, the geotextile is modeled as a 17

18 fabric material. The ultimate tensile capacity of geotextile (TGX) is considered 15 kn/m, which is within the range as reported in various studies [9, 11, 25, 39, 40]. The stability assessment of the reinforced soil wall is conducted for two conditions, namely considering and neglecting the drainage characteristics of the geotextile. To achieve this, transient seepage analysis is conducted for 240 hrs Results and Inferences from Stability Analysis Figure 18 illustrates the influence of drainage behavior of geotextile on the stability of reinforced soil wall. It is observed that beyond the barrier breakthrough time, the Factor of Safety (FoS) increases when the drainage function of the geotextile is considered. This is attributed to the fact that the geotextile layers drain out the excess pore-pressure generated due to the migration of saturation front, thus resulting in increased effective strength of the soil wall. The lateral earth pressure is computed near the facing of the soil wall, neglecting the reinforcement of the geotextile. Figure 19 highlights the influence of the drainage action of geotextile on the distribution of earth pressure under unsaturated condition. The lateral earth pressure, considering the unsaturated condition, is computed as per the model proposed by Fredlund and Rahardjo [41], which is given as 422 E ( h ua) ( v ua) (1 ) ( ua uw) 1 H [8] where, is Poisson s ratio, v is total normal stress in vertical direction, h is total normal stress in horizontal direction, E is Elastic modulus with respect to change in ( u a ), and H is elastic modulus with respect to change in ( u u ). a w In the present case, the Poisson ratio for sandy clay loam is adopted as 0.3, and the ratio of EHis assumed 0.17 [41]. Figure 19(a) shows that the lateral earth pressure distribution at 18

19 the 144 th hour, as conventionally calculated, without considering the effect of suction in unsaturated soil. Beneath the zone of saturation, the backfill is considered in dry state in the presence of drainage action of geotextile; while, the entire backfill remains in saturated state, when the drainage action is ineffective. Correspondingly, as shown in Fig. 19(b), consideration of suction (initial volumetric water content = 0.2) clearly represents the significant reduction of lateral pressure below the zone of saturation. Moreover, the effect of the drainage mechanism of the top two geotextile layers is reflected by significantly low values of earth pressure till a depth of 3 m, as compared to the same obtained when the drainage function is ineffective. Even in the case of higher initial volumetric content (0.23) of the soil, Figure 19(c) exhibits that the drainage action of the geotextile is effective in the partial reduction of the lateral earth pressure on the facing. Overall, the presence of geotextile layers as a drainage material leads to the reduction of lateral stress on the facing and enhancement of strength of the soil due to suction. These phenomena lead to the enhanced sustainability of the reinforced soil wall with marginal soil backfill CONCLUSIONS This article reports about the dual functionality of embedded geotextile (barrier and drainage) and their influence on the performance of reinforced soil walls. The influence of geotextile as a barrier material was investigated through transient seepage analysis of one-dimensional soil column subjected to rainwater infiltration. The influence of drainage characteristic of geotextile on the seepage and mechanical behavior of unsaturated reinforced soil wall, backfilled with marginal fill soil, was also investigated. Based on the present study, the following important conclusions are drawn: 1. It is understood that geotextile layers act as barriers to the saturated wetting front movement, leading to the accumulation of water in the soil overlying the 19

20 corresponding layer, until the breakthrough is attained. Under this condition, the underlying soil remains in an unsaturated state, exhibiting enhanced strength of the reinforced soil wall. 2. The sustainability of reinforced soil wall can be further enhanced by choosing marginal soil backfill with low initial volumetric water content, such that the breakthrough mechanism, and hence the migration of saturation front is further delayed. The barrier breakthrough mechanism occurs as the suction value at the soilgeotextile interface reaches the water-entry suction of the geotextile. Hence, choosing a geotextile having very low water-entry suction value substantially enhances the performance the long-term sustainable performance of the wall fill. 3. As the suction of the overlying soil attain the water-entry suction of the geotextile, the latter starts functioning as a drainage material by facilitating the movement of water in the direction of favorable hydraulic gradient. 4. Provision of an effective facing drain in reinforced soil walls substantially enhances the drainage characteristics of the geotextile, as well as prevents any backflow of water into the backfill. In this process, the rate of migration of wetting front to the depths of the soil wall is considerably diminished, thus prolonging the period of sustainable performance of the reinforced soil wall. 5. Presence of multiple geotextile layers in the backfill of the soil wall, and the alternate functioning of the successive geotextile layers as barrier and drainage, further results in delayed migration of the saturated water front. Such mechanism helps maintaining the suction characteristics of the reinforced soil wall for a longer duration, and thus enhances the long-term stability of the retention structure. 6. The combined functioning of facing drain and geotextile as drainage material helps in significant dissipation of infiltration induced pore-water pressure in the backfill soil. 20

21 This phenomenon helps in maintaining high suction in the backfill which substantially reduces the lateral earth pressure at higher depths of the soil wall, and the same is maintained for considerably larger durations due to the delayed migration of wetting front. Overall, the presence of multiple layers of geotextile acting as drainage material leads to the substantial enhancement of the sustainability of unsaturated reinforced soil walls with marginal soil backfill REFERENCES 1. Ingold TS (1983) Reinforced clay subject to undrained triaxial loading. Journal of Geotechnical Engineering ASCE 109(5): Fabian KJ, Fourie AB (1986) Performance of geotextile reinforced clay samples in undrained triaxial test. Geotextiles and Geomembranes 4(1): Christopher BR (1993) Deformation Response and Wall Stiffness in Relation to Reinforced Soil Design. PhD Thesis, Purdue University 4. Allen TM, Bathurst RJ, Berg RR (2002) Global level of safety and performance of geosynthetic walls: A historical perspective. Geosynthetics International 9(5 6): Zornberg JG, Arriaga F (2003) Strain distribution within geosynthetic-reinforced slopes. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 129(1): Tatsuoka F, Yamauchi H (1986) A reinforcing method for step clay using a nonwoven geotextile. Geotextiles and Geomembranes 4:

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23 Garcia EF, Gallage CPK, Uchimura T (2007) Function of permeable geosynthetics in unsaturated embankments subjected to rainfall infiltration. Geosynthetics International 14(2): Bathurst RJ, Siemens G, Ho AF (2009) Experimental investigation of infiltration ponding in one-dimensional sand-geotextile columns. Geosynthetics International 16(3): Bouazza A, Zornberg JG, McCartney JS, Singh RM (2013) Unsaturated geotechnics applied to geoenvironmental engineering problems involving geosynthetics. Engineering Geology 165(1): Zornberg JG, Azevedo MM, Pickles CB (2017) Evaluation of the development of capillary barriers at the interface between fine-grained soils and nonwoven geotextiles. In Geosynthetics, Forging a Path to Bona Fide Engineering Materials Geo-Studio Manual (2012) Seepage Modeling with SEEP/W, Geo-Slope International Ltd, Alberta 21. Bouazza A, Zornberg JG, McCartney JS, Nahlawi H (2006) Significance of unsaturated behaviour of geotextiles in earthen structures. Australian Geomechanics Journal 41(3): Zornberg JG, Bouazza A, McCartney JS (2010) Geosynthetic capillary barriers: Current state of knowledge. Geosynthetics International 17(5): van Genuchten MTh (1980) A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Science Society of America 44: Richards LA (1931) Capillary conduction of liquids through porous mediums. Journal of Applied Physics 1:

24 Iryo T, Rowe RK (2005) Infiltration into an embankment reinforced by nonwoven geotextiles. Canadian Geotechnical Journal 42(4): Stormont JC, Morris CE (2000) Characterization of unsaturated nonwoven geotextiles. Proceedings of Advances in Unsaturated Geotechnics, ASCE Stormont JC, Ramos RD (2004) Characterization of a fibreglass geotextile for unsaturated in-plane water transport. Geotechnical Testing Journal 27(2): Schaarp MG, Leij FJ (1998) Database-related accuracy and uncertainty of pedotransfer functions. Soil Science 163(10): Huang CC, Wu JTF, Huang BN, Leu AL, Jean GY (2003) Large-scale reinforced clay walls backfilled with clay at Cheng Kung University. In Reinforced Soil Engineering: Advances in Research and Practice Geo-Studio Manual (2012) Stability modeling with SLOPE/W. Geo-Slope International Ltd, Alberta 31. Nahlawi H, Bouazza A, Kodikara J (2007) Characterization of geotextiles water retention using a modified capillary pressure cell. Geotextiles and Geomembranes 25(3): Bouazza A, Freund M, Nahlawi H (2006) Water retention of nonwoven polyester geotextiles. Polymer Testing 25(8): Morgenstern NR, Price VE (1965) The analysis of the stability of general slip surfaces. Geotechnique 15: Krahn J, Price VE, Morgenstern NR (1971) Slope stability computer program for Morgenstern-Price method of analysis. User's Manual No. 14, University of Alberta, Edmonton, Canada 35. Fredlund DG, Morgenstern NR, Widger RA (1978) The shear strength of unsaturated soils. Canadian Geotechnical Journal 15(3):

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26 588 Table 1 Water characteristics parameters for soil and geotextile (Adopted from [21]) Material a (1/kPa) n Saturated water content s Residual water content r Saturated hydraulic conductivity (m/hr) Clay x10-3 Geotextile (GT-1) Table 2 Water characteristics parameters for different geotextiles Material α (1/kPa) n Saturated water content s Residual water content r Saturated hydraulic conductivity (m/hr) GT-2 [19] GT-3 [25] GT-4 [15] GT-5 [26] GT-6 [27] Table 3 Water characteristics parameters for marginal backfill soil and facing drain Material α (1/kPa) n Saturated water content s Residual water content r Saturated hydraulic conductivity (m/hr) Sandy clay loam [28] Drainage material

27 Fig. 1 One-dimensional finite element model of soil column (a) Without geotextile (b) With geotextile located at a depth of 30 cm Fig. 2 Hydraulic characteristics of soil and geotextile GT1 (a) Water retention curves (b) Hydraulic conductivity curves 27

28 Fig. 1 One dimensional water flow through soil column (a) without geotextile (b) with geotextile located at a depth of 30 cm 28

29 Fig. 2 Variation of volumetric water content with time at the selected time-history nodes (a) Without geotextile (b) With geotextile (Initial volumetric water content in soil is 0.12) Fig. 3 Variation of volumetric water content along the soil column (a) Without geotextile (b) With geotextile (Initial volumetric water content in soil is 0.12) 29

30 Fig. 4 Variation of pore-pressure along the soil column (a) Without geotextile (b) With geotextile (Initial volumetric water content in soil is 0.12) Fig. 5 Variation of volumetric water content with time (a) Without geotextile (b) With geotextile (Initial volumetric water content in soil is 0.23)

31 Fig. 6 Hydraulic properties of different geotextiles used in the present study: (a) Water retention curves (b) Hydraulic conductivity curve

32 Fig. 7 Variation of volumetric water content in the model soil columns using different geotextiles

33 Fig. 8 Geometry of geotextile reinforced soil wall Fig. 9 Water characteristic curves of marginal backfill material and facing drain Moisture characteristic curves (b) Unsaturated hydraulic conductivity curves (a) 33

34 Fig. 10 Seepage induced flow and wetting front through the reinforced soil wall Fig. 11 Time history nodes near topmost soil-geotextile interface 34

35 Fig. 12 Variation of (a) volumetric water content (b) pore-water pressure, at the topmost soil- geotextile interface (Initial volumetric water content of soil is 0.2) Fig. 13 Wetting front movement through the unreinforced soil wall (Initial volumetric water content of the soil is 0.23)

36 Fig. 14 Wetting front movement through the geotextile reinforced soil wall (Initial volumetric water content of the soil is 0.23) 36