Experimental evaluation of wicking geotextile-stabilized aggregate bases over subgrade under rainfall simulation and cyclic loading

Wicking geotextile has been increasingly utilized in field projects to mitigate water-related roadway problems. The previous studies showed that the wicking geotextile could provide mechanical stabilization, serve as capillary barrier, and enhance lateral drainage. The wicking geotextile differentiates itself from non-wicking geotextiles by providing capillary or wicking drainage in unsaturated conditions, whereas non-wicking geotextiles only provide gravitational drainage under saturated or near-saturated conditions. Although the previous studies have demonstrated the benefits of soil water content reduction by the wicking drainage, it is not well understood how the wicking geotextile stabilization improves overall performance of aggregate bases over subgrade under traffic or cyclic loading. This paper presents an experimental study where large-scale cyclic plate loading tests were conducted under different conditions: (1) non-stabilized base, (2) non-wicking geotextile-stabilized base, and (3) wicking geotextile-stabilized base, over soft and moderate subgrades. Rainfall simulation was carried out for each test section. After each rainfall simulation, a drainage period was designed to allow water to drain from the section. The amounts of water applied and exiting from the test section were recorded and are compared. Cyclic loading was applied after each drainage period. The test results show that the combined hydraulic and mechanical stabilization effect by the wicking geotextile reduced the permanent deformation of the aggregate base over the subgrade as compared with the non-stabilized and non-wicking geotextile-stabilized sections.     

Reexamination of traditional testing techniques for determining WRCs of woven geotextiles in full suction range

Woven geotextiles have been widely used in soil infrastructures for the reinforcement purpose. The hydraulic properties of a woven geotextile are not major reinforcement design parameters and the water retention capability of a woven geotextile is often ignored. The traditional testing techniques were designed for soils or nonwoven geotextiles, but not for woven geotextiles. Nowadays, a new type of woven geotextile with wicking fibers was developed which could be used for both drainage and reinforcement purposes. However, there are no proper testing techniques to determine the full-range water retention curve (WRC) for a woven geotextile, let alone for the wicking geotextile.This paper aimed at proposing a proper testing technique to determining the full-range WRC for the wicking geotextile and to compare the water retention capability of wicking and non-wicking geotextiles. Firstly, the traditional testing techniques were re-examined to check the suitability for characterizing the WRCs of woven geotextiles whose pore size distributions were anisotropic. Secondly, a proper testing technique was proposed and the WRCs of different types of woven geotextiles were determined. Thirdly, the WRCs of wicking and non-wicking geotextiles were compared to demonstrate the advantages of the wicking geotextile to hold and transport water under unsaturated conditions. Finally, the effect of wicking fiber on the water retention capability of the wicking geotextile was quantified.     

Laboratory tests to evaluate effectiveness of wicking geotextile in soil moisture reduction

A new type of woven geotextile, referred to as wicking geotextile, was developed and introduced to the market. Since this wicking geotextile consists of wicking fibers, they can wick water out from unsaturated soils in a pavement structure thus resulting in an increase of soil resilient modulus and enhance performance of roadways. In this study, a physical model test was developed to evaluate the effectiveness of the wicking geotextile in soil moisture reduction for roadway applications. A test box with a dimension of 1041 mm in length, 686 mm in width, and 584 mm in height was used in this study. Two HDPE plastic panels were used to separate the box into two sections, one containing a dehumidifier and the other backfilled with soil. The dehumidifier was adopted to collect the water, which was wicked out from the soil by the wicking geotextile and evaporated into air. Test results show that (1) the wicking geotextile wicked water out from the soil even at the moisture content close to the optimum moisture content and (2) the comparison of soil moisture contents before and after rainfall demonstrated that the wicking geotextile maintained the soil moisture contents after rainfall close to those before rainfall and had an effective distance for the soil moisture reduction.     

Field monitoring of wicking geotextile to reduce soil moisture under a concrete pavement subjected to precipitations and temperature variations

Wicking geotextile can reduce water contents in pavement layers under unsaturated conditions due to capillary action through grooves of wicking fibers. Reduction of soil water content under the pavement can minimize pavement distresses. So far, there have been limited use and verification of the wicking geotextile in reducing water content of soil under concrete pavements in the field. In this field study, moisture sensors were installed in three test sections under a newly-built concrete pavement during its re-construction. The base course in one test section had a higher percentage of small particles than those in other two sections. The wicking geotextile was used between the base course and the subgrade in two test sections while a nonwoven geotextile was used in one test section. All test sections were subjected to precipitations and temperature variations. Field monitoring data showed that the wicking geotextile reduced the volumetric water content (VWC) of an aggregate base more than the nonwoven geotextile and its wicking ability decreased as the content of small particles increased. In addition, the wicking ability of the wicking geotextile decreased as the temperature decreased due to the reduction in the evaporation rate and the increase in the water retention capacity of the soil at low temperatures.     

Evaluating wettability of geotextiles with contact angles

Geotextiles have been used for drainage purposes in pavements for many years. To drain water out of road sections, the geotextiles need to get wet first. In this study, the wettability of three different types of geotextiles, namely wicking woven (WW) geotextile, non-wicking woven (NWW) geotextile, and nonwoven (NW) geotextile, was investigated in terms of their contact angles dependent on water-geotextile interaction. Contact angle was observed by the VCA Optima XE tensiometer for up to 12 s after a water droplet was dropped at the center of a geotextile's surface. Water droplets of two different sizes (2 μL and 5 μL) were used to demonstrate the droplet size effect on the contact angles of water on undisturbed geotextiles. Test results show that the contact angle decreased to smaller than 90° and the droplet disappeared on the wicking woven geotextile within a few seconds after water dropping, while the contact angle remained larger than or approximately equal to 90° on the other two types of geotextiles within the observation period. This comparison indicates that water penetrated faster into the wicking woven geotextile than other geotextiles. Furthermore, this study investigated the effects of soil particle intrusion and geotextile or fiber deep groove flattening associated with compaction on the wettability of geotextiles.     

Working mechanism of a new wicking geotextile in roadway applications: A numerical study

Woven geotextiles are often to be used in roadways for reinforcement purposes due to their higher tensile strengths. In the design of a woven geotextile for practical applications, the focus is mainly put on its reinforcing effect, while its hydraulic behaviors are not major design parameters and the influence of hydraulic properties on the reinforcing effect is often ignored. However, woven geotextiles are predominantly made of polypropylene and polyester, which are hydrophobic. This characteristic can result in a capillary break effect which it is equivalent to raise the ground water table to the location where the geotextile is installed. Numerous researchers have reported that the moisture storage from a capillary break effect can be detrimental to the long-term performance of a pavement structure. Until now, no method is available to effectively resolve this issue.Recently a new type of wicking geotextile is produced which has the capability to laterally drain excess water in a roadway under both saturated and unsaturated conditions. Several field applications demonstrated its potential in improving pavement performance. This paper attempted to investigate the working mechanism of the wicking geotextile through numerical studies and quantify the benefits of the wicking geotextile in term of drainage performance in a pavement structure. A numerical model was developed and validated using column test results from existing literature. After that the drainage performance of the wicking geotextile under different working conditions was simulated and evaluated.     

土工布加固软土地基的机理试验研究

土工布与土体的界面摩擦力是衡量其对土体加固效果的关键因素,本文首先根据土工布与土体拉拔摩擦试验的机理,设计并制作了界面剪切测试仪,该试验测试仪具有试验精度高、应用广泛和操作步骤简单的特征,然后采用此测试仪对土工布与砂土的界面摩擦力进行了试验研究,试验参数包括不同的竖向压力、土工布层数和砂土含水率。试验结果表明:增加竖向压力能够显著提高土工布与土体的界面摩擦力;相同竖向压力作用下,土工布层数超过2层时,对其界面摩擦力提高并不显著;相同条件下,当砂土含水率超过5%时,提高砂土含水率能够显著降低土工布与土体界面摩擦力。这些研究结果可为土工布加固土体的设计和施工提供依据。     

Numerical parametric investigation of infiltration in one-dimensional sand–geotextile columns

Geotextiles are routinely used in separation and filtration applications. Design of these systems is currently based on saturated properties of the geotextiles and the surrounding soils. However, in the field, soil and geotextile can be in an unsaturated state for much of their design life during which they are essentially hydraulically non-conductive. Periodic wetting and drying cycles can result in rapid and large changes in hydraulic performance of soil–geotextile systems. The writers have reported the results from physical water infiltration tests on sand columns with and without a geotextile inclusion. The geotextile inclusions were installed in new and modified states to simulate the influence of clogging due to fines and to broaden the range of hydraulic properties of the geotextiles in the physical tests. This paper reports the results of numerical simulations that were undertaken to reproduce the physical tests and strategies adopted to adjust soil and geotextile properties from independent laboratory tests to improve the agreement between numerical and physical test results. For example the paper shows that the hydraulic conductivity function of the geotextile must be reduced by up to two orders of magnitude to give acceptable agreement. The lower hydraulic conductivity is believed to be due to soil intrusion that is not captured in conventional laboratory permeability tests. The calibrated numerical model is used to investigate the influence of geotextile and soil hydraulic conductivity and thickness as well as height of ponded water at the surface on wetting front advance below the geotextile and potential ponding of water above the geotextile due to a capillary break mechanism. A simple analytical model is also developed that predicts the maximum ponding height of water above the geotextile based on two-layer saturated media and 1-D steady state flow assumptions. The analytical model is used to generate a design chart to select geotextiles to minimize potential ponding of water above the geotextile. Ponding can lead to lateral flow of water along the geotextile in reinforced wall, slope, embankment and road base applications.     

Cyclic behaviour of dry silty sand reinforced with a geotextile

Recent studies on construction material technology have indicated that soil reinforcement improves resistance of soil against compression and tension. Due to the wide use of geotextile reinforcement in road construction, the potential benefit of geotextile reinforcement in cyclic loading should be investigated. In this study we performed a series of cyclic triaxial tests to examine dry silty sand reinforced with geotextile when subjected to dynamic loading. These tests were conducted on reinforced and unreinforced dry sand and sand mixed with varying amounts of silt (0–50%). The main factors affecting the cyclic behaviour, such as the arrangement and number of geotextile layers, confined pressure and silt content are examined and discussed in this paper. The results indicate that geotextile inclusion and increased confining pressure increase the axial modulus and decreased cyclic ductility of dry sand for all silt contents examined. Also, it was found that by increasing the silt content by up to about 35 percent the axial modulus in reinforced and unreinforced sand is decreased and cyclic ductility increased. With further increases in silt content, these values are increased for cyclic axial modulus and decreased for cyclic ductility.     

Two-dimensional consolidation analysis of geotextile tubes filled with fine-grained material

In this study, a two-dimensional consolidation solution for geotextile tubes filled with fine-grained material was presented. The solution is based on a combination of various methods that were modified or extended to take into account the change in tube shape, the nonlinear interaction between the soil and geotextile, and the water content distribution of the tube during consolidation. Using the proposed solution, the effect of various necessary input parameters was investigated. Thereafter, numerous dewatering and consolidation properties of various combinations of geotextiles and fill materials were obtained from several tests such as the half cross-section test, hanging bag tests, and geotextile tube demonstration test. Results of the study have shown that the method presented in this study can well-represent the consolidation behavior of geotextile tubes filled with fine-grained material.     

Centrifuge model tests of geotextile-reinforced soil embankments during an earthquake

The behavior of geotextile-reinforced embankments during an earthquake was investigated using centrifuge model tests, considering a variety of factors such as gradient of slope, water content of soil, geotextile spacing, and input shaking wave. The geotextile-reinforcement mechanism was revealed on the basis of the observations with comparison of the unreinforced embankment. The geotextile significantly decreases the deformation of the embankment and restricts sliding failure that occurs in the unreinforced embankment during an earthquake. The displacement exhibits an evidently irreversible accumulation with a fluctuation during the earthquake which is significantly dependent on the magnitude of input shaking. The peak strain of the geotextile exhibits a nearly triangular distribution in the vertical direction. The embankment can be divided into two zones, a restricting zone and restricted zone, where the soil and geotextile, respectively, play an active restriction role in the soil-geotextile interaction. The soil restricts the geotextile in the restricting zone, and this restriction is transferred to the restricted zone through the geotextile. The strain magnitude of the geotextile and the horizontal displacement of the geotextile-reinforced embankment decrease with increasing geotextile layers, with decreasing water content of the soil, with decreasing gradient of the slope, and with decreasing amplitude of the earthquake wave.     

The efficacy and use of small centrifuge for evaluating geotextile tube dewatering performance

Geotextile tube dewatering technology has been widely used over the past two decades for dewatering high water content slurries. The dewatering process in geotextile tubes aims to decrease the volume of the dewatered slurry, which helps in the transportation, disposal, and reuse of the dewatered material. Several researchers have emphasized the effect of the retained sediment (filter cake) properties, in particular final solids content and volume (height) change, on the feasibility of geotextile tube dewatering projects. Retained sediment properties are often evaluated using small scale tests such as rapid dewatering test, falling head test, pressure filtration test (PFT), and field scale tests such as hanging bag test (HBT) and geotextile tube demonstrations test (GDT). In this study, centrifuge test is introduced as an alternative for the widely used pressure filtration and falling head tests to evaluate retained sediments properties. Centrifuge test provides a mechanism for understanding the response of slurries to externally applied pressure in geotextile tube environment. Centrifuge test was used to evaluate maximal solids content of the retained sediments and change in slurry volume of four soils that represent typical dredged soils. Tully sand, Tully fines, Elliott silt loam, and kaolin slurries were used at varying solids concentrations. Slurries were subjected to external stresses between 0.1 and 40 kPa by applying centrifugal speeds between 300 rotation per minute (rpm) and 1800 rpm. Both centrifuge test and PFT were conducted with unconditioned and cationic polyacrylamide conditioned slurries. Centrifuge tests results were compared with PFT results with respect to retained sediments final solids content and volume change. Tests results indicated that the maximal solids concentration of the retained sediments in saturated conditions is unique for each soil and is independent of the initial slurry solids concentration. Tests results also indicated that there is linear relationship between the initial concentration of the slurry and the final volume change at any externally applied stress. Finally, a relationship between the total pumped slurry volume and the final height of the dewatered sediments in a geotextile tube is presented.     

Filtration performance of geotextile encasement to minimize the clogging of stone column during soil liquefaction

Stone columns, which are frequently employed to stabilize the liquefiable soil, are susceptible to accumulation of soil particles. The progressive accumulation of the soil particles causes clogging of the stone column which decreases its drainage capacity. The stone column can be encased with geotextile to sustain its long term drainage function. The encasement prevents the movement of the soil particles into the stone pores. In the present paper, a mathematical model is presented to assess the filtration performance of the geotextile encasement to prevent the clogging. The filtration capacity of the geotextile is related to its maximum pore size, porosity and soil characteristics. It is observed that the encased stone column dissipates the excess pore pressure at a faster rate compared to the stone column without encasement. The peak maximum excess pore water pressure (U_max) is not significantly affected due to selection of the opening size of the geotextiles for single earthquake. However, the opening size can significantly affect the peak U_max value for multiple earthquakes. Depending on the capture coefficient of the stone column, the clogging can be fully prevented for higher hydraulic gradient if geotextile with maximum opening size in between D₁₀ to D₅ is used as encasement.     

Three-dimensional numerical analysis of individual geotextile-encased sand columns with surrounding loose sand

Use of geotextile-encased sand columns (GESC) to improve weak soils is an emerging technology that has great promise for field applications. This paper contains the results of a numerical study with the goal of quantifying the benefits of geotextile encasement under different conditions. A three-dimensional finite difference method implemented in FLAC3D 5.01 was used to evaluate the performance of a vertically loaded individual GESC installed in loose sand. The numerical model was first verified using the results of experimental tests performed on 150-mm diameter GESC installed in loose sand. The influence of various parameters was investigated in this study, including GESC diameter and length, soil thickness, geotextile encasement length, geotextile stiffness, and friction angle and dilation angle of the infill material. The results of the numerical model showed that vertically loaded GESC of smaller diameter experienced less settlement and lateral expansion than those of larger diameter. The geotextile material with higher stiffness had a substantial influence on the performance of GESC. The maximum effective geotextile encasement length depended on the load on the column head or the compressibility of the column.     

Geotextile stabilization of seabeds: theory

Geotextiles are synthetic fabrics which may be substituted for graded aggregate to protect ocean and coastal structures from erosion and soil instability. Geotextiles are commonly used as a structural membrane and as a filter between an undisturbed sediment surface below and an erosion resistant coarse aggregate placed above. The dynamic response of a horizontally-layered soil system including a geotextile is examined analytically using the quasi-static Biot consolidation equations, the storage equation, and modelling the geotextile as a permeable elastic membrane. The influence of geotextile properties on soil stability are found to be rather small. The coupling of soil layer properties has a significant influence on soil stability. Maximum failure potential exists for specific combinations of the relative permeabilities and thicknesses of the soils.     

Investigation of geotextile–soil interaction under a cyclic vertical load using the discrete element method

Geotextiles are often used in roadway construction as separation, filtration, and reinforcement. Their performance as reinforcement in geotextile-reinforced bases depends on geotextile–soil interaction. This paper investigates the geotextile–soil interaction under a cyclic wheel load using the Discrete Element Method (DEM). In this study, soil was modeled as unbonded particles using the linear contact stiffness model, and the geotextile was modeled as bonded particles. The micro-parameters of the soil and the geotextile were determined using biaxial tests and a tensile test, respectively. The influence of the placement depth and the stiffness of the geotextile on the performance of the reinforced base was investigated. The DEM results show that the depth of the geotextile significantly affected the degree of interaction between the geotextile and the soil. Under the applied cyclic vertical load, the geotextile developed a low tensile strain. The effect of the stiffness of the geotextile on the deformation was more significant when the geotextile was placed at a shallower location than when placed at a deeper location.     

有纺土工织物覆土条件下的渗透特性试验研究

分别配制不同孔隙比的粉砂、标准砂和黏土试样,采用自主研制的一套多功能渗透试验装置,开展了一系列纯土和有纺土工织物覆土条件下的渗透试验,对比分析了这两种条件下渗透系数的差异,并探讨了有纺土工织物与土共同作用下的渗透机理。结果表明,有纺土工织物对于土体渗流略有一定的抑制作用,表现为覆土条件下的渗透系数略小于纯土的渗透系数,但是对于粉砂,当其孔隙比比较大、细砂颗粒的含量相对较多时,细砂颗粒则有可能在渗流作用下通过有纺土工织物孔隙而产生流失,使得覆粉砂条件下的渗透系数略大于纯粉砂的渗透系数。     

加筋软岩粗粒土路堤填料大型三轴试验研究

 为研究加筋粗粒土填料的强度变形特性及加筋效果,进行加筋强风化软岩粗粒土固结不排水和固结排水大三轴试验。试验表明：加筋填料的应力–应变关系表现为应变硬化型;轴向应变较小(ea＜1%)时,加筋填料效果不明显,随着轴向应变的逐渐增大加筋效果逐渐发挥。加筋填料的孔隙水压力均高于素填料,随着加筋层数的增加均有不同程度的提高。加筋效果系数均＞1.0,一层加筋填料加筋效果系数为1.09～1.21,二层加筋填料加筋效果系数为1.30～1.71,三层加筋填料加筋效果系数为1.31～1.72。加筋前后填料的内摩擦角j基本不变,填料的黏聚力增大。加筋填料的本构关系可以用Duncan-Chang模型来描述,依据试验结果求得模型参数。     

Laboratory evaluation of the behavior of a geotextile reinforced clay

To evaluate the behavior of cohesive soil reinforced with a geotextile, 144 unconfined and 72 unconsolidated–undrained (UU) triaxial compression tests were conducted. The moisture content of soil during remolding, relative compaction, soil type, confining pressure, type and number of geotextile layers were all varied so that the behavior of the sample could be examined. The results provide evidence that as the moisture content increases, the peak strength of both the reinforced and unreinforced samples decreases and the axial strain at failure increases. Moreover, with increasing relative compaction the peak strength of the sample and axial strain at failure increases, whereas the peak strength ratio decreases. The peak strength ratio is the ratio of the peak strength of the reinforced samples to that of the unreinforced samples. For soils with low plasticity indices the main cause of the increase in the strength is the increase in the cohesion of the reinforced sample. However, in soils of higher plasticity index, as the number of geotextile layers increases, the internal friction angle of the reinforced samples increases.