Centrifuge model study on geogrid reinforced soil walls with marginal backfills with and without chimney sand drain

The objective of this paper is to investigate the performance of geogrid reinforced soil walls with panel facing using marginal backfill with and without chimney sand drain subjected to seepage. A series of centrifuge model tests were performed at 40 gravities using a 4.5 m radius large beam centrifuge facility available at IIT Bombay. The results revealed that a geogrid reinforced soil wall with low stiffness geogrid and without any chimney drain experienced a catastrophic failure due to excess pore water pressure that developed in the reinforced and backfill zones at the onset of seepage. In comparison, a soil wall reinforced with stiff geogrid layers was found to perform effectively even at the onset of seepage. Provision of chimney sand drain effectively decreased pore water pressure not only at the wall toe but also at mid-distance from toe of the wall and thereby resulted in enhancing the wall performance under the effect of seepage forces. However, a local piping failure was observed near the toe region of the wall. The observed centrifuge test results were further analysed by performing seepage and stability analyses to evaluate the effect of thickness of sand layer in a chimney drain. An increase in thickness of sand layer in chimney drain was found to improve the discharge values and thereby enhancing the factor of safety against piping near the toe region. Based on the analysis and interpretation of centrifuge test results, it can be concluded that marginal soil can be used as a backfill in reinforced soil walls provided, it has geogrid layers of adequate stiffness and/or proper chimney drain configuration.     

Centrifugal tests on minimization of flood-induced deformation of levees by steel drainage pipes

The rise in the river water level in a levee raises the phreatic surface. This facilitates the development of positive pore water pressure in the region below the phreatic surface, and consequently, reduces the shear strength of the soil. Steel drainage pipes that can provide both drainage and reinforcement functions could be a better option for levee protection against flooding compared to the traditional method of protection which can provide only one or the other of these functions. This paper presents the results of a series of centrifugal tests for six cases conducted to investigate the effectiveness of newly designed steel drainage pipes for minimizing the flood-induced deformation of levees. The test results reveal that the installation of these steel drainage pipes (1) allows the levee to withstand a higher flood water head and extended flood duration and (2) is effective for limiting the continuation of the slip line in the slope. The quick drainage of the seepage water can restrict the development of positive pore water pressure in the slope, and the mobilization of the axial force in the pipes minimizes the flood-induced deformation of the levee.     

Centrifuge modeling of geotextile-reinforced slopes subjected to differential settlements

Today, geosynthetic-reinforced soil structures are widely used to support bridge abutments and approach roads in place of traditional pile supports and techniques. In such situations, foundation conditions have been shown to adversely affect the stability and deformation behaviour of overlying geosynthetic-reinforced slopes and walls. This paper addresses the response of geotextile-reinforced slopes subjected to differential settlements in a geotechnical centrifuge. Centrifuge model tests were carried out on model geotextile-reinforced sand slopes with two different types of reinforcement. A wrap-around technique was used to represent a flexible facing. In order to initiate failure in the reinforcement layers, the ratio of length of reinforcement to height of the slope was maintained as 0.85. One of the objectives of this paper is to present about a special device developed for inducing differential settlements during centrifuge test at 40g for a reinforced soil structure. A digital image analysis technique was employed to arrive at displacement vectors of markers glued to the reinforcement layers. The displacements were used to compute and analyze the strain distribution along the reinforcement layers during different settlement stages. Results of the centrifuge test indicate that even after inducing a differential settlement equivalent to 1.0 m in prototype dimensions, the geotextile-reinforced soil structure with a flexible facing was not found to experience a collapse failure. Analysis of geotextile strain results shows that the location of the maximum peak reinforcement strain occurs along the bottom-most reinforcement layer at the onset of differential settlements, at the point directly below the crest of the slope.     

A comprehensive method for analyzing the effect of geotextile layers on embankment stability

Commercial software is used widely in slope stability analyses of reinforced embankments. Almost all of these programs consider the tensile strength of geotextiles and soil–geotextile interface friction. However, currently available commercial software generally does not consider the drainage function of nonwoven geotextile reinforcement. In this paper, a reinforced channel embankment reinforced by a nonwoven geotextile is analyzed using two methods. The first method only considers the tensile strength and soil–geotextile interface friction. The second method also considers the drainage function. In both cases, the reinforced embankment is modeled in rapid drawdown condition since this is one of the most important conditions with regard to stability of channel embankments. It is shown that for this type of application, modeling a nonwoven geotextile reinforced embankment using commercial software which neglects the drainage function of the geotextile may be unrealistic.     

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.     

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.     

Analysis of geotextile filter behaviour after 21 years in Valcros dam

In 1970, nonwoven geotextiles were used for the first time in an earth dam. The geotextile acted as a filter for the toe drain and on the upstream slope below the rip-rap. In 1992, samples were taken from both locations and performance tests were conducted in the laboratory. This paper presents the main results of the hydraulic behaviour of the geotextile filter in association with the soil of the dam. Also microscopic analyses are presented and, as the filter is considered to be performing well, selected filter criteria are checked.     

含弱透水夹层饱和砂土坡体滑坡的实验研究

通过含有细砂夹层的饱和砂土在冲击载荷下的滑坡模拟实验，考察了土层参数不均匀的坡面的滑坡模式，坡体变形，破坏和不均匀渗流引起的滑坡，特别是散体粒径不均匀时出现的含水层和通道与滑坡的关系，各控制参数(载荷的和土体的)对滑坡的影响，并对这类滑坡中的现象给出了定性解释。     

堤防失事风险分析和风险管理研究

基于堤防渗透破坏和岸坡滑动失稳风险的数学模型，提出了场地砂土地震液化和堤防失事综合风险分析的数学模型，以及堤防风险管理的概念；以南京市板桥河左岸堤防加固工程为例，应用风险分析理论和方法对该段堤防渗透破坏风险、岸坡滑动失稳风险、场地砂土地震液化风险以及堤防失事综合风险进行了分析研究；通过1995年板桥河左岸堤防典型破圩实例建立整个板桥河堤防渗透破坏风险率监界值、岸坡滑动失稳风险率临界值以及堤防失事综合风险率临界值；同时基于风险分析的结果和所建立的风险率临界值，根据风险管理的理论，针对板桥河左岸堤防加固工程提出降低风险的措施，对堤防汛期洪水位运行管理提出建议。其理论、方法、思路和结论可供同类工程借鉴。     

On the hydraulic behavior of unsaturated nonwoven geotextiles

Geotextiles have been widely used in soil structures for separation, filtration, reinforcing, and drainage. They are often used to provide reinforcement and drainage for retaining walls and embankments. It has been reported, however, that geotextiles may not drain water as effectively as was initially expected. In this study, published data on the hydraulic properties of unsaturated geotextiles are compiled and analyzed in order to highlight the hydraulic characteristics of unsaturated geotextiles.

The application of the van Genuchten equations originally developed for the water characteristic curve and the hydraulic conductivity curve of unsaturated soil to unsaturated geotextiles is then examined and discussed. Finally, the drainage from a one-dimensional sand column having a horizontal geotextile layer was analyzed using the finite element method and the van Genuchten equations to assess the utility of this procedure for further study of unsaturated/saturated water flow within the soil–geotextile system.     

双层地基上堤身的渗流计算

黏土层覆盖下卧砂层的双层地基,在冲积平原的江河两侧普遍存在。对于双层堤基的渗流计算,已有相应于各种边界条件下的系统理论解;至于相应的堤本身的渗流计算,迥异于均质堤基和不透水堤基条件下的分析计算,理论解空缺。先按双层堤基计算出堤身下砂层的承压水位,堤身中点断面的水位非常接近承压水位;据此,并为便于求解,将整体堤身从中点断面分开为上下游两部分,每一部分的上下游边界视为延伸至无穷远;再应用保角变换法分别求出堤身上下游部分的渗流理论解。对于堤身上游坡1∶3、下游坡出逸点以下1∶5在中国常见的坡比,作出具体的数值计算和算例计算;堤身的下游部分为安全分析的重点,为应用方便计,根据详细的数值计算结果,提出一系列有一定精度的拟合式。经电拟试验结果比较,这一简化理论解的精度可靠,此外,论文的计算分析结果完全可推广至地表为黏土层覆盖的多层堤基上的堤身的渗流计算,也可推广至其它坡比的条件。     

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.     

同马大堤护坡防浪及渗流研究

同马大堤汇口段是 98洪水后安徽省堤防加固示范堤段 ,通过两典型断面的渗流计算对大堤的渗流控制措施进行了分析论证 ,给出了 1998年洪水期渗流场、设计断面压土方案下的渗流场及压土方案的优化措施 ;根据该段的风浪要素核算结果和波浪作用下的渗流计算结果 ,分析了砼板护坡的稳定结构形式 ,比较了开缝砼板护坡和全封闭砼板护坡、护坡下有无垫层等几种不同情况 ,进行了砼板护坡的厚度、强度和稳定性核算 ,提出了汇口段砼板护坡稳定形式。护坡工程建成后 ,经过 1999和 2 0 0 0年汛期洪水风浪的考验 ,运用良好     

长江细砂路基排水模型试验研究

水是道路危害最主要的自然因素之一。结合上海市外环线(浦东段)二期工程进行的长江细砂路基排水模型试验研究结果表明,长江细砂具有较强的饱水能力,在最佳含水率附近碾压不会产生排水现象。砂在注水情况下,路基密实效果不明显,不宜采用水密法施工方式;高含水率的长江细砂在碾压过程中会通过盲沟排水,排水历时与碾压历时基本一致。     

浅层排水处理施工工艺初探

对高速公路施工前的准备工作进行了阐述,介绍了开挖砂沟、铺设砂垫层、铺设S230横向塑料排水板和双复合土工布四种路面浅层排水的处理方案,并对它们的施工工艺进行了论述,最后对各方案施工中的相关问题进行了分析总结,供类似工程参考借鉴。     

Shaking table tests on gravel slopes reinforced by concrete canvas

The behaviour and performance of different reinforced slopes during earthquake loading were investigated through a series of shaking table tests. Concrete-canvas and composite reinforcement (geogrid attached to concrete-canvas) were proposed for reinforcing slopes. By considering the effects of different reinforcement methods, the seismic responses of the reinforced slopes were analysed, along with the accelerations, crest settlements, and lateral displacements. The failure patterns of different model slopes were compared using white coral sand marks placed at designated elevations to monitor the internal slide of the reinforced slopes. Both the concrete-canvas and composite reinforcement could increase the safety distance, which ranged from the slide-out point to the back of the model box. The composite reinforcement decreased the volume of the landslide and increased the failure surface angle as a result of the larger global stiffness in the reinforced zone. These results indicate that the recently developed concrete canvas has a better effect on restricting the slope deformation during seismic loading than the nonwoven geotextile reinforcement, and that the use of composite reinforcement could improve the seismic resistance of slopes.     

砂井地基固结的空间渗流和群井效应的解析分析

本文从Ｔｅｒｚａｇｈｉ－Ｒｅｎｄｕｌｉｃ三维固结方程出发，对砂井地基因结的空间渗流和群井效应作了分析。文中给出的砂井地基因结的解析解可考虑群井共同作用、空间渗流和井阻作用，满足软土的上下边界条件、各砂井中的连续和井阻条件。文末作了数值计算，讨论了空间渗流和群井效应对砂井地基中的孔隙水压力消散及地基因结的影响。     

武山尾矿坝无纺土工织物滤层化学淤堵问题初探

土工织物作为滤层在尾矿坝排渗设施中已得到广泛应用,但有些尾矿坝遇到了土工织物淤堵问题。本文针对武山铜矿尾矿坝排渗系统中发生的土工织物淤堵问题,通过一系列试验研究对无纺土工织物化学淤堵问题进行了初步探讨。研究结果表明氢氧化铁凝胶在织物纤维上的附着是化学淤堵的主要机理,尾矿中含有充分多的细粒黄铁矿且滤层处于非饱和渗流带或饱和- 非饱和交替变化渗流带是土工织物发生严重化学淤堵的必要条件     

江河大堤均匀砂基渗透破坏机理试验研究

为了摸清大堤堤基出现渗透破坏甚至溃决的原因,专门进行了模型试验。根据对堤基地质及水文地质的分析,将复杂的堤基归纳并概化为三种类型,并分别进行了各类模拟试验。阐述了均匀砂层堤基的渗透破坏特性及影响因素。均匀砂基的渗透破坏主要发生在堤基上部,特别是大堤与堤基的接触带,破坏水力比降决定于砂层土料本身的破坏水力比降,当大堤与堤基之间存在脱开问题,则破坏水力比降为砂层表面的冲刷水力比降。     

An optimal lightweight foamed mortar mix suitable for tunnel drainage carried out using the composite lining method

In this paper, an optimal lightweight foamed mortar mix suitable is proposed for facilitating tunnel drainage carried out using the composite lining method. A physical performance evaluation and pore structure analysis is provided in order to assess the performance of different lightweight foamed mortar mix proportions with various void fractions and foam solution concentrations. Furthermore, on the basis of an in-plane permeability test that simulated the permeation of fines of soil leaking with underground water, the formation and distribution of open-cell foams by measuring the outflow characteristics and outflow volume for each mix of the nonwoven geotextile and lightweight foamed mortar, which are the existing tunnel drainage materials was examined. Consequently, a mix employing a foaming agent with a dilution rate of 2% (#1) showed a better drain performance than all the other mixes, because it had the most appropriately formed and distributed open-cell foams, the key component for tunnel drainage. In other words, the mix of dilution rate of 2% was thought to have achieved stable closed-cell foams because of the decline in the surface tension of the foams, as well as the optimal formation and distribution of open-cell foams that possess excellent permeability because of the cohesiveness between the foams. Moreover, the thickness (drainage space) of it was approximately 17 times that of the nonwoven geotextile, and because the closed-cell and open-cell foams were connected like a spider web, it could be expected to reduce the blocking of drainage caused by the fines of soil.