Numerical study of strain development in high-density polyethylene geomembrane liner system in landfills using a new constitutive model for municipal solid waste

Tensile strain development in high-density polyethylene (HDPE) geomembrane (GMB) liner systems in landfills was numerically investigated. A new constitutive model for municipal solid waste (MSW) that incorporates both mechanical creep and biodegradation was employed in the analyses. The MSW constitutive model is a Cam-Clay type of plasticity model and was implemented in the finite difference computer program FLAC?. The influence of the friction angle of the liner system interfaces, the biodegradation of MSW, and the MSW filling rate on tensile strains were investigated. Several design alternatives to reduce the maximum tensile strain under both short- and long-term waste settlement were evaluated. Results of the analyses indicate that landfill geometry, interface friction angles, and short- and long-term waste settlement are key factors in the development of tensile strains. The results show that long-term waste settlement can induce additional tensile strains after waste placement is complete. Using a HDPE GMB with a friction angle on its upper interface that is lower than the friction angle on the underlying interface, increasing the number of benches, and reducing the slope inclination are shown to mitigate the maximum tensile strain caused by waste placement and waste settlement.     

Geosynthetic liner integrity and stability analysis for a waste containment facility with a preferential slip plane within the liner system

The paper examines the effects of settlement-induced downdrag on geosynthetic liner systems for a waste containment facility with steep side slopes for different design scenarios, and conducts the stability analysis of the waste mass during waste filling operations. Without the presence of a reinforcing layer above a geomembrane (GMB) liner, the liner experiences unacceptable tensile strains under both short- and long-term downdrag waste settlements. It is shown that an anchored high strength/stiffness geotextile (HS-GTX) reinforcement over the GMB can reduce the GMB tensile strains to less than 3%, but the HS-GTX itself may be overloaded. A geosynthetic slip layer over the full or partial HS-GTX reinforcement overlying the GMB can reduce the tensile strains of the GMB to less than 3% and of the HS-GTX to less than 5% by providing a preferential slip plane between the geosynthetic slip layer and the HS-GTX. A rupture of the geosynthetic slip layer is likely to occur resulting in the exposure of the HS-GTX to the waste, but the protection of the GMB by the HS-GTX is still expected. The results from the stability analysis show that, during waste filling operations under a given factor of safety, there is a critical relationship between the width of the top of the waste pile and the total waste thickness.     

HDPE土工膜端部张拉力的大规模实测数据分析

 垃圾填埋场边土工膜主要受到温度应力和垃圾压缩引起的张拉力的作用，为评价垃圾填埋场防渗系统中高密度聚乙烯(HDPE)土工膜的温度应力和垃圾填埋压缩引起的端部张拉力，以短纤维无纺布、HDPE土工膜组成防渗系统，进行大规模现场试验。试验结果表明：伴随环境温度的下降，HDPE土工膜中产生温度应力，当填埋高度较小时作用在HDPE土工膜端部的张拉力主要为温度应力；随着填埋高度的增加，压缩引起的HDPE土工膜端部张拉力增大。用有限元算法对压缩引起的端部张拉力进行分析，并采用温度与HDPE土工膜张拉力的关系对温度应力进行分析。计算结果与实测结果的对比情况表明，考虑填埋压缩引起的张拉力的应力松弛后计算结果更加接近实测结果。     

复杂荷载作用下填埋场HDPE土工膜受拉计算

采用FLAC程序对典型荷载作用下城市垃圾填埋场HDPE土工膜的受力状况进行了计算分析。3种荷载条件包括:36 m高垃圾堆体自重荷载(分层填埋);下卧软弱黏土层不均匀沉降引起的荷载;地震引起的动力荷载。计算结果显示:①土工膜内的拉应力随着垃圾土分层填埋、基础不均匀沉降、地震荷载的作用而积累;②基础不均匀沉陷是影响衬垫层土工膜局部拉应力的主要因素;③中等强度水平的地震动输入(例如,峰值加速度为0.25g)可使覆盖层土工膜端部锚固位置拉应力超过极限拉应力。     

基础局部沉降下土工格栅加筋衬垫系统变形特性的试验研究

基础局部沉降会引起垃圾填埋场衬垫系统中的土工膜产生较大的拉应变,有可能导致衬垫系统性能下降,因此正确评价衬垫系统的应变就显得非常重要。通过模拟试验,采用应变片和位移计对基础发生局部沉降后土工格栅加筋衬垫系统的变形进行试验研究。试验结果表明:环境温度对衬垫系统的变形影响较大;相同组成材料下土工格栅和土工膜叠放在一起比其他方案更能降低土工膜的应变;衬垫系统刚度对沉陷范围影响不大,但对最大应变值影响较大。所得结果对垃圾填埋场衬垫系统的设计具有一定的指导意义。     

Development of geomembrane strains in waste containment facility liners with waste settlement

The development of tensile strains in geomembrane liners due to loading and waste settlement in waste containment facilities is examined using a numerical model. Two different constitutive models are used to simulate the waste: (a) a modified Cam-Clay model and (b) a Mohr-Coulomb model. The numerical analyses indicate the role of the slope inclination on the maximum geomembrane liner strains for both short-term loading (immediately post closure) and long-term waste settlement. A geosynthetic reinforcement layer over the geomembrane liner is shown to reduce the maximum geomembrane liner strains, but the strain level of the geosynthetic reinforcement itself may become an engineering concern on steeper slopes (i.e., greater than 3H:1V) for cases and conditions examined in this paper. The paper considers some factors (e.g., slope inclination, use of a high stiffness geosynthetic over the geomembrane liner) and notes others (e.g., the designer selection of interface characteristics below and above the geomembrane, use of a slip layer above the geomembrane) that warrant consideration and further investigation to ensure good long-term performance of geomembrane liners in waste containment facilities.     

土工膜/土工织物界面大型斜坡模型试验研究

复合衬垫系统广泛应用于垃圾卫生填埋场,是防止渗沥液污染物渗漏扩散的重要屏障。在垃圾重力及沉降作用下易造成斜坡上复合衬垫系统拉伸破坏或沿其界面产生滑移而失稳。目前,由于缺乏对复合衬垫系统内部剪力传递机理的认识,仍难完全解决以上两大岩土工程问题。因此,设计并采用复合衬垫系统大型斜坡模型试验装置开展了其内部剪力传递机理的研究。该装置通过砂袋加载模拟填埋过程,采用手拉葫芦为核心的滑移控制系统再现了土工膜/土工织物界面的渐进累积破坏过程。试验结果表明：当外部剪力小于峰值强度时,界面不会进入残余状态,上覆的土工合成材料锚固端的拉力也非常小;但当外部剪力超过界面峰值强度时,界面就会逐渐进入残余状态,并最终达到残余强度。同时,薄弱界面上覆的土工合成材料锚固端的拉力也显著增加,严重时甚至被完全拉断。     

Geomembrane puncture and strains from stones in an underlying clay layer

Results from physical experiments are presented to assess the possible puncture of a 1.5-mm-thick HDPE geomembrane and, if not punctured, the maximum tensile strains in the deformed geomembrane from intentionally placed stone particles in an underlying compacted clay liner when subjected to applied vertical stresses. The influences of applied pressure, clay water content, stone size, stone burial depth and protection layer on the geomembrane tensile strains are reported. Except in one test conducted to a pressure of 2000 kPa, the geomembrane was not punctured in the short-term tests conducted; however, it was subjected to local indentations and tensile strains from the underlying gravel particles that may exceed proposed allowable long-term strain limits. Tensile strains for the specific 35 mm stones tested when initially flush with the clay surface were negligible, even up to pressures of 1000 kPa, provided the initial water content of clay was 12%. Increases in water content or stone size were found to increase the tensile strain. Placing the clay at the lower limit of acceptable water content was found to be beneficial in terms of reducing strains from buried stones; however, this was also found to make the geomembrane more susceptible to stone particles sitting on top of the clay surface and hence careful site inspection is required to remove all visible stones that sit on top of the clay surface.     

天津港南疆吹泥围埝工程试验段的试验研究

对天津港南疆 1#靠船墩改建吹泥围埝工程的施工方案及可行性展开了深入的探讨 ,进行了一系列室内常规试验、编织袋填充模拟试验、离心机模型试验 ,对施工过程中编织袋的形状和可能受到的最大应力进行了分析验算。研究结果表明 ,只有塑性指数小于 10的粉土才适用于吹泥围埝的施工方案 ,土工编织袋受到的最大拉应力小于其抗拉强度 ,建成围埝的整体稳定性满足要求 ,表明这一施工方案合理可行     

Geomembrane strains from coarse gravel and wrinkles in a GM/GCL composite liner

The physical response of a 1.5-mm-thick, high-density polyethylene geomembrane (GM) is reported when placed on top of a needle-punched geosynthetic clay liner (GCL), buried beneath 50-mm coarse gravel and subjected to vertical pressure in laboratory experiments. Local strains in the geomembrane caused by indentations from the overlying gravel and deflections of a wrinkle in the geomembrane are quantified. A peak strain of 20% was calculated when a flat geomembrane was tested without a protection layer at an applied vertical pressure of 250 kPa. Strains were smaller with a nonwoven needle-punched geotextile protection layer between the gravel and geomembrane. Increasing the mass per unit area of the geotextile up to 2200 g/m² reduced the geomembrane strain. However, none of the geotextiles tested were sufficient to reduce the geomembrane strain below an allowable limit of 3%, for the particular 50-mm gravel tested and when subjected to a vertical pressure of 250 kPa. Increasing the initial GCL water content and reducing the stiffness of the foundation layer beneath the GCL were found to increase the geomembrane strains. These local strains were greater when a wrinkle was present in the geomembrane. The wrinkle in the geomembrane experienced a decrease in height and width. The wrinkle deformations lead to larger pressures beside the wrinkle and hence producing larger local strains. A 150-mm-thick sand protection layer was effective in limiting the peak strain to less than 0.3% even with a wrinkle in the geomembrane, at a vertical pressure of 250 kPa.     

Geomembrane strains from wrinkle deformations

Methods to compute geomembrane strains caused by the deformation of a geomembrane wrinkle when subject to vertical overburden stress are examined. Thin shell theory, small strain-displacement, and large strain-displacement methods are developed to compute geomembrane strains from wrinkle deformations. The implementation of each method is validated for three hypothetical trial cases against results obtained with finite element analysis. It was found that it is necessary to employ large strain-displacement theory and explicitly consider both vertical and horizontal components of wrinkle displacement to compute strain. Results are then presented from six physical experiments where the vertical and horizontal components of wrinkle displacement were measured under simulated landfill base liner conditions. For the particular conditions tested, it was found that without a thick sand protection layer on top of the geomembrane, the largest calculated tensile strain due to wrinkle deformations of 8% was much less than the tensile strain caused by overlying gravel particles, and thus local strains from gravel contacts would govern in the assessment of maximum geomembrane strain; whereas, with a thick sand protection, the maximum tensile strain was from wrinkle deformations, but the measured values were below a proposed long-term allowable strain of 3%.     

An experimental evaluation of the behavior of footings on geosynthetic-reinforced sand

This research was performed to investigate the behavior of geosynthetic-reinforced sandy soil foundations and to study the effect of different parameters contributing to their performance using laboratory model tests. The parameters investigated in this study included top layer spacing, number of reinforcement layers, vertical spacing between layers, tensile modulus and type of geosynthetic reinforcement, embedment depth, and shape of footing. The effect of geosynthetic reinforcement on the vertical stress distribution in the sand and the strain distribution along the reinforcement were also investigated. The test results demonstrated the potential benefit of using geosynthetic-reinforced sand foundations. The test results also showed that the reinforcement configuration/layout has a very significant effect on the behavior of reinforced sand foundation. With two or more layers of reinforcement, the settlement can be reduced by 20% at all footing pressure levels. Sand reinforced by the composite of geogrid and geotextile performed better than those reinforced by geogrid or geotextile alone. The inclusion of reinforcement can redistribute the applied footing load to a more uniform pattern, hence reducing the stress concentration, which will result reduced settlement. Finally, the results of model tests were compared with the analytical solution developed by the authors in previous studies; and the analytical solution gave a good predication of the experimental results of footing on geosynthetic reinforced sand.     

Dynamic shear behavior of GMB/CCL interface under cyclic loading

The dynamic shear behavior of composite liner interface is of great importance for landfill seismic analysis. In this study, an experimental investigation of the shear behavior of the interface between smooth high density polyethylene (HDPE) geomembrane (GMB) and compacted clay liner (CCL) is presented. A series of displacement-controlled cyclic shear tests were conducted to investigate the effects of displacement amplitudes, normal stress levels and number of cycles on the GMB/CCL interface shear behavior. Cyclic loading with higher displacement amplitude will produce greater vertical contraction and lower interface initial shear stiffness. Also, significant shear strength degradation was observed within the first 5 shearing cycles, then followed by slight interface reinforcement in subsequent cycles. The dynamic shear modulus of GMB/CCL interface is dependent on both normal stress levels and displacement amplitudes, while the damping ratio is only affected by displacement amplitudes. Finally, a method considering the GMB/CCL composite liner as an equivalent soil layer was proposed, which is useful for landfill seismic analysis.     

Construction of unpaved rural road using jute–synthetic blended woven geotextile – A case study

Jute–high density polyethylene (HDPE) blended geotextile samples produced using HDPE slit-film in the machine direction and jute yarn in the cross direction for use in the construction of unpaved rural roads. Use of HDPE slit-film resulted in high productivity of jute-based geotextiles in modern high-speed machines, while jute (≈85%) in cross direction resulted in notable increase in modulus, breaking strength, CBR puncture resistance of the blended geotextile as compared to 100% HDPE geotextile. The optimized geotextile (plain-weave fabric with 111 tex HDPE in machine direction, 2 × 360 tex jute yarn in cross direction having area density, 316 g/m²) was used in a field trial. During road construction, the geotextile was covered with a layer of 10 cm thick laterite gravels as the sub-grade, compacted by rolling, and then finally covered with 10 cm small granular lateritic stones, and rolled again. The field trial showed that the monitored section where geotextile was used showed an even surface without any notable subsidence or rutting after 18 months. However in sections of the road constructed without the geotextile, 5–35 mm deep ruts were observed. CBR tests (carried out 11 months and 18 months after construction) showed a 67–73% improvement in the road due to the use of jute–HDPE blended geotextile than that obtained for the part of the road where geotextiles were not used.     

土工合成材料对路堤长期稳定性及工后沉降的负面影响分析

从土工合成材料作用机理着手,分析了在高速公路软基处理中使用土工合成材料会对路堤长期稳定性及工后沉降产生负面影响。土工合成材料的长期强度损失及模量的降低会降低路堤的长期稳定性。土工合成材料的长期变形和松弛对工后沉降也会产生一定的不利影响。某高速公路试验段实测资料表明,一层土工布对工后沉降的影响可忽略不计,两层土工布约增加工后沉降40～50mm。     

土工格室HDPE片材拉伸应变率相关特性

通过条带拉伸试验,分析了高密度聚乙烯(HDPE)片材的拉伸应力-应变关系,指出了该关系的应变率相关特性,并对HDPE片材的拉伸模量、最大拉伸应力、最大拉伸应力对应的拉伸应变、泊松比等物理力学参数与拉伸应变率的关系进行了详细分析,给出了HDPE片材应变率相关的应力-应变数学模型及其参数的物理意义.     

An analytical solution for geotextile-wrapped soil based on insights from DEM analysis

This paper presents a novel analytical solution for geotextile-wrapped soil based on a comprehensive numerical analysis conducted using the discrete element method (DEM). By examining the soil–geotextile interface friction, principal stress distribution, and stress–strain relations of the constituent soil and geotextile in the DEM analysis, a complete picture of the mechanical characterization of geotextile-wrapped soil under uniaxial compression is first provided. With these new insights, key assumptions are verified and developed for the proposed analytical solution. In the DEM analysis, a near-failure state line that predicts stress ratios relative to the maximums at failure with respect to deviatoric strain is uniquely identified; dilation rates are found to be related to stress ratios via a single linear correlation regardless of the tensile stiffness of the geotextile. From these new findings, the assumptions on the stress-state evolution and the stress–dilatancy relation are developed accordingly, and the wrapped granular soil can therefore be modeled as a Mohr–Coulomb elastoplastic solid with evolving stress ratio and dilation rate. The development of the proposed analytical model also demonstrates an innovative approach to take advantage of multiscale insights for the analytical modeling of complex geomaterials. The analytical model is validated with the DEM simulation results of geotextile-wrapped soil under uniaxial compression, considering a wide range of geotextile tensile stiffnesses. To further examine the predictive capacity of the analytical model, the stress–strain response under triaxial compression conditions is solved analytically, taking both different confining pressures and geotextile tensile stiffnesses into account. Good agreement is obtained between the analytical and DEM solutions, which suggests that the key assumptions developed in the uniaxial compression conditions also remain valid for triaxial compression conditions.     

Construction of an offshore dike using slurry filled geotextile mats

A study on the use of clay slurry filled geotextile mats to construct dikes for land reclamation at Tianjin Port, China, is presented in this paper. The dike so formed was covered by a thin layer of grouted geotextile mattress for protection. Through laboratory tests, a type of low plasticity clay was chosen to be the fill for the mats. A simple method for estimating the required tensile strength for the geotextile mat and the height of the mat was proposed. A preliminary design for the dike was made. Numerical analysis and centrifuge model tests were conducted to verify the design and assess the stability of the dike before construction. A field trial was also carried out in which a 100 m long and 4.8 m high dike was constructed on soft seabed. The dike has been stable and the settlement has been within the expected limit since the construction was completed in September 2001.     

Geotextile filtration opening size under tension and confinement

Nonwoven geotextiles have been used as filters in geotechnical and geoenvironmental works for half a century. They are easy to install and can be specified to meet the requirements for proper filter performance. There are situations where a geotextile filter may be subjected to tensile loads, which may alter relevant filter properties, such as its filtration opening size. Examples of such situations are silty fence applications, geotextile separators, geotextile tubes and geotextiles under embankments on soft soils. This paper investigates the effects of tensile strains on geotextile pore dimensions. A special equipment and testing technique allowed tests to be carried out on geotextile specimens subjected to tension and confinement. The results obtained showed that the variation in filtration opening size depends on the type of strain state the geotextile is subjected, under which the geotextile pore diameter may remain rather constant or increase significantly. However, confinement reduces the geotextile filtration opening size independent on the strain mobilised. An upper bound for the filtration opening size of strained nonwoven geotextiles is introduced and was satisfactory for the geotextile products tested.     

Bioengineering of river earth embankment using natural fibre-based composite-structured geotextiles

A Jute-HDPE composite structured geotextile was developed to improve the performance of earthen structure of river embankment. The optimized geotextiles (430 g/m²) containing 86% natural component (on weight) having better physical, mechanical (tensile strength, 10 kN/m (machine direction) and 18 kN/m (cross direction), index puncture (163 kN) and CBR (1.5 kN)), hydraulic (AOS 178 μ) and endurance properties than 100% HDPE geotextiles. A coconut fibre geotextile net was placed over jute-polyolefin geotextiles to resist washing-off of loose cover soil until the establishment of vegetation. Placing of continuous seamless geotextile tube (weight 196.2 kg/m) filled with moist river sand at the anchor trench-cum-toe guard assisted in safeguarding from eddies. It was observed that initially closed structure of the geotextile assisted in efficient filtration leading to soil stabilization through compactness of soil layer (14 cm thick). The uniqueness of work lies in conversion of closed structure of geotextiles to open-mesh of HDPE slit film on degradation of jute, remained beneath the cover-soil, through which grass root penetrated the geotextiles sheet and riveted both the layers of soil, the cover and the compacted back layers. The remnant synthetic part thus acts as durable reinforcing element and its increased porosity provides breathability for growth of soil flora and fauna. Bermuda grass turf provided very high nailing strength (658.8 kN/m²) with the soil through intertwining of grass roots with durable synthetic network.