Standsicherheitsnachweis für Oberflächenabdichtungssysteme von Deponien

Verification of slope stability of surface sealing systems of landfill structures. According to national and european standards slope stability of capping systems should be verified with the concept of partial safety factors. The revised german recommendation E 2‐7 “Analysis of planar surface failure in lining systems” of the TC “Geotechnics of Landfill Structures” requires design approach 3, where actions are treated as geotechnical actions and partial safety factors are applied to the shear resistance of the soil in the potential slip surfaces. The tension force of a geogrid in case of reinforcement should be designed according to this approach. The paper describes the different failure modes of rupture and excessive deformations, the fundamentals of the verification procedure with partial safety factors, the limit state conditions in infinite slope analysis of stratified final capping systems as well as the design of reinforced structures. Hence, central point of the analysis is less the method of calculation as rather the determination of the effective shear parameters, which can be mobilised for a long period of time considering changing conditions and material ageing effects in the contact plane of the system.     

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.     

Field behaviors of a geogrid reinforced MSW slope in a high-food-waste-content MSW landfill: A case study

A one-year field monitoring of a geogrid reinforced municipal solid waste (MSW) slope was conducted in the Xingfeng Landfill. Settlement tubes, strain gauges and earth pressure cells were used to measure the vertical settlement, the reinforcement strains and the vertical earth pressures in the reinforced MSW slope, respectively. During the monitoring period, the waste sliding occurred and the fresh MSW was dumped at the top of the reinforced slope. The vertical settlement along the reinforcement was nonlinear and the peak settlement occurred at the central part of the reinforcement. The reinforcement strains and the vertical earth pressures at various positions were affected by the sliding and the waste dumping to differing extents. Along the lengths of the geogrid reinforcements, the reinforcement strains showed single-peak distributions. The peak strains were attained in the central part of the reinforcements and the minimum strains were attained at the tail ends. The vertical earth pressures mainly depend on the overlying loads; however, the distributions of them along the reinforcement were nonlinear. Based on the monitoring results, the slope stability evaluation was conducted. It shows that the internal stability of the reinforced MSW slope might be sufficient, while the external stability was insufficient, meaning that this reinforced project was unsuccessful. Finally, various lessons and design suggestions learned from this unsuccessful project were discussed, which could provide valuable references for the future practice of geosynthetic reinforced MSW.     

土工格栅加筋边坡坡顶条基极限荷载的预测

通过土工合成材料加固的边坡,承载能力显著提高,因而获得广泛应用。为了合理的评价加筋边坡的坡顶条形基础的极限荷载,制作了足尺寸模型并进行了试验,采用延性较好但强度较低的聚丙烯(PP)土工格栅对边坡进行了加固,在坡顶通过条形基础(钢梁)施加荷载直至边坡破坏,获得了极限荷载以及边坡的变形和破坏规律,通过细致的测试手段,详细地捕捉到模型的力学响应。在此基础上,通过校验的FLAC数值模型,对土工格栅加筋边坡的承载能力进行了预测,得到了满意的结果。     

Investigations on pullout behavior of geogrid-granular trench using CANAsand constitutive model

Experimental and numerical investigations have been carried out on behavior of pullout resistance of embedded circular plate with and without geogrid reinforcement layers in stabilized loose and dense sands using a granular trench.Different parameters have been considered,such as the number of geogrid layers,embedment depth ratio,relative density of soil and height ratio of granular trench.Results showed that,without granular trench,the single layer of geogrid was more effective in enhancing the pullout capacity compared to the multilayer of geogrid reinforcement.Also,increasing the soil density and embedment depth ratio led to an increase in the uplift capacity.When soil was improved with the granular trench,the uplift force significantly increased.The granular trench improved the uplift load in dense sand more,as compared to the same symmetrical plate embedded in loose sand.Although it was observed that,in geogrid-reinforced granular trench condition,the ultimate pullout resistance at failure increased as the number of geogrid layers increased up to the third layer,and the fifth layer had a negligible effect in comparison with the third layer of reinforcement.Finite element analyses with hardening soil model for sand and CANAsand constitutive model for granular trench were conducted to investigate the failure mechanism and the associated rupture surfaces utilized.The response of granular material in the proposed model is an elastoplastic constitutive model derived from the CANAsand model,which uses a non-associated flow rule along with the concept of the state boundary surface possessing a critical and a compact state.It was observed that the granular trench might change the failure mechanism from deep plate to shallow plate as the failure surface can extend to the ground surface.The ultimate uplift capacity of anchor and the variation of surface deformation indicated a close agreement between the experiment and numerical model.     

顺层岩质路堑边坡破坏模式及设计对策

 在对大量顺层边坡进行详细调查研究基础上,总结提出顺层岩质路堑边坡的分类及8种顺层边坡破坏模式,并对较为常见的滑移–拉裂和滑移–弯曲破坏模式的稳定性分析方法进行讨论。指出在实际工程中,对于滑移–拉裂型顺层边坡,其稳定性分析可通过计算失稳岩层临界长度进行,为避免开挖过程中边坡出现顺层滑动,宜先加固后开挖或边开挖边加固;对顺层清方边坡,可能出现滑移–弯曲破坏,宜采用弹性受压板稳定理论进行边坡稳定性分析后,确定分级高度和加固措施。水是诱发顺层边坡失稳的主导因素,设计中应加强防排水措施。     

Experimental studies of the geosynthetic anchorage – Effect of geometric parameters and efficiency of anchorages

The soil reinforcement by geosynthetic is widely used in civil engineering structures: embankments on compressible soil, slope on stable foundations, embankments on cavities and retaining structures. The stability of these structures specially depends on the efficiency of the anchors holding the geosynthetic sheets. Simple run-out and wrap around anchorages are two most commonly used approaches. In order to improve the available knowledge of the anchorage system behaviour, experimental studies were carried out. This paper focuses on a three-dimensional physical modelling of the geosynthetics behaviour for two types of anchors (simple run-out and wrap around). The pull-out tests were performed with an anchorage bench under laboratory controlled conditions with three types of geosynthetic (two geotextiles and one geogrid) and in the presence of two types of soil (gravel and sand).The results show that there is an optimum length for the upper part of the geosynthetic for the wrap around anchorage.     

Performance of a 33m high geogrid reinforced soil embankment without concrete panel

Geosynthetic reinforced soil embankment are extensively applied in the construction of high-speed railway and highway in mountainous regions but limited field monitoring is conducted on high and steep cases. Aiming to acquire better understanding, a 33-m-high single-tiered wrapped-facing geogrid reinforced soil embankment with the slope of 1 V:0.5H in China was monitored for 2 years during and after construction. Vertical earth pressure, strain of geogrids, horizontal displacement and settlement per layer were recorded and analysed. The results show that the geogrid tensile strains gradually increased during construction. And they were still developing after completion due to creep and subsequent vehicle surcharge load. The predictions of reinforcement loads by the FHWA methods were much higher than the estimated ones from measured strains. The vertical earth pressures linearly grew during construction and then stabilized fast. The horizontal displacement increases with height and the largest value achieved around the top of the slope two years after the construction is 0.14% the total height approximately. The settlement per layer is larger in the lower and middle portion of the embankment and no obvious change is observed over time. This study hopes to serve as a case reference for design and construction of similar reinforcement projects in the future.     

Behaviour of geogrid reinforced soil retaining wall with concrete-rigid facing

Monitoring was carried out during construction of a cast-in-situ concrete-rigid facing geogrid reinforced soil retaining wall in the Gan (Zhou)-Long (Yan) railway main line of China. The monitoring included the vertical foundation pressure and lateral earth pressure of the reinforced soil wall facing, the tensile strain in the reinforcement and the horizontal deformation of the facing. The vertical foundation pressure of reinforced soil retaining wall is non-linear along the reinforcement length, and the maximum value is at the middle of the reinforcement length, moreover the value reduces gradually at top and bottom. The measured lateral earth pressure within the reinforced soil wall is non-linear along the height and the value is less than the active lateral earth pressure. The distribution of tensile strain in the geogrid reinforcements within the upper portion of the wall is single-peak value, but the distribution of tensile strain in the reinforcements within the lower portion of the wall has double-peak values. The potential failure plane within the upper portion of the wall is similar to “0.3H method”, whereas the potential failure plane within portion of the lower wall is closer to the active Rankine earth pressure theory. The position of the maximum lateral displacement of the wall face during construction is within portion of the lower wall, moreover the position of the maximum lateral displacement of the wall face post-construction is within the portion of the top wall. These monitoring results of the behaviour of the wall can be used as a reference for future study and design of geogrid reinforced soil retaining wall systems.     

土工格栅加筋边坡优化设计研究

基于加筋材料的拉拔试验结果和极限平衡理论,针对具体边坡工程进行了不同加筋方案的计算与分析,对比了计算模型和设计方法的适用性,给出了满足边坡稳定条件的最佳设计方案。计算结果表明:采用改进瑞典法或荷兰法的计算结果相近且较原瑞典法有明显的提高,更能体现加筋效果;地震效应和地下水对加筋结构有较大影响;水利法应用于稳定地基上加筋边坡目的性强,能获得满足稳定性条件的合理布筋量;当地下水位较高时,筋材宜通铺。双层加筋效果较单层加筋有明显提高,但并非后者的简单叠加。单层加筋时,铺设位置对于边坡稳定性的影响有限,若铺设于坡身更能减少布筋量,降低造价。对比分析还表明,无论采用何种加筋方式,加筋前后的最危险滑弧位置均会发生改变,后者会向边坡中心和地基深处发展,对于提高其稳定性有明显作用。     

Centrifuge model tests on the behavior of strip footing on geotextile-reinforced slopes

This paper examines the stability of geotextile-reinforced slopes when subjected to a vertical load applied to a strip footing positioned close to the slope crest. Vertical spacing between geotextile reinforcement was varied while maintaining a constant slope angle, load position, soil density and geotextile type. Small-scale physical tests were conducted using a large beam centrifuge to simulate field prototype conditions. After the model was accelerated to 40g, a load was applied to the strip footing until slope failure occurred. Digital image analysis was performed, using photographs taken in-flight, to obtain slope displacements and strain distribution along the reinforcement layers at different loading pressures during the test and at failure. Stability analysis was also conducted and compared with centrifuge model test results. The vertical spacing between reinforcement layers has a significant impact on the stability of a reinforced slope when subjected to a vertical load. Less vertical distance between reinforcement layers allows the slope to tolerate much greater loads than layers spaced further apart. Distributions of peak strains in reinforcement layers due to the strip footing placed on the surface of the reinforced slope were found to extend up to mid-height of the slope and thereafter they were found to be negligible. Stability analysis of the centrifuge models was found to be consistent with the observed performance of geotextile-reinforced slopes subjected to loading applied to a strip footing near the crest.     

土工格栅加筋路堤沉降及边坡稳定性分析

利用岩土工程有限元软件分析比较了路堤在加筋和不加筋两种情况下的沉降大小、不均匀沉降的分布以及边坡稳定性安全系数计算,指出了加筋能够减小路堤的沉降量和不均匀沉降,提高整体稳定性,减少路堤的变形发展,值得推广应用。     

Centrifuge model study on low permeable slope reinforced by hybrid geosynthetics

The objective of this paper is to study the performance of hybrid geosynthetic reinforced slopes, with permeable geosynthetic as one of its components, for low permeable backfill slopes subjected to seepage. Four centrifuge tests have been performed to study the behavior of hybrid geosynthetic reinforced slopes subjected to seepage, keeping the model slope height and vertical spacing of geosynthetic reinforcement layers constant. Centrifuge model tests were performed on 2V:1H slopes at 30 gravities. One unreinforced, one model geogrid reinforced and two hybrid geosynthetic reinforced slope models with varying number of hybrid geosynthetic layers were tested. The effect of raising ground water table was simulated by using a seepage flow simulator during the flight. Surface movements and pore water pressure profiles for the slope models were monitored using displacement transducers and pore pressure transducers during centrifuge tests. Markers glued on to geosynthetic layers were digitized to arrive at displacement vectors at the onset of raising ground water table. Further, strain distribution along the geosynthetic reinforcement layers and reinforcement peak strain distribution have been determined using digital image analysis technique. The discharge for the performed model tests is determined by performing seepage analysis. It was confirmed by the centrifuge tests that the hybrid geosynthetics increases the stability of low permeable slope subjected to water table rise. The hybrid geosynthetic layers in the bottom half of the slope height play a major role in the dissipation of pore water pressure.     

Laboratory investigation of bearing capacity behaviour of strip footing on reinforced flyash slope

The paper presents the results of laboratory model tests on bearing capacity behaviour of a strip footing resting on the top of a geogrid reinforced flyash slope. A series of model footing tests covering a wide range of boundary conditions, including unreinforced cases were conducted by varying parameters such as location and depth of embedment of single geogrid layer, number of geogrid layers, location of footing relative to the slope crest, slope angles and width of footing. The results of the investigation indicate that both the pressure–settlement behaviour and the ultimate bearing capacity of footing resting on the top of a flyash slope can be enhanced by the presence of reinforcing layers. However the efficiency of flyash geogrid system increases with the increasing number of geogrid layers and edge distance of footing from the slope. Based on experimental results critical values of geogrid parameters for maximum reinforcing effects are established. Experimental results obtained from a series of model tests have been presented and discussed in the paper.     

加筋高边坡的稳定分析

采用强度折减法对两个高度分别为 60 m 和 40 m 的土工格栅加筋高边坡的设计断面进行稳定分析,综合考虑塑性区贯通、特征点位移突变、计算不收敛,以及土工格栅的容许抗拉强度等确定相应的安全系数。计算表明采用不同的破坏标准,强度折减法会得到不同的安全系数;如果筋材强度始终得到保证,单纯由土材料的强度损失诱发边坡失稳,这种情况对应的安全系数是比较高的。考虑筋材强度,边坡会在较小的折减系数下因为筋材强度不足而失稳。有限元法能够得到不同情况下各层筋材的受力情况,可以据此进行加筋力的分配,这是极限平衡法所不具备的。     

The effect of geogrid reinforcement on unbound aggregates

Improvement of track, highway and runway unbound aggregate behaviour using geogrids is researched. Geogrid reinforcement into unbound aggregate in most cases will improve the performance of the unbound aggregate portion of a transportation support. Unfortunately, the optimal location and number of geogrid layers have not been established. Presented are experimental results for three different construction possibilities of geogrid reinforcement in the unbound aggregate layers. The aggregate layers were subject to both repeated loading and static loading. The advantages of the different construction methods are studied and field applications are discussed. Finally, conclusions are made regarding the optimal position of the geogrid reinforcement.     

A new approach to rock slope stability – a probability classification (SSPC)

The newly developed system presented in this paper is based on a three-step approach and on the probabilistic assessment of independently different failure mechanisms in a slope. First, the scheme classifies rock mass parameters in one or more exposures and allowance is made for weathering and excavation disturbance. This gives values for the parameters of importance to the mechanical behaviour of a slope in an imaginary, unweathered and undisturbed 'reference' rock mass. The third step is the assessment of the stability of the existing slope or any new slope in the reference rock mass, taking into account both method of excavation and future weathering. From the large quantity of data obtained in the field, the Slope Stability Probability Classification (SSPC) system has been proposed, based on the probabilities of different failure mechanisms occurring. Developed during 4 years of research in Falset, Tarragona province, Spain, it has been used with good results in Austria, South Africa, New Zealand and the Dutch Antilles.     

Case history on failure of a 67 M tall reinforced soil slope

Construction of this 67 m high RSS was completed in December 2006. After seven years in-service, a tension crack was observed at the top of the slope. In March 2015 this RSS structure catastrophically collapsed. This RSS structure collapsed in a compound failure mode; as the failure plane passed beneath, partially behind, and partially through the reinforced soil mass. The failure plane beneath the RSS was along a shale-claystone interface. The failure surface partially behind the RSS was along sandstone bedrock with water-seeping bedding planes dipping out of the rock mass. The failure surface through the upper portion of the RSS is where the geogrid reinforcement was overwhelmed by stresses originating from underlying deformation. The RSS collapse occurred after 8.3 years in-service as the shear strength along the shale-claystone interface decreased and approached the fully softened strength. The primary causative factors of this failure are: (i) an insufficient subsurface investigation program and interpretation of data for design and detailing; (ii) insufficient specifications and construction plan details for both foundation preparation and rock backcut benching; (iii) insufficient foundation preparation and rock backcut benching during construction; and (iv) adaptations to the design made during construction.     

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.     

大型多层次堆积体破坏模式及其稳定性

 边坡的破坏模式与稳定性评价是相辅相成的,稳定性评价可确定坡体的稳定性状态,而破坏模式则可有效确定坡体的加固方式。滑坡的破坏模式可分为牵引式、推移式和复合式滑坡,而不同破坏模式的滑坡体需采用不同的加固措施。在坡体当前应力状态的基础上,依据点安全系数的定义,获取潜在滑面上安全系数的分布形态,3种典型滑坡的计算结果表明：滑移面上安全系数的分布形态可很好的诠释滑坡的破坏特征及分段稳定性状态,并可据此提出有效的加固措施。最后将该方法应用在我国西部某大型堆积体中,在野外地质调查的基础上,对其工程地质结构、破坏模式及其稳定性进行了深入分析,综合评价了多层次堆积体的稳定性及破坏模式,并给出了有效的加固建议。