土工格栅加筋拓宽路堤有限元分析

采用二维非线性有限元法对软土地基上的拓宽路堤土工格栅加筋作用和效果进行模拟分析。计算中采用一维抗拉单元来模拟土工格栅,采用接触面单元考虑筋土界面的状态非线性。研究土工格栅嵌入老路基长度、格栅设置层数对格栅拉力、拓宽路堤不均匀沉降和水平位移的影响。计算结果表明,格栅嵌入老路基长度越长,拓宽路堤侧向位移和新老路基差异沉降越小,增加格栅层数并不会显著减小拓宽路堤的差异沉降。     

Measured and simulated results of a Kenaf Limited Life Geosynthetics (LLGs) reinforced test embankment on soft clay

For the first time, woven Kenaf Limited Life Geosynthetics (LLGs) were used for short term reinforcement of full scale embankment constructed on soft clay and their behavior is presented. The observed data in terms of settlements, excess pore water pressures and deformations or stresses in the reinforcements were compared with the simulated data. Two types of Kenaf LLGs were utilized, namely: coated and not coated with polyurethane. The coating can reduce water absorption and increase their life time. Subsequently, numerical simulations were performed on the behavior of Kenaf LLGs reinforced embankment using 2D and 3D finite element software. The rates of settlement from FEM 2D method overestimated the observed settlements data while the FEM 3D predictions agreed with observed settlements due to the three-dimensional geometrical loading of the embankment with length to width ratio (L/B) of 1.0. Regarding the maximum excess pore-water pressures at the locations of 3 m and 6 m depth, the FEM 2D analyses overestimated while the FEM 3D simulation yielded satisfactory agreement with the observed data. The reinforcement deformations and stresses in both coated and non-coated Kenaf LLGs reinforcement have higher values at the middle portions of the embankment and the predicted results from FEM 3D simulation yielded closer deformations of Kenaf LLGs reinforced than the FEM 2D simulation. Consequently, FEM 3D simulation captured the overall behavior of the Kenaf LLGs reinforced embankment with more reasonable agreement between the field observations and the predicted values compared to the FEM 2D simulation. The behavior of the sections on coated and non-coated LLGs were similar. The Kenaf LLGs can be applied for short term embankment reinforcement in order to improve the stability of embankment on soft clay.     

Investigation of an embankment failure in soft clay

Reported here are the results of an investigation into the failure of a low road embankment founded in soft Bangkok clay and flanked by retaining walls. A rotational slip appears to have occurred, the factor of safety of the embankment under its own weight being only about 1.2. While traffic loading was certainly a contributory factor, failure was probably caused by the excess pore pressures generated in the soft clay when long concrete piles were driven adjacent to the retaining wall as a remedial measure.     

Consolidation of soft clay foundation improved by geosynthetic-reinforced granular columns: Numerical evaluation

Soft clays are problematic soils as they present high compressibility and low shear strength. There are several methods for improving in situ conditions of soft clays. Based on the geotechnical problem's geometry and characteristics, the in situ conditions may require reinforcement to restrain instability and construction settlements. Granular columns reinforced by geosynthetic material are widely used to reduce settlements of embankments on soft clays. They also accelerate the consolidation rate by reducing the drainage path's length and increasing the foundation soil's bearing capacity. In this study, the performance of encased and layered granular columns in soft clay is investigated and discussed. The numerical results show the significance of geosynthetic stiffness and the column length on the embankment settlements. Furthermore, the results show that granular columns may play an important role in dissipating the excess pore water pressures and accelerating the consolidation settlements of embankments on soft clays.     

Mehrlagig mit Geogittern bewehrte Erdkörper über pfahlartigen Gründungselementen – Numerische Simulation des Verformungsverhaltens unter statischer und zyklischer Einwirkung

Numerical simulation of the deformation behaviour of multi‐layered geogrid‐reinforced embankments on pile foundations under static and cyclic loading. Embankments for traffic constructions above soft soil are often founded on piles and geogrids are inserted at the bottom of the embankment. In the framework of present design procedures the cyclic (dynamic) traffic loads are considered in a very simplified manner. They are replaced by a static load with a magnification factor. The established model perception for static loading is a redistribution of stress due to arches in the embankment and tensile stress in the geogrids. However it has to be expected that the load bearing and deformation behaviour of such soil structures will change during the life time of the structure (millions of cycles). The cycles cause an accumulation of deformations and changes of stresses in the soil. This may cause a large destruction of the arches and may lead to unexpected settlements. Numerical strategies and constitutive models for the investigation of the behaviour of soils under high‐cyclic loading using finite element method were recently developed. This paper presents the results of such calculations of multi‐layered geogrid‐reinforced embankments on soft soil for the 2D case. The results show that, depending on the position of the geogrids in the embankment, their contribution is unequally to the bearing behaviour and that the stress arches will actually be destroyed under cyclic loading.     

Performance of a road embankment on soft clay supported on a Geocell mattress foundation

A five kilometre-long road was constructed in the New Territories of Hong Kong between 1989 and 1992. Part of the road crossed an alluvial plain on an embankment, and at two locations on the alluvial plain this embankment was underlain by soft clay deposits. A geocell mattress was used as the means of supporting the embankment on these soft clay deposits. Wick drains were installed beneath the embankment to speed up consolidation of the soft clays, and using staged construction the embankment was built up to a maximum height of 10 m.

The embankment was fully instrumented with pneumatic piezometers, inclinometers, hydrostatic profile gauges, settlement plates, surface settlement markers, and lateral movement blocks. This paper presents the results of the instrumentation monitoring and the performance of the geocell mattress foundation during the construction of the embankment. At one section, unusually high excess pore water pressures and a slight heave of the toes of the embankment were recorded. The accompanying small lateral extension and the deflected shape of the geocell mattress indicated that it had behaved as a raft foundation to the embankment.     

Shear behaviour of a geogrid-reinforced coarse-grained soil based on large-scale triaxial tests

In China, weathered mudstone geogrid-reinforced coarse-grained soil is used extensively for road embankments. However, the microstructure and disintegration process of weathered mudstone remain unclear. Furthermore, few studies have investigated the shear behaviour of this kind of geogrid-reinforced fill through large-scale triaxial tests against grain size effects. To bridge this gap, this study reports results from large scale consolidated undrained (CU) and consolidated drained (CD) triaxial tests as well as scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and disintegration tests on weathered mudstone geogrid-reinforced coarse-grained soil. EDX spectrograms and SEM images show that coarse grains disintegrate rapidly mainly owing to the high clay mineral content and loose microstructure. Therefore, a suitable disintegration time (∼15 days) is recommended for embankment sits. The shear behaviour of this geogrid-reinforced fill is investigated in detail through large-scale triaxial tests. The shear deformation tends toward strain hardening behaviour with an increase in the number of geogrid layers and the confining pressure. Geogrids significantly improve the apparent cohesive strength of coarse-grained soil. The pore water pressure is found to develop rapidly in the 0%–4% axial strain phase but dissipate slowly in the 4%–12% axial strain phase. During shear, the pore pressure coefficient A values of 0.2–0.4 are indicative of the partial saturation of specimens. Consequently, pore water pressure development is mainly attributed to the movement and rearrangement of coarse particles in coarse-grained soil. Experimental data show that the geogrid-reinforcement coefficients increase with the number of geogrid layers, and a 20-cm separation between geogrid layers is recommended for embankment construction sites. The number of geogrid layers influences the geogrid–soil interface’s mobilization and the slip surface type. Test results revealed three types of slip surfaces related to the failure shapes of specimens. Then, based on CU experimental data, the parameters of the Duncan–Chang constitutive model are discussed.     

Mitigation of seasonal temperature change-induced problems with integral bridge abutments using EPS foam and geogrid

Expansion of bridge girders in summer moves integral bridge abutments toward backfill, causing high lateral earth pressures behind the abutment. Some backfill material slumps downward and toward the abutment when the abutment moves away from the backfill due to bridge girder contraction in winter. Placement of geogrids within the backfill can increase stability of the backfill while placement of compressible inclusions (e.g., Expanded Polystyrene (EPS) foam) can reduce lateral earth pressures behind the abutment caused by bridge girder expansion. In this study, six physical model tests were conducted with 30 abutment top movement cycles due to simulated seasonal temperature changes to study the performance of integral bridge abutments with different mitigation measures. The test results showed that geogrid reinforcements caused higher maximum lateral earth pressures at the same abutment movement, but geogrids with wrap-around facing significantly reduced the backfill surface settlements. The combination of the EPS foam and geogrids could minimize lateral earth pressure increase and backfill settlement. The EPS foam reduced the abutment toe outward movement when the abutment top was pushed against the backfill; however, the mitigation effects by the EPS foam was limited due to its small thickness and relatively high elastic modulus in this study.     

斜坡基底软土桩–网复合地基变形特性离心试验研究

为探究斜坡基底软土–桩网复合地基的变形特性,以沿海铁路线某深厚软土地基断面为原型,开展比尺为1∶100室内离心模型试验。通过对试验数据的整理分析,结果表明:(1)路堤顶部两侧的竖向和横向位移、两侧沉降差在运营期都有明显增长,但竖向和横向位移主要产生于路堤施工阶段,而沉降差基本产生于运营期内;(2)加筋垫层对其下方的地基表层横向位移有明显约束。地基右侧土体的竖向和横向位移关于路堤中心不对称,表现为右侧大于左侧,地基表面呈现出明显偏向斜坡外侧的非对称"沉降盆";(3)斜坡桩和悬浮桩的桩顶竖向和横向位移明显大于平台桩,平台桩潜在破坏模式为弯拉或弯剪破坏,斜坡桩为倾斜破坏,悬浮桩为倾斜+横移破坏;(4)桩发生明显上刺和下刺现象,且淤泥质黏土表现出流体特性发生绕桩横向流动。     

Sensitivity analysis of a scrap tire embankment using Bayesian influence diagrams

Scrap tires have several properties that make them preferable to other materials as fills for embankment construction, including light weight (the dry unit weight is 1/3 that of soils), high hydraulic conductivity (up to 23.5 cm/s), and low thermal conductivity. These properties of scrap tire fills result in low lateral pressures on the abutment wall and in reduced design and construction costs. The low thermal conductivity helps to prevent permafrost action of soil layers beneath it and failure of the subgrade due to frost penetration.However, scrap tires possess high compressibility, a property that leads to settlement of the fill and consequent failure of the embankment. Other undesirable attributes of scrap tire embankments are susceptibility to internal heating and leaching of substances into surrounding water.An efficient means of controlling such undesirable attributes in the field is by comparing them with those simulated from a model embankment developed using Bayesian influence diagrams. In this work, the essential responses simulated using the Analytica^® software program are the temperature, lateral pressure, settlements, and leachate characteristics. The most critical embankment characteristics, based on the maximum probability densities, are the settlement and horizontal pressures, which are relatively low at 0.428 and 0.0034, respectively, because the likelihood that these values will be exceeded in the field is high. Temperature response was not considered critical because the maximum probability density simulated was 0.9301. Limits for leachate concentrations were also obtained for the model embankment based on ASTM D 6270 (1998) standards.     

Spannungs‐ und Porenwasserdruckentwicklung infolge Schlitzwand‐Herstellung in weichen bindigen Böden

Stress and pore water pressure development during the construction process of dipahragm walls in soft clay. The influence of a diaphragm wall construction on the stress field in a soft clayey soil is investigated by the use of a three‐dimensional FE‐model of seven adjacent wall panels. The installation procedure comprises the excavation and the subsequent pouring of each panel taking into account the increasing stiffness of the placed fresh concrete. The soft clay deposit is described by a visco‐hypoplastic constitutive model considering the rheological properties and the small‐strain stiffness of the soil. The construction process considerably affects the effective earth and pore water pressures adjacent to the wall. Due to concreting, a high excess pore water pressure arises, which dissipates during the following construction steps. The earth pressure finally shows an oscillating, distinct three‐dimensional distribution along the retaining wall which depends on the installation sequence of the panels and the difference between the fresh concrete pressure and the total horizontal earth pressure at rest. In comparison to FE‐calculations adopting the earth pressure at rest as initial condition, greater wall deflections and surface ground settlements during the subsequent pit excavation can be expected, as the average stress level especially in the upper half of the wall is increased by the construction procedure of the retaining structure.     

Laboratory pull-out tests using bamboo and polymer geogrids including a case study

This study investigates the interaction between soil and geogrids by using both direct shear and pull-out tests and applied the results to a case study. A polymer geogrid and bamboo grids were used with clayey sand and weathered clay as backfill since these materials are readily available in Thailand. The results indicated that the interaction between soil and reinforcement consists of: (a) the adhesion between soil and reinforcement on the solid surface area of the geogrid; and (b) the bearing capacity of soil in front of all transverse members of the geogrids which behaved as a strip footing embedded in the soil. The proposed design procedure for pull-out resistance agreed fairly well with the laboratory pull-out test results. In addition, it was observed that bamboo grids have higher pull-out resistance per unit area than the polymer geogrids. Moreover, the cohesive fill proved to be quite effective when used with geogrid reinforcement. Finally, the proposed design procedure and test results were applied to a case study on an irrigation canal bank repaired by the Public Works Department of Thailand using cohesive backfill and Tensar SS2 geogrids resulting in much improved slope stability.     

桩网复合地基的模型试验加固效果对比研究

为了探究不同形式的桩网复合地基加固软土的效果,设计了3组复合地基进行室内模型试验.第1组为PVC管材加固软土地基,第2组为桩承式复合地基,第3组为PVC管材与砂桩复合地基.在模型箱钢板内侧壁涂上黄油,并覆以塑料薄膜来减小边界效应；在PVC管材和土工编织网(格栅)上粘贴应变片,来测试PVC桩体应力及加筋拉力；在桩间土、砂...     

Strength enhancement of clay by encapsulating geogrids in thin layers of sand

Large size direct shear tests (i.e.300 × 300 × 200 mm) were conducted to investigate the possibility of strength enhancement of clays reinforced with geogrids embedded in thin layers of sand. In this paper test results for the clay, sand, clay–sand, clay–geogrid, sand–geogrid and clay–sand–geogrid samples are presented and discussed. Thin sand layers with thicknesses of 4, 6, 8, 10, 12 and 14 mm were used to quantify their effect on the interaction between the clay and the geogrids. In this regard effects of sand layer thickness, normal pressure (i.e. confinement) and transversal members of geogrids were investigated. All the tests were conducted using saturated clay with no drainage allowed. Test results indicate that provision of thin layers of sand for encapsulating the geogrids is very effective in improving the strength and deformation characteristics of saturated clay. Maximum strength enhancement was derived at an optimum sand layer thickness of 10 mm which proved to be independent of the magnitude of the normal pressure used. For a particular sand layer thickness, increasing the normal pressure resulted in enhanced strength improvement. Results also showed that removal of the geogrid transversal members resulted in reducing the strength of the reinforced samples by 10% compared to geogrids with transversal members. Encapsulating geogrids in thin layers of sand not only will improve the performance of clays if used as backfill it would also provide drainage paths preventing pore water pressure generation on saturation of the backfill.     

柔性与硬性地工格网在砂土与风化泥岩围压下之力学特性

地工格网（以下称格网）用於加劲土壤时，除考虑无围压下的张力行为之外，围压下之力学性质更是设计考量的重点。实际工程应用而言，基於经济考虑，期以现地土壤作为回填材料。本研究分别以拉出、围压抗张与直剪三种试验来探讨格网放土壤中之力学行为；并利用凝聚性泥岩与非凝聚性细砂作为回填材料，评估两种回填材料对加劲成效之影响。结果显示，柔性格网之肋条在拉出过程中易扭曲，造成主应力面旋转的现象，以致拉出阻抗大放硬性格网；围压下格网抗张的应力-应变行为可分为三阶段，即束制阻抗期、张力发展期与破坏期。束制阻抗期大都於3％应变内即已完成；在低围压情况拉出阻抗达20％～60％之拉出强度（相同应变），在高围压下达150％。由直接剪力试验结果可以预测：（a）格网／泥岩加劲结构-低围压时，剪力破坏面应通过格网／泥岩之界面；而高围压时，剪力破坏面应通过泥岩上体。（b）格网／细砂加劲结构-低围压与高围压下剪力破坏面应通过格网／细砂之界面。     

软土地基加筋石灰土路堤离心模型试验数值模拟

 建立以离心试验几何尺寸的有限元数值模型,模拟变加速度加载下软土地基加筋石灰土路堤中的位移、土压力、孔隙水压力和加筋拉力随时间的变化规律,并与离心模型试验结果进行比较;同时,采用该数值模型计算了不加筋、加1,2层筋时路堤和地基位移情况。计算结果表明,加筋路堤沉降量、土压力、孔隙水压力和加筋拉力的计算值与离心试验实测值吻合很好或基本一致,表明该数值模型是合理的;不加筋路堤的中心沉降量和坡脚下地基水平位移比加1层筋时明显大一些,两者在加速度为100.0 g时地面坡脚处的水平位移差值达近2 mm,而加2层筋时位移与加1层筋接近。     

Settlements in Fine-Grained Soils Under Cyclic Loading

Based on the methods previously presented by the authors (Yasuhara et al., 1992, 1994, 1996) for predicting the degradation in strength and stiffness of soft clays in the course of cyclic loading, a methodology has been developed to estimate the cyclic loading-induced settlements. The method also includes not only immediate settlements but also post-cyclic long-term settlements due to dissipation of cyclically induced excess pore pressures in soft soils. The simplified formulae included in the proposed methodology are given as functions of the amplitude of cyclic-induced excess pore pressure normalized by the confining pressure, u/p'_c, plasticity index I_p and factor of safety against bearing capacity failure, F_s. The calculations of cyclic-induced settlements were conducted for soft soil deposits with different index and geotechnical properties. The results calculated using the proposed methodology are presented in the form of a design chart to give the settlement versus normalized excess pore pressure ratio relations including the effects of the plasticity index and safety factor for bearing capacity. An example of the calculated results using the proposed procedure for the earthquake-induced settlements of embankments founded on soft clay, is presented to demonstrate the practicality of the method for design at fields.     

中低压缩性土地区桩承式加筋路堤现场试验研究

将桩承式加筋路堤技术应用于中低压缩性土地区高速铁路桥台和涵洞之间填方路基的处理,通过逐渐改变CFG桩桩长形成刚度均匀变化的地基加固区,严格控制线路纵向差异沉降。通过现场试验对桥台、涵顶和路基中心地基沉降进行了长期观测,同时对桩承式加筋路堤桩间土沉降、孔隙水压力、格栅上下表面土压力和格栅变形进行了长期监测分析。研究结果表明:桩承式加筋路堤可有效减小中低压缩性土地基沉降,总沉降小且很快趋于稳定;桩承式加筋路堤通过土拱效应和张拉膜效应将路堤荷载向桩帽传递,格栅下桩土应力比明显高于格栅上,张拉膜效应明显,格栅上桩土应力比接近1.0,土拱效应较弱;格栅在路肩处发挥的作用强于线路中心处。     

加筋形式对桩承式路堤工作性状影响的试验研究

对无加筋和采用不同加筋材料、加筋层数下桩承式路堤的工作性状进行了三维模型试验研究,侧重分析了桩土应力比、应力折减系数、填土中竖向应力分布、地基沉降等内容。结果表明加筋材料的设置有利于荷载向桩顶的转移,可有效减小沉降,但不同加筋形式下桩承式路堤的工作性状有所不同。使用单层或双层土工布时,路堤的荷载传递机理主要是填土的土拱效应和加筋材料的拉膜效应,但拉膜效应发挥相对较晚。使用双层格栅时,加筋材料与周围砂土形成半刚性平台。单层格栅的作用介于两者之间。试验结果与常规拉膜效应设计方法的对比表明,若假设荷载只由相邻桩间的加筋材料条带承担,计算的拉力将偏大,过于保守。