A case study of geotextile-reinforced embankment on soft ground

Full-scale test embankments, with and without geotextile reinforcement, were constructed on soft Bangkok clay. The performances of these embankments are evaluated and compared with each other on the basis of field measurements and FEM analysis. The analyses of failure mechanisms and the investigations on the embankment stability using undrained conditions were also done to determine the critical embankment height and the corresponding geotextile strain. The high-strength geotextile can reduce the plastic deformation in the underlying foundation soil, increase the collapse height of the embankment on soft ground, and produce a two-step failure mechanism. In this case study, the critical strain in the geotextile corresponding to the primary failure of foundation soils may be taken as 2.5–3% irrespective of the geotextile reinforcement stiffness.     

Effect of reinforcement force distribution on stability of embankments

The effect of reinforcement force distribution on the stability of reinforced embankments is studied. The stability of reinforced embankments is analyzed using the extended generalized method of slices by incorporating the effect of reinforcement. The proposed method is capable of handling features such as a tension crack in the embankment, a varying soil strength profile in the foundation soil and a general slip surface. The method allows the tensile force distribution along the reinforcement to be varied. The stability analysis of reinforced embankments is solved using a spreadsheet optimization tool. The versatility of the proposed method is demonstrated through several cases of reinforced embankments. The results obtained from the proposed method are in good agreement with those obtained from other analytical or numerical methods. The assumed force distribution along the reinforcement appears not to affect the embankment stability in undrained condition. In drained condition, it has some effect on the location of the critical slip surface, but small effect on the factor of safety.     

Combined effect of PVDs and reinforcement on embankments over rate-sensitive soils

A numerical study of the behavior of geosynthetic-reinforced embankments constructed on soft rate-sensitive soil with and without prefabricated vertical drains (PVDs) is described. The time-dependent stress–strain-strength characteristic of rate-sensitive soil is taken into account using an elasto-viscoplastic constitutive model. The effects of reinforcement stiffness, construction rate, soil viscosity as well as PVD spacing are examined both during and following construction. A sensitivity analysis shows the effect of construction rate and PVD spacing on the short-term and long-term stability of reinforced embankments and the mobilized reinforcement strain. For rate-sensitive soils, the critical period with respect to the stability of the embankment occurs after the end of the construction due to a delayed, creep-induced, build-up of excess pore pressure in the viscous foundation soil. PVDs substantially reduce the effect of creep-induced excess pore pressure, and hence not only allow a faster rate of consolidation but also improve the long-term stability of the reinforced embankment. Furthermore, PVDs work together with geosynthetic reinforcement to minimize the differential settlement and lateral deformation of the foundation. The combined use of the geosynthetic reinforcement and PVDs enhances embankment performance substantially more than the use of either method of soil improvement alone.     

The mechanics of reinforcede bankments on soft soils

The mechanics of reinforced embankments on soft foundation soils are examined to try to establish appropriate methods for their analysis and design. The focus is the bearing capacity of the foundation under the combined vertical and shear loading derived from the embankment fill. Lower bound plasticity solutions exist for this case, and indicate the magnitude of the reduction in bearing capacity due to outward shear stresses, compared to the ‘smooth’ case, and the improvement due to inward shear stresses. The improvement is greatest for soil with strength increasing with depth, or relatively thin layers of soft soil.

The action of the reinforcement can be described in terms of the reinforcement firstly acting to carry the outward shear stresses generated by the embankment fill, and secondly providing inward shear stresses to restrain the foundation soil from lateral displacement.

The slip circle analysis for an embankment on soft soil can now be examined using the plasticity solutions as the benchmark. To calculate foundation stability the slip circle is used only in the foundation, with vertical and shear stresses applied on the foundation surface. In the unreinforced case the outward thrust in the fill must be included for the assessment of overall stability, and it is suggested that the thrust is besst calculated directly rather than with the same slip circle used in the foundation carried through the fill.

The conclusion is that the slip circle analysis is satisfactory in the case of deep deposits of soil with strength increasing with depth. Where the depth of the soft soil is limited, however, the slip circle analysis appears to overestimate stability and the use of the plasticity solutions as a basis for design is recommended.     

考虑填料–土工格室相互作用的加筋路堤力学响应研究

针对有砟轨道加筋路堤计算分析中难以考虑土工格室对碎石填料嵌锁及摩擦作用的现状,在大比尺直剪试验基础上建立考虑填料–格室相互作用的三维计算模型。通过室内模型试验验证该计算的可靠度和优越性,进而系统地分析有/无加筋情况下,填料强度、土工格室刚度及地基压缩性等参数对路堤力学响应的影响。结果表明:土工格室的嵌锁作用使得加筋对路堤响应的影响在其布设层发挥程度更大,该影响通过填料与格室及填料内的摩擦作用向周围传递且不断衰减;填料强度较低或地基较软时能加强土工格室对路堤响应的影响,对于填料内摩擦角为20°和地基变形模量为5 MPa的工况而言,加筋后基床侧向位移峰值分别降低60%和72%,应力峰值亦显著降低且分布区域明显扩大;土工格室弹性模量为0.2~2.5 GPa时,路堤力学响应受其影响较大。     

Generalized stability analysis of reinforced embankments on soft clay

By modifying Janbu's generalized procedure of slices (GPS), the stability of a reinforced embankment constructed on a non-homogeneous clay deposit of finite depth is analysed to compute the factor of safety. The non-circular critical surface corresponding to the minimum factor of safety is obtained by a sequential unconstrained minimization technique in conjunction with a conjugate direction method for a multidimensional search and a quadratic interpolation technique for a unidimensional search. The effect of the thickness of the desiccated zone and the variation of undrained shear strength of clay with depth on the factor of safety is considered in the analysis. The influence of the tensile reinforcement force on the location of the critical surface is presented. The effects of the tensile reinforcement force and its orientation as well as the number of reinforcing layers on the stability of embankment have also been studied. The results obtained from the present analysis are compared with solutions reported in the literature.     

Centrifuge model tests on the deformation behavior of geosynthetic-encased stone column supported embankment under undrained condition

A series of centrifuge model tests were carried out to investigate the performance of geosynthetic-encased stone columns (GESCs) supported embankment under undrained condition. The influence of stiffness of encasement, basal reinforcement and embankment loading on the deformation behavior of GESCs were also assessed. The centrifuge test results reveal that under undrained condition, compared to ordinary stone column (OSC) supported embankment, the settlement of column has reduced by 50% and 34% when columns were encased with high and low stiffness geogrids respectively. Moreover, under identical embankment loading condition, the stress concentration ratio has increased significantly upon inclusion of basal reinforcement in the GESCs supported embankment. In case of OSCs supported embankment, columns experiences bulging in the top portion, inward bending in the central portion and a noticeable shear at the bottom portion. However, when columns were encased with geogrid layer, bulging in the top portion was significantly reduced but the inward bending of columns were noticed. With the inclusion of basal reinforcement, bending curvature of columns increases thereby inducing higher settlement in columns and relatively lesser settlement in surrounding soil. The differential settlement between the encased column and the surrounding soil under embankment loading has been considerably reduced with the inclusion of basal reinforcement.     

均质软土地基上土堤稳定性的极限分析方法

为评估均质软土地基上土堤的稳定性,假设地基土体沿圆弧形滑动面破坏,不排水抗剪强度随深度线性增加,并假设土堤材料沿对数螺旋线形滑动面破坏,采用黏聚力和内摩擦角描述其抗剪强度,基于上限定理,建立了一种稳定性分析方法,用于计算得到土堤的无量纲化的稳定数和稳定图,并确定土堤的最小安全系数和临界滑动面。采用本文方法对土堤失稳现场试验工程案例开展了计算分析,结果表明,所得到的最小安全系数与土堤的实测失稳条件吻合,最小安全系数与临界滑动面的计算结果也与变分极限平衡法和条分极限平衡法所得到的计算结果吻合良好。     

成层软土地基上土堤填筑稳定性的塑性极限分析

针对成层软土地基上土堤填筑的稳定性分析问题,考虑地基土体的不排水抗剪强度既随土层变化又随深度任意线性变化,根据工程实践中揭示的堤基深层破坏模式,基于塑性极限分析的上限理论,构建满足运动许可的对数螺旋线与圆弧组合的转动破坏机制,建立了便于快速评估成层软土地基上土堤填筑稳定性的临界高度、无量纲化的稳定数、稳定图、最小安全系数和临界滑动面的分析计算方法,并将计算结果与2个土堤失稳现场试验工程案例的实测结果进行了对比。结果表明,采用本文方法计算得到的土堤最小安全系数接近于土堤失稳时的最小安全系数的理论值1.0,计算确定的临界滑动面也与实测的临界滑动面比较接近,从而验证了本文方法的适用性。     

袖阀管劈裂注浆加固粉土路基实验研究

通过室内大型模拟实验,对粉土路基强度、刚度在毛细水作用下的衰减规律和袖阀管劈裂注浆加固路基的效果进行研究。实验模拟新建路基、潜水位20 cm/50 cm的毛细水作用路基、袖阀管注浆劈裂土体加固路基等4个工况,得到粉土路基土体含水率的分布规律和物理力学指标;在分级循环加卸荷载的作用下,得到路基模型总体刚度、受力和变形的特性;挖除路基模型,观察袖阀微桩及浆液扩散的形态。结果表明：毛细水作用可使粉土路基模型总体刚度降低70%,土体抗剪强度下降,路基竖向塑性变形随荷载增大快速增加;水平分层劈裂土体凝固的浆液不仅阻断了毛细水的上升通道,而且与底端扩大的微桩形成空间骨架与土体共同作用,骨架与土体各承担荷载大约50%,袖阀管加固粉土路基效果显著。     

Influence of geosynthetic stiffness on bearing capacity of strip footings seated on thin reinforced granular layers over undrained soft clay

Thin granular fill layers are routinely used to aid the construction of shallow footings seated over undrained soft clay foundations and to increase their load capacity. The influence of time- and strain-dependent reduction in reinforcement stiffness on the bearing capacity and load-settlement response of a footing seated on a thin reinforced granular fill layer over undrained soft clay foundations is examined in this paper using finite-difference method (FDM) numerical models. The time- and strain-dependent stiffness of the reinforcement described by a two-component hyperbolic isochronous tensile load-strain model is shown to influence the bearing capacity and load-settlement response of the reinforced granular base scenario. The additional benefit of a reinforced granular layer diminishes as the time-dependent stiffness of the geosynthetic reinforcement increases. An analytical solution for the ultimate bearing capacity of strip footings seated on thin unreinforced and reinforced granular layers over undrained clay is proposed in this study. The main practical outcome from this study are tables of bearing capacity factors to be used with the analytical solution. The bearing capacity factors were back-calculated from the numerical analyses and account for the influence of rate-dependent properties of geogrid reinforcement materials and clay foundations with soft to very soft undrained shear strength.     

剪切应变速率对淤泥土力学性状影响试验研究

为研究剪切应变速率对高含水量淤泥土力学性状的影响,运用SPAX-2000真三轴测试系统进行了不同周围压力和不同应变速率的固结不排水剪切试验,分析了淤泥土不排水抗剪强度和孔隙水压力随应变速率的变化规律。试验结果表明,高含水量淤泥具有“应变速率软化”现象。在剪切过程中(应变速率为10^-6/s～10^-2/s),淤泥土不排水抗剪强度随剪切应变速率增长而增长,符合指数变化规律;根据试验数据,建立了经验方程式,并求得其应变速率参数η_e1为0.130 5,η_e2为0.131 9;当变率增长10倍时,淤泥土的不排水抗剪强度增长率为13.12%。不同于强结构性黏土,淤泥土的强度随剪切应变速率呈渐进性变化,未出现明显临界速率转折点。在加载初期,孔隙水压力增长和最大孔隙水压力受应变速率影响较明显;在试验过程中,孔隙水压力变化具有一定波动性和滞后性。     

Effect of principal stress direction during consolidation on the shear strength properties of natural granite residual soil

In numerous real-life civil engineering practices, including multi-stage embankment construction and foundation pit excavation, the direction of the major principal stress σ₁ becomes rotated. In these cases, the granite residual soil may be subjected to inclined consolidation (IC) with σ₁ being inclined, because of the relatively high permeability as a result of the fissures formed during weathering. While the effect of the σ₁ direction during the shear on the strength of granite residual soil (inherent strength anisotropy) has been primarily established, little is known about how the soil strength is affected by the direction of σ₁ during consolidation. This paper presents the effects of IC on the shear strength properties of natural granite residual soil through undrained hollow cylinder torsional shear tests. The effect of the soil structure is also considered by testing remolded soil specimens. The results reveal that while IC changes neither the shape of stress–strain curve nor the specimen features at failure, it leads to an increased ultimate shear strength in terms of both the undrained strength and stress ratio, with the remolded soil being more affected. The presented data provide new insights into the understanding of residual soil strength behaviors.     

台华高速公路路堤失稳原因分析与对策

台华高速公路采用干振碎石桩进行软土地基处理,在路堤填筑过程中发生了地基失稳滑移.通过数值方法对路堤填筑过程中地基沉降、超静孔隙水压力及地基塑性区的发展变化进行了模拟计算,并结合现场补勘数据,分析了引起路堤失稳滑移的原因.在此基础上,提出刚性桩地基处理加固方案.数值计算及现场勘查结果表明,在强度较低的软土中采用干振碎石桩进行地基处理是不成功的,施工不仅没有形成完整的碎石桩体,还因扰动降低了原状地基土强度,路堤填筑速度过快则直接导致了地基滑移破坏;当采用刚性桩加固软基时,桩间土仅承担小部分路堤荷载,而大部分路堤荷载由桩承担并传递给深层承载力较高的土层,因此路堤的稳定性高、沉降小,可实现快速填筑.数值分析及现场刚性桩试验段测试结果均表明：刚性桩方案可以满足路堤填筑要求.     

天津滨海软土不同应力路径下抗剪强度对比试验研究

现有基坑支护设计参数试验条件与实际不符。选取天津滨海新区海相软土为研究对象,进行三轴固结不排水试验、三轴不固结不排水试验和减p路径试验并对比分析了几种试验条件下土的强度参数。得到:滨海软土在不同应力路径下的强度与初期固结方式有关。减p路径对总应力状态下抗剪强度有影响,但对有效应力状态下抗剪强度基本无影响。在滨海软土基坑支护设计中,选用CU试验参数进行计算,设计偏于安全。     

Shear strength of geomesh reinforced clay

Consolidated undrained and consolidated drained triaxial tests were conducted on overconsolidated geomesh reinforced clay. The geomesh stiffness, number of reinforcing layers, and the confining pressure were varied. It appeared that the geomesh reinforcement enhances the strength in both the undrained and drained conditions. However, the clay-geomesh interaction is different from that known for sand. The enhanced confining stress concept is not applicable and the reinforcement stiffness does not significantly affect the strength. The A pore pressure parameter at failure is higher for reinforced than for unreinforced samples and it increases with an increasing number of reinforcing layers. The reinforcement effect in the undrained condition is due to an increase in the cohesion component of shear strength, whereas in the drained condition the effect is due to an increase in the angle of internal friction.     

Knicknachweis von Mikropfählen in geschichteten Böden

Buckling resistance of micropiles in varying soil layers. Micropiles are used as a foundation or supplementary foundation element. These piles with diameters less than 0.3 m are regulated in EN 14199. Slender micropiles subjected to axial compression are prone to buckling failure if the lateral foundation of the soil is insufficient. According to design codes the buckling resistance has to be checked if the undrained shear strength is less than 10 to 15 kN/m². Recently a design formula for the buckling resistance of a micropile under constant compression in a homogenous soil layer has been published and it has been shown that a buckling failure can also occur for higher undrained shear strengths. In this paper the design formula is extended to micropiles which penetrate varying soil layers and carry the load by skin friction.     

考虑参数空间变异性的无限长边坡可靠度分析

目前有关土体参数空间变异性对边坡稳定性影响的研究没有考虑抗剪强度参数随深度变化的影响。为此,提出了考虑土体抗剪强度参数均值随深度变化的无限长边坡稳定性概率分析方法。采用Karhunen-Loeve展开建立了表征土体空间变异性的随机场模型。探讨了考虑土体抗剪强度参数空间变异性时边坡失效概率和最危险滑动面的变化规律。最后,以无限长边坡稳定性概率分析问题为例验证了所提方法的有效性。结果表明：土体抗剪强度参数的空间变异性对无限长边坡失效概率有明显的影响,边坡失效概率随土体抗剪强度参数相关距离的增加而减小。对于不排水黏性边坡来说,边坡不排水抗剪强度随深度变化越明显,边坡失效概率越小。边坡最危险滑动面大部分都位于边坡底部。对于摩擦/黏性边坡来说,随着边坡抗剪强度随深度变化的增强,边坡失效概率有先减后增的趋势。抗剪强度参数随深度变化不同趋势对最危险滑动面分布规律有明显的影响。不考虑抗剪强度参数均值随深度变化将会导致最危险滑动面最可能出现在边坡底部。     

Reappraisal of loading rate effects on sand behavior in view of seismic design for pile foundation

This study re-visits the effects of loading rate on sand behavior in view of seismic design of pile foundation. Based on an extensive literature review covering both element testing and model testing, the paper summarizes the current understanding of the loading rate effects. The paper then describes the development of test apparatus used in this study and presents the data obtained from a series of monotonically increasing triaxial compression tests on Toyoura sand, varying the strain rate in the range of 0.005%/s to 250%/s. A total of 36 tests were conducted on dry and saturated sand, both under drained and undrained conditions with two confining pressures. The experimental results confirm that the soil strength and the soil stiffness increase as the strain rate increase. This paper also concludes that the internal friction angle and deformation modulus increase when the strain rate is high. The implications of the loading rate effect on pile foundation design are then presented.     

基于饱和土总应力强度指标的非饱和土强度 理论及其应用

 实际工程中求解和测定吸力比较困难,特别在不排水条件下,吸力并非常量,从而难以确定。而饱和度相对容易确定,选择饱和度代替吸力,建立关于饱和度的非饱和土的抗剪强度公式有着重要的实用价值。提出不排水(等含水量)条件下的基于饱和土总应力强度指标的抗剪强度公式,克服了以往相关模型只适合于较小范围的缺陷,能够实现从非饱和土状态到饱和状态的平稳过渡,同时更能反映土体的工程特性;结合非饱和土的土水特征曲线试验及常含水量三轴剪切试验成果,确定适用于云南红黏土的相关拟合参数;将所得抗剪强度公式应用于地基承载力分析中,得到地基承载力随饱和度的减少接近线性增加的结论。