基于蒙特卡罗法的加筋路堤稳定可靠性分析

软土路基上加筋路堤的稳定性可以采用圆弧滑动极限平衡法进行分析,加筋路堤的破坏可以归纳为圆弧内加筋的破坏、圆弧外加筋的破坏以及加筋路堤的整体破坏,建立了计算加筋路堤稳定系数K的3个不同的稳定系数表达式,运用蒙特卡罗法对设计参数随机变量与加筋土路堤稳定可靠指标的关系进行分析,通过数值计算发现密度越小,黏聚力、内摩擦角、筋材抗拉强度和筋土摩擦角越大,加筋土路堤稳定性越好,因此,在加筋土路堤可靠性设计中建议采用密度小、黏聚力和内摩擦角大的填料以及高强度筋材,并且重点考虑内摩擦角的变异水平。     

Stability analysis of stone column-supported and geosynthetic-reinforced embankments on soft ground

This study focuses on the stability of stone column-supported and geosynthetic-reinforced embankments on soft soil. An upper-bound limit state plasticity failure discretization scheme (known as discontinuity layout optimization (DLO)), which determines the embankment stability without pre-assuming a slip surface, is used. The relationships between the stability of stone column-supported and geosynthetic-reinforced embankments and various influencing parameters, including the soil strength, geometric configuration, reinforcement strength, and area replacement ratio, are analysed. It is found that geosynthetics provide a significant contribution to embankment stability. Two failure mechanisms of geosynthetics (i.e., rupture failure and bond failure) are revealed and the effect of geosynthetics on embankment stability is governed by the failure mode. The application of stone columns mitigates the risk of geosynthetic failure. To provide an analytical solution for primary design in engineering practice, an approach based on the limit equilibrium method is proposed. Validations are performed with the DLO solution to demonstrate the accuracy and reliability of the developed analytical approach.     

Two-dimensional soil arching evolution in geosynthetic-reinforced pile-supported embankments over voids

Soil arching and tensioned membrane effects are two main load transfer mechanisms for geosynthetic-reinforced pile-supported (GRPS) embankments over soft soils or voids. Evidences show that the tensioned membrane effect interacts with the soil arching effect. To investigate the soil arching evolution under different geosynthetic reinforcement stiffness and embankment height, a series of discrete element method (DEM) simulations of GRPS embankments were carried out based on physical model tests. The results indicate that the deformation pattern in the GRPS embankments changed from a concentric ellipse arch pattern to an equal settlement pattern with the increase of the embankment height. High stiffness geosynthetic hindered the development of soil arching and required more subsoil settlement to enable the development of maximum soil arching. However, soil arching in the GRPS embankments with low stiffness reinforcement degraded after reaching maximum soil arching. Appropriate stiffness reinforcement ensured the development and stability of maximum soil arching. According to the stress states on the pile top, a concentric ellipse soil arch model is proposed in this paper to describe the soil arching behavior in the GRPS embankments over voids. The predicted heights of soil arches and load efficacies on the piles agreed well with the DEM simulations and the test results from the literature.     

抗土洞塌陷的低填方加筋路基荷载传递机制及设计方法

低填方加筋路基对地基承载力要求较低,同时利用水平加筋法跨越尺寸较小的土洞能有效预防路堤出现突发式局部沉陷,提高路堤抗工后沉降和失稳的安全系数,正被逐步应用于岩溶土洞地区道路工程;但其作用机理复杂,现存设计方法大都偏于保守,考虑抗土洞塌陷的低填方加筋路基荷载传递机制的设计方法亟待提出。通过揭示受土洞塌陷影响的低填方加筋路基荷载传递机制,推导了考虑路基差异沉降引起土体应力偏转的竖向应力计算方法,假定塌陷区上方加筋体作用抛物线荷载,从而明晰了加筋体应力-应变状态;应对岩溶区不同形态的土洞塌陷,同时考虑设计需要满足的正常使用极限状态与承载能力极限状态,提出了抗土洞塌陷的低填方加筋路基加筋体及路堤填方高度设计方法,通过与现有设计方法的对比进行了合理性及准确性验证,可为空洞上方低填方加筋路基设计提供参考。     

关于土工合成材料加筋设计的若干问题

目前土工合成材料加筋技术被广泛应用,但人们对于加筋土中筋材与土间的相互作用的机理的认识还不够深入,因而在设计中总体上趋于保守。结合岩土工程的设计理论,指出土工合成材料在设计方法方面的不合理性;对于加筋挡土墙、加筋土坡、加筋软土地基上的土堤和桩网结构的设计分别进行了讨论;结合一些案例中的实测和预计的筋材应变和应力,进一步指出目前设计的保守性。最后指出,目前基于极限平衡法的设计不尽合理,而通过变形协调的筋土共同作用的研究,采用更能反映其相互作用机理的设计方法是非常必要的。     

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.     

3D numerical study of the performance of geosynthetic-reinforced and pile-supported embankments

Geosynthetic-reinforced and pile-supported (GRPS) systems provide an economic and effective solution for embankments. The load transfer mechanisms are tridimensional ones and depend on the interaction between linked elements, such as piles, soil, and geosynthetics. This paper presents an extensive parametric study using three-dimensional numerical calculations for geosynthetic-reinforced and pile-supported embankments. The numerical analysis is conducted for both cohesive and non-cohesive embankment soils to emphasize the fill soil cohesion effect on the load and settlement efficacy of GRPS embankments. The influence of the embankment height, soft ground elastic modulus, improvement area ratio, geosynthetic tensile stiffness and fill soil properties are also investigated on the arching efficacy, GR membrane efficacy, differential settlement, geosynthetic tension, and settlement reduction performance. The numerical results indicated that the GRPS system shows a good performance for reducing the embankment settlements. The ratio of the embankment height to the pile spacing, subsoil stiffness, and fill soil properties are the most important design parameters to be considered in a GRPS design. The results also suggested that the fill soil cohesion strengthens the soil arching effect, and increases the loading efficacy. However, the soil arching mobilization is not necessarily at the peak state but could be reached at the critical state. Finally, the geosynthetic strains are not uniform along the geosynthetic, and the maximum geosynthetic strain occurs at the pile edge. The geosynthetic deformed shape is a curve that is closer to a circular shape than a parabolic one.     

用统一强度理论求解岩土材料的动力强度参数

基于俞茂宏的统一强度理论，研究了岩土材料在复杂应力状态下的强度理论及确定其动力强度参数的新方法。根据循环荷载作用的特点，把循环过程中的动态应变变化和极限平衡理论耦合起来，提出了基于统一强度理论考虑中间主应力的动态极限平衡概念，探讨了它在岩土材料中的应用，并推导出了确定有效动强度参数C′d，φ′d的方程。进一步研究了统一强度理论下的偏应力、有效固结压力和动态平衡状态线的概念，提出了确定有效动强度参数C′d，φ′d的简便方法。该方法不仅可以适用于砂土、粉土，而且还适用于沉积软岩材料。     

Reinforcement load in geosynthetic-reinforced pile-supported model embankments

The stress conditions of geosynthetic reinforcements (GRs) are crucial in achieving the accurate serviceability design of geosynthetic-reinforced pile-supported (GRPS) embankments. However, the sensitivity of load distribution to the settlement process has been reported in geosynthetic-reinforced embankment overlying cavities. In this study, a three-dimensional model embankment was used to perform experiments and evaluate the load acting on the GR. A flexible pressure-mapping sensor was introduced to investigate the pressure distribution for two types of supporting conditions: partitioned displacement by multiple movable trapdoors and even trapdoor settlement underneath different subsoil materials. The results showed that the load on the GR was concentrated on the strip areas between adjacent pile heads along with the settlement. The measured load on the GR strip area was related to the settlement process and finally exhibited a U-shaped distribution after detachment from the support underneath. The soil arch height in the subgrade continuously increased with the settlement; meanwhile, the pile head load increased rapidly at first and then decreased slightly or remained stable depending on the foundation support stiffness. For both types of settlement behaviours, soil arching exhibited stress history-related characteristics that influence the load transfer in GRPS embankments.     

路堤下带帽疏桩处治地基桩土荷载效应分析

针对疏桩支撑路堤结构的作用特点,在一定简化的基础上,分别建立了上部路堤荷载转移分析模型和基底桩土相互作用计算模式,综合考虑上部路堤与疏桩地基的相互影响,根据基底位移连续条件,得出了一种计算带帽疏桩路基中桩、土荷载效应值的方法。通过工程实例现场实测结果对该计算方法进行了验证,与其它计算方法结果进行比较分析,结果表明采用该法计算的桩、土应力接近实测结果,说明该计算方法的合理性。同时,讨论了桩土荷载分担随填土高度、填土黏聚力和桩间距等因素的变化规律。     

Large-scale load capacity tests on a geosynthetic encased column

Stone columns have been used to minimize the settlement of embankments on soft soils but their use in very soft soils can become challenging, partly because of the low confinement provided by the surrounding soil. Geosynthetic encased columns (GECs) have been successfully used to enhance to reduce settlements of embankments on soft soils. This paper describes an investigation on the performance of encased columns constructed on a very soft soil using different types of encasement (three woven geotextiles with different values of tensile stiffness) and different column fill materials (sand, gravel and recycled construction and demolition waste, RCDW). The results of load capacity tests conducted on large-scale models constructed to simulate the different types of GECs indicate that the displacement method adopted during column installation can lead to an enhanced shear strength in the smear zone that develops within the very soft soil. In addition, breakage of the column fill material was found to affect the load-settlement response of gravel and RCDW columns. Furthermore, the excess pore water pressure generated in the surrounding soil during installation, was found to remain limited to radial distances smaller than three times the GEC diameter.     

Reliability-based settlement analysis of embankments over soft soils reinforced with T-shaped deep cement mixing piles

This paper presents a reliability-based settlement analysis of T-shaped deep cement mixing (TDM) pile-supported embankments over soft soils. The uncertainties of the mechanical properties of the in-situ soil, pile, and embankment, and the effect of the pile shape are considered simultaneously. The analyses are performed using Monte Carlo Simulations in combination with an adaptive Kriging (using adaptive sampling algorithm). Individual and system failure probabilities, in terms of the differential and maximum settlements (serviceability limit state (SLS) requirements), are considered. The reliability results for the embankments supported by TDM piles, with various shapes, are compared and discussed together with the results for conventional deep cement mixing pile-supported embankments with equivalent pile volumes. The influences of the inherent variabilities in the material properties (mean and coefficient of variation values) on the reliability of the piled embankments, are also investigated. This study shows that large TDM piles, particularly those with a shape factor of greater than 3, can enhance the reliability of the embankment in terms of SLS requirements, and even avoid unacceptable reliability levels caused by variability in the material properties.     

沿海滩涂软土路基堆载预压沉降变形特征与卸载时机研究

沉降变形与卸载标准是铁路路基堆载预压的主要问题。针对处于沿海滩涂软土路基之上的铁路专用线,采用插塑板与堆载预压相结合的方案进行地基处理,开展了大量的表层沉降、孔隙水压力、分层沉降等观测项目。据实测资料,总结分析了路基表面的沉降规律,并考虑了路基分层沉降与孔隙水压力的变化规律,全面地揭示了整个路基地层的沉降变化特征,运用双曲线合理地预测了工后沉降,且推算出路基整体的固结度,采用工后沉降与地基固结度建立卸载时机判别标准。为堆载预压的卸载时机判别标准研究提供了一种新的思路。     

Numerical study of creep effects on settlements and load transfer mechanisms of soft soil improved by deep cement mixed soil columns under embankment load

Deep cement mixed (DCM) soil columns have been widely utilized to improve soft soil to support embankments or seawalls. However, the influence of the time-dependent behavior of the soft soil on the performance of DCM column-supported embankments is not well understood. In this study, the finite element (FE) model was established to investigate the creep effects on settlements and load transfer mechanisms of the soft soil improved by DCM columns under embankment load. Comparisons were conducted for the cases of the soft soil with or without creep. The parametric analysis demonstrated that the area replacement ratio and Young's modulus of the DCM column can largely influence the long-term behaviors of the DCM column-improved composite ground. The numerical results were also compared with the results calculated by German design method (EBGEO) and British design method (BS 8006). Regarding the vertical stress taken by the DCM column, EBGEO method provides a lower limit while BS 8006 method provides an upper limit.     

Modelling of geosynthetic-reinforced column-supported embankments using 2D full-width model and modified unit cell approach

Soil-cement deep mixing (DM) columns combined with geosynthetic basal reinforcement are an accepted technique in geotechnical engineering to construct road and railway embankments over soft foundations. Both full-width and unit cell models have been used to numerically simulate the performance of geosynthetic-reinforced and column-supported (GRCS) embankments. However, the typical unit cell model with horizontally fixed side boundaries cannot simulate the lateral spreading of the embankment fill and foundation soil. As a result, the calculated reinforcement tensile loads using typical unit cell models are much less than those from matching full-width models. The paper first examines GRCS embankments using a full-width model with small- and large-strain modes in FLAC and then compares the calculated results from the full-width model with those using a typical unit cell model, a recently proposed modified unit cell model, and a closed-form solution. The paper also examines the influence of the soft foundation soil modulus, reinforcement tensile stiffness, and DM column modulus on the reinforcement tensile loads. Numerical analyses show that the reinforcement tensile loads from the modified unit cell model are in good agreement with those from the full-width model for zones under the embankment crest for all cases and conditions examined in the paper. Both the full-width model and modified unit cell model perform better than the typical unit cell model for the prediction of the reinforcement tensile load when compared to the closed-form solution. However, while the modified unit cell developed by the writers is shown to be more accurate than the typical unit cell when predictions are compared to results using full-width numerical simulations, the benefit of using this approach to reduce computation times may be limited in practice.     

3D coupled mechanical and hydraulic modeling of a geosynthetic-reinforced deep mixed column-supported embankment

Numerical studies were conducted to improve the understanding of the behavior of geosynthetic-reinforced column-supported embankments. Due to the complexity of the problem, so far, consolidation process and three-dimensional patterns of columns have not been well simulated in most published numerical studies. As a result, the time-dependant behavior and the serviceability of this system have not been well evaluated. In this study, a three-dimensional coupled mechanical and hydraulic modeling was conducted using FLAC3D to consider consolidation and three-dimensional arrangement of columns. This study was based on a well-documented bridge approach embankment reinforced by a layer of geotextile and supported by deep mixed (DM) columns. The foundation soils including soft clay and silt, the embankment fill, and the deep mixed columns were modeled as linearly elastic-perfectly plastic materials with Mohr–Coulomb failure criteria. The geotextile reinforcement was simulated by geogrid elements incorporated in the FLAC3D software, which can sustain in-plane tensile force only. The staged construction was simulated by building the embankment in lifts. The duration of each lift was the same as the actual construction time plus the lapse time between two consecutive stages. The development of settlement and tension in the geotextile with time is compared with the long-term monitoring data and yields good agreement. The generation and dissipation of excess pore water pressure during and after construction are presented and discussed.     

公路软基蠕变—固结耦合有限元分析

软土的蠕变特性常常导致路堤出现沉降过大、甚至失稳等现象。本文采用同时考虑蠕变和固结效应的修正的广义Kelvin蠕变—固结模型,对公路软基的时效性变形进行了有限元分析。在某软基上路堤填筑工程的变形分析中,该方法的计算结果和监测数据基本吻合,由此验证了该模型的有效性。本文针对该工程进行了一系列的参数分析,讨论了软土的蠕变效应、塑料排水板布置方式和堆载速率等因素对该路堤变形发展和路堤稳定性的影响。     

Comparison of predicted and observed behaviour of two test embankments

The results of finite element analyses of two test embankments, one geotextile reinforced the other a blank control section, are compared with field observations and the results of a simple limit equilibrium analysis. The results of both types of analyses indicate good agreement between predicted and observed failure heights. The geotextile reinforcement was found to significantly increase embankment stability for this case. The finite element analysis provided useful information concerning the development of plastic failure within the soil and the deformations which occurred as tension developed in the reinforcement. It is suggested that this information, which is not provided by simple limit equilibrium analysis, may be essential to the safe design of reinforced embankments.     

Geosynthetic reinforced piled embankment modeling using discrete and continuum approaches

Understanding the load transfer mechanism can support engineers having more economical design of geosynthetic reinforced piled embankments. This study aims to investigate the load transfer mechanisms by two different numerical methods including the Discrete Element Method (DEM) and the Finite Difference Method (FDM). The DEM model adopts (a) discrete particles to simulate the micro-structure of the granular materials and (b) coupled discrete element – finite element method (DEM-FEM) to capture the interaction between granular materials and geotextiles. On the other hand, the FDM model uses an advanced constitutive soil model considering the hardening and softening behaviour of the granular materials. The numerical results show that the geotextiles can only contribute to the vertical loading resistance in cases where the soils between piles are soft enough. In terms of design, an optimum value of the geotextile tensile stiffness can be found considering the load, the soft soil stiffness and the thickness of the embankment. Both the DEM and the FDM show that a high geotextile tensile stiffness is not required since an extra stiffness will slightly contribute to the efficiency of the geosynthetic reinforced piled embankments. Nevertheless, both models are useful to optimize the design of geosynthetic reinforced piled embankments.     

软土地区偏压基坑抗隆起稳定安全系数的强度折减法研究

针对偏压工况下软土地区基坑开挖的坑底隆起问题,建立了二维有限元强度折减分析模型。对偏压范围、偏压距离以及偏压荷载大小对抗隆起稳定安全系数的影响进行了数值模拟分析,同时对比了极限平衡法、极限分析法和上述数值方法在偏压工况下的抗隆起稳定安全系数。结果表明:偏压范围越大或偏压距离越小,基坑抗隆起稳定安全系数越小;不同深宽比下的基坑抗隆起稳定安全系数均随偏压荷载大小增加而线性减小;Terzaghi极限平衡法和极限分析法均高估了偏压基坑的抗隆起稳定性,偏危险;BjerrumEide极限平衡法和基于Prandtl机构的规范法低估了偏压基坑的抗隆起稳定性,偏保守。