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。     

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.     

Field study and numerical modelling for a road embankment built on soft soil improved with concrete injected columns and geosynthetics reinforced platform

Generally numerical modelling can provide an accurate and cost-effective approach to understand the behaviour of geosynthetic-reinforced column-supported embankment. When the problem geometry cannot be simplified to the two-dimensional plane-strain or axisymmetric, a full three-dimensional solution is required to obtain sensible results. This study presents a modelling of the geosynthetic-reinforced composite ground supporting a road embankment. Response of soft soil is captured by adopting Modified Cam-Clay model. In addition, Hoek-Brown constitutive model is considered to simulate non-linear stress-dependent yield criterion for Concrete Injected Columns (CIC) that describes shear failure and tensile failure by a continuous function. To assess whether the proposed numerical model can capture real behaviour of composite ground, field monitoring data of deep soft clay deposit improved by CIC from Gerringong Upgrade is used to validate the model. The settlement and lateral displacements of ground, stress transferred to column, and pore water pressure results for the embankment during and after the construction, measured using the field instrumentations including settlement plates, inclinometers, earth pressure cells on CIC, and pore pressure transducers, are compared with numerical predictions. In addition, the numerical results provide insights to investigate load transfer mechanism in the composite ground, capturing response of soil – column - embankment system.     

A closed-form solution for column-supported embankments with geosynthetic reinforcement

Soil arching effect results from the non-uniform stiffness in a geosynthetic-reinforced and column-supported embankment system. However, most theoretical models ignore the impact of modulus difference on the calculation of load transfer. In this study, a generalized mathematical model is presented to investigate the soil arching effect, with consideration given to the modulus ratio between columns and the surrounding soil. For simplification, a cylindrical unit cell is drawn to study the deformation compatibility among embankment fills, geosynthetics, columns, and subsoils. A deformed shape function is introduced to describe the relationship between the column and the adjacent soil. The measured data gained from a full-scale test are applied to demonstrate the application of this model. In the parametric study, certain influencing factors, such as column spacing, column length, embankment height, modulus ratio, and tensile strength of geosynthetic reinforcement, are analyzed to investigate the performance of the embankment system. This demonstrates that the inclusion of a geosynthetic reinforcement or enlargement of the modulus ratio can increase the load transfer efficiency. When enhancing the embankment height or applying an additional loading, the height of the load transfer platform tends to be reduced. However, a relatively long column has little impact on the load transfer platform.     

Displacements of column-supported embankments over soft clay after widening considering soil consolidation and column layout: Numerical analysis

The common challenges for constructing embankments on soft clay include low bearing capacity, large total and differential settlements, and slope instability. Different techniques have been adopted to improve soft clay, such as the use of foundation columns including stone columns, deep mixed columns, and vibro-concrete columns, etc. Due to increased traffic volume, column-supported embankments may be widened to accommodate the traffic capacity need. Adding a new embankment to an existing embankment generates additional stresses and deformations under not only the widened portion but also the existing embankment. Differential settlements between and within the existing embankment and the widened portion may cause pavement distresses. Limited research has been conducted so far to investigate widening of column-supported embankments. In this study, a two-dimensional finite difference numerical method was adopted. This numerical method was first verified against field data and then used for the analysis of widened column-supported embankments over soft clay. The modified Cam-Clay model was used to model the soil under the existing embankment and the widened portion. Mechanically and hydraulically coupled numerical models were created to consider the consolidation of the foundation soil under the existing embankment and the widened portion. Different layouts of foundation columns under the existing embankment and the widened portion were investigated. The numerical results presented in this paper include the vertical and horizontal displacements, the maximum settlements, the transverse gradient changes, and the stress concentration ratios, which depended on column spacing. The columns installed under the connection side slope were most effective in reducing the total and differential settlements, horizontal displacement, and transverse gradient change of the widened embankment.     

土工合成材料处治老路路基拓宽的数值分析

随着交通量的日益增加，老路拓宽已经成为公路建设非常重要的～种手段。为减少老路拓宽新老路基不均匀沉降，提高路基整体稳定性，土工合成材料在新老路基结合部处治中获得很大的成功。针对老路拓宽时采用土工合成材料处治结合部的特点，将新老路基、路面结构作为整体，建立数值模型。采用FLAC进行分析，主要从路基位移、路基项面应力及土工合成材料拉力分布等方面，研究土工合成材料处治新老路基结合部的作用机制，分析不同加筋层数和不同路基填土高度下的处治效果。计算结果表明，采用土工合成材料不仅可以有效地减小路基顶面的差异沉降和水平位移，而且使得路基反射到路面的平均应力更小且分布均匀。     

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.     

Internal stability analysis of reinforced convex highway embankments considering seismic loading

The seismic internal stability of reinforced, convex embankments that are three-dimensional in nature is analyzed. A limit equilibrium based three-dimensional rotational failure mechanism is adopted to calculate the required reinforcement strength to maintain the stability of convex embankments. The results are presented in the form of stability charts and the effects of various parameters on the three-dimensional solution are investigated. The calculation of the required strength and length of reinforcement is demonstrated by two examples using an approach consistent with AASHTO (2012). Comparing the strengths obtained under two and three-dimensional conditions, the results show that the two-dimensional results are more conservative with respect to the strength of reinforcement, but could be unconservative considering the required length of reinforcement, especially for reinforced convex embankments with gentle turning angles. The influence of seismicity causes greater three-dimensional effects when the reinforced convex embankment is vertical, but less so when the slope inclination is gentle.     

Response of stone column-improved ground under c-ϕ soil embankment

Problems with stone column-supported embankments are often addressed considering the pure granular soil properties of the embankment material. In practice, however, the embankment material may contain a certain percentage of fines which makes the material c-ϕ in nature. In the present paper, experimental and 3D numerical studies are conducted to investigate the effect of the fines content present in the embankment soil on the response of a stone column-improved ground. The numerical study is carried out using the finite difference software package FLAC^3D. The time-dependent behavior of the improved soil is modeled. It is observed that an increase in the fines content in the embankment soil brings about a decrease in the stress concentration ratio and an increase in the vertical displacement and the pore pressure in the soft soil. However, the vertical displacement of the soft soil does not change significantly above a fines content of 20%. The lateral displacement of the stone column increases at the top with an increase in the fines content and the point of the maximum lateral displacement shifts in the downward direction. It is also observed that the differential settlement increases at the ground level as well as at the embankment top with an increase in the quantity of fines in the embankment soil. Thus, the critical height of the embankment (a height where no differential settlement occurs on the embankment surface) increases due to the increase in the fines content.     

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.     

Overall stability of geosynthetic-reinforced embankments on soft soils

Overall stability of geosynthetic-reinforced embankments on soft soils is analysed using two different methodologies: application of a numerical model based on the finite element method; use of a limit equilibrium method. These two methodologies are described and also applied on three geosynthetic-reinforced embankments on soft soils. One of the cases is a case history constructed up to failure. Considering the analysis of the results, some conclusions are formulated on the limit equilibrium method accuracy, namely regarding the critical slip surface, overall safety factor and overturning and resisting moments.     

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.     

Analytical solutions for geosynthetic-encased stone column-supported embankments with emphasis on nonlinear behaviours of columns

This paper presents an analytical approach to predict the behaviours of geosynthetic-encased stone column (GESC)-supported embankments. The soil arching in the embankment and the nonlinear behaviours of stone columns are considered. Based on nonlinear elastic and elastoplastic constitutive models of stone columns, the nonlinear behaviours of GESCs, including settlement and radial deformation, are analysed. The deformations of GESCs, the surrounding soil, and the overlying embankment fill are compatible by applying stress continuity and volume deformation continuity at the bottom of the embankment fill. This method is verified via comparison with literature data and numerical analysis. The influences of parameters of the GESC, including encasement stiffness and column friction, on the performance of the embankment are investigated. Without considering the nonlinear behaviours of the column, the column-soil stress ratio is overestimated. It is more appropriate that the nonlinear characters of the column be considered in the analysis of GESC-supported embankments.     

A simplified model for the analysis of piled embankments considering arching and subsoil consolidation

An analytical model is presented for the design of geosynthetic-reinforced and pile-supported (GRPS) embankments in this paper. The originality of the proposed solution lies in the fact that it allows considering the influence of the subsoil consolidation on the soil arching and geosynthetic strain. A nonlinear function is implemented to describe the subsoil behavior with the consolidation process in a closed-form solution. A simplified approach is then presented to link the arching development with the subsoil consolidation. The arching theory is combined with the tensioned membrane theory and the soil-structure interaction mechanisms to provide a simple and suitable design approach that enables a realistic approximation for designing soil–geosynthetic systems. The analytical model is capable of performing an ultimate and serviceability limit state design of GRPS embankments. While current methods cannot fully address the important effects of the subsoil consolidation, the analytical results suggested that arching and differential settlements increase with an increase of the subsoil consolidation degree. The analytical model is compared to field measurements and five other design standards for several full-scale field tests to study its validity. The results showed a satisfactory agreement between the proposed model and measured data, and generally better results are obtained as compared with other design methods.     

Practical seismic fragility estimation of Japanese railway embankments using three seismic intensity measures

This study proposes a practical fragility estimation equation for Japanese standard models of railway embankments using the peak ground acceleration, peak ground velocity, and Arias intensity. The analytical models were implemented as unreinforced and geosynthetic-reinforced embankment models. A sensitivity analysis of the seismic fragility estimation of the embankment models was conducted on various embankment heights, average values of friction angles in the backfill soil, and tensile strengths of the primary reinforcement. Consequently, a unique formula for the fragility function in the presence of different seismic intensities was successfully presented. The parameters of the fragility function were successfully estimated using commonly used design parameters, such as the embankment height, average value of the friction angle of the backfill soil, and average value of the tensile strength. Additionally, another sensitivity analysis using different seismic databases was conducted to explore the effect of the seismic database on the fragility curve estimation of railway embankments. As a result, using different seismic databases, different fragility curves were obtained. These results highlight the importance of checking the sensitivity of the seismic database when developing the fragility curve.     

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.     

刚性桩加固软弱地基上路堤稳定性问题（Ⅱ）——群桩条件下的分析

针对刚性桩支承路堤，笔者已进行的对单桩位于路堤下不同位置时路堤稳定性的研究表明，复合抗剪强度极限平衡法不能反映不同位置单桩的强度及刚度对路堤稳定性的影响以及桩的弯曲破坏机理，将显著高估路堤稳定性。通过对群桩条件下刚性桩加固路堤分别采用三维和二维数值分析方法，研究了路堤填筑及趋于失稳破坏过程中，桩、土的内力与变形规律、桩的破坏形式等。结果表明：群桩条件下，不同位置的刚性桩的破坏模式不同，对路堤稳定性的贡献机理也相应不同，弯曲破坏比剪切破坏更易于发生，剪切破坏并非是桩最危险的破坏方式；提出了考虑路堤趋于失稳破坏过程中桩弯曲破坏的cut-off退出方法，在进行数值分析时可反映路堤趋于失稳过程中桩的破坏形式，从而合理地评估路堤稳定性。对素混凝土桩加固路堤的稳定分析，提出了桩可使用抗剪强度概念，并提出了按桩体可使用抗剪强度采用极限平衡法进行路堤稳定分析的方法。     

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。     

Field testing of geosynthetic-reinforced and column-supported earth platforms constructed on soft soil

This paper is focused on the behavior of geosynthetic-reinforced and column-supported (GRCS) earth platforms in soft soil. By analyzing the data of a 15-month long field monitoring project, the bearing behavior and effectiveness of GRCS earth platforms are discussed in detail. It can be found that the soil arching is generated when the filling reaches a certain height. The measured pressure acting on the soil in the center of four piles was smaller than that acting on the soil between two piles. The elongation and the tension of the geogrid located in the soil between piles are both larger than the corresponding values on the pile top. The skin friction of piles is relatively small in the soil layer with low strength and the load transfer of the axial force in those layers is significant; meanwhile, the opposite situation occurs in the soil layer with high strength. The pore water pressure at shallow locations increases slightly with the filling height and is greatly affected by the increasing filling load. The layered settlement is directly proportional to the filling height, and the corresponding amount is relevant to the locations and the properties of specific soil layers. Additionally, the lateral displacement of the embankment increases with greater loading and decreases with increased depth. These suggest that the use of GRCS system can reduce lateral displacements and enhance the stability of an embankment significantly.     

Construction and field measurement of high-speed railway test embankment built on Indian expansive soil “Black Cotton Soil”

The railway embankment applied to high-speed railways is required to have high performance in terms of strength and deformation characteristics. Especially in the case of railway embankments that support slab tracks, the allowable settlement is very small. There are two technical challenges in constructing high-speed rail embankments to support slab tracks in India. The first challenge is dealing with problematic black cotton soil (BCS), which is widely distributed in India but very unusual soil in Japan. The second challenge is posed by the strict deformation requirement in the construction of the embankments. In this study, a 6 m-high test embankment was constructed on BCS in India. The deformation of the embankment and changes in water content were measured over a period of 18 months. In the construction of the test embankment, two different BCS countermeasures were applied. The results of the tests on this embankment were compared with those from an embankment without countermeasures. Complicated deformation behaviors, including settlement and the uplift of embankment, were observed in the section without countermeasure. However, in the embankment with cement-mixed gravelly soil (CGS) slab improvement with geosynthetics, the much lower amplitude of embankment deformation is evidence of the effectiveness of this countermeasure. The cohesive non-swelling soil (CNS) layer applied immediately below the embankment to reduce the water content fluctuation of BCS was not effective enough for use for high-speed railway embankment. Besides determining the technical challenges for the BCS countermeasures, the results of this study confirmed that a high-performance embankment can be constructed with Indian embankment material by performing sufficient compaction management.