模拟堤坝荷载作用下软土的速率效应特性(英文)

用基于Perzyna超应力理论与修正剑桥模型的简单的弹黏塑性本构模型,耦合比奥固结理论,来模拟堤坝荷载作用下的软土的速率效应特性。以土工织布加固的堤坝为实例,提出从最初几个加载阶段下的沉降数据来确定黏性参数的反分析法。根据反分析的参数值来模拟,同实测值予以比较,并同文献中使用其他四个不同本构模型的模拟结果进行比较,比较研究表明:本文建议的模型具有优越性。特别研究了土工织布加固对堤坝下软土的滞后变形和稳定性的影响。良好的模拟结果反应了所提出的反分析法的可用性,同时展示了所使用的弹黏塑性本构模型在岩土工程中的实用性:弥补了修正剑桥模型不能模拟速率效应特性的缺点;跟其他黏塑性本构模型比较,本模型参数确定方法简单,模拟结果准确。     

软基上加筋防波堤的离心模型试验

通过离心模型试验，研究在有无土工织物加筋垫层条件下防波堤和软基的变形性状，得到了地基沉降、隆起及水平位移的分布规律。提出了一种量测筋材张力的新方法，对土工织物在离心试验过程中的应力状态进行了测试。研究结果表明：土工织物加筋垫层能有效地减小地基的侧向位移，有一定的加筋效果：土工织物张力的发挥水平与堤坝的沉降量密切相关，工作状态下其最大强度发挥水平约为52％。     

路堤荷载下双向增强复合地基荷载分担比及沉降计算

针对路堤荷载下双向增强复合地基受力变形特性,以单桩有效影响范围内的路堤与复合地基为分析对象,引入大挠度环形薄板考虑加筋垫层的“柔性筏板效应”与“拉膜效应”,同时通过假定桩土相对位移模式,考虑地基成层性,从而建立了路堤、水平加筋体、桩体、桩间土协调变形三维模型,获得了路堤荷载作用下双向增强复合地基的荷载分担比及沉降计算方法。采用某工程试验数据对该计算方法进行验证,同时分析了路堤高度、桩帽宽度、筋材抗拉模量对中性点位置、桩土差异沉降以及复合薄板中面最大拉应力的影响,结果表明该方法所求得的荷载分担比及沉降与实测值较为接近,证明了其合理性。     

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。     

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.     

A simplified model to analyze the reinforced piled embankments

It is an economic way to use the piled embankment for the construction of embankment over soft soil. The combination of piles and reinforcement can effectively reduce the differential settlement at the surface of embankment. The paper presents a simplified model for analysis of an embankment of granular fill on soft ground supported by reinforcement and piles. This model is based on consideration of the arching effect in granular material proposed by Hewlett & Randolph. The vertical equilibrium of the unit body at the center of pile caps immediately below the reinforcement is established. The refinements of the model are that the failure mechanisms of the arch both at the crown and at the pile cap were considered, three-dimensional situation was taken into account for reinforced piled embankment, calculation of the vertical stress carried by the subsoil due to arching effect and reinforcement for multi-layered soil was proposed. Using the simplified model, the influence of embankment height, one-dimensional compression modulus of subsoil, tensile stiffness of reinforcement on stress reduction ratio (SRR) and tensile force of reinforcement is investigated. It is found that the model can be used to assess the relative contribution of the reinforcement and subsoil. The results show that subsoil gives a major contribution to overall vertical equilibrium, while the reinforcement gives obvious contribution at relatively large settlement. The inclusion of the reinforcement can reduce the vertical stress acting on the subsoil. The simplified model is then evaluated by three case studies. The results of this model show good consistence with these cases.     

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.     

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.     

Numerical modeling of floating geosynthetic-encased stone column–supported embankments with basal reinforcement

The performance of the floating geosynthetic-encased stone column–(GESC)-supported embankments with basal reinforcement was examined using a 3-dimensional (3D) hydro-mechanical coupling finite element model. Comprehensive parametric analyses were performed on the governing factors such as consistency of substratum soil, tensile stiffness of basal reinforcement and encasement, and embankment height. The results indicated that a higher embankment load is transferred to the surrounding soil when a GESC was constructed on a weaker substratum. This causes larger increases in the settlement and lateral displacement of the GESC on the weaker substratum. The tensile strain of the basal reinforcement and hoop strain in the encasement also increases. In addition, high tensile stiffness in basal reinforcement and encasement is necessary to ensure feasible settlement reduction in a floating GESC-supported embankment with basal reinforcement.     

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.     

A comprehensive method for analyzing the effect of geotextile layers on embankment stability

Commercial software is used widely in slope stability analyses of reinforced embankments. Almost all of these programs consider the tensile strength of geotextiles and soil–geotextile interface friction. However, currently available commercial software generally does not consider the drainage function of nonwoven geotextile reinforcement. In this paper, a reinforced channel embankment reinforced by a nonwoven geotextile is analyzed using two methods. The first method only considers the tensile strength and soil–geotextile interface friction. The second method also considers the drainage function. In both cases, the reinforced embankment is modeled in rapid drawdown condition since this is one of the most important conditions with regard to stability of channel embankments. It is shown that for this type of application, modeling a nonwoven geotextile reinforced embankment using commercial software which neglects the drainage function of the geotextile may be unrealistic.     

高填方加筋新旧路堤现场试验与数值模拟分析

 结合山区高速公路拓宽工程,对土工格室处治高填方新旧路堤进行现场试验,分析加宽高填方路堤侧向位移、沉降及土压力变化规律,研究格室处治效果。在现场试验的基础上,采用三维薄膜单元模拟土工格室的立体加筋性能,建立三维弹塑性模型,分析土工格室受力特点,通过对相关参数的敏感性分析,揭示高填方加宽路堤的变形规律。结果表明,采用三维薄膜单元,能较好地反映土工格室处治现场高填方新旧路堤的规律。与现场试验相比,利用数值试验不仅能得到现场的加筋效果,而且还能通过分析筋材与填料参数的变化和筋材铺设间距来研究格室处治高填方路堤的规律,从而可进一步探讨格室加筋的机制。高填方路堤在加宽路基自重荷载作用下沉降主要集中在加宽路堤的中上部,侧向位移从路基顶面到底部依次逐渐减少。土工格室所在层位起到扩散荷载、减少侧向变形和不均匀沉降的作用。填料与筋材模量愈高,加筋间距愈小,加筋效果愈好,较为合理的铺设间距为2～3 m。该研究成果对高填方路堤加筋处理和新旧路基结合部处理均有借鉴意义。     

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.     

黄土高路堤沉降变形预测模型研究

以兰州—海石湾高速公路高69 m黄土路堤沉降资料为研究样本,采用经修正了非匀速填土和非等步长沉降观测时间的GM(1,1)灰色理论预测模型进行黄土高路堤工后最终沉降量预测,与等比级数曲线模型预测结果对比,认为这两种预测模型都能很好地预测黄土高路堤工后最终沉降量,且灰色预测模型能较等比级数曲线预测模型更好地反映黄土高路堤不均匀沉降趋势。同时认为,考虑了沉降观测时间非等步长性和路堤填土速度不均匀性的灰色预测模型其预测结果更符合黄土高路堤沉降变形趋势,可进行进一步的研究、推广和应用。     

A reinforcing method for steep clay slopes using a non-woven geotextile

The effect of non-woven geotextile reinforcement on the stability and deformation of two clay test embankments is examined based on their performance for about 3 years for the first embankment and about years for the other. Horizontal planar sheets of a non-woven geotextile are expected to work in three ways: for compaction control; for drainage; for tensile reinforcement. The degree of stability of the steep slopes of the test embankments decreased during heavy rainfall. It is found that the use of non-woven geotextile reinforcement may effectively improve embankment performance. Only the stability analysis in terms of effective stresses can explain the performance of the test embankments. The horizontal creep deformation of the embankments during 2–3 years, which is partly attributed to the creep deformation of the non-woven geotextile, was found to be small. The results of both laboratory bearing capacity tests of a strip footing on a model sand ground reinforced with the non-woven geotextile and plane strain compression tests on sand specimens reinforced with the non-woven geotextile show that the non-woven geotextile gives tensile reinforcement to soils.     

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.     

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。     

Analysis and modification of the British Standard BS8006 for the design of piled embankments

The piled embankment is an increasingly popular construction method. The Dutch Design Guideline for piled embankments (CUR 226) was published in the first half of 2010. Several existing models have been analysed to determine the calculation rules used in the Dutch Guideline. The British Standard BS8006 sometimes calculates tensile forces in the geosynthetic reinforcement that differ considerably from other models. For quite thin embankments in particular, BS8006 designs a relatively strong and thus expensive geosynthetic (basal) reinforcement in comparison with other design models. These differences are not always fully understood, leading to uncertainty. This paper analyses BS8006 and demonstrates why it behaves differently from other models. It also examines why this behaviour is different than would be expected. For example, it is shown that calculations using BS8006 are based on a higher load than the actual load.A modification to BS8006 is proposed, which is shown to give comparable results to the German Standard EBGEO for situations where there is no subsoil support.The results of BS8006, Modified BS8006, and the German/Dutch guideline are compared with finite element calculations and field measurements. It is concluded that the results given by the Modified BS8006 are more accurate to those using BS8006.     

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.     

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.