Bearing capacity of circular footing on geocell–sand mattress overlying clay bed with void

The potential benefits of providing geocell reinforced sand mattress over clay subgrade with void have been investigated through a series of laboratory scale model tests. The parameters varied in the test programme include, thickness of unreinforced sand layer above clay bed, width and height of geocell mattress, relative density of the sand fill in the geocells, and influence of an additional layer of planar geogrid placed at the base of the geocell mattress. The test results indicate that substantial improvement in performance can be obtained with the provision of geocell mattress, of adequate size, over the clay subgrade with void. In order to have beneficial effect, the geocell mattress must spread beyond the void at least a distance equal to the diameter of the void. The influence of the void over the performance of the footing reduces for height of geocell mattress greater than 1.8 times the diameter of the footing. Better improvement in performance is obtained for geocells filled with dense soil.     

Effect of geogrid-reinforcement in granular bases under repeated loading

The thickness of the base plays a crucial role in the stability of pavements and the lack of availability of good quality aggregates is a major concern in India and other countries. Loading on top of the base plays a crucial role in the design of pavements. Usually, the design of the pavement is done for standard axle load, however, in the field, in some of the cases, the vehicles are overloaded which results in a higher wheel load on the pavements. The current paper examines the performance of geogrid reinforced unpaved sections at higher stresses with the primary objective of reducing the thickness of base layer required in the field. Experimental studies were carried out using repeated plate load tests to obtain the optimum depth of placing the geogrid in granular base layer to achieve maximum reduction in rutting of pavement. Resilient deformation behavior of both reinforced and unreinforced sections are obtained and these values are utilized to predict the resilient modulus of the base sections. The paper also discusses the reduction in permanent deformation by the introduction of geogrid. Rut depth reduction studies were carried out in order to compare the performance of reinforced and unreinforced sections. The role played by the reinforcement in reducing the strains on top of the subgrade is studied in detail. A comparison is also carried out to understand the pressure distribution along the base layer and role played by the geogrids in reducing the pressure on the subgrade. Further, values of stress distribution angles were obtained for reinforced and unreinforced sections. It is evident from the studies that geogrids contributed to improved performance as well as reduction in thickness of the aggregate layer.     

DIC assessment of foundation soil response for different reinforcement between base and soft subgrade layer – Physical modeling

This paper presents insights from small-scale model tests on statically loaded strip footing on dense base sand supported by a single reinforcement layer. The selected reinforcement includes various lengths of wire mesh and a non-woven geotextile placed on soft subgrade sand. The influence of these inclusions on base and subgrade response (deformations and failure mechanism) is evaluated from the displacement pattern obtained by the digital image correlation (DIC) technique. The benefits of the reinforcement and geotextile were assessed by comparing the results obtained for the improved and unreinforced soil model. Additionally, the confining effect of the reinforcement was experimentally analyzed by comparing the sand displacements around the wire mesh reinforcement with different aperture geometry and geotextile without apertures. These systematically selected reinforcement geometries enabled the investigation of the aperture size influence on base and subgrade behavior during surcharge loading. Results confirm the contribution of reinforcement inclusions to the improved behavior of base and subgrade layers compared to the unreinforced soil. The test results with different reinforcement confirm the influence of the aperture geometry on the model response during the surcharge load application. Compared to large apertures, enhanced confining and membrane actions were obtained for reinforcement with relatively small apertures.     

Application of the two-layer system theory to calculate the settlements and vertical stress propagation in soil reinforcement with geocell

This paper presents a methodology for determining the surface settlements of the geocell-reinforced soil layer and the vertical stresses propagated to the foundation subgrade at the layers interface, on the subgrade. Based on the theory of equivalent thicknesses, which is an approximation of the theory of elasticity for layered systems, a generalized equation for determining settlements was proposed in a two-layer system composed of geocell-reinforced soil layer over the subgrade. The equation obtained is dependent only on the relations between the elastic parameters of these two layers, such as the deformation moduli and Poisson's ratio, and geometric parameters, such as geocell layer thickness and loading width,. The proposed equation generated very close results with rigorous solutions of the two-layer system from the theory of elasticity. It was applied, together with rigorous methods, in an instrumented field Plate load test allowing the determination of the geocell-reinforced soil layer modulus of deformation by retro analysis and the vertical stresses propagated to the subgrade. The results showed that the two-layer system theory from theories of elasticity and equivalent thicknesses can be used in a simple and efficient way for determining settlements and the propagation of vertical stresses. The proposed methodology also satisfactorily calculated these results when compared with the rigorous methods and with the values obtained in the field test.     

Uncertainties in determining the responses of reinforced flexible pavements using in-situ tests

A 225m long full-scale testing lane was constructed at a local road in Australia to evaluate the performance of the flexible pavements over a weak soft subgrade. The pavements were reinforced with three types of geosynthetic products: High-density polyethylene (HDPE) geogrid, HDPE geocomposite and fibreglass geocomposite. The road was divided into 15 sections with different configurations such as the thickness of the base course, reinforcement types and locations, and base course materials. A series of in-situ tests were conducted on each section to compare the behaviour of the pavement structures, such as the moduli of the subgrade, base course and asphalt layer. The comparison shows that there is a large variation in the properties of the structures and great uncertainties in determining the properties even within the sections with the same configuration. When the base course is weaker, the FWD tests may be able to detect the effect of the reinforcements below the asphalt seal layer. Smaller plates are recommended when determining the modulus of thinner base course layers using FWD or LWD tests to minimize the influences from the subgrade.     

土工格室于砂土之承载能力及动态特性试验研究

在沙漠地带中,在路基材料缺乏的状况下,利用土工格室与现场砂土可有效提高沙漠道路的承载力,并能达到经济效益与路基的设计要求。为了解土工格室于砂土路基的加筋行为,分别进行了静态承载板载重试验与动态载重试验,探讨土工格室加筋后的承载能力与动态特性。与未加筋砂土比较,土工格室发挥良好之加筋效果,其能提高砂土的承载能力;于静态承载试验中,承载能力随着土工格室高度增加而提升,并有效减少沉降量之产生。在动态试验指出,土工格室加筋于砂土受动态反复作用下,增加土工格室高度,并不相对能提升加筋的承载成效。以动态载重试验反应公路的反复载重条件,对于沙漠公路设计使用土工格室加固路基时,得以选择具有经济效益的优化方案。     

英国标准BS 1377与中国标准在铁路路基压实试验中的异同

对比了中国铁路标准和英国标准在铁路路基压实度、CBR(加州承载比)试验的异同点,分析了EVD(动态变形模量)试验与压实度和CBR试验之间的相关性。铁路标准根据填料粒径及测试层厚度选择压实度仪器,英国标准仅考虑层厚;铁路标准提供的CBR指标是一种等价于最优含水率的理想状态,英国标准则依据现场压实度进行选择;英国标准未涉及EVD试验,铁路标准在特殊路段有相关要求,但不同压实度下EVD和CBR值存在近似的线性关系。     

Mechanically reinforced granular shoulders on soft subgrade: Laboratory and full scale studies

A recently completed field study in Iowa showed that many granular shoulders overlie clayey subgrade layer with California Bearing Ratio (CBR) value of 10 or less. When subjected to repeated traffic loads, some of these sections develop considerable rutting. Due to costly recurring maintenance and safety concerns, the authors evaluated the use of biaxial geogrids in stabilizing a severely rutted 310 m tests section supported on soft subgrade soils. Monitoring the test section for about one year, demonstrated the application of geogrid as a relatively simple method for improving the shoulder performance. The field test was supplemented with a laboratory testing program, where cyclic loading was used to study the performance of nine granular shoulder models. Each laboratory model simulated a granular shoulder supported on soft subgrade with geogrid reinforcement at the interface between both layers. Based on the research findings, a design chart correlating rut depth and number of load cycles to subgrade CBR was developed. The chart was verified by field and laboratory measurements and used to optimize the granular shoulder design parameters and better predict the performance of granular shoulders.     

Laboratory model studies on unreinforced and geogrid-reinforced sand bed over stone column-improved soft clay

Results from a series of laboratory model tests on unreinforced and geogrid-reinforced sand bed resting on stone column-improved soft clay have been presented. The diameter of stone column and footing has been taken as 50 mm and 100 mm, respectively for all the model tests carried out. Load was applied to the soil bed through the footing until the total settlement reached at least 20% of footing diameter. As compared to unimproved soft clay, the increase in load-carrying capacity under different improved ground conditions has been observed. Influences of the thickness of unreinforced as well as geogrid-reinforced sand bed and the size of geogrid reinforcement on the performance of stone column-improved soft clay bed have also been investigated. Significant improvement in load-carrying capacity of soft soil is observed due to the placement of sand bed over stone column-improved soft clay. The inclusion of geogrid layer within sand bed further increases the load-carrying capacity and decreases the settlement of the soil. Due to the placement of sand bed, the bulge diameter of stone column reduces while the depth of bulge increases. Further reduction in the bulge diameter and increase in bulge depth are observed due to application of geogrid layer. The optimum thickness of unreinforced sand bed is twice the optimum thickness of geogrid-reinforced sand bed. Under specific material properties and test conditions, it is further observed that the optimum diameter of geogrid layer is thrice the diameter of footing.     

西安黄土基床系数变化规律及其测试方法的相关性分析

为确立西安黄土基床系数的取值范围和经验公式,根据西安地铁采用K_(30)平板载荷试验测得的基床系数资料,归纳西安黄土基床系数的范围及其随深度的变化规律,对比分析K_(30)平板载荷试验、旁压试验、标贯试验及室内固结试验的基床系数测试结果,探讨K_(30)平板载荷试验与其他试验方法间的相关关系。结果表明:在新黄土、古土壤及老黄土地层中基床系数随深度增加总体呈增大趋势,在黄土状土地层中基床系数随深度增加变化规律不明显;西安黄土的水平基床系数K_x以小于垂直基床系数K_v为主,K_x/K_v比值呈现正态分布,且约占试验数据总数81%的K_x/K_v比值分布在0.5~1.0范围;由K_(30)平板载荷试验测得的基床系数与旁压模量、标贯击数均近似呈线性关系,与压缩模量及压缩系数近似呈二元幂函数关系。这些结果可供西安地区隧道、基坑工程的勘察设计参考使用。     

减震层减震原理及跨断层隧道减震技术振动台试验研究

通过波函数展开法给出平面SV波入射下深埋圆形隧道"围岩—减震层—初期支护—二次衬砌"减震结构的动力响应解析近似解,分析了减震层厚度、弹性模量对衬砌结构动应力集中系数的影响,并开展了跨断层隧道抗减震研究大型振动台模型试验,通过分析跨断层及其设置减震层后隧道衬砌动力响应特性和破坏形态,得到以下有益结论:减震层与围岩弹性模量比越低,减震层厚度越大,衬砌动应力集中系数越小;减震层与围岩弹性模量的最优减震比在1/10~1/20,最优减震层厚度不宜大于0.2 m;跨断层破碎带隧道设置减震层可以明显降低跨断层衬砌结构加速度峰值和衬砌动应变幅值;断层处隧道衬砌裂缝分布数量多、复杂,多集中于拱脚、拱肩,并分布有剪切错动引起的环向裂缝,设置减震层后,断层处隧道衬砌裂缝明显减少,衬砌受力得到明显改善;断层处地表出现了平行断层方向为主的的贯通裂缝和大量斜裂缝,说明断层处以剪切破坏为主,设置减震层后,地表裂缝明显减少。     

Bearing ratio of reinforced fly ash overlying soft soil and deformation modulus of fly ash

This paper presents the laboratory study on the bearing ratio of unreinforced and reinforced fly ash overlying soft soil beds of a total of 11 fly ash samples collected from different thermal power plants located in the Eastern part of India. The thickness of the bottom clay layer (H_c) was maintained as 100 mm in the bearing ratio mould. The upper layer thickness of compacted fly ash (H_f) was varied. The values of the ratio H_f/H_c used were 0.75, 1.00 and 1.25 in this study. The fly ash layer was reinforced with single layer and double layers of geotextiles. The effects of (i) position and number of layers of geotextiles, (ii) thickness of the compacted fly ash layer overlying soft soil layer, and (iii) moulding water content of the soft soil, on the bearing ratio of fly ash are highlighted. The inclusion of geotextile into the compacted fly ash bed enhances the bearing ratio. An increase in the thickness of compacted fly ash layer over the soft soil layer also increases the bearing ratio of the compacted fly ash bed. The values of unconfined compressive strength and deformation modulus of all the fly ash samples are also presented. Empirical relationships to estimate deformation modulus of fly ash from unconfined compressive strength and relationships between initial tangent modulus and secant modulus of fly ash are presented. It may be concluded from this research study that reinforced compacted fly ash overlying soft soil with a geotextile layer at the interface can find potential application in the construction of roads over soft soil.     

Response of pavement foundations incorporating both geocells and expanded polystyrene (EPS) geofoam

The suitability of geocell reinforcement in reducing rut depth, surface settlements and/or pavement cracks during service life of the pavements supported on expanded polystyrene (EPS) geofoam blocks is studied using a series of large-scale cyclic plate load tests plus a number of simplified numerical simulations. It was found that the improvement due to provision of geocell constantly increases as the load cycles increase. The rut depths at the pavement surface significantly decrease due to the increased lateral resistance provided by the geocell in the overlying soil layer, and this compensates the lower competency of the underlying EPS geofoam blocks. The efficiency of geocell reinforcement depends on the amplitude of applied pressure: increasing the amplitude of cyclic pressure increasingly exploits the benefits of the geocell reinforcement. During cyclic loading application, geocells can reduce settlement of the pavement surface by up to 41% compared to an unreinforced case – with even greater reduction as the load cycles increase. Employment of geocell reinforcement substantially decreases the rate of increase in the surface settlement during load repetitions. When very low density EPS geofoam (EPS 10) is used, even though accompanied with overlying reinforced soil of 600 mm thickness, the pavement is incapable of tolerating large cyclic pressures (e.g. 550 kPa). In comparison with the unreinforced case, the resilient modulus is increased by geocell reinforcement by 25%, 34% and 53% for overlying soil thicknesses of 600, 500 and 400 mm, respectively. The improvement due to geocell reinforcement was most pronounced when thinner soil layer was used. The verified three-dimensional numerical modelings assisted in further insight regarding the mechanisms involved. The improvement factors obtained in this study allow a designer to choose appropriate values for a geocell reinforced pavement foundation on EPS geofoam.     

Cyclic behaviour of dry silty sand reinforced with a geotextile

Recent studies on construction material technology have indicated that soil reinforcement improves resistance of soil against compression and tension. Due to the wide use of geotextile reinforcement in road construction, the potential benefit of geotextile reinforcement in cyclic loading should be investigated. In this study we performed a series of cyclic triaxial tests to examine dry silty sand reinforced with geotextile when subjected to dynamic loading. These tests were conducted on reinforced and unreinforced dry sand and sand mixed with varying amounts of silt (0–50%). The main factors affecting the cyclic behaviour, such as the arrangement and number of geotextile layers, confined pressure and silt content are examined and discussed in this paper. The results indicate that geotextile inclusion and increased confining pressure increase the axial modulus and decreased cyclic ductility of dry sand for all silt contents examined. Also, it was found that by increasing the silt content by up to about 35 percent the axial modulus in reinforced and unreinforced sand is decreased and cyclic ductility increased. With further increases in silt content, these values are increased for cyclic axial modulus and decreased for cyclic ductility.     

混凝土密实度对矩形钢管混凝土短柱力学性能影响研究

为了考察混凝土密实度对矩形钢管混凝土短柱承载力和弹性模量的影响 ,进行了 16个不同混凝土浇筑方式下成型的矩形钢管混凝土短柱试验研究。结果表明 ,混凝土密实度对构件的承载力和弹性模量均有影响 ,密实度越好 ,构件的承载力和弹性模量越高。试验充分证明了混凝土密实度对矩形钢管混凝土短柱的重要性。     

Stress-dependent method for calculating the modulus improvement factor in geocell-reinforced soil layers

This paper presents an analytical method for determining the modulus improvement factor (MIF) in geocell-reinforced soil layers. Using a modified version of the hyperbolic soil model as a constitutive model, the method is developed based on the soil-reinforcement interaction relating nonlinear elastic soil behavior to the linear elastic response of the reinforcement. The proposed method, in an original way, explicitly takes into account the geometry of the geocell pocket, effects of soil and geocell-reinforcement stiffness, compaction-induced stresses, soil strength and strain compatibility. The method can be used both analytically and using simple and presented non-dimensional charts. Parametric analyses show that the reinforcement, soil relation and the stresses induced during the compaction procedure are the major factors influencing MIF. An evaluation using data from several laboratory, full-scale and field experiments in works is presented showing good predictive capability of proposed method. An application procedure for calculating MIF is presented.     

Impact of repeated loading on mechanical response of a reinforced sand

Pavements constructed over loosely compacted subgrades may not possess adequate California bearing ratio(CBR)to meet the requirements of pavement design codes,which may lead to a thicker pavement design for addressing the required strength.Geosynthetics have been proven to be effective for mitigating the adverse mechanical behaviors of weak soils as integrated constituents of base and sub-base layers in road construction.This study investigated the behaviors of unreinforced and reinforced sand with nonwoven geotextile using repeated CBR loading test(followed by unloading and reloading).The depth and number of geotextile reinforcement layers,as well as the compaction ratio of the soil above and below the reinforcement layer(s)and the compaction ratio of the sand bed,were set as variables in this context.Geotextile layers were placed at upper thickness ratios of 0.3,0.6 and 0.9 and the lower thickness ratio of 0.3.The compaction ratios of the upper layer and the sand bed varied between 85% and 97% to simulate a dense layer on a medium dense sand bed for all unreinforced and reinforced testing scenarios.Repeated CBR loading tests were conducted to the target loads of 100 kgf,150 kgf,200 kgf and 400 kgf,respectively(1 kgf = 9.8 N).The results indicated that placing one layer of reinforcement with an upper thickness ratio of 0.3 and compacting the soil above the reinforcement to compaction ratio of 97%significantly reduced the penetration of the CBR piston for all target repeated load levels.However,using two layers of reinforcement sandwiched between two dense soil layers with a compaction ratio of 97%with upper and lower thickness ratios of 0.3 resulted in the lowest penetration.     

高速铁路路基粗粒土B组填料大型动三轴试验研究

为研究高速铁路路基粗粒土B组填料的动弹性模量与阻尼特性,进行了大型动三轴试验。结果表明:动应变是影响粗粒土B组填料动弹性模量和阻尼比最主要的因素。随着动应变增大,动弹性模量呈幂指数减小、阻尼比呈S型曲线增加。随着固结围压或加载频率的增加,动弹性模量和阻尼比均增大。随着振动次数的增加,动弹性模量呈减小的趋势,应力水平越低衰减幅度越小。根据试验结果,建立了考虑围压、频率影响的动弹性模量及阻尼比关于动应变的关系模型,为高速铁路路基粗粒土B组填料的动力参数取值提供参考。     

交通荷载作用下软基加筋道路加筋效果分析

为了研究交通荷载作用下考虑软土软化效应的软基加筋道路加筋效果的影响因素,首先以室内动三轴试验为基础,通过回归分析得到了软土在循环荷载作用下动模量衰减的经验公式;然后编制了用户子程序将该公式导入有限元分析软件ABAQUS中,采用有限元分析了荷载形式、荷载频率、筋材模量、加筋位置、加筋层数、软土层厚度等对加筋效果的影响。结果表明,随着荷载频率的增大,加筋效果呈减小趋势。加筋效果会随着筋材模量的增大和加筋层数的增多而增大。当筋材铺设在面层和基层之间时,加筋效果最好。在软土层厚度较小时,加筋效果随软土层深度增大有明显提高;但在软土层厚度较大时,加筋效果随软土深度增加提高较少。     

关于天然地基弹性均压系数的确定

天然地基弹性均压系数是动力机器基础设计中最为重要的参数,根据天然地基弹性均压系数的定义,对当前确定弹性均压系数的几种方法进行了分析,指出GB 50040—96《动力机器基础设计规范》和采用地基土变形模量计算成层地基的弹性均压系数的方法在理论上均是不正确的,而应采用各土层的弹性模量去确定成层地基均压系数。为了便于工程应用,给出了与GB 50040—96相应地基承载力特征值对应的弹性模量值。最后,介绍了利用剪切波波速和标准贯入试验击数确定地基土弹性模量的有关资料。