Bearing capacity of square footings on geosynthetic reinforced sand

The results from laboratory model tests and numerical simulations on square footings resting on sand are presented. Bearing capacity of footings on geosynthetic reinforced sand is evaluated and the effect of various reinforcement parameters like the type and tensile strength of geosynthetic material, amount of reinforcement, layout and configuration of geosynthetic layers below the footing on the bearing capacity improvement of the footings is studied through systematic model studies. A steel tank of size 900 × 900 × 600 mm is used for conducting model tests. Four types of grids, namely strong biaxial geogrid, weak biaxial geogrid, uniaxial geogrid and a geonet, each with different tensile strength, are used in the tests. Geosynthetic reinforcement is provided in the form of planar layers, varying the depth of reinforced zone below the footing, number of geosynthetic layers within the reinforced zone and the width of geosynthetic layers in different tests. Influence of all these parameters on the bearing capacity improvement of square footing and its settlement is studied by comparing with the test on unreinforced sand. Results show that the effective depth of reinforcement is twice the width of the footing and optimum spacing of geosynthetic layers is half the width of the footing. It is observed that the layout and configuration of reinforcement play a vital role in bearing capacity improvement rather than the tensile strength of the geosynthetic material. Experimental observations are supported by the findings from numerical analyses.     

Comparison of bearing capacity of a strip footing on sand with geocell and with planar forms of geotextile reinforcement

Comprehensive results from laboratory model tests on strip footings supported on the geocell and planar reinforced sand beds with the same characteristics of geotextile are presented. The various parameters studied in this testing program include the reinforcement width, the number of planar layers of geotextile and height of the geocell below the footing base. Contrary to other researches, the performance of the geocell and planar reinforcement is investigated at the range of low to medium settlement level, similar to those of interest in practice. The results show that the efficiency of reinforcement was decreased by increasing the number of the planar reinforcement layers, the height of the geocell reinforcement and the reinforcement width. For the same mass of geotextile material used in the tests at the settlement level of 4%, the maximum improvement in bearing capacity (IF) and percentage reduction in footing settlement (PRS) were obtained as 2.73 and 63% with the provision of geocell, respectively, while these values compare with 1.88 and 47% for the equivalent planar reinforcement. On the whole, the results indicate that, for the same quantity of geotextile material, the geocell reinforcement system behaves much stiffer and carries greater loading and settles less than does the equivalent planar reinforcement system. Therefore, a specified improvement in bearing pressure and footing settlement can be achieved using a lesser quantity of geocell material compared to planar geotextile.     

Load-settlement characteristics of large-scale square footing on sand reinforced with opening geocell reinforcement

This paper describes load-carrying characteristics of a series of large-scale steel square footing tests performed on sand reinforced with two types of reinforcement methods. These are full geocell reinforcement (FGR) and geocell with an opening reinforcement (GOR). A thick steel square plate with 500?mm by 500?mm dimensions and 30?mm thickness was used as foundation. The parameters varying in the tests include the depth of geocell mattress (u), width of opening in geocell in the GOR type (w), relative density of sand (D_r) and number of geocell layers (N). The results revealed that the use of GOR and FGR methods enhances significantly the footing load carrying capacity, decreases the footing settlement and decreases the surface heave. It has been found that the use of GOR with an opening width of w/B?<?0.92, has the same improvement effect on the footing load-carrying response as the FGR has (B?=?footing width). Furthermore, with increasing the number of geocell layers from 1 to 2 in both GOR and FGR methods, the footing bearing pressure increases and footing settlement, surface heave and difference of performance between FGR and GOR mattress decrease.     

Strip footing on geocell reinforced sand beds with additional planar reinforcement

The results from laboratory model tests on strip footings supported by geocell reinforced sand beds with additional planar reinforcement are presented. The test results show that a layer of planar geogrid placed at the base of the geocell mattress further enhances the performance of the footing in terms of the load-carrying capacity and the stability against rotation. The beneficial effect of this planar reinforcement layer becomes negligible at large heights of geocell mattress.     

Behaviour of a square model footing on loose sand reinforced with braided coir rope

The effectiveness of horizontally placed braided coir rope reinforcement on the strength improvement and settlement reduction of loose sand is investigated for modeling footings using plate load tests in the laboratory. The influence of parameters such as depth of reinforcement embedment, length, number of layers and number of plies of braided coir rope was examined. The model test results indicate that up to about a six-fold improvement in strength and about ninety percent reduction in settlement (vertical displacement) can be achieved through the use of the proposed reinforcing method. The optimum value of embedment depth of a single layer of braided coir rope reinforcement was identified as 0.4 times the footing width. It was also found that optimal benefit was realized for a length ratio equal to about 3 and by reinforcing the zone of soil directly beneath the model footing upto a depth equal to about 0.6 times the width of footing. Increase in the number of layers within the significant depth leads to a proportionate increase in strength improvement ratio, while the optimal settlement reduction is realized with three layers of braided coir rope reinforcement. Regression analysis carried out with limited experimental data suggests the possibility of developing a predictive model to quantify the strength improvement.     

Geocell reinforcement for performance improvement of vertical plate anchors in sand

In this paper influence of geocell reinforcement on performance of vertical plate anchors is studied. A series of model tests were carried out in a test bed-cum-loading frame assembly. The anchor used was a steel plate of size 100 mm × 100 mm. With geocell reinforcement the anchor could sustain deformations as high as 60–70% of its height when the load carrying capacity was increased by four fold. The optimum length, width, and height of geocell mattress giving maximum performance improvement are found to be 5, 3 and 2.8 times the anchor height respectively. For adequate performance improvement size of geocell pocket opening should be close to the anchor size. The load dispersion angle that depicts the rigidity of the geocell mattress tends to increase with increase in its width, height and reduction in pocket size. A numerical study using fast Lagrangian analysis of continua was carried out. The agreement between observed and computed results is found to be reasonably good.     

Model testing and numerical investigation of interference effect of closely spaced ring and circular footings on reinforced sand

Due to heavy loads and the non-availability of suitable construction sites, engineers are often required to place footings at close spacing. These footings influence each other, including effects on load-settlement and bearing capacity behavior. In this research the bearing capacity of closely located ring and circular footings on reinforced sand has been investigated numerically and experimentally. The goal of this study is to evaluate the interference effect on the bearing capacity of adjacent circular and ring footings. Footings on reinforced and unreinforced sand have been investigated. In this research, interference effect of footings, shape effects, effect of spacing between footings and also the effect of reinforcement layer on the bearing capacity are studied. To achieve these objectives laboratory circular and ring footing models and also numerical models were used. Finite element computer code PLAXIS 3D Foundation was used for numerical modeling. Experimental and numerical analysis results show that the ultimate bearing capacity of two closely spaced circular and ring footings is greatest when they stand exactly beside each other and decreases with increase in the spacing to footing diameter ratio (Δ/D). It is found that for Δ/D > 4, the bearing capacity of each adjacent footing is almost the same as that for single footing. This means that for a center-to-center spacing greater than 4D, no significant interference effect was observed and each footing acted more or less independently, similar to a single footing.     

Effects of restraining conditions on the bearing capacity of footings near slopes

In evaluating the ultimate bearing capacity (q_u) of a strip footing adjacent to a slope, conventional correction formulas for the effect of load eccentricity may not be applicable because these formulas were developed exclusively for footings situated on horizontal grounds, where loads eccentric to opposite sides of the footing yield identical results for q_u. In this study, loading tests and analyses are conducted on a strip footing placed adjacent to a model slope with various slope angles. The experimental evidence shows that a load eccentric toward the heel of a footing leads to an increase in bearing capacity, whereas the analytical results based on conventional formulas show the opposite trend. To address this discrepancy, an approach is proposed that uses a bearing capacity correction formula for a footing with a setback from the crest of the slope. Results of a comparative study show that the experimental values for bearing capacity factor N_γ(test), with full corrections for load inclination, load eccentricity, and footing setback are comparable to the theoretical solutions. Furthermore, fully corrected values for N_γ(test) for the fixed footing approximately follow the line of the upper boundary; those for the free-rotating footing follow the lower boundary of the theoretical solutions reported in the literature. This discrepancy is due to the different failure mechanisms induced by the restraining conditions of the footing which have yet to be considered in engineering practice.     

砂土地基上圆形浅基础三维破坏包络面的理论研究

对砂土地基上圆形浅基础在竖向荷载V 、水平荷载 H 及力矩 M 复合加载条件下的承载力进行了系统的三维有限元分析。在分析中,砂土假定为纯摩擦材料,遵循基于 Mohr-Coulomb 破坏准则的理想弹塑性本构关系。首先,对圆形浅基础的竖向承载力进行了有限元计算,并与滑移线解法进行了对比,两种方法所得结果比较吻合。进而探讨了砂土内摩擦角对于基础在 V-H 、V-M荷载平面与V-H-M 三维荷载空间内的破坏包络轨迹的影响。计算结果表明,与不排水情况下软黏土地基上基础破坏包络面相比,砂土地基上圆形浅基础的破坏包络面形状有较大差异,但V-H 和V-M 平面内的破坏包络面形状仍具有较好的归一化特性。基于有限元计算结果,建立了圆形浅基础在V-H-M 三维荷载空间内的破坏包络面方程,该方程可用来合理评价复合加载条件下砂土地基上圆形浅基础的整体稳定性。     

临近基坑矩形浅基础地基承载力上限估算

临近基坑或边坡的既有建筑物地基承载力减损情况是一个值得关注的问题。文中利用极限分析中的机动法,构建了一个临近基坑矩形浅基础的三维机动许可破坏模式,按照地基承载力的传统表达方式,分别给出了单独考虑土的粘聚力、基础埋深(超载)和土的自重影响的承载力系数。根据极限分析上限理论,这种使影响地基承载力的每一个因素都达到最小的简化处理方法,已不是严格意义上的极限分析上限解答,但用于工程设计是可行的。最后将计算结果与已有文献资料进行了分析比较,并以地面水平时矩形基础地基承载力系数为基准,得到临近基坑矩形浅基础地基承载力减损因数上界估算值。     

Load-settlement response of shallow square footings on geogrid-reinforced sand under cyclic loading

To study the settlement and dynamic response characteristics of shallow square footings on geogrid-reinforced sand under cyclic loading, 7 sets of large scale laboratory tests are performed on a 0.5?m wide square footing resting on unreinforced and geogrid reinforced sand contained in a 3?m?×?1.6?m?×?2?m (length?×?width?×?height) steel tank. Different reinforcing schemes are considered in the tests: one layer of reinforcement at the depth of 0.3B, 0.6B and 0.9B, where B is the width of the footing; two and three layers of reinforcement at the depth and spacing both at 0.3B. In one of the two double layered reinforcing systems, the reinforcements are wrapped around at the ends. The footings are loaded to 160?kPa under static loading before applying cyclic loading. The cyclic loadings are applied at 40?kPa amplitude increments. Each loading stage lasts for 10?min at the frequency of 2?Hz, or until failure, whichever occurs first. The settlement of the footing, strain in the reinforcement and acceleration rate in the soil have been monitored during the tests. The results showed that the ultimate bearing capacity of the footings was affected by the number and layout of the reinforcements, and the increment of bearing capacity does not always increase with the number of reinforcement layers. The layout of the reinforcement layers affected the failure mechanisms of the footings. Including more layers of reinforcement could greatly reduce the dynamic response of the foundations under cyclic loading. In terms of bearing capacity improvement, including one layer of reinforcement at the depth of 0.6B was the optimum based on the test results. It is found that fracture of geogrid could occur under cyclic loading if the reinforcement is too shallow, i.e. for the cases with the first layer of reinforcement at 0.3B depth.     

Ultimate loads for eccentrically loaded model shallow strip footings on geotextile-reinforced sand

A series of tests were carried out with an eccentrically loaded model surface shallow strip footing on reinforced dense sand to investigate the decrease of the ultimate loads with increasing eccentricity and to compare the experimental results with commonly used approaches such as Meyerhof's effective width concept and the customary analysis. An experimental system was produced and used to run the tests. The experimental system consists of a tank, model footing, sand, loading mechanism, etc. A single woven geotextile sheet was placed horizontally below the footing's base at a depth of half of the footing's width. Geotextile reinforcement increased ultimate loads when compared to the unreinforced cases. This contribution decreases with increasing eccentricity. The measured decreases in ultimate loads with increasing eccentricities in the unreinforced tests within the core (kern, middle third) are in good agreement with Meyerhof's approach, while customary analysis is a little on the conservative side. Outside the core, Meyerhof's approach is on the conservative side in this case. Decreasing ultimate loads with increasing eccentricity for the reinforced tests cases were in general agreement with customary analysis, although they are slightly greater.     

临近基坑条形浅基础地基承载力减损的下限估算

利用极限分析有限元方法和下限定理,分析计算了考虑土体自重的临近基坑(边坡)条形浅基础c-?土地基极限承载力下限解。作为一种简化手段,分别按土的粘聚力、基础埋深(超载)和土的自重这3项承载力影响系数表达临近基坑地基极限承载力,获得了基础位置、基坑开挖的边坡角度以及土的内摩擦角取不同值时,相应的地基承载力系数下限解答,并对计算结果进行了讨论。最后将计算结果与已有文献资料以及利用多滑块Prandtl破坏模式得到的极限分析上限解答进行了分析对比,从上、下限给出了临近基坑既有建筑物地基承载力系数取值范围及其承载力上、下限平均值时的减损因数,分析了影响计算结果的因素。     

条形粗糙基础极限承载力求解与误差分析

利用滑移线法计算了粗糙条形基础极限承载力,计算时考虑了土的黏聚力c、内摩擦角φ和土体重度g的共同作用,避免了对破裂面形状的人为假定,并满足所有边界条件。将数值计算结果与其他学者的解答进行了对比,证明了解答的准确性。分析了地基承载力系数N_γ的影响因素,证实了N_γ除了与地基摩擦角φ有关外,还与超载比l有关。绘制了不同j值下N_γ随l的关系曲线,给出了N_γ的拟合公式,计算结果表明拟合公式的误差在±4%以内。最后对传统叠加方法计算承载力与精确解之间的误差进行了计算,总结了不同j值时误差e随l的变化规律,发现叠加计算结果比精确解小,且最大误差出现在l介于0.1~1之间。     

Laboratory investigation of bearing capacity behaviour of strip footing on reinforced flyash slope

The paper presents the results of laboratory model tests on bearing capacity behaviour of a strip footing resting on the top of a geogrid reinforced flyash slope. A series of model footing tests covering a wide range of boundary conditions, including unreinforced cases were conducted by varying parameters such as location and depth of embedment of single geogrid layer, number of geogrid layers, location of footing relative to the slope crest, slope angles and width of footing. The results of the investigation indicate that both the pressure–settlement behaviour and the ultimate bearing capacity of footing resting on the top of a flyash slope can be enhanced by the presence of reinforcing layers. However the efficiency of flyash geogrid system increases with the increasing number of geogrid layers and edge distance of footing from the slope. Based on experimental results critical values of geogrid parameters for maximum reinforcing effects are established. Experimental results obtained from a series of model tests have been presented and discussed in the paper.     

Ultimate bearing capacity of strip footing resting on soil bed strengthened by wraparound geosynthetic reinforcement technique

In the recent past, the wraparound geosynthetic reinforcement technique has been recommended for constructing the geosynthetic-reinforced soil foundations. This paper presents the development of an analytical expression for estimating the ultimate bearing capacity of strip footing resting on soil bed reinforced with geosynthetic reinforcement having the wraparound ends. The wraparound ends of the geosynthetic reinforcement are considered to provide the shearing resistance at the soil-geosynthetic interface as well as the passive resistance due to confinement of soil by the geosynthetic reinforcement. The values of ultimate load-bearing capacity determined by using the developed analytical expression agree well with the model footing load test values as reported in the literature.     

Bearing capacity of strip footings on sand slopes reinforced with geogrid and grid-anchor

This paper presents the effect of a new type of geogrid inclusion on the bearing capacity of a rigid strip footing constructed on a sand slope. A broad series of conditions, including unreinforced cases, was tested by varying parameters such as geogrid type, number of geogrid layers, vertical spacing and depth to topmost layer of geogrid. The results were then analyzed to find both qualitative and quantitative relationships between the bearing capacity and the geogrid parameters. A series of finite element analyses was additionally carried out on a prototype slope and the results were compared with the findings from the laboratory model tests and to complete the results of the model tests. The results show that the bearing capacity of rigid strip footings on sloping ground can be intensively increased by the inclusion of grid-anchor layers in the ground, and that the magnitude of bearing capacity increase depends greatly on the geogrid distribution. It is also shown that the load-settlement behavior and bearing capacity of the rigid footing can be considerably improved by the inclusion of a reinforcing layer at the appropriate location in the fill slope. The agreement between observed and computed results is found to be reasonably good in terms of load-settlement behavior and optimum parameters.     

复合加载情况下非均匀地基上矩形基础承载力研究

复合加载情况下,求解非均质地基上矩形基础的极限承载力,以及从理论上研究影响极限承载力的相关因素,具有很强的工程实用与理论参考价值。基于极限平衡原理,在Mohr-Coulomb破坏准则的基础上,将非均质地基上矩形基础极限承载力问题等价为一个边界待定的泛函极值问题。利用变分原理得到与平衡方程相等价的积分约束条件以及相应的欧拉方程与横截条件,在引入问题边界条件后,求得了复合加载情况下非均质地基破坏时的滑裂面方程。同时研究了土体内摩擦角、土体黏聚力、矩形基础长宽比、土层强度比与地下水位变化等因素对地基极限承载力的影响。     

双层地基极限承载力的极限分析上限法

极限分析方法已广泛应用于岩土工程稳定性分析中,目前已有很多学者探讨了极限分析上限方法计算均质土地基极限承载力问题,事实证明该方法是卓有成效的。而实际中常遇到的是层状地基,特别是持力层范围内主要有两层土的情况,对于双层地基极限承载力的计算,目前的研究还相当不成熟。将一种新的多块体离散模式的上限方法应用于双层地基极限承载力的计算,详细探讨了该多块体离散模式应用时相容速度场的确定方法。并应用该方法对一些双层地基土极限承载力问题进行了计算,将计算结果与已有计算方法做了对比,通过对比可以验证该方法是正确和有效的。     

基础形状和尺寸对增强土地基承载能力的影响

通过试验,初步探讨了基础形状和尺寸对于增强地基承载力的影响,试验结果表明,与非增强土系不同,基础形状不影响增强土地基的承载能力,关于增强土地基承载能力提高的有利影响,如同在小尺寸模型试验上观察到的一样,在一定尺寸范围内呈现出重复性和稳定性,在实际尺寸条件下承载能力的提高水平很可能与此类似。