Modified ultimate bearing capacity formula of strip footing on sandy soils considering strength non-linearity depending on stress level

Most of the contemporary ultimate bearing capacity (UBC) formulas assume a linear yield function in shear stress-normal stress space. However, experimental investigations have corroborated the non-linearity in the failure envelopes of sandy soils. This study focused on the assessment of the stress level effect on the UBC of surface strip footings ascribed to the soil unit weight (γ), footing size (B), and uniform surcharge load (q). The rigid plastic finite element method (RPFEM) was employed for the analysis. The analysis method was validated against the centrifuge test results from the published references in the case of various sandy soils with different relative densities. The RPFEM, using the mean confining stress dependence property of Toyoura sand, is utilized in non-linear finite element analysis of model sandy soil. The normalized ground failure domains in the case of the non-linear shear strength model are gleaned smaller than those in the case of the linear shear strength one. The numerical results are compared with the guidelines of the Architectural Institute of Japan (AIJ) and the Japan Road Association (JRA). The modification coefficients are ascertained for the frictional bearing capacity factor (N_γ) and surcharge bearing capacity factor (N_q), and a modified UBC formula is proposed. The performance of the proposed UBC formula is examined against the analysis results and various prevailing UBC guidelines.     

土工格栅加筋砂土复合体极限承载能力分析

土工合成材料加筋土桥台可以有效减小桥梁与路基之间的差异沉降,避免“桥头跳车”现象的发生。为了计算土工合成材料加筋土复合体在设计中承受荷载的安全冗余度,对其极限承载能力进行了分析。首先讨论了评价加筋土复合体极限承载能力的计算公式,并提出了该公式是否适用于评价加筋细颗粒土复合体承载性能的问题。然后在平面应变的条件下,进行了5组土工格栅加筋砂土模型试验和1组无加筋模型试验,考虑了加筋间距和筋材强度对加筋砂土复合体极限承载能力的影响,并将试验结果与公式的计算结果进行对比,发现该公式低估了加筋砂土的承载能力。基于莫尔库仑破坏准则,并假定加筋土的破坏面符合朗肯破坏面,提出了预测加筋砂土极限承载能力的分析模型,并将模型的计算值与试验值进行对比,发现两者基本吻合。     

Experimental and theoretical studies on the ultimate bearing capacity of geogrid-reinforced sand

Geosynthetic reinforced soil (GRS) structures have gained popularity in replacing concrete rigid piles as abutments to support medium or small-spanned bridge superstructures in recent years. This study conducted 13 model tests to investigate the ultimate bearing capacity of the GRS mass when sand was used as backfill soil. The GRS mass was constructed and loaded to failure under a plane strain condition. Test results were compared with two analytical solutions available in literature. This study also proposed an analytical model for predicting the ultimate bearing capacity of the GRS mass based on the Mohr-Coulomb failure criterion. The failure surface of the GRS mass was described by the Rankine failure surface. The effects of compaction and reinforcement tension were equivalent to increased confining pressures to account for the reinforcing effects of the geosynthetic reinforcement. The proposed model was verified by the results of the model tests conducted in this study and reported in literature. Results indicated that the proposed model was more capable of predicting the ultimate bearing capacity of the GRS mass than the other two analytical solutions available in literature. The proposed model can be used to predict the ultimate bearing capacity of GRS structures when sand was used as backfill material. In addition, a parametric study was conducted to investigate the effects of friction angle of backfill soil, reinforcement spacing, reinforcement strength, and reinforcement stiffness on the ultimate bearing capacity of the GRS mass calculated with and without compaction effects. Results showed that the ultimate bearing capacity of the GRS mass was significantly affected by the friction angle of backfill soil, reinforcement spacing and strength. Compaction effects resulted in an increase in the ultimate bearing capacity of the GRS mass.     

Analytical modeling of geogrid reinforced soil foundation

This paper aims at developing analytical solutions for estimating the ultimate bearing capacity of geogrid reinforced soil foundations (RSF) for both sand and silty clay soils. Failure mechanisms for reinforced soil foundations are proposed based on the literature review and the results of experimental study on model footing tests conducted by the authors. New bearing capacity formulas that incorporate the contribution of reinforcements to the increase in bearing capacity are then developed for both reinforced sand and silty clay soil foundations based on the proposed failure mechanisms. The predicted bearing capacity values are compared with the results of laboratory model tests on reinforced sand and silty clay soil. The proposed analytical solutions were also verified by the results of large-scale model tests conducted by the authors for reinforced silty clay and the data reported in the literature. The predicted bearing capacity values from analytical solutions are in good agreement with the test results.     

Bearing capacity of surface circular footings on granular material under low gravity fields

Low gravity fields have been simulated through magnetic acceleration to conduct experimental study on bearing capacity of circular footings on a type of crushable planetary regolith simulant,which has comparable density and particle size distribution of lunar soil.The load-settlement responses of surface spread footings are obtained by investigating the relative density,footing size and gravity effects.Applying the hyperbolic asymptote method,normalised foundation stiffness and ultimate bearing capacity are obtained by curve fitting and predicted by power functions using multivariate nonlinear regression.The results show that the nonlinear gravity effect is not negligible,related to stress condition,soil dilatancy and mobilised friction angle.A cone penetration test(CPT)-based method for prediction of bearing capacity is proposed with correlations between ultimate bearing capacity of footings and shallow penetration stiffness of CPTs,avoiding the uncertainties of soil property estimations.Analyses of allowable bearing capacity and footing influence zone in consideration of footing size and gravity effects could therefore improve the design of shallow foundations on the Moon and Mars,and provide new understandings and potential implications to the bearing capacity of shallow foundations on crushable granular material in both terrestrial and extraterrestrial geotechnical engineering.     

无黏性土斜坡地基承载力模型试验研究

采用缩尺模型试验对砂土斜坡地基的土压力分布、变形机制、破坏模式进行探索,并研究了斜坡坡角、基础尺寸、相对密度、基础形状对斜坡地基破坏形态及极限承载力的影响。结果表明：斜坡地基的破坏模式与Choudhury提出的破坏模式相近,破坏区域由不对称楔体、辐射向剪切区、被动楔体组成。斜坡地基的破坏区域长度随斜坡坡角、基础尺寸的增大而增大,但不随相对密度的变化而变化;而斜坡地基的极限承载力随斜坡坡角的增大而减小,随基础宽度、相对密度的增大而增大。对相同尺寸的基础而言,方形基础下的地基极限承载力和破坏区域长度均大于圆形基础。试验研究成果对斜坡地基变形特征、破坏形态和斜坡地基承载力影响因素的探究具有一定理论参考价值。     

砂土中锚板上拔三维物质点法模拟研究

土体中锚板的上拔过程存在复杂的锚土相互作用,掌握其变形及破坏机制对于确定锚板的极限承载力和优化设计具有重要的意义.采用三维物质点法(MPM)模拟了砂土中圆形锚板的上拔过程,探究了不同埋深条件下土体的位移场分布及锚板的上拔破坏机制,并结合极限平衡法研究了砂土密实度、锚板尺寸和埋深等因素对其极限承载力的影响.结果 表明,临...     

Failure surface for shallow foundations in sand using a classical bearing capacity

Much effort has been made to elucidate the ultimate capacity of shallow foundations under the general cases of vertical (V), lateral (H), and moment (M) loads in soils. The nature of the dependency of the ultimate capacity of shallow foundations on the combination of V, H, and M loads, the ratio of embedment to diameter, and soil properties has has not yet been analytically revealed. In this paper, an analytical investigation into failure effect of V, H, and M loads applied to shallow foundations in nonhomogeneous sand is made using a classical bearing capacity theory. Of the bearing capacity equations proposed thus far, the most appropriate bearing capacity equation which can predict experimental results of shallow foundations in sand is presented. A no-tension interface between a foundation base and a soil and an effective diameter of the foundation are used to analyze the base failure produced by the vertical stress of the soil below the foundation base when vertical and moment loads are applied. For shallow foundations with various embedment ratios in sand under the two different loading ways, the displacement-load curves and failure envelopes in the H-M, H-V, and M?V planes and in the H-M?V space are presented. For failure envelopes in the H-M plane under a constant vertical load for shallow foundations in sand, the results obtained from experiments are well predicted by the present method.     

扩底桩的抗拔承载力试验及计算

通过对干旱地区黄土中扩底桩的抗拔试验 ,测试了扩底桩在上拔荷载、水平荷载作用下的上拔位移和水平位移以及位移与荷载的关系。研究了极限上拔承载力和抗拔桩的破坏机理。在相同条件下 ,增加扩大端的高度对提高桩的极限上拔承载力是有效的 ,破坏机理为土的减压软化和损伤软化的渐进性破坏。提出了极限上拔承载力的理论计算模式 ,并与实测资料进行了对比 ,理论计算结果与实测值是吻合的     

砂土中螺旋锚上拔承载特性模型试验研究

螺旋锚基础因其能够利用深层土体抗力且具有快速安装和承载的优势而广泛应用于各类岩土工程问题中。多锚片螺旋锚上拔承载特性受埋深、锚片间距、数量、土质条件等因素影响。相邻锚片相互影响导致土体破坏区域重叠,从而影响破坏模式和极限承载力,然而多锚片螺旋锚承载特性的理论及试验研究有限。针对砂土中螺旋锚锚片间距及数量对上拔承载特性及极限上拔承载力影响进行室内1g模型试验研究。结果表明,在中密砂及密砂中,单锚埋深比分别超过6.0和10.5时可认为是深埋锚。中密砂中深埋多片螺旋锚锚片间距在3.0D~4.5D时,各锚片承载能力能够独立发挥,承载量破坏模式发生;密砂中浅埋多片螺旋锚保证承载量破坏模式的锚片间距超过6.0D,但间距为6.0D时,螺旋锚发挥效率超过90%。增加锚片数量可适当提高上拔承载力,但当锚片数量增加使得锚片间距小于某一临界值时,柱状破坏模式发生,螺旋锚承载力不再增加;中密砂中此临界间距约为1.5D,密砂中临界间距约为2.0D。     

Experimental and Numerical Studies on Push-Up Load Tests for Sand Plugs in a Steel Pipe Pile

This paper focuses on the bearing capacity of soil plugs (internal shaft resistance) through fundamental research on the bearing mechanism of dry silica sand plugs. Push-up load tests on the dry silica sand plugs inside a model pipe pile and DEM simulations were carried out to investigate the plugging behaviour. The influences of the packing state of the soil plugs (the relative density), and the height of the plugs on the bearing capacity were investigated. Prior to the push-up load tests, element tests on the silica sand and DEM analyses were performed to characterise the silica sand and to determine suitable DEM analysis parameters. The experimental and DEM results clearly show that the push-up force increases significantly with the increase in the aspect ratio of the soil plug, H/D, and with the relative density of the soil plug. The DEM analyses show a good agreement with the experimental results when the push-up force is small. Furthermore, the DEM results reveal that only the density of the soil plug in the lower portion, adjacent to the pile tip, increases gradually with the increase in the push-up displacement as well as the increase in H/D. Hence, it is the lower portion of the soil plug that mainly controls the capacity of the soil plug.     

Failure envelope considering the ultimate tensile capacity of suction caissons in sand

Little analytical work has been done to elucidate the ultimate capacity of suction caissons under vertical tensile (V), lateral (H), and moment (M) loads in soils. In this paper, in order to reveal the effect of vertical tensile, lateral, and moment loads on the ultimate capacity of suction caissons in sand, an analytical investigation was made using a traditional bearing capacity theory. Taking account of the vertical equilibrium of an annular element of a skirt, through the vertical tractions inside and outside the skirt of a suction caisson when a vertical tensile load is applied, the vertical displacement of the soils adjacent to the skirt of the suction caisson was presented. The most appropriate bearing capacity equation for predicting the experimental results was shown for suction caissons having an embedment larger than a diameter in sand. For the deformation-load responses of suction caissons with various embedment ratios in sand, subjected to inclined tensile loads, there was a good agreement between the results obtained from laboratory tests and those predicted by the present method. The failure surfaces, considering the ultimate tensile capacity in the H-M, H-V, and M−V planes, and in the H-M−V space, for suction caissons in sand, were presented.     

土工袋加固砂性土质边坡模型试验与上限解

 通过模型试验比较有土工袋加固和无土工袋加固时边坡的破坏形态及承载力,验证土工袋对边坡的加固效果,并基于试验结果建立边坡的许可破坏模式及其速度场,利用极限分析上限法求解了边坡的极限高度上限解,利用模型试验结果对边坡极限高度进行验证。试验及计算结果均表明：边坡越陡,有土工袋加固较无土工袋加固时边坡的极限高度和承载力提高越大,表明土工袋加固效果越好。试验结果与计算结果基本吻合,表明该计算方法的可信性,可为边坡的稳定性设计问题提供理论依据。     

加筋风砂土抗拔试验研究(II)——抗拔载荷能力计算分析

通过对典型沙地土样的分析试验和室内模型试验,对未加筋风砂土和土工格栅的15种不同加筋条件下风砂土地基扩展基础的上拔承载性能进行了试验研究.根据试验结果,确定了加筋风砂土地基扩展基础承受上拔荷载的计算模式和理论计算公式.研究了分别由风砂土和土工格栅引起的上拔承载能力,按计算值和实测值分项进行了对比分析,提出了有效的土工格栅加筋形式,即平铺一层和二层土工格栅.提出了进一步需要研究的"锚固长度"问题.     

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.     

Laboratory investigation of boundary effect on pressure-settlement behavior of foundation soil with limited thickness involving geosynthetics

Plate loading tests were conducted to investigate the effect of a bottom boundary condition on the pressure-settlement behavior of a footing on sand with a limited thickness involving geosynthetics. Seven boundary materials including geosynthetics were used to create different boundary conditions at the bottom of the sand. Interface direct shear tests were conducted first to determine the interface friction angles between these boundary materials and sand, followed by plate loading tests to determine their pressure-settlement curves. Test results show that the sand with a limited thickness by a rigid bottom boundary had a higher bearing capacity than that with a larger thickness. The ultimate bearing capacity of the footing on the sand with a limited thickness generally increased with the increase of the interface friction angle. Geosynthetics provided better lateral restraint than other materials. An equation was developed to describe the relationship of the bearing capacity ratio of the sand with a limited thickness to that with a large thickness versus the soil thickness and the interface interaction coefficient. The effect of the boundary material on the back-calculated modulus of the sand was also evaluated.     

月壤水平推剪试验的离散元数值分析

在未来的探月计划中,将进行月球基地的建设和月球资源的开发等活动,这些活动均涉及月壤开挖/开采问题,因此有必要研究月壤与设备间的相互作用。将月壤-设备相互作用简化为平板水平推剪过程,采用离散单元法模拟这一过程。考虑到月壤的颗粒形态及月球表面特殊的环境,将粒间抗转动作用和范德华力引入月壤微观接触模型,通过双轴压缩试验得到数值试样的强度指标。分别考虑月面环境、不考虑范德华力的月面环境、地面环境,对比分析了重力场和范德华力对水平推剪阻力、破坏面和能量消耗的影响。结果表明:随重力场增大,水平推剪阻力和能量消耗增加但不与重力场成正比,破坏面由圆弧面变为平直面;范德华力对水平推剪阻力和能量消耗影响很小,但使破坏面延伸范围更大。     

Effect of footing shape and load eccentricity on behavior of geosynthetic reinforced sand bed

This paper presents the results from a laboratory modeling tests and numerical studies carried out on circular and square footings assuming the same plan area that rests on geosynthetic reinforced sand bed. The effects of the depth of the first and second layers of reinforcement, number of reinforcement layers on bearing capacity of the footings in central and eccentral loadings are investigated. The results indicated that in unreinforced condition, the ultimate bearing capacity is almost equal for both of the footings; but with reinforcing and increasing the number of reinforcement layers the ultimate bearing capacity of circular footing increased in a higher rate compared to square footing in both central and eccentrial loadings. The beneficial effect of a geosynthetic inclusion is largely dependent on the shape of footings. Also, by increasing the number of reinforcement layers, the tilt of circular footing decreased more than square footing. The SR (settlement reduction) of the reinforced condition shows that settlement at ultimate bearing capacity is heavily dependent on load eccentricity and is not significantly different from that for the unreinforced one. Also, close match between the experimental and numerical load-settlement curves and trend lines shown that the modeling approach utilized in this study can be reasonably adapted for reinforced soil applications.     

局部剪切破坏模式下地基极限承载力的研究

基于土的极限平衡理论 ,探讨了局部剪切破坏模式下地基极限承载力的计算方法 ,并有效地解释了试验中所观察到的承载力系数的影响因素