Bearing Capacity Of Strip Foundation Research Articles

Bearing capacity factor for strip foundations on unsaturated clay

ABSTRACT The matric suction in unsaturated soil depends on the type of soil and various flux conditions i.e., infiltration, evaporation and no flow. The shear strength of unsaturated soil changes with change in matric suction leading to varying resistance of soil under various flow conditions. In this study, the bearing capacity of strip foundations resting over unsaturated clay considering variation of matric suction with depth for different surface flux conditions and position of water table, has been obtained using finite element lower bound limit analysis. Modified Mohr-Coulomb yield criteria based on unified effective stress approach has been employed to incorporate the contribution of matric suction stress to the failure condition. A non-dimensional bearing capacity factor was introduced to estimate the bearing capacity of strip foundations and presented as a function of different influencing parameters such as foundation width, water table depth, soil properties, various flow conditions and their flow rate, and surcharge pressure. The influence of variation of unit weight of soil in the unsaturated zone, air entry and pore size distribution parameters, residual degree of saturation of soil, specific gravity of soil solids, and foundation-soil interface roughness on the bearing capacity has also been examined and found to be insignificant.

Unsaturated bearing capacity of strip foundations by using the upper bound rigid block method

Shallow strip foundations are ubiquitous in the field of geotechnical engineering. However, most of the analyses regarding the strip foundations were performed without considering the unsaturated state of the soil. The present article aims to evaluate the upper bound collapse load of the strip footing by duly considering the matric suction developed above the water table. The effect of the vadose zone is considered by employing the suction-stress-based effective stress approach. The suction stress is modelled by employing van-Genuchten’s soil water characteristics curve and Gardner’s one-parameter hydraulic conductivity function. The kinematic admissibility of the problem is ensured by considering a Hill-type mechanism. A new bearing capacity factor is introduced into the formulation which takes care of the unsaturated soil properties and the climate changes. The obtained results are in good agreement with the available solutions.

Probabilistic bearing capacities of strip foundation on two-layered clay

The bearing capacity of a shallow foundation on two-layered soil has been widely studied using deterministic methods, without considering the spatial variation within layered soils. In this study, the bearing capacity of strip foundations on two-layered clay considering spatial variability is investigated by using the random finite element method (RFEM). The results show that the embedment ratio and the spatial variability parameters of the upper layer could significantly affect the normalized probabilistic bearing capacities. Based on the random forest algorithm, a model is established to predict probabilistic bearing capacities, which can be applied as an easy-to-use method without carrying out RFEM analysis.

Seismic Bearing Capacity of Strip Foundation on Rock Mass Obeying Modified Hoek-Brown Failure Criterion

The kinematic method of limit analysis theory was adopted in this paper to calculate the seismic bearing capacity of the shallow strip foundation on a rock mass obeying the non-linear modified Hoek-Brown failure criterion. The generalized Prandtl failure mechanism was chosen, which is different from the multi-wedge failure mechanism assumption commonly used in previous research. Three angle parameters were used to control the mechanism shapes, and the equivalent friction angle and equivalent cohesive were adopted to faithfully reflect the shape characteristics of the failure mechanism. The seismic action was considered using the pseudo-static method, which is simplified to the inertial force determined by the horizontal seismic coefficient. The validation of the present method was carried out by comparing with previous analytical results and the finite element model. Subsequently, the influences of the surface overload, the properties of the rock mass, and the seismic action on the shape and ultimate bearing capacity of the failure mechanism were investigated. For the convenience of practical engineering, this paper gives the ultimate bearing capacity of strip foundations on five representative rock foundations, and the variation trend of bearing capacity with the unit weight of rock mass, surface overload, and horizontal seismic coefficient.

Seismic Bearing Capacity of Strip Foundations with Nonlinear Power-Law Yield Criterion Using the Stress Characteristics Method

The computations of the bearing capacity of foundations have generally been carried out with the usage of the Mohr-Coulomb failure criterion—a linear yield envelope in a shear stress–normal stress plot. However, as noted from many experimental observations, the failure criteria associated with most geomaterials are generally nonlinear. A novel computational analysis, based on the stress characteristics method, has been performed in the current study to compute very accurately the seismic bearing capacity of a rough strip foundation considering a nonlinear power-law yield criterion. The analysis incorporates the effect of pseudostatic horizontal seismic inertial forces. The obtained results can, however, be used to include even the effect of the vertical component of the seismic inertial forces. The present formulation is based on the consideration of a curvilinear nonplastic trapped wedge below the footing base. By carrying out a detailed parametric analysis, the effects of different material shear strength parameters, overburden pressure, seismic forces, and foundation width on the bearing capacity factor Nσ have been examined. With the changes in horizontal seismic acceleration coefficients, the slip line patterns have also been explored. The results from the present analysis compare quite well with that reported from studies available in the literature.

Limit analysis solutions for the bearing capacity of strip foundations under seismic conditions

The limit analysis approach is used in this paper to analyse the problem of a strip foundation on cohesive-frictional soil under seismic conditions, based on the pseudo-static approach, considering different inclination factors. In particular, for a weightless c–ϕ soil, the upper-bound (UB) and the lower-bound solutions are found to be coincident, and then the exact solutions for the bearing capacity factors Nqe and Nce under seismic conditions are obtained. The effect of soil weight is considered with the UB method. A discussion is presented on seismic reduction factors and their superposition.

An Approximation Solution to Estimate the Ultimate Bearing Capacity of Smooth Strip Footings on Ponderable Soil

The procedure for calculating the bearing capacity of smooth footings is employed to calculate the bearing capacity of strip foundations on general c-φ-γ soil by the method of characteristics. The numerical results are in good agreement with other known computations. The surcharge ratio, λ, is found to be another good parameter for evaluation of the bearing capacity without superposition. The approximate formula for estimating the bearing capacity factor Nγ is then proposed considering the influence of λ in addition to φ with an error of no more than ±3 % compared to the numerical results. The error caused by the superposition method is systematically analyzed and the influences of λ and φ on the superposition error are discussed.

Study on Wide-Deep Strip Foundations Resting on Unsaturated Soils

With the development of high-rise buildings, wide and deep foundations have been extensively applied to practical projects during the last several decades. This research aims to develop a solution that applies to wide and deep foundations with the necessary accuracy. Considering the influences of matric suction, adjusted soil parameters, wall friction, and stress strength nonlinearities from high matric suction, a solution for estimating the bearing capacity of strip foundations on unsaturated soils is presented based on the unified strength theory of unsaturated soils and the upper bound method of limit analysis. The validity and applicability of the solution are verified compared with Meyerhof's solution and Zhang's solution. The influences of the breadth, depth/breadth ratio, wall friction, matric suction, and adjustment factor of soil parameters upon the bearing capacity are discussed. Furthermore, a simplified chart-lookup method is presented for practical application. Because it includes the wall friction and assumes only a single failure pattern in derivation, the solution developed for the bearing capacity of strip foundations applies to both shallow and deep foundations.

Influence of the number of reinforcement layers on the bearing capacity of strip foundation resting on sandy soil

Reinforced soil technology is considered one of the most important methods of soil improvement due to its simplicity, easy implementation and saving cost. One of the known soil reinforcement methods is using geogrids to improve the bearing capacity of the soil and reduce the settlement of the soil beneath foundation. In this study, a strip foundation made of a rigid stainless steel with dimensions of 490 mm length, 135 mm width, and 40 mm thickness and reinforced with geogrid (called Tensar SS2) was tested in a laboratory model to investigate the effect of the number of reinforcement layers on the bearing capacity and settlement. The soil was reinforced with one, two, three, and four layers of geogrid (Tensar SS2). The obtained results showed that the reinforcement using geogrid system significantly improved the bearing capacity while reducing the settlement under the strip foundation compared with unreinforced soil. The test result also showed a good improvement in the bearing capacity when the number of reinforcement layers increased from one to four layers. The bearing capacity of the foundation increased when the soil reinforced by four layers of geogrid to about 2.5 times compared with the case of one layer of geogrid. In addition, the maximum settlement decreased to about 2.0 times compared with the case of one layer of geogrid.

Seismic Bearing Capacity of Strip Foundations on Fiber-Reinforced Granular Soil

The present study investigated the bearing capacity of strip footings on a bed of granular fiber-reinforced soils. Analyses were performed by the stress characteristics method (SCM). The failure criterion considers anisotropic distribution of fiber orientation and ignores the rupture of reinforcements. Seismic effects were included in the stress equilibrium equations as the horizontal and vertical pseudo-static coefficients. Stress equilibrium equations were solved by the finite difference method. A computer code was provided to solve the problem. Using the soil and reinforcement input parameters, the code determines the characteristics network and calculates the bearing capacity. The bearing capacity was expressed as the bearing capacity factors of the soil unit weight and surcharge. Parametric analysis was performed to investigate the effect of soil and reinforcement parameters on the bearing capacity and the shape of the failure zone. The outcomes indicated that the SCM results lower ultimate bearing capacity as compared with the upper bound method. Also, the depth of the failure zone increases with an increase in the amount of reinforcement or decrease in the horizontal seismic coefficient. Considering the anisotropic distribution of fiber orientations leads to more bearing capacity than the isotropic distribution.

Ultimate Bearing Capacity of Strip Foundations in Unsaturated Soils considering the Intermediate Principal Stress Effect

The reasonable determination of ultimate bearing capacity is crucial to an optimal design of shallow foundations. Soils surrounding shallow foundations are commonly located above the water table and are thus in an unsaturated state. The intermediate principal stress has an improving effect on the unsaturated soil strength. In this study, the ultimate bearing capacity formulation of strip foundations in unsaturated soils is presented by using Terzaghi’s theory. The unified shear strength equation of unsaturated soils under a plane strain condition is utilized to capture the intermediate principal stress effect. Furthermore, two profiles of matric suction are considered and a hyperbolic function of the friction angle related to matric suction (φb) is adopted to describe strength nonlinearity. The validity of this study is demonstrated by comparing it with model tests and a theoretical solution reported in the literature. Finally, parameter studies are conducted to investigate the effects of intermediate principal stress, matric suction, and base roughness on the ultimate bearing capacity of strip foundations. Besides, the effect of strength nonlinearity is discussed with two methods representing the angleφb.

Coupled Bearing Capacity Factor for Strip Foundations on Cohesive-Frictional Soil Slopes under Static and Seismic Conditions

Abstract This paper investigates the bearing capacity of strip foundations resting on a cohesive-frictional soil sloping ground under both static and seismic conditions by applying the kinematic th...

The feasibility of PSO–ANFIS in estimating bearing capacity of strip foundations rested on cohesionless slope

This study aimed to assess particle swarm optimization (PSO) optimized with an adaptive network-based fuzzy inference system (ANFIS) for the prediction of the ultimate bearing capacity of strip footing resting on a sloping crest. To make datasets, a 2524 finite element model (FEM) simulation analysis was performed. In this sense, the k-fold validation technique selected for the selection of the testing and validation purposes. Also, the variables of the ANFIS algorithm (i.e., the number of clusters), and the PSO algorithm (the population size were optimized using a series of trial and error process. The second main objective of the current study was to assess the applicability of PSO–ANFIS in estimating the ultimate bearing capacity of the strip footing resting on a single cohesionless slope when it was subjected to an external vertical applied stress. The input parameters were the soil properties (Stype) (classified into five different classes from the weakest, shown as S1, to the strongest type, displayed in higher order), slope angle (β), setback distance ratio (b/B) (the distance of the footing from the slope crest to the footing width), and vertical settlement (Uy) of the footing, while the main output was the applied vertical stresses (Fy) that can be applied on the footing. Note that the ultimate bearing capacity (Fult) is the maximum vertical stress when Uy = 0.1 B. A thorough comparison between the measured FEM simulations, the proposed ANFIS and PSO–ANFIS models were carried out to demonstrate their effectiveness in predicting a reliable Fult resting on cohesionless slopes.

An exercise on the bearing capacity of strip foundations

This paper deals with the exact solution of the bearing capacity of strip foundations on weightless cohesive−frictional soil considering the inclination of the (a) foundation base, (b) ground surface and (c) loads on the foundation base and on the ground surface. It will be seen that the upper bound and the lower bound solutions coincide unless extreme conditions are considered. New definitions for the bearing capacity factors Nq and Nc will then be provided.

Spectral bearing capacity analysis of strip footings under pseudo-dynamic excitation

ABSTRACT Spectral seismic bearing capacity of strip foundations is evaluated by the limit equilibrium method (LEM). In this study, the pseudo-dynamic approach is followed to take into account seismic accelerations. The formulation of spectral pseudo-dynamic bearing capacity of strip foundations is presented for the Coulomb failure mechanism. The results are reported in the form of joint bearing capacity considering the contributions of cohesion, surcharge, and soil weight, simultaneously. A parametric study is conducted to evaluate the influence of soil friction angle, interface wall friction angle, dimensionless ratio of cohesion, depth factor, primary and shear wavelength values, amplification factor as well as the frequency on the seismic bearing capacity factor. The results of this study are shown in the form of design aids and tabular for the sake of simplicity to be used by engineers. The consistency of the results with those reported in literature shows the ability and efficiency of the methodology presented herein. It is shown that both the seismic loading characteristics and shear strength parameters affect the bearing capacity of shallow footings.

Approximation of undrained bearing capacity of strip foundations on heterogeneous marine clay

Approximation of undrained bearing capacity of strip foundations on heterogeneous marine clay

Reliability Assessment of Bearing Capacity of Cement–Iron Ore Tailing Blend Black Cotton Soil for Strip Foundations

A reliability estimate of bearing capacity values of cement–iron ore tailing treated black cotton soil was evaluated using a predictive model developed from laboratory results for specimens compacted at British Standard light, West African standard and British Standard heavy energy levels. Laboratory based geotechnical properties of the treated black cotton soil were incorporated into FORTRAN-based first-order reliability program to determine the reliability index values for the following parameters: cement content, C, Iron ore tailing content, IOT, cohesion, CO, friction angle, FA, percentage fine, PF, plasticity index, PI and unit weight, UW. Although cohesion, friction angle, plasticity index and unit weight were greatly influenced by the coefficient of variation, cement content C, IOT content IOT and percentage fine PF did not significantly affect the reliability index values. F-distribution test at 95% level of significance shows that compactive effort had significant effect on the outcome of the reliability index values. However, since reliability index (β) value of 1.0, which is considered adequate for serviceability limit state design, was not met, none of the compactive efforts could be used to model the behaviour of bearing capacity of cement–IOT treated black cotton soil. Hence, a more potent admixture should be used for improving the bearing capacity of strip foundation.

Probabilistic analysis of ultimate seismic bearing capacity of strip foundations

This paper presents a reliability analysis of the pseudo-static seismic bearing capacity of a strip foundation using the limit equilibrium theory. The first-order reliability method (FORM) is employed to calculate the reliability index. The response surface methodology (RSM) is used to assess the Hasofer–Lind reliability index and then it is optimized using a genetic algorithm (GA). The random variables used are the soil shear strength parameters and the seismic coefficients (kh and kv). Two assumptions (normal and non-normal distribution) are used for the random variables. The assumption of uncorrelated variables was found to be conservative in comparison to that of negatively correlated soil shear strength parameters. The assumption of non-normal distribution for the random variables can induce a negative effect on the reliability index of the practical range of the seismic bearing capacity.

Seismic Bearing Capacity of Strip Foundation Embedded in c - ϕ Soil Slope

Seismic Bearing Capacity of Strip Foundation Embedded in <i>c</i> - <i>ϕ</i> Soil Slope

Recent developments on bearing capacity of strip foundation on granular soil under eccentrically inclined load

Bearing capacity of shallow foundations is one of the most important areas of study in geotechnical engineering. Since the founding work of Terzaghi (1943), results of several theoretical and experimental investigations have been published. Most of these studies relate to the case where the foundation is subjected to centric vertical load. In the present paper, an attempt has been made to summarize the important developments related to the estimation of the ultimate bearing capacity of a shallow strip foundation on granular soil subjected to eccentrically inclined loading. Reduction factors to estimate the ultimate bearing capacity of foundations have been discussed.