Bearing Capacity Of Shallow Footings Research Articles

Estimation of bearing capacity of shallow footings on layered sand using finite elements analysis

PurposeThis paper aims to estimate the bearing capacity of a surface strip and circular footings lying on layered sand using numerical limit analysis.Design/methodology/approachLower and upper bound limit analysis, as well as finite elements and second-order conic programming (SOCP), are used in this analysis. The yield criterion of Mohr-Coulomb is used to model soil behavior. Using this technique, stringent lower and upper bounds on ultimate bearing capacity can be achieved by assuming an associated flow law.FindingsThe obtained results indicate that the exact collapse load is typically being bracketed to within 6% about a mean of both the bounds. The obtained results are compared with the existing literature wherever applicable.Originality/valueTo the best of the authors’ knowledge, no study has used lower and upper bound limit analysis, as well as finite elements and SOCP, to estimate the bearing capacity of a surface strip and circular footings lying on layered sand.

Effect of sheet pile wall on the load-settlement behaviour of square footing nearby excavation

ABSTRACT Due to the shortage of urban land, new buildings are constructed adjacent to old buildings. In this regard, there is limited information about the behaviour of the shallow foundations adjacent to the excavation. In this research, a series of experimental and numerical studies are conducted on reinforced and unreinforced granular soils adjacent to excavation loaded with square footings. The experimental results are in good agreement with the numerical study. Numerical investigations were carried out on sandy excavation by varying the footing distance from the edge of the different granular excavations. It was found that by using three reinforcing layers, the ultimate bearing capacity would increase. Additionally, excavation has no significant effect in the vicinity of 4B, in which B is the footing width. Furthermore, the need to build new large buildings in the vicinity of each other has reduced the distances between square footings and, as a result, has created the phenomenon of interference in the footings and excavation. This occurs through the interference of wedges and rupture surfaces. Since this phenomenon bears a substantial result on the bearing capacity of shallow footings, this work investigates the effect of interference of ruptured wedges on the carriage capacity, settlement, and deformation of square footings. The optimum interference factor is defined at spacings of 2B and 3B for dense and loose reinforced sands, respectively. Furthermore, by including three continuous geogrid layers underneath two square footing interferences, their behaviour will be improved.

A comparative study of bearing capacity of shallow footing under different loading conditions

ABSTRACT In the present study, the behaviour of a shallow rectangular foundation placed over the multiple layers of the geogrid reinforced sand under eccentric and oblique loads in both dimensions was studied. Various parameters were investigated in this study including the number of reinforced layers (N), eccentricity (e/B, e/L) and inclination of applied load (α). Bi-axial geogrid (Bx20/20)-reinforced sand and laboratory results were compared with PLAXIS 3D software keeping N, eccentricity ratios and angle of inclination as input parameters. The results present that the value of Ultimate Bearing Capacity (UBC) of model foundation found reduced as the axial eccentricity and inclination of applied loads increased. It also found that multiple geogrid-reinforced layers increased the UBC of about 75%. Finally, the model test results were analysed using PLAXIS 3D. A good agreement was originated between the laboratory results and numerical analysis.

Pseudo-static Seismic Bearing Capacity of Shallow Foundations in Unsaturated Soils Employing Limit Equilibrium Method

There exist many structures founded on unsaturated soil deposits. Shear strength augmentation due to the evolution of the matric suction within the unsaturated porous media enhances the bearing capacity of the overlying foundation. This paper presents the evaluation of the pseudo-static seismic bearing capacity of the shallow foundations resting on unsaturated soil deposits using limit equilibrium method. Adopting the Coulomb failure mechanism and Bishop effective stress concept, the bearing capacity equations are solved. The distribution of the matric suction beneath the footing is assumed to be linear. The results of the bearing capacity evaluation are validated against some experimental data found in literature for the static condition. For the seismic loading consideration, the pseudo-static method is utilized. The dual effect of the earthquake acceleration vertical component is thoroughly discussed and a suction transition point is introduced in which the minimum bearing capacity is observed to bear the same value for both upward and downward directions. The increase in the matric suction throughout the soil deposit leads to the increase in the soil shear strength, thus posing more resisting forces as well as higher ultimate bearing capacity. The offered solution is deemed a consistent and useful tool for the accurate prediction of the seismic bearing capacity of shallow footings resting on unsaturated soil deposits.

Seismic Bearing Capacity of Shallow Strip Footings on Sand Deposits with Weak Inter-layer

The main objective of this study is to evaluate the ultimate seismic bearing capacity of a shallow strip footing resting on a frictional soil stratum containing a weak intervening layer. The majority of the studies throughout the literature pertain to the static loading condition. The previous seismic analyses have also been devoted to the studies on the bearing capacity of shallow strip footings resting on a two-layered soil. The influence of weak middle layer on the pseudo-static seismic bearing capacity of shallow foundations is the main focus of the present study. To determine the seismic bearing capacity, the limit equilibrium method (LEM) was combined with the pseudo-static seismic loading approach. Bearing capacity was defined by a single equivalent coefficient which combines the contributions of cohesion, surcharge and soil weight. A two-wedge failure surface, known as the Coulomb failure mechanism, was adopted to model the slip lines in each layer to calculate the seismic bearing capacity of the overlying shallow strip footing. The Particle Swarm Optimization (PSO) algorithm was invoked to seek the optimal bearing capacity value under different strength and loading conditions. In order to verify the validity of the presented formulations, the results were compared with some Finite Elements Method (FEM) analyses available in literature. Furthermore, the influence of the embedment depth, thickness, and strength of the weak inter-layer on the seismic bearing capacity of the shallow footing is investigated in the presence of different seismic loading arrangements. The results of this study could be very helpful in the seismic analysis and design of shallow foundations overlying a soil medium containing a weak layer of various strengths.

Analysis of the suction evolution during direct shear test in a silty sand soil

Shear strength of soils is one of the essential parameters for analysing and solving divers geotechnical problems (e.g. the bearing capacity of shallow footings pile foundations, slope stability and earth embankments). In this study, a series of conventional large-scale (300 X 300 mm) direct shear tests were carried out on saturated and constant water content silty sand specimens at ei = 0.6 and 1.0 tested under applied vertical stresses of 100, 200, or 400 kPa to investigate the influence of matric suction on the shear strength characteristics of the tested material. A loading steel cap was modified to allow the direct measurements of the matric suction using two commercial available Equitensiometer suction probes (EQ3). The experimental program indicated that, for both studied void ratios, the obtained shear strength of specimens under constant water content is found to be distinctly greater than those obtained from saturated samples. The results showed that the samples compacted at ei = 1.0 exhibited collapse behaviour during saturation stage, whereas same samples did not show any volume change during stabilisation stage when tested under constant water content condition. The study results also showed that the matric suction reduction during consolidation stage depends on initial void ratio of the tested samples as well as the level of applied vertical stress. Moreover, the matric suction evolution during shearing process of both studied void ratios specimens decreased with increasing the level of applied vertical stress.

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.

Experimental and Numerical Modeling of the Effect of Groundwater Table Lowering on Bearing Capacity of Shallow Square Footings

The lowering of the groundwater table causes the area above the water table to become unsaturated and capillary phenomena to appear in this zone. This means that the bearing capacity of shallow footings will be influenced by capillary stress or matric suction. In this research, the effect of groundwater table lowering on the bearing capacity of a shallow square model footing on dense sand has been investigated by conducting plate load tests under different groundwater table conditions. Numerical simulations of the experiments also were performed using the finite element software Optum G2. The results of the experiments showed that lowering of the water table increased the matric suction. At a suction 0.5 to 4.5 kPa, the ultimate bearing capacity in the soil increased non-linearly from 2.5 to 4-times the bearing capacity of the saturated state. Numerical simulation of the experiments by assuming cohesion due to matric suction for the upper part of the groundwater table predicted the same behavior. Very good agreement was obtained between the predicted bearing capacity and the measured values.

Experimental and numerical investigation of the effect of pre-existing utility tunnel on the bearing capacity of shallow footing in sandy soils

PurposeThe purpose of this study is to investigate the effect of presence of buried flexible pipe on the bearing capacity of shallow footing. First, a model study is performed where shallow footing model is tested for its load settlement behavior, with and without the existence of buried PVC pipe lying vertically below the base of the footing.Design/methodology/approachThe experimental set-up consisted of a steel box filled with sand at two different relative density values [RD = 50 per cent (medium dense) and RD = 80 per cent (dense sand)] and vertical load was applied on the model footing through hydraulic jack and reaction frame arrangement connected with a proving ring. Test results are verified numerically using commercially available finite element code PLAXIS 2D. With due verification, a parametric study has been conducted, numerically, by varying the range of input parameters, such as unit weight, angle of internal friction, diameter of buried conduit and the Elastic modulus of the soil to assess the pre cent reduction in the capacity of the foundation soil because of the presence of underlying buried flexible pipe.FindingsResults show that for each footing, there exists a critical depth below which the presence of the buried conduit has negligible influence on the footing performance. When the conduit is located above the critical depth, the bearing capacity of the footing varies with various factors, such as geotechnical parameters of soil and location and diameter of the buried conduit.Originality/valueIt is an original paper performed to assess the presence of buried flexible pipe on the bearing capacity of the shallow footing.

Potential use of structural skirts towards improving the bearing capacity of shallow footings exposed to inclined loadings

ABSTRACT Though variety of remedial methods for soil stabilization is available and are well developed; however, they are prohibitively expensive and are restricted by some problems related to the site condition. Structural skirts hold promise as an alternative method of improving the bearing capacity of shallow footings subjected to inclined loads. In this study, a series of experimental model tests were carried out to measure the impact of structural skirts on the bearing of footings exposed to inclined loads. On the basis of these tests, modified inclination factors are proposed for footings with and without structural skirts subjected to inclined loads. The results presented the improvement of bearing capacity in the range of 1.2–3.5, depending on the particular geometry of both footings and skirts, the magnitude of the load and the inclination angle. This study provide good insights towards the need of more investigations of the impact of structural skirts on the bearing capacity of sub-structures exposed to different loads.

Prediction of ultimate bearing capacity through various novel evolutionary and neural network models

In the current study, various evolutionary artificial intelligence and machine learning models namely, optimized artificial neural network (ANN), genetic algorithm optimized with ANN (GA-ANN) and particle swarm optimization optimized with ANN (PSO-ANN), differential evolution algorithm (DEA), adaptive neuro-fuzzy inference system (ANFIS), general regression neural network (GRNN), and feedforward neural network (FFNN) were optimized and applied to predict the ultimate bearing capacity (Fult) of shallow footing on two-layered soil condition. Due to a lot of input variables such as (upper layer thickness/foundation width (h/B) ratio, footing width (B), top and bottom soil layer properties) finding a reliable solution for such a complex engineering problem is difficult. Most of the available solutions are based on very limited experimental works. To assess the capability of proposed methods a new ranking system called CER (color intensity rating) based on their result of above indices was developed. As a result, although all provided methods, after being optimized, could successfully predict the bearing capacity of shallow footing in the two-layer subsoil and PSO-ANN could perform better compared to other techniques. Based on RMSE, R2 and VAF, values of (0.01, 0.99, and 99.90) and (0.01, 0.99, and 99.90) were found, respectively, for the training and testing datasets of PSO-ANN model. In this regard, the accuracy of other hybrid algorithm of GA-ANN model with RMSE, R2 and VAF of (0.05, 0.99, and 97.80) and (0.06, 0.99, and 97.57), respectively, for the training and testing datasets was slightly lower than the PSO-ANN model. This shows the superiority of the PSO-ANN model in the prediction of a highly complex real-world engineering problem.

Scale Effects of Footings on Geocell Reinforced Sand Using Large-Scale Tests

The scale effect on bearing capacity of shallow footings supported by unreinforced granular soils has been evaluated extensively. However, the subject has not been addressed for shallow footings on geocell-reinforced granular soils. In this study, load-settlement characteristic of large square footings is investigated by performing large-scale loading tests on unreinforced and geocell-reinforced granular soils. The effects of footing width (B), soil relative density of soil (Dr), and reinforcement depth (u) have been investigated. The test results show that the scale effects exist in geocell-reinforced soils, like unreinforced soils, and the behavior of small-scale models of footings cannot be directly related to the behavior of full-scale footings due to the difference between initial conditions of tests and the initial state of mean stresses in the soil beneath the footings having different dimensions. Large footings create higher mean stresses in the soil, resulting in low soil friction angle and initial conditions of the test approach to the critical state lines. The results of tests indicate that model experiments should be conducted on low-density soil for better prediction of the behavior of full-scale footings, otherwise, the predicted behavior of full-scale footings does not seem conservative.

Seismic bearing capacity of shallow footings on cement-improved soils

Seismic bearing capacity of shallow footings on cement-improved soils

Bearing capacity of Skirt circular footing on sand

Skirts are used to improve the bearing capacity of shallow footings on sandy soil by constraining the soil beneath and containing the plastic flow of soil. They are used as an alternative to deep foundations in soils with low strength at the surface. As there has been available little work studying the performance of skirted foundation, we are performing eighteen laboratory experiments on circular steel footings of different diameters and different skirt lengths. The aim of these experiments is to shed some lights on the effects of skirts on the bearing capacity of shallow footings. The effects of skirt length and the relative density of sand on the ultimate load attained were investigated. From the accomplished laboratory tests, it was found that skirts improve appreciably the sustainability of shallow footings to applied load as they increase the ultimate load of shallow footings by some up to 6.25 times for the current study conditions and variables. The performance of skirted footing depends upon the relative density of sand and on the skirt length to footing diameter ratio. Skirts are more beneficial in case of footings on loose sand than in case of medium and dense sand.

Influence of Matric Suction on the Results of Plate Load Tests Performed on a Lateritic Soil Deposit

Abstract This paper evaluates the influence of soil matric suction on the results of plate load tests conducted in an unsaturated, lateritic soil deposit at a depth of 1.5 m. Soil suction was monitored during the tests with tensiometers installed at the bottom of the testing pit. Field test results show that small increases in matric suction lead to substantial increases in bearing capacity of the soil-plate system. In situ experimental loading-collapse (LC) and suction increase (SI) yield surfaces are proposed for the soil investigated. Changes in matric sucion were observed to significantly influence settlement response, particularly for high levels of surcharge load. The rate of settlement shows a non-linear decreasing trend with increasing soil matric suction.

Determination of bearing capacity of shallow foundations without using superposition approximation

The Terzaghi superposition assumption has been widely used to determine the bearing capacity of shallow footings. Although this assumption always errs on the safe side, a rigorous procedure to calculate the bearing capacity is still of engineering value. This paper presents such a procedure that is free from errors as a result of the superposition assumption. It demonstrates that the ultimate bearing capacity can be precisely expressed by the Terzaghi equation, except that the bearing capacity factor Nγis dependent upon the surcharge ratio. A recently developed numerical method, i.e., the critical slip field method, is used to calculate the modification coefficient for modifying Nγ. It is found that this modification coefficient increases with the surcharge ratio at small values of surcharge ratio and then remains constant for large values of surcharge ratio. However, the errors invoked by the superposition assumption do not exceed 10%. On the basis of numerical calculations, a simple closed-form expression of the modification coefficient is proposed that yields the theoretically rigorous ultimate bearing capacity. In the later part of the paper, errors in bearing capacity calculations owing to the use of conventional procedures are analyzed. It is concluded that the continued use of conventional procedures is justified, but the inherent errors should not be neglected in assessing the performance of shallow foundations.Key words: shallow foundation, strip footing, ultimate bearing capacity, critical slip field.

Effective width rule in calculations of bearing capacity of shallow footings

The classical solution to the bearing capacity problem predicts the limit load on symmetrically loaded shallow strip footings. A useful hypothesis was suggested by Meyerhof to account for eccentricity of loading, in which the footing width is reduced by twice-the-eccentricity to its ‘effective’ size. This hypothesis sometimes has been criticized as being overconservative. This paper examines Meyerhof’s suggestion and presents the bearing capacity of eccentrically loaded footings calculated using the kinematic approach of limit analysis. It is found that the effective width rule yields a bearing capacity equivalent to that calculated based on the assumption that the footing is smooth. For more realistic footing models and for cohesive soils the effective width rule is a reasonable account of eccentricity in bearing capacity calculations. Only for significant bonding at the soil-footing interface and for large eccentricities does the effective width rule become overly conservative. For cohesionless soils, however, the effective width rule may overestimate the best upper bound. This overestimation increases with an increase in eccentricity. ©

Influence of relative density and stress level on the bearing capacity of sands

AbstractA finite element approach is presented to determine the bearing capacity of shallow footings on silica sand deposits. The approach can take into account the effects of relative density and stress level on the shear strength of granular soil and is applicable to a fairly large effective stress range. The strength of sand is characterized by a non‐linear Mohr–Coulomb criterion which depends on maximum and critical friction angles, widely used parameters in engineering practice. The results of analysis indicate that this approach yields reliable predictions of bearing capacity and, in particular, it can model the volumetric behaviour of the soil at failure.

Inclination Factors for Bearing Capacity of Shallow Footings

An investigation is made of the inclination factors for estimating the ultimate bearing capacity of shallow strip footings under central load inclined in the direction of footing length. The corresponding theoretical inclination factors are found to be generally larger than the previous factors for a load inclined in the direction of the footing width. The theoretical values of the present inclination factors are compared with some experimental results of model footings on clay and sand.

Effect of strength anisotrophy on bearing capacity of shallow footing in a dense sand : Oda, M; Koishikawa, I Soils Found, V19, N3, Sept 1979, P15–28

Effect of strength anisotrophy on bearing capacity of shallow footing in a dense sand : Oda, M; Koishikawa, I Soils Found, V19, N3, Sept 1979, P15–28