Traditional Bearing Capacity Research Articles

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.

Determination of Efficiency Factors for Closely Spaced Strip Footings on Cohesive–Frictional Soils

The bearing capacity of closely spaced footings has become one of the important topics in geotechnical engineering research owing to the rapid development in urban areas around the world. In this paper, we propose three efficiency factors that can be used to describe the bearing capacity effects of closely spaced footings using Terzaghi’s traditional bearing capacity equation. With an advanced finite-element limit analysis of upper and lower bounds, both the closely spaced strip footings and the multiple closely spaced strip footings on cohesive–frictional soil with a surcharge effect were investigated. The numerical results showed that the efficiency factors were significantly influenced by the internal frictional angle and the spacing ratio. Several comparisons were made with those published in the literature. Furthermore, the failure mechanisms of closely spaced footings are presented, while design charts were produced with a wide range of practical parameters. This study should be of great interest to foundation engineering practitioners.

Failure surface considering the ultimate tensile and compressive capacities of suction caissons in clay

One of the important issues for suction caissons is to demonstrate the effect of the contact between the base and the soil on the ultimate capacities of vertical (V), lateral (H), and moment (M) loads. It has not yet been analytically revealed how the ultimate capacities of suction caissons are dependent on the contact conditions of tension and no-tension interfaces under combined loads including vertical compressive and tensile loads. In this article, an analytical investigation into the effect of interfaces on the ultimate vertical, lateral, and moment capacities of suction caissons in nonhomogeneous clay is made using a traditional bearing capacity theory with tension and no-tension interfaces. For failure envelopes of suction caissons in clay subjected to lateral and moment loads under vertical compressive and tensile loads, the results predicted by the present method are compared with those obtained from the finite element analysis. In order to compare failure envelopes obtained from a tension interface and those obtained from a no-tension interface under general loading, failure envelopes in the H-V, M-V, and H-M planes and in H-M-V space for suction caissons in clay are investigated.

Stability Factors F c , F s , and F γ for Twin Tunnels in Three Dimensions

This paper aims to study the three-dimensional (3D) tunnel heading pressure of twin circular tunnels in a drained cohesive–frictional soil using a stability factor approach and finite-element limit analysis (FELA). The primary concept adopted will be the use of a conventional equation that is similar to the traditional bearing capacity factors (e.g., Nc, Ns, and Nγ) in Terzaghi’s bearing capacity equation. For various spacing ratios (S/D) between the tunnels, the stability of the tunnels is expressed in terms of nondimensional tunnel stability factors (e.g., Fc, Fs, and Fγ), which are functions of the depth ratio (C/D) and soil internal friction angle (ϕ). For conciseness, the results will be presented in charts and tables with the tunnel stability factors that used the rigorous FELA upper (UB) and lower bound (LB) solutions.

Uncertainty quantification in the bearing capacity estimation for shallow foundations in sandy soils

ABSTRACT In geotechnical engineering, is well known that the theoretical Bearing capacity (BC) differs from the real behaviour of the foundation, especially when theoretical BC was obtained from in-situ test equations or through shear strength properties obtained by laboratory or correlations. The scope of this work is to quantify the uncertainty of the different BC methods of a shallow foundation supported by an anthropic sandy soil tested in the Texas A&M University. Uncertainty was classified according to a degree associated with human errors, laboratory tests, traditional BC models and BC obtained by in-situ test equations. The results show that the choice of the most appropriate friction angle (ϕ') correlation is ambiguous and can generate important uncertainties with possible BC overestimations. As the exploration and/or calculation method becomes more sophisticated (e.g. Finite elements (FE) and Random Finite elements (RFEM)), the bias will be lower. However, as evidenced, the FE and RFEM bias is linked to the quality of the laboratory results and the correlation length estimation in RFEM. Finally, it is concluded that the BC uncertainty is directly related to the model selection and the derivation of the input parameters and not the method degree of uncertainty.

Comparative study of theoretical methods for estimating pile capacity using 1-g model pile tests in cohesionless soil

The behavior of single pile is considered as the base of the most pile foundation design procedures. There are many different theoretical methods used to estimate the capacity of single pile embedded in cohesionless soil. In order to select the most accurate and reasonable method, 1-g model tests have been performed in this study. Different five procedures have been used to interpret the test results in order to select the suitable procedure. The statistical analysis showed that all procedures give approximate results except Chin’s procedure which yields inaccurate results with respect to the average value. Then, the results of five theoretical methods have been compared with the measured pile capacity from model tests (average value). The rank index analysis was conducted based on statistical parameters and probability distributions. The results of the analysis yielded that Coyle and Casttelo method gave the most accurate and reasonable results than the other theoretical methods considered in this study (Berezantsev’s method, Meyerhof’s method and traditional bearing capacity equation with some modifications). Finally, the author recommended using the two tangents procedure in interpreting of the pile test results. Also, the method proposed by Coyle and Casttelo can be used to estimate the pile capacity based on soil properties with accurate results.

A Numerical and Analytical Study on the Bearing Capacity of Two Neighboring Shallow Strip Foundations on Sand

The bearing capacity of shallow foundations is affected by both the shear strength and the geometry of the problem. In this regard, the neighboring of two or more shallow foundations can affect their bearing capacity, which has been studied in this research. The finite element and the limiting equilibrium methods are applied to investigate the problem in detail. In traditional bearing capacity problem, the effect of a uniformly distributed surcharge at the level of the footing base or, at the ground level, which is governed by the second bearing capacity term has been well studied. The main emphasis of this research is on the effect of a finite area in the proximity of the footing, which is loaded uniformly. The main goal of this research is to estimate the bearing capacity of neighboring strip footings on sand and to find distances maximizing or minimizing the ultimate bearing pressure. A series of correction factors have been presented which represent the effect of neighboring of two footings and they can be used to find the reduced and/or increased bearing capacity.

Comparison Analysis of Bearing Capacity Approaches for the Strip Footing on Layered Soils

The ultimate bearing capacity of shallow foundations is mostly based on the simplified idea that bearing layer is infinite and homogenous. However, in practice multilayered soils are mainly being used. Determination of reliable ultimate bearing capacity of foundation in multilayer soil stratum is required in the safety assessment and design analysis of a foundation structure. In present study, large number experimental, limit equilibrium, finite element analyses are performed. It has been found that a number of factors may influence bearing capacity of the soil, such as soil layer thickness, soil properties, applied stress, and type of design analysis which is not taken into consideration in the traditional bearing capacity theories. Such simplification causes structural design that is more conservative. In this study, ultimate bearing capacity of layered soil is investigated according to different conventional and recently developed techniques including finite element model, limit equilibrium methods, and experimental assessment. It is also found that the critical h / B point (first layer thickness to the foundation width) from analytical, numerical, and experimental methods are equal to 2.0, 2.5, and 1.5, respectively.

Estimating Bearing Capacity of Strip Footings over Two-Layered Sandy Soils Using the Characteristic Lines Method

A study on the bearing capacity of strip footings over sandy-layered soils has been conducted using the stress characteristic lines method. Traditional bearing capacity theories for specifying the ultimate bearing capacity of shallow foundations are based on the idea that the bearing layer is homogenous and infinite. In practice, layered soils are mainly being used. The stress characteristic lines method is a powerful numerical tool that can solve stability problems in geotechnical engineering. In the present paper, an appropriate algorithm is derived for estimating the static bearing capacity of strip footing located on two-layered soils using the stress characteristic lines method. Numerical and experimental examples are presented, to validate the proposed algorithm. Graphs and equations illustrate the effective depth of strip footings located on two-layered soils. If the friction angles of the top and bottom layers are 30° and 35°, respectively, the depth effect of the top layer is estimated to be 0.76 of the foundation’s width.

Undrained capacity of surface foundations with zero-tension interface under planar V-H-M loading

Undrained capacity of strip and circular surface foundations with a zero-tension interface on a deposit with varying degrees of strength heterogeneity is investigated by finite element analyses. The method for simulating the zero-tension interface numerically is validated. Failure envelopes for strip and circular surface foundations under undrained planar V-H-M loading are presented and compared with predictions from traditional bearing capacity theory. Similar capacity is predicted with both methods in V-H and V-M loading space while the traditional bearing capacity approach under-estimates the V-H-M capacity derived from the numerical analyses due to superposition of solutions for load inclination and eccentricity not adequately capturing the true soil response. An approximating expression is proposed to describe the shape of normalised V-H-M failure envelopes for strip and circular foundations with a zero-tension interface. The unifying expression enables implementation in an automated calculation tool resulting in essentially instantaneous generation of combined loading failure envelopes and optimisation of a foundation design as a function of foundation size or material factor. In contrast, the traditional bearing capacity theory approach or direct numerical analyses for a given scenario requires ad-hoc analyses covering a range of input variables in order to obtain the ‘best’ design.

Numerical modeling of an advancing hydraulically-driven pile in sand

The penetration of a model pile through sand was investigated via a numerical analysis. Data from nine triaxial compression tests on dense specimens at different stress levels was generalized and used to create an empirical non-linear plastic hardening stress-strain relation for use in the analysis. As the computer program used is capable of large displacement analyses in radial symmetry, we expected that the analysis would easily reproduce the tip resistance penetration profile of the model pile in sand of known density and stress. However, initial attempts led to over-prediction. Successful analyses required both successive reformations of the mesh and the complete elimination of the dilatant peak in soil strength, which is naturally eliminated under large confining stress directly beneath the advancing tip, and that soil in the far-field had strained insufficiently to reach peak strength. Thus, the soil around the shaft must have been sheared to a critical state as it flowed past the tip. The hypothesis that the resistance to displacement piles in sand is mainly a function of the deformability of the sand was again proven, and the use of peak strength in the traditional bearing capacity formulae was found to be inappropriate. Independent investigation in this direction is needed to quantify the hypothesis.

New Mechanism-Based Design Approach for Spudcan Foundations on Single Layer Clay

Spudcan foundations for offshore mobile drilling rigs are large saucer-shaped foundations that can penetrate several tens of meters into soft sediments. The penetration depth is typically predicted by considering a wished-in-place foundation at different depths and following traditional bearing capacity approaches to assess the depth at which the estimated capacity matches the applied loading. However, the geometry of the spudcan and its progressive mode of penetration lead to soil failure mechanisms that differ markedly from those relevant to onshore practice. This paper presents a new rational design approach for assessing spudcan penetration in single layer clays based on a study combining centrifuge model testing and large deformation finite-element (FE) analysis. The design approach takes account of the evolving failure mechanisms in the soil, which start with cavity formation and surface heave at shallow penetration, gradually transforming to backflow of soil over the spudcan. A detailed FE parametric study has explored the relevant range of normalized strength, strength nonhomogeneity, and spudcan base roughness, with results validated against centrifuge model test data. The penetration response curves are presented in terms of profiles of bearing capacity factors, forming nondimensional design charts along with simplified expressions for convenient use in practice. Comparisons with approaches suggested in the SNAME design code suggest an urgent need to update current practice.

Two- and three-dimensional bearing capacity of footings in sand

Bearing capacity calculations are an important part of the design of foundations. Many of the terms in the bearing capacity equation, as it is used today in practice, are empirical. Shape factors could not be derived in the past because three-dimensional bearing capacity computations could not be performed with any degree of accuracy. Likewise, depth factors could not be determined because rigorous analyses of foundations embedded in the ground were not possible. In this paper, the bearing capacity of strip, square, circular and rectangular foundations in sand are determined for frictional soils following an associated flow rule using finite-element limit analysis. The results of the analyses are used to propose values of the shape and depth factors for calculation of the bearing capacity of foundations in sands using the traditional bearing capacity equation. The traditional bearing capacity equation is based on the assumption that effects of shape and depth can be considered separately for soil self-weight and surcharge (embedment) terms. This assumption is not realistic, so a different form of the bearing capacity equation is also proposed that does not rely on it.

Bearing Capacity of Square and Circular Footings on a Finite Layer of Granular Soil Underlain by a Rigid Base

Traditional bearing capacity theories for the ultimate capacity of shallow foundations assume that the thickness of the bearing stratum is infinite. The presence of a hard layer within a certain depth below the foundation can significantly influence the unit load supported by the soil. Therefore the original bearing capacity equations should be modified to account for this condition in determining the ultimate bearing capacity. In order to evaluate this phenomenon further, model square and circular footing tests were performed on a bed of well-graded sand. Test beds were prepared at three different relative densities corresponding to loose, medium, and dense conditions: Dr =24 , 57, and 87%, using five different sand layer thicknesses, H ; H∕B values of 0.5, 1, 2, 3, and 4, where B is the footing width. Results of the model scale footing tests show that the bearing capacity factor, Nγ , should be modified up to H∕B=3 , instead of H∕B=1 , as previously suggested. The footing shape factor, sγ , should accou...

Bearing Capacity of Expanded‐Base Piles with Compacted Concrete Shafts

A data base of 70 loading tests on expanded-base piles with compacted concrete shafts has been collected to develop empirical design correlations based on standard penetration \IN\N-values. Both shaft and point resistances, \If\ds\N and \Iq\dp,\N were found to increase with depth and with increasing \IN\N-value. It appears that limiting values of \If\ds\N and \Iq\dp\N might be approached for initially loose sands at \ID\N/\IB\N (and \ID\db\N/\IB\db\N) ratios greater than 12; however, at greater relative densities, both \If\ds\N and \Iq\dp\N continue to increase with depth, but at a decreasing rate, indicating an important scale effect. A modification to traditional bearing capacity theory is used to show that the so-called scale effect arises because the bearing capacity factor \IN\dq,\N which is a function of the friction angle of the soil, decreases with depth or increasing effective overburden pressure; consequently, point resistance increases with depth at a decreasing rate. About 40% of the data base was used as an independent means of evaluating the accuracy of the empirical correlations developed.