Bearing Capacity Theory Research Articles

Test Study on Size Effect of Flexural Performance of RC Free Beams

Experimental studies on size effect of flexural behavior of reinforced concrete beams are not carried out fully. Standard values are based on test results of small-size components, which are different from large-size components in practical engineering. For size effect on quasi-brittle materials such as concrete, related researches have been carried out for many years, but related test study combined with concrete structures or components are not sufficient. This article is based on experimental study of 13 reinforced concrete free beams, obtaining test data during different loading stage, such as carrying capacity, deflection, steel and concrete strain etc. Test results show that size effect of flexural behavior of RC free beams is mainly reflected in reinforcement yielding stage and concrete crushing stage. The negative size effect due to concrete material is not significant. Yet internal force arm and longitudinal reinforcement have a positive size effect on flexural behavior, strength and ductility reserves show a growing trend with specimen size increasing. The safety of calculation theory of bearing capacity in code is verified indirectly.

Base Stability of Grout Pile–Reinforced Excavations in Soft Clay

Grout piles are used to reinforce base soil to increase the base stability of excavations in soft clay and to reduce excavation-induced ground movements. To propose a model for the base undrained stability of deep excavation in clay reinforced with grout piles, this paper presents an anisotropic strength criterion for clay reinforced with grout piles and the upper-bound bearing capacity theory for excavation. The proposed model is capable of determining the factor of safety against base heave for an excavation with or without ground improvement for various-strength anisotropy ratios of clay and clay reinforced with grout piles. The suitability of the model is verified with five field excavation cases. Among them, four were with no ground improvement, and one was with ground improvement. Generally, the larger the ground improvement rate, the more the material strength of a grout pile can be mobilized. The factor of safety also becomes larger when the ground improvement rate is higher. However, the factor of safety becomes insensitive to the undrained shear strength of a grout pile when the improvement ratio is less than 10%. Results from the model found that the increment of factor of safety against base heave and the increment of improvement ratio is generally 1∶2. Finally, because the proposed method does not consider the influence of displacement at the retaining wall bottom and the geometry of excavation, it can be used only in excavations with the width and depth of ground improvement zone larger than 1.41Dp and 1Dp (Dp is depth of the retaining wall minus depth of excavation), respectively.

Penetration testing for the Optical Probe for Regolith Analysis (OPRA)

The Optical Probe for Regolith Analysis (OPRA) is a spike-shaped subsurface analytical probe that will be delivered to a planet, asteroid, or cometary body by a lander and/or rover. OPRA will be pushed down into the subsurface to record near infrared spectra as a function of depth down to maximum of 50 cm. Therefore, knowledge of the required penetration force to specific depths can be helpful in estimating the length of the probe. Test probes covering the anticipated diameter (2.5, 1.9, 1.2 and 0.9 cm diameter) and tip angle (T.A. = 30°, 60°, 90° and 120°) of OPRA were inserted mechanically into dry playground sand. The results showed that tip angle does not have a major effect, while probe diameter and density of the regolith are the most important parameters. Increasing probe diameter from 0.9 to 1.9 cm (i.e. a factor of 2) leads to an increase in penetration force from 200 to 1000 N (i.e. a factor of 5) at 20 cm depth. An increase in bulk density (B.D.) from 1550 to 1700 kg m −3 leads to an increase in penetration force from 10 to 200 N at 20 cm depth. Bearing capacity theory was used to explain the downward movement of the penetrometer through regolith and showed good agreement with the experimental results. This model was then used to take into account the effect of gravity on other planetary bodies. We observed a good agreement between the theoretical model and results from penetration testings on the Moon by the Apollo missions. Since the maximum allowed force for penetration is the weight of the lander/rover on their targeted planetary surface, our results put a strong constraint on the maximum reachable depth without endangering the whole mission.

LRFD of shallow foundations at the service limit state

The design of shallow foundations typically involves two-steps: (1) the calculation of ultimate bearing capacity based on general bearing capacity theories and (2) the calculation of maximum contact pressure to produce an allowable total settlement. The allowable bearing capacity for the shallow foundation system is the lesser of the ultimate bearing capacity, divided by a factor of safety, and the computed maximum allowable contact pressure. Currently, these calculations utilise nominal values of soil strength and stiffness parameters and do not explicitly account for their uncertainty. This paper develops a method for the calibration of resistance factors for load and resistance factor design (LRFD) of shallow foundations at the service limit state. The bearing soil strength and stiffness parameters are assumed to be random variables and the standard elastic settlement equations are combined with the Monte Carlo simulation technique to develop a series of probabilistic pressure–settlement curves. Based on an allowable magnitude of total settlement, the pressure–settlement curves are analysed to compute the pertinent statistics for an allowable bearing resistance that are then utilised to develop the resistance factors. The computed resistance factors are observed to be highly variable and are dependent on design uncertainty and shallow foundation size.

Bearing capacity of reinforced foundations: Statistical approach and Sensitivity analysis

This paper pertains to a critical evaluation of two widely referred bearing capacity theories of reinforced foundation beds. An enhanced data-set (41 tests) have been used to carry out a refined linear and nonlinear multivariable regression analysis. The regression relations provide a statistical observational model for the prediction of bearing capacity of reinforced foundation beds. A sensitivity analysis is carried out to determine the measure of importance of the parameters towards the outcome of the model. It is found that the relative depth, number of layers of reinforcement and angle of internal friction of soil significantly influence the sensitivity of the system, while the other parameters have moderate influence. Relative length of reinforcement is found to affect the system negligibly.

TRANSIENT IMPACTING STRENGTH ANALYSIS OF MOTORCYCLE MODEL

Considering the disadvantages of analysis method singleness and result differentia in the conventional structural strength analysis of motorcycle, a method concerning transient impacting strength analysis was proposed, which is based on the finite element theory. Then the bearing capacity theory was researched, and three typical operating modes under the acting of transient impacting were confirmed. Under the circumstance of MSC.PATRAN/NASTRAN software, by applying the composite structure of rod, plate and shell which has a high calculation accuracy, the element subdivision of frame is done. The suspension is simulated by spring element, the engine is simulated by concentrate mass element, and then the whole simulation model was developed. The strength of the motorcycle frame was analyzed under the above-mentioned three operating modes. The results of the simulation meet well with the practical performance. Thus, the conclusion shows that this method has more important practical significance.

INVESTIGATION OF SHAFT AND BASE RESISTANCE OF A SINGLE BORED PILE USING OSTERBERG CELL

Evaluation of pile capacity in soil is an engineering problem of soil-structure interaction. Soil-pile interaction plays an important role in the analysis and design of pile foundations. Geotechnical engineers have recognized this role and many studies have focused on several aspects of the topics related to determination of pile resistance. There are different methods available to predict the ultimate load of a single bored pile, such as theory of bearing capacity of the soil foundation, based on pile load test, semi-empirical methods using in-situ test results, and empirical formulae were suggested by different codes of practice. Still the most reliable method to determine the bearing capacity of a pile is by static load test. This study investigates some of the important problems related to soil-pile interaction. Specific problems examined here include: identify of skin friction and end bearing resistance of pile separately, comparison between the measured results obtained from experimental methods presented by Osterberg cell, (Ocell), or conventional tests and these obtained from equations of Egyptian code for soil mechanics and foundations to evaluate and verify the applicability of different methods. The experimental results were used to study the effect of the different parameters such as pile diameter, D, pile length to pile diameter ratio (L/D), and relative density (Dr) on the pile shaft and pile base resistances of bored piles. Also, in this research, the coefficients of lateral soil pressure (KH) and bearing capacity factor (Nq) have been studied. From this study, It can be found that both pile shaft and pile base resistances have affected by the parameters D, L/D, and Dr.

Ultimate bearing capacity prediction of shallow foundations on cohesionless soils using neurofuzzy models

This study explores the potential of neurofuzzy computing paradigm to predict the ultimate bearing capacity of shallow foundations on cohesionless soils. The neurofuzzy models combine the transparent, linguistic representation of a fuzzy system with the learning ability of Artificial Neural Networks (ANNs). The data from 97 load tests on footings (with sizes corresponding to those of real footings and smaller sized model footings) were used to calibrate and test the model. Performance of neurofuzzy model was comprehensively evaluated with that of independent fuzzy and ANN models developed using the same data. The values of the performance evaluation measures such as coefficient of correlation, root mean square error, coefficient of efficiency, mean bias error, relative error and mean absolute relative error obtained through the neurofuzzy model are found to be good, which reveals that the neurofuzzy model can be effectively used for the bearing capacity prediction. The values of performance measures obtained for ANN and fuzzy models indicate that the neurofuzzy model significantly outperforms both fuzzy and ANN models. The predicted bearing capacity values obtained through the developed neurofuzzy, ANN and fuzzy models are compared with the values predicted by most commonly used bearing capacity theories. The results indicate that all the three models (i.e., neurofuzzy, ANN, fuzzy) perform better than the theoretical methods.

Correlation of California Bearing Ratio with Shear Strength Parameters

The California bearing ratio (CBR) test is still widely used in the design and analysis of pavements. The CBR test is relatively expensive and time consuming. A method is proposed for correlating CBR values with the undrained shear strength of clay soils, or the effective internal friction angle of noncohesive soils. Unconsolidated undrained triaxial tests on clays and quick drained-triaxial tests on sands can determine these parameters. These tests are much more economical and rapid than the CBR test. The correlation is based on foundation-bearing capacity theory. The configuration of the CBR test can be closely modeled in bearing capacity theory as a circular foundation with a surcharge. Actual test results are presented to support the correlations. The proposed method allows economical development of estimated CBR values from quick shear tests. The correlation between shear strength results and CBR has been established on the basis of a limited number of soil samples. Additional test results are needed to verify the correlation. In the interim, the proposed method should be used with good judgment and engineering experience to provide a quick method of determining sub-grade soil properties for pavement thickness design.

Bearing Capacity of Piled Rafts on Soft Clay Soils

The conventional design of a piled foundation is based on a bearing capacity approach, and neglects the contribution of the raft. As a consequence, piled foundations are usually designed by overconservative criteria. With respect to the conventional approach, a more rational and economical solution could be obtained by accounting for the contribution of the raft toward the overall bearing capacity, but this potential is not exploited due to the lack of theoretical and experimental research on the behavior of piled rafts at failure. Based on both experimental evidence and three-dimensional finite element analyses, a simple criterion is proposed to evaluate the ultimate vertical load of a piled raft as a function of its component capacities, which can be simply evaluated by the conventional bearing capacity theories. The results presented in the paper thus provide a guide to assess the safety factor of a vertically loaded piled raft.

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...

A plasticity model describing caisson behaviour in clay

This paper introduces a strain-hardening plasticity model that describes the behaviour of caisson foundations when subject to combined vertical, horizontal and moment loads in clay. Development and calibration of the model is based on centrifuge experimental results, theoretical knowledge (bearing capacity and upper bound plasticity theory) and numerical finite element studies. It is applicable to integrated wave-structure-soil analysis. This is demonstrated with example dynamic analyses of a proposed three legged jack-up in over 140 m water depth. Results presented show how the caisson model can be used to study (a) optimum caisson geometries, (b) penetration and relative movement of the foundations, including permanent deformation in extreme events, and (c) comparative behaviour between different footing types.

Undrained Bearing Capacity of Square and Rectangular Footings

The uniaxial vertical bearing capacity of square and rectangular footings resting on homogeneous undrained clay is investigated with finite element analyses, using both Tresca and von Mises soil models. Results are compared with predictions from conventional bearing capacity theory and available analytical and numerical solutions. By calibrating the finite element results against known exact solutions, best estimates of bearing capacity for rough-based rectangular footings are derived, with the shape factor fitted by a simple quadratic function of the footing aspect ratio. For a square footing, the bearing capacity is approximately 5% lower than that based on Skempton’s shape factor of 1.2.

Drag Force on Single Piles in Clay Subjected to Surcharge Loading

Piles driven into clay are often subjected to indirect loading as a result of the surcharge applied on the surrounding area. During the drained period, both the piles and the soil undergo downward movements caused by the axial and the surcharge loading, respectively. Depending on the relative movement of the pile–soil system, positive and negative skin friction develop on the pile’s shaft. Negative skin friction is the drag force that may be large enough to reduce the pile capacity and/or to overstress the pile’s material causing fractures or perhaps structural failure of the pile, and/or possibly pulling out the pile from the cap. A numerical model that uses the finite element technique combined with the soil responses according to Mohr–Coulomb criteria was developed for case simulation. The computer program CRISP (developed by Cambridge University) was used in this study. The numerical model was first tested against the results predicted by the bearing capacity theories for pile foundations in clay subj...

Bearing Capacity Characteristic of Dredged and Reclaimed Ground Reinforced by Bamboo Net

A series of large scale plate loading tests are performed to assess bearing capacity characteristic of dredged and reclaimed ground that is reinforced by bamboo net and geotextile as a surface strengthen method. Bearing capacity ratio is distributed from 3.46 to 6.03 for bamboo net and from 2.19 to 3.59 for Geotextile In case of bamboo net, BCR was obtained from 1.6 to 1.7 times than Geotextile. As the comparison of each bearing capacity from test and theory shows, bearing capacity theory for geotextile was not suited for bamboo net. The bearing capacity analysis reinforced by bamboo net shows a good relationship between sand mat thickness (H) / bamboo net space (S), and bearing capacity ratio (BCR).

Ultimate bearing capacity of footings on coal ash

Coal ash is recognized as an alternative fill material to the conventional natural soils near a coal fired thermal power station where its large deposits are available. This paper presents experimental investigations on footings on coal ash subjected to loads. A series of laboratory model tests on varying sizes of footings were conducted. The conventional bearing capacity evaluation methods applied for natural soils do not consider progressive failure. These effects are explained based on the non-linear strength behavior of the granular soil and occurrence of progressive failure. The classical bearing capacity theory was applied in relation to the relative dilatancy of coal ash to describe this phenomenon. Few novel observations presented here show that the extent of progressive failure of ash fills is a compressed function of material characteristics of the ash, size and depth of footing and the settlement ratio.

Soil Failure Patterns and Load–sinkage Relationships under Interacting Shallow Footing and Wheel Arrangements

This study explores possible methods, based upon generating interactions between footing/wheel arrangements, for supporting a greater proportion of surface applied loads in the soil surface layers, rather than at depth. Such a change would reduce the risk of deep soil compaction occurring under high machinery loadings. In this work, shallow footings were used to simulate wheel arrangements. Dual and triple footing combinations were examined at different spacings and in different relative positions. This was to determine the effect of interactions within the soil failure zones beneath the footings, on the nature of soil deformation, interaction behaviour and the load–sinkage relationships. The mode of interaction was very dependent upon the spacing between the footings. Three interaction modes were identified and the load–sinkage relationships indicated that, with interaction, higher loads could be supported in the surface layers at given sinkages. A mathematical model, based upon Meyerhof's bearing capacity theory for shallow footings, was developed to predict the likely interaction modes and the additional forces that could be generated as a result of the footing interactions at different spacings. The model showed an acceptable level of agreement with the experimental data for a range of multiple footing sizes and spacings.

Numerical Simulation of Bearing Capacity Characteristics of Strip Footing on Sand

This study aims at numerically simulating the bearing capacity characteristics of strip footing on sand, and consequently explaining the scale effects observed in a series of plane strain model tests carried out on a particular type of sand (Toyoura sand). The model tests were performed using different sizes of footing, with the largest width being 50 cm, under normal gravity and in a centrifuge. A constitutive model developed for Toyoura sand based on the results from an extensive series of plane strain compression tests is used. A number of factors which affect the strength and deformation of sand are taken into account, including; 1) confining pressure; 2) anisotropy; 3) non-linear strainhardening and strain-softening; 4) dilatancy; and 5) strain localization into a shear band(s). This material model is coupled with an isotropically hardening, non-associated, elasto-plastic material description. A widely used numerical technique, FEM, is applied to solve the non-linear equations. A parametric study is performed to evaluate the effects of using different assumptions for the material model. The simulation of bearing capacity compares well with the physical model test results. It is shown that the isotropic perfectly plastic modelling of soil property, assumed in most of the classical bearing capacity theories, is an overly simplified approximation to be used in FEM analysis of this issue. It is explained that the scale effect consists of the pressure level effect and the particle size effect.

Base Stability of Deep Excavation in Anisotropic Soft Clay

Base stability of a deep excavation in clay is affected not only by soil properties such as soil strength and strength anisotropy, but also by several construction factors, such as the stiffness and depth of the excavation support wall and the depth of excavation. However, not all these influencing factors can be dealt with by the limit equilibrium methods currently used to analyze the base stability of a deep excavation in clay. The method proposed in this paper takes into account the influence of strength anisotropy and nonhomogeneity of soft clay, and the depth of the wall. The method is derived from the upper bound theory of bearing capacity and an assumed failure mechanism and is applied to study the base stability of deep excavations supported with internal braces and a concrete diaphragm wall in soft clay. To describe the strength anisotropy of soft clay, a total stress anisotropic strength criterion is adopted. The suitability of this method for analyzing the base stability of deep excavations is verified by comparison with numerical study results of deep excavations in Boston and a base failure case in Taipei. Limitations of this method, such as the minimum depth of excavation wall required and the lower bound ratio between the depth of the excavation wall and the width of excavation, are discussed also.

Base Stability of Deep Excavation in Anisotropic Soft Clay

Base stability of a deep excavation in clay is affected not only by soil properties such as soil strength and strength anisotropy, but also by several construction factors, such as the stiffness and depth of the excavation support wall and the depth of excavation. However, not all these influencing factors can be dealt with by the limit equilibrium methods currently used to analyze the base stability of a deep excavation in clay. The method proposed in this paper takes into account the influence of strength anisotropy and nonhomogeneity of soft clay, and the depth of the wall. The method is derived from the upper bound theory of bearing capacity and an assumed failure mechanism and is applied to study the base stability of deep excavations supported with internal braces and a concrete diaphragm wall in soft clay. To describe the strength anisotropy of soft clay, a total stress anisotropic strength criterion is adopted. The suitability of this method for analyzing the base stability of deep excavations is verified by comparison with numerical study results of deep excavations in Boston and a base failure case in Taipei. Limitations of this method, such as the minimum depth of excavation wall required and the lower bound ratio between the depth of the excavation wall and the width of excavation, are discussed also.