Shallow Footings Research Articles

The use of the node-based smoothed finite element method to estimate static and seismic bearing capacities of shallow strip footings

The node-based smoothed finite element method (NS-FEM) is shortly presented for calculations of the static and seismic bearing capacities of shallow strip footings. A series of computations has been performed to assess variations in seismic bearing capacity factors with both horizontal and vertical seismic accelerations. Numerical results obtained agree very well with those using the slip-line method, revealing that the magnitude of the seismic bearing capacity is highly dependent upon the combinations of various directions of both components of the seismic acceleration. An upward vertical seismic acceleration reduces the seismic bearing capacity compared to the downward vertical seismic acceleration in calculations. In addition, particular emphasis is placed on a separate estimation of the effects of soil and superstructure inertia on each seismic bearing capacity component. While the effect of inertia forces arising in the soil on the seismic bearing capacity is non-trivial, and the superstructure inertia is the major contributor to reductions in the seismic bearing capacity. Both tables and charts are given for practical application to the seismic design of the foundations.

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.

A comparative study of ultimate bearing capacity of shallow rectangular footing subjected to eccentrically inclined loads

A comparative study of ultimate bearing capacity of shallow rectangular footing subjected to eccentrically inclined loads

A comparative study of ultimate bearing capacity of shallow rectangular footing subjected to eccentrically-inclined loads

A comparative study of ultimate bearing capacity of shallow rectangular footing subjected to eccentrically-inclined loads

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.

Evaluating the [formula omitted] coefficient for rough strip footing located adjacent to the slope using the stress characteristics method

The method of characteristics was used to estimate the coefficients Nγ for shallow strip footings located adjacent to the slopes. The soil-footing interface roughness (δ) was considered from perfectly smooth to fully rough state by 1/3 increments in each step. The effect of roughness factor (δ/φ) on the magnitude of Nγ has been evaluated for different friction angles (φ) and slope inclinations (β°). The arrays of the slip lines have been extracted and the geometric properties of plastic and non-plastic regions have been presented in tabular form. The results of the present research for both smooth and rough footings rested on the horizontal ground and adjacent to the slope showed a favorable consistency with findings reported in the literature by adopting various solution approaches. It was found that the Nγ coefficient increases and diminishes dramatically by increasing φ and β°, respectively. More than almost 95% of the Nγ magnitude in full roughness state is provided by mobilizing about 60% of the footing's roughness. The sizes of the plastic region as well as the elastic wedge beneath the rough footing extend with increasing both φ and δ/φ. An increase in β° leads to increasing and decreasing the plastic domains on the downslope zone and the horizontal ground part behind the crest, respectively. The dimensions of the non-plastic curved wedge slowly reduce and the intersection point of two plastic regions gradually tends toward the left edge of the footing. For β=20° and beyond, the maximum depth of the right plastic region from the right edge shifts toward the sloping ground face.

Use of Shear Wave Velocity for Foundation Design

This paper describes an approach for utilizing in-situ measurements of shear wave velocity Vs to carry out preliminary and check design calculations for shallow and deep foundations. For estimates of foundation movements, Vs can be used directly to estimate the small-strain stiffness of the soil or rock strata, while for ultimate capacity calculations, use is made of empirical correlations between Vs and penetration resistance measures, which in turn are correlated to the foundation resistance characteristics. The approach is applied to a series of published tests on shallow footings, and on a series of pile load tests for a very tall building. For these cases, comparisons of the calculated with the measured load—settlement behaviour indicates that the suggested approach provides a reasonable, albeit somewhat conservative, level of agreement.

Numerical Investigation of the Bearing Capacity of Strip and Rectangular Shallow Footings on Cohesive Frictional Soils Under Eccentric Loads

Numerical Investigation of the Bearing Capacity of Strip and Rectangular Shallow Footings on Cohesive Frictional Soils Under Eccentric Loads

Influence of a Thin Horizontal Weak Layer on the Mechanical Behaviour of Shallow Foundations Resting on Sand

The presence of minor details of the ground, including soil or rock masses, occurs more frequently than what is normally believed. Thin weak layers, shear bands, and slickensided surfaces can substantially affect the behaviour of foundations, as well as that of other geostructures. In fact, they can affect the failure mechanisms, the ultimate bearing capacity of footings, and the safety factor of the geotechnical system. In this research, numerically conducted through Finite Element Code Plaxis 2D, the influence of a horizontal thin weak layer on the mechanical behaviour of shallow footings was evaluated. The obtained results prove that the weak layer strongly influences both the failure mechanism and the ultimate bearing capacity if its depth is lower than two to four times the footing width. In fact, under these circumstances, the failure mechanisms are always mixtilinear in shape because the shear strains largely develop on the weak layer. However, the reduction in the ultimate bearing capacity is a function of the difference between the shear strength of the foundation soil and the layer. The presence of a thin weak layer decreases the ultimate bearing capacity up to 90%. In conclusion, this research suggests that particular attention must be paid during detailed ground investigations to find thin weak layers. Based on the obtained results, it is convenient to increase the soil volume investigation to a depth equal to four times the width of the foundation.

Groundwater effect on bearing capacity of shallow strip footings

A rigorous and practical solution aimed at accounting for the effect of groundwater in the assessment of bearing capacity of shallow strip footings in plane strain conditions has been developed using the method of characteristics.A steady water table in the foundation soil is considered and a bearing capacity factor Nγw to be used in the classical trinomial bearing capacity equation is introduced. The proposed bearing capacity factor depends on the angle of shear strength, on the footing roughness and on the water table depth and is also influenced by the soil unit weight.Normalizing Nγw with respect to the bearing capacity factor Nγ computed for the case of dry foundation soil, a corrective coefficient has been introduced for which best-fit empirical relationships have been proposed for both smooth and rough foundations. The size of the plastic mechanism under the footing has also been investigated and equations allowing to evaluate its relevant dimensions have been provided.The proposed solutions have been checked against results obtained via finite element analyses, finding a perfect agreement in terms of both corrective coefficients and shape and size of the soil plastic volume. Finally, the actual bearing capacity problem was solved showing that the trinomial formula, even in presence of a steady water table, still yields conservative values of the bearing capacity.

A comprehensive investigation on bearing capacity of shallow foundations on unsaturated fly ash slopes adopting finite element limit analysis

Present study investigates the upper bound bearing capacity of a shallow strip foundation on unsaturated fly ash slope overlying clayey deposit through finite element limit analyses incorporating suction stress based engineering parameters. Study has been conducted to incorporate uncertainties in the critical parameters including water retention characteristics curve (WRCC) parameters on the overall bearing capacity under a probabilistic framework through factorial design methodology. Relative significance of uncertain parameters has been quantified and a prediction model has been developed through multiple regression analysis. Probability of failure of a shallow footing, resting near a fly ash slope, has further been evaluated using Monte-Carlo (MC) simulation technique.

Experimental and numerical investigation of the effect of vertical loading on the lateral behaviour of monopiles in sand

The influence of combined loading on the response of monopiles used to support offshore wind turbines (OWTs) is investigated in this paper. In current practice, resistance of monopiles to vertical and lateral loading is considered separately. As OWT size has increased, the slenderness ratio (pile length, L, normalised by diameter, D) has decreased and foundations are tending towards intermediate footings with geometries between those of piles and shallow foundations. Whilst load interaction effects are not significant for slender piles, they are critical for shallow footings. Previous research on pile load interaction has resulted in conflicting findings, potentially arising from variations in boundary conditions and pile slenderness. In this study, monotonic lateral load tests were conducted in a geotechnical centrifuge on vertically loaded monopiles in dense sand. Results indicate that for piles with L/D = 5, increasing vertical loading improved pile initial stiffness and lateral capacity. A similar trend was observed for piles with L/D = 3, when vertical loading was below ≈45% of the pile’s ultimate vertical capacity. For higher vertical loads considered, results tended towards the behaviour observed for shallow footings. Numerical analyses conducted show that changes in mean effective stress are potentially responsible for the observed behaviour.

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.

Influence of using Micropiles as Retrofitting Method for Bridge Shallow Footing Numerical Study

Seismic retrofitting of the existing bridges, which was not properly constructed for seismic loading, is necessary. This paper examines and evaluates the effect of using micropiles as retrofitting method to improve seismic performance for bridge shallow footing. Numerical simulations using Plaxis-3D have been performed to study the effect of using micropiles to mitigate the seismic hazard and enhance the deformation characteristic of bridge shallow footing. Displacement and acceleration induced by seismic loading before and after retrofitting were assessed and compared. The results showed a significant influence of the micropiles on the control of the displacement and acceleration during the seismic period.

Experimental and numerical study for the optimization of bottom of foundation shapes on soft soils

Shallow foundations resting on a deep layer of soft saturated clay are considered a geotechnical challenge. The design engineer is faced with challenges in selecting safe and economic solutions for the foundations of these light weight structures by turning to the shell-shaped foundations to deal with such foundation problems and avoid the use of the costly deep foundations or other soil improvement schemes. This paper introduces different bottom of foundations shapes resting on extended soft clay layer in an effort to minimize the soil stresses and reduce the settlements. Elliptical, trapezoidal and inverted folded footings have been used as an alternative to the conventional flat bottom shallow footings. The purpose of the current study is to reach the optimal shape of the foundations, which achieves reduced stresses, reduced settlements and consequently cost. The geotechnical behavior of different bottom of foundation shapes resting on saturated soft clay is investigated experimentally and numerically. The experimental investigation started with studying a model for the conventional square flat footing shape resting over saturated soft clay. Elliptical, trapezoidal and inverted folded bottom of foundation models are tested and compared with the reference conventional square flat ones. In addition to the experimental study, a numerical analysis is performed using the finite element software ADINA. Verification of the numerical model is conducted first against the experimental results for all the studied cases. The verified numerical model is then extended to cover full-scale footing sizes of conventional and other studied footing cases and shapes. The optimum bottom of foundation shape is the one that gives the lowest soil stresses and consequently the lowest settlements.

Numerical Analysis of Shallow Foundations Resting on Granular Columns Embedded in Liquefiable Soil

Shallow footings are the most preferred foundations for buildings due to low cost and ease of construction. However, under seismic loads, these foundations may suffer excessive settlements, particularly when there is a risk of soil liquefaction. This paper explores the effectiveness of granular columns in mitigating the liquefaction-induced ground deformations under shallow foundations, using FLAC2D program. PM4Sand, a critical state-based bounding surface plasticity model is used to simulate the stress–strain response of sand to cyclic loading. The responses of granular columns are also simulated using the same soil model. Validation of the numerical model is presented against the experimental results of mildly sloping ground. The application of granular column groups resulted in maximum reduction of the settlement of footing by 60% when soil densification is included along with drainage and stress redistribution effects. Though excess pore water pressure is relatively low in treated deposit compared to the untreated deposit, its contribution to the reduction in settlement of footing is found to be minimal.

Long-term Field Heave Studies on Various Foundation Techniques in Expansive Soils

Expansive soils being susceptible for moisture variations undergo cyclic seasonal movements that pose serious stability concerns for structures resting on them, especially when they are lightly loaded ones. The concerted efforts made by various researchers across the world enabled them to develop remedial solutions to mitigate these problems with varied degree of success. The present work is an effort to study the relative performance of the different remedial techniques in vogue and those proposed in the recent years. These techniques were adopted below the shallow footings at a site on NIT Warangal campus where natural expansive clay bed is present. This study revealed that the sand cushion and CNS cushion could reduce the heave by 56 to 64%; but in case of sand cushion, an increasing trend of heave with number of cycles of wetting and drying was observed. The chemical alteration by lime could reduce the heave by 59% and by CaCl2 and CaCl2–Na2SiO3 the heave could be reduced by 70 and 73%, respectively. The recently promulgated concrete and granular anchor piled footings have shown the heave reduction up to 63 and 78%, respectively. Further, it is found that the combined effect of CNS cushion along with chemical modification and/or tension/anchor piled footing systems were found to be synergistically more effective than individual techniques in controlling the seasonal movements of footings and flooring/lining panels. Interestingly, none of these remedial techniques could overcome the shrinkage movements though these movements are observed to be insignificant in case of piled footings.

Settlement of Ring Footing Resting on Geocell Reinforced Sandy Soil under Cyclic Load

Ring footings are widely used as a foundation for water tanks, television antennas, silos, chimneys, oil storage tanks, etc. This paper is conducted to study the ring footing model's experimental cyclic behavior and circular footing resting on sandy soil reinforced with geocell. A group of ninety-six test models has been tested to investigate shallow footings' behavior beneath a cyclic loading of various loading rates. Four shapes of footing sand with three relative densities, two embedment depths of footing, two loading rates, and two widths of geocell were used. It was founded that as the footing depth increases, the settlement of soil due to cyclic loading decreases. Generally, when other variables are maintained to be the same, the footing bearing capacity increases when the foundation depth increases. The footing rebounds to some degree during the decay period of the load. The presence of geocell at the footing depth equals 100 mm will provide more improvement in all footing models more than using it at the surface, especially in ring 2 where the radius ratio is 0.4.

Rotational behaviour of bridge piers on shallow foundations

The behaviour of bridge piers supported on shallow foundations under static and cyclic rotations has been studied through several laboratory tests. A brief review on the observations made by several researchers on the responses of bridge piers on shallow foundation is presented in this paper. During an earthquake, the bridge elements are subjected to horizontal, vertical and rotational movements. In bridge analyses, simulation of bridge elements is done by simple models containing several springs and dashpots, wherein the springs are simulated by corresponding spring constants. Recently, the evolution of performance-based approaches in the design of bridge structures has lead to the incorporation of force–displacement (P-Δ) relationships in bridge design guidelines. The (P-Δ) relationships represent the response of bridge elements under translation movement. The rotational behaviour of bridge elements can be represented by Moment – rotation (M-θ) relationships. The M–θ relationships of bridge piers under rotation have been developed and studied by several researchers. In this paper, a detailed review of several shake table and centrifuge studies conducted to understand and analyse the behaviour of bridge piers supported on shallow foundations has been done in order to identify the various factors controlling the behaviour of shallow footings under static and cyclic rotations.

THE PENETRATION OF GROUND IMPROVEMENT INTO THE REALMS OF DEEP FOUNDATIONS

Ground improvement as a means for allowing the replacement of piled foundations with shallow footings is systematically being used by engineers for many decades. Some ground improvement techniques that are installed by piling rigs and include cementituous columnar rigid inclusions makes it difficult to distinguish where ground improvement ends and deep piled foundations commence. This paper assists the geotechnical engineer by reviewing the concepts of rigid inclusions, how they differ with piles, and presenting of case studies of very deep applications of rigid inclusions.