Adjacent Footings Research Articles

Numerical Study of the Ultimate Bearing Capacity of Two Adjacent Rough Strip Footings on Granular Soil: Effects of Rotational and Horizontal Constraints of Footings

In this paper, the numerical study of the ultimate bearing capacity (UBC) of two closely spaced strip footings on granular soil is investigated using the finite element method (FEM) and upper bound limit analysis (UBLA). Although the UBC of two adjacent footings has previously been studied in other experimental and numerical research, in all the previously reported studies, the footings were not allowed to rotate and move horizontally freely. Due to the deformation of the soil medium, two closely spaced footings are subjected to horizontal movements and tilting, even under central vertical loads. When the two adjacent footings are not permitted to rotate and move in the horizontal directions, the unwanted bending moment and horizontal force act on the footings. Indeed, the UBC of two closely spaced rough footings is evaluated under incorrect constraints in earlier research. In the present research, the UBC of two adjacent rough footings is evaluated with and without these incorrect constraints. The key finding of this study is that constraining the horizontal and rotational movement of the foundation artificially increases the UBC, which does not reflect field conditions. When foundations are permitted to rotate and move horizontally, there is no increase in UBC; however, there is an increased risk of differential settlement and structural instability.

Influence of consolidation on undrained capacity of two interfering footings on heterogeneous clays

The effect of consolidation on the undrained capacity of two interfering footings has not been investigated previously in a systematic manner, although this is particularly interest for offshore foundations, where consolidation will occur as a result of working load or preload. This paper presents results of a numerical investigation into the consolidated undrained bearing capacity of two interfering footings resting on heterogeneous clays. Finite difference code FLAC with the modified Cam Clay soil model is applied to couple the effect of consolidation and strength gain of clay beneath the footings. The results are expressed in dimensionless form of the gain in capacity, accounting for the coupling effects of the spacing between two footings, the magnitude and duration of applied preloads, and the clay strength heterogeneity. A design approach based on the data-fitting equations of the obtained solutions is proposed, and the failure mechanisms of the two adjacent footings after consolidation are also discussed.

Performance of Pile–Wall System Adjacent to Footings

The performance of a retaining wall is dependent on multiple factors including lateral earth pressure, which results from backfill soils and adjacent footings located behind a retaining wall. The prediction of a retaining wall’s performance in a footing–soil–wall system (FSPS) must incorporate the influences caused by the movement of a retaining wall. This study examines the performance of a retaining wall formed by driven, precast, concrete piles located adjacent to a concrete footing using two- and three-dimensional finite element analysis (2D and 3D FEA) by ANSYS 13.0 software. Both soil and concrete are assumed to behave as non-linear, elastic-perfectly plastic and rate-independent materials in compliance with the upper-bound model of Drucker–Prager yield criterion. Three backfill and foundation soils are considered: kaolin, silty clay, and kaolin–sand. Various conditions of soil type, footing shape ratio, pile width, and footing–pile distance through 180 FEA runs are investigated. The effects of 2D and 3D FEA on the behavior of the pile–wall system are compared. The lateral deflection and pressure distribution profiles along the pile–wall are studied and presented. Two empirical equations predicting lateral deflections at the pile toe and pile head and useful for pile structural design are developed under the ultimate pressure of the adjacent footing.

Numerical modelling of traffic-induced dynamic loading on a two-story residential building

BackgroundThis paper presents a numerical modelling procedure of dynamic loading due to traffic on a two-story residential building. In developing countries cities and towns, poor quality narrow road sections with high traffic density are common. In such cases, ground vibration due to traffic could be higher and lightweight buildings located closer are exposed to traffic-induced dynamic loading. Design codes require a proper assessment of such vibrations. However, a clear and definite procedure of assessment is not usually provided. This research presents an assessment procedure of dynamic loading due to traffic on a soil foundation system of light weight buildings based on numerical modeling. Traffic induced ground vibration acceleration amplitudes, frequencies and durations were measured, and the dynamic loads were calculated from measured vibration accelerations and vibrating mass of the vehicle. A two-story residential building with flexible square shallow footings was modelled together with the foundation soil using PLAXIS-2D. The dynamic load was modelled as harmonic loading considering the highest amplitude of vibration measured.ResultsOn the calculation stage, static loading analysis, dynamic loading analysis and free vibration dynamic analysis were carried out. A maximum increase in extreme total displacement of the soil to 22.03 mm was observed after the dynamic loading from 18.96 mm extreme total displacement due to static loading. Extreme effective mean stress in the soil increased to 112.81 kPa from 110.5 kPa, due to the dynamic loading. In addition, a differential settlement of 3.14 mm between two adjacent footings was observed after the traffic induced ground vibration. Furthermore, the Mohr–Coulomb plastic points were observed to be concentrated to the side of the soil-foundation system which the dynamic load was acting on.ConclusionThe regular exposure to traffic-induced vibrations may cause frequent change in stress and deformation response of the foundation-soil system. In areas where lightweight buildings are exposed to regular traffic induced vibrations, proper assessment of the effect should be carried out and measures should be taken to mitigate the problem. Improving road surfaces and limiting vehicle speed are possible remedial measures to reduce ground vibrations due to traffic.

Effect of Depth on the Stress-Settlement Behavior of Sandy Soil for Adjacent Footings

Effect of Depth on the Stress-Settlement Behavior of Sandy Soil for Adjacent Footings

Precising the Bearing Capacity of Rock Masses Subjected to the Loads of Adjacent Footings Considering Two Failure Mechanisms

Precising the Bearing Capacity of Rock Masses Subjected to the Loads of Adjacent Footings Considering Two Failure Mechanisms

Behavioural Assessment on Influence of Adjacently Placed Strip Footings at Different Embedment Level

Abstract The footings laid in close proximity imposes a definite change in the behaviour of the adjacent footing, subsequently changing the behaviour of the nearby footings. The present study emphasises the behaviour of the nearby strip footings embedded at a different level by adopting the commercially available finite element analysis program, ABAQUS. The load-settlement behaviour, ultimate bearing capacity (UBC), and the failure patterns of adjacent strip footings are assessed by considering the Mohr-Coulomb failure criterion. The UBC is of the nearby footings (left and right) are estimated and represented in terms of interference factors (ξL/ξR ) defined as the UBC of a footing in the presence of adjacent footing to that of same considered for equivalent isolated footing. The results reveal that a significant influence of the adjacent footing is experienced when the spacing between the footings (S/B) is lesser, and they behave as the single footing of greater width at S/B = 0.25 irrespective of the level of embedment depth. Furthermore, the influence of interference increases with the increase in the embedment depth of adjacent footing. It is found that the ξL is significantly more for a lower level of embedment depth, and the same increases with an increase in the embedment depth of the right footing but on the contrary ξR decreases. The increase in the peak interference factor, ξL-max for DL/B = 0.5 is 2.1% and 4.2% when DR/B = 0.75 and DR/B = 1.0, respectively.

Stability Assessment of Rock Mass System under Multiple Adjacent Structures

Numerical modeling is important for exploring the fundamental processes occurring in rock and for evaluating the real performance of structures built on and in rock mass system, and thus for supporting the design of rock engineering problems. Estimating the stability of rock mass foundation systems entirely based on a theoretical approach is a complicated task if there exists overlapping of their potential collapse modes. This paper applies finite element limit analysis to evaluate the bearing capacity of equally spaced multiple strip footings resting on rock mass obeying the modified non-linear Hoek–Brown failure criterion. Numerical solutions are expressed in terms of the efficiency factor that is dependent on the spacing between footings, as well as the rock mass properties. In addition, the effects of surface surcharge and footing roughness are quantified. The maximum spacing at which the interfering effect of adjacent footings becomes disappeared is evaluated and an algebraic expression for approximating the maximum spacing is proposed. Failure mechanisms for a few cases of rock mass under multiple strip footings are examined.

Bearing Capacity of Interfered Adjacent Strip Footings on Granular Bed Overlying Soft Clay: An Analytical Approach

In the present paper, the interference effects on bearing capacity of two and three closely spaced strip footings resting on granular bed overlying clay are being studied. A simple analytical model is proposed to predict the load-carrying capacity and the interference factor of an interfered footing, when adjacent strip footings are optimally placed on the surface of a Granular Bed (GB) overlying clay and both the footings are simultaneously loaded. A punching shear failure mechanism is envisaged in the analytical model. The load-carrying capacity of the footing is taken as the sum of total shearing resistances along the two vertical planes through the edges of the strip footing in the upper granular layer and the load-carrying capacity of the soft clay beneath the GB. Insights gained from finite element simulations are used to develop the new modified punching shear model for interfering footing. Bearing capacity can be easily calculated by using the proposed punching shear model for interfering footing. The analytical model is validated with numerical analyses and previous experimental results and found to be in reasonably good agreement. The influence of different parameters such as granular bed thickness, width of footing, number of footings are carried out in this study. Doi: 10.28991/cej-2021-03091723 Full Text: PDF

Subgrade reaction modulus of rock masses under the load of single and multiple footings

ABSTRACT Subgrade reaction modulus is an essential parameter in the structural design of footings. Determination of this parameter is difficult, especially in rock masses containing an irregular distribution of discontinuities. In this paper, the subgrade reaction modulus of rock masses obeying Hoek-Brown failure criterion was investigated. It was assumed that the rock masses are under the pressure of a single isolated footing and also two adjacent strip footings. The subgrade reaction modulus was determined from the pressure-settlement curve of the rock masses. This study’s results were presented in two forms, which include the subgrade reduction factor and the interference coefficient. The obtained results show that the subgrade reaction modulus of rock masses may be smaller than 50% of intact rocks. Likewise, based on the distance between the multiple footings, the subgrade reaction modulus may be in the range of 0.7 to 1.3 times the subgrade reaction modulus in the case of a single isolated footing.

Dynamic response of stand-alone and adjacent footing on saturated sand

This paper presents results from geotechnical centrifuge tests performed at the Japanese Institute of Occupational Safety and Health (JNIOSH) in Tokyo, Japan. A range of shallow footing configurations with different bearing pressures founded on saturated sand in a laminar box on a shake table was studied using harmonic excitations. The free-field condition was also studied as a reference. The study aimed to address the influence of stand-alone and closely adjacent shallow foundations on the dynamic response of the soil-foundation-structure system. A partially drained response of the soil and a minor dilative response for large values of shear strain were observed for the free-field case. In the case with footings, the settlement was found to be largely due to the footing penetrating the supporting sand. A dilative response of the soil beneath the footings was observed for the stand-alone configurations at the beginning of the base motions, this was not observed in the case of adjacent footings. Adjacent footings also tilt inwards consistently while stand-alone footings did not present a clear tilt pattern. In terms of the acceleration recorded in the near-surface soil beneath the footings, a reduction was observed after a few cycles in the free-field case. This reduction was considerably smaller in the case with footings with no significant differences between the response of the stand-alone and that of the closely adjacent footings.

Upper bound solution for the bearing capacity of two adjacent footings on rock masses

In this paper, the effect of the adjacency of two strip footings on bearing capacity of rock masses was investigated using the upper bound analysis and finite element method. The modified Hoek-Brown nonlinear failure criterion was used for the rock mass. The effect of various parameters such as the distance between the two footings, Hoek-Brown parameters, surcharge pressure and unit weight of the rock mass on the ultimate bearing capacity of rock masses were also investigated. The results show that if the distance between the two footings is significant, the failure lines occur beneath each footing will not interfere with the failure lines beneath the other adjacent footing and each footing behaves as a single one. The maximum distance for which the two footings affect each other can be obtained from the charts presented in this paper. By reducing the distance between the two footings, the bearing capacity will increase firstly and then decrease. Finally, when the two footings touch each other, the ultimate bearing capacity of the two footings will be equal to the ultimate bearing capacity of a single isolated footing with a width twice the width of each footing.

Study on Behaviour of Two Adjacent Strip Footings on Unreinforced/Reinforced Granular Bed Overlying Clay with Voids

This paper numerically studies the behaviour of two adjacent strip footings on unreinforced and reinforced granular bed (RGB) overlying clay having voids. The influence of different parameters such as granular bed thickness, length of reinforcement/s, number of layers of reinforcement, presence of void/s beneath the footing/s in the weak soil, on the behaviour of footings are carried out in this study. It is found that there is a certain optimum spacing between the two adjacent strip footings at which the footing/s carry the maximum load. The voids could be formed in weak soil due to various reasons and the presence of voids may affect the performance of footings. Such voids tend to reduce the load carrying capacity of the footing/s. However, providing a RGB over weak soil can be used as an effective method to maintain the good performance of footings, even when voids could be formed in future.

Study on Behaviour of Two Adjacent Strip Footings on Granular Bed Overlying Clay with a Void

This paper numerically examines the behaviour of two adjacent strip footings on granular bed over weak soil having a void. Voids could be formed in weak soil strata due to various reasons such as due to water leakage from the water supplying lines or sewer lines, poor drainage and erosion of soil, animal burrows, etc. A parametric study is carried out to understand the influence of granular bed thickness, width of footings, spacing between the footings, and the presence of a void beneath the footing in the weak soil, on the behaviour of footings. To understand the failure mechanism, shear strain contours, for different cases, are being studied. It is observed that there is a certain critical spacing between the two adjacent footings at which the footing/s carry the maximum load. This critical spacing depends on the type of loading, whether equal and simultaneous loading or unequal and sequential loading. The presence of voids also affects the performance of footings. Such voids tend to reduce the load carrying capacity of the footing/s and alter failure pattern.

Experimental modeling of strip footing adjacent to non-yielding retaining wall

A series of reduced-scale models were tested to assess the performance of non-yielding basement wall due to strip footing constructed adjacent to it. Construction design parameters such as strip footing width (B), its distance from non-yielding wall back (a), and footing embedment depth below the backfill surface (df) have been considered in this study. This paper presents results of five model tests that isolate effects of non-yielding wall on the strip ultimate footing bearing capacity. The opposite effect of the strip footing distance design parameters on the non-yielding wall responses is also investigated. Experimental results revealed that the strip footing load-carrying capacity increased, and the settlement decreased as its distance from the back of non-yielding wall increased. Furthermore, the vertically loaded strip footing imposed significant vertical and horizontal forces at the back of the non-yielding wall. Results also indicate that distance a > 4B between the footing and the back of the wall enabled the strip footing to mobilize ultimate load-carrying capacity comparable to both control strip footing test and value calculated with Terzaghi’s equation. Comparison of the measured and predicted lateral forces showed that using elastic theory method together with Jacky formula for at-rest lateral earth pressure coefficient led to about 23–35% underestimation of the measured lateral forces. However, including sand overconsolidation ratio, OCR = 2 resulted in an overestimation of the lateral forces by about 20–29% of the measured value. Finally, results of the experimental program are used to identify sources of conservativeness and non-conservativeness in the current design methodologies used to calculate lateral forces imposed on non-yielding walls due to adjacent strip footing and suggested necessary modifications.

Interference Studies of Adjacent Strip Footings on Unreinforced and Reinforced Sands

Interference effects of two and three strip footings placed adjacent to each other on unreinforced and reinforced sands are being studied in this paper. Effects on stresses in foundation soil, bearing capacity, settlements and tilts of footings are being looked into. Parameters varied in this study are (i) the number of footings (In the case of two footings loaded simultaneously, both experimental and numerical studies are conducted. In the case of two footings loaded sequentially and three footings, numerical studies are done.), (ii) loading conditions, (iii) clear spacing between the footings and (iv) number of reinforcement layers in foundation soil. With two footings, two loading conditions are considered. In the first loading condition, both footings are loaded simultaneously up to failure. In the second loading condition, one of the footings representing an already existing foundation is loaded with half of the estimated failure load of single strip footing and adjacent footing loaded up to failure. With two adjacent strip footings on unreinforced and reinforced sands, both loaded simultaneously, some laboratory-scale experimental studies are also conducted to compare with the numerical results. It is observed that there is a certain critical spacing at which the footing/s carry the maximum load. This critical spacing depends on the loading condition. Reinforcements in the foundation soil used are in the form of hexagonal wire meshes. The effect of providing the reinforcements in layers in the foundation soil is seen in the increased bearing capacity, reduced settlements and reduced tilts of the footings under simultaneous loading condition. Tilts are also found to be influenced by the loading condition.

Simple discrete models for dynamic structure-soil-structure interaction analysis

Simple models are presented for dynamic analyses of Structure-Soil-Structure Interaction (SSSI). Structures built on rigid, circular surface foundations resting on a homogeneous, elastic soil half-space are considered. The proposed models consist of discrete mass and spring elements that represent inertial, stiffness, and damping characteristics of an SSSI system. Frequency-dependent springs are developed to simulate Foundation-Soil Interaction (FSI) and Cross-Interaction (CI) between adjacent foundations, both being affected by the number and arrangement of neighbouring foundations. The stated springs are developed using a novel approach that approximates the through-the-soil interaction between adjacent footings using existing solutions to soil-structure interaction (SSI) analyses. Static stiffness matrices are obtained by means of compliance matrices using well known analytical solutions of surface displacement field to a ‘single-disk’ problem and a suggested displacement averaging rule. Dynamic stiffness matrices are created by applying modification factors to their static counterparts. To this end, a practical expression for dynamic modification factors that relate static and dynamic amplitudes of surface displacement field is provided. Equations of motion are formulated and solved in both time and frequency domains, taking into account time-lagging effects on input motion at separate foundations due to their vibration and wave passage. Validation results against more advanced numerical models prove the effectiveness of the proposed models. Finally, the limitations and potential improvements on the models are discussed.

Influence of shallow footings on the dynamic response of saturated sand with low confining pressure

This research addresses the dynamic response of closely adjacent shallow footings on saturated sand with low confining pressure. This was achieved by performing physical experiments. A large laminar box, filled with sand, on a shake table, was used to simulate the soil-foundation interaction. The subsurface acceleration and the excess pore-pressure were measured along with the acceleration of the footings. A stand-alone footing, three adjacent footings aligned with the axis of shaking and a cluster of six footings (two rows of three footings) were studied. Harmonic waves and seismic ground motions were used. Results show a modification of the response of the footings and sand dependent on the number and location of footings. Variation in excess pore-pressure was observed partly dependent on the length of the drainage path. For the stand-alone condition, the footing acceleration was larger than that of the shake table. However, in the case of these experiments, the acceleration of the footings reduced with an increasing number of footings.

Finite element analysis of two nearby interfering asymmetric footings embedded in cohesionless foundation medium

ABSTRACT The present study encompasses the behaviour of two closely spaced rigid, rough asymmetrical strip footings embedded in homogenous, semi-infinite cohesionless foundation soil medium. The geometry of the interfering footings with respect to their width is considered to be asymmetric. The analyses have been carried out using finite element software ABAQUS following the Mohr-Coulomb failure criterion obeying non-associated flow rule for foundation medium. The interference effect on the characteristic behaviour of the footing such as load-settlement (p-δ) behaviour, ultimate bearing capacity (UBC) and settlement have been evaluated. The UBC and settlement have been represented in terms of non-dimensional interaction factors defined as the ratio of UBC/settlement of left/right footing in presence of adjacent footing to that of the same considered for an isolated footing under identical condition. The results of the analyses reveal that the interaction factors with respect to UBC (ξL/ξR ) and settlement (ζL/ζR ) are affected significantly by the variation of clear spacing between the footings, embedment depth, soil friction angle and the width of adjacent footings. The interference effect is more prominent in the case of smaller footing having lesser embedment depth on the soil with higher friction angle in comparison with the footing of larger dimension.

Effect of geocell-reinforced sand base on bearing capacity of twin circular footings

The presence of a rigid layer underlying a soil bed layer supporting twin footings affects the footing bearing capacity and settlement. In this study, the bearing capacity and settlement of twin large-scale circular footings on unreinforced and geocell-reinforced sand layers with various thicknesses are investigated. The role of layer thickness, distance between two adjacent footings, and footing diameter indicates that the rigid base substantially influences the footing bearing pressure, settlement and tilt. The results show that the maximum bearing capacity is achieved when two footings are in contact and the rigid base is at shallow depth. The influence of the footing interference on settlement and tilting of closely spaced footings at a given load decreases with decreasing depth of rigid base and reinforcing the sand with geocells. This is observed for rigid bases located at less than about 2B, where B is the footing diameter.