Strip Footings Research Articles

Probabilistic stability analysis of geo-structures in anisotropic clayey soils with spatial variability

Theoretical solutions of geotechnical engineering problems are generally based on simplifying assumptions about soil homogeneity and isotropy. In reality, soil masses are usually both anisotropic and heterogeneous in situ and thus, the effects of soil deposition orientation and stratification should be taken into account. In this context, the present paper describes a contribution towards a general probabilistic limit analysis of geotechnical stability problems dealing with clays having inherent spatial variability and anisotropy of the undrained shear strength simultaneously. The static lower-bound theorem in conjunction with the finite element method and mathematical programming are utilized to provide limit analysis approach. Anisotropy of clay is imposed by the application of an iterative scheme and, its spatial variability has been considered via the application of random field theory. Based on the proposed method, the bearing capacity of a strip footing and the stability of a square tunnel have been investigated and the effects of simultaneous consideration of non-homogeneity and anisotropy of soil’s shear strength on collapse load have been studied.

Analysis of Load Carrying Capacity of Strip Footing Resting on Geogrid Reinforced Fly-Ash Over an Earthen Slope Using Plaxis2D

The current study deals with the use of material waste such as fly ash as a fill material or replaced layer placed over an earthen slope and strip footing is placed over the slope in order to explore the benefits of using geogrid sheets to strengthen the fly ash layer and to investigate its effects on the load carrying capacity. Using Program PLAXIS-2D v8.2, the fly-ash sheet is positioned over an earthen slope with slope inclination of 3H:2V, and multiple type of models were investigated, to study the integration impact of geogrid-reinforcement layers on load carrying ability. For optimal improvement in load carrying ability, models have been tested to evaluate the best location, length and configuration of geogrid layers and top fly-ash layer depth. In order to evaluate the effect of one parameter in each sequence the other factors were kept constant. The sequence of findings was illustrated using PLAXIS-2D on the strip base in both strengthened and un-reinforced cases. Test findings demonstrate that inclusion of geogrid significantly enhances efficiency of slope and footing.

Assessment of overall response of interacting skirted strip footings located on layered soil

Assessment of overall response of interacting skirted strip footings located on layered soil

Statistical and Numerical Study on the Performance of Skirted Foundations in Clayey Soil

Skirted foundation is considered as an alternative approach required for improving the bearing capacity of shallow foundations, by using structural skirts fixed to the edges of the foundation. The presence of vertical skirts at the base of footing generates a soil resistance on skirt side beneath the footing, which helps to resist sliding. For this purpose, a numerical analysis has been performed in order to explore the behavior of square and strip model footings with one and two sided skirts for different embedded ratio in soft clay. In this study, a vertical skirt depth (Ds) has been considered as the main parameter that significantly affects the bearing capacity. Finite element analysis using plaxis 3D Foundation has been performed to model the skirted foundation. This work indicated that, using structural skirts in one and two sides of footing has a paramount effect in improving the bearing capacity for both strip and square footings and a marginal precedence has been noticed for the strip once. Therefore, the results have demonstrated that the average improvement in bearing capacity in soft clay for the one and two sided skirts at skirted ratio of Ds/B=2 is 170 and 220% respectively. A derived equation has been proposed to calculate bearing capacity of skirted foundation and a feasible agreement has been obtained between the observed and there predicted test results where the coefficient of regression (R2) is found to be 90%.

Centrifuge testing of soil–structure interaction effects on cyclic failure potential of fine-grained soil

We describe two experiments performed on a 9-m-radius geotechnical centrifuge to evaluate dynamic soil–structure interaction effects on the cyclic failure potential of fine-grained soil. Each experiment incorporated three different structures with a range of mass and stiffness properties. Structures were founded on strip footings embedded in a thin layer of sand overlying lightly overconsolidated low-plasticity fine-grained soil. Shaking was applied to the base of the model container, consisting of scaled versions of recorded earthquake ground motions, sweep motions, and step waves. Data recorded during testing were processed and published on the platform DesignSafe. We describe the model configuration, sensor information, shaking events, and data processing procedures and present selected processed data to illustrate key model responses and to provide a benchmark for data use.

Controlling Soil Collapsibility Due to Water Intrusion by Rigid Foundation System with Reinforced Cushion

Since collapsible soils are been mostly transported by wind and deposited in arid or semi-arid regions, they founded in a state of unsaturated condition. In addition, engineering filling when placed in a certain none want density, undesired settlement will be predictable either due to wetting or due to loading on these soil deposits. Collapsibility study is important for the foundation design and construction on these soils. The most foundation systems used on these soils are isolated and strip footing connected with concrete tie beams. Therefore studying rigid foundation system resting on partially saturated collapsible soil/deposits is very important. The present work investigated using rigid strip footing resting in collapsible soils to study the effect of stress interference due to progressive wetting depth from leakage of surface water on collapsibility settlement. The study has been investigated the influence of different behavior of strip footing and inverted T-section strip footing rigidity system resting on unsaturated soil by numerical analysis using the finite element program PLAXIS 2D. The partially saturated collapsible soil is stimulated using the Mohr-Coulomb soil model. The significance parameters are considered two types of footing systems, collapsible soil thickness, use of sand cushion with geo-grid reinforcement at the bottom third of its thickness, and different clear spacing between source of surface water and strip footings on the stress-settlement relationship. The results of this study confirmed that the most important soil parameters in this problem are the use of reinforced sand cushion, decrease applied stress as well as rigid inverted T-section strip footing are more suitable for controlling Soil collapsibility, while the settlement is found to decrease. To avoid many observation of spread footing disaster that founded and rest on collapse soil. In addition, the results can be guide for design engineers, how to choose foundation type and the effect of spacing water resource.

A new approach to optimizing the geogrid layout to maximize the bearing capacity of strip footing

This study presents a new approach to optimizing the layout of the geogrid layers to achieve the maximum bearing capacity of the strip footing under different loading conditions (Vertical (V), Horizontal (H) and eccentric (M) loads) using a numerical method. To find the best location of the geogrid layers in the current method, the optimum depth of each layer is obtained separately, which was not considered in previous studies. The effects of parameters such as different loading combinations, numbers and layout of geogrid layers on the ultimate bearing capacity of the strip footing have been studied. The results of the analyses are plotted in the form of dimensionless graphs. For different loading combinations, the optimum layout and number of reinforcing layers have been determined. The results show that the presence of the reinforced layers, at the optimum layout, significantly increases the ultimate bearing capacity of the strip footing, especially in the V and VM loading conditions. The optimum number of geogrid layers was different for different loading conditions. Based on the analyses, 7, 4 and 4 geogrid layers were obtained as the optimum number of reinforcement layers for the V, VH and VM loading conditions, respectively. Also, it was found that the position of each layer depends on the number of layers. In this study, the position of the first layer from the foundation (u/B) was varied by increasing the number of reinforcement layers and the loading conditions. In the VM loading condition, the geogrid reinforcement effect on the bearing capacity is more prominent with respect to the VH loading conditions. The increase of the bearing capacity in the VM loading condition at the optimum layout of reinforcement (N = 4) is about 100 %, compared to the bearing capacity of the unreinforced soil.

Study on the ultimate bearing capacity of a strip footing influenced by an irregular underlying cavity in karst areas

This study investigates the ultimate bearing capacity and typical failure mechanisms of a rough strip footing constructed above an irregular cavity using the upper bound finite element limit analysis (UBFELA) method. A novel method based on the inverse discrete Fourier transform (IDFT) theory is employed to generate and quantitatively describe the irregular shape of natural cavities. Parametric studies are performed to study the influence of several variables, including the ratio of the cavity size to the strip footing width (D/B), the ratio of the cover depth to the footing width (H/B), the internal friction angle (φ), the direction angle of the long axis of the cavity (θ) and the shape descriptors of the cavity (D2, D3, and D8). Numerical results are presented in the form of dimensionless charts for the bearing capacity numbers (Pu/c). The obtained failure modes present significant asymmetry and a critical region is found to be greatly affected by the size of the cavity (D/B), the cavity depth (H/B) and the internal friction angle (φ).

Reliability analysis of strip footing under rainfall using KL-FORM

Spatial variability is an inherent uncertainty of soil properties. Current reliability analyses generally incorporate random field theory and Monte Carlo simulation (MCS) when dealing with spatial variability, in which the computational efficiency is a significant challenge. This paper proposes a KL-FORM algorithm to improve the computational efficiency. In the proposed KL-FORM, Karhunen-Loeve (KL) expansion is used for discretizing random fields, and first-order reliability method (FORM) is employed for reliability analysis. The KL expansion and FORM can be used in conjunction, through adopting independent standard normal variables in the discretization of KL expansion as the basic variables in the FORM. To illustrate the effectiveness of this KL-FORM, it is applied to a case study of a strip footing in spatially variable unsaturated soil under rainfall, in which the bearing capacity of the footing is computed by numerical simulation. This case study shows that the KL-FORM is accurate and efficient. The parametric analyses suggest that ignoring the spatial variability of the soil may lead to an underestimation of the reliability index of the footing.

Effect of using cemented sand as a replacement layer beneath a strip footing

ABSTRACT Soil replacement is an effective technique used in construction practices. Hence, several studies have considered the mechanisms to remove and replace mitigation in the past. This research represents an analysis of the efficiency of cemented stabilized soil replacement for improving the behaviour of weak soil. On the other hand, the optimum design for foundation depends on the soil properties beneath these foundations. One of the best solutions for this challenge is to increase the shear parameters and reduce the compressibility for the foundation soil using cemented sand mixture technique. An experimental program was conducted to study the impact of cement stabilization on the geotechnical characteristics of sandy soils, furthermore, five cement ratios (3%, 5%, 7%, 10% and 15% by dry weight of sand) have been mixed with sandy soil and tested under compaction and shearing. The results illustrated that the maximum dry unit weight of sand increases with the increase in cement content whereas the optimum moisture content decreases marginally with the increase in the cement content. In addition, increasing cement content leads to an increase of cohesion, friction angle and Young’s modulus. This paper is considered a serious attempt to estimate the optimum cement ratio required to improve the mechanical properties of soil replacement under a strip footing. Numerical modelling for this footing was done by Plaxis software. The results illustrated that adding the cement mixture improves the behaviour of footing. The effect of the replacement extension on the strip footing is also analyzed, where; the results indicated that the optimum replacement extension is equal to the thickness of replacement. This study is based on the limitation of the herein proposed numerical model and its properties.

Probabilistic Settlement Analysis of Strip Footing on Spatially Random Soil

The prediction of settlement in a traditional design approach usually uses a deterministic value of modulus of elasticity (E), which is estimated as an average value by testing the soil at selected locations. However, these deterministic properties of soil may not represent the actual properties of soil and site condition. Due to numerous sources of uncertainty, the properties of soil mass are spatially varying and anisotropic in the natural field condition. In this study, a random finite element method (RFEM) is used to evaluate the reliability of settlement of the strip footing on spatially random soil. Modulus of elasticity is the only considered random parameter. For this purpose, 2000 spatially random realizations of E-field are generated using Monte Carlo Simulation. Each of these realizations of heterogeneous soil profile is passed to FEM to analyze the settlement of footing. The final settlement results measured in all these realizations are then statistically evaluated and compared. The results of analysis show that the mean and standard deviation of the footing settlement are increased with increasing spatial correlation length. The large value of isotropic correlation length led to an increase in the mean settlement value by more than 25% as compared with the deterministic settlement calculations. Also, it is concluded that the rate of increase of settlements for anisotropic correlation length is lower than the one under isotropic condition.

Cracked Equivalent Beam Models for Assessing Tunneling-Induced Damage in Masonry Buildings

This paper proposes simple numerical models that simulate the influence of tunneling-induced ground movements on masonry buildings founded on shallow strip footings. The focus is on developing an improved understanding of the impact of structural discontinuities due to pre-existing cracks and joints between adjacent buildings. The soil–structure system is idealized with equivalent Timoshenko beams connected to a homogenous elastic half-space with rigid links. The flexibility of the equivalent beam at the location of the crack or building interface is determined using linear-elastic fracture mechanics. The proposed models are first applied to a generic numerical example. Then, they are used to interpret the detailed displacement and strain measurements gathered from a masonry church during nearby tunneling. The results show that pre-existing cracks can have variable influence on the soil–structure interaction depending on their extent and position relative to the building and settlement trough. For the investigated case study, the proposed models illustrate how pre-existing large cracks in the hogging zone localized deformations and how the interaction between adjacent buildings drastically altered response mechanisms.

Seismic Bearing Capacity for Strip Footings on Undrained Clay with Voids

ABSTRACT This paper investigates the seismic bearing capacity of strip footings on undrained clay with single continuous square voids. Numerical studies for a wide range of geometric parameters and material properties are performed using adaptive finite element limit analysis (AFELA). Based on the pseudo-static approach, rigorous upper and lower bounds of the seismic bearing capacity for the strip footing are obtained, with relative errors of 4% or better. The calculated results are presented in the form of design tables and charts, and the effects of different governing parameters on the seismic bearing capacity are evaluated, including the size and location of the voids, the dimensionless strength ratio and the horizontal seismic coefficient. In addition, typical failure mechanisms are examined, and comparisons with previous studies are also conducted.

Numerical parametric study of geosynthetic reinforced soil integrated bridge system (GRS-IBS)

A 2-D finite flement model was developed in this study to conduct a FE parametric study on the effects of some variables in the performance of geosynthetic reinforced soil integrated bridge system (GRS-IBS). The variables investigated in this study include the effect of internal friction angle of backfill material, width of reinforced soil foundation (RSF), secondary reinforcement within bearing bed, setback distance, bearing width and length of reinforcement. Other important parameters such as reinforcement stiffness and spacing were previously investgated by the authors. The performance of GRS-IBS were investgated in terms of lateral facing displacement, strain distribution along reinforcement, and location of potential failure zone. The results showed that the internal friction angle of backfill material has a significant impact on the performance of GRS-IBS. The secondary reinforcement, setback distance, and bearing width have low impact on the performance of GRS-IBS. However, it was found that the width of RSF and length of reinforcement have negligible effect on the performance of GRS-IBS. Finally, the potential failure envelope of the GRS-IBS abutment was found to be a combination of punching shear failure envelope (top) that starts under the inner edge of strip footing and extends vertically downward to intersect with Rankine active failure envelope (bottom).

Numerical study on bearing capacity of ring foundations for storage tanks on a rock mass

The bearing capacity of ring footings on a rock mass is investigated by employing finite element limit analysis (FELA). The rock mass obeys the modified Hoek-Brown failure criterion and an associated flow rule. Based on the FELA method, the bearing capacity factor Nσ is computed and presented in the form of design tables and charts that account for the variation of the geological strength index (GSI), the intact rock constant (mi), and the ratio of internal radius to external radius of the footing (Ri/R0). Typical failure modes associated with these parameters are compared and discussed. In general, the magnitude of Nσ increases with increasing GSI and mi, while the unit weight has negligible effect on the bearing capacity factor. As the value of Ri/R0 increases, the bearing capacity factor first increases, then decreases, and reaches the maximum value at Ri/R0 = 0.25, indicating that Ri/R0 = 0.25 may be the optimum ratio of the ring footing. In addition, computations of strip footings on rock masses are conducted for verification.

Upper bound shakedown analysis of foundations on cohesive-frictional soil based on a nonlinear programming method

In this paper, using the upper bound shakedown theorem by means of the displacement finite element method and nonlinear programming, a numerical method is proposed to obtain the shakedown limit of strip footing under repeated loading. Shakedown analysis is a powerful method that provides the possibility to determine the shakedown limit of a structure. The shakedown limit of a structure under repeated loading is a load limit below which the structure is in the safe zone and its behaviour is elastic. For cohesive-frictional soil, the Mohr-Coulomb yield criterion is considered without any linearization. The role of the unit weight of the soil in the shakedown limit of a footing under repeated load is studied. Also, the effect of repeated load on the reduction of the bearing capacity of footing is investigated in several examples.

Undrained Bearing Capacity of Two Strip Footings on a Spatially Variable Soil with Linearly Increasing Mean Strength

Abstract A probabilistic study on the bearing capacity of two parallel strip footings on a spatially variable soil where the mean of undrained shear strength increased linearly with depth was perfo...

A comprehensive study of waste coconut shell aggregate as raw material in concrete

The eminence exploitation of raw materials from the environment causes the natural sources to degrade and limits the future practices in the construction sector. The economy in this sector holds around 40% of the total flow of source materials into the GDP of the nation. The production of such materials are either consume a good amount of energy, responsible for causes of CO 2 , NO x , SO x, into the atmosphere or leads to the restriction on the usages of available natural sources in the future. Hence, the implementation of new practices for the incorporation of raw materials to save the extraction of available natural sources in the construction can be a beneficial foot step. The use of an agricultural by-product i.e. coconut shell can be a promising material for manufacturing the concrete in the partial replacement of coarse aggregates. Result indicates that coconut shell can be used as lightweight concrete which can be used in non-load bearing structures, strip footings and non-structural elements. Environmental concerns can also be minimizing by making such sustainable efficient practices by the use of these waste coconut shell materials.

Upper bound analysis of rectangular surface footings on clay with linearly increasing strength

The ultimate bearing capacity problem of rectangular surface footing over undrained clay with linearly increased strength is comprehensively analyzed with a numerical upper-bound method. For uniform soil, the present results are generally in accordance with available results and present the best upper-bound prediction when the three-dimensional effect is predominant. The obtained bearing capacity factor for nonhomogeneous soil decreases with the increase of width-length ratio, totally different from that of uniform soil. Accordingly, the shape factor also varies greatly with strength nonhomogeneity. Possible reasons are discussed based on the underlying velocity field. Finally, a strip footing based design equation considering such effect is proposed for the vertical bearing capacity of rectangular surface footings.

Numerical and Physical Modeling of Soft Soil Slope Stabilized with Stone Columns

There can be many reasons for engineers to place the footings near a slope such as leakage of suitable sites or architectural considerations. One of the approaches to increase the amount of bearing capacity, especially in soft soils, is adding stone columns to the soil. In this research, the behavior of a strip footing placed near a stone column reinforced clayey slope was investigated. For this purpose, some small-scale model tests were performed on a clayey slope reinforced with stone columns. The effects of the length of the stone column and the length of encasement on the footing were studied. Additionally, vertical encased stone columns in a group arrangement were investigated. Some numerical analyses were also performed using the Midas GTS NX finite element software, and the factor of safety was studied. Results show that the optimum length was equal to four times the diameter of stone columns. It was observed that by increasing the length of encasement, the bearing capacity of strip footing was also increased. The safety factor of slope showed an increase when stone columns were added to the slope, but the maximum influence on the factor of safety appeared when the stone column was in the upper middle of the slope.