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. How to cite this paper: Ali, N.A. (2021) Controlling Soil Collapsibility Due to Water Intrusion by Rigid Foundation System with Reinforced Cushion. Open Journal of Civil Engineering, 11, 451-462. https://doi.org/10.4236/ojce.2021.114027 Received: December 3, 2021 Accepted: December 28, 2021 Published: December 31, 2021 Copyright © 2021 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
Many of the engineering problems confronted due to collapse soils by water wetting, soaking. Related to increase in moisture content, these soils go through radical rearrangement of their particles, causing rapid collapse by higher reduction in its volume sudden changes, causing differential settlement of foundation. The foundation or infrastructure has been disaster or failure when directly founded on these soils due increase in their moisture content. Study collapsibility positional is important for the design and construction on these soils. The most foundation systems used on engineering compacted deposits and collapsible soils are spread footing connected with concrete tie beams. Deformation behavior of unsaturated soil under field conditions depends mainly on soil initial conditions, wetting and history of loading. The soil can practice a complex volume vary reaction related to the applied external load intensity. Thus, compacted soils wetted under load can collapse due to their conditions and the magnitude of vertical stress. These soils are often known as collapsible or met-stable, and the procedure of their collapsing is often called collapse due to inundation or hydro-collapse. These soils also, compress when wetted under no variation in applied stresses. Volume decreases due to excess of water under the same stresses in partly saturated natural soil deposits have been termed collapse, [1] [2] [3] [4] [5]. With recent advances in computer and computer technology, many researchers have applied numerical modeling to analyze the influence of collapsible soil settlement on foundations system, using the numerical approaches, particularly finite element (FE) analysis, an elasto-plastic constitutive model for unsaturated soil has been developed in reference, [6].
Collapse development is practical in semi saturated soil layers, as compacted soil deposits as well as dry natural soils deposits. The collapse settlement tends to transform meta-stable soil to stable and non-collapsible soil after wetting and saturation. In Egypt, urban has been increased broadly extended to many desert areas where many new cities are being constructed. The collapsible soil is formatted in these areas and there is much danger would be expected if soil is exposed to foundation loads and wetting from leakage of surface water to foundation. Many cases of distress and cracks in new building in different sites founded on compacted deposits have been observed and recorded due to leakage of excess surface water from irrigated landscapes area or bad drained of surface water in winter season.
Also, many researchers have been reported that for an accurate collapse settlement prediction it is required to verify the degree of saturation and wetting extent in the site. The main step to predict collapse settlement, is evalulated the After saturated of collapsible soils under foundation, significant settlements have induced in foundation of structures, which can lead to damage of structure.
Much observation recorded for spread footing and much non-rigid foundation system that founded and rest on collapse soil, disasters. Collapsible soils optional, wetting sensitive with increase in water content, which is chief, trigger system for reduction in these soils volume [9] [10].
Many researchers investigated and reported that collapse soil under constant applied load show decrease in its volume due to wetting, irrigation activities, induced collapse strain. The settlement that related to collapse strain in these cases studied which cause damage and structural distress consequent. Many studies were done to calculates the collapse strain and explain the observed collapse which has been occurred, [11] [12] [13] [14]. Also, in Egypt there are many cases of damaged recorded and observed in new and old building constructed on isolated footing rest on compacted filling and collapse soil [15].
Thus, the present research studies many attempts to predict the main parameters, which resist and control the collapse strain induced by wetting through near water source. These parameters effect in choose rigidity of foundation system rested on compacted filling and collapse soil related to water leakage source spacing, collapsible soil thickness and when using sand cushion with or without geo-grid reinforcement under foundation system. Two rigid footing types are analysis in this study, Inverted T-section Strip Footing, (ITSF), and rigid Strip Footing, (SF), as rigid foundation system to predicted behavior of foundation system under collapse soil settlement of and its improved behaviour.

Soil Characteristics and Simulation
In this study, the in situ soil sample, particle size analyses shown in Figure 1.
The single odometer test (ASTM D5333-03), [16], conducted to study the soil collapse potential. The results of conducted Odometer collapse test at different applied stress on undisturbed samples illustrate in Figure 2. Laboratory characterization of collapse soil illustrates in Figures 1-3. The results are agree with that increasing dry density and liquid limit of collapsible soil reduced collapse potential and many laboratory tests should be conducted to identify attribute of collapse potential. In addition, in situ many practical plate load tests have conducted in field to evaluated the soil collapsibility and all soil natural and post inundation properties. As reported in many researches, [ [21]. It easy to predicted that the collapse occurs in compacted unsaturated deposits at degree of saturation less than 60%, as shown in Figure 4 and Figure 5. For this reason, the numerical analysis in this study considered the equations in Figure 6, are represented the relation between different degree     near the source of water leakage, Figure 6. Collapse soil before wetting has been divided into S1, S2, S3 and S4 with degree of saturation, S% equal 10%, 20%, 40% and (60% -100%) respectively and underlying sand which is not affecting with wetting. For each degree of saturation, S%, and each collapse potential the modulus of elasticity can be calculated, [20].

Finite Element Model Description
Case studies of two type of footing system are numerically analyzed, Inverted T-section Strip Footing, (ITSF), and rigid Strip Footing, (SF), which resting on compacted collapsible soil. Finite element program PLAXIS 2-D performed using Mohr-Coulomb model considering compacted unsaturated soil. The results presented in terms settlement under footing, and differential settlement to compare for reasonable and best foundation system in such case. Water source ate distance, D, from first footing, F.A, which has been wetted the compacted unsaturated soil under foundation to simulate the leakage of surface water. The model has been considered top layer of compacted unsaturated collapse soil with thickness 4.0 m, overlying layer of medium sand extended to depth 6.0m under top layer. The rigid concrete foundations with 2, 3 and 4.0 m width, B, are considered. Three parallel strip footings resting on the surface of a compacted unsaturated collapse soil are studied the effect of stress interference on settlement and effect of spacing between strip footings Figure 7. Also, parametric study is performed for different clear spacing between the footings and sand cushion with and without a layer of geogrid reinforced at bottom third of sand cushion thickness. Footing density considered, 24 kN/m 3 , Poisson's ratio = 0.15 and Young's modulus = 2E + 07 kN/m 2 . Compacted sand cushion with thickness 1/4 and 1/2 the collapse soil thickness, were been studied considering constrained its yield modulus not less than 80 MPa. Compacted collapse soil has redefined due to the inundation by the reduced soil stiffness with the soil degree of saturation, S% from field plate test results.

Numerical Case Study
Plain strain analysis using finite element method (FEM) based software PLAXIS

Results and Discussion
Some results of this study record and illustrate in the flowing figures. Figure 8 and Figure 9 shown the total settlement below the loaded strip footing increase        inundation. In addition, it indicates that the collapse soil saturating and preloading together is best method in improves such soil collapsibility and limit the settlement induced due to inundation.

Conclusions
The developed Finite Element, (FE) model estimate the rigid foundation behavior resting on compacted collapsible soil deposits under water leakage.
The numerical approaches study applied using the field and Laboratory results to developed overall behavior of foundations rest on compacted collapse soil.
Also, take into account more factors than can possible in simpler methods can be investigated.
1) The (FEM) model developed can be help in predicting the overall behavior of footing system on compacted collapsible soil deposits and similar soils as, compacted partially saturated soil considering redefined the reduced soil stiff- 2) Result indicate that the collapse soil saturating and preloading together is the best method in improving soil collapsibility and limiting the settlement induced due to inundation.
3) Using sand cushion is the major parameter affecting on reduction of the total settlement, at thickness = B/4 reduces total settlement by 15%, and increasing sand cushion thickness to B/2 reduces total settlement by 35%.

4) Using inverted T-section strip footing, (ITSF) with different contact stress
gives the best results on reducing the total settlement than strip footing, (SF).
6) Using sand cushion reinforced with geogrid with great thickness up to B reduces the effect of the other parameters.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.