خاک دانه ای

Piled raft foundations are an effective and economical system in case of high-rise buildings and are a suitable alternative to conventional deep foundations. In this type of foundation, raft is in direct contact with the soil and hence it carries a significant portion of load. Regarding to the design of piled raft foundation, one of the uncertain issues is to determine the proportion of load sharing of the raft and piles. In this paper, Finite element analyses were carried out by developing three-dimensional models in ABAQUSE and with considering interaction effects for a piled raft foundation, the effective factors and their impact on load percentage carried by raft are investigated. In addition, a new equation for determining the load percentage carried by cap in granular soil is proposed for a group of piles for an alignment of less than 9×9 piles. The developed model is validated with available experimental results. The results of numerical analysis show that the distance of the piles from each other, the diameter of the piles, the internal friction angle of the soil and the number of piles in the pile group are among the most important and effective parameters in determining the load percentage carried by cap. In the current study, from the numerical analysis, it has been concluded that, the load percentage carried by cap, is 15% to 42%.

Geocell mattresses are one type of three-dimensional honeycomb soil reinforcements that is manufactured from polyethylene sheets using ultrasonically welded joints. This type of geosynthetics is commonly used to stabilize geotechnical problems in which the pullout failure is likely to occur. The present study has been conducted to evaluate the pullout behavior of a geocell by considering the effect of soil grain size. A series of 18 monotonic and 24 multistage geocell pullout tests were performed in sandy and gravely soil. The obtained geocell pullout loads were divided into two components: passive and frictional resistance. A previously made theoretical approach was used to measure the mobilized frictional and passive resistance components to evaluate the contribution of each mechanism. The results obtained from monotonic geocell pullout tests showed that the geocells exhibited hardening pullout behavior and the pullout failure occurred when the geocell material did not have any more pullout capacity to resist external load. Increases in the geocell height and soil grain size had significant effects on the passive component and it was seen that the contribution of geocell height developed passive resistance higher than the soil grain size. Furthermore, the multistage test results indicated that for removal of the geocell, the ultimate post-cyclic pullout load was less than the monotonic pullout load. This was the result of a reciprocating motion from loading caused by the interlock between the geocell infill soil and the surrounding material, which weakened and broke during the cyclic phase. As the coarseness of the soil increased, the interfacial strength increased. The theoretical approach did not single out the passive component between monotonic and multistage tests and the obtained passive resistance values were the same in these calculations. However, the cyclic loading could affect this component. Also, the soil particle size had a significant effect on the cumulative displacement during the cyclic phase.

Due to the non-homogeneous nature of cohesionless soils, and the complexity of the parameters related to settlement, generally the exact estimation of the foundation settlement involves many difficulties. This problem has been studied by many researchers and some equations have been proposed for estimating the settlement of shallow foundations. Because of the low accuracy of these equations, researchers are using more modern techniques like finite elements, finite difference, soft computing, and so on, for calculating the settlement. In the current study, a new model is proposed based on artificial intelligence to predict the settlement. The purpose of developing such models is to achieve more accurate and possibly simpler equations. In this paper, the performance of multi expression programming (MEP) method to predict the settlement of shallow foundations on granular soils is evaluated and the values obtained from the developed models are compared with those from the most accurate previous models. Then, the performance analysis, validation, parametric study and sensitivity analysis are performed. As can be seen from the results, the comparison between the MEP model and the most accurate models including ANN, GP, GEP and EPR in the technical literature showed that the model proposed in this study with correlation coefficient of 93.45 percent, performs better than most previously developed models.

Soil friction angle is one of the basic parameters in determining the shear strength of coarse-grained soils, which is often determined in the laboratory by spending a time and cost, and the use of field experiments has solved this problem. In this regard, Standard Penetration Test (SPT) is one of the most practical field tests that is commonly used in granular soils. In recent years many researches are proposed that present the correlation SPT and shear wave velocity, undrain shear strength and friction angle. Many correlations between SPT and soil friction angle are proposed but most equation with SPT numbers greater than 35 predict the right answers. The correlation between SPT results and soil bearing capacity has been studied and reported by various researchers.  In additional, some correlation between SPT and Soil friction angle have been proposed that each of those evaluated the material of specific area. In this paper new practical equation is proposed that presents an acceptable result in all SPT numbers. With use of this equation, the results can be predicted more accurately.

Under small strains, the shear-wave velocity (Vs) and its resultant maximum shear modulus (Gmax) are important parameters in geotechnical engineering soil dynamics analyses. In this regard, the evaluation of the influences of soil particle size on the dynamic behavior of soils during wave propagation has been an important issue in geotechnical engineering. According to the relevant literature, the influences of grain size on shear wave velocity of soil were completely different in various research studies. This research aims to experimentally examine the effects of a wider range of particle sizes, on maximum shear modulus in dry sandy soils, using a bender element apparatus embedded in a triaxial cell. The results indicated that maximum shear modulus of sand was considerably affected by changes in grain size so that in a particular range of grain size, shear modulus increased as the diameter of soil grains rose, while, in the other range, maximum shear modulus diminished with increasing grain diameter.

It has been of great interest among the researchers to investigate the behavior of soil_geogrid interface. Due to the wide use of geogrids between different layers of soil; these investigations are very important. This paper shows the results of experimental tests on soil_geogrid interface. This study conducts a series of large scale direct shear tests to investigate the interface shear strength of granular soil with various degrees of compaction and various sizes of geogrid apertures. The shear stress versus shear displacement curves and peak shear strength are important for evaluating the results. The interactions between soil and geogrid may include the following mechanisms: 1) shear resistance between soil and the surface of the geogrids; 2) internal shear resistance of the soil in the opening area; and 3) passive resistance of the transverse ribs. The value of α was defined to evaluate the effect of geogrid on the shear strength of the soil which is the ratio of geogrid_soil shear strength to internal shear strength of soil. The results showed that a larger degree of compaction reduces the resistance in soil_geogrid interface and shear strength of soil_geogrid interface will be reduced further by reducing the distances of transverse ribs of geogrid.

Soil reinforcing is one of soil improvement methods by using of reinforcing materials (with suitable performance in tension) in soil (with fairly compressive strength and weakness in tension). In recent years, numerous studies have been carried out about the application of reinforced soil and bearing capacity of granular soils. Generally, these studies are limited to reinforced embankment with geosynthetics and steel stripes and even plastic parts. So using of plastic waste to improve the bearing capacity of embankment has not been considered. Therefore, in this research the effect of plastic wastes to improve the bearing capacity of granular soils has been investigated. The variables considered are using one type of disposal plastic wastes in different weight percentages at irregular (random) reinforcement and regular reinforcement (with specified layers) in granular embankment. The research was carried out in small laboratory scale using CBR tests. According to results, placing plastic waste parts in sandy soil increases the bearing capacity remarkably. The optimum values were obtained at 2% to 2.5% plastic weight ratios to soil weight. Also the required energy conditions to attain optimum percentage were investigated. In this regard, the strain-stress behavior of soil was studied. The results show that increasing of plastic parts to 2%-2.5% at irregular and regular condition, respectively, raises the soil elasticity coefficient to 234 and 152% respectively.

Biological method is an unusual way for soil stabilization in civil engineering, particularly in the sectors of road. Microbial limestone deposits occur due to urea hydrolysis by an enzyme called urease. Through this reaction, PH of the soil increases and Crystals of limestone deposit between soil particles and reduce the permeability of soil. In this study, by using experimental results at the National Institute for Genetic Engineering and Biotechnology, a road improved sandy bed is modeled by biological methods. ABAQUS 6.11 in this study is used. The soil layer is modeled in elasto-plastic condition and Settlement is calculated by using elasto plastic model in various thicknesses. The results show a significant effect in reducing plastic deformation and increasing soil strength by using biological methods in soil improvement.

In many conditions, because of several restrictions, cantilever walls are the only way to stabilize the excavations. It is no doubt that one of the most important parameters in design of such walls is wall stiffness. Therefore, in this study, a large number of case histories are collected and the most commonly used range of wall thickness and stiffness are determined based on this database. In addition, validation of limit equilibrium method (LEM) in granular soils showed that this method can only estimate bending moment of rigid walls. Therefore, for more accurate estimating, a new equation is presented for the most commonly used range of wall stiffness and various types of granular soils. Moreover, LEM based equation is replaced with a modified version. The new equation was successfully validated using 70 numerical models and results lied in range of 85% to 115% times the predicted values obtained from FEM. According to the results, in loose and very loose soils, the common cantilever walls can only stabilize the excavations with depth less than 10 m. While if depth is more than 15 m, soil type should be dense or very dense with “E” more than about 70 MPa. The results also show that the effect of wall stiffness is negligible in bending moments less than 2000 kN.m.

The purpose of the present study is to evaluate mechanical {stress-strain and volumetric} behavior of granular materials considering their internal physical properties in particle scale. To this end, two-dimensional numerical modeling of drained compressive tri-axial te¬st, using Discrete Element Method (DEM) was utilized. DEM-2D computer code was adopted for modeling these tests based on Discrete Element Method formulation. Drained biaxial numerical tests on specimens with different void ratios and confining pressures were carried out and the results are compared with the associated laboratory tests results reported in the literature.