A comparison between performances of the behavioral models in evaluating load-bearing capacity of piles in fine-grained unsaturated soil

Department of Civil Engineering Isfahan (Khorasgan) Branch, Islamic Azad University Isfahan, Iran, Email: bandehzadeh.o@gmail.com Department of Water and Natural Environment, Isfahan Higher Education and Research Institute (IHEARI); Mimistry of Energy, Isfahan, IRAN, Email: msadeghi@iaht.ac.ir Civil Engineering Faculty, Isfahan University of Technology, Email: mohamali@cc.iut.ac.ir Department of Civil Engineering Isfahan (najafabad) Branch, Islamic Azad University Isfahan, Iran, Email: alborzhajian@pci.iaun.ac.ir Submitted: 22.12.2017. Accepted: 10.02.2018

Ground is found in the nature in the dried, wet, and saturated varieties.Most soil mechanics hypotheses have been formulated for dry or saturated soil and little attention has been paid to unsaturated soils.However, significant breakthroughs in modeling unsaturated soil behavior have been discovered in the past two decades.Unsaturated soils are soils that are to some extent saturated with water.Several soils display characteristic changes after absorbing or giving out water.{When it comes to such soil, any change of level of saturation {can result can cause changes of volume, shearing durability, and hydraulic properties of soil.These results cause the soil nonlinear tendencies.A soil behavioral model should be capable of showing soil behavior under changes of pore water and air pressure as well as stress variants (Alonso et al., 1990;Vaca, 2015).Any type of soil can be saturated or unsaturated, and unsaturation is soil state rather than a soil type as exhausted by Gens et et al. almost all of the traditional soil mechanics principles are suitable for saturated soils, the following points should be taken into account in formulating principles for unsaturated soils (Gens et al., 2006;Solowski & Sloan, 2015;Xu & Cao, 2015).Volume changes in relation to suction or changes of saturation degree: Soil shearing strength in relation to suction or variations of saturation degree In 1990, Alonso et al. proposed the first model of elasto-plastic behavior of unsaturated soil using the theory of plasticity, which was an oval-shaped model similar to the modified Cam-Clay Model (MCC).The structure of this oval differs depending on the suction level.This model is suitable for slightly to mildly expansive unsaturated soils such as soils containing sand, silt, clay, and clay with low-plasticity.Most structural piles are installed on top of the groundwater level, and these piles are placed on unsaturated soils.In this case, the negative pore water pressure caused by capillarity significantly influences the mechanical behavior of unsaturated soils (Rutqvist et al., 2011;Thakur et al., 2005).Structural suction equals the difference between the pore air pressure (ua) and pore water pressure (uw), and many studied have revealed that the engineering behavior of unsaturated soils (including load-bearing capacity) is mainly affected by structural suction.The design of pile foundations for engineering practice applications using the mechanics of unsaturated soils has been getting research consideration during the last 15 years (Chung & Yang, 2014;Georgiadis et al., 2003;Zhang et al., 2017).The key information required in outline of pile establishments incorporates the bearing capacity and settlement behavior (Ravichandran et al., 2013;Sheng et al., 2004).The load-settlement (i.e.p-δ) conduct of pile establishment is fundamentally affected by the stiffness and shear strength parameters of the surrounding soil and soil-pile interface (Han et al., 2016;Oh & Vanapalli, 2012).A few studies highlighted the huge commitment of soil suction towards the soil mechanical properties, for example, the shear quality (Vanapalli et al., 1996;S. K. Vanapalli, Fredlund, & Pufahl, 1996), modulus of elasticity (Lu & Kaya, 2014;Oh et al., 2009), and additionally the soil interface properties (Hamid & Miller, 2009;Yavari et al., 2016).Seldom do studies considered the impact of matric suction or capillary stresses on the load carrying capacity of pile foundations (Gui & Bolton, 1998;Vaca, 2015).Vanapalli and Taylan conducted a model test and suggested using a modified , and  method to contemplate the impact of matric suction on heap shaft limit.Their results also show that the shaft capacity of a single pile is fundamentally affected by the commitment of matric suction.The objective of this study is to investigate the pole limit of a little scale single, tube shaped jacked pile installed in a compacted unsaturated clayey soil (Ghorbani et al., 2016;Mohamed, 2014).Numerical investigations of the little scale single pile in unsaturated clayey soil were conducted by FDM.The FDM considered frictional strengths between the unsaturated soil and pile and was used to analyse load carrying capacity.To validate the FDM, analysis results were compared with results from small scale single pile load tests in the laboratory.

Testing program
A series of model pile load tests were performed in saturated and unsaturated compacted fine-grained soil under drained and undrained stacking conditions.The soil chosen for this study is a glacial till obtained from Indian Head, Saskatchewan, Canada.The key objective of the test program is to determine the influence of matric suction on the ultimate shaft capacity of model piles.

Soil properties
The properties of the tested soil are summarized in Table I.The shaft bearing capacity of model piles were proposed to be determined at a water content; 13% (dry of optimum).This water contents was chosen from the compaction curve data.The dry densities of the compacted soil at this water content was respectively equal to 14.5 kN/m 3 .The experiments were not conducted for the water contents on the wet of optimum side since the degree of saturation values were greater than 90%, which resulted in significantly low matric suction values (Sheikhtaheri, 2014).11.5 The soil-water mixture (hereafter referred to as soil) was placed in a tank (300 mm in diameter and 300 mm in height).The soil was compacted statically with 350 kPa push into the test tank using a specially designed compaction base plate.The compaction and model pile load tests were conducted using a conventional triaxial test loading frame (Fig. 1).
A comparison between performances of the behavioral models Ukrainian Journal of Ecology, 8(1), 2018

Numerical modeling
In this part of the paper the numerical modeling method is described.The Mohr-Coulomb, modified Cam-Clay, and Barcelona behavioral models were also used in this research.Tables ( 1) and ( 2) present the parameters of these behavioral models.The modeling conditions are designed to resemble laboratory conditions to the possible extent.Concerning the model dimensions, it has a height and width of 30 cm and the pile height is 20cm (Vanapalli & Taylan, 2011).To measure the load-bearing capacity of the wall in each state, 2mm of empty space is provided at the end of the pile.Numerical simulation of the process can be summarized as follows: (Al-Khazaali & Vanapalli, 2015;Fujiang et al., 2016;Ha & Hassanlourad, 2015;Han et al., 2016) 1. Construction of model geometry and pile.2. Boundary conditions and in-situ stresses.
3. Run the program to primary balance.4. Zero the amount of displacements in the direction of x and y. 5. Loading the pile with the corresponding rate.6. Assessment of displacements and forces pile and model.
A displacement of 1.5mm was applied on the top of the pile to simulate the loading process.Interface elements were placed around the pile to imulate the interaction between the pile and soil.Standard fixities were used to define the boundary conditions to provide stability conditions.A distributed load on the top of the small scale single pile was used to simulate a static load.Before applying the load to the head of the small scale single pile, the analysis model was assumed to be in an initial state of equilibrium.In this section, the load-bearing capacity of the pile is estimated experimentally using the Mohr-Coulomb behavioral model in the saturated and unsaturated drained states.Modeling was carried out using the finite difference method.Tables 1 and 2 present a list of parameters used in this model.The modeling conditions were designed to resemble laboratory conditions the most.Fig. 4 depicts the pile load-bearing capacity diagram resulted from the finite difference numerical modeling carried out using the Mohr-Coulomb behavioral model.As seen, the maximum pile load-bearing capacity in the unsaturated and saturated states is approximately 480 N and 150 N, respectively.

Summary and conclusion
In this paper, a comparison was drawn between the Mohr-Coulomb, modified Cam-Clay, and Barcelona behavioral models using the finite-difference method, and results of experiments conducted to estimate pile load-bearing capacity in the static state were presented.The behavioral model parameters matched the laboratory model's.To estimate soil saturation on the suction level of concern, the water-soil curve (showing suction behavior in relation to moisture) was used.Results of the finitedifference modeling using different behavioral models are presented in Table 3.The results show that the estimate of Barcelona behavioral model to estimate the capacity of the pile, to consider the impact of unsaturated soil suction is closer to reality.It is necessary to consider unsaturated soil behavior in engineering projects such as earth damps, road and railway body, water supply canals, solid waste landfills, etc.The 1990 BBM model is capable to predict soil hardening due to suction changes and plastic and irreversible response of soil against stress and suction, which lead to failure and collapse of soil, i.e. change in hardness.It can be used for unsaturated soils with low to moderate plasticity such as sandy-clay, clayey sand, silt, and cohesionless soil.Adding the BBM model to FDM application allows engineers to model soil projects more realistically.

Table 1 .
Properties of the tested soil

Table 2 .
Mechanical properties for soil

Table 3 .
Comparison of measured values and values resulted from numerical modeling in different states