HN10108480812
Evaluating the effects of fluctuating groundwater level upon settlement and stiffness of soil using finite elements method
Sajad Tavakoli
A member of scientific board in Ilam Azad University-branch of Mehran [email protected]
Abstract
Of parameters for designing foundations is settlement affected by parameters including shape and stiffness of foundation, sub-grad reaction module, soil type and groundwater level. Groundwater variation has various impacts on different soils. Regarding the different behavior in vicinity of the groundwater level, the present paper is to study this behavior at different states. To get this target, a sample of coarse soil was loaded (around Tehran) in different states of groundwater (inside and outside of slice failure domain) using Plaxis 3D Foundation and a behavior Hardening soil model which its characteristics were determined through Plate Load Test. A distribution of settlement and stiffness of soil under foundations was achieved with help of modeling in different conditions. Given the rising underground water level causes increasing pore pressure and consequently reducing the effective stress, the underground water level increases settlement and decreases stiffness of soil.
Keywords: settlement, finite elements method, groundwater level, sub-grade reaction module, Plaxis 3D Foundation.
1. INTRODUCTION
There are compress stresses in soil because of load effects and causes soil to be compacted compress settlement is because of compress deflection and movement of soil particles , exiting of air and water from soil void and other factors specially ground water level. Seepage of water into soil causes water to enter soil voids and soil slowly becomes saturated. According to relations about stress, rise of water level causes the increase in pure –water pressure and because of this effective stress of soil (that part of stress which is tolerated by solid particles of soil) decreases which is the most important part of stress in soil.
Existence of ground water in soil causes the decrease of module of elasticity which according to primary settlement relation in coarse soil (
F S S
e mI I
qB E S
12
), the decrease of this parameter leads to increase in settlement [1]. With regard to the importance of settlement in soil, this study analyzes the effect of ground water fluctuation under foundation (in or out of slice failure domain) on amount of settlement. To do this, first soil profile and foundation which is built on it is modeled to the domain of stress effect, by Plaxis 3D Foundation.
Then a set of nonlinear Elastic - plastic analysis was done with calculated code in which the amount of settlement in different situations was calculated by change of ground water level. Behavior model parameter was established with the help of Plate Load Test (PLT). According to settlement distribution graph in foundation and ground reaction distribution, estimation of amount and distribution of sub-grade reaction module (kS) was calculated.
2. F
INITE ELEMENT MODELING3-dimension mesh and elements number of finite elements with respect to ability of Plaxis 3D were selected in such a way that having high accuracy calculation time was optimum, a land with 11B width, 11B length and 10B depth was selected under foundation. The way of create mesh is shown in figure (1).
Figure 1. 3D-geometrical model of finite element analysis
3. T
HEH
ARDENING SOIL MODELPlaxis 3D gives a set of behavior model wish the most suitable one for this study is hardening soil model.
This model is a development of Mohr - Coulomb non-united model. This model removes the defects of Mohr - Coulomb model by adding a cap surface for modeling of plasticity flow under identical stress and also expressing plasticity flow before failure by identical hardening regulation. Hardening parameter is determined by deviatoric strain which controls deviatoric yield surface and also plasticity voluminal strain which controls cap region. Hardening module and elastic modules are considered as function of confining pressure characteristics of this model are as follow:
- Stress dependent stiffness according to a power law.
- Plastic straining due to primary deviatoric loading.
- Plastic straining due to primary compression.
- Elastic unloading/reloading.
- Failure according to the Mohr – Coulomb model.
Important parameters of model are as follow:
C: (Effective) cohesion [kN/m2]
: (Effective) angle of internal friction [degree]ψ
: Angle of dilatancy [degree]50
ref
: Secant stiffness in standard drained triaxial test [kN/m2]
50oed : Tangent stiffness for primary oedometer loading [kN/m2]
ref
ur
: Unloading / reloading stiffness [kN/m2]
m: Power for stress-level dependency of stiffness [6].
4. DETERMINING MODEL PARAMETERS
In this study, a kind of sandy soil considered which result of its plate loading test was available.
Characteristics of used load, settlement and time of loading on samples in plate loading test are shown in table (1). Parameter of this sandy soil is gained by using plate loading test simulation with hardening model in plaxis and adjusting results to plate loading test of this soil (figure 2). These parameters are shown in table (2).
Table 1- Results of PLT
S(m) T(min)
P(kPa)
2.44×10-3 30
283
5.49×10-3 30
566
8.09×10-3 1440
849
7.93×10-3 10
566
7.83×10-3 10
283
7.64×10-3 10
0
In which, P is stress on plate (kPa), T is time of load (minute) and S is settlement (m)
Table 2- Characteristics of sample
ψ (degree) Φ
(degree) C
(kN/m2) m
ref
Eur
(kN/m2)
ref
Eoed
(kN/m2) E50ref
(kN/
m2)
5 35
0.5 0.6
920000 115000
11500
Figure 2- Comparison between PLT and Plaxis modeling for sample
5.
EFFECT OF GROUND WATER LEVEL ON SETTLEMENT AND SUB-GRAD REACTION MODULETo study the effects of ground water level on settlement a square mat foundation with 10- meter dimensions which its characteristics are shown in table (3) is considered. Load on this foundation is 288 kPa. Depth of slice failure is calculated according to hypothesis of Hansen model and . According to this equation, depth of slice failure is 9.6 meter. Then in different stages by changing the water level, settlement-stress graph of soil is shown in figure (3). As it is shown in figure, when the level of ground water is in surface of foundation, the greatest settlement would occur. The reason of this process may be the decrease of elastic module of soil because of existence of water in top level.
Table 3- Characteristics of modeled foundation by Plaxis 3D
μ
E(kPa) t(m)
B(m)×B(m)
107
×
2 0.2 110×10
In which B is width of foundation (m), t is thickness of foundation (m), E is elastic module (kPa),
μ
is poison ratio.Figure 3- Results of settlement-stress of modeled foundation for different ground water level
Having amount and distribution of settlement, settlement is read from outputs of software which are shown in figure (4). In this figure, it’s shown that settlement in middle regions is less than those in side regions which is according to behavior of sandy soils. In these soils, elastic module is a function of confining stress. It is obvious that in central regions confining stress is more than side regions. It is obvious in this figure that in situation when the level of ground water was in surface level of foundation, the greatest settlement occurred.
Figure 4- Distribution of settlement under the foundation for different ground water level
To draw the graph of sub-grade reaction module, we do these steps:
Estimation of sub-grade reaction module distribution can be obtained from gained results. To do this
foundation into some areas that results are shown in figure (5). It is shown that distribution of sub-grade reaction module in under foundation is not steady and stiffness is more in central areas that side ones which is based on behavior of sandy soil. As it is obvious from figure, existence of ground water in high level causes about 30 percent decrease in sub-grade reaction module.
Figure 5- Distribution of sub-grade reaction module under the foundation for different ground water level
6. CONCLUSIONS
Following results can be obtained from study:
1- plaxis has a suitable and reliable application for foundation behavior modeling and its soil.
2- Settlement in side of sandy soil is more than its center and also sub-grade reaction module in foundation center is more than that of its corners which plaxis shown this clearly.
3- Existence of ground water level in high levels causes significant increase in settlement (about 40 to 45 percent).
4- Existence of ground water level in high levels causes 30 percent decrease in sub-grade reaction module.
7. REFERENCES
1. Bowles, J.E, (1999), “Foundation analysis and design.” McGraw-Hill Book Co. NewYork, 5th edn.
2. Brian, J.A and Townsend, M.F.C. and Rahelison, L., (2007), “Load Testing and Settlement Prediction of Shallow Foundation,” Journal of Geotechnical and Geoenvironmental Engineering, Vol. 133, No. 12.
3. Tong, L.I. and Ronald L.B., (2005), “Nonlinear Parameters for Granular Base Materials from Plate Tests,”
Journal of Geotechnical and Geoenvironmental Engineering, Vol. 131, No. 7.
4. Nabil, F and Ismarl., (1996), “Loading Tests on Circular and Ring Plates in Very Dense Cemented Sands,”
Journal of Geotechnical Engineering, Vol. 122, No. 4.
5. Ping-Sien, L and Li-Wen, Y. and Hsein, J.C., (1998), “Subgrade reaction and load-settlement characteristics of gravelly cobble deposits by plate-load tests,” Department of Civil Engineering, National Chung-Hsing University, Taichung 400, Taiwan, ROC.
6. Plaxis 3D Foundation manual.