Liquefaction Induced Ground Deformation of Slopes using Geostudio2007 Software Program Baydaa Hussain Maula a, Ling Zhang b

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1 dvanced Materials Research Online: ISSN: , Vols , pp doi:28/ 2 Trans Tech Publications, Switzerland Liquefaction Induced Ground Deformation of Slopes using Geostudio27 Software Program Baydaa Hussain Maula a, Ling Zhang b School of Civil Engineering, Harbin Institution of Technology, Harbin, China a Sun_2549@yahoo.com, b Xianzhang_ling@263.net Key words: liquefaction, Geo studio 27, earthquake, Lateral speared, Ground deformation. bstract. Liquefaction phenomenon which has produced severe damage all over the world was studied under earthquake record of.5g; one of the major effects of liquefaction is lateral spreading. Lateral spreading occurs in sloping grounds and can cause serious damage to structures and lifelines. The objective of this paper is to study the effect of earthquake shaking on soils and slope stability using Geo Studio software 27. The applicability of the analysis is demonstrated by analysis various slope and embankments subjected to earthquake shaking. This study investigated the effect of embankment slope angle and its geometry on liquefaction. Pore water pressure can be increased by % to 35% as β ranged from 35 to 45 respectively, it can see that liquefaction zone induced by earthquake can eliminate as flattering slope and mean while reduced lateral speared displacement. Three case studies are providing to evaluation cyclic stress ratio (CSR) due to earthquake and lateral speared for soils. Introduction Despite the progress made in understanding the behavior of embankment erected on soft clay ground in recent years, the optimal design of such embankment remains difficult and complex []. The failure of earth structures such as natural slopes or earth embankments and dams has resulted in heavy loss of life and property in communities' world wide, where the understanding of the mechanism of slope failure and its analysis has generally been in sufficient to prevent accidents occurring. t present, this problem remains incompletely resolved [2]. Slope stability analyses have received a great deal of studies by various researchers and a wide variety of analytical procedures have been developed over the years. They include limit equilibrium methods and finite element methods. There are number of important advantages in using the F.E.M. in slope stability analysis over the conventional limit equilibrium methods in analyzing the stability of slopes [3]. In most practical stability problems, the engineer is concerned with the factor of safety against failure, rather than local over stress. The most general definition of factor of safety which can be applied irrespective of the shape of the failure surface, is expressed in terms of the proportion of the measured shear strength that must be mobilized to just maintain a limiting equilibrium,[4]. Different methods were adopted for stability analysis using limiting equilibrium concept. Most of the limit equilibrium methods for slope stability analysis assume surface with particular shapes (e.g. circular or log-spiral). This is due to the observation that many of the slope failures had taken place along such surface shapes and to the fact that such assumption reduces laborious computation. Clearly, however, any slope movement must be a compound effect of the stresses induced in the soil and the resisting strength within the soil itself, [5]. For a slope comprising a complex sequence of non homogenous, isotropic materials, whose physical and chemical properties change with time, limit equilibrium methods can be unreliable,[3].this method is based on the assumption that slope failure is an instantaneous phenomenon occurring simultaneously along the entire length of the slip surface. The limit equilibrium methods used to evaluated stability of earth embankment assume that the F.S. along the slip surface is unique, which can be true only if the F.S. is a unity (i.e. at failure). Limit equilibrium methods have been widely adopted for slope stability analysis mainly due to the simplicity that the methods offer. In all such methods, it is assumed that the soil is in a state of ll rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, US-/5/6,:25:37)

2 34 dvances in Building Materials, CEBM 2 limiting equilibrium, but the methods differ in the assumptions concerning the shape of the failure and in the application of static's. The methods of slices has become the most common method due to its ability to a accommodate complex geometries and variable soil and water pressure conditions. Most of these methods like Bishop Method of slices [6].not satisfy all conditions of equilibrium, while other like [7] dose satisfy all these conditions. It is needed in any engineering analysis that the procedures for the analysis are not so rigorous, so as to simplify work. The finite element method is one of well known numerical methods in engineering practice. The basic idea of this method is replacing the continuum having an unlimited or infinite number of unknowns by a mathematical model which has a limited or finite number of unknowns at certain chosen discrete points called the "nodes". Many problem in soil mechanics are concerned with stress and deformations in the soil due to boundary and body, forces, therefore the F.E.M. is used for the evaluation of displacement, forces, strain or stress field starting from initial boundary force or displacement field. The F.E.M. of analysis can provide a very good prediction of the behavior of soil structure interaction problems if the different construction stages and the material behavior are simulated correctly and accurately in the analysis. The benefits of F.E.M. include its comprehensive ability to model deformations as well as to predict collapse []. Geotechnical engineers have recognized that the F.E.M. is a good tool by which many of their complicated problems can be solved. The main concepts in the F.E.M. are: Discretization of the region being analyzed into finite elements. These discrete elements are assumed to be interconnected only at the joints which are called nodes. The use of interpolating polynomials to describe the variation of a field variable within an element. Over the last decade, many numerical optimization routines to search for the min. F.S. have been developed, with comparison to limit equilibrium methods of analysis. The applications of stress-strain relationships to the analysis of slope stability have been successful in many studies. However, the use of factors of safety against local failures in the evaluation of slope stability has not yet received wide attention. [5], presented an approach to predict the critical slip surface of a slope stability using the minimum F.S. against local failure. The confirmed the validity of this proposed method using a two well established methods of slope stability analysis, [8] and [6]. The researchers also compared the location of a slip surface of a slope obtained using the F.E.M. with the slip surface obtained using slopes stability analysis methods. The results of their study show that the F.S. obtained by the proposed method are in good agreement with that determined by [6] and [8] methods. Soil Liquefaction and Lateral Spread Soil liquefaction is a process by which soil deposits, primarily sands and silts temporarily lose strength and behave as a viscous liquid rather than as a solid. The actions in the soil which produce liquefaction are as follows: Seismic waves primarily shear waves, passing through saturated granular layers, distort the granular structure, and cause loosely packed groups of particles collapse. Disruptions to the particulate structure generated by these collapses cause transfer of load grain-tograin contacts in the pore water. This transfer of load increases pressure, in the pore water, causing drainage to occur. If drainage is restricted, a transient build up water pressure will occur. If the pore water pressure rises to a level approaching the overburden pressure grain-to-grain contact stresses approach zero and the granular layer temporarily behaves as a viscous liquid rather than as a solid and liquefaction has occurred [9]. The common types of ground failures caused by liquefaction are lateral spreading, flow failure, loss of bearing strength and ground oscillation. Liquefaction may also enhance ground subsidence and sand boils. Lateral spreading involves lateral displacement of large, surficial blocks of soil as a result of liquefaction of subsurface layer. Displacement occurs in response to combination of gravitational and inertial forces generated by an earthquake. Lateral spreading generally develop on gentle slopes and move toward a free face such as an incised river channel. Horizontal displacement commonly range up to several meters, but where slopes are particularly favorable and ground shaking durations are long, displacements may range up to several tens of meters. The displaced ground usually breaks up internally, causing fissures, scarps, horsts, and grabens to form on the failure surface. Damage caused by lateral spreading, though seldom catastrophic, is severely disruptive and often pervasive. For example,

3 dvanced Materials Research Vols during laska earthquake, more than 2 bridges were damaged or destroyed by spreading of the floodplain deposits toward river channels. The spreading compressed the superstructures, buckled decks, thrust stringers over abutments, and shifted and tilled abutments and piers. Similar damages occurred during the Costa Rica earthquake and during previous large earthquakes. Evaluation of Liquefaction Soil liquefaction describes the behavior of soils that, when loaded, suddenly suffer a transition from a solid state to a liquefied state, or having the consistency of a heavy liquid. Liquefaction is more likely to occur in loose to moderately saturate granular soils with poor drainage, such as silty sand or sands and gravels capped or containing seams of impermeable sediments. During loading, usually cyclic untrained loading, e.g. earthquake loading, loose sands tend to decrease in volume, which produces an increase in their pore water pressures and consequently a decrease in shearstrength, i.e. reduction in stress. Deposits most susceptible to liquefaction.sands and silts of similar grain size (well-sorted), in beds at least meters thick, and saturated with water. Such deposits are often found along riverbeds, beaches, dunes, and areas where windblown silt (loess) and sand have accumulated. Some examples of liquefaction include quicksand, quick clay, turbidity currents, and earthquake liquefaction. Depending on the initial void ratio, the soil material can respond to loading either strain-softening or strain-hardening. Strain-softened soils, e.g. loose sands, can be triggered to collapse, either monotonically or cyclically, if the static shear stress is greater than the ultimate or steady-state shear strength of the soil. In this case flow liquefaction occurs, where the soil deforms at a low constant residual shear stress. If the soil strain-hardens, e.g. moderately dense to dense sand, flow liquefaction will generally not occur. However, cyclic softening can occur due to cyclic untrained loading, e.g. earthquake loading. Deformation during cyclic loading will depend on the density of the soil, the magnitude and duration of the cyclic loading, and amount of shear stress reversal. If stress reversal occurs, the effective shear stress could reach zero, then cyclic liquefaction can take place. If stress reversal does not occur, zero effective stress is not possible to occur, and then cyclic mobility takes place. The resistance of the cohesion less soil to liquefaction will depend on the density of the soil, confining stresses, soil structure (fabric, age and cementation), the magnitude and duration of the cyclic loading, and the extent to which shear stress reversal occurs.several approaches to evaluate the potential for liquefaction have been developed. The commonly employed methods are cyclic stress approach and cyclic strain approach to characterize the liquefaction resistance of soil both by laboratory and field tests. The cyclic stress approach to evaluate liquefaction potential characterizes both earthquake loading and the soil liquefaction resistance in terms of cyclic stresses. But, in the cyclic strain approach, earthquake loading and liquefaction resistance are characterized by cyclic strains.cyclic triaxial test; cyclic simple shear test and cyclic torsion shear test are the common laboratory tests. Further, standard penetration test, cone penetration test, shear wave velocity method, Dilatometer test are some of in situ tests to characterize the liquefaction resistance. Laboratory testing is recommended for determining grain size distribution, particularly the fines content (percent of passing the #2 sieve), plasticity, unit weight, and moisture content of potentially liquefiable layers. The case studies In order to investigate the liquefaction and lateral speared of slope due to earthquake using Geo- Studio 27 which using different search techniques, three examples were used for comparison with other research results. The investigated geometries and soil profiles ranged from simple to complex. For Geo-Studio 27 software, analysis demonstration by conjunction between two models (Quake/w analysis and Slope/w). This type of analysis is referred to as a Newmark analysis during an earthquake[] there will short moments in time when the inertial forces (mass times acceleration) plus the initial static forces will exceed the available shear resistance, and during these times the temporary loss of stability will lead to un-recoverable deformation. The accumulation of the un-recoverable deformation will manifest itself as permanent deformation after the shaking has

4 36 dvances in Building Materials, CEBM 2 stopped. Quake/w is a finite element program for analyzing the effect of earthquakes on embankments and natural slopes. Its computes the static plus dynamic ground stresses at specified intervals during an earthquake.. Case study no. The geometry of 6m height of embankment and slope angle equal to 45degree, strength parameters, and unit weight of soil, Strength parameters 2kPa while the friction angle φ varies from 45 the unit weight of soil layer was kept 2 kn/m 3. This case was taken from Pockoski and Duncan []. Results from Geo-Studio 27 were shown in Fig Pore-Water Pressure (kpa) Figure. Evaluation of Liquefaction as CSR and pore water pressure vs. Distance. B. Case study no.2 In this, effect of slope angle β will be investigated, height of slope H=2 m, unit weight of soil γ =25 kn/m3, young's modulus, E=Mpa, Poisson's ratio ʋ=, cohesion C=42 kpa, friction angle φ=7, Fig.2. Show that the effect of β for homogenous slope on liquefaction zone and cyclic stress ratio due to earthquake (CSR)eq as contour lines. While the Figure explains slope angle effect on CSR ratio and pore water pressure vs. distance, during shaking on consideration point inside media of shaking. It's clear from the figures that slope angle β has great role on evaluation liquefaction. Slope angle 35 Slope angle 45 Slope angle 35 Slope angle Pore-Water Pressure (kpa) Pore-Water Pressure (kpa) Figure 2. Evaluation of liquefaction CSR vs Distance.9 Slope angle 35 Slope angle Figure 2. Evaluation of liquefaction CSR vs Distance

5 dvanced Materials Research Vols H m H m S S T m T m a. Shape b. Shape 2 Figure 3. Basic Problem shapes Considered in case study no.3 Case I Case II Case III Case IV a. Evaluation CSR vs. Lateral speared for shape where( H=8,6, S :2, 2: and constant T= 3) Case I Case II Case III Case IV b. Evaluation CSR vs. Lateral speared for shape II where( H=8,6, S :2, 2: and constant T= 3) Figure 4. Evaluation CSR vs. Lateral speared for two shapes C. Case study no.3 In this example considered an embankment resting on soft clay soil foundation. Different geometries were selected and as shown in Fig.3, the embankment mass is composed of compacted sub base of (C= kn/m 2, φ=4 and γ=2 kn/m 3 ) which simulate a road way embankment that is resting on soft clay soil foundation of (C= 6 kn/m 2, φ = 6 and γ=8 kn/m 3 ). The case consists of two shapes each shape covered four different embankments ( case I, II, III, and IV ) subjected to same earthquake record of.5g, all examples cases have the same cycle stress ratio peak generated by earthquake motion (CSReq) about.55 to.2 for shape and about 5 to.2 for shape2. To investigate lateral spread during shaking, considering point inside media of shaking, see Fig.4. Disscussion nd Conclusion Soil profile of the investigated slopes ranged from lose, medium to dense sandy soil with different shapes and geometries. In Case no., the slope showed No liquefaction notice due to the increasing in pore water pressure as the effect of earthquake, we can get a fact that homogenous soil of good

6 38 dvances in Building Materials, CEBM 2 parameters (C, φ, and great slope height) can show stability against the dynamic loads. Case no.2, slope angle β have a great effect on each of liquefaction and pore water pressure, the results from figure 2 shows the increasing as β of slope decreasing liquefaction process. Point considered studying the effect of β and slope shaping on pore water pressure in Case no.3, two parameters investigated for layered slope having different shape, results from these figures showed that lateral speared can be controlled by increasing embankment height even sloping of embankment have great effect two its about ( -4) % due to height and sloping rate differences for shape and about (6-4) % for the same criteria for shape2. References [] C., Hird, I., Pyrah, D.Russell, and Cinicioglu, F., Modeling the Effect of Vertical Drains in Two Dimensional F.E.. of Embankments on Soft Ground, Canadian Geotechnical Journal 995; 32: [2] R. D., P. L,Espinoza,. Bourdeau and B. Muhunthan, Unified Formulation for nalysis of Slopes with General Slip Surface, Journal of Geotechnical Engineering, SCE, 994 ;2: [3] J., Zou, William, D., and Xiong, W., Search for critical slip surface based on finite element method, Conadian Geotechnical Journal, 995; 32: [4]. W., Bishop, and L. Bijerrum The Relevance of the Triaxial Test to the Solution of Stability Problems, Research Conf. on the Shear Strength of Cohesive Soils, SCE 96. [5] L.,Scott, Huang, and Yamasaki, K., Factor of safety approach. Journal of Geotechnical Engineering, 993, 9(2), [ 6]. W. Bishop, The use of the slip circle in the stability analysis of slopes. Geotechnique, 955, 5(), 7 7. [7] N., Janbu, Earth pressures and bearing capacity calculations by generalized procedure of slices, in 4th International conference on Soil Mechanics and Foundation Engineering. 957, 2, [8] S.K., Sarma, Stability analysis of embankments and slopes. Journal of the Geotechnical Engineering Division, SCE, 979, 5(GT 2), [9] T.L. Youd Liquefaction, ground failure and consequent damage during the 22 pril 99 Costa Rica Earthquake. bridged from EERI proceeding : US Costa Rica Workshop, 993. [] N. M., Newmark, Effects of earthquakes on dams and embankments. Geotechnique, 965,5(2), [] M. Pockoski, JM.Duncan. Comparison of computer programs for analysis of reinforced slops. Virginia Polytechnic Institute and State University; 2.

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