Influence of unloading soil modulus on horizontal deformation of diaphragm wall

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1 Influence of unloading soil modulus on horizontal deformation of diaphragm wall Hoang Viet NGUYEN 1 and Thi Dieu Chinh LUU 2 1 Lecturer, Soil Mechanics and Foundation Department, National University of Civil Engineering, Vietnam nhvdhxd@gmail.com 2 Lecturer, Water Resources and Hydropower Department, National University of Civil Engineering, Vietnam ABSTRACT This paper emphasizes the difference in the value of the soil modulus under unloading and loading conditions. The difference is used to explain the phenomenon why the theoretical analysis results of the horizontal displacement of diaphragm walls are usually much greater than the observation results when the soil moduli used for analysis are often estimated from the Oedometer test results. The cause was identified as the soil elements in the vicinity of deep excavation experienced the unloading stress paths during the excavation construction, which are opposite to the convention loading stress path of the compression tests. Based on the test results for the same soil under both unloading and loading stress paths, it is proposed a recommendation on the estimation of soil modulus for analysis the horizontal displacement of deep excavation based on the compression test results. Finally, the recommendation was applied to investigate the horizontal displacement of a case study. It is shown that the analysis results fit well with the displacement observed from the inclinometer tests. Keywords: unloading, soil modulus, deep excavation, horizontal displacement, stress path 1. INTRODUCTION Number of deep excavation projects is increasing due to the increase of urban population in the last decade. Deep excavations are often located inside cities near existing buildings, and diaphragm walls are widely used. Horizontal deformation of diaphragm wall during excavation process is one of the most important analyses, which play a crucial role in stability of surrounding existing constructions. However, prediction of diaphragm wall behavior during excavation construction period is one of most challenge of engineering practice. In generally, advanced triaxial stress path system is rarely applied to conduct soil experiment in common laboratory, especially in Vietnam. Therefore, the soil moduli obtained in geotechnical investigation reports are often interpreted from Oedometer or in-situ

2 0ctober 2013, Hanoi, Vietnam testing results. When the soil moduli applied to be input parameters for analyzing deformation of diaphragm wall by finite element method, the prediction horizontal displacement of retaining wall is usually considerable greater than the movement observed from the inclinometer test results. There are number of reasons which are recognized to cause the difference. One of the most important factors is the accuracy of input soil parameter used for analysis. In the case of deep excavation constructions, the soil modulus is an essential soil property which greatly affects the horizontal movement of retaining wall predicted by the finite element method (Rashed et al., 2012). The preciseness of the deformation prediction greatly depends on the properly soil modulus input data. Although, in engineering practice, the soil moduli often derived from the empirical correlation (Soils and geology procedures for foundation design of buidings and other structures n.d.; Whittle & Davies, 2006). On the other hand, it is obvious that in excavated earth structures, unloading is a major factor which affects on the behavior of soil mass. Excavation activities result in the decrease of horizontal stress in soil elements at sides and of vertical stress in soil elements at bottom of deep excavation. The change of stress state following unloading stress paths might induce plastic failure in the soil elements during construction stage. They are contrary to the loading stress path from the conventional triaxial compression test used to determine the soil properties in laboratory. There are several influences of unloading factor on excavated earth structures, which pointed out in literature. For instance, the lateral unloading due to excavation is one of two crucial external factors inducing landslides in unsaturated soils (Bao & Ng, 2000). Or Ng (1999) suggested that in order to improve the design and analysis of soil-structure interaction associated with deep excavation, it is important to understand about effective stress changes around excavations caused by both horizontal and vertical stress relief. In addition, the results of triaxial loading and unloading stress paths shear tests on both triaxial and true triaxial test systems pointed out that the soil properties such as stress-strain curve, elastic modulus, and failure characteristic during shear under unloading stress paths were different from those under loading stress paths (Chen et al., 2007; He et al., 2003; Yang et al., 2006; Zhang & Sun, 2005). The soil moduli estimated from the triaxial unloading stress path shear tests were considerably greater than those estimated from the triaxial loading stress path tests (Yuan & Nguyen, 2011). This paper will focus on the difference in the value of the soil modulus under unloading and loading conditions. Then the difference is used to explain why the theoretical analysis results of the horizontal displacement of diaphragm walls are usually much greater than the observation results when the soil moduli used for analysis are often estimated from the Oedometer test results. 2. UNLOADING MODULI TEST RESULTS New Technologies for Urban Safety of Mega Cities in Asia

3 Based on the results of the loading, unloading, and reloading Oedometer test series of Nguyen (2010), the constrained soil modulus corresponding to each level of applied vertical stress is obtained, then their values are shown in the following Table 1. Table 1: Oedometer soil moduli under unloading and reloading stress paths versus those under loading stress path Soil moduli (kpa) Vertical stress Loading Unloading Reloading EU/EL (kpa) EL EU ERe ERe/EL Average It is apparent that the soil moduli strongly depend on the level of vertical stress applied. Furthermore, at the same level of stress, the soil modulus determined from loading stress path is less than those determined from unloading and reloading stress paths, and the difference between them sharply increases when the vertical stress increases. The difference is quantified by the ratio of EU/EL and ERe/EL. It shows that in the stress range of common engineering practice the unloading moduli are greater than loading one at the same level of applied vertical stress about 5 to 7 times. On the other hand, Yuan and Nguyen (2011) also show that the undrained secant Young s moduli determined from the triaxial unloading stress paths test is obviously greater than those determined from the triaxial loading stress path at the same axial strain and confining pressure. This difference might be the reason that could explain the phenomenon why the theoretical analysis results of the horizontal displacement of diaphragm walls are usually much greater than the observation results when the soil moduli used for analysis are often estimated from the Oedometer test results. 3. CASE STUDY A case study of Complex Public services building, Condominium and Office Project on Le Duc Tho street, Mai Dich ward, Cau Giay district, Hanoi. This building is the complex of five buildings; two of them are 30 storeys luxurious condominiums. There are three basements designed on the whole area of these buildings. Influence of unloading soil modulus on horizontal deformation of diaphragm wall

4 0ctober 2013, Hanoi, Vietnam After carefully studying all the boring logs from the report (Geotechnical Report 2009), a representative boring K6 was selected for analysis. The engineering properties of the soils at the site of this project used in this case study are given in Table 2. The elastic moduli of saturated clays were obtained from Oedometer tests, for sands the elastic moduli were interpreted from in-situ test results. The underground water table at the depth of 17 meter was considered as a representative value for analysis. Layer Table 2: Soil layer properties at Complex Public services building Name, state Thickness Soil properties SPT γ W e PI LI E N (m) (kn/m³) % - % (kn/m 2 ) (blow) 1 Filled soil Clayey soil, low plasticity Clayey soil, plasticity, interferenced with sandy soil Clayey soil, low slightly plasticity Clayey soil, low plasticity Fine sand, dense Fine medium sand, dense very dense Gravel, very dense >100 The thickness of diaphragm wall was 600 (mm), and it was used for excavation down to 10.5 (m) below the ground with two levels of ground anchor systems at the different excavation depth respectively. The diaphragm wall toe was embedded down to 17 (m). The construction stages that were used in the numerical modeling correspond to those in the actual construction as follows: Table 3: Actual construction stages No. Stage Excavation level (m) 1 Installing diaphragm wall First excavation -3 3 Installing the first level of anchor with pre-stressed of 24 (T) -3 4 Second excavation -6 5 Installing the second level of anchor with pre-stressed of 31 (T) -6 6 Third excavation The plan view of Complex Public services building and all inclinometer testing points are demonstrated in Figure 1. There are six measured profiles of horizontal displacement of diaphragm wall obtained from the six inclinometer testing points, although they are not too different from each other. Hence the measured New Technologies for Urban Safety of Mega Cities in Asia

5 displacement profiles from ICL4 inclinometer testing point are selected to be representative measured value used to compare with the following analysis results. Figure 1: Plan view of Complex Public services building and inclinometer testing positions Figure 2: Measured horizontal displacement obtained from inclinometer testing point - ICL4 A two dimensional model of the diaphragm wall and the associated construction stages was implemented by using Plaxis version 8 to obtain the horizontal deformation (presented in Figure 3). In the model, the diaphragm wall was modeled by plate element with Normal Stiffness (EA) of (kn 10 3 ), and Influence of unloading soil modulus on horizontal deformation of diaphragm wall

6 0ctober 2013, Hanoi, Vietnam Flexural Rigidity (EI) of 540 (kn 10 3 m 2 ). The ground anchor was modeled by node-to-node anchor element. The minimum value of Normal Stiffness (EA) was 9.12E+04 (kn). After installing the anchor, the anchor was pre-stressed until the design value. However, the measured tension force in each anchor was about 60% of the prescribed load in average. Thus only 60% of the prescribed load is applied to the pre-stressed load of ground anchor in the Plaxis model. Figure 3: Geometry model in Plaxis When analyzing the behavior of diaphragm wall by finite element method, the Mohr-Coulomb (MC) and Hardening Soil (HS) models are often used (Vo & Ngo 2011). The horizontal displacement profiles of diaphragm wall when the clayey soil layers modeled with MC and HS models are plotted in Figure 4 a. In the case of HS model, as average values for various soil type, Eur = 3E50, and Eoed = E50 are suggested as default settings (Plaxis Geotechnical Software - Material Models Manual n.d.). Although, for this case study, unloading is the dominant factor which greatly influence the behavior of diaphragm wall, therefore it is assumed that Eur = 5Eoed as suggested by Ismail and Teshome (2011). Figure 4 a shows that the horizontal displacement of diaphragm wall when the clayey soil layers modeled by HS model is much close to the results from inclinometer test than that when modeled by MC model. However, the difference in the displacement when using HS model compared with the measured results is still remarkable. In order to find out the main factor which causes the considerable different between the horizontal displacement of diaphragm wall predicted by finite element method and the inclinometer testing result, the soil modulus value initiated in model of MC and HS are varied to investigate. One of the different points of HS model from MC model is Eur, which is used to simulate the unloading behavior of soil. This difference might be used to explain the more appropriate result when modeled by HS model than that when modeled by MC model depicted in Figure 4 a. To conduct a survey on the influence of the value New Technologies for Urban Safety of Mega Cities in Asia

7 of Eur in HS to the horizontal deformation of retaining wall, two other values of Eur of 15 Eoed and 19 Eoed were applied to compare. The corresponding horizontal displacement of retaining wall are presented in Figure 4 b. (a) (b) Figure 4: Horizontal deformation of diaphragm wall when clayey soil modeled by MC and HS model (a) (b) Figure 5: Horizontal deformation of diaphragm wall when clayey soil modeled by MC with EMC model = 1E, 3E, 5E, 7E and 6E Influence of unloading soil modulus on horizontal deformation of diaphragm wall

8 0ctober 2013, Hanoi, Vietnam Figure 4 b shows that in this case study, modeling clayey soil layers by HS model is more appropriate than by MC model, this result is appropriate with other published results (Vo & Ngo, 2011). Furthermore, the displacement is more close to the measured result when the Eur is greater. However, the displacement when using Eur = 19 Eoed seems to fit that when using Eur = 15 Eoed, but both of them are still remarkably different from the inclinometer testing results. Meanwhile, Plaxis only accepts Eur < 20 Eoed (Plaxis Geotechnical Software - Material Models Manual n.d.). It seems that the superiority of HS model over MC model in modeling the unloading behavior could not explain for the considerable difference in horizontal displacement of retaining wall between analysis and measured results when the soil moduli estimated from Oedometer test results. In order to prove that the difference in soil moduli between unloading and loading stress paths is the main factor, which causes the difference in horizontal displacement of retaining wall. Three different value of EMC model = 3E, 5E, 6E, 7E are applied to analysis. The results are shown Figure 5 a, b. They show that the predicted horizontal displacement of case E = 6E is most fit the measured result. Therefore it could be stated that, if unloading soil moduli are applied for analyzing the displacement of excavation constructions, the predicted displacement will fit well with the inclinometer testing results, at least with this case study. 4. CONCLUSION AND RECOMMENDATION In this paper, the difference in value of soil moduli under unloading and loading conditions is pointed out. Then the difference is proved to be the major factor, which causes the remarkable difference between predicted horizontal displacement of diaphragm wall by finite element method and measured result when the soil moduli used for analysis are often estimated from the Oedometer test results. In the case of displacement of retaining wall analysis, if there no triaxial unloading stress path shear test carried out to determine the unloading soil modulus; it is recommended that the unloading modulus value could be deduced equally 5 to 6 times of the value of Oedometer soil modulus. REFERENCES Bao, C. G. & Ng., C. W. W., Some thoughts and studies on the prediction of slope stability in expansive soils. In Rahardjo, H., Toll, D. G. & Leong, E. C. (editors), The first Asian Conference on Unsaturated Soils, Rotterdam, Balkema, Chen, S. X., Ling, P. P., He, S. X. & Yang, X. Q., Experimental study on deformation behavior of silty clay under unloading. Yantu Lixue/Rock and Soil Mechanics, vol. 28, Geotechnical Report, 2009, Hanoi Construction Design Investigation Consultants Joint Stock Company, Hanoi. New Technologies for Urban Safety of Mega Cities in Asia

9 He, S. X., Han, G. S., Zhuang, X. S. & Wu, X. G., Experimental researches on unloading deformation of clay in excavation of foundation pit. Yantu Lixue/Rock and Soil Mechanics, vol. 24, Ismail, A. & Teshome, F., Analysis of deformations in soft clay due to unloading. Master Thesis at Chalmers University of Technology, Sweden. Ng, C., Stress Paths in Relation to Deep Excavations. Journal of Geotechnical and Geoenvironmental Engineering, vol. 125, no. 5, Nguyen, H. V., Shear strength and stress-strain behavior of saturated and unsaturated soil under unloading condition. Master Thesis at Hohai University, Nanjing, China. Plaxis Geotechnical Software - Material Models Manual n.d., Plaxis. Rashed, A., Bazaz, J. B. & Alavi, A. H., Nonlinear modeling of soil deformation modulus through LGP-based interpretation of pressuremeter test results. Advanced issues in Artificial Intelligence and Pattern Recognition for Intelligent Surveillance System in Smart Home Environment, vol. 25, no. 7, Soils and geology procedures for foundation design of buidings and other structures n.d., Departments of The Army and The Air Force. Vo, P. & Ngo, D. T., Study on the effects of soil constitutive model on the displacement prediction of excavation induced lateral wall deflections. Vietnam Geotechnical Journal, vol. 14, no. 2, Whittle, A. J. & Davies, R. V., Nicoll Highway Collapse: Evaluation of Geotechnical Factors Affecting Design of Excavation Support System. International Conference on Deep Excavations, Singapore. Yang, X. Q., Zhu, Z. Z., Han, G. S. & He, S. X., Deformation and failure characteristics of soil mass under different stress paths. Yantu Lixue/Rock and Soil Mechanics, vol. 27, Yuan, J. & Nguyen, H. V., Laboratory Study on Soil Shear Stiffness and Strength Under Unloading Conditions. Journal of Testing and Evaluation, vol. 39, no. 5, Zhang, M. X. & Sun, J., Unloading-induced deformation and strength properties of loess during construction. Yanshilixue Yu Gongcheng Xuebao/Chinese Journal of Rock Mechanics and Engineering, vol. 24, LIST OF SYMBOLS EL Oedometer soil modulus under loading stress path EU Oedometer soil modulus under unloading stress path ER Oedometer soil modulus under reloading stress path γ Unit weight of soil w Water content Specific gravity e Void ratio PI Plasticity Index LI Liquidity Index E Deformation modulus of soil N SPT in-situ testing value Eur The unloading reloading modulus Influence of unloading soil modulus on horizontal deformation of diaphragm wall

10 0ctober 2013, Hanoi, Vietnam E50 The secant modulus at 50% strength Eoed Oedometer modulus Ux Horizontal displacement of diaphragm wall EMC model Deformation modulus initiated in Mohr-Coulomb model New Technologies for Urban Safety of Mega Cities in Asia