Author(s) Jeong, Sang-Seom; Kim, Young-Ho; Ki. Symposium on Backwards Problem in G.

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Problem in G Engineering and Monitoring of Geo-C 24 Issue Date 11 URL

1 TitleFailure Case Study of Tiebak Wall Author(s) Jeong, Sang-Seom; Kim, Young-Ho; Ki Proeeding of TC2 Symposium Osaka Citation Symposium on Bakwards Problem in G Engineering and Monitoring of Geo-C 24 Issue Date 11 URL Right Type Artile Textversion publisher Kyoto University

International Symposium on Bakward Problems in Geotehnial Engineering TC2-Osaka 11 Failure Case Study of Tiebak Wall in Urban Area, Korea S. S. Jeong & Y. H. Kim Dep.

2 International Symposium on Bakward Problems in Geotehnial Engineering TC2-Osaka 11 Failure Case Study of Tiebak Wall in Urban Area, Korea S. S. Jeong & Y. H. Kim Dep. of Civil Eng, Yonsei University, Seoul, Korea M. M. Kim (orresponding author) Dep. of Civil and Env. Eng., Seoul National Univ., Seoul, Korea ABSTRACT: In this study, a numerial analysis was performed to reprodue the sequential behavior of an anhored retaining struture in an urban area. The numerial analysis was verified through omparisons between the predition and a field failure ase. The emphasis was plaed on the wall behavior and the loation of the sliding surfae based on the elasto-plasti method and the shear strength redution method. Through the omparison study, it is found that oupled analysis using the shear strength redution method an be effetively used to perform the bak alulation analysis to find a ritial surfae in the anhored wall strutures, whereas unoupled analysis by the elasto-plasti method an be appliable to the preliminary design of a retaining wall with a suitable safety fator. 1 INTRODUCTION South Korea suffers from serious lak of land spae due to its high population of about 47 million people on a little less than 1, km 2 of land and the fat that 7% of the land spae is mountainous. Sine 198, a number of huge exavation projets suh as underground spae for onstruting power stations, subways, high-speed railways, and many lifeline onstrutions have been performed in urban areas. The retaining strutures suh as the slurry wall, C.I.P wall and H-pile are frequently used in those areas as temporary exavation proeeds. The analysis tehnique for analyzing the sequential behavior of anhored retaining wall has been developed for deades (Haliburton, 1968; Clough, 1971; Clough and Tsui, 1974; Briaud and Kim, 1998; Jeong and Seo, 4). Although these methods make slightly different assumptions, they an generally be lassified into two main groups: (1)elasto-plasti method, (2)finite-element method (shear strength redution). The objetive of this study is to suggest the oneptual methodology of anhored retaining wall design by onsidering the oupling effet between soil and struture. Comparisons are made between the soil wall behavior omputed by elasto-plasti, and finite-element analysis whih onsider the mehanism of shear strength redution using the material properties of real failure ase. 2.1 method The elasto-plasti method is based on beam-olumn theory, whih has been studied for many appliations in engineering pratie. Governing equation for the horizontal beam modeling of anhored retaining wall is as follows: 4 d y A E' EI x pi k 4 s dx L x (1) where, E is the elasti modulus of retaining wall; I is the inertia moments of retaining wall; A is the area of bore hole; E is the elasti modulus of bore hole; L is the length of bore hole; p i is the initial earth pressure; k s is the horizontal subgrade reation; x is the depth; y is the horizontal wall defletion. This method is developed for rigid beam with infinite length and is assumed that the soil is rigid and perfetly plasti. Thus, this method may not represent the behavior of atual wall in the field: this method does not take into aount the atual behavior of finite flexible beams, soil arhing and soft soil, et. (Hassiotis, 1997; Cai et. al., ) Shemati drawings of the elasto-plasti modeling for a retaining wall is shown in Figure 1. 2 ANALYTICAL METHODS FOR ANCHORED RETAINING WALL 19

3 International Symposium on Bakward Problems in Geotehnial Engineering TC2-Osaka 11 x spring (k t) Exavation line Passive earth pressure (p p) ed retaining wall Ground surfae line Ative earth pressure (p A ) Initial soil pressure (p i ) Passive earth pressure (p p) Soil spring (k sy ) the system is stable. Then the value of F is inreased by F in values until the retaining wall fails. 2.3 Failure surfae A typial failure surfae used in the elasto-plasti analysis is shown in Figure 3. The failure surfae defined in the shear strength redution as the urve whih passes the elements having maximum shear strain ratio(figure 4). Fig. 1 Shemati drawing of elasto-plasti modeling y 2.2 Finite element method (shear strength redution) The finite element method has been used to investigate the soil-struture system, whih is analyzed as a ontinuous elasti or elasto-plasti medium using finite element formulations. This method provides oupled solutions in whih the wall and slope response are onsidered simultaneously. For retaining wall, the fator of safety F is traditionally defined as the ratio of the atual soil shear strength to the minimum shear strength required to prevent failure. The shear strength redution tehnique was used as early as 197 by Zienkiewiz et al., and has been applied by Naylor (1981), Donald and Giam (1988), and Won (), et. In the shear strength redution method, the shear strength of soil is redued and the fator of safety is realulated until ollapse ours. To alulate the fator of safety of a retaining wall struture defined in the shear strength redution tehnique, a series of stability analyses are performed with the redued shear strength parameters and φ (Figure 2)defined as follows : Exavation surfae Distane to failure surfae Install Angle o / Free Length Safety length 2 Virtual failure surfae Fixed Length Fig. 3 Failure surfae in Rankine s ative zone Failure Surfae Weathered Weak Hard Inreasing shear strain ratio 1 1 tan tan Fs T F s Fig. 2. Relationship between the atual strength and the strength redued by a fator of safety. 1 ' ' (2) F 1 ' artan tan' F (3) where, φ are the real shear strength parameters and F is a fator of safety. Usually, initial F is set to be suffiiently small so as to guarantee that Fig. 4 Failure surfae in finite element method 3 MODELING OF ANCHORED RETAINING WALL 3.1 An example ase of wall failure In this study, an analysis setion was adopted from a onstrution field in urban area where the failure aident took plae. As shown in Figure, the retaining wall onsisted of 18.3m CIP (ast-in plae pile) wall plus 21.7m H-pile, and was braed with nine earth anhors (various free length) and thirteen

4 International Symposium on Bakward Problems in Geotehnial Engineering TC2-Osaka 11 rok bolts downwards. Based on the in-situ and laboratory soil tests, soil profile was onstruted as being omposed of medium to dense sand in the upper 17.m underlain by weathered rok and weak to hard rok. Speially, the frature zone was noted in the hard rok layer at depths below.4m. Sequential exavation proeeded until the wall failure ourred (38m exavation). Based on the onstrution sequene, the exavation stage and the water table loation are summarized in Table m (CIP wall + H-pile) Failure surfae Fill Sand Medium Sand Residual Soil 17.m Weathered 18.m Weak 6.6m 12.8m properties were taken from the assoiated soil properties surrounding the wall using the strength redution fator (R i ) as follows: i Ri soil, tan i Ri tan soil (4) where i and φ i are the ohesion and frition angle of the interfae, soil and φ soil are the ohesion and frition angle of the soil mass. This model was seleted in the element library of PLAXIS (), the ommerial finite element pakage used for this work. Table 2 shows the anhor fores and their dimensions, and Table 3 shows material properties used in numerial analyses. Table 1. Constrution stages for the failure site Exavation Stage - Constrution Retaining wall & H-pile Exavation Water Table (m) (m) Remark m (H-pile) Exavation surfae (38.m) Hard Frature Zone Hard 27.7m.4m 34.9m 1 st nd No rd No th No Fill Sand Fig. Analysis setion of anhored retaining wall 3.2 Numerial simulation To obtain detailed information on the behavior of the anhored retaining wall, a series of numerial analyses were performed for the site of failure. The response of an anhored retaining wall was analyzed in parallel by the elasto-plasti method and by the 2D nonlinear finite-element method. First, the elasto-plasti analysis (SUNNEX, 2) was exeuted whih is based on an iterative and inremental proedure to take into aount the hanges of the earth pressures and wall displaements at eah onstrution stages. Seond, the Finite Element Method modeled the site under the plane strain ondition. The finite-element mesh for a typial ase is shown in Figure 6. The mesh onsisted of 2D six-noded solid triangular elements and was assumed to be resting on a rigid layer. The wall element remained elasti at all times, while the surrounding soil was idealized as a Mohr Coulomb elasto-plasti material. The interfae element was omposed of 1D quadrati three-node elements. The Coulomb s fritional riterion was used to distinguish between elasti behavior, where small displaements an our within the interfae, and plasti interfae behavior (slip). The interfae th No th No. No th No th No th No.8, th No th No th No th No th No th - No th Bolt No th No. No h No th No th No th No th No Medium Sand Residual Soil Weathered Weak Hard Frature Zone Hard 21

International Symposium on Bakward Problems in Geotehnial Engineering TC2-Osaka 11 Fill sand Medium Sand Residual Soil Weathered Soft (free length) (fixed length) Bolt 4.

5 International Symposium on Bakward Problems in Geotehnial Engineering TC2-Osaka 11 Fill sand Medium Sand Residual Soil Weathered Soft (free length) (fixed length) Bolt 4.2 Bending moment and earth pressure Figure 8 and Figure 9 show the predited bending moments and earth pressures mobilized in the retaining wall. The overall distributions are alike for the finite element method and the elasto-plasti method until the middle onstrution stage. As the onstrution stage proeeds to the final stage, the distributions beome dissimilar eah other espeially at the lower part of the exavation Frature zone Hard CIP + H-Pile Wall Fig. 6. Element mesh and boundary onditions 4. COMPARISON OF ELASTO-PLASTIC METHOD WITH FINITE ELEMENT METHOD 4.1. Lateral defletion of retaining wall Figure 7 shows the predited and measured lateral defletion profiles from initial to final onstrution stages. The measured defletion profiles were obtained from field inlinometer data. It is seen in the figure that the finite element Table 3. Material properties and geometries method predits the wall defletion relatively well ompared with those of the elasto-plasti method: 4.3 Failure surfae Figure 1 ompares the measured failure surfae with the failure surfae predited by the after the final exavation stage. From this omparison, it is onfirmed that the shear strength redution method simulates well the field failure surfae. In Figure 11, the possible failure surfaes are shown for the middle stages of exavation. Table 2. fores and properties Earth Fixed Length (m) Free Length (m) Number of Steel Wire Fore (kn) No No No No No No No No Soil Condition Model Type γ t (kn/m 3 ) (kn/m 2 ) φ (deg) E (kn/m 2 ) ν R i Fill Sand M.C Drained 18 No , Medium Sand M.C Drained 18 31,.3 - Residual Soil M.C Drained ,.3 - Weathered M.C Drained 1,9.3 - Soft M.C Drained ,6. - Fature Zone M.C Drained ,7.3 - Hard M.C Drained ,,.2 - CIP wall Beam non-porous x H-pile Beam non-porous - -.6x Reinforements Model Horizontal Spaing (m) Size (mm) Angle (deg) E A (kn) ν Earth anhor (No.1~3) Node-to-Node 2.7 Φ12.7x3 o 19,742.2 Earth anhor (No.4) & 1.8 Φ12.7x3 o 32,968.2 Earth anhor(no.~9) Geogrid 1.8 Φ12.7x4 o 236,223.2 Bolt (No.1~13) Geogrid 1.8 Φ o 272,

6 International Symposium on Bakward Problems in Geotehnial Engineering TC2-Osaka 11 Wall Displaement (mm) Wall Displaement (mm) Wall Displaement (mm) m Exavation Measured 17.41m Exavation Measured 28.24m Exavation (a) 3.79m Exavation (b) 17.41m Exavation () 28.24m Exavation Mesured Fig. 7 Measured and Predited lateral defletion of anhored retaining wall Bending Moment (kn.m) Bending Moment (kn.m) Bending Moment (kn.m) m Exavation 17.41m Exavation 28.24m Exavation (a) 3.79m Exavation (b) 17.41m Exavation () 28.24m Exavation Fig. 8 Bending moment distribution of anhored retaining wall Soil Pressure (kn) Soil Pressure (kn) Soil Pressure (kn) m Exavation 17.41m Exavation 28.24m Exavation (a) 3.79m Exavation (b) 17.41m Exavation () 28.24m Exavation Fig. 9 Earth pressure distribution of anhored retaining wall 23

The finite element method predited well the loation of the field failure surfae, whih is not possible for the elasto-plasti method. Exavation (38.m) Figure 1.

7 International Symposium on Bakward Problems in Geotehnial Engineering TC2-Osaka 11.m 23.m F.E.M. (Shear strength redution) strength redution tehnique predited wall defletions losely ompared with the measured values. However, the result of elasto-plasti method showed large disrepany. 2. The finite element method predited well the loation of the field failure surfae, whih is not possible for the elasto-plasti method. Exavation (38.m) Figure 1. Comparition of failure surfaes (a) 18m Exavation () 31m Exavation Field failure surfae (b) m Exavation (d) 38m Exavation Figure 11 Failure surfaes at the middle stages of exavation. CONCLUSIONS The main objetive of the analysis desribed herein was to ompare the sequential behavior of anhored retaining wall predited by the finite element method and elasto-plasti method with the observed behavior in the field. From the omparisons, the following onlusions are drawn: 3. In overall, it was onfirmed that the finite element method utilizing shear strength redution tehnique an be effetively used to perform the bak alulation analysis of the anhored wall strutures. REFERENCES Briaud JL, Kim NK. (1998). Beam-olumn method for tiebak walls. J. Geoteh. Geoenviron. Eng. ASCE. 124(1) Cai F, Ugai K. (). Numerial analysis of the stability of a slope reinfored with piles. Soils Found., Jpn. Geoteh. So. (1) Clough GW, Dunan JM. (1971). Finite element analyses of retaining wall behavior. J. Geoteh. Eng. ASCE. 97(12), Clough GW, Tsui Y. (1974). Performane of tiebak walls in lay. Pro J. Geoteh. Div ASCE. 12(1), Donald IB, Giam SK. (1988). Appliation of the nodal displaement method to slope stablilty analysis. Proeedings of the th Australia New Zealand onferene on geomehanis, Sydney, Australia Haliburten TA. (1968) Numerial analysis of flexible retaining struture. Pro ASCE. 94(6), Hassiotis S, Chameau JL, Gunaratne M. (1997). Design method forstabilization of slopes with piles. J. Geoteh. Geoenviron. Eng.,ASCE. 123(4) Jeong, S, Seo, D. (4). Analysis of tiebak walls using proposed P-y urves for oupled soil springs. Computers and Geotehnis, Vol. 31, Jeong S, Kim B, Won J, Lee J. (3). Unoupled analysis of stabilizing piles in weathered slopes. Computer & Geotehnis. Vol Korean Geotehnial Soiety. (3). Korea speifiation for substrutures (in Korean), KGS, Seoul. Naylor DJ. (1981). Finite element and slope stability. Num Meth Geomeh. Proeedings of the NATO Advaned Study Institute, Lisbon, Portugal PLAXIS (). Program manual, version 8, PLAXIS In. SUNEX (2). Program manual, 8th edition for Ver w.3, Geogroup In. Won, J, You, G., Jeong, S., Kim, S. (). Coupled effets in stability analysis of pile slope systems. Computer & Geotehnis. Vol Zienkiewiz OC, Humpheson C, Lewis RW. (197). Assoiated and nonassoiated viso-plastiity and plastiity in soil mehanis. Geotehnique. (4) The finite element method utilizing the shear 24

FINITE ELEMENT MODELLING OF STEEL-CONCRETE COMPOSITE BEAMS STRENGTHENED WITH PRESTRESSED CFRP PLATE

FINITE ELEMENT MODELLING OF STEEL-CONCRETE COMPOSITE BEAMS STRENGTHENED WITH PRESTRESSED CFRP PLATE H.Y. Omran 1, P. Zangeneh 2, and R. EL-Haha 3 1 PhD student, Department of Civil Engineering, University