Finite Element Analysis of Lattice-Shaped Ground Improvement by Cement- ... Figure 2: Flow Chart for Design of Deep Mixing Method for Liquefaction Mitigation .... soil cement walls at this site consist of 55cm diameter column reinforced with.
Liquefaction Mitigation by Deep Soil Mixing Method (DSM) ECI 284 Winter 2010: Theoretical Geomechanics Professor: Boris Jeremic Term Project by Thang Van Nguyen
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Liquefaction Mitigation by Deep Mixing Method (DMM) I)
INTRODUCTION
Site improvement by Deep Mixing Method (DMM) has been widely adopted in many projects in Japan. This liquefaction mitigation method proves to be quite effective in stabilizing potential liquefiable soil. Commonly, grid line of stabilized soil/cement mixture will be constructed at sites which are sensitive to strong ground motion. The grid pattern soil/cement mixture act as confined shear-box which can provide additional shear strength for site to withstand strong ground motion. This paper will present the literature review of current procedure for modeling the dynamic response of DMM at liquefiable sites, a summary of results of series centrifuge model tests, and selective case studies. Developing a frame work for standardizing design procedure for liquefaction mitigation by DMM method and implement it into commercial graphic user interface (GUI) design software will be part of my future work. II) PAPERS REVIEWED • Deep Mixing Technology For Liquefaction Mitigation • Reduction of liquefaction hazards by deep soil mixing, T.D. O’Rourke, S.H. Goh • Finite Element Analysis of Lattice-Shaped Ground Improvement by CementMixing For Liquefaction Mitigation. 1) Deep Mixing Technology For Liquefaction Mitigation by Ali Porbaha, Kouki Zen, and Masaki Kobayashi, 1999 This paper provides a comprehensive review of important development deep mixing technology, research in progress, and areas need of further study. Selected key points of this paper will be reported in this paper. Deep mixing method is grouped together with grouting method, premixing method, and quicklime pile method in the solidification technique according to Tanaka (Tanaka et al. 1991). It is an in-situ soil stabilization technique based on the solidification principle. The in-situ soil will be mixed with cementicious reagent, most commonly cement. Injected as a slurry to improve the
3 engineering characteristics of the liquefiable ground. The loose soil at the bottom of the structure is replaced by underground solid grid-type walls which aim to restrain shear deformation below the structure. The most common configuration used for liquefaction mitigation is the lattice pattern.
Figure 1: Deep Mixing Lattice Pattern The improved ground is modeled as an underground structure having much higher stiffness than the surrounding soil. Thus, only the unimproved surrounding soils are prone to liquefaction under earthquake motions. Forces acting on the underground rigid body are: (1) the body and inertia forces, (2) reaction of liquefiable soils and water pressure, and base reaction; and (3) surcharge. The dynamic water pressure is estimated using Westergaard’s formula. It is worth to mention that the interaction of improved ground with surrounding soil during earthquake is not completely understood. Therefore, there are many research are in progress. Most of current understandings of the seismic behavior of improved ground by deep mixing method and the effect of dynamic earth pressure have been the result of physical tests such as: shaking table, centrifuge model complimented by numerical simulations. Matsuo et al. (1996) gave a general outline of deep mixing design to prevent liquefaction-induced damage
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Figure 2: Flow Chart for Design of Deep Mixing Method for Liquefaction Mitigation (Matsuo et al. 1996)
2) Reduction of liquefaction hazards by deep soil mixing, T.D. O’Rourke, S.H. Goh This paper presents a simplified analytical modeling method to the earthquake performance of DSM grids. The base of this simplified approach is the estimation of cyclic shear strain in DSM-reinforced ground and correlation of these strains to excess pore water pressure. This approach look at 2-D models of DSM panels oriented both parallel and perpendicular to the direction of induced ground motion to account for 3-D
5 grid distortion. Although this simplified model is conservative, it helps in maintaining computational efficiency. Plane Strain Analysis As O’Rourke pointed out in his paper, it is necessary to transform the compartmentalized soil-wall into an approximate equivalent homogeneous in order to successfully perform a plane strain analysis. Figure 3, 4, and 5 show us the step-by-step of the transformation procedure:
Figure 3: Original Soil-Wall System (O’Rourke et al. 1997)
Figure 4: Original Soil-Wall System (O’Rourke et al. 1997)
Gt
Figure 5: Steps in obtaning transformed section (O’Rourke et al. 1997) A representative 2-D unit of the soil-wall mass is selected and shown in Figure 5(i). Using the method of transformed sections, this representative soil mass can be converted into an equivalent wall of thickness (Gs/Gw)b. Further transformation
6 The following relationship illustrates the transformation:
Gw A' w = Gt Aunit A' w = [t + (Gs / Gw )b]a Aunit = [t + b]a Hence, Gt =
G w A' w G w [t + (G s / G w )b] = Aunit [t + b ]
One drawback of this analysis which O’Rourke acknowledged in his paper is that this transformation would result in a dominating shear deformation. Plate Analysis: The flexural component of total deformation is computed by plate bending analysis method. This analysis is performed with a finite element code based on Mindlin plate formulation:
u = zθ y
εx = z
v = zθ x
ε y = −z
∂θ y ∂x ∂θ x ∂y
∂θ ∂θ γ xy = z y − x ∂x ∂y
γ yz =
∂w −θx ∂y
γ zx =
∂w −θ y ∂x
The DSM soil-wall unit is modeled as a plate with both vertical edges and bottom edges are fixed the top edge is unrestrained. The applied pressure is obtained from the plane strain analysis above. However, in doing so this model does not take into account the variation of horizontal soil stresses. More advanced methods of analysis are needed to obtain realistic models of DSM grids wall. Total deformation We first combine the displacements along the centerline of the wall obtained from plate analysis and from the plane strain analysis generate resultant profiles. Empirical relations were used to correlate these resultant profiles the maximum and average shear strains along the depth of the wall. These strain values can further be correlated
7 to the buildup of pore pressure. Several trial analyses are required to assure compatible soil modulus. 3) Finite Element Analysis of Lattice-Shaped Ground Improvement by Cement-Mixing For Liquefaction Mitigation by Tsutomu Namikawa, Junichi Koseki, and Yoshio Suzuki, 2007 This is one of the most comprehensive papers reporting the current advancement in numerical simulation of lattice-shaped DSM grid. The improved soil grid serves dual functions. It prevents the unimproved sand deposits that are bounded by the soil grids from liquefaction by impeding their shear deformation during an earthquake. In addition, the DSM wall should also resist the inertial force of the unimproved soil mass surrounded by the improved soil grids and the dynamic earth pressure exerted from the original soil located on the outside of the improved zone. A schematic of external forces acting on the improved soil grids during an earthquake is shown below
Figure 6: External forces applied to improved soil grids during an earthquake Finite element tests were conducted using dynamic effective stress analysis code, and the results were compared with a case history: The Oriental Hotel Kobe during Kobe Earthquake 1995. Two phase formulation (u-U) formulation was used to compute the mitigation of excess pore water pressure. Eight node isoparametric elements were selected for FE. Iterative initial stress method was used incrementally for solving the equilibrium equations. Two models, an elastic model and an elasto-plastic model, were used for assessing the behavior of the DSM wall. The elasto-plastic model was purposely used to capture the behavior of the cement-treated soil under a general 3-D stress state which was not done in simplified method described by O’Rourke. The advantage of this
8 model is that it is capable in both capturing tensile and shear-strain softening response in localized zones after peak stress state. The behavior of the saturated sand layer was described by densification model based on Mohr Coulomb’s yield criterion (Shiomi at al. 1993). All of the soil parameters used in this model were calibrated for the prescribed liquefaction resistance ru at 20 cycles. Each node is associated with an appropriate modal damping factor. The modal damping factor of the Rayleigh damper, hr is given by the following
hr =
1 α + βωr 2 ωr
The stress strain relationships of the improved soil were confirmed by mean of simulating the triaxial compression, triaxial extension, plane strain compression, and bending tests on cement-treated sand samples.
Figure 7: Unconfined stress strain relationship of elasto-plastic model for improved soil
Finite Element Analysis results suggest that treatment area ration is the most effective parameter followed by the elastic modulus of the improved soil in liquefaction mitigation. In addition, even under partial failure of the improved DSM girds, the
9 displacement of the unimproved sand deposit bounded by the DSM grids does not increase significantly. III) CASE HISTORIES 1) Nishinomiya Conference Center Soil Profile
Figure 8: Subsurface Profile of Nishiomiya Conference Center
DSM columns were installed directly under the mat foundation as shown in figure 9. Spacing between the total 68 DSM columns is between 1m and 1.3m. The treatment ratio is estimated to be 10%. . This site is located roughly 33 km from the epicenter of the Hyokogen Nanbu Earthquake (1995). The closest distance to the surface projection of the fault rupture zone is approximately 10 km. Peak ground acceleration is in the order of 0.31g recorded by various recording stations.
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Figure 9: Plan view of the improved foundation Sand boils and 50 cm ground settlement were recorded in the adjacent unimproved areas. Slightly differential settlement was observed at the building. 2) Shimagami Pumping Station Near Hyogo Wharf
Figure 10: Subsurface profile, shear wave velocity, and fine content (Shimagami Station)
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According to the subsurface profile, loose fill and sandy soil were encountered to the depth of 9m, underlain by 3m of silty clay and sandy soil grading from loose to dense with depth. Low average SPT (N1)60 implies that this site is prone to liquefaction. The soil-cement walls were originally intended for excavation support for the basement, but remain in place following building construction. The 45cm wide soil cement walls at this site consist of 55cm diameter column reinforced with steel H-piles and were set in an external square around the perimeter with the offset of 2m. The treatment ratio was estimated at approximately 5%. Figure 11 below shows the foundation plan layout.
Figure 11: Foundation Plan Lay Out (Shimagami Pump Station)
12 This site is located approximately 17km from the epicenter of the Hyokogen Nanbu earthquake. The closest distance to the surface projection of the fault rupture zone is approximately 2km. Recorded PGA is in the order of 0.35g. The building performed well during the earthquake. No piles were damaged; no building damage and no settlement was documented. However, the entire site translated toward the sea due to upward movement of the sea wall. The upper part of the soil cement wall on the shore site buckled and displace 24cm toward the shore with 3% tilt. The backfill soil between the soil cement wall and the building on the sea side subsided uniformly by 1.1 m to 1.5 m. On the other side, away from the sea wall the backfill soil settled about 10 to 20 cm. Nevertheless, Hamada and Wakamatsu (1997) reported that the soil cement wall was influential in preventing damage to the foundation. Boulanger (1997) point out that on the south side of the building, the 11 m deep basement has no liquefiable beneath it. The presence of the basement and piles contribute dominantly to the good performance of this site. Still, the soil cement wall partially adds another level of assurance to the performance of this essential structure. IV)
CONCLUSION
The papers gave a good understanding of current development of liquefaction-induced damage prevention by deep mixing method. Simplified analytical method, numerical finite element analysis method supplemented by physical tests such as shaking table and centrifuge test. However, the interaction of stabilized ground with surrounding soil is not completely understood. For the case in which generated excess pore pressure is less than the effective stress, the earth pressure can only be estimated; this might hinder the reliability of the analysis. Moreover, failure mode due to cracking has not been considered exclusively in many analytical analyses. On top of that, many available finite element analysis software are not very user-friendly. Looking at failure mode due to cracking, and developing a more user-friendly finite element analysis software will be part of my future work. Data from the Hyokogen Nanbu earthquake and centrifuge data supplied by Dr. Kitazume can be applied in my future research.
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V)
REFERENCES
Hamada, M. and Wakamatsu, K (1997). “ Liquefaction, ground deformation and their caused damage to structures, “ The 1995 Hyogoken-Nabu Earthquake, Japan Society of Civil Engineers, Tokyo, Japan, June, pp. 45-92 Matsuo, O. and Shimazu, T., Goto, Y., Suzuki, Y., Okumura, R., and Kuwabara, M. (1996). “Deep Mixing Method as a liquefaction prevention measure.” Proc., 2nd Int. Symp. on ground improvement geosystems, Tokyo, 521-526. Namikawa, T. Koseki, J. and Suzuki, Y. (2007). “Finite Element Analysis of Lattice – Shaped Ground Improvement by Cement-Mixing for Liquefaction Mitigation”, Soils and Foundations, Japanese Geotechnical Society, Tokyo, June, pp 559-576. O'Rourke, T.D. and Goh, S.H. (1997), "Reduction of Liquefaction Hazards by Deep Soil Mixing", Proceedings of the NCEER-INCEDE Workshop, March 10-11, Buffalo, New York. Porbaha, A., Zen, K. and Kobayashy, M. (1999). “ Deep Mixing Technology for Liquefaction Mitigation.” Journal of Infrastructure System, ASCE, Reston, Va p21-34. Shiomi, T., Shigeno, Y. and Zienkiewicz, O. C. (1993), “Numerical prediction for model No.1.” Verification of Numerical Procedures for the Analysis of Soil Liquefaction Problems (eds. by Arulanandan & Scott), Balkema, 213-219.
Tanaka, Y., Nakajima, Y. and Tsuboi, H. (1991). “Liquefaction control works.” Symp. on control of soil liquefaction, Japanese Soc. of Soil Mech. and Found. Engrg, Tokyo 33-38 (Japanese).
