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Centrifugal Study on Effect of Water Content of Clayey Backfill on the Stability of MSEW Huei-Tsyr Chen 1, Wen-Yi Hung 2, Bo-Wen Chen3 and Chung-Jung Lee 1 1 Associate Professor, Department of Civil Engineering, National Central University, Taiwan, ROC 2 Graduate Student, Department of Civil Engineering, National Central University, Taiwan, ROC 3 Former Graduate Student, Department of Civil Engineering, National Central University, Taiwan, ROC ABSTRACTS: Mechanically Stabilized Earth Wall (MSEW) requires the use of high quality granular soil as reinforced backfill materials. However, in many cases the low quality in-situ clayey soil is frequently used instead to cut down the cost, which obviously violates the design assumption. In addition, the clayey soil backfill may block the draining path, resulting in increase in water content during heavy rainfall and thus affecting the stability of MSEW. In this study, centrifugal tests of MSEW with clayey soil as backfill are performed. By varying the water contents of the clayey soil backfill, the effect of water content on the stability of MSEW is investigated and the countermeasure is proposed to maintain the stability at high water content of the backfill. The results indicate the increase in water content of the clayey soil backfill will make MSEW become unstable and the stability can be maintained by using 35% of the reinforcement spacing designed according to Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines (FHWA-NHI-00-043). 1. INTRODUCTION 2. TEST SYSTEM Because of its flexibility and capability to absorb deformations due to poor conditions in the foundations and a higher resistance to seismic loading than rigid concrete structures, many reinforced soil structures have been constructed all over the world. The Mechanically Stabilized Earth Wall (MSEW) is a type of wall which is constructed by introducing reinforcing into the backfill of wall via mechanical means such as metal strips and rods, geotextile strips, grids and sheets, or wire grids. Although it is required to use high quality granular soil as reinforced backfill materials of MSEW (Elias et al, 2001), the low quality in-situ clayey soil is frequently used instead to cut down the cost which obviously violates the design assumptions. However, in-situ clayey soil backfill may block the draining path, leading to increase in the water content in MSEW and then affect the stability of MSEW. There was a report of failures of three MSEW with clayey soil as backfill after heavy rain in Taiwan. (Huang, 1994) In the past years many centrifuge studies (Porbaha and Goodings, 1996; Zornberg, Sitar and Mitchell, 1996; Zhang, Lai, and Xu, 2000) have been performed to investigate the behavior of MSEW with clayey soil as backfill, but none of them studied the effect of water content. Thus, in this study, centrifugal tests of MSEW with clayey soil as backfill are performed. By varying the water contents of the clayey soil backfill, the effect of water content on the stability of MSEW is investigated and the countermeasure is proposed to maintain the stability at high water content of the backfill. The centrifuge model tests were conducted using the 100g-ton centrifuge (Acutronic 665-1) in the Experimental Center of Civil Engineering of National Central University shown in figure 1. The effective radius is 3.0m and the maximum centrifugal acceleration is 200g when the weight of the model is 550 kgf. Because of the setting of control system, the acceleration of centrifuge will increase gradually to 10g automatically after starting. Then, the acceleration can be further increased through the control panel according to the test requirements. The soil container used is a rectangular box made of aluminum plates, which has the internal dimensions of 223 mm wide, 820 mm long and 580mm deep. Figure 1. The centrifuge in the Experimental Center of Civil Engineering of National Central University 3. EXPERIMENTAL PROGRAM dimension is put in an Ng field, it is equivalent to the case of the same MSEW model built of material with a unit weight of N where is the unit weight of the chosen material in 1g condition. In this study we adopted this concept to design the MSEW models. The Guidelines used is the Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines (FHWA-NHI-00-043). Before starting the design process, the properties of geosynthetic reinforcement are determined first. In order to ensure the larger deformation at the designed acceleration level, the geosynthetic reinforcement must be weak enough and water-proof to assure that the mechanical properties will not change both in dry and in wet conditions to accord with the need of this experiment. Shown in figure 3 is the geosynthetic reinforcement used in this study which is manufactured by Seven States Enterprise Company, LTD. It is a kind of composite material which combines geotextile with polymeric grids. It has the limit tension strength of 1.62 kN/m at a limit extended strain of 9.1%. After the strength of the reinforcement had been obtained, the size of the model of MSEW is then chosen considering the size of the container. The ensuing design steps follow the adopted guidelines as stated before except the weight of the clayey backfill being multiplied by N since the model is to be placed in Ng states. In this study the N is taken as 20. Table 1 depicts the dimensions of the design MSEW. 3.1 Preparation of soil sample The soil used in the model is clayey soil of Linkou area, and its fundamental characteristics are specific gravity 2.67, liquid limit 40% and plastic index 22.6. The clayey soil is purged first to ensure its homogeneity and then ventilated to the desired water content in the open air. Figure 2 shows the variation of the water content with days of ventilation. It can be seen that the water content decreases as the day of ventilation becomes longer. Also shown in the figure is an equation derived from regression analysis which is used in this study to predict the days of placement required for the desired water content. However, it should be noted that the actual water content of the model tested is measured before each test. Water content (%) 70 60 50 40 30 y = -4.0057x + 59.575 20 2 R = 0.9479 10 0 0 2 4 6 8 Days Figure 2. Relationship between water content and day of ventilation 3.2 Model construction and test procedures In performing the centrifuge model test, an important step is to derive the scaling law for the type of problem investigated. For the MSEW, one of the important scaling factors that must be determined is that for the strength of reinforcement. Based on the requirement that the model and the prototype must have the same factor of safety against failure, Zonberg et. al. (1996) proposed a reduction factor of 1/N for the MSEW in Ng field where g is the gravitational acceleration, while Lord, Jr. (1987) proposed the factor to be 1/N2. The difference between Zonberg’s and Lord’s result lies in whether the width dimension of the MSEW is taken into consideration when computing the scaling factor. Amid these conflicting results, we have performed several tests to see what the scaling factor should be by using the feedback analysis from the tests with concept of modeling of models. These tests showed that the strength scaling factor is different from those proposed by the previous investigators. Thus, further study is still needed to clarify this issue. On the other hand, Sawicki (1998) considered the centrifugal model as a small model placed in a high g condition. That is, if the MSEW model of selected Figure 3. Reinforcement material Table 1. Geometry characteristics of the models Physical quantity Dimensions Height of the wall 30cm Length of the reinforcement 20cm Spacing of the reinforcement 4cm Thickness of the surcharge soil 2cm The depth of the foundation 15cm Before constructing the MSEW model, the inside vertical boundaries of the container are sprayed with silicon and overlain with a sheet of thin rubber to reduce the boundary friction effects. The foundation 2/6 1.455 82.6 82.4 82.2 3 Dry density(g/cm ) 1.450 1.445 82.0 1.440 81.8 81.6 1.435 81.4 1.430 81.2 81.0 1.425 34 36 38 40 42 44 Average degree of relative compaction (%) of wall model is prepared first by proctor compaction method to ensure the global stability of the wall and adequate bearing capacity. After the foundation is constructed, several boards are piled up vertically on top of the foundation to provide lateral support for MSEW construction. The MSEW is prepared by rolling compaction method. The rolling concrete cylinder used is 210 mm long and has a diameter of 150 mm with weight of 8.452kgf. The compaction is made for a soil layer of 2cm thick each time with concrete cylinder rolling forwards and backwards 5 times to make the degree of relative compaction greater than 80%. The relationship between the dry density and the water content of soil in the models by rolling compaction method is shown in figure 4. This process is repeated until the model wall reaches the designed height. Deformation of the walls during the test is measured using LVDTs whose locations are shown in figure 5. adopting such a scheme is to ensure that stresses between soil and reinforcement are transferred evenly at each stage and the significant behavior of the wall such as the development of crack or sliding can be recorded at the actual acceleration. In the course of increasing acceleration, the deformation of the MSEW is measured by LVDTs. When the acceleration reaches 20g, it is maintained at that level for about 5 minutes until the readings of LVDTs become stable. During tests a CCD camera is also employed to monitor the deformation of the model. When the test is completed, the water content of the soil is measured to see if there is significant change. In addition, the undrained shear strength of the soil is also determined using torvane shear test and uniaxial compression test. 4. TEST RESULTS AND ANALYSIS In this study 10 tests were performed which are classified into two groups. Group 1 is designed to investigate the effect of water content on the stability of MSEW, while the purpose of Group 2 is to study the effectiveness of reducing the reinforcement spacing in maintaining the stability of MSEW at high water content. To facilitate the following discussions, the names of different parts of MSEW used in this paper are defined as shown in figure 6. Crest of the facing Top of the wall Facing MSE wall 46 Water content(%) Figure 4. The relationship between the dry density and the water content of soil by rolling compaction in 5 times. Spacing 100mm 100mm 100mm Foundation Figure 6. Generic cross section of centrifugal model 48mm 48mm 625mm 295mm 200mm 916mm Figure 5. Layout of the model In performing the test, the model is accelerated to 10g at first. Then, it proceeds with an increase in acceleration of 2g for each stage which is maintained for about 30 seconds until 20g. The reason for 4.1 Effect of water content on the stability Table 2 depicts the fundamental characteristics for each test in group 1. In this group the reinforcement spacing for all tests is 4cm; only the water content of clayey soil backfill varies from 28.4% to 53.1%. The one with 28.4% water content will be denoted as the standard model. Test 2 and Test 3 have almost the same characteristics, since they are used for investigating the reproducibility of test. From Table 3, it can be seen that the tests are reproducible. Shown in figures 7, 8 and 9 are the deformations of the model after the tests for clayey soil backfill for water content 28.4%, 43.1% and 53.1%, respectively. For water content of 28.4%, the deformation is a type of forward tilting of wall face. As the water content increases to 43.1%, the deformation becomes a type of bulging in wall face. Finally the deformation changes to overturning when the water content reaches 53.1% and it should be pointed out that this wall actually collapses at an acceleration of 18g. Figure 8 Deformation of MSEW for water content 43%. Table 2. Fundamental characteristics of the models MSE wall Test Water Undrained content shear (%) strength (kPa) Foundation spacing Sv (cm) Water content (%) Undrained shear strength (kPa) 1 28.4 44.5 4 28.9 58.3 2 35.1 33.1 4 28.1 61.6 3 36.1 31.2 4 29.2 56.3 4 43.0 23.8 4 30.0 56.3 5 53.1 11.6 4 31.8 47.7 Figure 9 Figures 10 and 11 depict the settlement and horizontal displacement of the crest of the facing. It can be seen that both the settlement and horizontal displacement increases with increasing water content. Also from these tests, it is observed that the width of tension crack on top of the wall increases with increasing water content. Table 3. The results of repeatability test Test2 Test3 35.1 36.1 33.1 31.2 28.1 29.9 61.6 56.3 4 4 2.6 2.9 1.6 1.8 0.8 0.9 5 4 Settlement(cm) Water content in MSE wall (%) Undrained shear strength in MSE wall (kPa) Water content in foundation (%) Undrained shear strength in foundation (kPa) Reinforcement spacing (cm) Maximum settlements of the crest of the facing (cm) Horizontal displacements of the crest of the facing (cm) Horizontal displacements of the crest of the facing without the distances of sliding (cm) Maximum settlements of the top of the wall (cm) Maximum horizontal displacements of the facing (cm) Maximum horizontal displacements of the facing without the distances of sliding (cm) Deformation of MSEW for water content 53.1% 3 2 1 0 2.6 1.6 2.9 1.8 0.8 0.9 0 20 40 60 Water content(%) Figure 10. Settlement of the crest of the facing for different water contents Displacement (cm) 5 4 3 2 1 0 Figure 7 0 Deformation of MSEW for water content 28.4% 20 40 60 Water content (%) Figure 11 Horizontal displacement of the crest of the facing for different water contents 4/6 Table 4. Fundamental characteristics of the models Measurements Water content in MSE wall (%) Reinforcement spacing (cm) Test5 Test6 53.1 52.6 4 2 Settlements of the crest of the facing (cm) 2.8 Horizontal displacements of the crest of Collapse the facing (cm) at 18g Maximum horizontal displacements of the facing (cm) Maximum settlements of the top of the wall (cm) Settlements of LVDT 1 (cm) 2.5 Maximum settlements of LVDT (cm) 5.2 soil backfill of 43% water content behaves almost the same as the standard model. Table 5. Fundamental characteristics of the Group 2 models Test 7 8 9 10 4 MSE wall Foundation Water Undrained Reinforcement Water Undrained content shear spacing content shear (%) strength Sv (cm) (%) strength (kPa) (kPa) 43.1 22.7 1.5 30.1 59.5 42.4 25.1 2 31.2 57.8 44.0 20.9 2.5 28.0 61.6 42.8 24.2 3 31.9 47.4 43.0 23.8 4 31.8 52.0 5 Settlement (cm) 4.2 Improvement by varying spacing on the stability As stated in previous section, the MSEW with clayey soil backfill of water content 53.1% (Test 5) collapsed at 18g which is 2g less than the desired acceleration level. In order to see if such a situation can be improved by reducing reinforcement spacing, Test 6 is then constructed under the same conditions as Test 5 except that the reinforcement spacing is reduced to 2cm. From Table 4 and figure 12, it can be seen that although lager deformation can be observed for the MSEW of Test 6, it does not collapse as Test 5. This indicates the reduction of reinforcement spacing can effectively improve the stability of MSEW at high water content. However, in reality the water content may not be as this high; therefore, a second group of test was designed with clayey soil water content of 43% and the reinforcement spacing varying from 4cm to 1.5cm as described in Table 5. 3 2 1 0 0 1 3.7 2 3 4 5 Spacing (cm) 4.5 Figure 13. Settlements of the crest of the facing for different reinforcement spacing 2.7 Displacement (cm) (a) Test 5 4 (b) Test 6 4 3.5 3 2.5 2 1.5 1 0.5 0 0 1 2 3 4 5 Spacing (cm) Figure 12. Deformation of Wall: (a) test5 and (b) test6. Figures 13 and 14 show the settlement and horizontal displacement of the crest of the facing for the reinforcement spacing considered. On these two figures, the horizontal arrows indicate the deformation for the models having 28% water content and 4 cm spacing (standard model), while the vertical arrows indicates the reinforcement spacing required for the MSEW model with 43% water content to achieve the same deformation as that for standard model. It can be seen that using 35% of the reinforcement spacing as determined from the guideline, the MSEW with clayey Figure 14. Horizontal displacements of the crest of the facing for different reinforcement spacing 5. CONCLUSIONS In this study, centrifugal tests of MSEW with clayey soil as backfill were performed. The effect of water content of backfill on the stability of MSEW was investigated and the effectiveness of reducing reinforcement spacing in maintaining the stability of MSEW at high water content was also studied. From this study, the following conclusions can be drawn. (1) The increase in water content of the clayey soil backfill will lead to decrease in stability of MSEW. (2) If the low quality in-situ clayey soil is used as reinforced backfill materials, the reinforcement spacing should be reduced in order to maintain the stability of MSEW at high water content. In this study the use of 35% of the reinforcement spacing determined from the guidelines is proposed for clayey soil of Linkou area. 6. ACKNOWLEDGEMENTS The writers appreciate some of the fundamental issues regarding this study raised by Dr. Nelson N. S. Chou, the general manager of Genesis Group/Taiwan and would like to express our sincerest thanks to Mr. C. H. Wang, the general manager of Seven States Enterprise Company, LTD. for his generosity in providing the reinforcement materials used in this study. We are also grateful for the help from members of centrifuge modeling group of the Experimental Center of Civil Engineering of National Central University. REFERENCES Elias, V., Christopher, B.R., and Berg, R.R. (2001): Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines, U.S., Department of Transportation Federal Highway Administration, Publication No. FHWA-NHI-00-043. Huang, C.C. (1994): Report on Three Unsuccessful Reinforced Walls, Proceedings of Recent Case Histories of Permanent Geosynthetic-Reinforced Soil Retaining Walls, pp.219-222. Lord Jr., A.E. (1987 ): Geosynthetic/soil studies using a geotechnical centrifuge, Geotextiles and Geomembranes, Vol. 6, pp.133-156. Porbaha, A., and Goodings, D. J. (1996): Centrifuge modeling of geotextile - reinforced cohesive soil retaining walls, Journal of Geotechnical Engineering, pp.840~848. Sawicki, A. (1998): Theoretical analysis of centrifugal model tests on reinforced earth structures, Geotechnique, Vol. 48, No. 4, p.p. 563-567. Zornberg, J.G., Sitar, N., Mitchell, J.K. (1996): Performance of Geosynthetic Reinforced Slopes at Failure, Journal of Geotechnical and Geoenvironmental Engineering, Vol.124, No.8, pp.670-683. Zhang, W., Lai, Z., and Xu, G. (2000): Centrifuge model tests of geosynthetics reinforced retaining walls, China Civil Engineering Journal, Vol.33, No.3, pp.84-91. 6/6