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Static structural behavior of geogrid reinforced soil retaining walls with a deformation buffer zone
Geotextiles and Geomembranes xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem
Static structural behavior of geogrid reinforced soil retaining walls with a deformation buffer zone He Wang, Guangqing Yang∗, Zhijie Wang, Weichao Liu School of Civil Engineering, Shijiazhuang Tiedao University, Shijiazhuang, Hebei, 050043, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Geosynthetics Geogrid reinforced soil retaining wall Deformation buffer zone Structural behavior Model tests
To understand the structural behavior of geogrid reinforced soil retaining walls (GRSW) with a deformation buffer zone (DBZ) under static loads, the model tests and the numerical simulations were conducted to obtain the wall face horizontal displacement, vertical and horizontal soil pressures, and geogrid strains. Results showed that compared with the common GRSW, the horizontal displacement of GRSW with DBZ decreased, and the horizontal soil pressure acting on the face panel of GRSW with DBZ increased. The vertical and horizontal soil pressures showed a nonlinear distribution along the reinforcement length, and the value was smaller near the face panel. The horizontal soil pressure acting on the face panel of GRSW with DBZ was greater than that of the common GRSW in the middle portion. The cumulative strain of the geogrid had a single-peak distribution along its length; the maximum strain of the geogrid was 0.45%, the maximum tension was approximately 29.12% of ultimate tensile strength.
1. Introduction Geosynthetic reinforced soil retaining walls are increasingly applied in highway and railway construction because of their high strength, small deformation, improved earthquake resistance, and deformation accommodation. They have some superiorities in saving the land and environmental protection. The reinforced soil retaining wall can adopt fabricated construction to speed up the construction pace. The advantages above can result in great economic and social benefits. In recent decades, many laboratory and field tests have analyzed the deformation and stress characteristics of reinforced soil retaining walls with different types of face and under different surcharge loads. The post-construction displacement of the reinforced soil wall is about 0.3%–1% H (Latha and Manju, 2016; Yang et al., 2012, 2014; Zevgolis, 2018). Bathurst et al. (2010) compared the measured value with the recommended value by different design specifications and observed that the FHWA and AASHTO design specifications gave more rational limits. The coefficient of lateral earth pressure acting on face panel is 0.6–1.3 and can be divided into three zones, the earth pressure coefficient decreases with an increasing reinforcement ratio (Jacobs et al., 2016; Udomchai et al., 2017). The distribution of the earth pressure for geobag retaining walls is within the range of Rankine's and Coulomb's earth pressure after construction (Shin et al., 2017). The evolution of
reinforcement strain and failure surface is also revealed (Balakrishnan and Viswanadham, 2016; Krystyna, 2005; Xue et al., 2014; Xiao et al., 2016). And the deformation modes are also different among reinforced soil retaining walls (Correia et al., 2012; Liu et al., 2011; Liu, 2012; Ren et al., 2018; Wang et al., 2014, 2016; Yu and Bathurst, 2017), which the maximum cumulative horizontal displacement occurred at different portions. Numerical analysis is also important for understanding the characteristics of reinforced soil retaining walls and has elucidated the distributions and controlling factors for the stability of reinforced soil retaining walls. Reinforcement stiffness and length were identified as influential parameters affecting the horizontal movement at a specific MSE wall height, and the maximum displacement and tensile load in the reinforcement increased with an increase in the reinforcement spacing (Kibria et al., 2014; Ling and Leshchinsky, 2003; Marián et al., 2016). The currently available design guideline tends to over-estimate the surcharge load-induced reinforcement forces, and the surcharge load-induced reinforcement strains exponentially decrease with depth (Yoo and Kim, 2008). Some studies provide detail on how material properties are selected and how computer modeling is carried out (Purkar and Kute, 2015; Rabie, 2016; Yu et al., 2016). Barani et al. (2018) presented a new approach for back analysis of a geogrid reinforced soil wall failure.
∗ Corresponding author. School of Civil Engineering, Shijiazhuang Tiedao University, No. 17 North 2nd-Ring East Road, Shijiazhuang, Hebei Province, 050043, China. E-mail addresses: [email protected] (H. Wang), [email protected] (G. Yang), [email protected] (Z. Wang), [email protected] (W. Liu).
https://doi.org/10.1016/j.geotexmem.2019.12.008
0266-1144/ © 2019 Elsevier Ltd. All rights reserved. This is an open access article under the#lictext# license (http://creativecommons.org/licenses/#lictextcc#/#licvalue#/).
Please cite this article as: He Wang, et al., Geotextiles and Geomembranes, https://doi.org/10.1016/j.geotexmem.2019.12.008
Geotextiles and Geomembranes xxx (xxxx) xxx–xxx
H. Wang, et al.
Table 1 Major physical characteristics of wall filling. Items
Indexes
Name Model Natural unit weight (kN/m3) Saturated unit weight (kN/m3) Cohesion (kPa) Internal friction angle (°) Maximum dry unit weight (kN/m3) Elasticity modulus (MPa) Poisson ratio
Sand Mohr-Coulomb 17.4 20.0 0 36.7 1.789 12.00 0.30
Table 2 Major physical characteristics of geogrids. Items
Indexes
Model Tensile strength (kN/m) Tensile strength @ 2% strain (kN/m) Tensile strength @ 5% strain (kN/m) Peak strain (%) Axial stiffness @ 2% strain (kN/m)
Elastic 35.0 18.48 34.59 5.1 924.0
Table 3 Filling requirements. Items
Index
Coefficient of loose layer Thickness of loose layer (cm) Compaction thickness (cm) Compaction times Compaction degree (%)
1.33 20.0 15.0 4.0 ≥85
procedure of GRSW with DBZ. Fig. 1. Structure plot of GRSW with DBZ: (a) overall structure; (b) detail of reinforcement connection.
Step 1: Place the face footing and fix it. Lay the ground soil and compact it. Step 2: Lay the 1st reinforcement layer and the 1st block layer. Connect the two pieces of reinforcement using a connecting bar and reserve the requested length reinforcement for wrapping. Tighten the reinforcement and fix it on the ground soil using the soil nail. Step 3: Lay the soil and the soil bags on the 1st reinforcement and compact it. Wrap the soil bag using the reinforcement. Step 4: Lay the 2nd reinforcement layer according to step 2 and connect the wrapped reinforcement to the 2nd layer. Step 5: Repeat the above steps to the top of the wall.
Research into the evolution of wall structural behavior has aimed to control the wall's deformation and guarantee the wall's strength, providing important information for the design and construction of reinforced soil retaining walls. Deformation control is critical for flexible reinforced soil retaining walls. In conventional reinforced soil retaining walls, the reinforced soil is in contact with the panel. Lateral soil pressure acts on the panel directly, which results in the panel horizontal displacement. If the horizontal displacement is large, it is negative for structural stability and great settlement of the retaining wall will occur, impacting its practical applications. However, a deformation buffer zone can be installed between the reinforced soil and the panel to relieve horizontal displacement. The deformation buffer zone is also reinforced soil. The model structure is shown in Fig. 1. Model tests of GRSW with DBZ have been performed and the aim is to evaluate the stress and deformation characteristics during construction and under top loading after construction.
2.2. Model size and monitoring instrument arrangement As is shown in Fig. 2, the model of GRSW with DBZ was 3.0 m long, 1.0 m wide, and 1.8 m high, and the slope ratio of the wall face was 1:0.05. To reduce the boundary effect, the model box wall was painted with lubricant to reduce friction. The model box was enhanced and no deformation occurred on the box wall during the test, which ensured plane strain conditions. The vertical spacing was 0.3 m between the lower five layers and 0.15 m between the 5th and 6th layers in order to increase the shearing resistance to top loading. The laid length of the reinforcement was 2.0 m. A steel plate (0.7 m long, 0.5 m wide, and 0.03 m thick) was used to bear the surcharge loads. The wall panel was built with concrete blocks. A DBZ (0.3 m width) was placed between the panel and the reinforced soil, and was built using reinforced soil. The connection between the reinforcements and the panel is shown in Fig. 1 (b). The panel foundation was restricted. The arrangement of the monitoring instruments is shown in Fig. 2.
2. Experimental study and numerical modeling 2.1. Wall filling and reinforcement characteristics The filling of the retaining wall was sand, whose main characteristics are listed in Table 1 according to the direct shear tests. The reinforcement in DBZ and reinforced soil was high density polyethylene (HDPE) uniaxial geogrids, whose main characteristics are listed in Table 2. The controlling indexes of compaction are listed in Table 3 according to the compaction tests. Following was the installation 2
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Table 4 Model parameters. Items
Filling in soil bags
Wall face and face footing
Name Model Natural unit weight (kN/m3) Saturated unit weight (kN/m3) Cohesion (kPa) Internal friction angle (°) Elasticity modulus (MPa) Poisson ratio
Gravel soil Mohr-Coulomb 20.60 22.00 0 37.00 25.00 0.25
Concrete Linear Elastic 23.00 25.00 – – 3.0 × 104 0.20
Fig. 2. Arrangement of monitoring instruments.
2.3. Experimental procedure Step 1: Fill and compact each respective layer. Measure the horizontal static soil pressure, the vertical static soil pressure, and the reinforcement strain after filling each layer. Step 2: Let the retaining wall rest for a time after construction. Measure the stress and deformation without any surcharge load until constant values are obtained. Step 3: Apply local uniform loads to the top of the wall in six levels: 10 kPa, 20 kPa, 30 kPa, 40 kPa, 50 kPa, and 60 kPa. During loading, measure and record the data every 30 min. When the difference between two neighboring experimental data is zero, end the loading progress of one level. Then, conduct the loading of subsequent levels until all levels are completed.
Fig. 4. Distribution of cumulative horizontal displacement along the wall height.
3. Experimental results and analysis under static loads 3.1. Deformation characteristics of the wall face The measured distribution of cumulative horizontal displacement of the wall face along the entire wall height is shown in Fig. 4. The maximum cumulative horizontal displacement was 1.13 mm, which was 0.063% of the wall height. At lower loads (p ≤ 30 kPa), the cumulative horizontal displacement of the wall face increased with wall height and the maximum displacement occurred at the top of the wall. However, at higher loads (p > 30 kPa), the maximum cumulative horizontal displacement occurred in 3H/4. The GRSW with DBZ and the common GRSW horizontal displacement were calculated through the numerical simulation with different loads (p = 30 kPa, p = 60 kPa, p = 90 kPa, p = 120 kPa, p = 150 kPa, and p = 180 kPa) and different acting positions (d = 0.4 m, d = 0.8 m, and d = 1.2 m). The parameter d is the distance from the wall face to the load edge, as shown in Fig. 2. The distribution of horizontal displacement along the wall height, which is represented when d = 1.2 m, is shown in Fig. 5. And the comparison of the maximum horizontal displacement is listed in Table 5. The horizontal displacement of the common GRSW was always greater than that of GRSW with DBZ regardless of the load position. And the difference was getting greater as d was getting smaller when the load was located in the effect scope of DBZ (d = 0.8 m and d = 1.2 m). But when the load was located outside the effect scope of DBZ (d = 0.4 m), the difference was very small due to the weaker effect of DBZ. From the data listed in Table 5, when the load was constant, the horizontal displacement increased gradually with the load position approaching the wall face. For GRSW with DBZ, the increase was smaller from d = 1.2 m to d = 0.8 m, when the load was located in the effect scope of DBZ. However, the increase was greater from d = 0.8 m to d = 0.4 m, when moving the load position outside the effect scope of DBZ gradually. For the common GRSW, with the load position approaching the wall face, the horizontal displacement trend was just
2.4. Numerical modeling Conduct numerical simulation using a finite element program to compare the deformation and stress of GRSW with DBZ with that of the common GRSW. The numerical model of GRSW with DBZ is shown in Fig. 3. The common GRSW has not the soil bags and dose not form the DBZ. The parameters of numerical models are listed in Table 1, Table 2, and Table 4. Fig. 4 compares the measured face horizontal displacement with the simulated face horizontal displacement of GRSW with DBZ under loading p = 30 kPa and p = 60 kPa. The two results were quite close, indicating that the parameters chosen were reasonable.
Fig. 3. Numerical model of GRSW with DBZ. 3
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Fig. 5. Distribution of GRSW with DBZ and common GRSW horizontal displacement (d = 1.2 m). Table 5 Maximum horizontal displacement of GRSW with DBZ and the common GRSW. Load location
Load (kPa)
Maximum horizontal displacement (mm) GRSW with DBZ
Common GRSW
Increase rate (%)
d = 0.4
30 60 90 120 150 180
1.52 3.05 4.84 6.52 8.66 10.67
1.51 3.20 5.10 6.87 9.08 10.99
−0.66 4.92 5.37 5.37 4.85 3.00
d = 0.8
30 60 90 120 150 180
0.84 2.01 3.37 4.78 6.61 8.34
0.95 2.35 4.15 6.29 8.95 11.83
13.10 16.92 23.15 31.59 35.40 41.85
d = 1.2
30 60 90 120 150 180
0.43 1.12 2.58 4.16 6.19 8.40
0.42 1.29 2.77 5.01 7.36 10.08
−2.33 15.18 7.36 20.43 18.90 20.00
Fig. 6. Development of vertical soil pressure: (a) h = 0.3 m; (b) comparison of different layers.
some stress, the horizontal soil pressure applied to the panel was very small. The maximum horizontal soil pressure applied to the panel was 6.0 kPa and occurred in the bottom soil. Fig. 8 compares the horizontal soil pressure acting on the face of GRSW with DBZ with that of the common GRSW. The horizontal soil pressure acting on the top portion was smaller than 1.0 kPa and had little difference between the two walls. The horizontal soil pressure acting on the middle portion (ranging from H/4 to 5H/6) of the common GRSW was smaller than that of GRSW with DBZ, which was because the common GRSW occurred the greater horizontal displacement than GRSW with DBZ to release more stress. While the horizontal soil pressure acting on the bottom portion also had little difference between the two walls.
opposite compared with that of GRSW with DBZ.
3.2. Characteristics of vertical and horizontal soil pressure Fig. 6 and Fig. 7 show the development of vertical and horizontal soil pressures. The development of soil pressures on one layer is represented by that where h = 0.3 m. The parameter h is the wall height. The soil pressure comparison of different layers is represented by that on the center line of the load. The distribution of vertical and horizontal soil pressures along the reinforcement length during construction and loading phases was nonlinear. During construction, differences among the soil pressures within each layer were small. After applying the load, the soil pressure near the loading position increased rapidly whilst that far from the loading position increased more slowly because of the diffusion and attenuation of additional stresses. The development of vertical and horizontal soil pressures presented a linear increase during construction and loading phases, and an almost constant soil pressure during the resting phase. The ratio of the soil pressure increase was almost equal at every monitoring point during construction, and clear differences were observed during the loading phases. During the loading period, the increase of horizontal soil pressure in the top and bottom portions was greater than that in the middle portion. Because of wall face horizontal deformation leading to release
3.3. Characteristics of geogrid strain Fig. 9 (a) shows the development of the geogrid cumulative strain during construction and loading periods, which is represented by that where h = 0.3 m. Fig. 9 (b) shows the distribution of the geogrid cumulative strain along the reinforcement length. The predominantly single-peak distribution of strain was caused by the shearing action generating near the potential failure surface. Moreover, the strain gradually decreased with approaching the end of the reinforcement, which resulted in a gradual stiffness transformation from reinforced areas to non-reinforced areas in order to avoid large differential 4
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Fig. 9. Distribution of geogrid cumulative strain along the reinforcement length: (a) h = 0.3 m; (b) comparison of different layers.
Fig. 7. Development of horizontal soil pressure: (a) h = 0.3 m; (b) comparison of different layers.
transmission path of stress and deformation. Thus the horizontal displacement decreased, and the force on the face panel was more uniform to avoid a greater tilt angle. In this model test, the filling of DBZ was sand, the same as reinforced soil. In actual engineering, the DBZ can be made up of the coarse aggregate to play a drainage function, which also can enhance the reinforcing effect of DBZ. About the GRSW with DBZ, this experiment is an initial study and focuses on the structural behavior. There are some other aspects that will be studied by following model tests and numerical simulations in the future, as the comparison between GRSW with DBZ and other types GRSW on the structural behavior, economic benefits, the structural behavior of GRSW with DBZ under dynamic loads, the most suitable width of DBZ, etc. 5. Conclusions
Fig. 8. Comparison of horizontal soil pressure acting on the face panel between GRSW with DBZ and common GRSW.
Based on the results of model tests and numerical simulations, a GRSW with a DBZ can meet the strength and the deformation requirements under static loads. The following conclusions were obtained.
settlement. The strain increased rapidly during construction because of compaction efforts, during which the reinforced soil generated large stiffness. The maximum strain was 0.45%. The maximum tension was approximately 29.12% of the ultimate tensile strength of the geogrid.
(1) The maximum cumulative horizontal displacement was approximately 1.13 mm (0.063% of the wall's height), and was located at a height of 3H/4. The cumulative horizontal displacement of GRSW with DBZ was smaller than that of the common GRSW, and the difference became greater with approaching to the face panel. (2) The distribution of vertical and horizontal soil pressures was
4. Discussions Like a rubber plate, the DBZ widened the range of soil to bear the load and consumed the energy. The wrapped structure interrupted the 5
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nonlinear along the reinforcement length, and the value was smaller near the face panel. During construction and loading periods, both the vertical and horizontal soil pressures increased linearly. The horizontal soil pressure acting on the face panel of GRSW with DBZ was greater than that of the common GRSW in the middle portion. (3) The cumulative strains of geogrids had a single-peak distribution along the reinforcement length. The strain was greater near the soil bags and decreased progressively closer to the end of reinforcement. The maximum strain was 0.45% and the maximum tension was 29.12% of the ultimate tensile strength of the geogrid.
Kibria, G., Hossain, M.D.S., Khan, M.S., 2014. Influence of soil reinforcement on horizontal displacement of MSE wall. Int. J. Geomech. 14 (1), 130–141. Ling, H.I., Leshchinsky, D., 2003. Finite element parametric study of the behavior of segmental block reinforced-soil retaining walls. Geosynth. Int. 10 (3), 77–94. Liu, H.B., Wang, X.Y., Song, E.X., 2011. Reinforcement load and deformation mode of geosynthetic-reinforced soil walls subject to seismic loading during service life. Geotext. Geomembranes 29 (1), 1–16. Liu, H.B., 2012. Long-term lateral displacement of geosynthetic-reinforced soil segmental retaining walls. Geotext. Geomembranes 32, 18–27. Latha, G.M., Manju, G.S., 2016. Seismic response of geocell retaining walls through shaking table tests. Int. J. Geosynth. Ground. Eng. 2 (1), 7. Marián, D., Jozef, V., Martina, H., Ladislav, K., 2016. Analytical and numerical evaluation of limit states of MSE wall structure. Civ. Environ. Eng. 12 (2), 145–152. Purkar, M.S., Kute, S., 2015. Finite element analysis of a concrete-rigid wall retaining a reinforced backfill. Int. J. Geo-Eng. 6 (1), 14. Rabie, M., 2016. Performance of hybrid MSE/Soil Nail walls using numerical analysis and limit equilibrium approaches. HBRC J. 12 (1), 63–70. Ren, F.F., Zhang, F., Wang, G., Zhao, Q.H., Xu, C., 2018. Dynamic assessment of saturated reinforced-soil retaining wall. Comput. Geotech. 95, 211–230. Shin, E.C., Park, K.W., Shin, H.S., Ham, K.W., 2017. Behavior of full scaled geobag retaining wall structure by field pilot test. J. Korean Geosynth. Soc. 16 (4), 21–31. Udomchai, A., Horpibulsuk, S., Suksiripattanapong, C., Mavong, N., Rachan, R., Arulrajah, A., 2017. Performance of the bearing reinforcement earth wall as a retaining structure in the Mae Moh mine, Thailand. Geotext. Geomembranes 45 (4), 350–360. Wang, H., Yang, G.Q., Wu, L.H., Liu, H.B., Xiong, B.L., 2014. Experimental study of geogrids reinforced retaining wall under overhead loading. Chin. J. Rock Mech. Eng. 33 (12), 2573–2581 (in Chinese). Wang, H., Yang, G.Q., Xiong, B.L., Wu, L.H., Liu, H.B., 2016. An experimental study of the structural behavior of reinforced soil retaining wall with concrete-block panel. Rock Soil Mech. 37 (2), 487–498 (in Chinese). Xue, J.F., Chen, J.F., Liu, J.X., Shi, Z.M., 2014. Instability of a geogrid reinforced soil wall on thick soft Shanghai clay with prefabricated vertical drains: a case study. Geotext. Geomembranes 42 (4), 302–311. Xiao, C.Z., Han, J., Zhang, Z., 2016. Experimental study on performance of geosyntheticreinforced soil model walls on rigid foundations subjected to static footing loading. Geotext. Geomembranes 44 (1), 81–94. Yoo, C., Kim, S.B., 2008. Performance of a two-tier geosynthetic reinforced segmental retaining wall under a surcharge load: full-scale load test and 3D finite element analysis. Geotext. Geomembranes 26 (6), 460–472. Yang, G.Q., Liu, H.B., Lv, P., Zhang, B.J., 2012. Geogrid-reinforced lime-treated cohesive soil retaining wall: case study and implications. Geotext. Geomembranes 35, 112–118. Yang, G.Q., Liu, H.B., Zhou, Y.T., Xiong, B.L., 2014. Post-construction performance of a two-tiered geogrid reinforced soil wall backfilled with soil-rock mixture. Geotext. Geomembranes 42 (2), 91–97. Yu, Y., Bathurst, R.J., Allen, T.M., 2016. Numerical modeling of the SR-18 geogrid reinforced modular block retaining walls. J. Geotech. Geoenviron. Eng. 142 (5) 04016003. Yu, Y., Bathurst, R.J., 2017. Probabilistic assessment of reinforced soil wall performance using response surface method. Geosynth. Int. 24 (5), 524–542. Zevgolis, I.E., 2018. A finite element investigation on displacements of reinforced soil walls under the effect of typical traffic loads. Transport. Infrastruct. Geotechnol. 5 (3), 231–249.
Acknowledgements This research is supported by the National Natural Science Foundation of China (Grant No. 51709175), Natural Science Foundation of Hebei Province (Grant No. E2018210097), Science and Technology Research Projects of Universities in Hebei Province (Grant No. QN2018255), and the Cooperative Innovation Center of Disaster Prevention and Mitigation for Large Infrastructure in Hebei Province (Shijiazhuang Tiedao University). We would like to thank Editage [www.editage.cn] for English language editing. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.geotexmem.2019.12.008. References Bathurst, R.J., Miyata, Y., Allen, T.M., 2010. Facing displacements in geosynthetic reinforced soil walls. Earth Retention Conference, 2010 384. pp. 442–459 208. Balakrishnan, S., Viswanadham, B.V.S., 2016. Performance evaluation of geogrid reinforced soil walls with marginal backfills through centrifuge model tests. Geotext. Geomembranes 44 (1), 95–108. Barani, O.R., Bahrami, M., Sadrnejad, S.A., 2018. A new finite element for back analysis of a geogrid reinforced soil retaining wall failure. Int. J. Civ. Eng. 16 (4), 435–441. Correia, A.A.S., Pinto, M.I.M., Lopes, M.L.C., 2012. Design of brick-faced retaining walls reinforced with geotextiles: face deformation. J. Geotech. Geoenviron. Eng. 138 (5), 629–632. Jacobs, F., Ruiken, A., Ziegler, M., 2016. Investigation of kinematic behavior and earth pressure development of geogrid reinforced soil walls. Transport. Geotech. 8, 57–68. Krystyna, K.F., 2005. A case study of a geosynthetic reinforced wall with wrap-around facing. Geotext. Geomembranes 23 (1), 107–115.
6
Contents lists available at ScienceDirect
Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem
Static structural behavior of geogrid reinforced soil retaining walls with a deformation buffer zone He Wang, Guangqing Yang∗, Zhijie Wang, Weichao Liu School of Civil Engineering, Shijiazhuang Tiedao University, Shijiazhuang, Hebei, 050043, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Geosynthetics Geogrid reinforced soil retaining wall Deformation buffer zone Structural behavior Model tests
To understand the structural behavior of geogrid reinforced soil retaining walls (GRSW) with a deformation buffer zone (DBZ) under static loads, the model tests and the numerical simulations were conducted to obtain the wall face horizontal displacement, vertical and horizontal soil pressures, and geogrid strains. Results showed that compared with the common GRSW, the horizontal displacement of GRSW with DBZ decreased, and the horizontal soil pressure acting on the face panel of GRSW with DBZ increased. The vertical and horizontal soil pressures showed a nonlinear distribution along the reinforcement length, and the value was smaller near the face panel. The horizontal soil pressure acting on the face panel of GRSW with DBZ was greater than that of the common GRSW in the middle portion. The cumulative strain of the geogrid had a single-peak distribution along its length; the maximum strain of the geogrid was 0.45%, the maximum tension was approximately 29.12% of ultimate tensile strength.
1. Introduction Geosynthetic reinforced soil retaining walls are increasingly applied in highway and railway construction because of their high strength, small deformation, improved earthquake resistance, and deformation accommodation. They have some superiorities in saving the land and environmental protection. The reinforced soil retaining wall can adopt fabricated construction to speed up the construction pace. The advantages above can result in great economic and social benefits. In recent decades, many laboratory and field tests have analyzed the deformation and stress characteristics of reinforced soil retaining walls with different types of face and under different surcharge loads. The post-construction displacement of the reinforced soil wall is about 0.3%–1% H (Latha and Manju, 2016; Yang et al., 2012, 2014; Zevgolis, 2018). Bathurst et al. (2010) compared the measured value with the recommended value by different design specifications and observed that the FHWA and AASHTO design specifications gave more rational limits. The coefficient of lateral earth pressure acting on face panel is 0.6–1.3 and can be divided into three zones, the earth pressure coefficient decreases with an increasing reinforcement ratio (Jacobs et al., 2016; Udomchai et al., 2017). The distribution of the earth pressure for geobag retaining walls is within the range of Rankine's and Coulomb's earth pressure after construction (Shin et al., 2017). The evolution of
reinforcement strain and failure surface is also revealed (Balakrishnan and Viswanadham, 2016; Krystyna, 2005; Xue et al., 2014; Xiao et al., 2016). And the deformation modes are also different among reinforced soil retaining walls (Correia et al., 2012; Liu et al., 2011; Liu, 2012; Ren et al., 2018; Wang et al., 2014, 2016; Yu and Bathurst, 2017), which the maximum cumulative horizontal displacement occurred at different portions. Numerical analysis is also important for understanding the characteristics of reinforced soil retaining walls and has elucidated the distributions and controlling factors for the stability of reinforced soil retaining walls. Reinforcement stiffness and length were identified as influential parameters affecting the horizontal movement at a specific MSE wall height, and the maximum displacement and tensile load in the reinforcement increased with an increase in the reinforcement spacing (Kibria et al., 2014; Ling and Leshchinsky, 2003; Marián et al., 2016). The currently available design guideline tends to over-estimate the surcharge load-induced reinforcement forces, and the surcharge load-induced reinforcement strains exponentially decrease with depth (Yoo and Kim, 2008). Some studies provide detail on how material properties are selected and how computer modeling is carried out (Purkar and Kute, 2015; Rabie, 2016; Yu et al., 2016). Barani et al. (2018) presented a new approach for back analysis of a geogrid reinforced soil wall failure.
∗ Corresponding author. School of Civil Engineering, Shijiazhuang Tiedao University, No. 17 North 2nd-Ring East Road, Shijiazhuang, Hebei Province, 050043, China. E-mail addresses: [email protected] (H. Wang), [email protected] (G. Yang), [email protected] (Z. Wang), [email protected] (W. Liu).
https://doi.org/10.1016/j.geotexmem.2019.12.008
0266-1144/ © 2019 Elsevier Ltd. All rights reserved. This is an open access article under the#lictext# license (http://creativecommons.org/licenses/#lictextcc#/#licvalue#/).
Please cite this article as: He Wang, et al., Geotextiles and Geomembranes, https://doi.org/10.1016/j.geotexmem.2019.12.008
Geotextiles and Geomembranes xxx (xxxx) xxx–xxx
H. Wang, et al.
Table 1 Major physical characteristics of wall filling. Items
Indexes
Name Model Natural unit weight (kN/m3) Saturated unit weight (kN/m3) Cohesion (kPa) Internal friction angle (°) Maximum dry unit weight (kN/m3) Elasticity modulus (MPa) Poisson ratio
Sand Mohr-Coulomb 17.4 20.0 0 36.7 1.789 12.00 0.30
Table 2 Major physical characteristics of geogrids. Items
Indexes
Model Tensile strength (kN/m) Tensile strength @ 2% strain (kN/m) Tensile strength @ 5% strain (kN/m) Peak strain (%) Axial stiffness @ 2% strain (kN/m)
Elastic 35.0 18.48 34.59 5.1 924.0
Table 3 Filling requirements. Items
Index
Coefficient of loose layer Thickness of loose layer (cm) Compaction thickness (cm) Compaction times Compaction degree (%)
1.33 20.0 15.0 4.0 ≥85
procedure of GRSW with DBZ. Fig. 1. Structure plot of GRSW with DBZ: (a) overall structure; (b) detail of reinforcement connection.
Step 1: Place the face footing and fix it. Lay the ground soil and compact it. Step 2: Lay the 1st reinforcement layer and the 1st block layer. Connect the two pieces of reinforcement using a connecting bar and reserve the requested length reinforcement for wrapping. Tighten the reinforcement and fix it on the ground soil using the soil nail. Step 3: Lay the soil and the soil bags on the 1st reinforcement and compact it. Wrap the soil bag using the reinforcement. Step 4: Lay the 2nd reinforcement layer according to step 2 and connect the wrapped reinforcement to the 2nd layer. Step 5: Repeat the above steps to the top of the wall.
Research into the evolution of wall structural behavior has aimed to control the wall's deformation and guarantee the wall's strength, providing important information for the design and construction of reinforced soil retaining walls. Deformation control is critical for flexible reinforced soil retaining walls. In conventional reinforced soil retaining walls, the reinforced soil is in contact with the panel. Lateral soil pressure acts on the panel directly, which results in the panel horizontal displacement. If the horizontal displacement is large, it is negative for structural stability and great settlement of the retaining wall will occur, impacting its practical applications. However, a deformation buffer zone can be installed between the reinforced soil and the panel to relieve horizontal displacement. The deformation buffer zone is also reinforced soil. The model structure is shown in Fig. 1. Model tests of GRSW with DBZ have been performed and the aim is to evaluate the stress and deformation characteristics during construction and under top loading after construction.
2.2. Model size and monitoring instrument arrangement As is shown in Fig. 2, the model of GRSW with DBZ was 3.0 m long, 1.0 m wide, and 1.8 m high, and the slope ratio of the wall face was 1:0.05. To reduce the boundary effect, the model box wall was painted with lubricant to reduce friction. The model box was enhanced and no deformation occurred on the box wall during the test, which ensured plane strain conditions. The vertical spacing was 0.3 m between the lower five layers and 0.15 m between the 5th and 6th layers in order to increase the shearing resistance to top loading. The laid length of the reinforcement was 2.0 m. A steel plate (0.7 m long, 0.5 m wide, and 0.03 m thick) was used to bear the surcharge loads. The wall panel was built with concrete blocks. A DBZ (0.3 m width) was placed between the panel and the reinforced soil, and was built using reinforced soil. The connection between the reinforcements and the panel is shown in Fig. 1 (b). The panel foundation was restricted. The arrangement of the monitoring instruments is shown in Fig. 2.
2. Experimental study and numerical modeling 2.1. Wall filling and reinforcement characteristics The filling of the retaining wall was sand, whose main characteristics are listed in Table 1 according to the direct shear tests. The reinforcement in DBZ and reinforced soil was high density polyethylene (HDPE) uniaxial geogrids, whose main characteristics are listed in Table 2. The controlling indexes of compaction are listed in Table 3 according to the compaction tests. Following was the installation 2
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Table 4 Model parameters. Items
Filling in soil bags
Wall face and face footing
Name Model Natural unit weight (kN/m3) Saturated unit weight (kN/m3) Cohesion (kPa) Internal friction angle (°) Elasticity modulus (MPa) Poisson ratio
Gravel soil Mohr-Coulomb 20.60 22.00 0 37.00 25.00 0.25
Concrete Linear Elastic 23.00 25.00 – – 3.0 × 104 0.20
Fig. 2. Arrangement of monitoring instruments.
2.3. Experimental procedure Step 1: Fill and compact each respective layer. Measure the horizontal static soil pressure, the vertical static soil pressure, and the reinforcement strain after filling each layer. Step 2: Let the retaining wall rest for a time after construction. Measure the stress and deformation without any surcharge load until constant values are obtained. Step 3: Apply local uniform loads to the top of the wall in six levels: 10 kPa, 20 kPa, 30 kPa, 40 kPa, 50 kPa, and 60 kPa. During loading, measure and record the data every 30 min. When the difference between two neighboring experimental data is zero, end the loading progress of one level. Then, conduct the loading of subsequent levels until all levels are completed.
Fig. 4. Distribution of cumulative horizontal displacement along the wall height.
3. Experimental results and analysis under static loads 3.1. Deformation characteristics of the wall face The measured distribution of cumulative horizontal displacement of the wall face along the entire wall height is shown in Fig. 4. The maximum cumulative horizontal displacement was 1.13 mm, which was 0.063% of the wall height. At lower loads (p ≤ 30 kPa), the cumulative horizontal displacement of the wall face increased with wall height and the maximum displacement occurred at the top of the wall. However, at higher loads (p > 30 kPa), the maximum cumulative horizontal displacement occurred in 3H/4. The GRSW with DBZ and the common GRSW horizontal displacement were calculated through the numerical simulation with different loads (p = 30 kPa, p = 60 kPa, p = 90 kPa, p = 120 kPa, p = 150 kPa, and p = 180 kPa) and different acting positions (d = 0.4 m, d = 0.8 m, and d = 1.2 m). The parameter d is the distance from the wall face to the load edge, as shown in Fig. 2. The distribution of horizontal displacement along the wall height, which is represented when d = 1.2 m, is shown in Fig. 5. And the comparison of the maximum horizontal displacement is listed in Table 5. The horizontal displacement of the common GRSW was always greater than that of GRSW with DBZ regardless of the load position. And the difference was getting greater as d was getting smaller when the load was located in the effect scope of DBZ (d = 0.8 m and d = 1.2 m). But when the load was located outside the effect scope of DBZ (d = 0.4 m), the difference was very small due to the weaker effect of DBZ. From the data listed in Table 5, when the load was constant, the horizontal displacement increased gradually with the load position approaching the wall face. For GRSW with DBZ, the increase was smaller from d = 1.2 m to d = 0.8 m, when the load was located in the effect scope of DBZ. However, the increase was greater from d = 0.8 m to d = 0.4 m, when moving the load position outside the effect scope of DBZ gradually. For the common GRSW, with the load position approaching the wall face, the horizontal displacement trend was just
2.4. Numerical modeling Conduct numerical simulation using a finite element program to compare the deformation and stress of GRSW with DBZ with that of the common GRSW. The numerical model of GRSW with DBZ is shown in Fig. 3. The common GRSW has not the soil bags and dose not form the DBZ. The parameters of numerical models are listed in Table 1, Table 2, and Table 4. Fig. 4 compares the measured face horizontal displacement with the simulated face horizontal displacement of GRSW with DBZ under loading p = 30 kPa and p = 60 kPa. The two results were quite close, indicating that the parameters chosen were reasonable.
Fig. 3. Numerical model of GRSW with DBZ. 3
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Fig. 5. Distribution of GRSW with DBZ and common GRSW horizontal displacement (d = 1.2 m). Table 5 Maximum horizontal displacement of GRSW with DBZ and the common GRSW. Load location
Load (kPa)
Maximum horizontal displacement (mm) GRSW with DBZ
Common GRSW
Increase rate (%)
d = 0.4
30 60 90 120 150 180
1.52 3.05 4.84 6.52 8.66 10.67
1.51 3.20 5.10 6.87 9.08 10.99
−0.66 4.92 5.37 5.37 4.85 3.00
d = 0.8
30 60 90 120 150 180
0.84 2.01 3.37 4.78 6.61 8.34
0.95 2.35 4.15 6.29 8.95 11.83
13.10 16.92 23.15 31.59 35.40 41.85
d = 1.2
30 60 90 120 150 180
0.43 1.12 2.58 4.16 6.19 8.40
0.42 1.29 2.77 5.01 7.36 10.08
−2.33 15.18 7.36 20.43 18.90 20.00
Fig. 6. Development of vertical soil pressure: (a) h = 0.3 m; (b) comparison of different layers.
some stress, the horizontal soil pressure applied to the panel was very small. The maximum horizontal soil pressure applied to the panel was 6.0 kPa and occurred in the bottom soil. Fig. 8 compares the horizontal soil pressure acting on the face of GRSW with DBZ with that of the common GRSW. The horizontal soil pressure acting on the top portion was smaller than 1.0 kPa and had little difference between the two walls. The horizontal soil pressure acting on the middle portion (ranging from H/4 to 5H/6) of the common GRSW was smaller than that of GRSW with DBZ, which was because the common GRSW occurred the greater horizontal displacement than GRSW with DBZ to release more stress. While the horizontal soil pressure acting on the bottom portion also had little difference between the two walls.
opposite compared with that of GRSW with DBZ.
3.2. Characteristics of vertical and horizontal soil pressure Fig. 6 and Fig. 7 show the development of vertical and horizontal soil pressures. The development of soil pressures on one layer is represented by that where h = 0.3 m. The parameter h is the wall height. The soil pressure comparison of different layers is represented by that on the center line of the load. The distribution of vertical and horizontal soil pressures along the reinforcement length during construction and loading phases was nonlinear. During construction, differences among the soil pressures within each layer were small. After applying the load, the soil pressure near the loading position increased rapidly whilst that far from the loading position increased more slowly because of the diffusion and attenuation of additional stresses. The development of vertical and horizontal soil pressures presented a linear increase during construction and loading phases, and an almost constant soil pressure during the resting phase. The ratio of the soil pressure increase was almost equal at every monitoring point during construction, and clear differences were observed during the loading phases. During the loading period, the increase of horizontal soil pressure in the top and bottom portions was greater than that in the middle portion. Because of wall face horizontal deformation leading to release
3.3. Characteristics of geogrid strain Fig. 9 (a) shows the development of the geogrid cumulative strain during construction and loading periods, which is represented by that where h = 0.3 m. Fig. 9 (b) shows the distribution of the geogrid cumulative strain along the reinforcement length. The predominantly single-peak distribution of strain was caused by the shearing action generating near the potential failure surface. Moreover, the strain gradually decreased with approaching the end of the reinforcement, which resulted in a gradual stiffness transformation from reinforced areas to non-reinforced areas in order to avoid large differential 4
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Fig. 9. Distribution of geogrid cumulative strain along the reinforcement length: (a) h = 0.3 m; (b) comparison of different layers.
Fig. 7. Development of horizontal soil pressure: (a) h = 0.3 m; (b) comparison of different layers.
transmission path of stress and deformation. Thus the horizontal displacement decreased, and the force on the face panel was more uniform to avoid a greater tilt angle. In this model test, the filling of DBZ was sand, the same as reinforced soil. In actual engineering, the DBZ can be made up of the coarse aggregate to play a drainage function, which also can enhance the reinforcing effect of DBZ. About the GRSW with DBZ, this experiment is an initial study and focuses on the structural behavior. There are some other aspects that will be studied by following model tests and numerical simulations in the future, as the comparison between GRSW with DBZ and other types GRSW on the structural behavior, economic benefits, the structural behavior of GRSW with DBZ under dynamic loads, the most suitable width of DBZ, etc. 5. Conclusions
Fig. 8. Comparison of horizontal soil pressure acting on the face panel between GRSW with DBZ and common GRSW.
Based on the results of model tests and numerical simulations, a GRSW with a DBZ can meet the strength and the deformation requirements under static loads. The following conclusions were obtained.
settlement. The strain increased rapidly during construction because of compaction efforts, during which the reinforced soil generated large stiffness. The maximum strain was 0.45%. The maximum tension was approximately 29.12% of the ultimate tensile strength of the geogrid.
(1) The maximum cumulative horizontal displacement was approximately 1.13 mm (0.063% of the wall's height), and was located at a height of 3H/4. The cumulative horizontal displacement of GRSW with DBZ was smaller than that of the common GRSW, and the difference became greater with approaching to the face panel. (2) The distribution of vertical and horizontal soil pressures was
4. Discussions Like a rubber plate, the DBZ widened the range of soil to bear the load and consumed the energy. The wrapped structure interrupted the 5
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nonlinear along the reinforcement length, and the value was smaller near the face panel. During construction and loading periods, both the vertical and horizontal soil pressures increased linearly. The horizontal soil pressure acting on the face panel of GRSW with DBZ was greater than that of the common GRSW in the middle portion. (3) The cumulative strains of geogrids had a single-peak distribution along the reinforcement length. The strain was greater near the soil bags and decreased progressively closer to the end of reinforcement. The maximum strain was 0.45% and the maximum tension was 29.12% of the ultimate tensile strength of the geogrid.
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Acknowledgements This research is supported by the National Natural Science Foundation of China (Grant No. 51709175), Natural Science Foundation of Hebei Province (Grant No. E2018210097), Science and Technology Research Projects of Universities in Hebei Province (Grant No. QN2018255), and the Cooperative Innovation Center of Disaster Prevention and Mitigation for Large Infrastructure in Hebei Province (Shijiazhuang Tiedao University). We would like to thank Editage [www.editage.cn] for English language editing. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.geotexmem.2019.12.008. References Bathurst, R.J., Miyata, Y., Allen, T.M., 2010. Facing displacements in geosynthetic reinforced soil walls. Earth Retention Conference, 2010 384. pp. 442–459 208. Balakrishnan, S., Viswanadham, B.V.S., 2016. Performance evaluation of geogrid reinforced soil walls with marginal backfills through centrifuge model tests. Geotext. Geomembranes 44 (1), 95–108. Barani, O.R., Bahrami, M., Sadrnejad, S.A., 2018. A new finite element for back analysis of a geogrid reinforced soil retaining wall failure. Int. J. Civ. Eng. 16 (4), 435–441. Correia, A.A.S., Pinto, M.I.M., Lopes, M.L.C., 2012. Design of brick-faced retaining walls reinforced with geotextiles: face deformation. J. Geotech. Geoenviron. Eng. 138 (5), 629–632. Jacobs, F., Ruiken, A., Ziegler, M., 2016. Investigation of kinematic behavior and earth pressure development of geogrid reinforced soil walls. Transport. Geotech. 8, 57–68. Krystyna, K.F., 2005. A case study of a geosynthetic reinforced wall with wrap-around facing. Geotext. Geomembranes 23 (1), 107–115.
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