Research on Deformation of Soil Nailing Structure with Flexible Facing

Transcription

1 2017 International Conference on Transportation Infrastructure and Materials (ICTIM 2017) ISBN: Research on Deformation of Soil Nailing Structure with Flexible Facing Tao Sun 1, Yanfeng Sun 2, Qiang Liu 3 * 1 Professor, Shandong University of Science and Technology, Qingdao, , China; sunystao@qq.com 2 Master student 3* Corresponding Author, Assistant Professor, Shandong University of Science and Technology, Qingdao, , China; sunnyseasea@163.com ABSTRACT: Soil nailing is a widely-used technology for retaining cutting works in soil. A significant portion of the cost of construction is related to the construction of a reinforced concrete face. The possibility of replacing the flexible facing with reinforced concrete facing in soil nailing was evaluated by field test. A woven geomembrane was used as a substitute for concrete. In the field test, a slope was made by cutting. During the loading, horizontal displacement and vertical settlement of the slope top, earth pressure of slope top and surface, tension force of woven geomembrane and horizontal displacement of inner slope were observed. The flexible facing performed well, since the facing did not have large vertical and horizontal deformations. The lateral earth pressure on the slope surface was also quite small. When the cutting work was finished, total surcharges of kpa was applied on the slope top by six steps and no obvious displacement was observed. Finally, artificial rain fall was conducted on the slope top. After 2 hours durations, only shallow surface failure occurred, but the overall slope is stable. Therefore, the flexible facing can replace reinforced concrete in practice. But such kind of improvement should be limited to non-critical structures where large vertical and horizontal deformations are acceptable. INTRODUCTION Soil nailing is an earth reinforcement technique that can be used to support existing slopes or excavation for ensuring the stability and restrict developing of displacement through interaction between nails and unreinforced soil or rock. Comparisons with other supporting technologies, the advantages of soil nailing are a short construction time and low construction cost (Watkins and Powell, 1992). Therefore, soil nailing is used widely in the world. The basic components of a soil nailing include the reinforced mass of soil with nails and facing. According to the new code EN 14490:2010(EN, 2010), three types of facing can be defined as, hard facing, flexible facing and soft facing. As one of the important components of soil nailing, the effect of facing on the performance of soil nailing system was studied earlier by several investigators. Regarding theoretical analysis on soil nailing structure, Schlosser and Unterreiner (1991) made some analysis on soil nailing structure for temporary and permanent soil nailed wall. Several functions of facing in a soil nailing structure have been proposed. The functions of facing were that: it provides a lateral confinement for the soil between the nails, and it carries external loads such as decorative plane. Joshi (2003) indicated facing stiffness is one of the important factors that affect the soil nail head by analysis of the force of the soil nail head using FHWA calculation method. Shaw-Shong (2005) discussed the design methods of soil nailing structure. He proposed that the relatively low flexural facing stiffness and comparative high nail head support stiffness will

2 encourage effective arching effect resulting in highly non-uniform pressure distribution between the mid-span of facing and nail head. In addition to theoretical analysis, some model tests on soil nailing structure were conducted. Tei et al. (1998) carried out a series of centrifuge model tests on soil nailing slope by varying the nail type, roughness of nail surface, nail inclination, facing stiffness and facing roughness. Rigid facing and flexible facing showed the different deformation characteristics. Comparison with rigid facing, horizontal displacement of soil nailing structure with flexible facing including rotation, sliding and curving which due to the expansion of the slope surface. Sanvitale et al. (2013) conducted a series model tests to investigate the effect of facing type on soil nailing slope in excavation and surcharge. It was reported that the stiffness of facing has effect on stress distribution of nails, and the more flexible facing the larger displacements were observed. Rotte and Viswanadham (2013) performed several centrifuge tests to investigate the effects of nail inclination and facing type on soil nailing. It was concluded that facing can significantly prevent partial failure of slope. In the same way, Giacchetti et al. (2013) indicated that flexible facing is not like a rigid facing, uniting the soil nailing, to prevent deformation of slope. Rotte and Viswanadham (2014) carried out several centrifuge model tests and numerical models to investigate facing effect on soil nailing slope subjected to seepage. Two different facings were considered: (a) woven geotextile; and (b) 2 mm thick Aluminum sheet. It was concluded that stiffness of slope facing can significantly affect the deformation behavior of soil-nailed slopes. In addition, with stiff facing, the pattern of force mobilization is found to be different from a soil nailed slope with flexible facing. However, with the focus on the environment, substituting flexible or soft facing for shotcrete facing of soil nailing has been paid more attention. Pokharel et al. (2011) carried out both numerical and physical models to study the performance of soil nailing structure with flexible facing. It was concluded that soil nailing structure with flexible facing could result large deformation of the facing and settlements of the surface. Giacon (2012) performed a series of numerical models by varying the nail spacing, slope inclination and facing type. It was reported that stress concentration of facing are in the positions of the soil nail head and the middle of two rows soil nailing, but the stress is far less than tensile strength of facing. It was concluded that flexible facing for the stability of the slope didn't have a structural effect, only rise to prevent erosion and the soil nail penetrated. With the same facing, anchored geosynthetic slope system has common characteristics to soil nailing structure with flexible facing. Meanwhile, performance of anchored geosynthetic system was studied by several investigators. Rajabian et al. (2012, 2013) conducted several centrifuge tests to investigate the performance of anchored geosynthetic slope under seepage condition. It indicates that anchored geosynthetic slope can significantly maintain the stability of slope and control deformation of slope, which consists with the research by Chi et al. (2012). Most of the studies mentioned above have been focus on model test and numerical analysis. Field test for flexible facing with soil nailing structure is insufficient. In this study, field tests were performed to examine the behavior of soil nailing with flexible facing. In the field test, a slope was made by cutting. During the loading, horizontal displacement and vertical surface settlement of the slope, earth pressure of slope top and surface, tension force of woven geomembrane and horizontal displacement of inner slope were observed.

3 SOIL NAILING SYSTEM WITH FLEXIBLE FACING The soil nailing with flexible facing, consists of three parts, nail, flexible facing and connecting component for the former two (FIG. 1). The nail can be grouted bar or fiber reinforced polymer (FIG. 2). The flexible facing, which replaces reinforced concrete facing, can be geogrid, geomembrane or just a steel mesh (FIG.3). The connecting component plays a key role in the whole system, since it can transfer the lateral earth pressure on the flexible facing to the part of nail located in resistant zone. The connecting component can be made either by relative rigid materials, e.g. Bolt and nut, welded short steel bar, or flexible materials, e.g. fiber or nylon rope having high strength and modulus. When the rope is used, one side of the rope will be binding on the anchor, the other side through the axial force conversion ring and tide to nearby rope (FIG. 4). The axial force conversion ring is a steel or plastic ring. The hole on the flexible facing where the rope pass through, is easily to produce stress concentration. Without the ring, the flexible facing may be torn. The rain will come into slope through the torn parts. Therefore, the axial force conversion ring can prevent the failure of flexible facing by reducing this effect. slope top flexible facing connecting component nail Figure 1. Soil nailing with flexible facing. Figure 2. Fiber reinforced polymer. The working mechanism of the flexible facing supporting technology is like that of the traditional nail and shotcrete support technology. As shown in FIG. 5, when the lateral displacement of slope occurs, the tension force of the nail will resist the slope sliding. The flexible facing will resist parts of the earth pressure between two nails. The tension generated by the flexible facing plays a role in maintaining the local stability of the slope. facing rope force conversion ring Figure 3. Geosynthetic. Figure 4. Connecting component.

4 flexible facing connecting component tension on facing nail earth pressure concrete slide plane tension anchor force Figure 5. Mechanism of soil nailing structure with flexible facing. FIELD TESTING The project was in Qingdao, Shandong province of China. Main soil layer in field was a kind of silty clay. The natural water content was 23.9%, the unit weight was 19.6 KN/m 3, the void ratio was 0.726, and the specific gravity of the soil was This soil had a liquid limit of 30.8 and plastic limit of 15.5, resulting in a plasticity index of The inner friction angle was 12.4 degree and cohesion was 24.8 kpa. The average counts of standard penetration test were 8. The cutting slope was 20 m in longitude, 6 m in height and inclination was 1:0.25. Three rows of nail were installed by drilling hole which diameter was 13 cm, in the slope. The nail orientation in relation to the horizontal was 15 degree. The length of nail was 6, 5 and 4 m from top to bottom of slope. The horizontal spacing was 2 m and the vertical spacing was 1.5 m. On the slope top, 1.5 m away from the shoulder, one row of nail was vertically installed to 1 m depth. The surcharge was applied within an area of m step by step. Two kinds of load, soil filling and precast concrete block were used (Table 1). The sketch and view of surcharge on slope top are shown in FIG. 6 and FIG. 7. In the process of stepwise loading, load at the slope top, earth pressure at the slope surface, tension of the flexible facing, horizontal displacement and settlement at slope top and deep horizontal displacement were measured. Each monitored point is shown in FIG. 8. Load and earth pressure were measured by load cell. Tension were measured by tension meter, which were vertically and horizontally installed separately. The deep horizontal displacement was measured by the inclinometer. The total length was 8 m and 1 m beneath the bottom of the slope. The horizontal displacement and vertical settlement were measured by electronic total station. Figure 6. Sketch of surcharge on slope. Figure 7. View of surcharge on slope.

5 Earth pressure cell Reinforcement Deformation monitor Flexible facing deformation monitor Flexible facing internal force monitor Inclinometer tube Figure 8. Layout of monitor point. Table 1. Steps of load. Step Materials Load/ (KN) I Soil (1 m thick) 739 II Soil (1 m thick) 739 III Soil (1 m thick) 739 IV Precast concrete block (14 blocks) 700 V Precast concrete block (13 blocks) 650 VI Precast concrete block (17 blocks) 850 RESULTS AND DISCUSSIONS The pressure distribution on the slope top is shown in FIG. 9. The pressure increase linearly with increasing load. However, it is not a uniform distribution on the slope top. The pressure on the crest side is smaller than the pressure on the opposite side. The center of the load area has the largest pressure value. Even though uniform load is applied on the slope top, the results show the stress concentration occurs at the center line of the slope top. Figure 9. Variation of pressure with load a) the first column, b) the second column, c) the third column.

6 Fig. 10 shows the earth pressure of the slope surface at different depth. The earth pressure increase with the load addition on the slope top. Under each load step, the earth pressure distribution in the mid lower portion parts of slop is like letter C, where the lowest earth pressure appears in the middle along the depth. When load is lower than 1478 KN, the earth pressure almost equal to zero. It means when the load is lower than that value, the slope is stable and there is no large displacement. Earth pressure begin to increase with the load continue increasing and the increased magnitude is uniform. After maximum load (4416 KN), the earth pressure of slope increasing uniformly and maintain stable. Therefore, under the load of 4416 KN, the soil nailing with flexible facing supporting system can maintain the stability of slope. Figure 10. Variation of Earth pressure with depth a) the first column, b) the second column, c) the third column. The variation tendency of deep horizontal displacement with load increasing is shown in FIG. 11. When the first three steps of load were added to the slope top, the deep horizontal displacement has no change. When the load becomes larger than 2864 KN, the horizontal displacement of upper part of the slope is mm, the lower part still does not move. When load reached 3714 and 4416 KN, the maximum displacement is 13.6 and 24.4 mm separately. The displacement is small, where the depth is deeper than 4 m. Figure 11. Variation of horizontal displacements of slope with depth.

7 The settlement curve of slope top has a stepwise characteristic (FIG. 12). After the second step load, the settlement is about 25% of the total settlement. The settlement caused by third and fourth step load is about 58% of the total settlement. When the fifth and sixth step load was applied, the settlement is about 17% of the total settlement. The sixth step load is larger than the allowable bearing capacity of the ground per the theoretical and calculated results. Therefore, the soil nailing with flexible facing protect the slope from shearing failure caused by applied load on slope top. Figure 12. The settlement of slope top. The horizontal and vertical tension of the flexible facing fluctuated varying with the increase of the load, and the range is 0-35g (FIG. 13). This value is much smaller comparing with the earth pressure of slope surface. However, after 1 hour s artificial rainfall simulation, shallow part of slope surface collapsed. The reading of tension gauge is out of its range. That means the tension of flexible facing is constant when the vertical load on the slope top is smaller than the allowable load. In other words, only when the slope has obvious deformation, the facing will be stressed. CONCLUSIONS Figure 13. Variation of tension of facing with load. This study presents out the field test result of soil nailing with flexible facing. The construction technique is a cost effective and a low environment impact solution. Especially, the technique can avoid dust during shotcreting. The flexible materials like geotextile can replace shotcrete as facing in the soil nailing system, while guarantee the stability of the slope and the erosion control. Under the protection of the technique, the cutting slope maintains overall stability even if large surcharge is applied on the slope top. The results of the

8 tension of the flexible facing confirm that the woven geotextile used as facing is providing a structural function and requires deformation and mobilization of the slope to become functional. It means that significant deformation is required to mobilize the tensile strength of the facing. REFERENCES Chi, P.C., Fon, K.Y., Chen, R.H., (2012). "Model tests for anchored geosynthetic slope systems under dry and seepage conditions." Geosynthetics International, 19, EN, B., (2010).14490: "Execution of special geotechnical works. Giacchetti, G., Grimod, A., & Cheer, D. (2013). Soil Nailing with Flexible Structural Facing: Design and Experiences. Landslide Science and Practice. Giacon, L., (2012). "Flexible facings for soil nailing retaining systems." Joshi, B., (2003). "Behavior of Calculated Nail Head Strength in Soil-Nailed Structures." Journal of Geotechnical & Geoenvironmental Engineering, 129, Pokharel, S.K., Parsons, R.L., Pierson, M.C., Han, J., Willems, I., (2011)."Use of Flexible Facing for Soil Nail Walls." Finite Element Method. Rajabian, A., Viswanadham, B.V.S., Ghiassian, H., Salehzadeh, H., (2012). "Centrifuge model studies on anchored geosynthetic slopes for coastal shore protection." Geotextiles & Geomembranes, 34, Rajabian, A., Viswanadham, B.V.S., Ghiassian, H., Salehzadeh, H., (2013). "Centrifuge study of anchored geosynthetic slopes." Geosynthetics International, 20, Rotte, V.M., Viswanadham, B.V.S., (2013). "Influence of nail inclination and facing material type on soil-nailed slopes." Proceedings of the Institution of Civil Engineers Ground Improvement, 166, Rotte, V.M., Viswanadham, B.V.S., (2014). "Centrifuge and Numerical Model Studies on the Behaviour of Soil-Nailed Slopes with and without Slope Facing." Geotechnical Special Publication, Sanvitale, N., Simonini, P., Bisson, A., Cola, S., Role of the facing on the behaviour of soil-nailed slopes under surcharge loading. ICSMGE, International Conference on Soil Mechanics and Geotechnical Engineering, Paris. Chlosser, F., & Unterreiner, P. (1991). Soil nailing in France: research and practice. Transportation Research Record, 1330, Shaw-Shong, L., (2005)."Soil nailing for slope strengthening." Geotechnical Engineering, Gue & Partners Sdn Bhd, Kuala Lumpur, Malaysia. Tei, K., Taylor, N.R., Milligan, G.W.E., (1998)."Centrifuge model tests of nailed soil slopes." Journal of the Japanese Geotechnical Society Soils & Foundation, 38, Watkins, A., Powell, G., (1992). "Soil nailing to existing slopes as landslip preventive works." Hong Kong Engineer, 20,