Analysis and Design of Nailed Soil Wall—A Case Study
ANALYSIS AND DESIGN OF NAILED SOIL WALL—A CASE STUDY
K. Premalatha
Assistant Professor, Department of Civil Engineering, College of Engineering, Anna University Chennai, Chennai, India.
Email: .
M. Muthu Kumar
Post Graduate Student, Department of Civil Engineering, College of Engineering, Anna University Chennai, Chennai, India.
E-mail:
D. Mohan Babu
Project Engineer, Ganga Foundations Pvt. Ltd., Chennai–17, India.
ABSTRACT: Soil nailing is an in situ reinforcement technique which is used to retain excavations or to stabilize slopes. During past fifteen years, significant development has been taken place in the techniques of in situ reinforcement using nails. Especially, soil nailing has gained worldwide acceptance in both theory and practice due to its economy, technical advantages and construction speed. Insufficient availability of land in cities and sub soil profile necessitates the designer to provide basement floors. This is mainly used for car parking facility in metro cities. Basement construction requires excavation of soil up to 6 to 9 m. For this depth, soil is retained by nailed wall. Nailed wall is used as a temporary structure for execution before the construction of permanent retaining wall.. This paper summarizes the recommendations that are made for a nailed wall construction in Chennai city. This analysis was carried out using SNAILZ computer program.
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Analysis and Design of Nailed Soil Wall—A Case Study
1. INTRODUCTION
Soil nailing is an in-situ soil reinforcement technique that has been used during the last three decades mainly in France and Germany to retain excavation or slopes. The origin of soil nailing can be traced to a support system for underground excavations in rock referred to as the “New Austrian Tunneling Method”. One of the first applications of soil nailing was in 1972 for a railroad widening project near Versailles, France; Where an 18 m high cut slope in sand was stabilized using soil nails. The first research program on soil nail walls was undertaken in Germany from 1975 through 1981 by the University of Karlsruche and the construction company Bauer. In France, the Clouterre research program involving private and public participants was initiated in 1986. The first published applications of soil nailing in the United States was the support of the 13.7 m deep excavation in dense silty lacustrine sands for the expansion of the Good Samaritan hospital in Portland, origin in 1976.
In India, Srinivasa Murthy et al. (2002) reported the use of a nailed soil wall as permanent retaining wall for subway underneath a busy national highway. Patra (2005) used the optimization technique for the optimum design of nailed slope and also established a generalized procedure (2008) for the design. Sivakumar Babu (2007) used this nailing technique to stabilize a vertical cut in a hilly terrain.
2. LOAD TRANSFER CONCEPT IN SOIL NAIL WALLS
The response and load transfer mechanism takes place during a conventional soil nailing construction is shown in Figure 1. Soil excavation is initiated from ground surface and immediately after the excavation of phase 1, by utilizing the temporary ability of soil to stand unsupported, the first row of nails (nail 1) are installed. The load derived from the deformation of the upper soil is transferred to these nails through shear stresses along the nails and translate in to axial forces. The axial force distribution in nail 1 at the end of excavation phase 1 is also shown in Figure 1. As excavation proceeds to excavation phase 2, the uppermost and the unsupported portions of the soil nail wall deforms laterally. Another potential sliding surface is originated from base of excavation phase 2. The critical failure surface for excavation level 2 is different from excavation level 1. Nails 2 are then installed. Movements of the soil above the phase 2 depth transfers additional loads to nails 1 and generate loads in nails 2. The temporary facing supports the excavation surface and provides connectivity between adjacent nails. Nails are extended beyond the potential failure surface. Increase in size of the retained zone increases the stresses at the soil/nail interface and the axial forces in the nail increases. The induced tensile stresses are transferred behind the retained zone in an anchorage effect and these stresses stabilize the potentially sliding mass. Also there is a variation in the tensile force mobilization as the excavation depth increases. The contribution of upper nail diminishes as the failure surface becomes deeper and larger.
3. DESCRIPTION OF CASE STUDY
(a) Details of structure
Type of Building: Shopping mall and hotel
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Analysis and Design of Nailed Soil Wall—A Case Study
Fig. 1: Potential Failure Surface and Soil Nail Tensile Force (FHWA, 2002)
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Analysis and Design of Nailed Soil Wall—A Case Study
No. of Basement: 3
No. of storey: 3 for shopping mall and 13 for Hotel
(b) Details of Subsurface Investigation
SPT test in borehole of 150 mm diameter
Rotary drilling
(c) Soil Profile
0 – 1.5 m made-up soil
1.5–6m loose silty sand
(f = 29.6° and γ = 13 KN/m3)
6.0–9.5 m Medium dense silty sand (f = 39.4°, γ = 17 KN/m3)
9.5–10.8 m Shale
>10.8 m weathered granite
Ground water table at 1.5 m depth
(d) Details of Required Retaining System
Depth of excavation 9 m, no surcharge near the excavation
(e) Type of Retaining System Selected
Nailed soil wall after
dewatering up to 10.5 m below G.L
4. ANALYSIS AND DESIGN OF NAILED SOIL WALL USING SNAILZ PROGRAM
The analysis and design of nailed soil wall was carried out by SNAILZ programming developed by California Department of Transportation Engineering service center.
4.1 Basic Theory
The program uses a bi-linear wedge analysis for failure planes exiting at toe of wall and tri-linear for failure planes developing below and beyond the wall toe. It is a fully balanced force equilibrium equation with only soil inters lice forces included, based on a mobilized φ and c. For failure planes day lighting beyond the wall toe, a three part wedge is used. The third wedge develops at specified depths directly below the wall toe. Resistance is determined by passive earth pressure principles wherein the passive force is inclined at an angle of 1/3 the mobilized φ on the vertical plane.
4.2 Program Execution
Based on the input data’s, a graphical representation was made by this program. Different possible failure surfaces were considered using ‘Search limit’ command. The number of failure planes considered was 560 depending upon the horizontal distance behind the wall. For each level of nail, the nail forces were also calculated based on the pull out force and the frictional resistance. It is possible to view and save all the results.
The analysis and design of nailed soil wall was carried out by SNAILZ programming developed by California Department of Transportation Engineering service center.
4.3. Details of Input for Case Study
Table 1 shows the details of input for present case study.
Table 1: Details of Input for Case Study
S. No. / Property / Value1. / Vertical wall height, H, m / 9
2. / Soil type / Silty sand
3. / Grade of steel / Fe 415
4. / Drill hole diameter DDH, mm / 100
5. / Ultimate bond strength qu, kpa / 100
6. / Unit weight, KN/m3 / 13 and 17
7. / Friction angle in degrees / 29.6 and 39.4
8. / Nail length LN, m / 7.2
9. / Punching shear capacity, KN / 150
The length of the reinforcement was taken as 0.8H (H- Vertical height of wall) Ultimate bond strength and punching shear capacity was taken from design guidelines.
4.4. Validation of Program
To understand the limitation and relative accuracy of the procedure used to calculate the global factor of safety for a selected wedge, the output results of SNAILZ program was compared with the results obtained from Plaxis software and hand computations using FHA guidelines. The case study referred for validation is of G.L. Sivakumar babu and Vikas Pratap Singh. (2009). Table 2 shows the properties of that site. The global factor of safety obtained by the above procedure is summarized in Table 3.
Table 2: Properties of Site
S. No. / Property / Value1. / Vertical Height of wall H, m / 6, 12 and 18
2. / Soil Type / Silty sand
3. / Cohesion c, kpa / 5
4. / Friction angle / 35°
5. / Unit weight KN/m3 / 18.9
6. / Grade of Steel / Fe 415
7. / Nail spacing SVXSH, mxm / 1.0 × 1.0
8. / Nail inclination / 15°
9. / Drill hole diameter DDH, mm / 100
10. / Nail Length LN, m / 4, 8.5 and 13
11. / Diameter d, mm / 16, 20 and 22
Ground water table encountered at a depth of 1.5 m below the existing ground level.
Table 3: Comparison of Results for Global Factor of Safety
Wall Height, H, m / Hand Computation / PLAXIS Software / SNAILZ Program6 / 1.35 / 1.81 / 1.50
12 / 1.35 / 1.79 / 1.51
18 / 1.35 / 1.78 / 1.51
The obtained factor of safety is comparable with other results.
4.5 Output Received
For optimum design of nailed soil wall, the parameters like nail spacing, diameter, and inclination were varied for this analysis. This was continued until the required minimum factor of safety was obtained. The output includes the forces in the nail and a complete graphical illustration with all details such as critical failure surface, global factor of safety and soil profile. The sample of output is shown in the
Figure 2.
5. RECOMMENDATIONS
For the selected case study, an optimum design of nailed soil wall was done using the SNAIL program for a factor of safety of 1.43. The other details of the design are tabulated in Table 4.
Table 4: Details of Design Parameters
S. No. / Property / Value1. / Horizontal and Vertical spacing / 1.5 m and 1.5 m
2. / Nail Inclination / 0° to Horizontal
3. / Diameter of Nail / 16 mm
4. / Diameter of grouted hole / 100 mm
5. / Length of Nail / 7.2 m
6. / No of Reinforcement level / 6
5.1 Wall Facing
Nails are connected at the excavation surface (or slope face) to a facing system. The components of facing system are listed below.
Type of reinforcement-WWM (Welded Wire Mesh)
Thickness of shotcrete : 100 mm
Bearing plate grade : Fe250
Dimensions : 225 × 225 × 25 mm
Punching shear capacity : 150 KN
Temporary shotcrete applications are constructed using both WWM or fiber reinforcement and bars. WWM is the preferred method because it requires less time to install while the excavated face is unsupported. Restriction in the usage of land necessitates recommending for vertical excavation. If it is difficult to have a vertical excavation, a slope 10° is allowed.
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Analysis and Design of Nailed Soil Wall—A Case Study
Fig. 2: Sample of Output
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Analysis and Design of Nailed Soil Wall—A Case Study
5.2 Drainage Installation
This temporary nailed wall is not designed for seepage flow; the migration of ground water towards the excavation must be prevented. Depending upon the season of this construction, the following measures could be considered to avoid failure due to seepage. Vertical geocomposite strip drains are installed behind the wall facing to prevent groundwater pressure to build up. Strip drains must be spliced at the bottom of each excavation lift an must have at least a 300 mm (12 in.) overlap such that the water flow is not impeded. The groundwater collected at strip drains must be removed by a series of footing drains at the bottom of the excavation. The footing drain consists of a trench at the bottom of the excavation, which should be filled with aggregate free of fines with PVC slotted collection pipe. Weep holes has to be installed through the wall facing at the lower portions of the wall. In special situation when the groundwater behind the proposed soil nail wall is high, conventional, deeper horizontal pipe drains are necessary.
6. CONCLUSIONS
Recommendations were made for the site for the construction of a nailed soil wall. The site was inspected during execution and design was found to be satisfactory with 10° of inclination.
REFERENCES
Barley A.D. (1993). “Soil Nailing Case Histories and Developments”, Retaining structures, Thomas Telford, London, pp. 559–573.
Jewell R.A. and Pedley M.J. (1990a). “Soil Nailing: The Role of Bending Stiffness”, Ground Engineering, Mar, pp. 30–36.
Jewell R.A. and Pedley M.J. (1990b). “Soil Nailing: The Role of Bending Stiffness”, Ground Engineering, July-Aug, pp. 33–33.
Jian Xin Yuan and Yuwen Yang (2003). “New Approach to Limit Equilibrium and Reliability Analysis of Soil Nailed Wall”, International of Geomechanics, 3(2).
Juran, I., Gerge. B., Khalid F. and Elias V. (1990b). “Design of Soil Nailed Retaining Structures, Design and Performance of Earth Retaining structures”, J. of Geotechnical Engineering, ASCE, Vol. 116 pp. 54–71.
Mittal, S., Gupta, R.P. and Mittal, N. (2005). “Housing Construction on inclined cuts”, Asian Journal of Civil Engineering, Vol. 6, No. 4, pp. 331–346.
Muthu Kumar, M. (2009). “Design of Nailed soil wall based on Nail rigidity number”, M.E. thesis submitted in Anna University Chennai.
Patra, C.R. (1998). “Sequential Minimization Technique in the Optimum Design of Slopes With or Without Nails”, Ph.D. Thesis Submitted in the Indian Institute of Technology, Kanpur, India.
Patra, C.R. and Basudhar, P.K. (2005). “Optimum Design of Nailed Soil Slopes”, Geotechnical and Geological Engineering, pp. 273–296.
Siva Kumar Babu, G.L. and Vikas Pratap Singh (2009). “Appraisal of Soil Nailing Design”, Indian Geotechnical Journal, Vol. 39, No. 1, pp. 10–17.
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Analysis and Design of Nailed Soil Wall—A Case Study
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