STABILIZATION OF EARTH SLOPES BY USING SOIL NAILING

The basic concept of soil nailing is to reinforce and strengthen the existing round by installing closely spaced steel bars, called “Nails”, into a slope as construction proceeds from “top-down”. This process creates a reinforced section that is in itself stable and able to retain the ground behind it. Soil nailing technique is used to support new very steep cuts with advantage of strengthening the slope with excessive earth works to provide construction access and working associated with commonly used retaining systems. In the present research work a parametric study has been made using commercial computer program “Slide 6“, which utilize different methods for solving slope stability problem, Bishop method has been used herein to analyze un nailed and nailed slopes with granular soil, different slope heights and angles have been considered. Some of nails parameters have been studied here in, positions of nail, length of nail, angle of nail inclination


Introduction
The fundamental concept of soil nailing consists of placing in the ground passive inclusions, closely spaced, to restrain displacements and limit decompression during and after excavation. Slope stability analysis procedures have been developed to evaluate the global stability of the soil nailed mass and/or the surrounding ground, taking into account the shearing, tension or pull-out resistance of the nails crossing the potential failure surface. In the present research work a parametric study was conducted to study the following variables of nailed slope system: 1. The best location of nail and nails group within the slope. 2. The optimum length of nails. 3. The optimum angle of nails inclination. 4. The optimum spacing of nails.
Commercial software, well known (Slide 6) was used herein to conduct the parametric study. Verification of program (Slide6) results was made with Finite Element Solution.

Design of Soil Nailing
The following documents have been widely referred by designers in designing the soil nailing strengthen in works: a) BS 8006:1995 code of practice for strengthened reinforcement soils and other fills. b) Federal Highway Administration (FHWA): Manual for Design and Construction Monitoring of Soil Nail walls. c) BS 8081:1989 code of parametric for ground Anchorage.

Review of Literature
The available design procedures involve different definitions of safety factors and different assumptions with regard shape and failure surface the type of soil-nail interaction, and the resisting forces in the nail.Limit-Force equilibrium methods were developed by Stocker et. al. (1979) assuming a bilinear sliding surface and by Shen et. al. (1981) considering a parabolic surface. Both methods take into account only the tension resistance and pull-out capacity of the inclusions. A more general solution, integrating the two fundamental mechanisms of soil-inclusion interaction (i.e., lateral friction and passive normal soil reaction) was developed by Schlosser (1983). This solution considers both the tension and the shearing resistance of the inclusions as well as the effect of their bending stiffness. A kinematical limit analysis design approach was presented by Juran et. al. (1990) and Byrne (1992). This approach allows for the valuation of the effect of the main design parameters (i.e., structure geometry, inclination, spacing, and bending stiffness of the nails) on the tension and shear forces generated in the nails during construction.The kinematic limit analysis approach was applied by Askari et. al. (2009) to study the three dimensional stability of reinforced slops.

Method of Analysis and Method used
Program well known (SLIDE 6) has been used in the present research work to analyze un-nailed slopes. The program utilize Bishop Limit equilibrium analysis method has been used.

Nail Implementation in Program 2.1.1 Intersection with Slip Surface
In order for the nail to have an effect on a given slip surface, the nail must intersect the slip surface. If the nail does not intersect a slip surface, then no nail force will be applied to the slip surface, and the nail will have no effect on the safety factor of that slip surface.

Location of the Applied Force
When nail intersects a slip surface, a force is applied at the point of intersection of the slip surface with the nail (i.e. to the base of a single slice). The applied force is simply a line load, with units of force per unit width of slope.

Orientation of Applied Nail Force
The orientation of the applied force is assumed to be parallel to the direction of the nail, as shown in Figure 3.

Factor of Safety Calculations
The factor of afety is defined as the ratio of the forces resisting motion, to the driving forces. Driving forces include the mass of each slice accelerated through gravity, seismic forces, and water in a tension crack. Resisting forces arise from the cohesion and frictional strength of the slip surface. Active Support is included in the Slide analysis as in Eq. By this definition, Passive Support is assumed to increase the resisting force provided by shear restraint, in the factor of safety equation. Soil nails or geo-textiles, which only develop a resisting force after some movement within the slope has taken place, could be considered as passive support.

Verification of the Method and Program used
Verification of the method used was done by comparing the results of un-nailed and nailed slopes by both limit equilibrium method (slide method) and Finite Element Method. The finite element solution was conducted using 2D Plaxis software. The properties of the model solved are in Table 1, and the results are presented in Table 2. The results show very good agreement among the limit equilibrium method, finite element method, and the used method results (i.e. slide 6). Figure 4 shows slide 6 model of the verification problem.

Parametric Study
A parametric study is conducted to investigate the influence of the various parameters of the nailed slope system. The parameters are nail position, nails length, nail inclination angle, and nails spacing. Un-nailed and nailed slope were modeled by program (SLIDE 6). Soil extent for both sides of each model were enough to be not intersect by any possible failure surfaces (SLIDE 6 Manual). Three different Height of slopes were considered (H = 10, 15, 20 m), and three different angle of slopes were considered (β =30 o ,45 o ,60 o ). Granular soil was used with Ø =30, small cohesion value was introduced (C = 5kPa). The small cohesion value prevent the development of small failure surfaces which gives a small unreal value of F.S. Nails properties used are presented in Table 3 (FHWA, 1998). Figure 5 shows un-nailed slope model with (H = 20m, β =30).

Optimum Angle of Nail Inclination
Many models have been used to investigate the optimum angle of nail. The properties of the used models are presented in Table 4. The length of nail has been selected depending upon the variable (K) as stated in previous article (2.3) in the present study. Figures 12, 13, and 14 show the relationship between the angle of nail inclination (θ), (measured from the horizon), and the factor of safety improvement percent F.S%. These curves show obviously that the larger improvement in Factor of Safety can be produced with different nail inclination angles (θ), and it ranged between 10 o to 25 o . Figure 15 shows the relationship between the angle of earth slop (β) and the percent of the tangent of nail angle to the tangent of slope angle (δ= tan θ/ tan β). The Figure 15 could be used in the design procedure to obtain the optimum angle of nail inclination.

Optimum Spacing of Nails
Three angles of earth slopes have been investigated (β=30 o , 45 o , 60 o ) with height of slope (H=20m), the length of nails were used depending on K variable (article 2.3. in the present study). Inclination angle of nails (θ=15 o ). Figure 16 shows the relationship between F.S% and nails spacing for the three different angles of slope. This relationship indicates that the influence of nails spacing was the same regardless of slope angle until the spacing become less than 2 m, different influence obviously noticed. The behaviour of nails and in between soil become like a block, when spacing less than 2 m, and the failure surface cannot pass through the nails group. In this case the length of nails govern the value of F.S.