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CHAPTER 2.0 Soil Nailing with the SOIL SCREW®
Retention Wall System
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2.1 Designing with the SOIL SCREW® Retention Wall System The methodology for designing a soil nail wall using the SOIL SCREW® Retention Wall System is based upon the same limit equilibrium approach used to determine the stability of reinforced soil slopes and the internal stability of mechanically stabilized earth walls. An overview of the design methodology is provided in section 3.3. 2.1.1 Facing Deformation - The fundamental mechanisms and behavior of a soil nail wall reinforced with screw anchors will not differ appreciably from other soil nail systems in that controlling deformation of the wall face requires a skilled and efficient operator during excavation, soil nail installation and placement of the facing. To aid in limiting facing movement, it is common, for all systems, that spacing of the nails in the upper portion of the wall be minimized. And as discussed earlier, providing tiebacks in the upper rows can help to mobilize the soil strength and minimize wall movement. Controlling wall deformations is achieved by using proper installation techniques, a logical design layout and spacing, and by providing tiebacks where deformation control is critical. 2.1.2 Pullout Resistance - The screw anchors used in soil nailing are specially designed with 8-inch helices placed on approximately 2.5-foot centers. This spacing of helices at a distance of more than 3 helix diameters, center to center, helps to ensure that each individual helix acts in bearing without significantly affecting the capacity of the others. For design, the pullout resistance, both in front of and behind a potential failure plane, is analyzed. Design analyses for pullout resistance are provided in section 3.5.2. One benefit of a Chance screw anchor is that its pullout resistance can also be estimated during installation. Experience with Chance screw anchors has shown that the ultimate capacity of a screw anchor in pounds is approximately equal to 10 times the torque in foot pounds needed to install the anchor. Therefore, monitoring of the torque during soil nail installation can assist in determining soil nail capacity. Numerous pullout tests have been performed on screw anchors in granular and cohesive soils and have been documented in the literature. These have been summarized for use with the SOIL SCREW® Retention Wall System in section 3.5.2. 2.1.3 Tensile Strength of a Screw Anchor - The ultimate tensile strength of a Chance SS-5 screw anchor is 70,000 pounds minimum. This is 2 to 2.5 times greater than the typical soil nail design strength used and is actually controlled by the shear strength of the bolts that connect the shafts. The ultimate tensile strength of the 11/2-inch square shaft itself, away from the coupling area, is about 3 times the 70,000 pound strength of the assembly due to the need to resist torsional stresses during installation. The steels used in Chance type SS screw anchors are proprietary alloys designed to resist such stresses. In determining the actual allowable design strength of a soil nail (Table 3.5.2), both the original tensile strength and the sacrificial steel or loss of cross sectional area over the life of the project, need to be taken into account. The size and strength of Chance screw anchors are governed primarily by the strength required to resist torque during installation, and not the tension required for soil reinforcement. Therefore, Chance screw anchors provide more-than-sufficient sacrificial steel and tensile strength for long-term performance when compared to the actual design requirements. Calculations and further discussion on how the allowable design strength for Chance screw anchor soil nails is developed are provided in section 3.5.1. 2.2 Data Required for Soil Nail Design To perform a soil nail wall design, knowledge of the soil behind the wall face and the foundation soils supporting the wall (Figure 2.2.1) is required. It also requires knowledge of the project geometry, loading and surcharge conditions, groundwater conditions, and the properties of the soil nails. The purpose of this section is to discuss each of these areas that must be considered prior to performing a design. 2.2.1 Soil Parameters - As is the case for Mechanically Stabilized Earth Walls (MSE), the quality of a soil nail wall system will be a function of the soil being reinforced. Since a soil nail wall is comprised of over 98% soil, the characteristics of that soil (shear strength, consolidation, permeability, corrosion potential) will greatly influence the soil nail design and the wall performance. The shear strength of the retained soil must also be determined since this will determine what load will be applied to the back of the soil nail wall. The shear strength of the foundation soil will determine what length the soil nails will need to be to resist bearing and sliding failure modes for a wall of a given height. In general, the key input parameters, in terms of soil properties needed to perform the analyses in section 3.0, are shown on Figure 2.2.1, and are as follows: Soil Shear Strength - The two components that make up the effective shear strength, s’, of a soil are the internal friction angle (Ø') and cohesion, c', of the soil as represented in the equation: s' = c' + sn x tan f' where : sn = effective overburden pressure The angle at which soil resists shearing is termed its friction angle. The cohesion of the soil is the internal bond within the soil, which is not a function of the overburden pressure. The cohesion may decrease with time in a soil structure, and therefore, oftentimes is ignored in long-term designs. It is important to accurately determine the friction angle of the reinforced soil, retained soil and foundation soils. The friction angle of the soil is best determined from consolidated undrained triaxial compression tests which measure pore water pressures and drained direct shear tests performed at rates slow enough to ensure that pore water pressure does not occur during the test. The friction angle of a soil can also be estimated from grain size analyses, standard penetration testing and cone penetration testing for preliminary designs, but is best determined from actual laboratory or field testing for final designs. The dry and moist density of the soil should also be determined in order to provide an accurate assessment of the loading from each element. These values are best obtained from direct measurements of the soil density and moisture from Shelby tube samples. These values can also be estimated based on prior knowledge of the local soils or from grain size analyses. Consolidation/Creep - The tendency of a soil nail to creep in soil will be a function of the consolidation characteristics of the soil being reinforced. In general, if the soil is fine grained, the potential for soil nail movements in the long term is greater than that for granular soils. For permanent soil nail applications, soil nailing should not be performed in soils with moderate to high plasticity, such as soils classified as MH or CH, and caution should be used for temporary applications. Soil Corrosion Potential - The corrosion potential for a soil can be determined by running tests on the resistivity, pH, sulfates and chlorides, as discussed in detail in section 3.5.1. In general, soils with a resistivity of greater than 3000 ohm-cm and a pH between 5 and 10 are good candidates for screw anchor soil nails. Tests as described in Table 3.5.1 should be carried out on the soil to be reinforced to determine if the soil is suitable for nailing. 2.2.2 Surcharges and Loading Conditions - To accurately perform stability analyses for a soil nail wall, the geometry of the wall cross section is required. This includes the slope at the toe of the wall, the top of the wall and the wall batter (if any). Slopes as flat as 3(H):1(V) at the top or bottom of a wall can have a significant effect on the global stability of a wall. Other surcharge loads can include dead and live loads such as:
While all of these conditions are not incorporated in the design charts in Section 3, these can be analyzed using commercially available slope stability and soil nail design software, i.e., SNAIL, GOLDNAIL, STABL. 2.2.3 Drainage and Groundwater Conditions - The location of the permanent groundwater table is critical to a successful design. Soil nailing is best suited to applications above the water table. Excess seepage that cannot be controlled by strip drains during construction can deteriorate the excavated face, prevent shotcrete from bonding with the soil and provide excess pressure on the wall face. Therefore, soil nailing may not be feasible in areas where a permanent phreatic surface exists in the proposed wall volume. Seepage from surface infiltration can be controlled with well-designed drains (Figure 2.3.1), such as a lined interceptor ditch placed at the top of the wall and a subsurface drain placed inside the wall face. Details on drainage design are discussed in section 4.1.4. Prior to design, the type of facing for temporary and permanent walls needs to be identified. While shotcrete facing is most commonly used, depending upon the site conditions and the ultimate wall batter or slope, there are other options that may be desirable. Each of these is discussed below. 2.3.1 Temporary Facings - Temporary facing systems that can be used with the SOIL SCREWTM Retention Wall System include shotcrete and welded wire mesh; welded wire mesh, steel channels and geotextiles; and timber shoring. The most effective is shotcrete, since it creates a bond with the soil and fills in voids which may develop due to sloughing of soil at the wall face. For projects involving near vertical walls where minimal wall movement is required, this is the best option. Typically a 3 to 4 inch layer of shotcrete is applied. The shotcrete is lightly reinforced with welded wire mesh, as shown in Figure 2.3.1. Drainage can be provided, if needed, between soil nails at less than a 50% area coverage to allow for bond of the shotcrete with the soil. For sloping walls or for sites where vertical cuts are not required to install soil nails (cut and fill situations), use of a welded wire mesh facing or timber walers may be effective (Figure 2.3.2). In these situations where soils have an apparent cohesion and are cut on a slope, and soil sloughing is not a problem, the facing can be designed to contain the fill rather than provide a structural face to span nails in flexure. 2.3.2 Permanent Facings - Permanent facing systems that can be used with the SOIL SCREW®Retention Wall System for near vertical walls include reinforced shotcrete, cast-in-place and precast concrete panels, concrete masonry segmental wall units (Figure 2.3.3), and gabions. These facings must be designed to structurally support the soil loading applied between soil nails and be attached with a connector that is strong enough to resist punching failure of the nail at the wall face. The design of the permanent shotcrete or concrete facing for flexural stiffness and punching is adequately covered in FHWA-SA-96-069. For soil nailed slopes where the slope facing is stable without reinforcements, i.e., the soil nails are being used to increase the deep seated slope stability (Figure 1.3.4), a facing consisting of an erosion mat and vegetation consistent with the area can be utilized. |
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