Design Considerations
Helical soil nails are passive bearing elements which rely on movement of the soil mass to mobilize the soil shear strength along the nail. As a result, soil nail walls typically experience more lateral movement than tieback walls of similar height. By allowing this movement, the highest stress in the soil nail is near the failure plane, centered between the opposing tensile forces.
Conversely, the highest stress in a tieback is at the wall face. Therefore, soil nails have less nail head force than tiebacks for a similar size wall, which results in potential cost savings by using soil nails due to reduced wall thickness requirements.
The following should be considered when designing soil nail walls.
- Not all soil conditions are suitable for construction of helical soil nail walls. Excavations are generally made in 3 to 5-foot steps, depending upon soil type and strength, and the soil should be able to stand unsupported for a period of at least one day after the vertical cut is made. Soil conditions that may not be favorable for helical soil nail wall construction include:
- Dry, poorly-graded cohesionless soils; e.g., clean sands or sands with SPT N-values less than 5 blows/foot
- Highly plastic clays, expansive soils, organic soils, or soils with a liquidity index of 0.2 or greater
- Clays with SPT N-values less than 4 blows/foot
- Soil profiles with high groundwater levels - dewatering may be required to facilitate installation
- Soil with cobbles, boulders or weathered rock lenses
- Highly corrosive soils and Collapsible soils
- Very dense sands and hard clays - may be difficult to penetrate without pre-drilling a pilot hole
- A failure plane generally develops at the top of the wall at a horizontal distance of about 0.7 to 0.8 times the height of the wall away from the wall face (Lazarte, Elias et al. 2003). This distance may be reduced by battering the wall face. Any structure, utility, roadway, etc. that would be impacted by the wall movement and/or failure plane should be considered during the design phase.
- Top of wall lateral movements on the order of 0.2% to 0.3% of the wall height should be expected with soil nail lengths to wall height ratios between 0.7 to 1.0, negligible surcharge loading and a design including a global factor of safety of at least 1.5. As a general guide, the soil mass located between the failure plane and the wall facing may slump approximately 1/8-inch laterally and 1/8-vertically for each 5-foot depth of excavation.
- Soil nail walls may be designed with a slight batter rather than vertical to account for anticipated lateral wall movement.
- There may be restrictions to the design soil nail lengths, including property lines, right-of-way (ROW), underground utility corridors, bridge abutments or existing structures.
- Consider temporary and/or permanent surcharge loads from adjacent structures, roadways, construction equipment, fill placement, etc.
- Maximum wall heights for helical soil nail walls are practically limited to 20 to 30 feet depending upon soil conditions. Increased heights may be considered with a stepped wall design.
- Helical soil nails are typically installed in a grid pattern, spaced 3 to 5 feet vertically and 4 to 7 feet horizontally.
- Helical soil nails are typically installed at an angle of 10 to 15 degrees downward from horizontal, although a batter in not required. The downward installation angle is a carryover from grouted nail design where an angle is required to prevent wet grout from flowing out of the hole.
The design procedure for helical soil nails is similar to that for grouted nails. For a helical soil nail, the bond stress with the soil is assumed to act along a cylindrical surface area defined by the outside edge of the helix plates.
As the construction of the wall progresses, the upper soil nails become less important for the stabilization of the soil mass, and depending upon wall height, may not contribute to the global stability at the final excavation phase.
However, the upper soil nails are instrumental in providing stability during the early phases of excavation and contribute to limiting wall deflections.
Figure 22 shows the distribution of tensile force in Nail 1, cumulative wall movement and the critical failure surface as the soil nail wall construction progresses. The upper schematic of Figure 22 illustrates the tensile force distribution along the top soil nail as construction continues through the various excavation phases. Phase N in the upper schematic does not reflect the maximum soil nail tensile force since additional loading occurs after construction to reach long term equilibrium of soil nail forces.
