Most research to date has been done in homogeneous soils. However, there is no reason why the concept cannot be applied to heterogeneous soil masses if proper consideration of soil properties is made and rationally applied to the selection of nail length and spacing.
To be economical, soil nailed walls should be constructed in ground that can stand unsupported on a vertical or steeply sloped cut of 3 to 6 ft (1 to 2 m) for one to two days, and can maintain an open drill hole for a few hours. Soils considered favorable to soil nailing are as follows:
1. Naturally cohesive materials (silts and low-plasticity clays that are not prone to creep)
2. Naturally cemented sands and gravels
3. Weathered rock
4. Fine to medium, homogeneous sand with capillary cohesion of 60 to 100 lb/ft2 (2.9 to 4.8 kPa) associated with a water content of at least 5 to 6 percent
According to FHWA Report RD-89-108, soil nailing is generally not considered cost — effective or applicable in the following soils:
1. Loose granular soils with field standard penetration N values lower than about 10 or relative densities of less than 30 percent
2. Granular cohesionless soils of uniform size (poorly graded) with uniformity coefficient (D60/D10) less than 2, unless found to be very dense; nailing of these soils may be impractical because of the necessity of stabilizing the cut face (by grouting or another permanent technique) prior to excavation
3. Soft cohesive soils with undrained shear strengths of less than 500 lb/ft2 (24 kPa), because of the inability to develop adequate pullout resistance
4. Highly plastic clays (LL > 50 and PI > 20 percent), because of their potential for excessive creep deformation
5. Expansive (swelling) and highly frost-susceptible soils
Soil nailing is also not recommended for the following conditions:
1. In ground with water pressure present at the face
2. Below the groundwater table, unless the slope can be effectively dewatered prior to excavation
3. Loose fill, granular soil with no apparent cohesion
