Soil nailing, nail walls

Soil nailing is used to secure steep slopes or walls of excavation pits. Nailing the soil increases its shear strength to such an extent that the nailed soil body can be regarded as a monolith and verified accordingly by calculation. The load-bearing behavior of this monolith or composite body can be compared to that of a gravity wall.

To secure the slope, shotcrete, for example, is applied on the excavation face so the soil nails  safely transfer the stresses resulting from the earth pressure into the soil. Soil nailing can be carried out in almost all soil types. Concrete-affecting substances in groundwater that may be present must be ruled out by prior investigations.

Soil nails can be used as temporary and permanent support. The necessary corrosion protection measures must be observed in each case. The corrosion protection for permanent soil nails must be applied at the factory.  

The excavated soil must ensure an unsupported temporary stability. If this cannot be expected, soil improvement measures in the form of grouting must be provided. Depending on the soil type, the excavation can advance the supporting measure by approx. 1.5 m. After applying the reinforcement, shotcrete is applied in a necessary thickness to be verified.     

In the following step, the soil nails are produced. For this purpose, suitable drilling methods are selected depending on the existing soil. The tension members are completely grouted with the soil using a cement injection. The maximum nail spacing is 1.5 m in horizontal and vertical direction. The soil nails are screwed against the shotcrete with an anchor slab and a nut, which results in a slight tensile or prestressing force.

If the soil nails are produced in very stable soil, they can also be produced prior to reinforcement and application of the shotcrete.

Soil nails differ from anchors in that they do not have a free anchor length and are installed only slightly prestressed.

For the soil nails, steel with an official approval is used, e.g. as B500B multi-rib reinforcement bars (GEWI) or as steel support members made of fine-grained structural steel as hollow sections (TITAN micropiles) in various diameters required for structural purposes. Detailed information can be found in the suppliers’ general building supervision approvals.

Cements with special properties according to DIN 1164-10 and cements according to DIN EN 197-1 shall be used as raw materials for the cement mortar for grouting the tension members, taking into account further specifications in the suppliers’ general building supervision approvals.

Approvals from various suppliers require that the facing made of shotcrete or precast concrete elements must at least correspond to the strength class of a C 25/30 concrete.

Further technical details on spacers, plastic casing, etc. can be found in the suppliers’ general building supervision approvals.

Verifications for soli nailing is not regulated in a standard. However, the “Institut für Bautechnik”, DIBt, has issued general building supervision approvals for soil nailing systems from various suppliers, which contain information on the calculation.

The calculation of soil nailing essentially requires the following verifications:

a. Verification of the internal stability by means of moving body analysis, generally with a fracture mechanism consisting of two bodies. According to the applicable approvals, the design of the nails (tension members) is governed by the verification that results in the largest nail forces. The following analyses are necessary for this purpose:

• load components in the nails due to the moving body analyses
• load components in the nails from the earth pressure on the shotcrete facing (outer skin)

However, the force from a tension member may only be taken into consideration if the following conditions specified in DIN 4084:2009 are met:

"A tension member is considered to be self-tensioning if the moving body in which the head of the tension member is located, moves approximately as a rigid body on a slip surface and the angle ψA between the axis of the tension member and the most unfavorable slip surface achieves

• for loose, non cohesive soils or soft cohesive soils: ψ_A = 75°,
• for firm cohesive soils: ψ_A = 80°,
• for medium-dense non cohesive soils and for semi-solid cohesive soils: ψ_A = 85°,
• for dense non cohesive soils: ψ_A = 90°.

b. verification of sliding safety

c. verification of the safety for loads with large eccentricities

d. verification for the bearing resistance safety

e. dimensioning of the shotcrete or precast facing concrete elements (partial area loading, punching).

The verifications are to be carried out for construction stages and for the final state.

For the example shown below of an embankment secured with soil nailing and loaded at the top of the embankment with a live load of p = 16.6 kN/m², the following video shows how the computer program GGU-STABILITY can be used to verify internal stability, sliding safety, the safety for loads with large eccentricities, bearing resistance and slope failure safety for the design situation DS-P.