Aspects of design

B.1 General

B.1.1 This annex is not intended to be a design guide, but its purpose is to introduce the soil nail design concepts to those involved in the execution of soil nail works.

B.1.2 Design of soil nailing is not explicitly covered by EN 1997. However, some general geotechnical matters, which are common to soil nailing and to other techniques, are included. This annex provides some guidance on the design of the soil nails.

B.1.3 EN 1997 defines three Geotechnical Categories: 1, 2 and 3. The design advice given in this annex would generally apply to a Category 2 structure. Category 3 structures are very large or unusual and require individual consideration and provision for special factors in addition to the design requirements of Category 2.

B.2 General design philosophy

B.2.1 Normally it is necessary to carry out calculations to assess the stability of the proposed works. In general, a limit equilibrium analysis is used and has been proven to give satisfactory results. One key parameter that requires careful consideration is the determination of the pullout resistance between the nail and the ground, as this is fundamental to the nailing process. The retention effect of soil nails is demonstrated in Figure B.1.

B.2.2 Any stability analysis should assess all the potential failure surfaces passing through the zone into which soil nails are proposed, and those passing behind and below the proposed zone of soil nailing, (Figure A.1). If any of the potential failure surface results in a lower factor of safety than required, then the proposed lengths and spacing of the nails should be adjusted until a satisfactory value can be achieved.

B.2.3 A variety of potential failure mechanisms may be used in a stability analysis (for example: single wedge, two-part wedge, log spiral and slip circle). However, the chosen mechanism should be compatible with the slope geometry and geology and external factors such as seismic activity.

B.2.4 Normally it is necessary to carry out the stability analysis for each stage of construction, as the completed works may not be representative of the lowest factor of safety.

B.2.5 The use of numerical modelling methods, such as the finite element method, may also be used for soil nail design and may provide useful predictions of movement. Analysis based on the finite element method is most efficient in cases where the relevant mechanism of failure differs from conventional circular failure surfaces.