7 Design considerations
7.1 General
7.1.1 A soil nail construction relies on the composite interaction of the soil, the reinforcing elements, and the facing (if applicable) to perform satisfactorily for the duration of its design life.
NOTE The stabilising force provided by a soil nail is generated passively.
7.1.2 The design of soil nailing works should be based on the general principles of EN 1990, EN 1991, EN 1997-1 and EN 1997-2.
NOTE Further guidance on the design of the soil nail system is given in informative Annex B.
7.1.3 Where the observational method is applied, particular attention shall be paid to 2.7 of EN 1997-1:2004.
7.1.4 The design shall consider the performance of the works, both during the temporary stages of construction and for the period of the design life, once construction is complete.
7.1.5 The soil nail system shall be compatible with the existing and anticipated ground conditions.
7.1.6 If the soil nail system to be used is not specified in the design, the design brief shall emphasise the particular requirements of the project. These requirements shall determine the final selection decision.
7.1.7 The design of the soil nail construction shall consider all relevant limit states in accordance with EN 1997-1.
7.1.8 Considerations should be given to the ability of the soil nail works to tolerate the expected magnitudes of total and differential settlements, frost heave, deformations and movements. Where necessary, such settlements, deformations and movements of soil nail constructions should be monitored during and after construction (see Clause 9).
7.1.9 For flexible facing the manufactures recommendation should be considered.
7.2 Design output
7.2.1 The design output should contain all the relevant information required for construction, some of which may be used in the preparation of the technical specification.
7.2.2 The design of a soil nailed construction should include the following:
- a) design life of the construction;
- b) Geotechnical Category of the construction in accordance with EN 1997-1:2004;
- c) geometry: plan view, representative cross sections, elevation;
- d) type of construction: for example, new construction or remedial work of an existing retaining structure;
- e) stages of the works;
- f) construction tolerances;
- g) suitability of the ground to mobilise adequate forces within the soil nails;
- h) sacrificial and production nails testing;
- i) serviceability requirements: for example, permissible deflection and/or displacement of the works and retained ground;
- j) monitoring of deflection and/or displacement;
- k) possible existing and expected constraints, imposed by site boundaries and adjacent buildings or structures to the nailed zone;
- l) buried services within or adjacent to the nailed zone;
- m) relevant specification of materials or products assumed in the design.
7.2.3 The ground investigation report shall, when applicable, contain the following information according to EN 1997-2:
- a) mechanical and physical properties: strength parameters, unit weight, grading curve, permeability, water content, compressibility, stone and boulder content;
- b) hydrogeology, water levels, frost susceptibility where appropriate;
- c) chemical, electro-chemical and biological properties of the ground and groundwater;
- d) suitability of the ground excavated to stand unsupported, in the temporary condition, as the works are undertaken.
7.2.4 The design report shall include, when relevant, the following specifications about the soil nail system:
- a) type, configuration and stagger pattern (nail section, length, orientation and spacing);
- b) short-term and long-term design strength, pullout capacity;
- c) installation of test nails and sacrificial nails;
- d) type of grout and strength;
- e) design life and corrosion protection system;
- f) restrictions on installation methods.
7.2.5 The design report shall include, when relevant, the following specifications about the facing system:
- a) mechanical requirements;
- b) environmental requirements;
- c) aesthetic requirements.
7.2.6 The design report shall include, when relevant, the following specifications about drainage system:
- a) type and specification;
- b) construction phases;
- c) durability and maintenance of the drainage system.
7.3 Design amendments
7.3.1 Design amendments may be necessitated by either unforeseen conditions or planned modifications.
7.3.2 Design amendments necessitated by unforeseen circumstances, such as changes in ground or hydraulic conditions, shall be reported immediately in accordance with Clause 4.
7.3.3 If construction activity requires a change of the final construction, as defined in the design documents, this shall only be carried out after the design has been checked and modified accordingly.
