8.4 Nail installation

8.4.1 General There are two principal methods of installing soil nails: direct installation (driving), and drilling and grouting. Direct installation can be performed by percussive, vibratory or ballistic methods. For drilling and grouting technique, grouting methods can involve either gravity or pressure grouting procedures. The nail installation method shall be appropriate to the ground conditions. Soil nail components shall be handled with care during transportation, storage and installation. Inspection shall be carried out to ensure the integrity of the components before installation, with particular attention to coatings and corrosion protection components. Nail installation shall be carried out in a controlled manner with minimum disturbance and detriment to the stability of the ground or previously installed nails. Nail installation shall be carried out to the tolerances and the sequence required by the design. Generally attainable construction tolerances are reported in Annex A. Nails shall be installed so that the reinforcing element projects a sufficient distance beyond the face of the slope to allow the connection to the facing system in accordance with the design requirements. All locking nuts, plates and other fasteners shall be securely fastened. If couplers are used to join sections of reinforcing elements, precautions shall be taken to ensure proper and durable coupling. If the presence of obstructions (or unexpected ground conditions) prevents the complete installation of a nail or causes it to deviate from the design alignment, then the installation method shall be reviewed and consideration given to relocation of the nail. Nails already or partially installed shall not be removed.

8.4.2 Driven installation methods Nails may be driven into the ground using jacking, screwing, percussive, vibratory or ballistic action to displace the soil. The reinforcing element is normally in direct contact with the ground. To avoid buckling during installation, the reinforcing element shall be sufficiently stiff, with regard to its length, the nature and state of compaction of the soil and the power of the driving tools used.

NOTE The reinforcing element may be guided whilst being driven.

8.4.3 Drilled installation methods General Nails can be installed into the ground using rotary or rotary-percussive drilling methods to remove the ground. The drilling method used and the rate of drilling shall ensure that the nominal hole diameter specified in the design is achieved along the entire length of the nail. The borehole shall be drilled to a depth sufficient to ensure that the design length of the reinforcing element can be installed. If the nail transfers load between the ground and the reinforcing element by grout bond, unless the cover is assured otherwise, spacers may be fitted to ensure that the minimum annulus of grout required by the design is formed around the reinforcing element. If the nail relies on grout as a part of the corrosion protection system, then spacers shall be fitted to ensure that the minimum grout cover, specified by the design, is achieved. Uncased drilling Open hole drilling can be used in stable ground or in unstable ground with suitable stabilising fluid.

NOTE 1 Where ground is stable, air-flushing techniques are commonly used with rotary, rotary-percussive or down- the-hole hammer drilling techniques.

NOTE 2 In certain ground conditions, the use of water as a flushing medium may be more appropriate, due to its greater density, capacity to support the borehole and its increased efficiency in drill spoil lift at lower flushing rates. Water should not be used if it increases the risk of soil mass instability and decreases the bond capacity between ground and nail.

NOTE 3 Where a dense drill fluid is required to support the hole in less stable soils, cement grout as a flush medium and also as the load transfer medium is commonly applied. Associated with its use are demands for efficient recovery of the flush returns, effective removal or partial removal of the drill spoil from the grout return, utilisation of re-circulation pumping systems and disposal of excess grout and grout contaminated spoil. Where a non-return flush condition is encountered, the drill bit or down-the-hole hammer should be withdrawn until flush return is recovered. If introducing the reinforcing elements or corrosion protection ducts into the borehole, care should be taken to ensure that they are not smeared against the sides of the hole and contaminated by the ground. It is recommended that soil nail system is installed in boreholes immediately after drilling. Cased hole drilling and hollow stem auger drilling Cased hole drilling and hollow stem auger drilling are methods, which should be used in ground conditions where the borehole will not stand open for its entire length before grouting. Open hole drilling may continue to the base of the hole beyond the drill casing, where the borehole will stand open for part of its length before grouting. Where the flush return is lost, the drilling equipment should be withdrawn into the casing to recover flush so that flush penetration of the soil mass does not occur. If the ground is not stable, then grouting of the borehole shall be carried out prior to removal of the casing or augers. If a reinforcing element is pushed into a freshly grouted hole, measures shall be taken to ensure that it enters the hole centrally and that contaminants are not introduced. Self-drilled hollow bar soil nails With this technique, the reinforcing elements are fitted with a drill bit and they are installed into the ground during drilling. Hollow bar soil nails are typically installed using rotary percussion.

NOTE The installation of reinforcing elements by rotary drilling and simultaneously flushing with grout is sometimes referred to as self-drilled hollow bar soil nails or simultaneously drilled and grouted. The rate of drilling, grout pressure and flow rate should be adjusted to suit the ground conditions to ensure the correct borehole diameter.

NOTE In some ground conditions, simultaneous drilling and grouting can result in an enlarged grout body, compared to the size of the drill bit. If self-drilled hollow bar is used in unstable ground, a suitable stabilising fluid shall be used.

NOTE Water should not be used if it increases the risk of soil mass instability and decreases the bond capacity between ground and nail. A flush return should be observed at all times during drilling when the drill is advanced. If lost, the drill string should be retracted until the flush returns. If structural grout is not used as stabilising fluid, then the structural grout shall be introduced when the final borehole depth is achieved, and it shall be visually observed that the stabilising fluid is replaced by the structural grout.

EN 14490:2010 Execution of special geotechnical works – Soil nailing