8 Execution
8.1 General
8.1.1 The sequence of soil nail construction can include five main processes:
- a) preliminary work;
- b) excavation / face preparation;
- c) nail installation;
- d) drainage installation;
- e) facing installation and connection with nail heads.
NOTE The order of these processes depends on compliance with the design construction sequence and requirements and can vary.
8.1.2 Unforeseen circumstances, such as changes in ground or hydraulic conditions, shall be reported immediately in accordance with Clause 4.
8.1.3 Where material properties, or construction processes, can be adversely affected by climatic conditions (e.g. extreme heat, cold or heavy rain), then consideration shall be given to the implementation of special measures to mitigate the detrimental effects.
8.2 Preliminary work
The following preliminary works can be required:
- a) preparation of the existing vegetation and facing;
- b) setting out the position of the slope;
- c) construction of safe and appropriate access systems (for example: scaffolding platforms, excavated benches, access by crane);
- d) installation of drainage to control surface runoff and groundwater to allow soil nailing to be undertaken;
- e) installation and testing of nails to verify the design characteristics and assumptions (see Clause 9);
- f) installation of geotechnical monitoring instrumentation (see Clause 9) and verification required for control of the soil nailing works and of their effects on adjacent structures and services.
8.3 Excavation / Face preparation
8.3.1 The stability of the excavation, the face preparation and any adjoining or adjacent land property or services shall be examined before the beginning of the soil nailing work.
8.3.2 Excavation may comprise a bulk operation followed by trimming of the face (see Annex A).
8.3.3 Restrictions can be imposed on progressive levels of excavation.
8.3.4 Superficial soils, near the ground surface, are often of made ground and poor quality and may require special measures to prevent their collapse.
8.3.5 Even if defined in the design, the stand up time of the cut face should be verified. From this, an estimate should be made of the maximum length of the face, which can be trimmed in advance of soil nail installation (see Clauses 5 and 9).
8.3.6 The period between trimming of the face, soil nail installation and facing construction should be minimised to prevent risk of face collapse.
8.3.7 Where stand up times are short and face stability cannot be maintained, then consideration should be given to the use of excavation in small sections and using berms for temporary support.
8.3.8 Excavation tolerances, such as slope angle, temporary bench levels and line of the proposed excavation, shall be agreed before the start of excavation.
8.3.9 Excavation limits shall be agreed in advance of the start of excavation.
8.3.10 Before the start of excavation, monitoring procedure shall be in place with the required action to be taken to ensure that the agreed excavation limits are not exceeded.
8.3.11 Bulk excavation may proceed in advance of the benching and local face trimming for soil nail and facing installation, subject to the stability and displacement requirements.
8.3.12 Where the stability of the cut face has not been assessed, it shall be investigated prior to full excavation by trial pits and/or observation of the initial bulk excavation.
8.3.13 If soil nail installation or facing construction does not follow face trimming within the anticipated stand up time, then consideration shall be given to local back filling of the face to maintain stability.
8.3.14 In the event of actual face collapse, an investigation shall take place and actions shall be taken to prevent a reoccurrence.
8.3.15 At each level of excavation the soil type and level of any groundwater encountered should be observed and compared with those anticipated from the ground investigation. Where differences are observed, these shall be reported and any necessary actions to be taken.
