14 Construction procedure

14.1 Support of temporary excavations

14.1.1 General

Temporary excavation support systems should be selected, for one or more of the following purposes:

  • the protection of persons within and around the excavation;
  • the control of movement of the ground around the excavation perimeter; or
  • to minimize the excavated area.

14.1.2 Performance criteria

When selecting and designing ground support systems the following should be considered:

  • the capability of the chosen system to prevent water ingress through the sides and/or the base of the excavation;
  • the capability of the chosen system to accommodate service crossings, variations in plan layout of the excavation and stop ends;
  • access into the excavation and the provision of adequate working space in the excavation;
  • where necessary, the potential for installing, supporting and removing the support system, incrementally as work progresses;
  • installation of the support system so that operatives are always working from within a protected area;
  • the use of ground anchors in place of strutting and propping to reduce the potential obstructions within the excavation;
  • soil nailing incorporating geotextile mesh as necessary to support earth faces (see CIRIA C637 [73] and BS 8006-2);
  • the means for handling structural members particularly in wide and/or deep excavations, during installation and removal, and for supporting them during use; and
  • provision of a suitable upstand or barrier to reduce the risk of falls into the excavation.

Suppliers of support systems, their literature and websites should be consulted for information on the types, performance capabilities and capacities of systems which are available; systems may include trench boxes to BS EN 13331-1 or be of traditional construction utilizing materials such as timber, steel sections or precast reinforced concrete. BS 6031:1981 should be referred to for guidance on the use of timber as the dominant structural material.

14.1.3 Buildability criteria

There are a number of issues relating to the buildability of the support system which should be considered:

  • means and sequence of installation;
  • maintenance of the system when in use;
  • backfilling requirements;
  • ease of striking parts or all of system without compromising safety;
  • protection against falls of system components due to unintended removal of load;
  • temporary support and reinstatement of service crossings.

NOTE Practical guidance on these issues can be found in HSE "Safety in Excavations" HSG 185 [70], BS 6031:1981 and suppliers' literature and websites. BS 6031:1981 is obsolescent, but is considered to contain important information on timber support, and is still available from BSI.

14.1.4 Inspection criteria

NOTE There is a legal duty to inspect the support system to ensure its continuing integrity in use.

An inspection should identify:

  • signs of overstressing, movement or loosening of support members;
  • excessive continuing deflection;
  • mechanical damage;
  • abnormal ingress of water or ground;
  • any monitoring system(s) relevant to the stability of the excavation or adjacent structures; and
  • fluid loss, where proprietary hydraulic systems are employed.

14.2 Temporary support by permanent structure

Consideration should be given to using components forming part of the permanent structure as support for temporary excavations: such components include diaphragm walls and secant or contiguous piled walls for ground support along with floor slabs to support the walls.

15 Trenches

15.1 Construction methods

When designing a trench, consideration should be given to the following factors which influence the method of excavating, supporting and backfilling it:

  • purpose and location of the trench;
  • size of the trench including due allowance of access, egress and adequate working space for construction of the permanent work;
  • the length of trench to be open at any one time and the period for which it is to remain open; no trench should be left open longer than necessary;
  • nature of ground including information from any trial pits and intermediate boreholes, etc.;
  • avoidance of excavation in loose or made ground or alongside earlier backfilled excavations, all of which circumstances require extra care;
  • removal of ground water (see 13.4 and 15.5);
  • statutory obligations (see 15.2);
  • obstructions above and below ground level;
  • buried services.

15.2 Statutory obligations

General recommendations and guidance on the statutory bodies and infrastructure owners that should be consulted in advance of excavation works being undertaken are given in Clause 5.

The requirements of the New Roads and Streetworks Act 1991 [74] including requirements for backfilling and reinstatement should be taken into account.

15.3 Excavation procedure

15.3.1 Trenchless techniques

The use of trenchless techniques should be considered as a minimally disruptive alternative to open trench excavation. Such techniques should also be considered when obstructions on the line of trench cannot be disturbed, or where the depth is too great for open trenching to be cost effective or practicable. (See CIRIA SP147 [75]).

15.3.2 Methods available

The trenching techniques including excavation equipment and support regimes should be compatible with the purpose of the trench and the environment in which the trench is excavated.

15.3.3 Trenches with vertical sides

When excavating a trench with vertical or near vertical sides and for which it is intended to provide additional support, the following should be considered.

Support should be provided for all vertically sided trenches of any depth where ground conditions so require it, including shallow trenches where operatives are required to work kneeling.

Support systems should be selected and installed in a manner that does not involve risks to operatives due to instability of the sides of the trench. Details of various methods for supporting vertical sides or providing a protected working space are given in proprietary shoring manufacturers' literature or in HSG 185 [70].

15.3.4 Bottoming of trenches

Whatever the permanent work, some manual trimming can be necessary in the trench bottom. During this operation disturbance of the soil at formation level, particularly in clays, silts and fine sands should be minimized. Depending upon the purpose of the trench consideration should be given to laying gravel, broken stone or weak concrete as soon as the formation is exposed to form a protective layer over such soils. Groundwater should meanwhile be kept below formation level.

In rock, excavation should be taken down to below formation level and then uniformly compacted sand or other fine granular material, or concrete placed to produce a true bottom. Where the longitudinal gradient is steep, the material placed should be sufficiently coarse to resist erosion by a permanent flow of groundwater along the base of the trench.

15.4 Hand excavation of trenches

Only when it is not practicable to excavate mechanically should hand excavation be undertaken.

Given the variety of sizes and types of excavator now available and the availability of trenchless techniques, conditions necessitating large-scale hand excavation should be extremely rare but could include:

  • a) ground too steep for a machine, or working space very restricted;
  • b) road and railway crossings where a machine would interfere with traffic;
  • c) sites where cables, mains, drains and other obstructions are known to exist;
  • d) paved surfaces or lawns where damage to the surface by a machine cannot be tolerated;
  • e) very bad ground which is incapable of supporting the weight of a machine.

NOTE The Manual Handling Regulations 1992 [76] apply to hand excavation.

15.5 Methods of dewatering trenches

Water should not be allowed to enter or accumulate in the bottom of a trench or excavation, and pumping or dewatering facilities should be provided to deal with ground water or surface water inflow. The disposal of water from trench excavations might require measures to prevent pollution of watercourses (see 7.5); CIRIA C515 [77] should be consulted for further information.

15.6 Backfilling and reinstatement of surface

Backfilling and compacting around any pipes or conduits should be done in accordance with the asset owner's requirements or a nationally recognized specification (see also Clause 8).

NOTE The New Roads and Streetworks Act 1991 [74] contains requirements for backfilling and reinstating excavations in highways.

16 Pits and shafts

16.1 General

Pits and shafts should be constructed on the principles described in Clause 12, Clause 13, Clause 14, and BS 6164.

16.2 Methods of support of excavations

Depending on the depth required, ground and groundwater conditions, etc., the following methods of support for pits and shafts should be considered:

  • cantilevered or propped, contiguous or secant piling or diaphragm waling;
  • cantilevered or propped, sheet piling;
  • segmental concrete segments (placed by underpinning or caisson sinking techniques) or sprayed concrete;
  • excavation support systems described in Clause 12;
  • tubular steel casing;
  • the use of ground anchors.

The excavation of pits and shafts should be carried out in accordance with the recommendations of this standard.

In addition, the recommendations of BS 6164 should be complied with.

Where the lining of pits or shafts is supported by ring walings or ground anchors; and is intended to be self supporting or to be supported without the use of props, consideration should be given to applying the "observational method" (CIRIA R185 [16]).

16.3 Wells and hand excavated piles

Wells and piles in inaccessible locations can be sunk as small diameter deep shafts by hand excavation techniques. The recommendations of BS 6164 should be strictly adhered to.

BS 6031:2009 Code of practice for earthworks