Construction Technology - EXCAVATIONS AND TIMBERING

EXCAVATIONS AND TIMBERING

Before a foundation can be laid it is necessary to excavate a trench of the required depth and width. On small contracts such as house extensions this is effectively carried out by hand, but on large works it will be more economic to use some form of mechanical excavator. The general procedure for the excavation of foundation trenches is illustrated in Fig. 1.4.1.

TIMBERING

This is a term used to cover temporary supports to the sides of excavations and is sometimes called planking and strutting. The sides of some excavations will need support to:

• protect the operatives while working in the excavation;
  
• keep the excavation open by acting as a retaining wall to the sides of the trench.
  

The type and amount of timbering required will depend upon the depth and nature of the subsoil. Over a short period many soils may not require any timbering, but weather conditions, depth, type of soil and duration of the operations must all be taken into account, and each excavation must be assessed separately. Suitable timbers for this work are:

• Scots pine;
  
• Baltic redwood;
  
• Baltic whitewood;
  
• Douglas fir;
  
• larch;
  
• hemlock.
  

Typical details of timbering to trenches are shown in Figs 1.4.2–1.4.6

THE CONSTRUCTION (HEALTH, SAFETY AND WELFARE)

REGULATIONS 1996

This document establishes objectives for employers, the self-employed and employees, to ensure safe working and support in excavations.

Regulation 5 – Safe places of work

This is a general requirement applying not least to work in the ground. It places an obligation on all involved to assess the risks and to ensure reasonably practicable means to safeguard work in excavations.

Regulations 12 and 13 – Excavations

Timber or other suitable material must be provided and used to prevent danger from a fall or dislodgement of materials forming the sides of an excavation. Underground services must be foreseen (if possible), located, identified and assessed for risk to operatives working in excavations. Positive action is necessary to eliminate any risk and to prevent injury.

Deep excavations, tunnels, cofferdams and caissons require special consideration and must be correctly designed, constructed and maintained with regard for prevailing conditions. (See Chapter 3 of Advanced Construction Technology.)

Regulations 28, 29 and 30 – Training, inspection and reports

These require excavation support to be installed or supervised by adequately trained, suitably knowledgeable and experienced personnel. Prior to general access being given, an inspection of excavations by a competent person (usually the main contractor’s safety supervisor) must be made. That person must ensure that work can proceed safely. Following inspection, a written report must be filed for presentation if requested by the Health and Safety Executive. Further documented inspections should be undertaken daily or at the beginning of each shift or after severe weather conditions. Changes to the trench format and unexpected falls of earth should also be monitored.

BUILDING REGULATIONS

Notice of commencement and completion of certain stages of work. Building Regulation 14 requires that the building control office of the local authority is notified by a person carrying out building work prior to commencement and at specific stages during construction work. The notice should be given in writing or by such means as may be agreed with the local authority.

Notices of the stages when statutory inspections are required under this regulation occur as follows:

• Commencement 48 h
  
• Foundation excavation 24 h
  
• Foundation concrete 24 h
  
• Oversite preparation, before concreting 24 h
  
• Damp-proof course 24 h
  
• Drains before backfilling (foul and rainwater) 24 h
  
• Drain test after covering 5 days after
  
• Occupation before completion 5 days before
  
• Completion within 5 days
  

With the exception of notice of commencement, drain testing and completion, the amount of time required before progressing with other parts of the work is one day. Weekends and bank holidays are excluded.