trial pit during excavation

RSA GEOTECHNICS LTD ASHBURNHAM HOUSE 1 MAITLAND ROAD NEEDHAM MARKET IP6 8NZ

Trial Pitting Safety Guidance, April 2011 TELEPHONE (01449) 723 723 FAX (01449) 723 907

TRIAL PITTING SAFETY GUIDANCE

Unsupported excavations can be unstable and can collapse without any warning. Operatives working near or within an unsupported or unstable trial pit or trench may risk death or serious injury. Other significant hazards may include: contact with buried or overhead services; material, equipment or personnel falling into excavation; contact with plant; hazardous substances; water influx; entrapment in soft soil; stability of adjacent structures; fumes or toxic ground gases and/or lack of oxygen.

Careful planning and authoritative supervision are the essential aspects required for safe trial pitting. This document provides procedures for the safe digging of trial pits and associated ground investigation works. The following control measures must be followed:

• A competent person must supervise the work and operatives must be given clear instructions on working safely when near excavations.

• If there is a risk of contact with buried services then up-to-date utility drawings must be consulted prior to carrying out any ground-works.

• A cable avoidance tool should be used to locate the buried cables and a hand dug trial pit shall be excavated if in doubt.

• Do not work in vicinity of overhead power lines if plant could come within 9m of the power line.

• The excavated sides must be prevented from collapse either by digging a safe angle or by shoring with timber or support systems.

• If manned entry cannot be avoided then suitable access and egress provisions must be in place.

• No unsupported access shall be allowed if the sides are unstable or the excavation is deeper than waist height.

• If it is necessary to leave the excavation open or unattended, a secure cover or a suitable barrier or fence must be provided.

• Whenever possible, the workings should be covered outside work hours.

Mechanical excavators are capable of moving large amounts of soil in a matter of seconds. They will also inflict major damage to anything and anybody that gets in their way. Whilst it is important for the operator to hold evidence of training it is equally important that other site personnel are familiar with safe working practices when working alongside machine excavators. The key safety factors are summarised below:

• Never approach a working mechanical excavator until the operator has seen you.

• Always stay in clear view and ensure that the operator is aware of your intentions.

• Never stand under a raised bucket and always stand well clear of the arm or backhoe.

• Always stand at the end of an excavation furthest from the machine.

• The sides of the trial pit or trench can be unstable and the operative should never attempt to stand along an unsupported long face of the excavation.

• If manned entry is required then plant should be switched off or kept away from the excavation to avoid the risk of movement or engine fumes.



Designers of ground investigations are duty bound to avoid siting excavations in close proximity of potentially dangerous buried or overhead utility services, unless absolutely necessary. If an excavation has to be located close to known services then careful hand digging is the safest means of proceeding.

Everyone involved in trial pitting, especially the operative supervising the work, should be informed of any known or suspected services. If there is a risk of contact with underground services then up-to-date utility drawings must be available. A cable avoidance tool (CAT) should be used to locate electricity cables and metallic pipes. However, the CAT will not detect plastic, clay, concrete or fibre optic services.

Any excavation deeper than waist height will present a risk of an operative being trapped, seriously injured or killed should a side collapse. An unsupported side of an excavation will always eventually collapse. The unknown factor is ‘when’.

Waist height must be the maximum depth for an unsupported man-entry pit. No manned entry should be made into an unstable or unsupported pit deeper than waist height. Where pits are machine dug, soil samples can be recovered from excavated material at ground level clear of the open pit.

Ground investigation surveys will often require detailed observations, depth measurements and photographs to be taken giving rise to the temptation of treading close to the edges of an unsupported pit with the risk of the edge giving way and the logger falling into the pit. The deeper the pit, the greater the risk becomes. In some circumstances the risk can be minimised by using boards to span the pit. Alternatively the sides could be battered to a safe angle. A simple bar placed across the pit (over which a measuring tape can be draped) will eliminate the need to reach over whilst taking depth measurements.

If manned entry is required within an excavation deeper than waist height the pit must be supported. A wide range of proprietary trench support systems are available and may be preferred to traditional ‘timbering’. Hydraulic shoring systems can be installed from ground level thereby removing the need for entry into an unsupported or partly supported pit.

Before entering any excavation the supervising operative must ensure that there is a suitable means of access and egress and that there are no plant, equipment or materials stockpiled immediately adjacent to the excavation.

Andy Symis Operations Director, RSA Geotechnics Ltd 14 April 2011

trial pit during excavation

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