Part 9: Excavations

Construction Health & Safety Management

Simon Smith (University of Edinburgh)

& Philip Matyear (Balfour Beatty)

Overview

General

Legislation

HSE view

Excavations

Planning

The Hazards of Excavation Work

Collapse of the sides

Materials falling into excavations

People and vehicles falling into excavations

People being struck by plant

Undermining nearby structures

Contact with underground services

Access to the excavation

Suffocating, toxic and explosive gases

Protecting members of the public

Supervision and Inspection

Safe plant - Quick Hitch Buckets

Summary

Further reading

General

Fact - Excavation work is one of the most hazardous operations on site.

Fact - If an incident on site is excavation related, death or serious injury is the likely outcome.

Fact - Up to seven people die each year working in and around excavations.

Fact - One cubic metre of soil can weigh over one tonne ( the same as a new mini car)

Legislation - Principle Requirements

Health and Safety at Work Act etc 1974

Requires employers and self employed workers to;

•Ensure they provide and maintain workplaces, equipment and systems of work that are , so far as is reasonably practicable , safe to workers and the public.

•Employees to co-operate and take care of their own and others health and safety.

Management of Health and Safety at Work Regs 1999

Requires employers and self employed workers to;

•Identify the control measures they need to take when reviewing the hierarchy of control (Reg. 4 ) to produce risk assessments

•Institute safety management systems

•Appoint competent persons to assist in health and safety management

•Ensure co-ordination and co-operation

•Provide information and relevant information training to employees

Construction, Design and Management Regs 1995

Applies to all stages of a construction project and places duties upon clients, designers and contractors. The regulations aim to ensure:

•Reduction of risk at the planning and design stages

•Selection of competent appointees and provision of adequate resources

•Effective management of health and safety throughout the project

Construction (Health, Safety and Welfare) Regs 1996

Regulation 12 & 13. Excavations, Cofferdams & Caissons

•All practicable steps shall be taken, where necessary to prevent danger to any person from accidental collapse.

•Identify and prevent risk from underground cables and services

•Ensure cofferdams and caissons properly designed, constructed and maintained.

•Regulation 29/30 – Inspection and Reporting Schedule 7.

HSE view

Comments by a HSE inspector when the ‘Be Safe and Shore’ guidance notes were issued in 1999, that are still valid today;

An HSE spokesperson said:

"Digging foundations and trenches for drains is one of the first jobs carried out on construction sites. For too many workers, it is also the last job they do.”

"To most people, being buried alive is the stuff of horror trims, yet every day workers either knowingly take that risk or are placed at risk by their employers.

"If those managing and supervising such work would stop to consider that the average cubic metre of soil weighs a tonne - and it is quite common for that volume to collapse into an unsupported excavations - they might treat it with the respect it deserves."

THINK BEFORE YOU DIG !

Excavations

No two excavations are the same, they can range from a small hole for a fence post to a long pipe line project and yet size doesn’t always matter when you consider the hazards. Here are some typical examples.

PlanningAll excavation work must be planned before work commences on site - This is essential if the work is to be carried out in a safe manner. Consider the picture belowHow much planning has gone into this

operation?

Would you enter this excavation?

Do you think they have considered the following?

•Ground conditions and adequate support

•Surcharge loads from spoil and mechanical equipment

•Existing structures above and below ground

•Location of underground services

•Ingress of ground / surface water

•Access and egress to the excavation

•Edge protection and materials falling from height

•Protection of the public and others

For more information on planning see the Health and Safety Procedures FM-H&S-033 (01). Excavation Checklist No: 1

Excavation – Hazards

1. Collapse of the sides

Make sure the necessary equipment needed such as trench sheets, props, baulks, etc are available on site before commencing work.

Prevent the sides and ends from collapsing by battering them to a safe angle or support with timber, sheeting or proprietary support systems

DO NOT go into unsupported excavations

Never work ahead of the support

Remember that even work in shallow trenches can be dangerous. You may need to provide support if the work involves bending or kneeling within the trench

Excavation – Hazards1. Collapse of the sides – Correct Installation of Trench Boxes

1.Assemble box with correct spindle assembly to desired trench width

2.Excavate ground to about 1m or deeper as ground permits and lower box into trench using 4 leg chains of the correct SWL

3.The excavator then progressively excavates between the two faces of the box plates and pushes down on each of the four corners of the box using the machine bucket.

4.This method continues until the desired depth is reached

5.For extension tops the procedure is the same.

6.Extraction is the reverse of above.

Note. This the general principle – Some makes of box do not rotate at the spindle joint.

Excavation – Hazards1. Collapse of the sides – battering back & safe slope angles

Graph of typical safe slope angles – All angles are from the horizontalExtract from – Be safe and shore – HSG 185

Excavation – Hazards2. Materials falling into excavations – Edge Protection

Do not store other materials close to the sides of the excavations. The spoil may fall into the excavation and the extra loading will make the sides more prone to collapse

Make sure the edges of the excavation are protected against falling materials. Provide toe-boards or similar.

Excavation – Hazards3. People and vehicles falling into excavations

Take steps to prevent people falling into excavations. If the excavation is 2m or more deep, provide substantial barriers, e.g Guard rails and toeboards.

Keep vehicles away from excavations where ever possible. Use brightly painted baulks or barriers where necessary

Where vehicles have to tip materils into excavations, use stop blocks to prevent them from over running. Remember that the sides of the excavatiion may need extra support

Excavation – Hazards4. People being struck by plant

Keep the operatives separte from moving plant such as excavators and vehicles removing spoil

Equip vehicles with audible reversing alarms and rear visibility aids such as rear view cameras, fish eye and convex mirrors.

Plant operatives should be competent with a card / certificate to confirm their level of ability such as CTA, CITB, & the new CPCS.

Ensure equipment is well maintained and operators inspect at regular intervals – visual check daily, recorded every 7 days

Excavation – Hazards5. Undermining nearby structures

Make sure the excavaton does not affect the footings of scaffolds or the foundations of nearby structures. Walls have very shallow foundations which can be undermined by even small trenches

Decide if the structure needs temporary support before digging starts. A survey of the foundations and the advice of a structural engineer may be needed.  

Excavation – Hazards6. Contact with underground services

Look around for obvious signs of underground services, e.g. valve covers or patching of footpath and road surfaces

Use cable locating devices to track any services and mark the ground accordingly

Make sure that the person supervising the excavation work has service plans and knows how to use them. Use the ‘permit to dig’ procedure, safe digging practices and make sure the emergency procedures are known to every one

in the team. 

Correctly support any existing services that encroach the excavation

Excavation – Hazards

7. Access to the excavation

Provide good ladder access or other safe means of getting in and out of the excavation.

Ladders must extend 5 rungs / over 1.0m above topside ground level – same criteria as scaffold

Tie and secure ladders at all access points

Install a short ladder on shallow excavations – discourage operatives jumping in and climbing out of trenches

 Consider the length of excavation and number of access points for general and emergency use, space at regular intervals on long trenches

Keep access points clear of materials, remove excess mud etc from ladder rungs as it builds up

Excavation – Hazards8. Suffocating, Toxic and Explosive Gases.

When chalk and limestone comes into contact with acidic water it can liberate carbon dioxide and Glauconitic sand (Thanet sand) can oxidise causing oxygen deficiency.

Other gases such as methane and hydrogen sulphide can seep into trenches from contaminated ground or damaged services (Transco etc).

Fumes from petrol and diesel engine equipment can collect within trenches if sited near excavation - this can lead to asphyxiation and / or poisoning. (Such equipment should never be taken into excavations)

Tests for gas must be carried out before work is started and detectors positioned within the working space to provide constant monitoring.

Excavation – Hazards9. Protecting members of the public

Fence off all excavations in public places to prevent pedestrians and vehicles falling into them.

Where children might get onto a site out of hours, take precautions (e.g. backfilling or securely covering excavations) to reduce the chance of them being injured

Works on the public highway should be guarded and signed in accordance with the New Roads and Street Work Act guidelines

Excavations close to public thoroughfares need to be well signed and lighted during the hours of darkness 

Keep the operation within the confines of the site boundary

DANGER

DEEP EXCAVATION

!

Excavations – Supervision and Inspection

Supervision

a competent person must inspect excavations:

At the start of each shift before work begins

After any event likely to have affected the strength or stability of the excavation

After any accidental fall of rock, earth or other material

A written report should be made after most inspections

STOP work if the inspection shows the excavation to be unsafe

Use form (FM-H&S-019 (01) shown on next page to record excavation inspections  

A ‘competent’ person must supervise the installation, alteration or removal of excavation support

People working in excavations should be given clear instructions on how to work safely.

Inspection

Excavations – Inspection Form

With Reference to ‘The construction (Health, Safety and Welfare) Regulations 1996’, the completion of this form constitutes an Inspection of a place of work (Excavation) which should be completed prior to any access by employees or persons under control of the employer and regularly thereafter.

EXCAVATION INSPECTION REPORT Site Address:

Site Safety Supervisor (on behalf of AWG)

Name / Signature

Inspection Time / Date

Excavation Reference or Description

Result of Inspection (Brief Summary)

Action Required Person

undertaking Inspection

Name / Signature

Name / Signature

Name / Signature

Name / Signature

Name / Signature

Name / Signature

This report must be retained for a minimum period of three months following the completion of the inspection.

‘Quick Hitch’ BucketsThe past has shown that we cannot rely upon the hydraulic ram located within the quick hitch body to safely lock and secure the bucket to the lower arm of the excavator.

Quick hitch systems are now the ‘norm’ on most excavators and while the locking mechanisms and technology are improving, buckets still come off - sometimes with disastrous consequences.

Safety pin fits here - Not Here

New type of system from Caterpillar No safety pin – Integral locking bar

Summary

No excavation can be

considered safe,

however shallow !

Always consider the hazards and PLAN the work.

Use the company procedures and this presentation as guide to help plan and control your excavation work.

Further ReadingAvailable from HSE Books

Consider This !!

Even Grave Diggers use trench support