The purpose of this occupational health and safety procedure is to establish generic instructions and guidelines for carrying out excavations and trenching activities at the construction project site.
Project manager and health and safety department is primarily responsible for the implementation of this safety work instruction in coordination with competent persons, all site personnel and subcontractors etc.
Competent person is an individual who has immediate knowledge of the subject activity, either by years of experience in the specific field, formal education, or specialized training pertaining to:
- Job activity being performed
- Operation and use of specific equipment
- Potential hazards associated with specific jobs
- Applicable Safety, Health and Environmental Standards
- Project HSE procedure pertaining to the tasks being undertaken
General Excavation Requirements
Almost all construction work involves some form of excavation, in particular for foundations and drainage. In carrying out excavations the ground conditions can vary widely often over very short distances.
No soil, whatever its condition can be relied upon to support its own weight. In addition loads imposed upon the edges of excavations by plant, buildings, men and materials increase the danger of collapse.
A relatively small fall of earth could involve a cubic meter of soil, which in terms of mass could weigh over a ton of material.
Hazard Identification and Excavation Risk Assessment
Before commencing any excavation works, a suitable and sufficient risk assessment should be carried out by a competent person. The risk assessment should identify hazards associated with the works and lay down procedures or control measures to eliminate or reduce the risks.
Where risks cannot be eliminated or reduced to an acceptable level then a safe system of working (including where necessary the provision of personal protective equipment) should be implemented as a means of safeguarding the workplace or person.
The following are a summary of the likely risks which may be encountered.
Risk Checklist
i. Falls of earth or trench collapse
ii. Falls into excavations
iii. Exposure to ground contamination.
iv. Contact with ground contamination.
v. Exposure to toxic or oxygen deficient atmospheres.
vi. Reduced stability of nearby buildings or structures.
vii. Falls of materials or plant onto persons in an excavation.
viii. Unauthorized access by children or young persons to excavations.
ix. Entry and Exit from excavation.
Control Measures Related to Excavation Risks
A safe method of working should be established in conjunction with the site risk assessment.
This safe method or method statement should be in writing and outline what procedures are required to ensure the safety of those persons concerned with the works and any other persons likely to be affected.
All work on, or in excavations should be supervised by a competent person.
Drawings or plans showing the location of all services should be obtained. A procedure can then be implemented to establish the “true” location, and such services exposed or marked.
Where electricity, gas and other underground services cannot be diverted and are in such a position that they could be damaged by the excavation process, they should be exposed by hand digging, and where necessary protected.
Before any excavation is started an excavation permit is required. Validity to excavate should not exceed one week.
All excavations in excess of 1.2m should be either:
- Benched i.e. cut to form steps so that a line from the top of the bench to the bottom of the excavation should be equal to the batter required.
- Properly supported, i.e. shored or sheeted etc. The system of support should be designed taking into account the loads imposed by ground conditions and any additional loads due to the proximity of plant, roadways, buildings, water table, etc.
- Excavated to a safe batter, this can be done in almost all soils provided that sufficient space is available, and a safe temporary slope known.
In poor ground conditions, it may be necessary to support excavations less than 1.2m deep.
In water-logged ground a system of de-watering may be necessary to improve stability; tests are usually needed, carried out by an experienced person, to show whether or not the ground is capable of being de-watered.
Where excavations are supported, the following conditions should be met:
- Timber packing should be used between metal props and metal sheets or piles.
- Poling boards, trench sheets, etc. should be set vertically.
- Any voids behind poling boards should be loosely backfilled.
- All walling should be horizontal.
- Ground props should rest on sole plates.
- Wedges should be driven between the poling boards, trench sheets, etc. and the walling. Each poling board trench sheet should have a wedge driven from the top of the waling and on alternate poling boards, trench sheets from the bottom.
- Where timber struts are used against a waling then lipping blocks should be used.
- When using expanding metal props only the proper pins should be used. Pins should be inserted from the top of the prop.
- No struts should be removed unless a suitable alternative has been previously installed.
- Persons should not be permitted to work beyond the protected area of an excavation, or in close proximity to the bucket of any excavator.
- Materials including spoil should not be stored closer than 1.5m from the edge of the excavation, and stacked so they cannot be accidentally displaced.
- Plant, where possible, should be positioned remote from the edge of the excavation and positioned so that it cannot fall or roll into the excavation.
- Proper lighting should be catered for when doing night work.
If any machine is required to tip material into the excavation, stop blocks or anchorage should be provided. If restraining ropes are used for such stop blocks or anchorage, they should be secured at least four times the depth of the excavation from the edge.
Persons should not remain in excavation or in the vicinity of tipping operations.
Where persons can fall from a fall risk position and there is a risk to cause injury or harm put in edge protection into an excavation or where other hazards exist at a lesser depth, i.e. starter bars, water etc. a rigid barrier should be erected and maintained.
Excavations, to which any member of the public, especially children, could gain access, should have a greater degree of protection. In addition, special thought should be given to visually impaired persons.
During hours of darkness, edges of excavations should be illuminated, especially where they are adjacent to public thoroughfares.
Where lighting is required for work in the excavation such lighting should be installed with equipment suitable for safe use i.e. intrinsically safe and/or protected against breakage.
The only means of access/egress into or out of excavations is by strategically placed ladders. Such ladder should be secured at the upper resting place, or at the bottom by knocking in a piece or steel parallel to the ladder and securing with binding wire, and reach a minimum of 1.070m above the landing.
On no account should access/egress be gained by climbing sheeting, timbering or other support work.
Walkways across excavations should be of sufficient width, at least 635mm, fitted with guardrails and toe boards, and secured to prevent displacement.
Only experienced and competent persons are allowed to install support systems for excavations, they and those persons authorized to work in the excavations should have their names recorded by the manager, or person nominated by the site manager.
Excavations should be kept clear of suffocating, toxic or explosive gases, these may be natural gases like hydrogen sulphide, methane and Sulphur dioxide, or exhaust gases from nearby plant or leaks from nearby pipes, sewers or installations.
Leakage of propane and butane from LPG cylinders inside excavation/trench is potentially very dangerous, because the gas will sink to the lowest point and form an explosive concentration.
One of the most effective methods of keeping the atmosphere healthy, is to use ventilating equipment to blow clean air into the excavation, shaft or tunnel, in sufficient quantities to dissipate any contaminant.
Note! On no account should compressed oxygen be used for the above purpose.
Tests should always be carried out in advance of work starting and be continued throughout the period of work. (Air Monitoring)
Thorough examinations are required to be carried out by the competent person in the following circumstances, before first entry, then again:
i. Daily prior to shift no record required
ii. Every seven days;
iii. After the use of explosives;
iv. After heavy rainfall;
v. After any damage to support work
The results of these thorough examinations to be entered in the site excavation register or site records where appropriate.
In addition to the above, excavations should be inspected each shift prior to work commencing, for which no record need be kept.
Systems of Support for Excavations
Adequate support of excavations depends upon the type and depth and the nature of the ground and ground water conditions.
A variety of materials and methods for excavation support may be used, and would include:
Timber Supports
Timber support is seldom used these days as a support material for excavations, other materials having taken its place. However it still finds a use in such as tunneling, and for shafts and headings.
Steel Trench Sheeting
Trench sheeting has the advantage of easy driving in poor ground conditions, prior to the excavation being commenced. In hard ground, it can be part driven initially, and then further driven as the excavation proceeds.
Steel Sheet Piling
This type of support system is obtainable in a variety of sectional stiffness, it is interlocking and being much stiffer than trench sheeting, it can be pre-driven to much greater depths before any excavation is started.
Its use also has the following advantages:
i. By pre-driving, the main structural support is in place before any excavation is started.
ii. The interlocking is reasonably watertight, making its use ideal where waterlogged or very unstable ground is to be supported.
iii. Because of its structural strength , steel sheet piling is the only really satisfactory material where a cantilever support is used.
iv. Where impermeable conditions exist below a waterlogged strata, sheet piling can be used to “ cut-off” the water bearing layer from the excavation. Where no cut-off is economically possible and wet material is to be supported, the design of the piling should allow sufficient penetration to avoid the bottom of the excavation “heaving” or “blowing”.
In all cases where steel sheet piling is the method of support, the calculations which determine the specification should be carried out by a competent person.
H-Piling or Soldier Piling
In this system, steel universal “H” column sections are pre-driven, at pre determined centres prior to any excavating.
If the ground conditions make driving difficult or, if vibration or noise is to be avoided, the steel sections can be positioned in pre drilled holes with the bottom end held in concrete.
As the excavation proceeds, the exposed face is supported by timbering or trench sheeting set horizontally between the piles.
The “H” piling method has the following advantages:
i. The main structural support is in place before the excavation commences.
ii. The sheeting can be kept tight up with the excavation, which need not be left unsupported for any length of time.
iii. Where services cross an excavation, the “H” pile method is particularly adaptable. The piles can be installed between service locations and horizontal sheeting located above and below. If the resulting gap is excessive, vertical sheeting can be installed behind the horizontal members to fill the gaps.
iv. “H” piling can be used for both double and single sided methods of support. It is not however applicable to cantilever support or in ground conditions which are waterlogged. As an “engineered system” it should be designed by competent persons.
Proprietary Excavation Support Systems
These systems are specifically designed and used so that during the installation of the support there is no need for persons to enter the excavation.
Examples of types of proprietary systems and their intended use are as follows:
Hydraulic Frames, Walling and Shores
Are made up of sheeting or structural sections permanently attached to hydraulic struts.
The systems can be installed and jacked against the sides of the excavation without anyone entering the trench.
Square or rectangular plan walling with two-way jacking are available for pits and manholes.
Proprietary waling frames are normally used in conjunction with trench sheeting.
Where these are crossing services, small gaps may be left between the appropriate trench sheets.
Boxed Systems
These are modular strutted support walls which are installed by lowering them into a pre-dug trench (thereby acting as a safety box to protect workmen) or by digging them in progressively and therefore providing positive applied support to the trench face.
Boxes of this type can be extended in width, and by adding other boxes vertically can provide a safe workplace at increased depths.
Plate Lining Systems
Generally consist of vertical soldier posts kept apart by strutted members.
Wall plates are then inserted, spanning between the soldiers to support the trench face.
Such systems may be used to form a continuous wall, and are usually installed by the dig and push method.
Extra care is needed when the line of the excavation crosses services and where a manhole construction is required.
Since the system can be dismantled into smaller components it is far easier to remove from tight ground than box systems.
Relevant Records
- Permit to Excavate
- Inspection Reports