Hazards Associated With Excavations
There are about seven deaths each year due to work in excavations. Many types of soil, such as clays, are self-supporting but others, such as sands and gravel, are not Many excavations collapse without any warning, resulting in death or serious injury. Many such accidents occur in shallow workings. It is important to note that, although most of these accidents affect workers, members of the public can also be injured.
The specific hazards associated with excavations are as follows:
- the collapse of the sides;
- materials falling on workers in the excavation;
- falls of people and/or vehicles into the excavation;
- workers being struck by the plant;
- specialist equipment such as pneumatic drills;
- hazardous substances, particularly near the site of current or former industrial processes;
- the influx of ground or surface water and entrapment in silt or mud; proximity of stored materials, waste materials or plant;
- the proximity to adjacent buildings or structures and their stability;
- contact with underground services;
- access and egress to the excavation;
- fumes, lack of oxygen and other health hazards (such as Weil’s disease).
Clearly, alongside these specific hazards, more general hazards, such as manual handling, electricity, noise, and vibrations, will also be present.
Precautions & Controls Required For Excavations
The following precautions and controls should be adopted:
At all stages of the excavation, a competent person must supervise the work and the workers must be given clear instructions on working safely in the excavation.
The sides of the excavation must be prevented from collapsing either by digging them at a safe angle (between 5 ° and 45 ° dependent on soil and dryness) or by shoring them up with timber, sheeting or a proprietary support system. Falls of material into the workings can also be prevented by not storing spoil material near the top of the excavation.
The workers should wear hard hats.
If the excavation is more than 2 m deep, a substantial barrier consisting of guardrails and toe boards should be provided around the surface of the workings.
Vehicles should be kept away as far as possible using warning signs and barriers. Where a vehicle is tipping materials into the excavation, stop blocks should be placed behind its wheels.
It is very important that the excavation site is well lit at night.
All plant and equipment operators must be competent and non-operators should be kept away from moving the plant.
PPE must be worn by operators of the noisy plant.
Nearby structures and buildings may need to be shored up if the excavation may reduce their stability. Scaffolding could also be de-stabilized by adjacent excavation trenches.
The influx of water can only be controlled by the use of pumps after the water has been channeled into sumps. The risk of flooding can be reduced by the isolation of the mains water supply.
The presence of hazardous substances or health hazards should become apparent during the original survey work and, when possible, removed or suitable control measures adopted. Any such hazards found after work has started must be reported and noted in the inspection report and remedial measures taken. Exhaust fumes can be dangerous and petrol or diesel plant should not be sited near the top of the excavation. The presence of buried services is one of the biggest hazards and the position of such services must be ascertained using all available service location drawings before work commences.
As these will probably not be accurate, service location equipment should be used by specifically trained people. The area around the excavation should be checked for service boxes. If possible, the supply should be isolated. Only hand tools should be used in the vicinity of underground services. Overhead services may also present risks to cranes and other tall equipment. If the supply cannot be isolated then ‘ goal posts ’ beneath the overhead supply together with suitable bunting and signs must be used.
Safe access by ladders is essential, as are crossing points for pedestrians and vehicles. Whenever possible, the workings should be completely covered outside working hours, particularly if there is a possibility of children entering the site.
Finally, care is needed during the filling-in process.
Wells and disused mine shafts are found during construction work and must be treated with caution, and in the same way as an excavation. The obvious hazards include falling in and/or drowning and those associated with confined spaces – oxygen deficiency, the presence of toxic gases and the possible collapse of the walls. Controls include fencing off the well and covering it until the situation has been reviewed by specialists. Shallow wells would normally be drained and filled with hardcore whereas deeper ones would be capped.
Inspection and reporting requirements
The duty to inspect and prepare a report only applies to excavations which need to be supported to prevent accidental fall of material. Only persons with a recognized and relevant competence should carry out the inspection and write the report. Inspections should take place at the following timing and frequency:
- after any event likely to affect the strength or stability of the excavation;
- before work at the start of every shift ;
- after an accidental fall of any material.
Although an inspection must be made at the start of every shift, only one report is required of such inspections every 7 days. However, reports must be completed following all other inspections. The report should be completed before the end of the relevant working period and a copy given to the manager responsible for the excavation within 24 hours. The report must be kept on site until the work is completed and then retained for 3 months at an office of the organization which carried out the work.