Excavation safety

                                        Excavation safety

1.     Excavations:

Ø  There are about seven deaths each year due to work in excavations.

Ø  Many excavations collapse without any warning, resulting in death (or) serious injury.

Ø  The specific hazards associated with excavations are as follows:

ü  Collapse of the sides.

ü  Materials falling on workers in the excavation.

ü  Falls of people and/or vehicles into the excavation.

ü  Workers being struck by plant.

ü  Specialist equipment such as pneumatic drills.

ü  Hazardous substances, particularly near the site of current (or) former industrial processes.

ü  Influx of ground (or) surface water and entrapment in silt (or)mud.

ü  Proximity of stored materials, waste materials (or) plant.

ü  Proximity of adjacent building (or) structures and their stability.

ü  Contact with underground services.

ü  Access and egress to the excavation.

ü  Fumes, lack of oxygen and other health hazards (such as weil’s disease).

ü  Contaminated ground.

Precautions and controls required for excavations:

The following precautions and controls should be adopted:

v  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.

v  The sides of the excavation must be prevented from collapsing either by digging them at a safe angle (between 5º and 45º dependent on soil and dryness) (or) by shoring them up with timber, sheeting (or) a proprietary support system.

v  Falls of material into the workings can also be prevented by not storing spoil material near the top of the excavation.

v  The workers should wear hard hats.

v  If the excavation is more than 2m deep, a substantial barrier consisting of guard rails and toe boards should be provided around the surface of the workings.

v  Vehicles should be kept away as far as possible using warning signs and barriers.

v  Where a vehicle is tipping materials into the excavation, stop blocks should be placed behind its wheels.

v  The excavation site must be well lit and night.

v  All plant and equipment operators must be competent and non-operators should be kept away from moving plant.

v  PPE must be worn by operators of noisy plant.

v  Nearby structures and buildings may need to be shored up if the excavation may reduce their stability.

v  Scaffolding could also be stabilized by adjacent excavation trenches.

v  The influx of water can only be controlled by the use of pumps after the water has been channeled into sumps.

v  The risk of flooding can be reduced by the isolation of the mains water supply.

v  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.

v  Any such hazards found after work has started, must be reported and noted on the inspection report and remedial measures taken.

v  Exhaust fumes can be dangerous and petrol (or) diesel plant should not be sited near the top of the excavation.

v  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.

v  As these will be probably not be accurate, service location equipment should be used by specifically trained people.

v  The area around the excavation should be checked for service boxes.

v  If possible the supply should be isolated.

v  Only hand tools should be used in the vicinity of underground services.

v  If the supply cannot be isolated then ‘goal posts’ beneath the overhead supply together with suitable signs must be used.

v  Safe access by ladders is essential, as are crossing points for pedestrians and vehicles.

v  Whenever possible, the workings should be completely covered outside working hours particularly if there is a possibility of children entering the site.

v  Finally care is needed during the filling in process.

v  Wells and disused mine shafts are found during construction work and must be treated with caution, and in the same way as an excavation.

Inspection and reporting requirements:

      The duty to impact and prepare a report only applies to excavations which need to be supported to prevent accidental fall of material.

      Only persons with a recongnized and relevant competence should carryout the inspection and write the report.

      Inspections should take place by a competent person 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 inspection 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 24hours.

      The report must be kept at the inspection site until the work is completed and then retained 3 months at an office of the organization which carriedout the work.

      Almost all construction work involves some form of excavation (for foundations, drains, sewers etc) and every year people are killed (or) seriously injured while working excavations.

      Designers and those specifying the work should always consider the use of trenchless techniques, such as micro-tunneling, directional drilling and pipe bursting etc, which replace the need for excavation, apart from the launch and reception pits.

      They also reduce risks to members of the public from open excavations and subsequent traffic disruption.

      If excavations are required, the work must be properly planned and carriedout to prevent accidents.

      There is almost no ground that can be relied upon to stand unsupported in all circumstances and the risk in self-evident when you consider that it is quite common for one cubic metre of soil to collapse into an unsupported excavation, and this can weigh as much as one tonne.

      Before digging any trenches, pits, tunnels(or)other excavations, decide what temporary support will be required and plan the precautions that are going to be taken against:-

§  Collapse of the sides.

§  People and vehicles falling into the excavation.

§  Materials falling onto people working in the excavation.

§  Undermining nearby structures.

§  Underground and overhead services.

§  The inflow of ground and surface water.

Collapse of the sides (or) roof:

Ø  The need for adequate support will depend on the type of examination, the nature of the ground and the ground water conditions.

Ø  Install support without delay as the excavation progresses.

Ø  Any unsupported excavation will be safe without support only if its sides are battered back sufficiently (or) if the excavation is in sound rock.

Ø  Battering back the sides of an excavation to a safe angle is a simple and acceptable means of preventing instability.

Ø  In granular soils the angle of slope should be less than the natural angle of repose of the material being excavated.

Ø  A competent person who fully understands the dangers and necessary precautions should inspected excavation at the start of each shift.

Ø  Excavations should also be inspected after any event that may have affected their strength (or) stability (or) after a fall of rock (or) earth.

People and vehicles falling into excavations:

      Prevent people from falling by guarding excavations.

      Edges of excavations should be protected with substantial barriers where people are liable to fall into them.

      This can be achieved using guard rails and toe boards, which can be inserted into the ground immediately next to the supported excavation side (or) using fabricated guard rail assembles that connect onto the sides of the trench box.

      All excavations in public places should be suitably fenced off to prevent members of the public approaching them.

      Prevent vehicles from falling into excavations by keeping them out of the area.

      Vehicles passing close to the edges of excavations may also overload the sides leading to collapse.

      Where necessary use baulks (or) barriers to keep vehicles away from excavated edges.

      Where vehicles have to tip materials into excavations, prevent them from overrunning into the excavation by using properly secured stop-blocks.

      These must be placed at a sufficient distance from the edge of the excavation to avoid the danger of the edge breaking away under the weight of the vehicle.

      The sides of the excavation may also require extra support.

Materials falling into excavations:

      Do not park plant and vehicles (or) store excavated spoil and other materials close to the sides of excavations.

      The extra lodgings from spoil, vehicles etc can make the sides of excavations more likely to collapse.

      Loose materials may fall from spoil heaps into the excavation.

Undermining nearby structures:

      Make sure excavations do not undermine the scaffold footings, buried services (or) the foundations of nearby buildings (or) walls.

Underground and overhead services:

      Many serious accidents have occurred when buried services have been damaged during excavation work.

      Excavation work should not start until steps have been taken to identify and prevent any risk of injury arising from underground services.

      Burns and electrocution can result if raised tipper truck bodies (or) excavators touch (or) come close enough to overhead power lines to cause arcing.

      The need to undertake excavation work close to (or) below such lines should be very carefully considered and avoided where possible.

Ground and surface water in flow:

      Depending on the permeability of the ground, water may flow into any excavation below the natural ground water level.

      The supports to the side of the excavation should be designed to control the entry of ground water and the design should take any additional water loading into account.

      Water entering the excavation needs to be channeled to sumps from where it can be pumped out; however the effect of pumping from sumps on the stability of the excavation should be considered.