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Excavation work: Methods and Safety Considerations
Carefully planned and professionally accomplished excavation is the key to a successful construction project.
Being a major part of the site preparation, excavation can make or break the stability of your future building.
First things first. Do not commence any excavation work before you have a clear map of underground utilities: cables, pipes, etc. Mark the services and make sure everyone in the excavation team knows where the buried services are. Also, test the soil and conduct site evaluation. Armed with this information, you will be able to choose methods and safety measures that fit specific excavation works.
Here is a list of methods you should consider to take your project to its full and safe completion.
A trench is a type of excavation that is generally deeper than it is wide at the top.
Trenches are used in archeology, civil engineering, and military engineering for various purposes. In residential construction, they are excavated mainly to provide a base for buildings. If the trench is going to be at least 1.5 meters deep, it must be properly supported from all sides to make it safe against collapse and ensure the safety of workers.
A number of methods are used to provide a support system:
These systems function as risk controls and are often combined to produce the most efficient option for a given situation and project-specific excavation materials.
A series of steps in the vertical surface of an excavation are created as a safeguard against collapsing and sliding. Benching reduces the wall height and ensures stability.
The excavated slope is safe when the ground is stable. If, for some reason, the stability of the ground is lost (due to heavy rains or flooding, for example), appropriate control measures should be taken, such as temporarily suspending work or providing ground support.
When battering or benching are impracticable due to the location or depth of the excavation, shoring is used.
Shoring is used when unstable ground conditions are encountered. These may include: soft ground or ground liable to be wet during excavation. In such cases there is a risk of a person being buried, struck or trapped by dislodged material.
If shoring is being installed as the excavation works progress, no worker should enter any part of an unsupported trench.
In case excavations are less than 2 meters deep and where each section of the excavation is open for a short period of time (less than one week), you might need to consider shoring that uses soil arching.
Shoring that uses soil arching
When you slightly compress soil without allowing the soil to move outward, it will produce an arching effect.
You will need a competent person’s recommendation for each specific case; however, generally the closer the shoring is, the better soil arching works.
With specific lifting points for installation and removal, shields and boxes are used in trenches to withstand the earth pressures and protect workers within them. Shields can be permanently installed or designed to be moved along as work progresses.
De-watering as a control measure
Necessary control measures should be provided before anyone is allowed to work in excavations where water has collected or is collecting.
Water may collect because of:
surface run-off after heavy rain;
a nearby swamp, dam, lake, river or storm water drain;
a high groundwater table seeping into the excavation.
Excavation work may involve controlling large quantities of water; in such cases steel piling or closed sheeting are not always the best solution, and often the most effective way is to drain the ground before excavation begins.
Zone of Influence
Zone of influence is the volume of soil around the excavation affected by an external load (for example, vehicles, plant, excavated material).
The influence of any loads near the excavation may cause ground collapse. Unless specifically indicated, no excavated material, mechanical plant, or other heavy loads should be in the zone of influence.
Here is how existing nearby structures may restrict the excavation.
Temporary spoil must be placed no closer than 2 ft (0.61 m) from the surface edge of the excavation (the distance is measured from the nearest base of the spoil to the cut not from the crown of the spoil deposit).