Excavations and trenches are necessary part of the construction process in order to provide the infrastructure necessary for new projects.
The term “excavation” can be referred to as any man-made cut, cavity, trench, hole, or depression made in the earth’s surface, formed by earth removal while a Trench is a narrow excavation made below the surface of the ground. Excavation is recognized as one of the most hazardous construction operations. A basic rule for excavation is “plan your dig, then dig your plan.” It is believed that if you encounter something unplanned for, then stop and re-plan.
PLANNING FOR EXCAVATION
If an excavation work is to be carried out, the following should be considered:
Drawings to show the depth, distances, location and surrounding environment.
Identification of existing underground services e.g. power cables, gas/water lines, sewer, etc.
Soil nature and specification
Surrounding features of building etc. that can be potentially affected by excavation.
Access and egress by workers
Method of excavation- mechanical or hand digging.
Identification of equipment to be used.
Excavated soil movement and disposal.
Type of exaction wall protection.
Competency of persons involved.
CHECKING FOR THE SOIL CLASSIFICATION
The soil is classified as:
Class A-very cohesive, e.g. clay, silty clay, clay loam.
Class B- Semi cohesive example; silt, silty loam, silt clay loam, etc.
Class C- Non cohesive example; sand, gravel, loamy sand, etc.
Stable rock-example; granite and sandstone.
The strength of soil is dependent on the type of soil, the amount of moisture in the soil and whether the soil has been previously disturbed.
The main hazards of excavation includes:
Equipment, plant, vehicles or people falling in.
Collapse of excavation wall (cave-ins)
Overhead power lines contact
Unprotected edges, incorrectly positioned access ladders, badly constructed vehicle ramps and materials stored or vehicles parked too close to the edge are main causes of equipment, plant or vehicles falling in.
Wrong judgement of soil type, additional weight due to equipment and vehicles parked too close, inadequate support of side walls, vibration of machinery and unstable walls caused by rain are the main causes of wall collapse or cave-ins.
PRECAUTIONS WHILE UNDERGOING EXCAVATION WORK
- Workers should guide against unsafe attitudes.
- Underground services such as power cables, water/gas lines, sewer should be identified, isolated and protected. Buried utilities could be located and traced using detective devices.
- Excavators and power tools should not be used within 0.5meters of an indicated line of cable/pipe. Hand digging should be employed when nearing the assumed line of cable/pipe.
- Excavations more than 4feet (1.2 meters) should be tested for oxygen deficiency, highly combustible gas concentration such as methane and high levels of other hazardous substances such as hydrogen sulphide before an employee goes in. presence of water in the excavation is very hazardous, refer to our previous post on “WHY THIS EXCAVATION WAS CARRIED OUT THIS WAY” for this reason, groundwater must be removed before entry.
These and more can be used as precautions when embarking excavation jobs. Your personal contribution is welcomed.