Safety ground rules
Leighton Bennett, a Safety First Association committee member from Benrisk Consulting, delves into excavation safety and takes a look at southern Africa’s typical soil profile.
The 2014 Occupational Health and Safety Act’s Construction Regulation 13(1a) requires that all excavation work must be performed under the supervision of a competent person – an excavation supervisor or inspector.
This inspector should evaluate site conditions in terms of traffic, the location of structures and their soil type and condition, the surface and ground water and the water table, as well as overhead and underground utilities and the weather. It’s, thus, no surprise that excavation and trenching designs aren’t simple to complete.
When a trench or other excavation is made in soil, the residual forces in the ground work to restore the soil to a more stable configuration. If those residual forces (soil load and gravity) are greater than those forces holding the trench or excavation walls where they are, a cave-in or sidewall collapse occurs. Because of these forces, the evaluation of the soil conditions and structure is critical for safe operations.
Southern Africa has a generic soil profile, which includes the fill, transported soil, a pebble marker, residual soil and bedrock.
• The fill describes all refuse, added materials (such as brick paving and its base materials) and excavated ground used for filling a depression, or raising the ground level.
• The transported soil (gravels, sand, silts and clays) are soils that have been transported by water. This occurs when the flowing water velocity slows and the heavier particles, which it carries, are deposited.
• As a rule, the contact zone between the transported soils and the underlying residual soils is a pebble marker layer of deposited, transported stones and unweathered pebbles of the original bedrock.
• Beneath that is the residual soil, or sub soil, which has developed where the action of the elements on the original bedrock has produced a soil with little tendency to move. The characteristics of the residual soil develop according to the parent rock type, which decomposes to form the residual soil.
• Towards the residual soil layer’s base, highly weathered fragments of the original bedrock material is often encountered. The bedrock varies from very soft to hard rock. Its strength also depends on its weathering, and whether it is cracked, layered, inclines at dip angles or has rock cleavages. These factors can impact on the ground stability of rock, which could result is a land slide or a collapsed excavation.
It’s clear that excavation stability of soils depends on many variables that must be considered by an excavation supervisor to ensure excavation safety.