PAMPHLET Reference No: IP004 Originator: Leighton Bennett EXCAVATION SAFETY & THE SOIL PROFILE The Occupational Health & Safety Act’s 2014 Construction Regulation 13(1a) requires that all excavation work must be performed under the supervision of a written appointed competent person, an Excavation Supervisor or Inspector. The appointed excavation supervisor must take into account the specific site conditions in terms of traffic, the location of structures and their condition soil type and condition, surface & ground water, the water table, overhead & underground utilities and the weather. This makes the design of excavations & trenching not simple, as soil stability can change with changing weather, while the soil profile & location of utilities can present a safety threat to the people excavating. 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. When an excavation is cut, the soil residual forces system is disturbed & is therefore potentially unstable. If those residual forces (soil load & gravity) are greater that 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 & structure is critical to safe operation. A TYPICAL SOUTH AFRICAN SOIL PROFILE DIAGRAM Ground surface Fill Transported soils l ranite Date : January 2015 Pebble Marker Residual soils Bedrock In Southern Africa there is a generic soil profile, which is disturbed when excavations are made. The main soil profile elements are shown in the adjacent soil profile on granite bedrock. The “Fill” or "Made up Ground” describes all refuse, added materials (eg. brick paving & its base materials), excavated ground used for filling a depression or raising the level of the ground and it overlies or is dug into the transported soils The “Transported soil” (gravels, sand, silts & clays) are soils that have been transported by water on the basis that as the flowing water velocity slows the heavies particles that it carries are deposited first with the silts then clays being the last particles to be deposited, The upper section of this layer usually has a layer of humus & plant roots. Normally at the contact zone between the transported soils & the underlying residual soils is a "Pebble Marker” layer of deposited transported stones & unweathered pebbles of the original bedrock. Sometimes this layer contains Laterites of insoluble hydroxides of iron (ferricrete) or aluminium (bauxite) or calcium (calcrete) in the arid areas The “Residual soil” or “Sub soil” are soils developed where the action of the elements on the original bedrock has produced a soil with little tendency to move. The nature of the residual soil developed depends on the parent rock type, which decomposes to form the residual soil. (eg. granites produce a coarse sandy gravel to sandy silt soil, while sedimentary shales & igneous dolerites produce a silty clay or one of the 3 types of clay residual soil types depending on the properties of the clay mineral content). Towards the residual soil laye’s base highly weathered fragments of the original bedrock material is often encountered. The “Bedrock” varies from very soft rock to hard rock depending on the amount of weathering & as rock can be massive to fractured or cracked &/or can be layered & inclined at dip angles which depending on the rock strength, rock cleavages, angle and weathering, etc such that these factors can impact on the ground stability of rock, which could even land slide or collapse into an excavation. The bedrock may not be of a homogeneous nature but may commonly consist of bedrock layers of sedimentary shales & sandstones, in the RSA. Clay particles in soils have sizes less than 0.002mm in diameter, silt particles between 0.002 & 0.06mm, sand from 0.06 to 2.0mm, gravels & pebbles from 2.0mm to 60mm, cobbles to 200mm & boulders over 200mm in diameter. Excavation stability of soils depends on many variables which must be considered by an excavation supervisor to ensure excavation safety (eg. the cohesion or lack of cohesion of the soil, tension cracks, moisture content, vibration, etc). THE INSTITUTE OF SAFETY MANAGEMENT, THE PROFESSIONAL BODY OF CHOICE FOR THE DISCERNING OCCUPATIONAL SAFETY PRACTITIONER www.iosm.co.za