Download IP004 - Institute of Safety Management

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).
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