BS 5975-1:2024 - Book - Page 64
BS 5975‑1:2024
e)
f)
BRITISH STANDARD
surcharge or other loading around the trench/excavation:
1)
magnitude, distance and dimensions of loading due to site plant, delivery vehicles,
stockpiles, storage areas, structures, railways, etc.;
2)
possibility of compaction or vibrating plant working near the support scheme;
3)
any vertical loads to be carried by the temporary retaining wall; and
4)
possibility of impact loading (e.g. from machinery) or preferred mitigation measures to
prevent failure of components; and
length of time that the trench/excavation will be required for:
1)
indication of approximate timescale for the work to be completed (earth and water
pressures on a retention scheme can vary with time); and
2)
residual risks and preferred measures if work has to be suspended for a significant period
of time.
NOTE 1 Excavations can fail suddenly and without warning and particular care is required for the safety of
workers within. Guidance is available in CIRIA R97 Trenching practice [7] and CPA’s shoring technology guidance
[8], [9], [10] and [11].
NOTE 2 Trenches are significantly longer than their depth or width and most tend to be relatively shallow. They
are often required for installing below-ground utilities. Most trenches can be excavated, the work within completed
and then backfilled, within a relatively short period of time (days).
Excavations can be square in plan and deep so support is generally required (space required for battering/stepping
will be extensive). They are often used for major work such as basement construction and can be open for a
significant period of time before the work within is completed (months, years).
Cofferdams can be considered as a type of excavation support from which water has to be excluded, to allow the
works to be completed. They are often used for major work such as basement construction (in soils with a high
water table) or for constructing bridge foundations in bodies of water (rivers or sea).
NOTE 3 Often a ground investigation is commissioned by the permanent works designer for the purpose of
designing the permanent works. The sampling locations selected and techniques used for investigation and testing
might not be particularly relevant to the design of any associated temporary works. Additional investigation might
be necessary to help mitigate the risks.
NOTE 4 If the trench is being excavated in good cohesive materials then “short-term” (total stress) soil parameters
might be appropriate for the design of a support scheme. This is because the rate of the load being applied is much
quicker than the rate at which pore water can drain out (due to the low permeability of the soil). However, it is
difficult to justify the use of short term soil parameters in granular soils where the permeability is high.
Work in large excavations and cofferdams generally tends to take a significant period of time (months or years) so it
is more likely that “long-term” (effective stress) soil parameters are appropriate as it can be difficult to justify the
use of short-term soil parameters as the pore water has time to drain.
NOTE 5 BS EN 1997-1 requires that both short- and long-term design situations are considered. The use of the net
pressure method does not necessarily require the consideration of both long-term and short-term parameters and
the choice is left to the designer.
NOTE 6 Interpreting the site investigation information and establishing a geotechnical model for analysis is critical.
NOTE 7 Requirements for detection, verification and location of underground services are specified in PAS 128.
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