Section 3: Temporary excavations, trenches, pits and shafts
13 Temporary excavations
COMMENTARY ON CLAUSE 13
This section covers the design and construction of temporary excavations with vertical or near vertical slopes which require some form of excavation support to be stable. Temporary excavations which are designed to be self supporting (with soil reinforcement e.g. soil nails, where necessary) are covered by Section 2.
Procedural controls of temporary works for excavations should be undertaken in accordance with BS 5975, Section 2.
All temporary excavations should be provided with safe means of access and egress including means of escape in an emergency. There should be adequate working space in the excavation along with walkways and ladders between working areas where necessary. All temporary excavations should be subject to an assessment of the risks involved as described in 4.2. This should be in proportion to the complexity of the excavation being undertaken.
The upper perimeter of the excavation should be adequately guarded to protect persons both in and around the excavation and to prevent vehicles from falling in.
NOTE 1 Temporary excavations can be confined spaces depending on their layout (see Confined Spaces Regulations ACoP ).
NOTE 2 Further guidance on safe working in excavations can be found in HSE publication HSG 185 Safety in Excavations .
13.2 Design considerations
The design of the excavation support system should take into account:
- the extent and nature of the works to be undertaken in it;
- the extent and nature of the support system;
- ground and groundwater conditions;
- requirements for adequate working space and clearance between supports within the excavation;
- the method and sequencing of backfilling along with the removal of the support system; and
- plant available for construction of the excavation.
13.3 Site investigation
Site investigation for temporary excavations should be undertaken and reported on, in accordance with the principles in Clause 6.
The investigation should be in proportion to the complexity of the temporary excavation and its support system, prevailing ground conditions and the sensitivity of adjacent structures to disturbance.
As part of these investigations, ground water levels should be monitored over as long a period as possible to assess seasonal or tidal variations which could be relevant and to eliminate misleading data arising from the monitoring process.
13.4 Ground conditions
Knowledge of the ground conditions gained from the site investigation should be used:
- to determine the appropriate method of excavation and related plant requirements;
- to determine the appropriate form of support to the sides of the excavation and to ensure its adequacy;
- to determine suitable means of maintaining the excavations free from ground water;
- to ensure that any potentially buoyant structures which might be constructed within the excavations will not be subjected to water pressures sufficient to cause uplift forces at any stage of their construction.
13.4.2 Influencing factors on construction methods
The following factors should be considered when determining the methods of excavation and excavation support system required:
- the purpose of the excavation;
- the ground profile;
- short term stability of excavated faces;
- geotechnical factors (see 13.4.3);
- the depth and extent of the excavation;
- groundwater table and fluctuations in its level;
- variability in ground permeability, including the risk of piping/artesian conditions;
- adjacent structures and services; and
- the driveability of sheet piles or other lining systems.
13.4.3 Geotechnical factors
Geotechnical factors affecting the safety of the excavation which should be considered include:
- the nature of the ground;
- the short- and long-term soil strength;
- the ground water regime;
- the presence of rock, and discontinuities of the rock mass;
- the presence of made or previously disturbed ground.
NOTE More comprehensive recommendations and guidance are given in Clause 7.
13.4.4 Sources and control of ground water
Consideration should be given to controlling water which can adversely affect the stability of an excavation including:
- rainfall and surface runoff;
- shallow subsoil water;
- land drainage;
- inflow from below the ground water table;
- artesian conditions;
- inflow from damaged services.
The provision of dewatering, drainage or pumping schemes should be considered as appropriate to ensure the excavation remains suitably dry. Likewise dry conditions can and may be achieved by the construction of an impermeable curtain around the excavation. The effects of such activities on the local groundwater regime and adjacent structures should be considered.
13.5 The design of stable slopes and supports to excavations
For routine excavations, in stable ground conditions, information on the loads to be resisted or the load resisting capabilities of the excavation support system should be available from suppliers' literature or websites or by reference to standard texts such as CIRIA R97 .
In other situations geotechnical engineering principles using strength parameters obtained from site investigation work should be used to design the excavation support system in accordance with accepted procedures.
13.5.2 Magnitude and distribution of lateral soil pressures
Lateral pressures on the support systems of excavations should be calculated from first principles (allowing for soil structure interaction) or in the manner described in CIRIA C517 .
13.5.3 Stability of base of excavation
Consideration should be given to preventing base failure in deep excavations from:
- uplift or "boiling" of granular soils due to large seepage forces caused by high hydrostatic heads; or
- by heave or shear deformation of soft saturated cohesive soils due to overstress.
22.214.171.124 Water-bearing permeable soils
Ideally, sheet piles supporting an excavation below the ground water table in granular soils should have a sufficient depth of penetration below the base of an excavation to reach an impermeable stratum and thus provide a cut-off to the flow of water beneath the toes of the piles preventing upward seepage at the base of the excavation.
Where this is not possible sheet piles should be driven to a prescribed depth of penetration below the base of the excavation to restrict inflow of ground water.
Care should be taken to ensure the head of water outside the sheet piling is not sufficient to cause a sufficiently steep hydraulic gradient over the length of the seepage path to give a velocity of upward seepage which could cause instability of the soil particles.
The design of deep excavations in water-bearing granular soils should also take into account that boiling can occur as a result of strong flow from a permeable layer underlying less permeable soil at the base of the excavation.
If necessary, ground water lowering methods should be used to lower the external head of ground water, or alternatively the sheet piling should be driven to a deeper penetration to lengthen the seepage path so decreasing the hydraulic gradient and thus reduce the tendency of the ground to boil.
126.96.36.199 Soft cohesive soils
Failure by heaving can occur in deep excavations in soft cohesive soils through overstressing of the soil in the region of the base of the excavation. Conventional methods of stability analysis should be used to predict the likelihood of base failure by shear deformation. It is recommended that a conservative approach is taken to setting the characteristic strength of the soil.
188.8.131.52 Movements at base of excavation
The following should be considered when determining the magnitude and rate of upward movement at the base of an excavation:
- a) the reduction in vertical stress caused by the removal of soil from within the excavation;
- b) the nature of the strata underlying the base of the excavation;
- c) the ground water conditions;
- d) upward movements which take place are caused by immediate elastic strain which occurs simultaneously with the deepening of the excavation and by long term volumetric strains due to moisture content changes; in stratified cohesive deposits which display high horizontal permeabilities, heave caused by volumetric strain can be rapid;
- e) variations in the magnitude of upward movement which are generally greater at the centre of the base of the excavation than at the periphery; the magnitude of heave may be predicted by elastic theory but the rate of heave cannot be reliably predicted on the basis of theory and few field measurements have been made.