Section 11: Review during and after construction
COMMENTARY ON CLAUSE 64
There is an inherent difficulty in forecasting ground conditions from ground investigations carried out before the works are started because, no matter how intensive the investigation and whatever methods are used, only a small proportion of the ground is examined. It is also the case that for many investigations the exact locations of the proposed structures have not been determined and so the ground conditions relevant to the individual structures is not always known.
The ground exposed during construction and the behaviour of structures during and after construction should be monitored to verify that the predictions made during design remain valid.
65 Purpose of review
Review during construction should be carried out to determine, in the light of the conditions newly revealed, to what extent conclusions drawn from the ground investigation are required to be revised, if at all. For maximum benefit, this review should be directed by the geotechnical adviser (see Clause 6).
Where additional and/or different information is revealed during construction, the design and construction should be reviewed and the design or the construction procedures might need to be amended as a result of the review.
NOTE In certain cases it might be appropriate to initiate a site procedure in the early stages of the contract, so that correct and agreed records are kept during the duration of the contract by both the designer and the contractor.
The information collected during construction should be used to:
- a) check the adequacy of the design;
- b) check the safety of the works during construction and to assess the adequacy of temporary works;
- c) check the findings of the ground investigation and to provide feedback so that these findings can be reassessed;
- d) check assumptions about ground conditions related to construction methods, which might include groundwater;
- e) check the suitability of instrument installations;
- f) enable the best use to be made of excavated materials;
- g) reassess the choice of construction plant and equipment; and
- h) provide agreed information about ground and groundwater conditions in the event of dispute.
The results of the investigation which is carried out after construction should be used to verify that the structure is behaving in accordance with the predictions made by the designer.
66 Information required
66.1 Soil and rock
Accurate engineering descriptions of all strata encountered below ground level should be made in accordance with Section 6. The soil and rock profile revealed on site should be recorded and compared with that anticipated from the ground investigation. The descriptions should be made by a geotechnical engineer or engineering geologist competent in geotechnics.
NOTE It might be advantageous to arrange for the site to be inspected by the organization that carried out the ground investigations, particularly if conditions appear to differ significantly from those described in the ground investigation.
Accurate information about the groundwater should be obtained during construction and compared with information recorded during the investigation. The information should include the flow and static conditions in all excavations, any seepage from slopes, any seasonal variations, any tidal variations in excavations or tunnels near the sea or estuaries, suspect or known artesian conditions, the effect of weather conditions on groundwater, and any unforeseen seepage under or from water-retaining structures. The effect of groundwater lowering should also be recorded in observation holes to determine the extent of the cone of depression. The effect of groundwater lowering should also be recorded in observation holes to determine the extent of the cone of depression and changes to ground level for sensitive structures as required.
COMMENTARY ON CLAUSE 67
Instruments and subsequent observations used during and after construction can include measurement of pore pressure, seepage, earth pressure, settlement or heave and lateral movements.
Monitoring by means of inspection or instrumentation should be carried out wherever appropriate; this might require the installation of appropriate instruments additional to those that were installed as part of the pre-construction investigations. If existing instruments are used, their correct functioning should be verified and this might require fresh calibrations. Readings of the instrumentation can be usefully continued after construction in order to observe the performance of the project. This is particularly necessary in the case of earth dams for maintaining a safe structure under varying conditions, and in other cases for gaining valuable data for future design.
NOTE Monitoring might be necessary to check that construction works can proceed safely or on large or critical structures such as earth dams, embankments on soft ground, large buildings with underground construction, deep excavations or tunnels.
Monitoring should be carried out in accordance with
BS EN 1997-1:2004+A1:2013, 2.7 and the following should be identified:
- who makes observations and where, and who reads which instruments and when;
- the party that receives the observations;
- the review to be carried out, checking against the acceptable limits of behaviour; and
- the contingency actions to be adopted if the monitoring reveals unacceptable performance.
These requirements should be set out in the monitoring and maintenance report that is included within the safety file and passed to the client.
The findings of all construction and post-construction investigations should be incorporated into or appended to the GIR and GDR as appropriate.