5.4.1 Introduction
In Section 4, BSEN 1997-1 requires that all geotechnical construction processes, including the workmanship applied, must be supervised, that the performance of the structure must be monitored, both during and after construction, and that the finished structure must be adequately maintained. The amount of supervision, monitoring and maintenance will depend on the nature of the project and the ground conditions.
These requirements are fulfilled by:
- Ensuring safety and quality through supervision, monitoring and maintenance. The nature and quality of the supervision and monitoring prescribed for a project must be commensurate with the degree of precision assumed in the design, and in the values of the engineering parameters chosen and the partial factors used in the calculations. If the reliability of the design calculations is in doubt, it may be necessary to prescribe an enhanced regime of construction supervision and
monitoring.
- Communicating what must be done formally in contract documents and records such as the Geotechnical Design Report (GDR);
- Supervising, monitoring and maintaining in an orderly, planned manner, with the keeping of records.
Clause 4.1(1)P Clause 4.1(8)P Clause 2.8(1)P
It is the designer’s responsibility to specify the requirements. Clearly, it cannot be the designer’s responsibility to ensure that post-construction monitoring and maintenance are carried out on the structure; however, it is the designer’s responsibility to prepare and communicate specifications for any such monitoring and maintenance, so that assumptions made in the design can be confirmed during and after construction.
Section 4 contains no specific details about construction workmanship and the reader is advised to check in the appropriate BSEN ‘execution’ standard for special geotechnical works (see Section 6 of this Guide).
As in other parts of BSEN 1997-1, Section 4 does not specify precise contractual arrangements. It does, however, indicate what tasks must be undertaken and that an appropriate flow of information must be maintained. Section 4 is concerned principally with the actions of the designer and with information others should make available to him.
BSEN 1997-1 specifically requires that extracts from the GDR concerning any post-construction monitoring and maintenance must be provide to the owner/client.
Clause 2.8(6)P
A list of the more important items to be considered for supervision during construction is given in Annex J of BSEN 1997-1. The reference in Annex J to ‘…due consideration of geotechnical limit states’ particularly concerns matters such as the collapse of an excavation in which men are working. Item J.2.1(4) of this Annex refers to the ‘safety of workers’ which is not
specifically covered by the Code as it is a matter for National health and safety regulation. Reference should be made to the Construction (Design and Management) Regulations - the CDM Regulations (Health and Safety Commission, 1994).
5.4.2 Supervision
‘Supervision’ means checking both the design and the construction. A plan is required, to be included in the GDR, that will depend on the scale and complexity of the project, as may be represented by Geotechnical Categories (GCs). In the case of GC1, a visual inspection of the ground, superficial quality controls and a qualitative assessment of the performance of the structure during and immediately after construction, may be all that are required. For GC2, measurements of the properties of the ground or the movement of the structure may be necessary. Tests to check the movement under load and the quality of piles, or tests of the density of fill behind a retaining wall are examples of the monitoring that might be required for GC2 projects.
‘The construction shall be inspected on a continuous basis…’. ‘Continuous’ means that inspection should not be so infrequent that important features of the ground or of the installation of foundations, for example, may be missed.
Clause 4.2
That inspection records should systematically be kept and made available to the designer is an important requirementof BSEN 1997-1. The following features must be recorded, as appropriate:
- Significant ground and groundwater features; - The sequence of works;
- The quality of materials; - Deviation from design; - As-built drawings;
- Results of measurements and their interpretation; - Observations of the environmental conditions;
-
Unforeseen events.Clause 4.2.2(5)P
5.4.3 Checking ground conditions
BSEN 1997-1 describes the manner in which the checking of ground and ground water conditions is carried out by inspection, description and recording.
Clause 4.3
5.4.4 Checking construction
This must include checks that site operations comply with those specified in the GDR.
Clause 4.4(1)P 5.4.5 Monitoring
The purposes and objectives of monitoring are listed; again, compliance with
any requirements stated in the GDR is emphasised. Clause4.5(4)
5.5 Fill, Dewatering, Ground Improvement and Reinforcement
5.5.1. Introduction
because the subject matter generally concerns improving the properties of the ground. The Section provides few specific requirements except for some aspects of the use of fill. The Section effectively comprises a checklist of items which must be considered in the design.
In BSEN 1997-1, fill placed during construction works is regarded as part of the ‘structure’. BS EN 1997-1 defines a ‘structure’ as ‘an organized
combination of connected parts, including fill placed during execution of the construction works, designed to carry loads and provide adequate rigidity’.
Clause 5.1(1)P
Clause 1.5.2.4
5.5.2. Fundamental requirements
Filled, dewatered, improved or reinforced ground has to satisfy the same fundamental requirements as natural ground, namely that it must be capable of sustaining the actions arising from its function and from its environment.
Clause 5.2(1)P 5.5.3. Constructing with Fill
The Code covers the selection, placement, compaction and checking of fill in the following circumstances:
− beneath foundations and floor slabs, − in excavations,
− behind retaining structures
− for general landfilling, including hydraulic fill, − for landscape mounds
− for the placement of spoil heaps
− for embankments for small dams and transportation infrastructure.
It provides a checklist of items to be covered in specifications for earthworks.
Clause 5.3
Clause 5.3.2(3)P
Design calculations require the assessment of the characteristic values of the fill properties which are cautious estimates of the values affecting the occurrence of the limit state. It should be noted that, at the time the design is made, the fill to be used may not have been identified, though its properties will have been specified.
5.5.4. Dewatering
‘Dewatering’ means the abstraction of water to improve the behaviour of the ground and to facilitate construction in it. Groundwater ‘recharge’ may also be required, as part of a dewatering scheme, to prevent the lowering of the water table.
The importance of the control of groundwater is emphasised in several places in BSEN 1997-1 and the Code provides a checklist of conditions to be met when designing a dewatering scheme. It is particularly important to check that groundwater lowering has no unwanted consequences for structures in the vicinity. The design of dewatering schemes often utilises the Observational Method.
5.5.5. Ground improvement and reinforcement
BSEN 1997-1 discusses the design of ground improvement and ground reinforcement only in very general terms, with some aspects being listed. In many cases, the design of ground improvement and ground reinforcement would be classified as a GC3 problem and, as such, would be covered by the requirements of BSEN 1997-1 only in general terms; design would be carried out by a geotechnical specialist.
In order that the effectiveness of improvement may be checked, the Code requires that an initial geotechnical investigation be carried out to determine ground conditionsbefore the improvement technique is applied.
e.g. clauses 2.4.2(9)P and 10.5(1)P Clause 5.4(3) Clause 5.5(2)P Clause 5.5(3)P
6. EUROPEAN GEOTECHNICAL CONSTRUCTION STANDARDS