7 Design of earthworks
7.1 General
7.1.1 Introduction
Clause 7 should be consulted for information relating to the design of earthworks that are intended to be self supporting (excavations requiring temporary support are covered in Clause 13).
Earthworks that incorporate some degree of reinforcement should satisfy the recommendations of BS 8006-1 or BS 8006-2, in addition to this standard, where relevant.
7.1.2 Concept of BS EN 1997-1:2004 geotechnical categories
Earthworks should be designed in accordance with the principles and application rules of BS EN 1997-1:2004, which is intended to be used in the United Kingdom as a general basis for the geotechnical aspects of the design of buildings and civil engineering works.
NOTE BS EN 1997-1:2004, Section 5 (fill, dewatering, ground improvement and reinforcement), Section 11 (overall stability) and Section 12 (embankments) are particularly relevant to the design of earthworks whilst Section 1, (general), Section 2 (basis of geotechnical design) and Section 3 (geotechnical data) provide guidance on the application of design rules and the selection of parameters used in the design.
BS EN 1997-1:2004, 2.1 introduces the concept of limit state design to earthworks, typically the ultimate limit state (ULS) and the serviceability limit state (SLS); earthworks should be designed such that the relevant limit states are not exceeded during their design life.
COMMENTARY ON 7.1.2
Instead of using global factors of safety that have been adopted previously in traditional earthworks design, BS EN 1997-1:2004 adopts the approach of applying partial factors to actions (loads) and the effects of actions, materials (soil parameters) and earth resistances.
To establish the geotechnical design requirements, BS EN 1997-1:2004 recommends the classification of geotechnical structures into three geotechnical categories according to the complexity of the structure, the ground conditions, the loading and the level of risk that is acceptable.
The Geotechnical Categories are used to establish the extent of site investigation required and the amount of input to the design. The Geotechnical Category should be checked throughout the design and constructions process and the Category amended if necessary as information becomes available.
Geotechnical Category 1 – for small and relatively simple structures for which it is possible to ensure that the fundamental requirements will be satisfied on the basis of experience and qualitative geotechnical investigations. Category 1 structures carry negligible risk.
Geotechnical Category 2 – encompasses conventional geotechnical structures with no exceptional risk or difficult ground or soil loading conditions, most earthworks will fall into this category.
Geotechnical Category 3 – includes very large or unusual structures which are not included in Geotechnical Categories 1 and 2. Examples of such Category 3 structures (which could be earthworks) are given as: structures involving abnormal risks or unusual or exceptionally difficult ground or loading conditions; structures in highly seismic areas; structures in areas of ground instability or persistent ground movements that require separate investigation or special measures.
BS EN 1997-1:2004 permits the use of three design approaches but the National Annex adopts Design Approach 1 (DA1). DA1 includes two combinations and the application of these combinations is discussed in 7.3.3.
7.1.3 Design input at GI stage
The scope and extent of the ground investigation should reflect the BS EN 1997-1:2004 Geotechnical Category of the project. For Category 2 or 3 projects the GI will normally be undertaken as a staged process (see 6.4); this process might require a certain amount of design to be undertaken as the GI develops in order to confirm the scope of certain aspects of the GI.
7.1.4 Detailed design using GI data
The detailed design should be undertaken based on the GI data obtained; BS EN 1997-1:2004, 2.8 requires that a geotechnical design report is prepared (see 6.4.2) that identifies how the GI data has been interpreted into the design including justification for the design values adopted for soil and rock properties. The geotechnical design report should also identify ground condition issues that should be checked during construction.
7.1.5 Geotechnical certification
Consideration should be given to the adoption of a formal system of geotechnical certification (see 4.3), for recording formally the Geotechnical Category and for certifying the design process at specific stages; BS EN 1997-1:2004 Section 4 provides recommendations for the procedures to be adopted for supervision, monitoring and checking during the construction process.
