6 Site conditions and investigations

6.1 Site investigation for earthworks

6.1.1 Scope

The site investigation [comprising desk study, geomorphological mapping, topographic survey and physical ground investigation (GI) as appropriate] should be planned and implemented to ensure that the site conditions are adequately understood. In particular it is essential that the GI provides adequate and sufficient information for design and construction, and for investigation of existing earthworks and slopes (BS EN 1997-1:2004, 2.1.8 and 2.1.9 have to be satisfied in this regard).

NOTE The degree of the investigation will depend on the complexity of the project. For small-scaled earthworks, such as simple re-grading or reshaping of the ground profile, a simple trial-pitting investigation with in-situ testing might be appropriate; for larger schemes a more detailed geotechnical investigation is likely to be needed.

The scope of the site investigation should be determined by the project's geotechnical engineer with consideration given to all parties that will be involved with the earthworks in future, particularly the earthworks contractor (input from whom at an early stage is to be encouraged).

The recommendations and guidance within this section apply to both new construction and the investigation of existing earthworks; the GI should be planned to suit the project.

6.1.2 Planning

NOTE BS EN 1997-2:2007 identifies the need to plan the investigation to provide sufficient information for the different stages of design; further guidance is given in BS 5930:1999+A1.

The geotechnical designer should be consulted and contribute to all stages of the investigation.

When planning a phase of ground investigation, it is important to consider the needs of all those who will use the data obtained, at a later phase of the scheme, e.g. if the project will go to design and build tender, what information will be required to evaluate the scheme?

If the observational method is the design approach, certain requirements for the GI should be addressed at an early stage (see BS EN 1997-1:2004, CIRIA R185 [16]). However, the GI and design process for earthworks projects should be responsive to ground conditions encountered, so there should always be an element of observation and response. Therefore, the GI may be planned accordingly from an early stage, which can be advantageous, (e.g. position of instrumentation to ensure future use through the project).

The investigation of failed earthworks should be given special consideration when planning investigations. Of paramount importance are the on-going risks posed by the failed earthwork and the economic implications for the earthworks owner. The investigation should be designed to identify the failure mechanism and provide sufficient information to design an engineered solution to the problem.

Existing earthworks that will be subject to modification e.g. embankment widening should be given special consideration when designing a GI to take account of the potential for differential settlement, increased porewater pressures and reduced slope stability.

6.1.3 Testing

The testing regime, both in situ and within the laboratory, should form an integral part of any GI to enable site characterization and material classification for both design and construction purposes.

The choice of testing method to control the works should maximize the volume of usable material and minimize disruption to the works. The use of laboratory relationship testing (MCV : mc : dry density : strength) in advance of the works is advisable; this approach will often enable control of earthworks by MCV which minimizes disruption of the works.

Relationship testing to identify an acceptable range of moisture contents for the material to be used in earthworks should be undertaken under Category 2 and Category 3 projects. An adequate amount of soil should be recovered as bulk samples to enable a range of testing to be undertaken at a set of moisture contents, allowing an assessment to be made of the acceptable range of moisture content of the material for use in earthworks (see HA44/91 [17] and HA70/94 [18] for details). An assessment may then be made for the appropriate treatment of marginal material falling outside of this acceptable moisture content range, or one of the other required material suitability criteria such as grading. Additional testing may be required as part of this assessment of marginal materials.

Table 2 provides a summary of the earthworks testing that is most commonly used to follow the SHW [1], and appropriate tests from Table 2 should be selected for the investigation and design stage.

COMMENTARY ON 6.1.3

The nature of soils tests undertaken for earthworks are detailed within BS EN 1997-2:2007 and the various supporting documents. Experience has shown that more meaningful results can be obtained for earthworks by modifying tests to take account of local soils and conditions. This subclause is provided to identify the tests most commonly recommended to control earthworks (especially when following the SHW [1]) and to comment on some of the main issues that ought to be considered both during GI and design of the earthworks.

Further details on some of the issues are provided within HA 44/91 [17] and HA 70/94 [18], which provide information on the selection and assessment of appropriate testing for the control of earthworks and the design assumptions that form part of the SHW [1]. This topic is summarized in 7.6.4. The information on earthworks control shown in

Table 2 is only provided to illustrate how the test might be taken into the construction stage.

6.1.4 Geotechnical reporting

Site investigation and design should be undertaken as a phased process in order to ensure that the ground conditions of the site are adequately understood for the works to be constructed (see Site Investigation in Construction [20] for guidance ⁶⁾). The geotechnical reporting should be dependent on the complexity of the scheme, as identified in the subsequent sections, but in most cases this will also follow a phased process.

All phases of the geotechnical reporting should feed back into the geotechnical risk register (see Clause 4) as appropriate to the scale of the project.

NOTE The reporting approach required to satisfy BS EN 1997 (both parts) includes the preparation of reports at various phases in the ground investigation, design and construction process. These are set out in 6.3, 6.4 and Figure 2 in the context of earthworks.

6.1.5 Soil and rock descriptions and classification

6.1.5.1 Description

Soils should be described in accordance with BS EN ISO 14688-1:2002 and BS EN ISO 14688-2:2004.

Rocks should be described in accordance with BS EN ISO 14689-1.

6.1.5.2 Classification

Earthworks materials should be classified in accordance with Table 6/1 of SHW [1].

The classification of the materials involved during excavation, transportation and deposition can vary, hence soil/fill may be classified at any of the following stages:

• in situ – classification in undisturbed condition prior to excavation;
• on excavation – disturbed material after excavation; and
• on deposition – classification following placing and prior to compaction.

The option for classifying soil/fill should be selected which is most appropriate for the particular project logistics and materials to be worked with, and should maximize the potential to win suitable fill from the site.

Classification should be based on both descriptive determination and standard use of materials, such as SHW [1] Table 6/1.

6.2 Site characterization and investigation

NOTE 1 See BS EN 1997-2:2007, 2.1 and 2.2.

The stages of a geotechnical investigation should include a desk study (sometimes referred to as a preliminary sources study), a preliminary investigation, to characterize the site in general terms, and wherever required, subsequent phases of design investigation to provide detailed information for specific elements of the design. These phases are briefly described within the following subclauses.

NOTE 2 Under certain circumstances, it is impracticable to use a phased approach to a geotechnical site investigation beyond the desk study. The ground investigation phases have to be combined. This is particularly true for investigations in a railway environment or major trunk road where opportunities for access are limited.