6.3 Desk study

6.3.1 General

A desk study is an essential prerequisite for planning a ground investigation and informing subsequent design. Some general guidance on desk studies that should be consulted is provided in BS 5930:1999+A1 and BS 1997-2:2007, 2.1.1.

Feedback (through an iterative process) should be made from the desk study to the overall conception and design of the project, where warranted by its complexity. Preliminary information may be fed back into the project risk register at an early stage to aid conceptual design and preliminary planning application(s).

The desk study for earthworks projects should include an evaluation of both internal and external geotechnical influences on the project. This may include both current and historical land use, ground stability, an examination of existing earthworks, surface and groundwater conditions, contamination, etc. The desk study should include an adequate site-based inspection of the site and adjacent land by the geotechnical engineer; the use of geomorphological mapping techniques for this activity can prove advantageous.

The desk study should provide a design for the preliminary investigation based on the information available at that point in time.

The desk study should enable an initial input to geotechnical and project risk register(s) (see Figure 2).

Where appropriate an initial review may be undertaken based on the desk study information obtained of the various engineering options available for the scheme; when undertaken this generally forms the first phase of the project planning process.

6.3.2 Extent of desk study

6.3.2.1 General

The scope and extent of a desk study should be commensurate with the proposed works. The desk study for widening of an existing highway embankment may be limited to a walk-over survey and review of existing as-built drawings, whereas the desk study for 5 km of new flood defence embankment through an industrial area may require enquiries of multiple sources.

Figure 2 The iterative feedback processes of a site investigation
The iterative feedback processes of a site investigation

A) Optional. See text for recommendations on good practice.

B) Dependent on complexity of project (see 6.5).

6.3.2.2 Existing earthworks

A desk study for projects involving existing earthworks should seek to make maximum use of construction, inspection and maintenance records available for the earthwork, and give consideration to common performance problems with that particular type of earthwork (e.g. likely method of construction given age of earthwork).

A thorough search of drainage plans and records is essential when dealing with existing earthworks. Any maintenance records of infrastructure supported by the earthworks should be included in the desk study as these might highlight areas with potential underlying problems.

6.3.2.3 New-build earthworks

In addition to the recommendations of 6.3.1, attention should be paid to areas where land drains are present, as these might require interception. Topographical low points should be identified as these might be underlain by relatively soft, recent or weathered soils. The presence and extent of alluvium should be gleaned from geological maps and memoirs; this is particularly important when studying flood plains of large, mature river systems.

6.3.2.4 Reporting of desk study

COMMENTARY ON 6.3.2.4

In the UK, the desk study report (DSR) may be presented as a preliminary sources study report (PSSR) in accordance with HD 22/08 [11].

The presentation of desk study findings should be dependent on the scale of the project. Desk study information should be collated in a coherent form and be made available for all interested parties, including designers of ground investigations and designers of the earthworks. This may be achieved by the preparation of a formal DSR. However, if a DSR is not prepared, the information should be collated within the project file and made available to the project team; one option is to include the data within the ground investigation report (GIR).

NOTE The potential benefit of compiling the desk study information and interpretation into a formal DSR depends on the size of the project and complexity of the ground conditions. The DSR can be an important document in itself and may be used for preliminary planning applications, design, etc.

6.4 Ground investigation

6.4.1 Preliminary investigation

COMMENTARY ON 6.4.1

See BS EN 1997-2:2007, 2.3.

The preliminary investigation is undertaken once the general scope of the project has been identified to provide an overall understanding of the ground conditions at the site and thereby minimize the geotechnical risk associated with the project (see Clause 4). The outputs from the preliminary investigation are likely to include some or all of the following:

  • ground investigation report;
  • feedback to geotechnical risk register;
  • feedback to project risk register (option analysis);
  • proposals for design investigation.

6.4.1.1 Purpose and extent of preliminary investigations

The specific parameters that should be evaluated depend on the nature of the works; with different priorities being applied to new build, modification and repairs. Four broad categories of earthworks related assessment should be considered.

  • a) Shear strength and stiffness (consolidation) characteristics of in-situ soils that will be subject to changes of in-situ stress; e.g. soils below formation level in cut zones and fill zones, soils that will form batter slopes in areas of cut. This may include existing earthworks that are to be incorporated into new works; e.g. raising of an existing embankment.
  • b) Characteristics of fill materials after compaction. This may include classification of fills in terms of SHW Table 6/1 and establishing relationships between moisture content and other compaction control related parameters. It may include estimation of shear strength and stiffness parameters.
  • c) Assessment of ground for excavation. This may cover estimation of volumes of suitable, or potentially suitable, fill material available from identified borrow areas or other sources. The assessment may also include rippability of material to be excavated to form cuttings.
  • d) Assessment of the hydrological and/or hydrogeological conditions that may influence the design and construction of the earthworks.

A preliminary investigation for existing slopes and earthworks should include:

  • a check for existence of shear zones;
  • gathering data to enable back analysis of existing slopes for comparison with laboratory parameters;
  • highlighting potential access difficulties for remedial works; and
  • consideration of the possible need for specialist plant.

6.4.1.2 Reporting of preliminary investigations

Preliminary investigations should be presented within the GIR. The degree of detail in the GIR should be consistent with the objective of the particular phase of ground investigation.

As required by BS EN 1997-2:2007, 6.1, the GIR should include both factual information and evaluation.

COMMENTARY ON 6.4.1.2

BS EN 1997-2:2007, 6.1 states that "the GIR shall form a part of the Geotechnical Design Report". It goes on to state that:

"(2) The Ground Investigation Report shall consist of the following:

  • a presentation of all appropriate geotechnical information including geological features and relevant data; [commonly referred to as a "factual report" in the UK]
  • a geotechnical evaluation of the information, stating the assumptions made in the interpretation of the test results. [historically included within an "interpretative report" in the UK].".

"(3) The information may be presented as one or as separate parts."

6.4.2 Design investigation

COMMENTARY ON 6.4.2

Design investigation is undertaken to clarify ground conditions at selected locations or to clarify particular issues to enable the design (there can be as many phases of design investigation as are required).

See BS EN 1997-2:2007, 2.4.

6.4.2.1 General

The design investigation should concentrate on specific aspects of the geotechnical design that mitigate the risks identified in the geotechnical/project risk register(s), where appropriate.

6.4.2.2 Purpose and extent of design investigation

The design investigation for an earthworks project should include classification testing, soil relationship testing and marginal materials assessment (see 6.1.3). The design investigation should elaborate on and develop the classification and relationship testing undertaken as part of the preliminary investigation.

At this stage the designer may give consideration to the likely approach to the earthworks and arrange for appropriate specialist testing. Sufficient numbers of large bulk samples, particularly from trial pits, should be specified in the design investigation to enable extensive laboratory trials to be carried out. These may include strength and stiffness testing and undertaking stabilization trials (modification of marginal materials at varying additions of binder). These trials consume large volumes of material but it should always be considered that reducing the number and size of bulk samples can be a false economy; this is especially the case where factors beyond the control of the project team preclude further GI.

Where appropriate, the design investigation should include the installation of monitoring instrumentation such as piezometers and inclinometers. Further guidance is given in Dunnicliff [21].

The design investigation should be designed to meet and, if needs be, enhance/expand on the requirements of BS EN 1997-2:2007.

6.4.2.3 Reporting of design investigation

A geotechnical design report (GDR) should conform to BS EN 1997-1:2004, 2.8. HD 22/08 [11] has been drafted in compliance with BS EN 1997 (both parts), with a particular emphasis on the requirements of a major earthworks project.

The following items might be of particular relevance to an earthworks design and should be considered for inclusion in the GDR as appropriate.

  • a) Draft specification of fill materials. This may be in the form of draft numbered appendices in accordance with the SHW [1]. In particular, the designer should be looking to set upper and lower limits for identified parameters in Table 6/1.
  • b) Formalized charts for relationship testing of fills.
  • c) The proposed scheme for monitoring and measuring compliance with the specification. This may be in the form of a draft Appendix 1/5 in accordance with the SHW [1].
  • d) Deformation limits and associated monitoring requirements if an observational design approach is being followed. The designer should make clear any assumptions made in the design. In particular, the GDR should make clear any assumed sequence of works as this may affect the routing of haul roads or selection of plant.

6.4.3 Further investigation during construction

It should be appreciated that no ground investigation, however carefully done, ever examines more than a very small proportion of the ground. It is essential that the soil conditions revealed during progress of the excavations are checked to see that they correspond with those forming the basis for earthworks design as interpreted from the ground investigation; it might be necessary to undertake further investigation to determine the extent of anomalous conditions.

NOTE Guidance is given in BS 5930:1999+A1.

6.5 Geotechnical feedback

The iterative feedback process that should be followed, including the production of a geotechnical feedback report (GFR) if appropriate, is illustrated in Figure 2.

COMMENTARY ON 6.5

BS EN 1997-2:2007 does not include a requirement for a GFR to be produced to record the construction works phase of a project, however BS EN 1997-2:2007, Annex B, under "Execution of works", does advise a "report from inspection, supervision and monitoring", and this would at least in part be satisfied by the production of a GFR. As stated at Clause 4, the GFR is a document of value for capturing geotechnical data from an earthworks project (this would also be a valuable part of the CDM health and safety file).

A GFR is usually prepared on completion of construction. Generally two broad types of information are contained in a GFR:

  • a) geotechnical design changes during construction; and
  • b) results of monitoring and testing conducted during construction.

It forms an ideal vehicle that may be used for reporting the results of compliance testing (e.g. in-situ density measurement, plate load tests), distribution of fill classes within an earthwork, and data from monitoring instruments (e.g. piezometers, inclinometers, settlement gauges). The report should give a commentary on how the results of testing and monitoring have compared with the expected or specified range of values. This is particularly important where the observational method of design has been applied. The commentary should include a time-line that allows a chronological understanding of events, e.g. relating instrument readings to filling operations.

NOTE The benefits of preparing a geotechnical feedback report are both for maintenance of the site and to capture "lessons learned" for future projects: as-built drawings alone generally do not capture all the information relevant to earthworks that will be relevant to future maintenance of the earthwork; an appropriate range of information to be included within the geotechnical feedback report is detailed within HD22/08 [11].

BS 6031:2009 Code of practice for earthworks