19 General considerations in the selection of methods of ground investigation
19.1 General
The selection of the appropriate equipment for the investigation of ground, groundwater and ground gas conditions (see BS 8576) should take into account the phase of investigation and likely design requirements for the geotechnical category of structure or structures that are proposed.
NOTE 1 BS EN 1997 places greater emphasis on the need to undertake serviceability limit state design. This requirement might require greater consideration of the stress-strain behaviour of the ground with ground investigations including more in-situ and advanced laboratory testing.
The geotechnical parameters that are required for design should inform decisions relating to the following:
- type of intrusive equipment (e.g. drilling rigs) to be used;
- type of samplers;
- in-situ testing; and
- monitoring installations.
Regardless of whether in-situ testing is undertaken or samples are recovered, the results obtained should be of the highest possible quality in relation to the likely ground conditions that exist at the site. For soil and rock samples, the choice of appropriate sampler should be considered in relation to the required quality class of the sample. This in turn should be dictated by the type(s) of laboratory testing that is to be performed in order to establish the required design parameters.
NOTE 2 The availability of equipment and personnel or the cost are not reasons to compromise achieving the required design parameters.
19.2 Site constraints
19.2.1 General
When planning the scope for a ground investigation, there are a number of potential constraints (see 19.2.2 to 19.2.7) that should be taken into account by the designer.
NOTE The constraints can influence the type of ground investigation equipment to be used and methods of sampling to be employed. In addition, some site constraints have health and safety as well as environmental implications for the ground investigation contractor's workforce as well as for the general public. It is important for the designer to be aware of any site constraints as early as possible, so that they can be identified to the ground investigation contractor.
19.2.2 Terrain constraints
COMMENTARY ON 19.2.2
There are many possible physical constraints that might affect a ground investigation. These could include access restrictions in terms of the ground conditions as well as the width and height available for the equipment.
The ground surface conditions and obstructions on the site and accesses to the site should be taken into account in the planning of investigations; these factors could include soft or marshy ground, uneven ground, sloping ground, cliffs or breaks in slope, possible flooding, buildings, gateways, trees and mature shrubs. The area around the investigation positions should include adequate space for working, storing tools and stockpiling excavated materials.
Land affected by shallow mining and quarrying activities should be assessed to identify any shafts, crown holes or areas of potential instability. Land affected by natural cavities such as caves or swallow holes should be assessed to identify any areas of instability. The use of ground investigation equipment in these areas should be carefully assessed.
19.2.3 Structure constraints
COMMENTARY ON 19.2.3
In urban areas physical constraints that might affect the placement of investigation points include the presence of existing buildings immediately adjacent to the site to be investigated. There might be overhanging structural elements or basements that extend beyond the apparent footprint of the existing buildings.
The need for access between and around existing structures should be taken into account in relation to width and headroom; this is particularly the case for investigation points that have to be accessed through other structures.
Exploration of the ground should take account of the possible presence of buried basements, cellars, walls, foundations, tunnels, shafts and pipelines. Where present, structural plans should be consulted to identify the extent of such features prior to designing the investigation.
The stability of existing structures (e.g. retaining walls, pipes, shallow tunnels) should be adequately assessed prior to the undertaking of ground investigation, with particular attention paid to the ability of the existing structure to cope with pressures exerted by a drilling rig or other plant.
Special considerations which apply to investigation points sited within existing structures should be taken into account. Confined space working means that there is the potential for the build-up of dangerous gases emitted from the investigation equipment being used; provision should be made to adequately vent all noxious gases and fumes so as to protect the workforce. In some cases, remotely or electrically powered equipment can be used in order to minimize the risks of exhaust gases in the confined space. The noise associated with operating equipment in confined spaces should also be taken into account.
19.2.4 Utility constraints
COMMENTARY ON 19.2.4
There is a plethora of buried and overhead utilities both in rural and urban areas. Although the concentration of such utilities is far greater in urban areas, the same procedures are used to identify their location prior to any investigation taking place.
The locating of utilities should be carried out by desk study, specialist search, on-site search survey, on-site location and inspection holes, as appropriate. The designer of the investigation should identify the location of utilities prior to any investigation taking place; this task may be delegated to the contractor.
NOTE 1 Certain utilities impose minimum distance criteria for investigation depending on the proposed investigation techniques. For example, there is a minimum distance for the formation of cable percussive and rotary drilled boreholes from buried cast iron water and gas pipes. There is also a restriction on the distance of a rig mast from overhead power lines. General guidance on minimum distances is available from the Health and Safety Executive documents HSG47 [3] and HSE AIS No 8 [19]; individual utility companies might have specific requirements.
NOTE 2 Vibration monitoring might be required by utility companies for those exploratory hole positions that could potentially impact adjacent buried pipelines and services.
Once the likely types of utility present on a site have been identified, the ground investigation designer should make due allowance in their siting of investigation points.
Overhead utilities (pylons, telephone wires, etc.) should be taken into account during pre-planning site desk studies and field reconnaissance. During such reconnaissance visits, any evidence of services in the form of accesses or signage (such as manhole covers, inspection pits, hedgeline markers) should be noted and reported as this assists with the identification of buried utilities and thus the siting of investigation points.
Underground utility detection of varying levels of detail should be carried out in accordance with PAS 128.
Utilities within the top two metres of ground are usually searched for using inspection pits, cable avoidance tool (CAT), scanning and ground probing radar (GPR). For deeper structures, such as many sewers and tunnels, such methods are not suitable and they might not be identified or shown on Ordnance Survey maps; therefore reference should be made to utility organizations. Specialist search companies may be used for this as the right contacts should be used to ensure that the search is comprehensive. An appropriately designed geophysical survey should deploy techniques that are able to resolve features at greater depth, and the potential use of these technologies should also be evaluated.
19.2.5 Environmental constraints
COMMENTARY ON 19.2.5
On any brownfield site and on many greenfield sites (e.g. agricultural land), there is a possibility that agents or substances potentially harmful to humans, or other potential receptors such as local flora and fauna, the water environment or structures, might be present. Alternatively, or in addition, naturally elevated concentrations of potentially hazardous substances (e.g. arsenic, methane, radon) might be present.
To ensure the safety and protection of site staff, the public and the environment, the geotechnical adviser should assess and communicate the potential risks arising from contamination or the presence of naturally elevated concentrations of potentially hazardous substances in the ground or groundwater.
A desk study and field reconnaissance should be carried out in accordance with BS 10175 to properly identify the potential for hazardous substances to be present, either naturally or as a result of human activity.
All sites should be categorized by the designer as either green, yellow or red (see BDA guidance [2]).
NOTE 1 Green sites are those where there is little potential to cause permanent harm to humans; yellow sites are those where the substances are not sufficiently harmful to cause death but nevertheless require protection to be worn; red sites are those where the substances could subject persons to risk of injury, impairment or death.
If a site is categorized as red, the geotechnical adviser should select methods to obtain information on the ground conditions without causing disturbance or movement of existing contamination and should ensure the health and safety of the ground investigation contractor's workforce.
NOTE 2 The use of rotary drilling techniques in contaminated ground can spread contaminants both laterally and vertically unless careful control of the flushing medium is exercised.
NOTE 3 For further guidance on environmental constraints, see BS 10175.
19.2.6 Seasonal and ecological constraints
Seasonal constraints such as fields being ploughed, planted or harvested or animals' breeding seasons should be taken into account.
NOTE 1 There might also be environmental constraints imposed by sites having a protected status, such as Sites of Special Scientific Interest (SSSI).
The potential presence of invasive or noxious species should be taken into account. Care should be taken to avoid:
- a) the spread of invasive or injurious plants (e.g. Japanese knotweed [Fallopia japonica], Himalayan balsam [Impatiens glanulifera], Giant hogweed [Heraculeum mantegazzium]);
- b) the spread of infective agents, including for example those causing foot/hoof and mouth disease (Aphatae epizooticae) and rhizomania (Benyvirus — Beet Necrotic Yellow Vein Virus [BNYVV]);
- c) the spread of genetically modified (GM) crops outside of areas approved for their growth.
Such potential risks should be identified and at an early stage and addressed by site management procedures and appropriate guidance provided to site operatives, including in respect of species such as Giant hogweed that can cause harm to humans. Specialist advice should be sought at the desk study and field reconnaissance stage and at other stages of the investigation as necessary.
NOTE 2 The listing above of invasive, noxious and injurious plants and infective agents is not exhaustive.
NOTE 3 Guidance on the management of Japanese knotweed on development sites has been provided by the Environment Agency in their document The Knotweed Code of Practice [20]. Background information on invasive and injurious species can be found at:
<www.gov.uk/japanese-knotweed-giant-hogweed-and-other-invasive-plants> [last viewed 24 June 2015].
19.2.7 Special constraints
COMMENTARY ON 19.2.7
On any site there exists the possibility that there are constraints that lie outside those that would normally be expected. Such special constraints might include but are not limited to, unexploded ordnance (see CIRIA C681 [21]), radioactive waste, unusually difficult access or working environments.
The geotechnical adviser should identify any special constraints and assess whether they preclude the use of certain investigatory techniques. If there is doubt over what techniques can be used on such sites, ground investigation contractors should be consulted in order to confirm what techniques are appropriate and what the associated risks might be.
