Section 3: Planning ground investigations

15 Types of ground investigation

15.1 Sites for new works

Investigations of sites for new works should aim to provide information to assist in the selection of the most suitable location for the works. They should also aim to provide the data required for designing the geotechnical aspects of the works.

NOTE 1 The type of information required varies, depending on the nature of the proposed works.

Where a building is to be constructed, for example, a knowledge of the subsurface strata and groundwater conditions should include:

  • a) the strength of the various layers so that a stratum capable of supporting the loads imposed by the foundations can be identified;
  • b) the stiffness of those strata which are subjected to an increase in stress to allow the magnitude of settlement to be assessed;
  • c) the potential for shrinkage or swelling of clay soils due to weather, transpiration of trees and shrubs, and heat, for instance from furnaces;
  • d) the potential for soil movements associated with freezing, which can occur naturally or artificially;
  • e) the degree of aggressivity of the soil to materials that might be used for foundations or other buried elements of the works; and
  • f) the presence and possible implications of and on vegetation, including damage by the investigation.

Where an excavation is required, the investigation should enable decisions to be made on:

  • 1) how material is to be removed;
  • 2) excavation stability and support requirement;
  • 3) the groundwater conditions and any groundwater control or other requirements;
  • 4) whether the excavated material has suitable engineering and chemical properties to be retained, reused and/or reinstated as part of the earthworks design, and whether the nature of the excavated material is likely to change, such as an increase in volume (bulking); and
  • 5) whether any of the soil or groundwater contains hazardous substances, therefore requiring special controls on excavation, movement, disposal and additional safety measures.

Investigations should also take account of changes which can potentially be caused by the proposed new works, whatever their nature, for example:

  • i) impeded drainage, which can result in a rise in the groundwater level; this can cause softening of fine grained strata and a reduction of bearing capacity of permeable strata, as well as giving rise to increased pore pressures affecting the stability of slopes and retaining walls; swelling can result in ground heave;
  • ii) alteration in stream flow of a waterway, which can cause undercutting of banks or scouring of foundations of walls, bridges and piers, and can be due to works carried out some distance away;
  • iii) siltation of the approaches of harbour works or the changing of navigation channel alignments;
  • iv) disturbance of contaminated ground, which can allow aggressive leachates or noxious gases to migrate through the ground or into the environment;
  • v) environmental or ecological considerations that might impose any constraints on the scope of the new works; and
  • vi) the compatibility of the environmental considerations with the geotechnical design.

For design purposes, assessments should be made of considerations such as bearing capacity and settlement of foundations, slope stability of embankments and cuttings, earth pressures on supporting structures, and the effect of any chemically aggressive or hazardous ground conditions.

For the design of new works, the range of conditions, including the least favourable conditions, should be known. Not only should a study be carried out of the degree of variability in the strata over the area of the site, but account should also be taken of the possible effects of groundwater variation and weather conditions on the properties of the various strata.

NOTE 2 Where works require excavations into or within rock, the orientation and nature of discontinuities in the rock can be the most important factor.

NOTE 3 The requirements for investigations of extensions on or reuse of existing sites are in most respects similar to those above except that the location of the proposed works is already substantially known.

15.2 Impact on existing structures


Existing structures close to, or even at a distance from, the proposed site might need to be investigated, to decide whether they are likely to be affected by changes in the ground and groundwater conditions brought about by the new works.

The effect of new works on existing structures should be taken into account as part of the investigation process; possible changed conditions include the following:

  • a) excavations or demolitions in the immediate vicinity, which could cause a reduction in support to the structure, either by general ground deformation or by slope instability;
  • b) mining or tunnelling operations in the neighbourhood, which could cause ground deformations and subsidence and the effect of tension and compression on the structures and drainage;
  • c) stresses that the new structure might impose on the foundation strata below adjacent structures or upon earthworks and supporting structures;
  • d) vibrations and ground movement resulting from traffic, vibratory compaction, piling or blasting in the immediate vicinity;
  • e) lowering of the general groundwater level by pumping from wells can increase effective stress in the ground, which could cause damaging settlement of adjacent structures. Also, the design of a filter for wells so that any leaching of fines from the subsoil does not result in excessive settlement of structures, which might occur at a considerable distance from the well; and
  • f) changes in groundwater temperature, for example, shallow geothermal systems.

Given problems of this kind, it should first be established what changes are likely to occur. Knowledge of the subsurface strata should be determined from the ground investigation and samples should be examined and tested to assess the effect that the changed conditions are likely to have on these strata.

NOTE 1 Detailed analysis might be needed to estimate the effect of the changed conditions on the safety of the existing works.

NOTE 2 On occasions, existing structures are demolished but their foundations left intact and re-used to support a replacement structure. In these circumstances, an investigation would be tailored to the specific requirement (see Butcher, Powell and Skinner, 2006 [12] and CIRIA C653 [13]).

15.3 Defects or failures of existing works


The investigation of a site where a failure has occurred is often necessary to establish the cause of the failure and to obtain the information required for the design of remedial measures.

Where a failure has occurred, observations and measurements of the structure should be carried out to determine the mode or mechanism of failure; these might suggest the origin of the trouble, or at least indicate whether the ground conditions were partly or wholly responsible. If this is the case, the ground investigation should ascertain the condition of the strata and the groundwater conditions, both as they exist in situ and, if possible, as they existed before the works were constructed. Each failure should be evaluated individually.

NOTE 1 Indications of the probable cause of a failure often result in detailed attention being directed to a particular aspect or to a particular stratum of soil.

NOTE 2 In the case of slope failure, or where such failure is considered imminent, it is common practice to monitor movements on the surface and underground. This is achieved by conventional survey methods and by instruments such as slip indicators, tilt meters, inclinometers, etc. Automatic data recorders and warning systems can be used to monitor potentially unstable conditions. These techniques are described in BS 6031 and Section 8.

NOTE 3 An investigation to determine the causes of a failure might be much more detailed than an investigation for new works.

15.4 Material for construction purposes


The investigation of a site might be required where the main objective is to establish whether it is a potential source of materials suitable for use in the construction of works elsewhere.

Investigations of sites to identify materials for construction purposes should include the following, as necessary:

  • a) assessment of the suitability and quantities of material that become available from excavations or dredgings for construction work and fills in other places, e.g. whether spoil from cuts can be used for embankments in roads and railway works;
  • b) ascertaining the presence of suitable material for specific purposes, e.g. borrow pits for earthworks, aggregate for concrete and road construction (see BS EN 12620 or, where ecological or environmental impact is an issue, see BS ISO 15176);
  • c) assessment of the chemical characteristics of ground that might need to be treated or removed to protect the environment;
  • d) ascertaining the location of suitable storage or disposal sites for excess spoil, dredged materials and groundwater, for which attention is drawn to current waste management regulations; and
  • e) assessment of the suitability of groundwater for use for construction purposes.

NOTE In works such as railways and roads, a prior knowledge of the strata on the line of the route might influence design, e.g. by suggesting deeper cuts at places where the material is particularly suitable for fill construction, or a reduction of cuts in unsuitable material. It might be possible to assess the suitability of the material by visual inspection. Otherwise, the suitability requires testing (field and/or laboratory) to identify the best use of the material (see Manual of contract documents for highway works, Vol. 1 [14]). Records of the British Geological Survey are recommended for investigations of this kind (see Section 2).

16 Geological mapping

The potential impact of the local geology on the project should be understood as a part of the investigation. Whatever information on local geology is obtained from published geological maps, memoirs and models or from commercially available extracts at the desk study stage (see Section 2), additional geological mapping or modelling should be carried out where appropriate (see C.2.1.1 for further guidance).

NOTE The object of geological mapping is the elucidation of the character, distribution, sequence and structure of the soils and rocks underlying the area. Interpretation of the geological conditions at the site might, therefore, require the mapping of a larger area.

An understanding of geomorphological features should inform the interpretation of the nature and distribution of soils and rocks. Hydrogeological conditions and man-made features should be recorded as part of the mapping.

Information from natural exposures and man-made exposures, such as quarries and cuttings beyond the site, might provide data on the material and mass characteristics, including, for example, the character of discontinuities, weathering profiles and the nature of the junction between soil and rock formations; such information should be used as a guide to conditions likely to be present at and beneath the site. Interpolation should be made with care and with knowledge of units and local geological conditions; geological deposits can vary laterally and very important geological structures such as faults and other major discontinuities might have only a restricted extent.

The information obtained from geological maps and mapping at all stages of the project should be used to assist with the formation and evolution of the ground model.