17.7 Depth of exploration

17.7.1 General

The minimum depth of investigation points should be in accordance with BS EN 1997-2:2007, Annex B, unless there is good reason not to do so, which might include the structure, experience and prior knowledge of the ground conditions.

NOTE 1 Additional rules for low rise buildings are given in BS 8004.

The depth of exploration depends on how significantly new work affects the ground and groundwater, or is affected by them. For example, exploratory holes for a building should normally be taken down to a depth that includes all ground that might be stressed to a significant level by the foundation. This might include investigation of weaker strata overlain by a stronger layer.

If rock is encountered, the depth of penetration into the rock should be taken into account; this depends on several variables including the type of structure to be constructed and the anticipated ground conditions. Potential voids (either natural or man-made, see Annex F) should be taken into account in assessing the required depth of investigation.

NOTE 2 It might also be necessary to advance more than one borehole by an appropriate method to penetrate sufficiently far into the "hard stratum" to establish whether bedrock or a boulder has been encountered, unless prior knowledge of the local geology obviates this.

There are likely to be numerous combinations of geological strata that require the depths of investigation points to be reviewed on a project by project basis. No guidance can be given here for such cases, but where doubt arises, consideration should be given to drilling a deeper hole, or making an in-situ examination in an excavation. More specific recommendations are given in 17.7.2 to 17.7.8.

NOTE 3 It is not always necessary that every exploration be taken to the depths recommended in 17.7.2 to 17.7.8. In many instances, it is adequate if one or more boreholes are taken to those depths at an early stage of the field work to establish the general ground profile, and then the remainder sunk to shallower depths to explore more thoroughly the zone near the surface which the initial exploration had shown to be most relevant to the problem in hand.

Within strata of either low strength or high compressibility, within made ground, or within ground that has been weakened by the effects of mining and/or ground subsidence, the depth to base of this ground should be proven unless known from the latest ground model.

17.7.2 Foundations for structures Spread foundations

For spread foundations, the depth of exploration should be in accordance with BS EN 1997-2. The depth of exploration should cover the zone of ground stressed by the foundations associated with the proposed structure(s); it is usually related to the width of the loaded area.

NOTE 1 The factors relating thickness of zone to width of foundation are given in BS EN 1997-2:2007, Annex B.

NOTE 2 The exception to abiding by the depth/width relationship is where the imposed stress change becomes insignificant when compared with the strength and stiffness of the ground at a lesser depth, e.g. in competent rock. Piled foundations

Where piled foundations are considered a possibility, a preliminary analysis of the possible lengths of different types and sizes of pile should be made using the desk study information. Investigation depth for piled foundations should be in accordance with BS EN 1997-2: this would generally require that a conservative assumption of pile depths is made in scheduling the depths of investigation.

NOTE 1 If the information from the investigation shows that deeper piles are required than previously assumed, additional investigation is likely to be appropriate.

In addition, the following factors should be taken into account when planning the depth of exploration.

  • a) Pile groups stress the ground below the pile points to a greater depth than an individual isolated pile. The depth of exploration should be sufficient to identify any weak strata beneath the pile points, which might affect the bearing capacity of the pile groups.
  • b) Made ground and compressible soils seldom contribute to the shaft resistance of a pile and might add down drag to the load on it. The whole pile load, possibly with the addition of down drag, should be borne by the stiffer strata below these materials.
  • c) In the case of end-bearing piles in strong rock, boreholes should be of sufficient depth to establish conclusively the presence of suitable founding rock. The rock should then be further explored, usually by means of rotary drilling, to such a depth that the geotechnical advisor is satisfied that there is no possibility of weaker strata occurring beneath, which could affect the performance of the piles.
  • d) There are some instances where consideration should be given to shortening the investigation holes compared to the depths given in BS EN 1997-2:2007, Annex B. An example of this would be pile-supported rafts on clays which are being used solely to reduce settlement and where relatively incompressible strata occur within the zone of influence of the "imaginary raft". In this case, the depth of exploration is governed by the need to examine all strata that could contribute significantly to the settlement.
  • e) In rocks, the exploration should be taken through the weathered materials and should penetrate a sufficient depth into the relatively unweathered rock to prove its continuity.

Based on the information of the ground model obtained from the desk study, the general guidance given in a) to e) and the assessment of the types of pile likely to be considered, the geotechnical advisor should determine the depth of exploration and be ready during the course of the field work to modify this depth as necessary.

NOTE 2 If any structure requiring piled foundations is likely to be affected by subsidence due to mining or any other causes, greater exploration depths than those recommended in this clause might be required so that basic geological data can be obtained.

17.7.3 Embankments

The depth of the exploration for embankments should be sufficient to check possible shear failure through the foundation strata and to assess the likely settlement. In the case of water-retaining embankments, investigation should explore all strata through which piping could be initiated or significant seepage occur.

17.7.4 Cuttings, quarries and opencast mines

For cuttings, quarries and opencast mines, the depth of exploration should be sufficient to permit assessment of the stability of the proposed slopes and the base of the excavation; this might necessitate proving the full depth of any relatively weak strata.

Groundwater conditions should be determined to ensure stability of the slopes and the base of the excavation for the design life.

17.7.5 Roads and airfields

For roads and airfields, the depth of exploration should be sufficient to determine the strength and frost susceptibility of possible subgrades and the drainage conditions.

17.7.6 Pipelines

For deep pipelines, the depth of exploration should enable groundwater control to be designed if necessary and allow any likely difficulties in excavating trenches and supporting the pipelines to be investigated.

NOTE Exploration to significant depths below the invert level might be advisable. Large pipelines, especially those in ground of low bearing capacity or slope instability, require special consideration.

17.7.7 Maritime works

For maritime works, tidal variations should be taken into account, together with the appropriate considerations from 17.7.2 to 17.7.6.

17.7.8 Tunnels

For tunnels, the exploration should be taken to an adequate depth below the proposed invert level, both because changes in design might result in the lowering of the level of the tunnel, and because the zone of influence of the tunnel can be extended by the nature of the ground at a greater depth. Some of the boreholes should be taken to a depth sufficient to establish the overall geological structure (see 17.5 on the location of the boreholes).

An investigation point should be formed close to the location of any tunnel access shafts, portals, etc. In the case of access shafts, the depth of investigation should be sufficient to identify the potential for hydraulic uplift of the base and for any consequential cut off walls (temporary or permanent works) extending below shaft invert level.