Section 4: Exploratory holes
23 Surveying of investigation points
Investigation points should be set out on the ground prior to making the excavations or boreholes.
NOTE 1 The most common methods of setting out are:
- a) using survey equipment including such as GPS (see 22.5) to establish a point on the ground at the coordinates established while planning the investigation; and
- b) by sight lines and distances from local features where these are present and on the ground and identifiable on plans of the proposed locations.
The location and elevations of investigation points should be established in relation to an appropriate coordinate system and datum, such as the National Grid and Ordnance Datum. The accuracy required of the surveying should be specified and/or agreed before the start of the investigation.
NOTE 2 GPS can achieve surveying to an adequate accuracy, provide coordinates and levels to a global system and thence to a national one.
NOTE 3 Local grids are sometimes appropriate, necessitating the use of traditional surveying instruments or conversion from a national system. National Grid references and Ordnance Datum allows the data to be used in future investigations.
As investigation points are sometimes moved from the initial set out position (for instance, to avoid underground services), the correct "as drilled" locations should be determined and reported, which might require further surveying (see Section 10).
24 Excavations and boreholes
Utilities should be identified before any intrusive work commences (see 19.2.4).
24.2 Inspection pits
COMMENTARY ON 24.2
Inspection pits are normally used to demonstrate the absence of buried utilities. They are usually small.
Inspection pits should be excavated using suitable hand tools or vacuum excavation techniques. A record should be kept of any utilities that are encountered or a null record made if none are encountered. Inspection pits should be described and sampled in accordance with the recommendations of this British Standard.
NOTE Such pits can also be used to inspect structural foundations, although these pits are also often referred to as trial pits.
24.3 Trial pits and trial trenches
COMMENTARY ON 24.3
Trial pits and trial trenches can be used for making a rapid check of the condition of the ground. They permit examination of both horizontal and vertical faces exposed as the pit is dug.
Trial pits and trenches should not be entered. Trial pits and trenches should be dug using an hydraulic wheeled back-hoe loader or a tracked 360° excavator.
NOTE 1 The maximum depth of excavation is generally 4 m to 5 m, but might be up to 7 m to 8 m where specialist plant is available.
The following should be taken into account to ensure the safety of both the excavator and the personnel involved with logging the pit and taking samples:
- a) location of excavator in relation to surrounding structures or above or below ground utilities;
- b) location of excavator in relation to excavation itself;
- c) positioning of spoil heap;
- d) position of geologist/engineering geologist during excavation and when logging; and
- e) exclusion zone for third parties.
If there are any indications of instability in the ground including those due to groundwater, excavation should be stopped, the risks assessed and a method of continued working developed. Pits that are unsupported could collapse soon after being dug, so any logging, sampling and in-situ testing should be carried out as, or immediately after, the pit is excavated.
NOTE 2 Guidance on safety aspects is provided by the AGS.12)
Working from ground surface, the geologist or engineering geologist should prepare a log of the strata and take disturbed samples from the excavator bucket. The field record should include a plan giving the location and orientation of the pit with details of which face(s) was logged, and a dimensioned section of each side and the floor (see BRE Digest 381 ). Whenever possible, the record should include photographs (see Annex H).
NOTE 3 Further details of reporting are given in Section 10. Trial pits can readily be extended into trenches in order to trace any particular feature, and in suitable ground this method is very efficient and economical.
Pits should be backfilled as soon as possible after logging, sampling and testing have been completed, since open pits can be a hazard to the general public (see 24.13).
24.4 Observation pits, trenches and shafts
COMMENTARY ON 24.4
Observation pits are those where personnel entry is required. By providing access for taking samples and carrying out in-situ tests, observation pits allow the in-situ condition of the ground to be examined in detail both laterally and vertically; they also provide a means of determining the orientation of discontinuities in the ground. Tube samplers can be driven into the floor of the pit and then extracted by the excavator or they can be pushed into the side walls. In-situ testing, such as vane shear strength or in-situ density, can also be carried out.
Deep shafts might be constructed by hand or machine excavation using traditional methods for supporting the sides. In suitable ground conditions, shafts approximately 1 m in diameter can be bored using large power-driven augers. In suitable conditions, augered shafts provide a fast and economical expedient for inspection, sampling and testing in situ.
Temporary support of the pit or shaft sides through trench boxes, timber supports and whalings are deemed to be "temporary works" and are used in unstable ground to provide the necessary protection for personnel. When making inspections, however, it is necessary to expose as much of the ground as possible, and considerable judgement and experience is required, since the excessive use of liners or trench sheets can lead to delay and to additional danger as a result of the build-up of water pressure behind them.
For many types of ground, excavations below the water table present serious problems both in maintaining a dry investigation and in stabilizing the sides. In such circumstances, the water table could be the maximum depth for which this method is feasible.
As personnel are required to enter observation pits, the sides should be made safe, particularly from sudden collapse. A risk assessment should be carried out prior to commencement of the works to identify the possible modes of collapse and any mitigation measures that can be implemented to make a safe working space.
NOTE 1 These measures are often carried out to provide support to the pit sides, which can readily be provided by purpose-made metal frames that can be quickly inserted and extracted. Alternatively, it might be possible to excavate the sides to a safe profile by means of a series of benches (see BRE Digest 381 ).
NOTE 2 Observation pits can vary from shallow unsupported pits to deep supported pits formed to enable detailed logging of the strata, structural inspections, in-situ testing or sampling. Both shallow and deep pits can be formed using either hand excavation, machine excavation or a combination of the two.
Unsupported large area pits up to about 1 m depth may be entered, provided that the risk assessment has concluded that the soil or rock being excavated is likely to remain stable. Alternatively, pit sides may be benched or battered back to produce a stable face. Such benches or battered sides should be risk assessed by a competent person prior to person entry. However, no depth is completely "safe" and every pit should be assessed in its own right with regard to likely stability, even within the depth range ground level to 1 m.
Working in any excavation is dangerous and the appropriate safety precautions should be followed (see the guidance in BS 8008). Attention should be given to the possible occurrence of injurious or combustible gases or of oxygen deficiencies, and the correct inspections and precautions should be established. Oxygen-consuming engines that emit toxic exhaust fumes, such as petrol-driven pump motors, should not be employed in pits or shafts.
Pits should be backfilled as soon as possible after logging, sampling and testing have been completed, since open pits can be a hazard to the general public (see 24.13). There might be advantages in leaving pits open (at least overnight and possibly longer), as this can allow the excavated surface to partially dry, exposing fissures and fabric better than immediately after excavation; however, any pits to be left open and unattended should be securely fenced off and have hazard signage attached.