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3.7.2 Safe digging practice

Contents
3.7.2.1 Avoidance of underground services
3.7.2.2 Avoidance of buried (dangerous) materials

Safe digging on a nuclear-licensed site has three main aspects associated with it:

  • Avoidance of underground services.
  • Avoidance of buried (dangerous) materials, e.g., munitions.
  • Radiological monitoring to protect workers and minimise the spread of contamination.

The avoidance of underground services and materials are discussed below. Radiological monitoring issues during intrusive investigations are discussed in Section 3.7.2.1. In addition to these aspects, hazards appropriate to working on a conventionally contaminated site must also be considered (for example, civil engineering risks and protection against chemical contamination).

3.7.2.1 Avoidance of underground services

Safe digging practices should be used during the intrusive investigation, as underground services typically present the greatest hazard during the intrusive phase of a site investigation. Because of this, the general process for determining if it is safe to excavate is repeated below:

  • Collect and review service plans of the area in which the works are to be undertaken (either from the site owners/occupiers or from appropriate utility companies).
  • Identify the positions of all services using non-intrusive techniques (geophysical surveys, a cable avoidance tool (CAT) and signal generator and tracing of services between visible features such as manhole covers).
  • If a planned excavation is close to the location of services, consider relocating it (provided the location is not critical to the site investigation).
  • If excavating close to the position of a suspected service dig carefully by hand.
  • Excavate carefully and stop should anything unusual be discovered.

It should be noted that:

  • Service plans may be inaccurate.
  • Not all services may be shown on the service plans.

Nuclear-licensed sites will generally have site procedures for excavations, which must be followed. A typical procedure for undertaking excavations at a nuclear-licensed site is given in Table 3.44. The quality of service plans for land outside the main security fence of a nuclear-licensed site may be poorer than those for services within the site. If excavating in public access areas owned by a nuclear-licensee, it is recommended that the main utilities providers for the region are contacted. This is to ensure that their service location plans can be checked for agreement with the site plans.

3.7.2.2 Avoidance of buried (dangerous) materials

Buried (dangerous or hazardous) materials may be present on nuclear-licensed sites. If the desk study has indicated that these materials could be a potential hazard at a site, a procedure must be put into place to ensure that drilling into such objects does not occur. It is recommended that site-specific advice be sought from a specialist.

During site characterisation, the greatest hazard could arise from drilling into the soil and encountering (dangerous) materials especially at defense sites, e.g., munitions. Therefore it is advised to take into account in the historical site assessment to review a list of locations where possible (dangerous) materials can be buried or present in the sub-surface and at greater depths.

In some circumstance, the obstruction to drilling may not be identified and drilling may continue on the assumption that a piece of concrete has been encountered. The hazard can be decreased by trial pitting on such sites, e.g., munitions could be rapidly identified and works stopped.

Step Description
1 Production of a plan showing the areas of proposed excavations.
2 Production of service plans of the areas.
3 Selection of proposed excavation positions by the contractor, taking into account the service plans. Agreement of this plan with the stakeholders.
4 Cable avoidance tool survey of the proposed excavation positions. If the proposed excavation positions are free of services, excavation positions are marked out using spray paint (i.e., avoid penetrating the ground at this stage). If services are found to be present, alternative positions are agreed with the stakeholders.
5 Confirmation by the stakeholders that the excavation positions marked on the ground correspond with the proposed positions, and that the cable avoidance tool survey has been completed.
6 Production of an excavation permit by the site owner. The excavation permit would typically include a second set of service drawings and approvals from all interested parties (health physicists, appropriate building managers, etc.) for the excavations to proceed.
7 Issue and signing off of excavation permit by the site owner.
8 Issue and signing off of permit to work by the site owner’s project manager.

Table 3.44 A typical procedure for undertaking excavations at a nuclear licensed site
Notes:

  • In addition to the procedures listed above, a cable avoidance tool should be on site and used regularly during the excavations by a suitable qualified and experienced person.
  • Should any excavation need to be relocated, this entire procedure would need to be repeated for the new location. However, the permits would only require modification rather than re-issue.
  • Approvals are required from interested parties such as health physicists so that, if necessary, special instructions can be given on issues such as radiological hazards and monitoring requirements.

A procedure for investigating a site containing possible buried (dangerous or hazardous) materials is given below:

  • Undertake a historical site assessment (see Section 2.4) of the area to evaluate the potential for (dangerous or hazardous) materials to be present. If the historical site assessment indicates a high potential for (dangerous or hazardous) materials to be present it is advisable to consult a specialist on the expected materials. The results of the historical site assessment would be unlikely to change the overall characterisation approach. However, if there is a high risk that (dangerous or hazardous) materials may be present, greater care should be taken during the excavation process.
  • Undertake a geophysical survey across the site to identify the positions of buried metallic (ferrous) objects. Appropriate geophysical techniques for detecting buried metallic objects are described in Section 3.6.2. However, advice from a specialist geophysical contractor should be sought in order that the most appropriate geophysical technique for the site is employed. The geophysical survey should produce a map showing the locations of buried metallic and other objects.
  • The results of the geophysical survey can be used either to plan the site characterisation so as to avoid all areas with buried metallic objects, or to ensure that, if excavation must be undertaken in the vicinity of buried metallic objects, the appropriate level of caution is exercised. In the majority of cases, buried metallic objects will not be munitions.
  • Excavation to identify buried metallic object should be undertaken with care. Borehole drilling methods are not appropriate. An appropriate method would be to use an excavator to carefully remove approximately 20 cm thick layers of soil to expose the metallic object(s). A banksman should be present.
  • To observe the excavation and determine if the object has been located. This method of approach should allow (dangerous) materials to be identified at an early stage, prior to them being significantly disturbed or punctured. If the (dangerous) materials discovered, are munitions or objects that may be munitions, the site police must be informed. The site police will then involve the appropriate civilian and military authorities. It should be noted that the civilian authorities will make the occurrence public, and media interest may result. The licensee should inform national regulatory agencies according to the national procedures.