Intrusive Investigation Techniques

An Intrusive Investigation may comprise the following aspects:-

  • Excavation of exploratory holes using a range of techniques.
  • Logging of profiles in accordance with BS5930:1999+A2:2010.
  • Recovery and analysis of soil, surface water, groundwater and/or ground gas samples for testing against a site specific range of potential contaminants identified within the CSM.
  • In-situ gas and groundwater monitoring if required.

TGEN will select the intrusive investigation technique(s) most appropriate to the site, anticipated conditions (geology etc.) and the requirements for sampling (depth, volume, installations etc.). TGEN have experience of a wide range of investigation techniques, including the following:-

Trial Pits and Trenches

These can be progressed by hand digging (to 1.2 mbgl) or using wheeled/tracked excavators. Where a cross section is required trial pits are extended laterally to create trenches. In some cases trial pits are a pre-cursor to other mechanical techniques, the benefits of which are as follows:-

  • Provide a good visual representation of ground conditions.
  • Easy to obtain discrete samples (where entry is appropriate) and bulk samples.
  • Rapid and inexpensive.
  • Applicable to a wide range of ground conditions.
  • Can be used for integrated contamination and geotechnical investigations.

Spike Holes

A narrow diameter hollow rod is driven to form a hole. Disposable tubing runs down the centre of the rod which connects to monitoring/sampling equipment, the benefits of which are as follows:-

  • Inexpensive method of testing for the presence of soil vapours.
  • Quick method of delineation and monitoring of near surface soil vapour concentrations.
  • Easy to take samples.
  • Allows assessment of immediate hazards.

Hand Augering

A range of augers for different soil types, conditions and sampling requirements, the benefits of which are as follows:-

  • Highly portable and useful for progressing shallow exploratory holes across sites with poor access.
  • Allows examination of soil profile and collection of samples at pre-set depths.

Power Driven Auger Boreholes

This typically involves rotary drilling using a solid stem auger, the benefits of which are as follows:-

  • Greater depths can be reached than hand augers.
  • More rapid than hand augering for shallow investigations and less susceptible to refusal due to compact soils or obstructions.
  • Can be used to install shallow gas monitoring wells if hole remains open after withdrawal of auger.

Hollow Stem Auger Boreholes

Uses a continuous flight auger with hollow central shaft. Withdrawing centre bit and plug allows access down the stem for sampling and/or installing monitoring standpipes. The benefits of this technique are as follows:-

  • Forms a fully cased hole avoiding potential problems of cross-contamination arising with percussion techniques.
  • Soil samples can be taken at reqular intervals through the hollow stem allowing accurate estimation of depth.
  • Typically sample recovery is possible for very coarse samples (e.g. river terrace gravels) compared to percussion techniques.

Dynamic Sampling Using Window or Windowless Sampling Tubes

1m long cylindrical steel tubes (with internal plastic sleeves) are driven into the ground by a percussive hammer mounted on small wheeled or tracked rigs. Some rigs are also equipped with rotary drilling capabilities. For sites with restricted access hand-held window sampling equipment may be used. The benefits of these techniques are as follows:-

  • Allows the collection of continuous undisturbed samples.
  • Can be used for installation of water and ground gas monitoring wells.
  • Compact rigs can be used inside buildings or where space is limited.
  • Can be used for shallow sampling and to depths of up to 10mbgl.
  • Requires less time than some other techniques such as cable percussion.
  • Does not require flush to be used, minimizing the risk of cross-contamination and waste generated.
  • Effective at retaining volatiles, especially in cohesive soils where a plastic liner is used and because a relatively undisturbed sample can be cut from the extruded core.

Cable Percussion Boreholes

Often referred to as shell and auger boreholes, these consist of a tripod derrick with a winch driven by a diesel engine. These type of drilling rigs are capable of drilling to depths of +50m. This technique involves driving lengths of steel casing into the ground to stabilize the sides of the borehole as soil is removed using shell and clay cutter tools. Boreholes are usually advanced using 150mm or 200mm casing, although hole sizes can be increased to 300mm diameter where required.

  • Allows greater sampling depth than trial pits or hand augers.
  • Enables installation of permanent sampling/monitoring wells.
  • Can penetrate most soil types.
  • Less potential for adverse effects on above-ground environment than trial pits.
  • Allows the collection of undistrubed samples.
  • Allows integrated sampling for contamination, geotechnical and gas/water sampling and the installation of groundwater and ground gas monitoring pipes.

Sonic/Rota-sonic Drilling

Involves the use of high frequency energy which shears and displaces the soil particles.
Two types of rig are typically used: sonic and rota-sonic with rota-sonic having the ability to combine rotary and sonic drilling capabilities, the benefits of which are as follows:-

  • Near 100% core recovery in the majority of ground conditions.
  • Faster drilling progress is possible where conditions permit compared with cable percussion boreholes.
  • Permits recovery of undisturbed samples.
  • The use of drilling flush is not always necessary.
  • Rota-sonic drilling can penetrate all soil types and also hard rock, concrete and other obstructions (where sonic drilling alone can be subject to refusal).

Cone Penetration

This technique consists of pushing an instrumented cone, with the tip facing down, into the ground at a controlled rate. These can be static, dynamic or environmental cones and can be used to determine a wide range of geotechnical soil properties.