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To Assist With Project Planning, What Geotechnical Information Should be Obtained When Designing a Complex HDD

07.02.17

The importance of defining the crossing geology should never be underestimated. This will allow for the downhole tooling, drilling fluids and schedule to be accurately developed and costed. I would suggest that initially a site visit and geological review would be undertaken to provide a geological overview of the area, which should then be used to determine the site specific investigations. The objective of the site investigation for a trenchless crossing is to obtain sufficient reliable information to permit the safe and economic design of installation and permanent works. The investigation should be designed to verify and expand upon any information previously collected. Site investigations for all drills should include site inspection/surface investigation, topographic survey, identification of existing services and a geotechnical assessment. Where the client prepares the investigations, the HDD contractor should assess the completeness of the information provided and ensure it provides sufficient information for planning and execution of the bore. Where further information is required, the HDD contractor should provide a proposal to the client outlining the objectives, requirements, and budget costs for any additional works required.

When planning the geotechnical investigations scope the engineer should consider some basic guidelines:

  • The level of geotechnical investigation required is a function of the length of the bore and the anticipated complexity of subsurface conditions. While typical spacing is at least every 150–250m along the bore alignment, a minimum of two geotechnical boreholes is required for each bore where the bore length is greater
    than 300m.
  • Boreholes should be located to track stratigraphy and to detect the geological sequence, structure, and areas of significant change. When results indicate other anomalies or highly varying strata, then additional boreholes may be required.
  • The boreholes should penetrate through an elevation at least 3-5m below the depth of the proposed bore profile to provide information for HDD design modifications and pilot hole deviations.
  • Boreholes should be offset perpendicularly from the HDD centreline where practical by 10m.
  • Investigations should describe the soils and rocks encountered and recover samples for laboratory testing. Where soils are encountered, in-situ standard
    penetrometer testing (SPT) sampling should be undertaken at selected depth intervals within the borehole.
  • Where frac-out modelling is required, the geotechnical parameters required for undertaking the modelling (e.g. unit weight, shear strength, friction angle, cohesion and Youngs Modulus) should be determined during the geotechnical investigation.
  • The likelihood of soil/groundwater contamination should be determined prior to undertaking any investigations. If contamination is suspected (i.e. near electrical transformers, fuel storage, petrol stations, industrial land), samples should be tested for likely contaminants in accordance with the relevant guidelines for contaminated sites.
  • Boreholes should be backfilled to minimise the possibility of drilling fluid migration along the borehole during subsequent HDD operations. The upper 1.5m of land-based boreholes should be backfilled with the surrounding soil. Below 1.5m, a backfill mixture containing cement grout and a bentonite product to promote expansion is recommended. Cuttings from the drilling operation may be incorporated into the backfill mixture if considered beneficial.
  • A geotechnical report addressing the sampling program, laboratory analysis (including strength testing and  particle size distribution), interpretation of geotechnical engineering properties, bore logs and a profile of the subsurface conditions shall be produced. Reduced levels of borehole data shall be included on the HDD profile drawings.

Probably 90 per cent of the work is happening below ground, so thorough and well thought out site specific investigations are the most basic and essential requirement for planning any trenchless construction project. All formations can be drilled reliably if the soil conditions have been properly defined and considered
during the design phase.

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Charles Stockton
Managing Director/HDD Specialist

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+61 (0) 400 623 441
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charles@stocktondrillingservices.com
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158B Gaudrons Road Sapphire Beach NSW, 2450, Australia

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