Rock Mass Properties and Their Potential Impacts on Trenchless Projects

By David J. Scarpato, P.E., Haley & Aldrich Inc. | August 2012, Vol. 67 No. 8
HDD pilot bore road crossing exit pit in moderately weathered tuffaceous siltstone in Utah.

A pre-construction understanding of the basic rock mass properties will help to significantly minimize adverse affects from structural geologic and overall rock mass controls. Many of the potential geotechnical challenges presented herein can be foreseen with an adequate field investigation program, including rock mass characterization, targeted use of instrumentation, and a laboratory testing program, completed by experienced geotechnical and rock mechanics practitioners. An appreciation and basic understanding of the prevailing discontinuity and rock mass properties can help the geotechnical and trenchless engineering design team better evaluate:

  • Required drilling trajectory geometry like optimal bore diameter, elevation, entry/exit angles, horizontal and vertical curves, and overall pipeline orientation during feasibility or scoping level evaluations;
  • Desired drilling fluid properties like the need for additives, viscosity control and need for any ground pre-treatment (e.g. grouting);
  • The possibility of encountering mixed-face ground conditions;
  • Drilling bit selection and characteristics of any back/over-reaming during pull-back, if required;
  • Cutter-head selection with respect to abrasion resistance and desired production rate;
  • The most appropriate trenchless method of product installation;
  • Ideal entry/exit pit and receiving shaft locations with respect to ground conditions;
  • Optimal direction of drive for tunneling-type operations, with respect to prevailing discontinuity orientations;
  • Additional product pipe abrasion and puncture resistance demands;
  • Ideal opening size with respect to discontinuity geometry and frequency;
  • The need for tunnel support (e.g. lining, shielding); and
  • MTBM lubrication demands;

Drill and blast tunnelling under a dam in Pennsylvania. Note the geologic structures on the rock sidewall.

ABOUT THE AUTHOR: David Scarpato, P.E., is a senior rock mechanics engineer for Haley & Aldrich in Bedford, NH. He can be contacted at, (603) 361-0397.

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