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.
  • Ground failures - In the case of larger diameter underground excavations, structurally-controlled rock block stability could result in crown (roof) failures, sidewall failures or even block instability at the advancing face. This becomes even more of a potential issue when shielding at a minimum, particularly in cases of larger-diameter machine-aided and drill and blast tunnels;
  • Under-designed jacking force – In cases where a thrust force is required for advancement of the trenchless feature, unstable ground conditions like roof falls or cave-ins may bear-down on the microtunneling frame and result in underestimation of required jacking force(s); and
  • Failure of jacking pit, access/jacking shafts or reaction block bearing – Structurally-controlled failures in bedrock may also impact ancillary open-cut excavations, vertical jacking shafts or dedicated reactionary blocks. In addition to the effects from a temporary loss of excavation support and potential influx of water, reaction loads from the jacking operation may need to be considered for further evaluation depending on the orientation and spacing of the prevailing discontinuities.

Geotechnical challenges

For trenchless methods that rely on drilling mechanics as the primary means of advancement, the following geotechnical challenges must be anticipated in rock masses with structurally-controlled behavior: