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.

Rock mass properties will almost always have a profound influence on the excavatability of any feature to be constructed in bedrock.

In-situ earth material properties can affect the economic viability, construction schedule and overall feasibility of virtually any project, including excavations for civil, mining, petroleum and energy projects. Rock mass properties and even more importantly, their variability, must be reliably understood before undertaking efforts in project design and preparation of contract documents.

In this regard, the geotechnical challenges associated with the relatively new field of trenchless technology are not unique; rock mass properties will impact the constructability of virtually all trenchless projects that are constructed within bedrock. Similarly to drilling petroleum or water wells, blast holes or deep exploration holes, horizontal directional drilling (HDD) projects will be impacted by drill hole wander, due in-part to rock mass properties. Likewise, utility tunneling, auger boring and microtunneling projects will be affected by rock mass properties, for example, by influencing cutting efficiency at the advancing face and by differences between the design and actual jacking (thrust) force requirements. An initial understanding of rock mass properties, and their potential impacts on project design and construction, can help to significantly reduce the chances for differing site conditions claims and change orders, and can reduce the likelihood of painful schedule delays.