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.

Soils vs. rock analysis
In most cases, geotechnical analyses applied to soils is not appropriate for analysis of rock, as the latter is frequently (but not always) analyzed using a discontiuum approach depending on the scale of the project with respect to prevailing rock mass properties. Soils engineering frequently adopts a continuum approach to analyses, based on an assumption of uniformity with in-situ material properties. Scale-dependencies may impart some level of control on whether a continuum or discontinuum approach is more appropriate. For example, in cases where an excavation diameter is less than anticipated rock block size as defined by discontinuity (e.g. joints, fractures) intersections, a continuum approach may be more appropriate for the specific situation. For projects locations consisting of jointed bedrock with relatively high intact rock strength, often the most important factor that controls behavior are the geologic structures within the rock mass. General rock mass type failures are also possible in bedrock conditions comprised of numerous joint sets and/or weathered rock, whereby failure occurs not only along discontinuities but also through intact rock that is in a weakened state due to the weathering process.

The phrase “intact rock properties” refer to a continuum of rock where flaws or discontinuities do not significantly impact the mechanical properties of the rock. In this regard, intact rock properties, if utilized as the sole basis for geotechnical design, would be operating under assumptions based on continuum approach. The phrase “rock mass properties” refers to the intact rock properties in addition to all of its flaws (e.g. joints), and is generally discontinuous in character. Intact rock properties can be markedly different from rock mass properties; using intact rock properties as the sole basis for design may result in a non-conservative assessment (i.e., over-statement) of in-situ material strengths. Actual rock mass strength parameters may be significantly lower than those derived primarily from intact material properties, and trenchless designers are advised to be clear on this distinction.

In the case of tunneling-type operations, trenchless projects could encounter the following geotechnical challenges associated with adverse rock mass conditions: