Enhancing Pipeline Project Management With Refined Rock Excavation Forecasting

Accurate rock-excavation forecasting is one of the geotechnical risk factors that challenge successful management of cross-country pipeline projects. Pipeline construction personnel with local experience typically estimate rock excavation requirements for economic feasibility, permitting, and contracts. Where the excavation is paid on a “classified” basis, construction bid and contract documents typically call for excavation of “ditch” rock to be paid per lineal foot, whereas “area” or right-of-way (ROW) grading rock is paid per cubic yard. This paper briefly reviews the desktop procedure for estimating rock excavation quantities presented at IPC2012 and describes refinements to the procedure that expand its utility for pipeline project managers and planners. Input for the desktop study consists of digital GIS files of topography, geology, soil survey, pipeline alignment, and construction ROW layout and width. Publically available topographic data commonly has a horizontal resolution of 10-m; therefore, the pipeline centerline is subdivided into 10-m-long segments, the endpoints of which are used to summarize the data and perform calculations. Profiles of elevation, maximum ground slope, apparent ground slope across the ROW perpendicular to segment alignment (sidehill slope), and the relative sidehill slope direction are plotted for visual reference. A virtual geologic field reconnaissance along the alignment is performed using Google Earth Pro to supplement digital geology and soil survey data. Bedrock type is interpreted for general ease of excavation (granite versus shale) and soil survey map units are used to identify shallow cemented zones or bedrock that form the basis for an overall rock excavation index factor, which is expressed in terms of estimated mean and standard deviation of depth to rock and rock-like material. Rock factors vary based on the range of pipe size and ground conditions for a particular pipeline project or segment. Rock Factor 0 on a recent project corresponded to a mean-minus-one standard deviation (−1σ) depth to rock that was below pipeline depth, whereas Rock Factor 3 corresponded to a −1σ rock depth that was above pipeline depth. Refined rock excavation calculations consider equipment parameters (boom reach, track offset), trench configuration parameters (working pad width, bench offset, two-tone geometry), centerline distance to adjacent pipelines, and direction of lay, as well as pipe diameter and minimum cover depth. Desktop rock excavation results can be further refined by field examination and seismic refraction surveys to check depth to blast rock in trench excavations which is interpreted to be seismic velocities >1,220 to 1,370 m/s. Construction records of actual blasting details are needed to further improve the rock excavation model.