Enhancing Pipeline Project Management With Improved Rock Excavation Forecasting Available to Purchase

Accurate rock-excavation forecasting is one of the geotechnical risk factors that challenge successful management of cross-country pipeline projects. Rock excavation requirements commonly are estimated by pipeline construction personnel with local experience. Construction bid and contract documents typically call for excavation of “ditch” rock to be paid per lineal foot, whereas “area” or right-of-way grading (ROWG) rock is paid per cubic yard. Rock excavation forecasting tends to be used for economic feasibility more than for selecting contractors or preparing contracts. Recent pipeline projects in Pennsylvania and New Jersey used geotechnical estimates of rock excavation to update detailed cost and schedule projections after construction was underway because ROWG rock excavation exceeded the expected volume. Geotechnical estimates of rock excavation were based on a rapid desktop study followed by field observations and measurements. The desktop study used available digital data manipulated with geographic information management software (GIS). Topographic data (digital elevation models) at 10-m resolution and the pipeline centerline in 10-m-long segments were used to plot alignment elevation profile and ground slope, as well as to calculate slope aspect and apparent ground slope across the ROW perpendicular to centerline. The centerline was plotted in Google Earth Pro for a virtual geologic field reconnaissance to identify areas where rock was likely or unlikely to be encountered within ditch depth. Digital geology was used to assess bedrock type along the alignment and digital soil survey data were classified to identify soil units with shallow cemented zones or bedrock. These complementary data types were combined into an overall rock excavation index factor (0 = uncemented soil; 1 = cemented soil; 2 = weathered rock near ditch bottom; or 3 = nearby rock outcrop, weathered rock near top of ditch, or unweathered rock at any ditch depth). GIS polygons of “rock factors” were converted to a grid so that values could be extracted at points along the pipeline centerline. Ground-condition variability was considered subjectively for each rock factor by assigning length-based and area-based percentages where rock was considered likely to be encountered for both “average” and “maximum” rock conditions. Rock factor areas were used to select locations for 115 or 230-ft-(35 or 70-m-) long seismic refraction surveys. Seismic velocities > 4,000 to 4,500 ft/s (1,220 to 1,370 m/s) were considered blast rock in trench excavations. Locations where the 4,000-ft/s contour was shallower than ditch depth were used to refine subjective ground variability estimates. Additional construction records of actual blasting details are needed to further improve the rock excavation model. Unique aspects of geology may require model parameters to be modified for other settings.