DETERMINING THE FEASIBILITY OF CROSSING THE SHENANDOAH RIVER (PAGE COUNTY, VA) WITH HDD IN KARST TERRAIN

In the Grove Hill area of southern Page County, Virginia the Columbia Gas Transmission LLC pipeline crossed the South Fork of the Shenandoah River by means of a subsurface trench. To replace the 70-year-old pipeline Columbia contracted GeoConcepts (now Terracon) to conduct a feasibility study for horizontal direction drilling (HDD) installation by means of subsurface exploration and geological reconnaissance. The assessment consisted of the drilling of two subsurface borings near the planned HDD entrance pit including field and laboratory analysis of the recovered soil samples and rock core returns, a geophysical survey using ERT, review of publicly available geological data, and surface reconnaissance of the local geology and geomorphology.

The existing pipeline was installed over 70 years prior to our work using coffer dam and trenching, and it was hoped that the new crossing could be undertaken using HDD methods. The borings and well logs suggested the original pipe had been installed in an area of deep sediments flanked by rock outcrops both upstream and downstream of the alignment. Geological mapping of the site area revealed that the bedding was overturned in the Conococheague Formation due to the proximity of the Stanley Fault, a high angle reverse (thrust) fault to the west of the project area. This changed our understanding of the site’s geomorphology and suggested that an inferred karst feature lineament parallel to the area of deep sediments did not extend from southwest to northeast but may have developed parallel to the strike of the Conococheague Formation. It was proposed to move the HDD alignment to an area of known bedrock to the west, but observations of bedrock exposed by a railroad cut revealed numerous solution-enlarged fractures and open joints, any of which could cause loss of HDD drilling fluid circulation. Upon review of the assessment by Columbia Gas HDD subject matter experts and engineers it was concluded that HDD was not going to be feasible at this location. Therefore, the crossing was made at the location of the original pipeline using coffer dam and trenching methods.