Inadvertent Slurry Returns during Horizontal Directional Drilling: Understanding the Frequency and Causes

Abstract

Horizontal directional drilling (HDD) is an innovative and attractive construction method for installing pipelines and utilities within rights-of-way. Regulatory agencies often consider HDD as the least environmentally disruptive alternative because construction operations can be isolated from environmentally sensitive areas. However, bentonite slurries used in HDD are known to inadvertently migrate to the surface, which may introduce foreign material into wetlands or increase turbidity in surface waters. The Gas Research Institute (GTI) sponsored a survey of large HDD installations to identify the frequency and possible causes of inadvertent returns. Survey results would provide planners information to aid in critical decision making. Questionnaires were sent to HDD Contractors to assimilate information about crossing parameters and inadvertent return details. Ultimately, 54 individual HDD installations of various lengths and in an assortment of geotechnical conditions were studied. Inadvertent return events occurred on approximately half of the HDD installations. Inadvertent returns occurred most frequently within 200 ft. of the entry or exit point and where there was less than 40 ft. of overburden. No correlation was observed between lack of normal returns, slurry properties, or pumping rates and the inadvertent return events. Disturbed ground increased the likelihood of inadvertent returns. Inadvertent returns are a phenomenon which cannot be ignored. Construction rights-of-way should be obtained for remedial clean-up in the first and last 200 ft. of any HDD alignment. Most events studied produced manageable amounts of material that can be contained and disposed without interrupting the construction operations.