Skin friction increases associated with pipe-jacking stoppages modelled using direct shear interface tests

In long pipe-jacking drives used for installing utility pipelines, field experience has shown that transient peaks in skin friction arise upon recommencement of jacking after stoppages; these forces are often very large and difficult to predict, presenting a significant risk for contractors. In this paper, the problem is replicated in the laboratory using direct shear interface tests with a concrete specimen in one half of the apparatus and various sand/bentonite slurry mixtures in the other. Once critical state conditions were reached in these tests, stoppages of various durations (from 30 mins up to ≈2 weeks) were incorporated and the increase in shear stress upon recommencement of shearing was noted. The experimental results revealed that these increases are dependent on bentonite slurry content, and there appears to be a threshold stoppage duration beyond which the skin friction increase appears to plateau, suggestive of a time-limited process within the bentonite. Shearometer tests measuring the gel strength of aged bentonite slurry samples, in addition to small and erratic (in time) consolidation magnitudes during the direct shear test stoppages together suggest that bentonite thixotropy is a key contributor to the stoppage-induced skin friction increases. A simple model capturing this behaviour provided a safe upper bound to the stoppage-induced skin friction increases extracted from a selection of field data, offering much better jacking force predictions than standard prediction models used in industry.