Pipe-jacking stoppages modelled using interface shear testing
Date
2024-01-26Embargo Date
2025-01-24
Author
O’Dwyer, Kevin G.
Metadata
Show full item recordUsage
This item's downloads: 0 (view details)
Abstract
In recent years, there has been an increased resort to microtunnelling/pipe-jacking as a means
of constructing underground conduits (for water, sewage, gas, and other utilities) to avoid on street disruption in urban areas. The difficulty in identifying suitable intermediate shaft
locations in built-up areas means that long tunnel drives are often necessary; maintaining
jacking forces at manageable levels can be a challenge in these drives. The total jacking force
consists of the force at the face of the tunnel boring machine and the frictional force between
the pipe string and the surrounding soil, with the latter typically the main contributor to the
total jacking force. The introduction of a lubricant into the overcut (the annulus formed on
account of the TBM having a larger diameter than the concrete pipes) is well known to be an
effective means of reducing skin friction (the frictional force divided by the embedded surface
area of the pipe string). While studies have been undertaken to develop a better understanding
of the pipe-jacking processes and the factors affecting skin friction, significant gaps remain
which are worthy of further research. This study analyses both field and physical modelling
results with a primary focus on stoppages, but the impact of buoyancy, bentonite slurry content
of material adjacent to the pipeline and shearing rate on the interface shear strength were also
examined.
Drive data were obtained from 15 pipe-jacking sites in various ground conditions from Ireland,
the UK, and Canada. Retrospective analysis of drive data published in the literature has tended
to focus on isolated learnings from a single drive/site, whereas a broader view was possible in
this study. Once the drives were considered suitable (i.e. screened to check for standard jacking
rates, limited amount of excessive deviations from line, etc.), the following conclusions were
drawn.
A traditionally used method of separating face resistance and skin friction may be flawed.
There is a general tendency for skin friction to reduce with normalised lubrication ratio
(i.e., the lubrication volume ratio (injected bentonite volume normalised by overcut
volume) normalised by recommended lubrication volume ratios (Praetorius and Schößer
(2017)), but this is complicated by over administration of lubrication in fine soils and under
administration in coarse soils.
Pipe buoyancy was demonstrated by estimating the effective normal stress on the pipes as
a proportion of the vertical effective stress. Thus, the stability of the overcut could be
quantified.
Skin friction increases were noted for stoppages as short as 20 minutes and shown to depend
on stoppage duration.
Following the literature review and field data analysis, the primary gap identified was
quantification of the effect of stoppages on skin friction, which has the potential to increase
risk for pipe-jacking contractors, in addition to the underlying mechanism. The effect of
bentonite slurry content of the material at the interface (i.e. a proxy for overcut stability) and
the shearing rate were considered integral to the stoppages problem and therefore were
considered in tandem. Interface shear tests were conducted in both a standard direct shear
apparatus and a newly-constructed Interface Shear Facility. The latter facility was designed to
incorporate a concrete section cut from a jacking pipe and was capable of shearing at typical
field penetration rates.
The relationship between shear strength and bentonite slurry content was dependent on whether
or not an interface was present. In the absence of an interface, a transition zone was observed
where samples were sand-governed up to ≈30% bentonite slurry and clay-governed above
≈80% bentonite slurry. On the other hand, a linear reduction in interface shear strength with
bentonite slurry content was observed when the interface was present. The effect of shearing
rate on the interface shear strength for a range of sand-bentonite slurry mixtures has not
previously been investigated. A more pronounced shearing rate effect was observed for mixes
containing greater volumes of bentonite slurry. As shearing rates increased, samples transition
from a drained state to an undrained state, which significantly reduced the interface shear
strength.
To investigate the effect of stoppages on skin friction, a stoppage was incorporated once critical
state conditions were reached in these tests, and the increase in shear stress upon
recommencement of shearing was noted. These increases were found to be dependent on
shearing rate, interface surface roughness and bentonite slurry content at the interface, but 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 on aged
bentonite slurry samples, in addition to consolidation magnitudes during stoppages which are
small (relative to pre-shearing magnitudes) and erratic (with time) combine to suggest that
bentonite thixotropy is, at least in part, the mechanism behind the stoppage-induced skin
friction increases.