Pipe-jacking stoppages modelled using direct shear interface tests

Abstract

In long pipe-jacking drives used for installing utility pipelines, maximum jacking load requirements are usually governed by skin friction at the pipe-soil interface. In addition, field experience has shown that transient peaks in skin friction arise upon recommencement of jacking after stoppages; these stoppage durations can be short (due to the addition of a pipe to the string) or long (due to weekend stoppages or breakdowns) and constitute a risk for pipe-jacking contractors. In this paper, the problem is replicated in the laboratory using direct shear interface tests using a concrete specimen in one half of the apparatus and sand/bentonite 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. From the experiments, 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. These skin friction data are shown to provide an upper bound to corresponding stoppage data from pipe-jacking drives in sandy ground conditions.

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ICPMG 2022
10th International Conference on Physical Modelling in Geotechnics
19(Mon) – 23(Fri) September 2022 / KAIST, Daejeon, Korea
Pipe-jacking stoppages modelled using direct shear interface tests
1Bryan A. McCabe, 1Kevin G. O’Dwyer, 2Brian B. Sheil, 3Padraig F. Burke
1Civil Engineering, School of Engineering, National University of Ireland, Galway, Ireland.
bryan.mccabe@nuigalway.ie, k.odwyer2@nuigalway.ie
2Department of Engineering Science, University of Oxford, United Kingdom. brian.sheil@eng.ox.ac.uk
3Ward and Burke Construction Limited, Kilcolgan, Co. Galway, Ireland. padraig.burke@wardandburke.com
ABSTRACT
In long pipe-jacking drives used for installing utility pipelines, maximum jacking load requirements are usually
governed by skin friction at the pipe-soil interface. In addition, field experience has shown that transient peaks in
skin friction arise upon recommencement of jacking after stoppages; these stoppage durations can be short (due to
the addition of a pipe to the string) or long (due to weekend stoppages or breakdowns) and constitute a risk for
pipe-jacking contractors. In this paper, the problem is replicated in the laboratory using direct shear interface tests
using a concrete specimen in one half of the apparatus and sand/bentonite 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. From the experiments,
there appears to be a threshold stoppage duration beyond which the skin friction appears to plateau, suggestive of a
time-limited process within the bentonite. These skin friction data are shown to provide an upper bound to
corresponding stoppage data from pipe-jacking drives in sandy ground conditions.
Keywords: bentonite, interface, pipe-jacking, skin friction, stoppages
1 INTRODUCTION
Pipe-jacking is now the preferred means of
installing utility pipelines in urban areas, with some
substantial jacked lengths achieved in recent times. For
example, Ward & Burke Construction Ltd. have
constructed approximately forty drives in excess of 500
m in length since 2013 in Ireland, U.K., Finland, U.S.A.
and Canada. The total jacking force comprises the force
at the tunnel boring machine (TBM) face and the shear
force along the pipe-soil interface, with the latter
usually dominant in longer drives, dictating the total
force requirement (O’Dwyer et al., 2018). Furthermore,
field experience has shown that transient peaks in the
skin friction (i.e. the shear force divided by the
embedded pipe surface area) arise upon restarting
jacking after a stoppage; these stoppages can be short
(due to the addition of a pipe to the string, for example)
or long (due to weekend work breaks or breakdowns).
The selection of field data from the literature in Table 1
illustrates that longer stoppages give rise to greater skin
friction increases upon restart in any one drive,
representing increased risk for the pipe-jacking
contractor. However, there is no consistency in the
magnitudes of these increases across the variety of
ground conditions represented in Table 1, although it is
acknowledged that the type and extent of lubrication
will also have an impact on these values. This hampers
accurate prediction of jacking force requirements.
Table 1. Skin friction values reported in the literature for
stoppages of various durations.
Ground
conditions
Stoppage
duration
(h)
Skin friction
increase
(kPa)
Reference
Marl, sandy
gravel marl
< 3h 0.8 Pellet-Beaucour
& Kastner
(2002)
> 12h 2.2
Sl. clayey
fine sand
< 3h 0.7
> 12h 1.1
Clay < 1.5h 0.26
Curran &
McCabe (2011)
> 12h 0.30
Gravelly
clay
< 1.5h 0.8
> 12h 0.84
Sand/gravel < 1h 0.52
Cheng et al.
(2017)
> 12h 0.99
Clayey sand < 1h 0.44
> 12h 1.0
Sand/silt
< 2 0.05
O'Dwyer et al.
(2020)
2–5 0.12
12–20 0.16
> 20 0.31

ICPMG 2022
10th International Conference on Physical Modelling in Geotechnics
19(Mon) – 23(Fri) September 2022 / KAIST, Daejeon, Korea
An exclusive focus on the retrospective analysis of
pipe-jacking drives as a means of improving our
understanding of the relationship between skin friction
and stoppage duration may have limitations, for reasons
summarised by Sheil et al. (2020). The calibration of
load cells used in conjunction with jacks in the launch
shaft and the steering cylinders (and at intermediate
jacking points, where present) is not routinely checked
on working sites. Moreover, where steering cylinder
forces (located just behind the TBM face, providing an
indication of face force) are not measured, and
therefore skin friction and face resistances cannot be
separated directly, the widely cited Pellet-Beaucour &
Kastner (2002) empirical method for separating these
components can be used but is somewhat subjective.
Greater certainty in data quality can be achieved
through laboratory testing. Interface shear testing has
been used to measure the skin friction at typical
pipe-jacking interfaces (e.g. Iscimen, 2004; Staheli,
2006; McGillivray, 2009; Shou et al., 2010). However,
none of those studies has considered the impact of
stoppages on interface behaviour. In this paper, direct
shear interface tests with a concrete specimen in one
half of the apparatus and either sand, a 1:1
sand/bentonite mix or bentonite in the other are
reported. These tests cover the spectrum of overcut
stability scenarios arising in practice; the sand-concrete
interface is deemed to represent a collapsed overcut
which is unlubricated, the sand/bentonite mix models a
collapsed overcut with lubrication, while the
bentonite-only case models an open lubricated overcut.
Upon reaching critical state conditions, stoppages of
various durations were imposed and the net increase in
shear stress upon recommencement of shearing was
noted. The results offer some new insights into the
process underpinning the skin friction increases.
2 MATERIAL CHARACTERIZATION
2.1 Leighton Buzzard sand
A medium to coarse ‘2EW’ Leighton Buzzard sand
was selected for the direct shear interface tests. The
sand is sub-angular to rounded. It has a mean particle
size (D50) of 0.51 mm, and according to ISO 14688-2
(2017), the combination of its uniformity coefficient
(Cu) of 1.81 and its coefficient of curvature (Cc) of 0.97
suggest that it is uniformly graded.
2.2 Lubricant
Baroid Tunnel-Gel Plus, a bentonite slurry widely
used in the pipe-jacking and microtunnelling industries,
was chosen as the lubricant. A 5% slurry solution,
corresponding to concentrations used in Ward & Burke
Construction pipe-jacking drives, with pH measured as
9.3, was mixed at 500 rpm for 20 mins using a Stuart
SS20 high speed stirrer; the speed was subsequently
reduced to 100 rpm and was left to hydrate for 4 hours,
following recommendations by Praetorius & Schoesser
(2017). The Marsh funnel times fell in the range 10-20
mins, more consistent with Marsh funnel times on site
(e.g. 38 mins quoted by Cheng et al. (2017) in clayey
sand) than reported in laboratory studies (e.g. 43 s in
Reilly (2014); 130 s in Shiu & Liu (2010)).
Sand/bentonite combinations considered in this
study were (i) sand only; (ii) 1:1 sand:bentonite and (iii)
bentonite only. Both the bentonite (given its thixotropic
nature) and sand were stored at approximately constant
temperature and humidity (20oC and 50% respectively)
until time of testing in a laboratory with ambient
temperature of approximately 19oC.
2.3 Concrete
A concrete coupon was manufactured using a 30
MPa mix comprising 17 % CEM II cement, 14.6 %
water, 32.8 % fine aggregate (≤ 2.36 mm) and 35.6 %
coarse aggregate (≤ 8 mm). The coupon had a 59 mm
square plan area and a thickness of 19 mm to ensure a
snug fit within the bottom half of the direct shear
chamber. The concrete surface roughness was measured
using a Bruker's NPFLEX 3D optical profilometer in
vertical scanning interferometry mode. This device has
a vertical resolution of 3 nm and can measure up to 10
mm in height. Average surface roughnesses (Ra) of 10.3,
4.4 and 6.1 m were determined for zones of 0.439 mm
× 0.330 mm located at the centre and near two corners
of the coupon. These values were a close match for Ra
values for a section of a typical concrete jacking pipe
(supplied by an Irish precast concrete manufacturer)
using the same optical technique. The concrete coupon
and an example surface profile are displayed in Fig. 1.
Fig. 1. Concrete coupon used for interface tests and topo-
graphical image of 0.439 mm × 0.330 mm zone.
3 DIRECT SHEAR INTERFACE TESTING
3.1 General
A 60 mm × 60 mm × 40 mm Wykeham Farrance
apparatus was used to carry out the direct shear tests
according to BS 1377-7 (1990). The shear force was
measured using an AEP Transducers Type TS 0.5t load
cell; horizontal and vertical displacements were
measured using PY-2-F-025-S02M-XL0465 and PY-2-
F-010-S02M-XL0465 transducers respectively, supplied
by GEFRAN. All measurements were recorded using a
Controls Group Geodatalog 8 data acquisition system at
a rate of 1 Hz. Normal stresses of 50, 150 and 250 kPa
were achieved by applying dead weights to the rigid

ICPMG 2022
10th International Conference on Physical Modelling in Geotechnics
19(Mon) – 23(Fri) September 2022 / KAIST, Daejeon, Korea
steel top cap. For specimens containing bentonite, the
consolidation process was monitored and was
considered complete when the top cap displacement
ceased. Sand and sand-concrete interface tests were
conducted five times each to confirm repeatability. All
tests incorporating bentonite were effectively replicates
up to the point at which the stoppages were imposed.
3.2 Sand-only direct shear tests
For the sand-only tests, dry samples of Leighton
Buzzard sand were placed in the apparatus following
the steps in Miura et al. (1997); the sand was poured
through a funnel with the tip in contact with the base
initially, and then was carefully raised to minimise drop
height. The surface was levelled before placing the
perforated plate on top. This process created a loose
sand sample with a density of 1.53 Mg/m3 (relative
density, Dr = 20±2 %). This relative density was chosen
for consistency with that of the sand in O’Dwyer et al.
(2020) pipe-jacking study. A shearing rate of 1 mm/min
was chosen, recommended for sands by Bolton (1991)
to ensure drained shearing.
3.3 Interface direct shear tests
To facilitate interface testing, the concrete coupon
was placed in the bottom half of the shear box, ensuring
that the top of the concrete was flush with the shear
plane. The top half was filled with either sand only, a
1:1 sand/bentonite mixture, or bentonite only. The 1:1
sand/bentonite mixture was achieved by mixing 50 g of
sand and 50 g of bentonite by hand in a container.
Subsequently, 70 g of the mixture was poured into the
shear box, resulting in a density of 1.62 Mg/m3. In the
bentonite-only tests, a syringe was used to place the
bentonite directly on the concrete coupon, with a
density of 1.1 Mg/m3 achieved. Modified top and base
plates were used to limit leakage in tests with bentonite.
The plates and all contact surfaces were lightly greased
to reduce friction and prevent bentonite leakage from
the pre-defined failure plane. Filter paper was also
placed between the perforated plate and the porous stone
to prevent bentonite egress while allowing excess water
to escape during the consolidation phase.
From direct shear tests on kaolin clay conducted at
0.05 mm/min (shown to be drained) and 1 mm/min,
Doan (2019) showed that peak shear stresses
normalised by normal effective stress (pk′n) were rate
dependent but critical state shear stress normalised by
normal effective stress (crit′n) were not. Assuming
comparability of kaolin and bentonite based on their
high clay contents, rate effects were also considered to
be negligible in the bentonite, enabling the faster 1
mm/min rate to be adopted for the interface tests
sheared to critical state in this study. Unless otherwise
stated, shear stresses crit were calculated as the mean of
all values between 10% shear strain and ≈18%, to
ensure critical state conditions had been achieved.
All tests incorporating stoppages were conducted at
a normal stress of 50 kPa, reflecting the modest soil
cover typical of microtunnel drives. In these tests, the
motor was stopped at 12% shear strain upon
verification that critical state conditions had been
achieved. One of eight different stoppage durations was
then imposed: 30 mins, 6, 24, 48, 75, 112, 150/(163 in
one case) and 333 hours, after which shearing resumed
to ≈18% shear strain. Any consolidation occurring
during the stoppage was also recorded.
Several methods for calculating the stoppage-
induced increase in shear stress were explored.
Ultimately the increase was determined as the
difference between the shear stresses immediately
before and the peak value immediately after the
stoppage, in keeping with how real pipe-jacking drives
are interpreted (e.g. O’Dwyer et al., 2020); see example
in Fig. 2 for the bentonite-only 75 hour stoppage case.
A combination of stress relaxation and load fluctuations
due to diurnal temperature variations arose during most
stoppages; the example in the inset to Fig. 2 displays
primarily the latter. Stress relaxation is a natural
phenomenon in both sands (e.g. Lade, 2009) and clays
(e.g. Oda and Mitachi, 1988) under constant strain.
Phillips et al. (2019) and Phillips (2022) have also
observed stress relaxation from instrumentation in
concrete pipes during pipe-jacking stoppages.
Therefore no correction for stress relaxation was
applied. A ‘Time Series Decomposition’ approach was
used to ‘detrend’ the shear stress values for temperature
effects using Python programming; the full details are
presented in O’Dwyer (2022).
Fig. 2. Determination of shear stress increase upon restart of
jacking after a 75 h stoppage; bentonite-only case
4 RESULTS
4.1 Sand and sand/concrete interface
The shear stress-strain relationship for dry sand is
shown in Fig. 3 for normal stresses of 50 kPa, 150 kPa
and 250 kPa, demonstrating excellent repeatability
(coefficients of variation (COV) for crit less than 3%).

ICPMG 2022
10th International Conference on Physical Modelling in Geotechnics
19(Mon) – 23(Fri) September 2022 / KAIST, Daejeon, Korea
The average critical state friction angle (′crit) from five
tests was 33.3ᵒ (Fig. 4), slightly above the 31ᵒ-32ᵒ
recommended by BS8002 (2015) based on its
angularity and grading. Values of 33ᵒ, 35.3ᵒ and 32ᵒ
have been reported by Houlsby & Hitchman (1988),
Lings & Dietz (2004) and White et al. (2008)
respectively for Leighton Buzzard sand, the spread of
values probably reflecting angularity and grading
differences also.
Fig. 3. Development of shear stress with shear strain for
sand-only tests.
Fig. 4. Friction angle for sand and interface friction angle for
sand-concrete.
Fig. 5. Development of shear stress with shear strain for
sand-concrete interface tests.
The shear stress-strain relationships for the sand-
concrete interface are shown in Fig. 5, also showing
strong repeatability (COV for crit less than 4.1%). The
relative differences between pk and crit are lower in the
interface tests, in keeping with slightly reduced dilation
evident from the vertical displacement measurements.
The critical state interface friction angle crit was
determined (from the average of five tests) to be 31.3o
(Fig. 4). The combination of the crit/′crit ratio of 0.94
and the average Ra/D50 ratio of 0.013 is broadly in
keeping with values for concrete interfaces presented
by Knappett & Craig (2012) based on two separate
studies; differences may be attributable to the
techniques used to determine roughness.
4.2 Tests incorporating stoppages
All interface tests were conducted at a normal stress
of 50 kPa. The specimens incorporating bentonite
exhibited contraction and the absence of a peak stress.
The mean and COV of crit′n values averaged between
10% and ≈12% strain (just before the stoppages) were
0.39 and 8.6% respectively for the 1:1 sand/bentonite
case, and 0.17 and 23.1% for the bentonite-only tests.
Vertical displacements arising from consolidation
prior to shearing were 2.6–4.0 mm for the 1:1 sand/
bentonite mix, and 2.9–5.2 mm for the bentonite-only
mix. The extent of consolidation that had occurred by
the end of each stoppage (ongoing in many cases) was
relatively minor compared to the final values prior to
shearing. On average, the ratio of consolidation
occurring during a stoppage to the initial value was
≈0.02 for the 1:1 sand/bentonite mixture and ≈0.1 for
the bentonite-only case. There was no apparent pattern
between the amount of consolidation during the
stoppage and the stoppage duration.
The relationships between the stoppage-induced
shear strength increase and the stoppage duration are
presented in Fig. 6. There are no time-related increases
for the dry sand case, as would be expected for drained
shearing. In general, the shear stress variation with
stoppage duration data for the bentonite-only case plots
slightly above that for the 1:1 sand/bentonite case.
However, both sets of data are consistent in showing an
increasing relationship between shear stress increase
and stoppage duration up to ≈ 3–3.5 kPa after 5–6 days,
after which the shear stress appears to plateau.
Fig. 6. Relationship between shear stress increase after stoppage
and stoppage duration: direct shear data.

ICPMG 2022
10th International Conference on Physical Modelling in Geotechnics
19(Mon) – 23(Fri) September 2022 / KAIST, Daejeon, Korea
5 DISCUSSION
It is acknowledged that the magnitudes of shear
stresses (crit) alluded to in Section 4.2 exceed the
values of skin friction measured in field drives, where
values below 1 kPa are routinely achieved with modern
automated lubrication systems (e.g. Curran & McCabe,
2011; Cheng et al., 2017; O’Dwyer et al., 2020). This is
because jacking pipes are designed to exploit buoyancy
from the presence of bentonite within the annular
overcut, formed as a result of the TBM having a
slightly larger diameter than the pipes. However, the
primary focus of this study is on the magnitude of skin
friction increases due to stoppages; their variation with
stoppage duration modelled here (Fig. 6) is believed to
be representative of practice.
The shorter duration stoppage data are reproduced
in Fig. 7, with the time axis plotted on a log scale due to
the apparent skin friction intercept in Fig. 6. Also
included are the skin friction increases quoted in Table
1 relevant to sandy soils (i.e. Pellet-Beaucour and
Kastner 2002, Cheng et al. 2017 and O’Dwyer et al.
2020) superimposed. The arrow annotations on some
field datapoints denote that the skin friction increases
relate to stoppage durations less than or equal to
(arrows pointing left), or greater than or equal to
(arrows pointing right) those plotted. Error bars are
used where a discrete time range is represented (with
the point plotted at mid-range). The interface shear data
shows good agreement with both the Pellet-Beaucour
and Kastner (2002) and Cheng et al. (2017) field data.
The fact that the O’Dwyer et al. (2020) plots lower
reflects a highly effective lubrication strategy for that
drive; those authors note that the volume of lubricant
injected amounted to over six times the annular volume
throughout the drive, which is over double the ratio
recommended for tunnelling in sands and silts by
Praetorius and Schoesser (2017).
Fig. 7. Relationship between shear stress increase after stoppage
and stoppage duration: direct shear and field data.
Various researchers have offered explanations for
the increases in skin friction arising during a stoppage:
(i) Chapman & Ichioka (1999) proposed that there was
potential for bentonite consolidation during longer
stoppages, such as nights and weekends. While
consolidation occurred during the stoppages in this
study, the amounts were small and no correlation
with stoppage duration was established.
(ii) Reilly & Orr (2017) highlight the role of bentonite
pressure in reducing local effective stress on
jacking pipes. The corollary, as suggested by
Zhang et al. (2018), is that a reduction in bentonite
pressure during a stoppage may cause the skin
friction to increase, leading to cavity contraction,
thereby increasing the effective stress acting on the
concrete pipes. Such a decrease in pressure could
also counteract pipe buoyancy (Choo & Ong,
2012). However, the significant lubricant pressures
used in modern microtunnelling (e.g. Phillips 2022)
means that this phenomenon is likely to be
secondary.
(iii) In the context of stoppages, Zhang et al. (2018) also
infer differences in the coefficients of static and
dynamic friction. The time-dependent nature of the
measured data in Fig. 6 would appear to oppose this
explanation for these tests.
The time-dependent increase in skin friction in Fig. 6
may be a result of thixotropic strength gain within the
bentonite. For example, using both a fall cone and shear
vane, Shahriar et al. (2018) report an increase in
thixotropic strength ratio (defined as the multiplier on
initial strength) with time, plateauing after ≈150 hours.
Noori et al. (2019) carried out triaxial tests on
sand/bentonite mixtures in which an increase in
undrained strength with time was observed up to 100
hours. The undrained strength showed a dependence on
bentonite content, and exhibited a reducing rate of
strength gain with time. There are clearly parallels
between such trends and the interface shear data
presented in Fig. 6. Research is ongoing at NUI Galway
using a large-scale interface shearing facility
incorporating a section of a jacking pipe to consider
further the mechanism behind the stoppage-induced
skin friction increases (O’Dwyer 2022).
6 CONCLUSIONS
In this paper, direct shear interface testing has been
used to model the effect of pipe-jacking stoppages on
skin friction increases along pipe-sand/lubricant and
pipe-lubricant interfaces. The maximum stoppage
duration observed of 2 weeks was deliberately longer
than typically arises in field situations. The maximum
increases (≈ 3–3.5 kPa) were found to be small when
benchmarked against critical state shear strengths, but
could potentially represent very significant increases in
jacking force requirements in field scenarios, where
average skin friction values of less than 1 kPa are
routinely achieved. The increases were dependent on
stoppage duration up to 5-6 days, but appear to plateau
thereafter, with a mild effect of bentonite content noted.

ICPMG 2022
10th International Conference on Physical Modelling in Geotechnics
19(Mon) – 23(Fri) September 2022 / KAIST, Daejeon, Korea
The results suggest that a temporal process, such as
thixotropic strength gain in bentonite with time, may be
behind the observed behaviour, but further research is
needed to confirm the underpinning mechanism(s). The
laboratory data presented appear to represent a
(conservative) upper bound to corresponding
stoppage-induced skin friction increases from three
case histories presented.
ACKNOWLEDGEMENTS
The second author was funded under the Irish
Research Council Enterprise Partnership Scheme (ID:
EPSPG2017244), with Ward & Burke Construction
Limited as Enterprise Partner. The third author is
supported by the Royal Academy of Engineering (U.K.)
under the Research Fellowship Scheme.
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