2012, Hunt & Del Nero, Microtunneling in Gravel, Cobbles and Boulders
2012, Hunt & Del Nero, Microtunneling in Gravel, Cobbles and Boulders
2012, Hunt & Del Nero, Microtunneling in Gravel, Cobbles and Boulders
FIGURE 1
MTBM choked with
gravel and cobbles
TECHNICAL PAPER
Microtunneling in Gravel, Cobbles and Boulders
By Steven Hunt and Don Del Nero
M-12
!"!#!$%!&'()**+++!$
(GCB). This ground condition increases the risk of potential impacts such
as: a jammed excavation chamber (see
Figure 1); high torque and microtunnel boring machine (MTBM) stalling;
www.trenchlessonline.com
!!,!,$-!$+++!-./&.!/+01
Groundwater Conditions
Wet, cohesionless, high permeability
GCB tends to cause several important
geotechnical challenges. This ground
condition has one of the lowest standup
times possible. As the groundwater head
increases, the standup time decreases
and challenges of providing face stability increase. Unbalanced heads as small
as 1 m (0.1 bar) can cause flowing
ground and overmining. Overmining or
ingesting more ground than displaced
by the MTBM and jacked pipe tends to
cause excessive settlements and excessive flow of ground into the excavation
chamber.
In order to apply an effective face
pressure when tunneling in open gravel or GCB, the subsurface investigation
program and geotechnical instrumentation should adequately indicate the
range in permeability expected and
groundwater pressure to be resisted. In
many cases, too few piezometers are installed and permeability tests performed
to adequately predict groundwater conditions along the entire alignment.
Cutterhead Opening
The cutterhead opening ratio (COR),
which is the percentage of open area
on the cutterhead, and size and distribution of openings from the center
are critically important considerations
for microtunneling in GCB. MTBMs
typically have CORs ranging from 20 to
more than 50 percent. Larger CORs are
desired in cohesive soils (firm or slow
raveling ground) and smaller CORs in
cohesionless soils (flowing or fast raveling ground).
Where the ground has sufficiently
low permeability, no active groundwater head and sufficient strength to be
stable in an open-face condition, a larger COR is also desired in GCB with a
www.trenchlessonline.com
!"!#!$%!&'()**+++!/
M-13
!!,!,$-!$+++!-./&.!/+01
M-14
!"!#!$%!&'()**+++!2
MTBM Torque
The thrust and torque required for an
MTBM to effectively advance through
GCB is dependent on many factors including: soil density; clast volume ratios;
clast sizes and strengths; energy required
to fracture, pluck and crush clasts; muck
flow friction in the MTBM chamber; intakes and slurry mucking system; and
friction between the ground and MTBM
and jacked pipe. GCB with higher density or that is weakly cemented tends
to increase the torque required to cut,
pluck and pass cobbles and boulders.
As the clast volume ratio increases, the
torque required increases. In addition
to clast volume ratios, the size and unconfined compressive strength of the
clasts also influences torque demand.
A boulder will generally require more
torque to cut and pluck than scattered
cobbles for the same clast volume ratio.
Torque spikes above that required for
general excavation will result when the
cutters impact boulders at the face. The
sustained energy and torque required to
cut and fracture or pluck clasts at the
heading increases as the unconfined
compressive strength of the rock increases [2].
After cobbles and boulders are partially cut, plucked and passed into the
MTBM excavation chamber, the energy
required to crush the clasts to a gravel
size for slurry mucking is very high and
increases with both increasing clast volume ratio and increasing unconfined
compressive strength of the clasts.
Torque spikes may also occur when a
large, high strength clast is engaged by
the rock crusher.
www.trenchlessonline.com
!!,!,$-!$+++!-./&.!2+01
Summary of Potential
Mitigation Measures for
Microtunneling in GCB
The risks of microtunneling in GCB
can be mitigated by a variety of potential measures. To microtunnel in highpermeability gravel and to reduce risk of
choking and stalling along with the risk
of severe overmining and sinkholes, the
flow of ground through the cutterhead
into the excavation chamber must be
restricted to a rate that the rock crusher
and slurry mucking system can handle by
one or more of the following methods:
www.trenchlessonline.com
!"!#!$%!&'()**+++!&
face equal to at least the groundwater
pressure and estimated active earth
pressure.
and has the maximum torque available from manufacturers for that diameter.
These and other special measures
discussed above should help make
microtunneling in GCB more manageable and minimize the risk of get-
References
Boyce G.,Wolski M., Zavitz R. & Camp
C. 2011. Chemistry And Physics Behind
Microtunnel Slurries, Proceedings of
North American No-Dig 2011, NASTT,
paper A-2-01, (2011.6) 10p.
Hunt S.W. & Del Nero, 2011. Microtunneling in Cobbles and Boulders. Microtunneling Short Course,
Colorado School of Mines, Golden,
CO. 36p.
Milligan G.W.E., 2000. Lubrication
and soil conditioning in pipe jacking
and microtunnelling. Tunnels & Tunnelling International, July 2000, pp.
22-24.
!!,!,$-!$+++!-./&.!&+01