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Bored tunnels are some o the
most demanding and challenging
projects a waterprooer will ever
work on.Unlike tunnels close to the surace,
which are usually constructed using
cut-and-cover techniques, bored or
mined tunnels are constructed deep
below grade, where sometimes just
getting to the worksite is a major ordeal.
Additionally, the pressures, depths, and
lack o repair options inherent in this
type o work make doing the job right
even more critical than usual.
Because o these challenges, boredtunnels are usually economical only
when other construction methodssuch
as cut-and-covercant be used, usually
because the jobsite is too deep. Its not
unusual or bored tunnels to reach 160
eet below the surace. At those depths,
the tunnel is almost always carved
through solid bedrock. In other cases,
bored tunnels are preerred because the
surace cannot be disturbed.
Basic tunneling methods are airly
standard. First, the tunnel is excavated
using heavy equipment, drill-and-blast
mining techniques, gigantic tunnelboring machines (BMs) or some
combination o the above. I needed, soil
nails and rock bolts are then installed
to strengthen and stabilize the newly
exposed ace. Also, nearly every bored
tunnel receives a layer o shotcrete that
serves the same purpose.
Most o the popular waterproong
methods take advantage o this shotcrete
layer to simpliy the waterproong
process. Once this is in place, workerscan install any drainage and/or
waterproong membranes. Finally, a
grid o reinorcing steel is set up, and a
nal tunnel lining is cast. Tis lining is
usually either another layer o shotcrete
or a traditional concrete mix thats cast-
in-place using sliporms.
While each jobsite is unique and
each project requires a customized
waterproong system, these systems can
be categorized into just a ew basic types.
Crystalline Systems
Perhaps the least complex methodis to incorporate a crystalline admixture
into the shotcrete so that the concrete
itsel becomes waterproo and sel
sealing. In past decades, the product
was usually applied as a powder or a
liquid aer-the-act. Recently though,
a better method has been developed.
(For more on crystalline admixtures, see
Understanding Integral Waterproofng in
the Spring 2010 issue.)
Ann Martucci, a marketing executive
Waterproofng Techniques
For Bored Tunnels
PhotoCourtesyICSPenetron
Bored tunnels, suchas this Pacifc Coast
Highway tunnel nearDevils Slide, Caliornia,
can be a challengeor waterprooers.
Typically, the stakes arehigh, the jobsite remote,
and project size isenormous.
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on the Devils Slide tunnel, construction
ollowed the typical pattern. Workers
installed a double mat o rebar to match
the areas seismic requirements. A rail-
mounted slip-orm was used to cast the
nal tunnel lining.
CECO has recently developed a
series o PVC membranes specically
or bored tunnels. Te CoreFlex 60
integrates a 60-mil thermoplastic
membrane with DuPont's Elvaloy-
KEE (Keytone Ethylene Ester) to seal
any leaks that develop. Its reportedly
the only thermoplastic membrane
composite with this reactive, sel-sealing
perormance eature.
Te company also makes 80- 100-
and 120-mil membranes without the sel
sealing liners.
Te tunneling industry really
preers these durable PVC membranesbecause o the longer service lie, reports
Stacy Byrd, National Products Manager
at CECO.
Drainage and WaterstopsMany tunnel designsespecially
deep tunnel designsallow or a certain
amount o leakage.
Designers typically shy away rom
speciying drain sheets, as pressures
deep within the earth are extreme, andthey dont want anything compressible
in the design. Still, some sort o drainage
is needed to reduce hydrostatic pressure
on the waterproong system. Engineers
typically turn to a system o waterstops,
drain pipes, and sump pumps.
Te goal is to direct the fow o water
to the drain lines or discharge as quickly
as possible.
I a PVC membrane is used,
thermoplastic waterstops are standard,
as they can be welded to the membrane.
PVC waterstopsfexible, impermeable
barriersare installed vertically at
regular intervals to limit the distance
water can migrate behind the membrane.
Byrd reports that theyre typically
installed every 30 eet or so, dividing
the tunnel into drainage sections o
no more than a thousand square eet.
Tese waterstops are dierent rom the
conventional cold-joint waterstops used
in regular concrete work. Tey have
a dierent prole, and are so sti that
they cant easily be rolled. Tey typically
arrive at the jobsite in long straight
sections with separate actory-abricated
connections.
Te preerred solution or drain
lines is surprisingly mundane: the
standard perorated-pipe-in-gravelevery waterproong contractor is
amiliar with. On the 25-mile Channel
unnel connecting England to France,
the perorated pipe was laid in the gravel
ballast under the rails.
I possible, the pipes are run to
daylight and drained by gravity. On the
Channel unnel and other deep tunnels,
a standard sump pump setup is used.
Polyurethane GroutSystemsLike every type o project, bored
tunnels have an Achilles Heel. Cold
joints, expansion joints, and connections
to other tunnelssuch as ventilation
shas or cross passagesare prone to
leakage and require extra care.
One popular method or secondary
waterproong or remedial repair is
polyurethane grout. Teyre especially
useul when access shas or other
penetrations create geometric shapes
that are too awkward or sheet goods to
seal adequately. Tis grout is usually
injected as a liquid resin that reacts with
water to orm a watertight, permanent
seal.
Grace Construction Products
markets a sheet membrane that comes
with a grid o polyurethane grout
injection ports pre-installed. Aer the
nal tunnel lining is installed, the grout
can be injected at any location to ensure
a watertight bond.
CECOs line o thermoplastic
membranes can also be installed with
integral grout tubes.
Additional ConsiderationsWaterproong is critical in deep
tunnel applications because the stakesare so high. Maintenance or repair
work typically would cause a serious
interruption in service, so the job has to
be done right. A ailed waterproong
job, though, is not only inconvenient,
but dangerous. In a mountain highway
tunnel, or instance, any water or ice
buildup is a serious saety hazard.
Peter DAntonio, who oversees
tunnel sales or Sika Sarnal, notes,
Proper waterproong o tunnels isone o the most cost-eective ways to
increase the useul lie o these structures
and enhance saety. Water seeping into
a structure endangers the building
materials through the reeze-thaw
cycle, with water reezing inside cracks
and causing them to enlarge. Further,
preventing water rom reezing on road
suraces increases saety. n
P h o
t o C o u r t e s y X y p e x
The 25-mile-long Channel Tunnel, connecting England and France,used more than 500 tons o joint waterproofng.
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