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Cut and Cover
Tunnels
he vast majority o below-grade
tunneling projects today
are actually created using cut-
and-cover construction methods. Tis
methodin which a trench is excavated
rom the surace, then re-coveredisusually more economical and more
practical than mined or bored tunneling.
Its especially practical at shallow depths
(30 or 40 eet) but depths to 60 eet are
not uncommon.
Cut-and-cover tunneling is
extremely versatile. Most o the worlds
subways have been built using cut-and-
cover techniques. Its the overwhelming
method o choice or traffic, rail and
pedestrian underpasses, utility tunnels,and a host o other applications.
Construction MethodsDepending on the situation, a variety
o different construction methods can
be used. Stacy Byrd, National Products
Manager at CECO, says, Tere are so
many different methods o constructing
a tunnel and the waterproofing solution
should be designed to fit that particular
project and the expected jobsite
conditions.
Byrd continues, o determine
the best waterproofing solution, one
must consider how the tunnel will be
constructed. Will it be constructed withcast-in-place concrete, precast, shotcrete,
or a combination such as a precast
tunnel roo placed onto cast-in-place
walls? Access is also a defining actor in
product selection. Will the waterprooer
have the space to install the membrane
rom the exterior or will it be blindside
construction?
Major actors determining the
construction method include the
excavation depth, soil type, elevation
o the water table, site access, and the
amount o time which the final at grade
surace can be disturbed.
In general, the many different
construction method used to build cut-
and-cover tunnels can be divided intothree major categories. Each o them
requires an entirely different approach to
waterproofing.
Bottom-Up Construction: Te most
common technique is the bottom-up
construction. Here, a trench is excavated
rom the surace downward. Once the
final depth is reached, the tunnel floor
is built, then the walls and roo are put
into place. Finally, the entire structure is
buried and the surace restored.I the construction site has plenty
o room, the trench is usually cut back.
Walls are typically cast in place, and
the waterproofing can be applied to the
P h o t o C o u r t e s y C E T C OMost tunnels are actually built as trenches using cut-and-cover
techniques. This St. Louis subway tunnel was built using thebottom-up method, using cast-in-place concrete walls and
precast roof segments.
Photo Courtesy City of Calgary
The Calgary light rail tunnels
reached 60 feet below grade, and
used a combination of soil nails,
shotcrete, and cutback slopes that
waterproofers had to work around.
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outside ace o the concrete with minimal
problems.
Usually, though, theres not enough
room or positive side waterproofing.
Especially in urban settings, where most
cut-and-and cover tunnels are built, land
is at a premium, so the excavation is cut
vertically and reinorced with soldier
piles or soil nails. In these cases, the
waterproofing is done blindside. With
this construction method, the walls
(usually shotcrete or using removeable
orms on the exposed ace) are poured
afer the waterproofing is finished.
Roos can be poured in place or
ormed with precast planks. Te roo
waterproofing is then tied into the wall
waterproofing and drainage, and then
the whole system is buried.
Bottom-up construction offers
several advantages: First, its well
understood by contractors, so finding
skilled labor isnt a problem. Additionally,
waterproofing and drainage systems can
be applied to the outside surace o the
structure. Te floor o the excavation is
easily accessible or equipment and or
the delivery, storage and placement o
materials.
For these reasons, bottom-up
construction is perhaps the most
economical method o building large,
shallow tunnels, but has the disadvantage
o making the site unusable longer than
either o the other methods detailed
below.
Dave Polk, owner o Epro Services,
has been involved with a number o
complex tunnel projects, including the
Calgary light rail extension.
Calgary was cut-and cover, says
Polk, at Epro, but it was a little different
because portions o it were open-cut.
Polk adds that the deepest areas reach
60 eet below grade, and that parts o the
line are a dual tunnel layout.
We did underslab on everything,
says Polk. Its a 60-mil HDPE film, and
that underslab system is bath-tubbed up
the wall to water table height. Once the
walls were in place, the roo was poured
in place in 15-oot sections using slip
orms.
Te roo experienced a tremendous
amount o construction traffic during
the backfill process, so the membrane
All of the joints in the precast roof
segments of the St. Louis subway
tunnel at left were detailed and
sealed before the waterproofing and
drainage layers were installed.
PhotoCourtesyCETCO
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chosen had to be extremely durable.We started with a spray-applied
liquid membrane, and then laid down a
abric reinorcement, says Polk. Ten
we spray another membrane over that,
which creates a reinorced monolithic
membrane that will stand up to site
conditions. Tat was covered with a
15-mil polyolefin film, and then a heavy
duty drainage composite, which we tied
into the composite on the walls.
CECO was involved withwaterproofing the recent extension o
the below-ground St. Louis light rail,
which used bottom-up cut-and-cover
construction or the walls and a pre-cast
roo.
Byrd explains. Because it was
going through residential sections o the
community, they couldnt overexcavate
and had to pour the tunnel walls
directly onto the soil retention system.
We provided a bentonite-geotextile
membrane that was installed to the
soil retention wall prior to the concrete
placement. Te bentonite geotextile
membrane was also used as an underslab
barrier over the gravel substrate.
Te precast arched concrete
segments or the roo were waterprooed
using a peel-and-stick butyl rubber
membrane that tied into the system used
on the walls.
Modular Tunnels: For small
tunnels, such as utility or sewer access
tunnels and pedestrian underpasses, a
variation on bottom-up construction can
be used. Called modular tunnels, this
construction method uses corrugated
steel or pre-cast concrete segments to
create the structure. Tese segments are
usually waterprooed offsite and craned
into place.
One o the primary challenges with
waterproofing precast tunnels, then,
is ensuring that the membrane is not
damaged between when it is applied and
when the tunnel segments are set into
place. o ensure the waterproofing and
drainage components stay in constant
contact with the bottom o the precast
section, designers ofen call or a concretemud slab instead o loose gravel at the
bottom o the excavation.
Waterprooers must also pay careul
attention to the many joints between the
pre-cast sections to ensure the tunnel
remains dry.
A utility corridor built on the
campus o the University o Connecticut
in 2006 used this method. Each segment
measured ten eet square and eight
eet long, and was craned into placeonto a our-inch thick mudslab, then
waterprooed. First, the joints between
each segment were sealed using Ultraseal
B, a product that combines bentonite
clay with a hydrophilic polymer. Ten,
workers covered the roo and walls o
the tunnel with Volclay, a similar sheet
product that also contains a bentonite-
polymer alloy.
Top-down Construction: Te top-
down construction method calls or the
walls and roo o the tunnel to be put into
place beore the actual excavation takes
place. Te advantage o this method
is that the surace is disturbed or a
minimal amount o timean important
consideration i the tunnel is being built
under a heavily used traffic artery.
In the top-down method, the first
step is getting the temporary walls in
place. Tese could be steel sheet piles
or a concrete slurry wall. Once the walls
are completed, the roo o the tunnel is
constructed and the surace (usually a
roadway) is restored.
Te surace is then available or
normal use while the tunnel excavation
is carried out.
op-down cut-and-cover tunnels
use blind-side waterproofing techniques.
Te waterproofing membrane is typically
applied between the temporary shoring
wall and the permanent tunnel lining.
Te tunnel floor slab and the underslab
barrier are typically the last part o
construction to be completed.
Te underpass tunnel leading to
the San Jose Airport was built using this
method. Site conditions were difficult,
to say the least; the water table was onlysix eet below grade, which meant even
the top o the tunnel would be below the
water table.
Polk, at Epro Services, explains
how the job went. Te first thing that
needed to be done was the installation
o construction wells and pumpsa site
dewatering system, says Polk. Tey
built the top o the tunnel first. Tey
drove pilings in, then poured corbels and
reinorced concrete beams on that. Oncethat was is place, we could waterproo
the top. By finishing the top first,
surace roads could be opened sooner
and minimize the traffic disturbance.
We used our spray membrane with
a HDPE [high-density polyethylene]
film over the top, says Polk. We knew
it would be covered by paving, so we just
used strip drains or drainage.
Ten they began excavating under
PhotoCourtesyCETCO
This college utility tunnel used
modular precast sections. It was
waterproofed using a bentonite-
polymer alloy after the segments
were placed in their final location.
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the roo, installing post-tensioned
tiebacks as the work progressed to keep
the pilings stable. Te tunnel bottom had
to withstand 20 eet or more o hydrostatic
pressure, so the waterproofing system
here was absolutely critical. Tey used
a seamless, spray-applied membrane
sandwiched between two layers o
HDPE.
Te wall waterproofing was done
blindside. Polks crew first applied
a dimple drain sheet, then careully
sealed around the tiebacks and other
penetrations. Tey tied this into the top
and bottom waterproofing systems, then
sprayed a final layer o waterproofing
on the walls. Ten it was enclosed in
shotcrete. Completed about ten years
ago, its still as watertight as the day it
was completed.
WaterproofingConsiderations
Te type o waterproofing used
on cut-and-cover tunnel installations
usually hinges on the type o soil
retention system used on the sides o
the excavation. As noted above, open
cut excavations are the easiest. Soldier
piles and sheet piles are more difficult.Soil nails retention systems, which
can involve detailing around literally
hundreds o different points, are the
most involved.
Regardless o the system used,
waterproofing is essential. Not only does
it reduce corrosion and maintenance and
extend the lie o the structure, but it can
also improve saety. In colder climates,
leaking tunnels can create hazardous
ceiling icicles or ice patches on roadways.Additionally, once a tunnel is
placed in service, theyre extremely
inconvenient to close, which imposes
severe constraints on remedial
waterproofing options. Particularly with
rail tunnels, options or getting toand
repairingleaking sections become
extremely limited.
A ew years ago, the U.S. Department
o ransportation published a white
paper explaining good design o cut-
and cover tunnels. It places special
emphasis on waterproofing. Te
conception o the waterproofing system
in cut-and-cover tunnels demands an
exact planning o the system, a good
collaboration and discussion between
producer, designer o the project, main
contractor and installer, it reads. oo
ofen the importance o waterproofing
is underestimated by the designer with
the consequence that the execution o
the waterproofing works are immensely
difficult to achieve.
Teir study indicates that the best
results can be achieved by paying special
attention to joint sealing, waterproofing
o penetrations, reinorcement opti-
mization, and drainage in area o the
ceiling/crown. n
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*All Rhino urethanes are 100% solids. Some epoxy and polyaspartic products may contain less than 100% solids. The photos depicted in this advertisement are shown for
illustration purposes only and are not intended to represent particular applications or methods. All businesses are independently owned and operated. 2012 Rhino Linings
Corporation. All rights reserved. Waterproof Winter12 6272
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