cut and cover tunnels

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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

method—in which a trench is excavated

rom the surace, then re-covered—isusually more economical and more

practical than mined or bored tunneling.

It’s 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 world’s

subways have been built using cut-and-

cover techniques. It’s 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 the“bottom-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.

20 WATERPROOF!



outside ace o the concrete with minimal

problems.

Usually, though, there’s 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, it’s well

understood by contractors, so finding

skilled labor isn’t 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. “It’s 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.

P h o t o C o u r t e s y C E T C O

Winter 2012 21

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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 couldn’t 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 concrete“mud 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 time—an 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 pumps—a 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

P h o t o C o u r t e s y C E T C O

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.

22 WATERPROOF!



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. Polk’s 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, it’s 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, they’re extremely

inconvenient to close, which imposes

severe constraints on remedial

waterproofing options. Particularly with

rail tunnels, options or getting to—and

repairing—leaking 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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