London Underground

New London Underground slab track cast in-situ

You might think that B2F stands

for “Back to the Future”. In fact, it

is the acronym chosen by London

Underground to signify its project

to renew the track in tunnels

between Baker Street and Finchley

Road on the Metropolitan line.

Back to the future, though, would

not be a bad interpretation as the

project is all about an innovative

process that LU has developed, in

conjunction with its partner Tarmac,

and which shows promise for many

more applications in the future.

The task facing LU was how to

replace 3.2km of life-expired bullhead

rail, timber sleepered, ballasted

track in seven single-bore tunnels.

These range in length between 185

and 720 metres. The track drainage

had largely failed and the conditions

arising from that were making track

maintenance increasingly difficult.

The solution that LU aspired to was

to replace all the ballasted track in

the tunnels with a concrete slab track

form, giving the benefits of minimal

maintenance requirements and a

design life of 100 years. But how

to achieve this economically with

short track access periods on this

intensively used underground line

close to central London? The answer

was to be found by deploying a very

specific type of concrete (and by LU

granting itself slightly longer night-

time access periods than the norm).

Product PQ-X Cement

Client London Underground

SPECIAL MIX

Tarmac’s Pozament brand which

provides high performance building

products, had developed concrete

mixes for similar situations. These

had been successfully used for

surface bay replacements on the

M42 and M54 motorways and on

runways at Birmingham International

and London City airports.

So Tarmac was approached by LU

in the latter half of 2015 to develop

a modification of its concrete

specification to meet some particular

performance requirements. These

were to not only have a good

strength gain over time and to

be placeable within a limited

time frame, but also to achieve

these within a wide temperature

range from 10°C to 30°C.

The newly designed mix, which would

cope with this temperature range

and reach a strength of 15 N/mm2

two hours after placement, was ready

for the work to commence in May

2016, the start of a two year project.

Roger Eke, technical sales manager

for Pozament, explained that, because

it was not feasible to transport a

wet mix into the tunnels, LU wanted

a dry concrete mix that could be

produced within the tunnels and have

water added close to the point of

placement. Therefore the pavement

quality PQ-X mix, previously used on

the M42 and other projects, was re-

engineered for the specific application

of the track slab in tunnels.

PQ-X is mixed with 0/4mm washed

sand and 4/20mm aggregate from

the Tarmac owned Tyttenhanger

Quarry, near St Albans. The

materials from this source were

specially selected because of the

regular particle size distribution,

which results in a homogeneous

and workable concrete mix.

Tyttenhanger Quarry has allocated

specific reserves to achieve

consistent quality from the extracted

materials for the life of the project.

Using land-based aggregate rather

than a marine source also avoids

any potentially adverse chloride

reaction. The PQ-X cement is

manufactured at Tarmac’s specialist

facility at Swains Park in Derbyshire.

As part of the final refinement and

approval of the modified PQ-X

cement, the London Underground

project representatives were

invited to Swains Park to witness

a typical pour of PQ-X based

concrete and to agree on the

appropriate slump characteristic.

The concrete performance was

then re-engineered by Pozament’s

laboratory which is responsible

for research and development,

together with quality control.

The actual mixing of the concrete takes place

within the tunnel on an engineering delivery train which includes a special volumetric mixer wagon.

TRACK REPLACEMENT

The actual mixing of the concrete

takes place within the tunnel on an

engineering delivery train which

includes a special volumetric mixer

wagon. The rail-mounted wagon

is one of a range manufactured

by the Italian company, Blend.

This wagon has compartments

for segregating the constituent

materials, a cement hopper for up

to 5.0m3 of the PQ-X cement, a

sand hopper of 8.0m3 capacity, an

aggregate hopper of 9.0m3 capacity

and a 5,000 litre water bowser.

The volumetric mixer can be

programmed to deliver a specific

mix from these compartments.

Sand and aggregate are gravity fed

onto a conveyor belt and are then

transferred to an auger rotating at

300rpm, at which point the cement

and water are added. The high speed

of the auger mixing process ensures

that sufficient energy is imparted

to activate the superplasticisers

and other additives included in

the mix design. The output from

the auger is then dropped onto

another conveyor belt at a steady

rate and delivered to the placement

area. The mix is designed to have

an S4 slump characteristic.

David Sloane, project manager for LU,

explained the planning and delivery

arrangements. Extended engineering

possession hours of 22:00 to 05:30

each night on Monday, Tuesday

and Wednesday were taken on the

Metropolitan line for installation

of the track slab. Within this time

period it is possible to install 10.5

metres of new track per night.

The works are dependent on an

engineering train, which consists of

locomotive/GP wagon/ mixer wagon/

DISAB/GP wagon/locomotive. The

first operation is to use the DISAB

vacuum machine to excavate all the

old track ballast from the length to

be replaced that shift. The DISAB

process is also able to remove all the

old drain components along with the

ballast. All material up to the soffit

of the rail baseplates is removed, the

bullhead rail and chairs having been

previously replaced by flat bottom

rail and baseplates in readiness for

formation of the new track slab.

Once the area under the rail and

baseplates downwards has been

cleared, shuttering is placed ready

for casting the track slab. A base

reinforcement mesh is also positioned

75mm from the tunnel invert. The

overall depth of the concrete slab

is typically 400 to 480mm.

Mixing, delivery, placement and

vibration of the concrete can be

achieved within 45 minutes. It is

interesting that, in the early stages

of the project, it was found that the

concrete was not remaining workable

for long enough and LU asked the

Pozament team to slow down the mix.

It was impressive how they were able

to do this very accurately whilst still

preserving the required specification

and properties of the finished

product. Another striking feature of

the process is that, in programming

the concrete mix, the volume of

concrete required for each pour

can be produced very accurately.

The site team is made up

of 16 personnel from Track

Partnership (a Balfour Beatty

and London Underground JV).

INTEGRAL DRAINAGE

A new square-section U-shape

four-foot drain is cast into the top

surface of the slab and is provided

with a GRP cover. This drain runs

to the end of each tunnel section

and then goes through a transition

slab at the beginning of each open

track section where the drainage

crosses into the six-foot drain.

Prior to the works in the tunnels

commencing, trials of the concrete

placement had been carried out

on a mock-up of the tunnel cross-

section. This was a full-scale model

approximately 15 metres in length,

constructed at an LU facility at

Northwood. The purpose of these

trials was to ensure the feasibility of

placing and compacting the mix into

the fairly constricted area between

the tunnel invert and the shuttering

for the surface drain, and to prove the

curing and strength gain aspirations

of the process were achieved.

In addition to the mix design for the

main work of the track slab, Pozament

also produce a rapid-setting flowing

repair concrete for application on

small sections where it is necessary

to effect localised patching and

small section slab track pours of

approximately three linear metres.

Once the project is complete by

mid-2018, London Underground

should expect very low maintenance

within these tunnel sections for a

number of years, having achieved

the transition to an innovative

track form with minimal disruption

to Metropolitan line customers.

Tarmac can be proud that its

expertise and skill in designing

specialised concrete mixes has

been further refined and applied to

a new situation and to one which

looks capable of extension more

widely within the LU network and

on other metro systems worldwide.

As featured in Rail Engineer Magazine

- December 2016

TARMAC.COM

For more information on our range of

Cement solutions please visit pozament.co.uk