The use of lattice girders in the construction of tunnels
Komselis C.
Bekaert OneSteel Fibres Australasia (Brisbane Australia)
Blayney N.
ROMTECH Ltd (Witham UK)
Hindle D.
London Mining and Mineral Consultants Ltd (London UK)
ABSTRACT:
In recent years considerable advances have been made worldwide in the design and construction of sprayed
concrete tunnel linings. Included in these advances has been the move away from traditional support using
heavy rolled steel arches in linings, to lighter, more manageable lattice girders, steel mesh and/or steel fibres
providing a continuous support. The development of lattice girders has provided engineers and contractors
with greater options in design, increased flexibility and a more cost effective method of construction.
This paper outlines the development of lattice girders within the tunnelling industry, advantages of the
product, other applications and specific project details from the United Kingdom and Australia.
1. HISTORY OF STEEL SUPPORT IN TUNNELS
At the turn of the 20th century steel supports were
being used sporadically in tunnelling, usually
patterned after the timber sets that were in
widespread use at the time. In the late 1920's and
early 1930's, steel began to be adopted more widely
as its superiority was gradually recognised. By this
time metallic linings such as cast iron were in
virtually universal usage in soft ground tunnelling.
With this major advance in lining an expansion of
deep urban metro tunnels had been seen, particularly
in London, which contributed generally to the
increased use of steel.
The most common form of steel support used was
rolled steel "I" or "H" section beams bent, welded
and bolted to form an arch to fit the particular tunnel
profile and secured in place by wood packing and
wedges that span the remaining gap formed by over
break. Horizontal tie bars connect adjacent arches
and wood or steel sheeting supports the exposed face
behind. The completed arrangement is often
referred to as "ribs and lagging" and is still in use
today, particularly in the mining industry. In terms
of construction the main disadvantage of this support
method is the unwieldy nature of the heavy arch
sections that often have to be manhandled in place.
From a design point of view the support system is
essentially passive; i.e. ground movement must take
place following installation for any load transfer to
occur. If the arches are not correctly set or the
blocking and lagging becomes loose or is
insufficient, failure of the ground can occur lo cally
placing severe loadings on individual supports that
then deflect.
Increasing emphasis on key elements of tunnel
support such as: safety, speed, economy,
performance and surface settlement led to major
innovations during the late 1960's and 1970's with
the development of tunnel boring machines with
bolted, gasketted and fully grouted steel reinforced
precast concrete segments and the rapid spread of
steel reinforced sprayed concrete or "shotcrete".
These methods offered systems that are quickly
installed in full contact with the ground and are,
therefore, able to provide early support that can
restrict ground movement by structurally interacting
with the surrounding ground to provide an active
support system.
The first recorded application of shotcrete in
tunnelling was in the USA during the early 1920s
but it was not until the mid to late 1960s that it
gained significant recognition following the
development of new and innovative forms of
tunnelling for the Snowy Mountains hydroelectric
power scheme in Australia and the New Austrian
Tunnelling Method in Europe. This followed
pioneering work carried out in 1954 by a little
known Austrian mining engineer, Anton Brunner,
who patented a support system using the method.
2. DEVELOPMENT OF SHOTCRETE AND
LATTICE GIRDERS
Initially, shotcrete was applied either unreinforced,
often in conjunction with rock bolting, or with more
conventional steel arches embedded in a shotcrete
lining. However, as the method was developed
further, the main support was no longer provided by
the steel arch sets but by the shotcrete shell in
connection with the ground, which as a consequence
required a thinner overall structural section than was
possible with discrete arches. The steel arch became
increasingly used only as a temporary supporting
element to protect the face workers from unstable
ground until the shotcrete was fully set. More
suitable "V" section rolled steel arches became more
popular for the thinner shotcrete lining and are still
in sporadic use today, however, as is the case with
conventional steel sections they are difficult to fully
embed in the shotcrete shell without shadowing.
The problem of providing a steel arch that has both a
low profile section and offers a minimal barrier to
the placement of the shotcrete shell led to the
development in the late 1970s of the lattice girder.
Their ease of manufacture, transport, storage and
handling underground ensured their rapid gain in
popularity with tunnelling engineers and crews with
additional benefits accruing to the design of the
shotcrete lining itself. In tunnelling, the lattice
girders retain many of the basic functions of
conventional steel arch ribs requiring a degree of
strength and rigidity that is efficiently provided by
the 3-dimensional steel lattice configuration.
Two basic types of lattice girder have been
developed and relate to the number of main support
members present. The most commonly
manufactured three-chord lattice has an isosceles
triangular section with a larger bar diameter (25 40mm) at the apex and two smaller diameter bars
(20 - 32mm) at the base corners. The apex bar is
separated from the side-bars by small diameter (10 12mm) sinusoidally bent bars that are welded at
each node to the main bars. The entire fabrication is
radiused about the apex of the triangle with the apex
being located either on the inside or outside of the
curve. The basal bars are separated and braced by
straight 16mm diameter cross -bars welded at the
node locations of the "sinusoidals". The much less
frequently used four-chord lattice has four equally
sized bars (20 - 40mm diameter) located at the
corners of a square or rectangular section. Pairs of
bars on each side of the rectangle separated and
braced by sinusoidal bars and the two pairs are
cross-braced by 16mm diameter cross-bars in a
similar manner to the 3-chord lattice. The
sinusoidally braced sides are normally radiused or
form the upright member in a straight lattice girder.
In both forms of lattice girder the ends are secured
by steel plates either butt welded to the main bar
ends or fillet welded to form base plates or
connections.
3. FUNCTION AND DESIGN OF LATTICE
GIRDERS
Lattice girders provide the following important
functions in the tunnelling process:
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
Emergency temporary support/restraint for
unstable ground.
Accurate template guide for profiling the
tunnel excavation.
Rigid fixing and support for steel fabric
reinforcement to the intrados and extrados of
the shotcrete lining shell.
Cantilever fixing for spiling ahead of the
advancing tunnel face.
Temporary support to the shotcrete as it is
being applied and until it gains sufficient
strength to support itself.
Accurate guide to the thickness of the
shotcrete application.
Contribution to the overall structural steel
reinforcement in the completed lining shell.
In structures such as ring-beams the lattice
girder may provide the primary steel
reinforcement member.
Accurate fixings for tunnel convergence
monitoring stations.
In determining the relational dimensions of the
various bars that make up the lattice the following
aspects of lattice design must be considered:
i.
ii.
iii.
iv.
Required rigidity, capacity and moment
characteristics.
Avoidance of shotcrete voids (shadowing)
for complete encapsulation.
Required shotcrete lining thickness and cover
to steel.
Minimum required arch radius.
The optimisation of the lattice design is in part
calculated but is largely based on experience.
Consequently, manufacturers usually offer a
comprehensive range of lattice configurations that
have known structural properties from which the
tunnel designer can select the girder size that best
suits the tunnel diameter and lining thickness. The
separation distance between girders is usually equal
to the advance length of each tunnel round and the
girder may extend around the crown arch of the
tunnel only or continue below the springing line to
the tunnel invert. Where full invert closure is
critical to the tunnel construction the lattice girder
may be completed across the tunnel invert.
In larger tunnels a sequential excavation sequence is
used with the full tunnel profile being excavated in
stages to form a top heading, bench and invert,
sometimes involving single or multiple side drift
excavations that are staggered longitudinally by
several metres. The versatility of the lattice girder
provides a particularly useful means of setting out
and temporarily supporting complex excavation
sequences. For this the complete lattice girder arch
is fabricated in manageable sections that can be
bolted or pinned together at the end plates to form
full moment connections when fully encapsulated in
shotcrete.
There is some debate as to whether to orientate the
apex of a three-chord lattice girder towards the
intrados or extrados of the lining. Structurally there
is little difference between the two options and there
is some argument against intrados orientation as the
single bar may induce a crack to form in the
unrestrained, exposed lining surface. On the other
hand this orientation provides distinct advantages in
terms of providing a more efficient load transfer
between the formation and the girder. In addition,
by placing the girder apex towards the intrados the
insertion of a spile through the lattice located at the
advancing tunnel face and cantilevering it against
the preceding girder's thickest bar is certainly
simpler and more efficient.
4. 1 Materials
Materials used in the manufacture process must
comply with the relevant Engineers specification
and the national standards of that country providing
full traceability back to a reputable source. The
material strength will determine the load carrying
capacity of the section and the chemistry the ability
to bend the section to the desired dimensions and its
ability to be welded.
4.2 Manufacture
Personnel involved in the manufacturing process
will have had to undertake an induction p eriod on
the construction of lattice girders. A suitably
recognised welding qualification with proven
competence in welding are also required. The
welding of the sections determine the performance
and the tolerance of the sections.
Purpose built jigs should be used for repetitive
construction which will improve tolerances and
manufacture speed. Individually made girders will
require additional measurement checks for tolerance
control.
4. 3 Quality Control
Stringent documented procedures are enforced to
ensure that each component is completed to
specification. The documents cover issues of:
i. traceability for each uniquely numbered girder
from material source through to all stages of
manufacture
ii. weld quality and strength through non destructive
testing
iii. tolerance and dimensional check requirements
depending on the complexity and number of
sections.
4. MANUFACTURE OF LATTICE GIRDERS
To ensure that the lattice girders fulfil their design
criteria to function in a safe and predictable manner,
as a temporary support and profile former, it is vital
that the product is manufactured in a quality
controlled factory environment.
Photo 1. Typical Lattice Girder Sections
5. INSTALLATION OF LATTICE GIRDERS
The lattice girder is usually the first structural
member to be installed in the tunnel excavation
either as soon as the excavation round is completed
or a "flash'" layer of shotcrete is applied to protect a
potentially unstable profile. The girder is usually
installed, often using a laser setting out guide, as
close to the tunnel face as is practicable without
interfering with the excavation of the next tunnel
round. This is to enable the following shotcrete
lining to be completed as far forward as possible to
minimise the length of unsupported ground and to
provide a profiling guide for the next excavation
advance. The girder is usually offered up in place
manually, sometimes from mobile cradles, or it can
be elevated and manoeuvred into place using special
attachments fixed to the excavator. The girder can
be temporarily held in place by propping or pinning
it to the formation. Alternatively, it can be bolted to
a girder that has been already secured.
Traditionally welded wire fabric (WWF) is placed
behind the girder and tied in place lapping to the
WWF from the previous round. An initial layer of
shotcrete is then applied to cover the WWF half
encapsulating the lattice girder. The second, inner
layer of WWF can then be fixed in place and the
shotcrete layer completed to the required thickness.
This staged installation improves the penetration of
the shotcrete though and around the steel without
overloading the girder with shotcrete that has not
reached sufficient strength to begin to support itself.
Also the initial shotcrete layer should not have cured
sufficiently to prevent the second layer from
bonding to it. In addition, the application of the
second shotcrete layer normally falls short of the
newly installed girder in order to stagger the
construction joint between rounds and reduce water
penetration. The process also provides a safe
working environment minimising the exposure of
unsupported ground.
With the development of Fibre Reinforced Shotcrete
(FRS) the need for two passes of plain shotcrete to
fully encapsulate the lattice girder and mesh has
been eliminated. Full thickness single pass FRS can
achieve full encapsulation and increase the speed of
tunnel advance. The FRS is tapered towards the
end of advance to offer protection and create a
lapping section for the next section of spray. The
use of single pass layers is discussed further in
section 9.
Where the tunnel heading is constructed in side
drifts a temporary lattice girder supports the section
of the main arch in the crown and is removed as the
second side drift is excavated. An easily dismantled
pinning arrangement is normally incorporated in the
design of this temporary connection whilst the
permanent bolted connection with the adjacent
girder in the crown arch must be protected from
becoming encapsulated by shotcrete until the arch is
completed. Where the lattice girder is extended
down the sidewall of a bench, the design must
incorporate some form of temporary longitudinal
support to the base of the crown arch that is
undermined by the bench excavation. Alternatively
a horizontal lattice girder is placed below the crown
arch girder that bridges the gap between the bench
and the last installed sidewall girder. This
configuration is termed a "wall plate" from the
similar arrangement in traditional "ribs and lagging"
support and is a particularly appropriate use of a 4chord lattice girder. The base of the lattice girder
arch is formed simply from a butt-welded steel plate
that can be packed underneath to temporarily
support the structure until it is encapsulated in
shotcrete.
The key to efficient construction of the lattice girder
arch is the close consideration needed for the lattice
girder geometry and connection detail in relation to
the tunnel geometry and excavation sequence.
Safety and handling considerations are also
important, as it is often the ease of setting out and
erecting the girder in a confined space and in a
limited time period that dictates the accuracy and
functionality of the completed lining.
6. SPECIAL SUPPORT SYSTEMS AND
APPLICATIONS
In addition to simple tunnelling applications, the
versatility of the lattice girder/shotcrete lining
system allows complex underground openings to be
created even in soft ground. Multiple junctions into
shafts, tunnels and caverns are possible using
specially fabricated lattice girders that form the
opening "eye" or ring beam support. In addition, the
"eye" beam can be further supported by the insertion
of rock bolts and spiles through the lattice which
when encapsulated in shotcrete forms a structural
connection and intimate bond with the ground.
In addition to bored tunnel construction a lattice
girder and reinforced shotcrete shell can be used in
cut and cover tunnel construction. Here the lattice
girder arches are fully erected in an open cutting and
braced by the fixing WWF to the intrados and
extrados sides. Rough, temporary shuttering panels
are then fixed to the extrados and the sprayed
concrete is applied from inside the structure. Once
the shotcrete has cured sufficiently to be selfsupporting, the shuttering is removed and further
sprayed concrete is applied to the exterior face. The
resultant reinforced concrete shell can then be
loaded with backfill once it is fully cured. This
technique has also been used to construct portal
canopies and even surface structures and buildings,
further demonstrating the versatility of the method in
the forms and geometries of construction that are
possible, which would require complex shuttering
and casting using conventional construction
techniques.
the tunnel heading being advanced with support
from lattice girders and shotcrete.
Photo 2. North Downs Tunnel London Portal
Tunnel refurbishment is an increasing field of
application for lattice girder and shotcrete support.
For example brick or masonry lined tunnels can be
strengthened by the incorporation of reinforced
concrete arches formed by local removal of the
existing lining to form a slot in which a lattice girder
can be erected. Infilling with shotcrete provides a
good structural bond with the adjacent lining
improving its overall load carrying capacity through
the formation of reinforced concrete arches.
7. PROJECTS IN THE UK
7.1 Channel Tunnel Rail Link, Contract 410, North
Downs Tunnel
The tunnel, constructed by a joint venture of Miller
Civil Engineering, Beton-Und-Monierbau and
Dumez GTM, is 3.2Km long with a tunnel face area
of 165m2 is the largest tunnel ever constructed on
the United Kingdom mainland.
With cost and time major factors the JV decided to
proceed with a sprayed concrete primary lining
comprising of lattice girders, 2 layers of steel mesh,
spiles and rock bolts.
The advance rates varied between 1 metre and 2.2
metres providing a recorded daily advance of up to
14 metres.
This allowed the cast concrete secondary lining to be
started early, culminating in the concrete works
being completed within budget and ahead of
schedule.
The lattice girders provided commencement profiles
for the portals as indicted in Photo2. Photo 3 shows
Photo 3. North Downs Tunnel Country portal
7.2 Greenway Pumping Station, Storm Water Surge
Shaft
Constructed during 1998 the 21 metre deep x 15
metre diameter surge shaft would traditionally have
been constructed using a precast segment lining. An
alternative solution was accepted by the client
Thames Water, using a sprayed concrete lining,
reinforced with lattice girders at 2 metre vertical
centres and two layers of steel mesh. The support
method selected proved to be successful with
construction being significantly faster and cheaper
than traditional precast methods.
7.3 Heathrow Express rail link from London to all
four Heathrow Airport terminals
With the main running tunnels constructed using a
precast segmental lining, it was decided that the
more complex areas of the tunnelling works would
be constructed using the NATM method of
construction in London Clay. These areas included
two Terminal 4 platforms, Terminal 4 crossover,
Terminal 4 turnout, and a turnout for the future
provision of a fifth terminal and various adits and
escalator shafts.
Lattice girders used in the turnouts and crossover
were a complete ring at 1 metre centres with two
layers of steel mesh. The method of construction
used either a single side drift and enlargement or two
side drifts and enlargement.
The construction of the first turnout was complex
requiring 75% of the lattice girder sections to having
varying lengths and radii every metre. The lattice
girder dimensions were rationalised on further
elements to increase speed of installation.
Photo 4 – Buranda Tunnel Southern Portal
Modified lattice girders also proved to be useful as
support to the water proofing membrane and to
provide additional reinforcing for the final lining in
the southern portal section, shown in figure 5.
8. RECENT PROJECTS IN AUSTRALIA
8.1 Buranda Tunnel
The Buranda Tunnel forms part of the Water Street
to O’Keefe Street section of the South East Transit
Busway. The driven tunnel was 190 metres in
length being advanced in two stages, heading and
bench, with a tunnel width varying between 19.2
metres and the southern portal and 12.6 metres an
the northern portal. There were challenges in
driving the tunnel from the southern portal due to the
rail freight line situated approximately 2.5 metres
above the tunnel crown and the large tunnel width at
this point, shown in photo 4. With the low
overburden, canopy tubes were installed for the first
30 metres of the tunnel as temporary support. Lattice
girders and steel fibre reinforced shotcrete (SFRS)
were then used as the primary support. The lattice
girders were installed in 4 sections utilising bolted
connections. Adjustable footing supports where
used on the lattice to account for the varying profile
of the excavation. Tie rods where used to
transversely position the girders. The tunnel was
advanced in 1metre intervals from the Southern
Portal.
Photo 5. – Membrane Support Lattice Girders
The northern portal lattice girders were installed at 1
metre intervals and increased to 1.5 metres as the
ground conditions improved. The lattice girders
were first installed and shotcreted in the top heading
of the tunnel. Once the bench was excavated lattice
girder legs were installed and shotcreted, extending
the lattice girder to the invert level. Photo 6
indicates the connection point between heading and
bench.
fibres in place of the WWF reinforcement
particularly for crack control and fire protection
purposes the need for lattice girders to provide
profiling will still exist.
Photo x – Northern Portal Buranda Tunnel
8.2 Vulture Street Tunnel
The Vulture Street Tunnel forms part of the City to
Woolloongabba section of the South East Transit
Busway. The section comprises of a 410 metre
driven tunnel with particular interest to the 34 metre
length of Y junction located in the tunnel. The
purpose of the Y junction was to allow for future
transport and dual use provisions. The Y junction
provided complexities in both design and
construction due to the width of the section and low
overburden to the main road above. The section was
advanced in multiple stages, with a central pillar to
be removed once the two tunnel sections were
developed. The multiple advance, in 1 metre
intervals, also allowed installation of rock bolts,
cables, lattice girders and SFRS. The lattice girders
were installed in 5 sections with the bolted and
sliding joints used to accommodate excavation
tolerances. The sliding joint was the central
connecting point once the pillar was removed. The
sliding connection was suggested by the consultant
as a practical method of installing the lattice girders
in difficult conditions within complex geometries.
The lattice girder was modified further to allow the
rock bolts to be installed through the top chord of
the girder eliminating large cumbersome bolt plates.
9. FUTURE POTENTIAL OF LATTICE GIRDERS
AND SHOTCRETE SUPPORT
Significant moves towards single pass permanent
shotcrete linings have been made in recent years
following improvements in the wet-mix sprayed
concreting process and shotcrete mix designs, where
the placed concrete quality can equal or even exceed
cast-in-situ reinforced concrete in terms of strength,
compaction permeability and finish. Although there
has been a parallel development of the use of steel
New uses of the method are being developed for a
wide range of civil engineering applications but it is
perhaps the mining industry, where the method first
began, that is likely to provide the main future
growth. To date lattice girder and shotcrete support
has had limited use in this industry with a tendency
to use more traditional methods such as ribs and
lagging, but the benefits of safety, economy and
speed will, in the long run, prove attractive.
10. REFERENCES
-
Baumann Th. & Betzle M. 1984.
Investigation of the Performance of Lattice
Girders in Tunnelling. Rock Mechanics and
Rock Engineering 17, pp 67-81.
-
Baumeister A.E. & Ertel J. 1985. Lattice
Girder Construction and Dimensioning.
Tunnel 2/85, May 1985.
-
Betzle M. 1987. Lattice girders giving arches
a dig in the ribs. Tunnels & Tunnelling,
November 1987.