Christchurch Western Interceptor Phase II - Micro Tunnelling in a
Confined Urban Environment
E. Ayre, McConnell Dowell Constructors (NZ) Limited
Keywords: Microtunnelling, pipejacking, urban, sewer
Abstract
The Western Interceptor is a 4 kilometre long 1.35 metre internal diameter sewer
running west to east through the centre of the City of Christchurch constructed
using the pipe jacking method. This paper provides an overview of the technical
aspects and some of the challenges encountered. The Western Interceptor is
being constructed using a slurry shield micro tunnel boring machine over 10
drives up to 650 metres in length. The alignment of the tunnel runs through the
centre of the city under one of the main thoroughfares, close to a number of
historic buildings and has the potential to impact upon significant city
infrastructure including the bus station, high voltage distribution power lines,
major sewer, water and telecom infrastructure and also passing under a live
railway line. From an environmental point of view the tunnel passes within one
metre of the bed of the Addington Brook and under the largest park in the city.
The geology the tunnel has encountered is varied, with ground ranging from
cobbles to fine silts and clays with significant amounts of timber encountered,
and a cover which at best was 5 metres and drops to as low as 1.8 metres for
extended periods. Significant guaranties were made at tender time regarding
maximum footprint size, maintaining the traffic flow through city streets and
minimising impact on the city parks. Despite a very tight working environment
and some close scrutiny from residents and authorities the tunnel is progressing
with minimal impact on the community.
INTRODUCTION
The Western Interceptor Stage 2 Sewer forms part of a larger scheme, The Christchurch
Major Sewer Upgrade (MSU). The purpose of the Project is to increase the capacity of
Christchurch City's trunk sewer network by providing two new pipelines - the Western
Interceptor Stage 2 and the Fendalton Duplication. This scheme aims to reduce wet weather
sewer overflows into the city's rivers and to allow for residential growth in new build western
suburbs.
The Western lnterceptor Stage 2 requires approximately 4,000 m of micro tunneled, 1,350
mm diameter gravity sewer, falling from Dalgety Street in the west to Fitzgerald Avenue in
the east. The alignment initially runs under Blenheim Road, through Hagley Park and along St
Asaph Street, roughly parallel to the existing Southern Relief sewer, as shown in figure 1.
The client is Christchurch City Council, the consulting engineer is CH2M BECA Limited
with co consultant GHD. The Contractor is McConnell Dowell Constructors Limited.
The tunnel was constructed over 10 separate drives with lengths up to 650m. 11 Shafts were
constructed for launching and receiving the Tunnel Boring Machine.
Figure 1 – Tunnel Alignment (WI Stage II Contract Drawings)
1. EQUIPMENT
1.1 Tunnel Boring Machine
The Tunnel Boring Machine (TBM) is an up skinned AVN1200T supplied by Herrenknecht
GmbH, figure 2. It comprises two cans, one containing the cutterhead, articulation joint,
hydraulic drive motors, electric control panel and slurry injection reticulation. The second can
contained the hydraulic power pack and oil tank, along with the oil cooling system.
The electric power for the TBM was supplied at 1000v from a surface control cabin, which
also provided power and control for 1 number slurry feed pump and 3 number slurry
discharge pumps.
The TBM was guided using 3 different systems depending on the drive lengths. For the
shorter (less than 400m) straight drives a pit bottom drifting laser and active target unit
mounted in the TBM were used, with a hydrostatic water level system to control the vertical
alignment. Where the drive lengths were longer, and also curved, 2 different gyro theodolite
systems were employed.
One was a Northstar 24(N24) north seeking gyro compass. This was a mechanical gyro that
took a measurement of the angle of the TBM with respect to north when requested. These
measurements were undertaken every 1m to 1.5m. The resultant angles, combined with the
distance travelled measured from a travelling wheel in pit bottom, provided an update to the
machine position. Due to the mechanical nature of this equipment, the machine had to be
stopped, and the hydraulic motor in the TBM turned off to prevent vibrations while the gyro
was taking its measurement. Each measurement generally took around 5 minutes.
Figure 2 Herrenknecht AVN 1200T (Photo R Martin)
The other gyro was an optical Measure While Driving (MWD) north seeking gyro compass
employing a fibre optic bundle that formed a ring laser. With no moving parts, the MWD gyro
did not require the TBM to be shut down to maintain accuracy. Due to nature of the ring laser,
it continually returned a yaw angle, rather than the snapshot the N24 Gyro gave. The position
updates were calculated as above, except that they were calculated every 100mm as opposed
to ever 1 to 1.5m. The other benefit of the system was that the system was calculating as the
TBM advanced. Hence there wasn’t the need to stop for 5 minutes to actually perform the
survey.
The MWD gyro was found to be an order of magnitude more accurate than the N24 gyro,
both systems would produce the required accuracy for the project.
Pitch and roll of the TBM was measured by inclinometers inside of the TBM. This
information, along with the attitude of the articulation joint, measured by extensometers
inside the rams, was fed back to a visualisation screen in the control cabin.
1.2 Separation Plant
With options from various international suppliers, the chosen plant was a PSD plant
comprising a milchem shaker, PSDP450 and an S4E centrifuge. Nominal flow rate of the
plant was 300m3/hr limited by the primary screen. The desander processed 450m3 of fluid
per hour by means of an overflow and recirculation. The design and arrangement of the
desander allowed for half the process capacity to be taken off line for pump cleaning and
maintenance while the other stream was still operating.
Due to the fine sands expected in the ground, the screen sizes were modified across the bed of
the linear motion shaker to effectively capture all particles down to 63 microns without
flooding the screen or getting an overly wet discharge.
The centrifuge would generally be considered to be oversized, but with the possibility of
significant amounts of silts and clays through the alignment of the project the projected down
time through cleaning the slurry was higher than the increased cost of the larger centrifuge
which had a much high throughput when separating either clays or fine sands/silts. The
decision was justified when a motor on the desander dropped a bearing, taking the desander
off line. The soil at the time was fine sands and silts. With the primary screen doing virtually
nothing, the centrifuge was able to keep up with the tunnel production. This occurred when
the TBM was 550m into the first drive, and without the increased capacity there is a
significant chance we would have lost the drive and been forced to sink another shaft to
recover the TBM.
1.3 Bentonite
On a previous project we had gone to market and done an evaluation of different bentonite
types and suppliers, determining the best cost and performance solution. The selected product
was a beneficiated calcium bentonite, with a short hydration time and good yield. A true
sodium bentonite was considered as a superior product, but the additional cost outweighed the
practical benefits.
The mix water was drawn from a potable source and water chemistry tested was carried out to
double check the properties. Soda ash was added to the mix water prior to adding the
bentonite to aid the hydration and viscosity of the slurry. Where an emergency mix was
required, due to hitting an unexpected patch of cobbles and a fast increase in the viscosity was
required, additional polymers were added to the slurry. Generally though, these were not
favoured as they had the potential to bind up the screens on the desanding plant, stopping the
production cycle and wasting valuable polymer.
The feed into the centrifuge, when fine particles needed to be removed or water need to be
recovered from the system, was treated with a coagulant. Generally it was found that a
coagulant was more effective in the ground conditions than a flocculent.
1.4 Noise constraints
With the separation plant located in the city centre, stringent noise constraint limits were set
for the plant and all night time tunnelling activities to prevent disturbance to local residents.
Initially a lined building with closing doors was envisaged for the separation plant, with
enough capacity inside for a shifts production and spoil storage area. This necessitated a very
tight footprint for the separation plant to prevent the building becoming too big and
prohibitively expensive. While these options were being investigated, an industrial plot closer
to the jacking pit became available. This site was better screened from local residents and
therefore only required a noise fence around the plant as opposed to a full enclosure. There
was a multistage noise mitigation plan prepared which was a staged full enclosure for the
separation plant, should field noise measurements show noise reduction be required.
As the plant was to be located in an industrial car park, the footprint available was severely
restricted, as was the available storage for spoil. The result was a muck bay with 1,8m high
walls and still maintaining a shifts full capacity.
1.5 Pipes
The pipes for the project were manufactured in New Zealand using dry cast VT technology.
They had a 1350mm internal diameter and a 1630mm external diameter. Due to the sewer not
running full the majority of the time, the pipes were lined with a 2mm hdpe liner to protect
against hydrogen sulphide attack. The pipes were designed for a 650 tonne axial load.
Compliance with this requirement also ensured that the pipe met all the relevant standard
design criteria for buried pipes.
The joint between the pipes was sealed with an EPDM “sharks tooth” seal that was design to
withstand a test pressure of 1 bar at the factory.
2. Shafts
2.1 Jacking Shafts
The tunnelling operation was carried out from seven separate locations along the alignment.
Three of the shafts were used for drives in two directions, requiring a combined launch and
jacking wall design. Due to the high water table all the shafts were dewatered during
construction, either by well points, or in areas of high water inflow by a submersible pump in
a deep well.
The shafts were all constructed using steel sheet piles and waling frames. Alternative shaft
construction methods, including caissons and CFA piling were investigated, but were
discounted due to the space requirements for either the shaft structure or the equipment itself
that was required.
The main design constraint for three of the jacking sites was that they were located in the road
corridor of St Asaph Street. St. Asaph Street is a four lane one way street which comprises
part of the main one way traffic distribution system around Christchurch City centre. Two
lanes are generally trafficked, with a parking lane either side of the road. Two trafficable lanes
had to be kept open for the duration of the project. This led to a maximum site width of 7.5m,
and an internal shaft width of 4m. Occasional closures of a third land were permitted, but a
full road closure was to be avoided if at all possible.
The width of the launch shafts was constrained by the proximity of services in the street,
including a 100 year old sewer, fibre optic cables, 66kv power lines, and the requirement to
have a crane to lift equipment and pipes into the shaft.
Due to the very limited width, a crawler or mobile crane would not be able to swing round in
the site without swinging into the road or hitting over head power lines. The decision was
made to procure a goliath type gantry crane commissioned specifically for the shafts, figure 3.
A requirement of this crane was that it needed to have a low horizontal profile, a high lift
height to be able to double stack pipes if required and meet the required noise limits. In
addition it needed be easy to dismantle and reassemble for the six times it was moved during
the project.
Figure 3 - Gantry Crane Testing (Photo E. Ayre)
The resulting crane had a total lift of 10 T. This meant the jacking shaft could be completely
fitted out with the crane. It had a clear internal width of 5.6m, but an overall width of only
6.3m, and could be dismantled, moved and reassembled over a weekend.
Capacity to lift the TBM into the shaft was considered, but increasing the crane lift and
footing capacities from 10 to 17 tonnes for just ten lifts was not economic.
When an external crane was required for lifting the TBM, a temporary lane closure was
implemented, requiring the lane to only be closed for twenty minutes at a time.
2.2 Reception Shafts
Five reception shafts were constructed during the scheme. Four of these were straightforward
boxes where the TBM was either recovered into the shaft from both ends, or where we tied
into the existing network.
Shaft 20 in the middle of the project provided more of a challenge as it intersected an existing
750mm sewer line which had a peak wet weather flow rate of 300 l/s. This shaft was enlarged
to encompass a top down manhole on the existing sewer line for over pumping, and the
facility to reinstate the 750 line and shut down the over pumping when the TBM had passed
through the shaft.
3. TUNNEL OPERATIONS
3.1 High consequence services – 66kv and asbestos cement water mains
For over half the alignment, (2200m) the tunnel was in close proximity to a buried 66kv oil
filled power line, crossing underneath it in 3 locations and had shaft sheet piles driven 1m
away from of the cables. Close consultation with the local network provider was required to
optimise alignment, pit locations and physical cable identification allowing us to minimise the
potential risk to this essential service.
Eight hundred metres of the alignment ran directly underneath a 300mm diameter asbestos
cement water ring main which had a feed pressure of 7 bar with vertical clearances down to
900mm at times. Observations from the first drive had shown that there was a significant risk
of forming a sink hole should a large pocket of timber be encountered. The consequence of a
catastrophic breakage of the ring main was estimated to form a crater some 15m long and 3
metres wide in a matter of minutes, with subsequent loss of water supply to the surrounding
area.
To mitigate against this risk, a switching plan was developed whereby a particular section of
the water main was isolated from the rest of the system. A single feed was left open into the
main to enough water flow to cover sprinkler demand for the buildings that were fed off of
the main. A twenty four hour a day emergency contact protocol was established in case
damage to the main occurred. During the construction of the tunnel, a number of timber
clusters were encountered, and the rising main was compromised, but only to the extent of a
crack as opposed to a complete failure.
3.2 Ground conditions
Timber
Timber was identified in the geotechnical baseline report as occurring occasionally during the
drives, with an incidence of one 7.5m long cluster every 1000m of tunnel. During the first
drive of 650m we had evidence of wood coming through the separation plant for over 150m
of the drive. This total is currently over 600m and climbing.
The timber encounters fell into two main groups with one exceptional occurrence. A type 1
incident was where timber was encountered while mining through a sand, gravel and cobble
matrix. In these conditions the timber was firmly held by the ground and was broken up easily
by the head of the TBM. The timber was easily caught on the primary screen of the plant, and
there was less adverse effects on the TBM operation or advance rate.
In a type 2 incident, the timber was encountered while mining through silty sand, sand silt or
a soft clay. Here the timber wasn’t held securely within the ground and it tended to be more
worked by the cutterhead. The timber tended to be of a more fibrous type material. This had
the effect of forming a mat over the inlet ports into the excavation chamber, chocking the
slurry flow and stopping the machine. High pressure water jets were built into the head of the
TBM to aid with the mining of timber. With a number of these jets aimed at the inlets, we
were able to mobilise the mat of timber over the inlet ports, and continue mining. In practice,
this jetting was effective, but caused some ground loss caused by washing out the fine
material in the face. At these times, the ground changes were not immediately visible to the
operator, until there was no material coming off the separation plant. Instead there was a slow
increase in slurry weight. With the volume of slurry in the system being around 100m3, a
small increase in mud weight equated to a significant amount of ground being removed.
When it was apparent that the head was blocking with timber, contingency traffic
management plans were implemented over the TBM head to mitigate the possible formation
of a sink hole or settlement. On a number of occasions the sink hole formation took more than
24 hours to show at the surface, so traffic management was kept in place for a number of
days.
One timber strike almost caused the loss of the TBM. During the mining of a pipe, the ground
changed from a silty sand to clay with small pieces of timber. The pitch of the TBM went
from -3mm/m to -45mm/m in 1.5m of advance as the TBM shoved into a very soft patch,
possibly the root ball of the tree. Maximum steering was applied to the TBM and 1m later the
TBM passed out the other side of the bad timber patch and started to climb. The pitch
finishing at -15mm/m
Cobbles
Mining through cobbles and sands was relatively trouble free. The primary screen took up to
95% of the material out of the slurry. As the drive progressed, keeping the cobbles in
suspension proved a greater challenge as it required greater viscosity in the slurry. This was
further compounded by any change in ground conditions, especially when going from sandy
silt with timber, requiring little in the way of viscosity and material carrying capacities, even
benefitting from a low mud weight to enable the silts to be taken in suspension, to large,
sometimes single sized cobbles which require very high viscosities to keep particles moving.
When this transition happens 600m in by as well, there is up to 100m3 of slurry which needs
to be improved up to enable tunnelling to continue.
Significant delays were encountered when tunnelling through cobbles which were single sized
deposits of an old river bank. These formations would cause blockages in the slurry lines
between the TBM and the first inline pump. Various measures were introduced, including
back flushing of the lines, and stationing someone in the tunnel to reduce the down time of
stripping out the slurry lines. The ultimate answer was to keep the viscosity high with the
addition of bentonite extending polymers.
Silts
Sandy silt and silty sand formed significant amounts of the pipe jack drive. Generally they
would not cause a significant problem to the jacking operation, with the desander keeping the
sand content of the slurry at zero. In addition, the centrifuge controlled mud weight without
the addition of any additives. The primary screen was doing very little work at this point, only
picking up a small pieces of timber that were sometimes present.
During one drive there was a motor failure on the desander shaker deck, taking the desander
off line. During this time, the oversized centrifuge came into its own. With the addition of a
coagulant, the centrifuge could keep up with the tunnelling rate of advance, provided that the
10 hours shifts were adhered to and the 4 hours down time between the shifts could be used to
condition the slurry prior to the next shift starting.
3.3 Natural Hazards
The alignment passes under the Addington Brook in the middle of Hagley Park. The brook
drains into the River Avon and then into the Estuary. The cover from the tunnel alignment to
the bed of the brook was approximately 300mm, and therefore the brook required surcharging
to prevent slurry blowout at the tunnel face. The brook provides significant Christchurch City
storm water capacity, so blocking it completely was not an option. A haul road was required
over the brook to the middle of the park behind, so a crossing point was incorporated into the
brook crossing surcharge. Reject jacking pipes were used to maintain the flow in the brook,
two parallel pipes being installed to match a nearby throttle where the brook passes through
an 1800mm ID culvert.
3.4 Jacking forces
2
1.9
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1.7
1.6
1.5
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1.2
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normalised jacking load T/m2
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Figure 4 Normalised Jacking loads drive 2
3.5 Lubrication
To reduce the skin friction between the TBM and the ground, the annulus of the pipe jack was
filled with a bentonite polymer slurry. This slurry was mixed by a colloidal mixer, and was a
much heavier slurry than that used for the transport of the slurry.
The bentonite was injected through ports cast in the jacking pipes, using an automated system
that controlled injection of bentonite at each port. The volume and pressure at each injection
port could be monitored from the control cabin, and individual ports switched on and off as
required.
This proved very useful in the first drive, as the aftershocks of a recent 7.1 magnitude
earthquake had opened fissures in the ground through which the bentonite could travel to the
surface. These fissures were usually hidden by the flexible asphaltic road pavement. Blow
outs of bentonite would soon become visible when the pavement started to lift, then crack,
releasing bentonite onto the surface of the road. The lubrication system could be disabled in
these specific location from the control cabin to minimise the escape of bentonite to the
surface.
The first of these blowouts occurred inside the barricaded site, so spill kits and measures to
protect the storm water system could be deployed easily, and there was no impact on the
general public. However, with this risk identified, mitigation measures were implemented to
reduce the impact of further blowouts. These measures included having a dedicated bentonite
spill kit available and ready to be deploy, a sucker truck on 24hr call out to clean up any spills
and emergency traffic management plans formalised to protect the traffic and to stop the
bentonite being tracked down the road by wheels.
3.6 Drive through manhole
The Western Interceptor tunnel interfaced with the other half of the project, the Fendalton
Duplication at manhole WIMH24A. To mitigate programme requirements and external
council requirements, manhole 24A was constructed prior to the construction of the pipe jack
in the area. The manhole was constructed with entry and exit eye structures, then the entire
manhole was filled with a weak fill material for the TBM to mine through.
Once finished, the soft fill is to be removed, the pipe broken out and the inside of the manhole
to be benched to the finished requirement.
This presented a significant challenge for the pipe jack alignment as the manhole was 300m
from the launch shaft, and located on the inflexion point of two opposite handed 1500m
radius curves.
4. Natural Disasters
During the course of the Project, Christchurch City was subjected to two significant
earthquake events, a magnitude 7.1 event in Darfield, 30km from the city centre, and a
subsequent 6.3 aftershock 10km from the city centre, figure 5.
Fig. 5 Seismograph trace from the Feb 22 aftershock. (NZ Geonet Project)
The Darfield Earthquake occurred when one shaft was only partially excavated and caused
relatively little disruption to the project works. However one building next to a proposed shaft
suffered significant structural damage, and no assurances could be made about the
susceptibility of the building to withstand the vibrations from sheet piling operations. The
shaft was therefore moved approximately 80m to a better location. This did not delay the
project as the drives were carried out in a different order, allowing time for decisions and
construction.
The subsequent February 22nd aftershock had an epicentre closer to the city and hence had a
much greater impact on the city and the project. Fortunately at the time we were not mining
and the TBM was above ground between drives. Observed effects on the project were as
follows:


An excavated shaft, without the base slab poured, was completely backfilled by
liquefaction, a volume of around 80m3
The already excavated tunnels flooded due to loss of dewatering when the power went
off, and subsequent appropriation of generators by civil defence. Inflow volumes were
calculated in the region of 400 litres per second for the 15 minutes immediately after
the shock.


Pipes on brackets inside the tunnel had subsequently been found in the invert of the
tunnel
Absolute level changes of the shafts and tunnel were in the order of 180 to 300mm
Following this aftershock a risk review of the tunnel project from a safety point of view was
undertaken with the new earthquake information and highlighted risks. The review focused
around the potential for people to be trapped in the tunnel during an earthquake event, or be
drowned by water inundation. The main changes to the project were to shorten the drive
lengths, isolate one section of tunnel from another to cope with the broken water mains, and
upsize the pumping capacity in the shafts to over 400l/s.
Figure 6 Isolation plug in the tunnel (Photo R Martin)
5. CONSCLUSIONS
While ostensibly a 1350 pipejack should be a fairly straightforward project, the geographical
extent, ground conditions, number of stakeholders, proximity to existing assets and numerous
significant earthquakes turned this into a very complex project. In turn these complexities
required careful planning, risk mitigation and innovative solutions to ensure that the project
will be completed to the quality and satisfaction of the client, Christchurch City Council.
6. ACKNOWLEDGMENTS
I would like to thank the McConnell Dowell project staff for their assistance in compiling in
compiling this paper, and Christchurch City Council for allowing the paper to be presented.
Biography
Edward is a Senior Engineer with McConnell Dowell Constructors Ltd. Graduating from
Camborne School of Mines in Cornwall, England, he started tunnelling at the Terminal 5
development at Heathrow Airport working on various TBM’s and underground shotcrete
structures. For the last 5 year he has been working for McConnell Dowell Constructors in
New Zealand on various tunnelling projects, including the then longest pipe jack in
Australasia at 900m long with a 1.8m internal diameter, a 3km long 2.8m internal diameter
Earth Pressure Balance TBM drive on the North Shore in Auckland and currently working on
a 4km long pipe jacked sewer scheme in Christchurch.