1. The document discusses risk management strategies for tunneling projects, using the Vancouver Canada Line tunnel as a case study. It faced risks from unstable soil and proximity to buildings. 2. To mitigate risks to nearby towers, tunnelers lowered the alignment, increased spacing between tunnels, improved soil conditioning, and located maintenance areas in safer locations. These steps allowed tunneling without damaging the towers. 3. Proper risk management through measures like controlling excavation parameters, monitoring ground movements, and adapting tunnel design based on geological conditions helped complete the challenging tunneling project successfully and safely.

1. 1
I would like present the realization of Vancouver Canada line as example
of risk management for urban tunneling.
The tunnel has been realized by Seli in Vancouver downtown from 2005
to 2008.

2. 2
Some basic concepts of risk management.
The objectives are: to increase the safety during the excavation and to attain the job
targets which are usually time and costs.
The definition of risk is damage times probability.
One can reduce the risk intervening on the damages or on the probability or on both.
The residual risk should be accepted.
A particular case of high, often unacceptable damage, with extremely low probability
is here indicated as Sword of Damocles. As example we can think to the death of the
inhabitants of a building that must be always avoided.
From the types of damages I want to mention the loss of confidence from the
Authority and the public opinion because as we all know it can be really complicated
completing an excavation after unforeseen events, even if without damages.

3. 3
In order to make a risk management we have to: establish al the facts necessary to
identify the hazard scenarios. So we need to know not only the geology and the
tunnel geometry, but also the structural details of the objects interested by the
excavation.
Then it’s fundamental to understand the mechanism governing the hazard in order to
design the appropriate measures.

4. 4
Here we have some photos just to get an idea of the possible damages
due to excessive settlements or to the formation of a sinkhole following
the collapse of the face.

5. 5
Tunneling in soil, cohesive or not, surface settlement cannot be
completely avoid, but using the proper procedures can be mitigated and
maintained within acceptable range.
Indeed uncontrolled excavation may result in excessive subsidence
mainly due to over excavation, which means that a larger volume of
material is excavated by the TBM compared to the required volume for
the tunnel.
In certain cases exceptional over excavation, due for example to
unexpected objects or soils, can also produce a sinkholes on the
surface.
Long term settlements due to water table drawdown must also be
avoided in urban tunneling.
Finally the most dangerous phenomena is the collapse of the face for
insufficient or not uniform support pressure, that often brings to the
formation of a crater on the surface (without warning).

6. 6
Here some data of the twin EPB tunnel of Vancouver Canada Line.
The tunnels have an external diameter of 6m and an interaxis of around
12m.
The geology is characterized by the glacial till, composed by a clay, silt
and sand matrix and the substratum of sedimentary rock with favorable
strength parameters.
The water table is 10 to 15m over the tunnel crown.
In the Till was expected to encounter boulders up to 2m of diameter and
saturated sand lenses.
This presentation is going to focus on two stretch, the first one consist in
crossing a fiord, named False Creek, and the second one in
underpassing with reduced overburden the downtown.

7. 7
The main hazard was to flood the tunnel during the excavation.
Such an event is only possible under the combination of the 2 conditions:
• First it’s necessary an hydraulic connection between the fiord and the face of the
tunnel, for example by rock faults, abandoned unfilled boreholes or the presence
of permeable layers in the till.
• Second the water together with the soil can flow freely through the screw conveyor
only in if the permeability of the muck is to high.
In this condition a proper operation mode of the EPB machine, that means right
conditioning, sufficient support pressure and continuous observation of the
parameters and of the muck, can exclude the possibility of flooding the tunnel
irrespective of the possible presence of a hydraulic connection.

8. 8
A nice picture of the Brava Towers

9. 9
Here we have the plan, the longitudinal and transversal sections of the
original alignment of the tunnel.
As one can see we had really difficult situation, with the tunnels
underpassing the towers very close to the footings and excavating in the
till with about 15m of hydraulic pressure.

10. 10
Based on the geology the main hazards crossing the Till are:
• The presence of high strength boulders that can induce over excavation when
extracted by the TBM head from the soil and also not being destroyed by the
cutting tools they remain in the chamber requiring maintenance and disturbing
the formation of a proper muck.
• The possibility to encounter sand lenses under high hydraulic pressure that could
flow uncontrolled though the screw conveyor causing the collapse of the face.
• The occurrence of external object that similarly to the boulders may damage the
cutting tools.
The most important remedial to reduce the effect of such hazards are:
• Applying the right conditioning to the muck in order to obtain the correct plasticity
and permeability. This is very important since these parameters influence the
capacity of the muck to exert a uniform pressure on the face and to avoid the
inflow of water in the chamber.
• Controlling continuously the TBM parameters such: face pressure, thrust and
torque, to identify quickly changes in ground conditions and therefore to react
properly.
• Finally we applyed a constructive remedial, a thick matter pump that avoided
uncontrolled flow through the screw conveyor reducing the possibility of face
collapse in case of sand lenses.
Screw conveyor: it’s the part of the EPB that controlling the outflow of the material in
form of muck from the chamber, regulate the pressure exerted on the face.

11. 11
Despite all the proposed measures, considering the small distance
between the crown and the footing of the Towers, the first suggestion
was to change the alignment in order to avoid the building.
Unfortunately was only possible to lower the tunnel of 8m.
To cope with potential sand lenses under high hydraulic pressure and
increase the face stability underpassing the towers, higher face pressure
was applied.
Finally monitoring the ground surface before the Towers and the building
provided the information to confirm the design and to intervene to avoid
severe damages.

12. 12
200m before the Brava Towers occurred a sudden change of the
geological conditions, instead of the Till the tunnel encountered a highly
permeable sand layer.
In this conditions was impossible to create an impermeable cake that
allows any intervention in the chamber for ordinary maintenance.
In fact when first it was attempted the presence of a local collapse was
discovered and for stability reasons the chamber was filled again and the
excavation resumed.
The impossibility to replace worn cutting tools involved the risk of TBM
stoppage once reached the mixed face condition or in case of big
boulders, with the difficulty or quite impossibility to intervene for
maintenance underpassing the Towers..
In the end with adequate control of the TBM parameters, they were able
to complete the stretch even if the achieved a slow advancement speed
and major settlements were recorded at the Tower footing.

13. 13
Based on the experience from the 1° drive, it was necessary to reduce
the risk of new settlement that would have caused damage to the
Towers.
The first remedial measure was to increase the interaxis of the tunnels to
reduce the possibility of adverse interaction between the 2 tubes.
A new proper conditioning was studied able to lower the muck
permeability and therefore limit the inflow of water in the chamber.
Finally new boreholes were made to reassess the geology and on this
basis a proper zone was located for maintenance before underpassing
the tower.

14. 14
The new conditioning was partially effective, allowing to refurbish the
cutting head in the located zones.
With cutting tools in good condition the operator was able maintain the
design face support pressure continuously, thus balancing the
hydrostatic pressure and avoiding any inflow in the chamber of extra
ground or water.
This proper execution brought null settlement avoiding any damages to
the Towers.
In the longitudinal section we can see the reassessed geology and the
lowering of the alignment.
One can only imagine what could have happened without a proper risk
management to such high and inhabited building.