General overview of the management and mitigation techniques to address groundwater infiltration within underground rock excavation.

1. Management and Mitigation of Groundwater infiltration in
underground rock excavation
Angelo Indelicato

2. Underground excavation often involves dealing with groundwater infiltration. The
management and mitigation of water inflow is important during tunnelling work, as it
affects both the construction area and the surroundings. This session will focus on the
following key elements:
• Where groundwater comes from
• How to measure water infiltration
• Mitigation measures
• Past Cases
• Conclusions
Synopsys

3. Why is groundwater infiltration important?
Essentially because we want to avoid that this….… turns into this.

4. Groundwater Infiltration Problems
Groundwater-related problems can be direct or indirect and can be subdivided into 5
categories (Loew, S. et al. 2010):
1. Excavation and safety problems;
2. Instability problems;
3. Impacts on surface springs and wetlands;
4. Settlements of sensitive surface layers (soils);
5. Large scale rock mass deformation due to deep tunnel drainage and pore pressure
drawdown.
NB: Designer specification regarding water inflow has to be also included (This do not affect
the stability of the tunnel. However, if the water inflow exceeds the design limit, it must be
addressed.)

5. Where does groundwater come from?
Groundwater finds its own way
inside the underground excavation
through rock joints and faults. It
also reach the shaft/tunnel
excavation through cracks and
crevices present in the shotcrete.
The water infiltration can be also
facilitated by operations such as
probe or blast holes drilling.
50L/min. water inflow during probing

6. Examples of how to measure groundwater infiltration during
rock excavation
From the underground area
(quantitative methods):
• Kibble method
• Pumping method
• Pipe channel
• Probing
• V-notch weir
From the surface area
(qualitative methods):
• Piezometers
Shaft excavation
Tunnel excavation
Shaft & Tunnel

7. Kibble Method
This method consists of using a sealed bucket
(kibble) to collect water from the shaft bottom.
During the weekend, when no operation was
carried out within the shaft, the water pump was
switched off to allow the groundwater to
accumulate at the end of the last excavation.
The water was then pumped into the kibble and
carried up to the surface, where the total
volume was calculated.
At times, more than one kibble was needed to
rid the shaft of water.
Pro: Very effective in estimating the water
inflow in the shaft
Con: No operation can be carried out during
the calculation

8. Example of volume
calculation using
the kibble method
As the water inflow is
higher than the design
limits, the area needs
remedial works

9. Pumping Method
This method is carried out on the sinking stage
inside the shaft and a container of known volume is
used to collect the water.
The water pump is placed inside the container and
the time it takes to pump 100 litres of water into the
container is measured.
Pro: The measuring can be done without switching
the water pump on and off and slowing down
production.
Con: The stage is not fixed. It shifts in height and
prevents the pump from working at a constant rate. If
the stage is very high, the force of gravity will burden
the pump as it draws the water out. This can affect
both the validity of the test and the calculation of the
volume of water infiltration.

10. Example of
pumping
method test

11. Pipe Channel Method
This method uses a catchment pipe ring
around the shaft walls, which is
embedded into the shotcrete layer.
Any water leaking from the shaft wall
surface can be collected within this
structure and subsequently quantified.
Pro: Same as pumping method
Con: The catchment ring only lasts for
a very short time and most of the water
is not collected by this system, making
water inflow calculations imprecise.

12. Probing
The main purpose of probe drilling is to identify potential geological or hydrogeological
hazards. After the initial holes are completed, a hydraulic inflatable packer is installed at 2-
3 metres from the collar and the water inflow rate is calculated. If the water inflow is below
the design criteria, the holes are filled with grout. If the water inflow exceeds the design
criteria, more holes are drilled. This operation is carried out until the last holes (control
holes) have an acceptable water inflow.
Material used to grout holes: colloidal silica, microfine or nanofine cement.
Pro: Very effective in assessing the ground water in both shaft and tunnel.
Con: Expensive in terms of grouting and it takes several shifts to drill and grout the holes
affecting the production cycle.

13. V-notch weir uses a metal slab
with a V-shape opening. Water is
collected in a pool and after the
slab is opened, the water level is
measured so that the water inflow
can be calculated. This type of
measurement is used mainly to
check if the post grouting work has
been effective.
Pro: It is quite cheap and easy
Con: Not always very reliable as
water is used inside the tunnel for
excavation and construction so it
is difficult to distinguish its origin.
V-notch Weir

14. Piezometers
Piezometres are used to monitor changes in the
piezometric head, which indicates changes in
groundwater conditions. Their great advantage is
that they are small in scale and provide useful
information.
Piezometers are normally used together with
settlement markers. They are located in the
buildings and streets adjacent to the underground
excavation. Their levels are measured before
excavation starts and periodically during the
excavation. They do not measure water but they
record any subsidence caused by excavation and
related to groundwater infiltration.
Pro: they provide valuable data regarding any
possible subsidence caused by the underground
work.
Con: They do not give us information about
groundwater infiltration.

15. Mitigation Measures
• Pre-excavation Grouting
• Post Grouting
• Concrete Lining
Example of high pressure water inflow
through shotcreted crown (Zhang C. et al.
2016)

16. Pre-excavation Grouting
If probing shows a water inflow higher than
the client’s established limit, extra holes must
be drilled around the shaft.
They are then filled with grout and they will
form a “fan’ around the excavation area,
reducing the water inflow to an acceptable
level. Once completed, control holes are
drilled to ensure water infiltration is reduced to
a proper level.
Sometimes initial probing shows low levels of
water infiltration and the initial probe holes are
considered enough. But not the entire
excavation area is probed, leaving some
zones untreated. If issues are detected later,
post grout work may be required.
Typical length of probe holes 25m with an angle of 8°

17. Pre-excavation Grouting in Tunnel

19. Post Grouting
This technique is used as remedial work if the previous did not achieve substantial results
in mitigating groundwater infiltrations.
This system involves the injection of grout into semi-horizontal holes drilled in the specific
areas where water infiltration was most severe in order to reduce it.

20. Concrete lining
Concrete lining is a permanent structure
that can be cast-in-place or made by a
precast unit.
The type is chosen according to the
ground conditions of construction areas,
the tunnelling method and the use of the
underground areas.
A layer of waterproof material is placed
between the excavated rock and the
concrete lining. Sometimes other
materials such as gravel and mortar can
be placed around the concrete to facilitate
the movement of the groundwater to the
level below.

21. Hazards related to groundwater mismanagement
Case 1
Loetschberg Tunnel – Switzerland
Tunnel completed in 1912, it is 13.7km
long.
The tunnel was flooded after the
excavation reached the contact
between limestone and alluvium of the
Kander torrent 201m above. Within
seconds, the excavated area filled up
with water, boulders, silt mud and
sand, causing the death of 25 miners.
The main cause of the accident was
the poor knowledge of the
hydrogeologic conditions of the area.

22. Case 2
Seoul Metro Line 5 – Phase 2, South Korea, January 1993
Construction of metro tunnel with drill and blast method
Tunnel collapse after removing spoil and an water inflow of up to
300 Lt/min was recorded.
High groundwater pressure was among the possible causes.
Remedial work included backfilling and grout work.
Sinkhole in Yongdungpo, Seoul from Shin J. et al. 2006

23. Case 3
Ping Lin Pilot Tunnel – Taiwan 1996
The Ping Lin is a 12.9km long tunnel excavated by TBM. During the excavation of the
pilot tunnel in 1996, groundwater infiltration reached 750 Lt./s, causing a severe
collapse with a huge volume of debris flow. The TBM was trapped with the consequent
delay of work. Freeing the TBM and water sealing/consolidating grouting lasted several
months, considerably affecting the budget (Tseng D. et al. 2001, Kipko E. et al. 1998).
Plan view of the 10th stoppage of pilot tunnel TBM, Tseng D. et al. 2001

24. Case 4
HATS STAGE 1 – Tunnel C and F, Hong Kong 1995 - 2000
Both tunnel have been excavated using TBM. Tunnel C is approximately 5.3km and
Tunnel F is 3.57km in length. During the first phase of excavation a water inflow of
1400lt/min was recorded. Tunnel excavation was stopped for almost 2 years without any
concern about the water in the tunnel. A settlement up to 750mm affected building within
a 1.5km radius occurred as result.
General ground lowering at TKOIE (Endicott J. 2013)

25. Conclusion
The combination of the methods above
manage and mitigate the groundwater
infiltration within shafts, tunnels and
generally all underground excavations.
Keeping in mind Burland’s triangle and its
principle, we can say that:
1. ground investigation can help to identify
any potential hazards, generating
ground models and helping with the
design.
2. Field tests and observations aid in
verifying the hydrogeological model and
the relationship with the actual ground
profile.
Important: A contingency plan is also
considered wise, in case of high water
infiltration exceeding the design or more
serious issues.
Idealisation,
conceptual, physical
or analytical model
Ground
profile
Appropriat
e model
Observed
behaviour
Precedent
Experience
Ground exploration
and description
Observation,
measurement,
lab & field testing

26. References:
• Benton I. 2015 - A comparative engineering geological study of HATS stage 1 Tunnels C and F, Hong Kong
University, MSc Dissertation;
• Indelicato A. 2012 – Management & Mitigation of Groundwater within Deep Excavation Shaft – The HATS2A
project experience in 32nd Annual Seminar, Geotechnical Division, Hong Kong Institute of Engineers, pp.111-
117.
• Kipko E., Spichak Y., Pozolov Y. 1998 – Water Sealing of Fault Zones at Ping Lin Pilot Tunnel in Taiwan in
Proceedings of International Mine Water Association Symposium, Johannesburg, South Africa, 1998, pp.141-
148.
• Loew S., Barla G. and Diederichs M. 2010 - Keynote Paper: Engineering Geology of Alpine Tunnels: Past,
Present and Future. In Proceedings of the International Association of Engineering Geologists. Auckland, New
Zealand, September 2010, 34pgs.
• Shin J., Lee I., Lee Y., Shin H. 2006 – Lessons from Serial Tunnel Collapse during Construction of the Seoul
Subway Line 5 in Proceedings of the World Tunnel Congress and 32nd ITA Assembly, Seoul, Korea, April
2006, pp. 22-27.
• Stratskraba V. 1984 – Groundwater as a nuisance in International Journal of Mine Water, Vol.3 (1), pp.25-40.
• Tseng D., Tsai B., Chang L. 2001 – A case study on ground treatment for a rock tunnel with high groundwater
ingression in Taiwan in Tunnelling and Underground Space Technology, Vol.16, Issue 3, pp.175-183.
• Zhang C., Liu N., Chu W. 2016 – Key technologies and risk management of deep tunnel construction at
Jinping II hydropower station in Journal of Rock Mechanics and Geotechnical Engineering, Vol. 8, pp.499-512.