Groundwater Control Techniques for Tunnelling and Shaft Sinking

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GROUNDWATER CONTROL TECHNIQUES FOR
TUNNELLING AND SHAFT SINKING
Dr Martin Preene
Preene Groundwater Consulting
May 2015

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GROUNDWATER CONTROL TECHNIQUES
Synopsis
• Background and definitions
• Groundwater control techniques:
– by pumping
– by exclusion
• Guidelines for selecting the best technique
• Some tunnelling and shaft sinking problems
• A bit of dewatering design and philosophy

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PRACTICE PROFILE
Preene Groundwater Consulting is the Professional Practice
of Dr Martin Preene and provides specialist advice and design
services in the fields of dewatering, groundwater engineering
and hydrogeology to clients worldwide
Dr Martin Preene has more than 25 years’ experience on
projects worldwide in the investigation, design, installation
and operation of groundwater control and dewatering
systems. He is widely published on dewatering and
groundwater control and is the author of the UK industry
guidance on dewatering (CIRIA Report C515 Groundwater
Control Design and Practice) as well as a dewatering text book
(Groundwater Lowering in Construction: A Practical Guide to
Dewatering)

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GROUNDWATER CONTROL
Definition
Groundwater Control
“The process of temporarily dealing with groundwater, to allow
excavations to be made in dry and stable conditions below natural
groundwater level”
May be known as Dewatering or Construction Dewatering or
Groundwater Lowering

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GROUNDWATER CONTROL
Additional Definitions:
Permeability = coefficient of permeability = hydraulic conductivity
(units of m/s). Typically given the symbol k
High to moderate permeability:
Gravel, sand and gravel, sand, silty sand
Porous and fractured rock
Low to very low permeability:
Silt, clay
Unfissured and ‘tight’ rock
Drawdown = amount of vertical lowering of groundwater level due to
pumping (units of metres)

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GROUNDWATER CONTROL
Two main philosophies of groundwater control:
• Pumping: Arrays of wells or sumps (construction
dewatering)
• Exclusion: Physical cut-off walls

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GROUNDWATER CONTROL BY PUMPING
• Typically relies on
arrays or groups of
pumped wells and/or
sumps, acting
together, to lower
groundwater levels
over a wide area
• Commonly known as
dewatering

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GROUNDWATER CONTROL BY PUMPING
Available Techniques
• Sump pumping
• Wellpoints
• Deepwells
• Ejector wells
• Relief wells
• Horizontal wells
• Collector wells
• Electro-osmosis
• Artificial recharge

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SUMP PUMPING
Sump pumping
during construction
of a large diameter
shaft in Sherwood
Sandstone

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WELLPOINTS
From CIRIA Report
C515 (2000):
Groundwater Control:
Design and Practice

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DEEPWELLS
From CIRIA Report
C515 (2000):
Groundwater Control:
Design and Practice

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EJECTOR WELLS
From CIRIA Report
C515 (2000):
Groundwater Control:
Design and Practice

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RELIEF WELLS

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HORIZONTAL (HDD) WELLS

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• Shaft or caisson
constructed
• Perforated ‘laterals’
driven out from
shaft
• One pump (located
in the shaft) can
achieve high yields,
can be much more
widely spaced than
conventional
deepwells
COLLECTOR WELLS

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ELECTRO-OSMOSIS
From CIRIA Report C515
(2000): Groundwater
Control: Design and
Practice

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ARTIFICIAL RECHARGE

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EXCLUSION: VERTICAL CUT-OFF WALLS
Cut-off walls penetrate
into underlying low
permeability stratum

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EXCLUSION: CUT-OFF WALLS AND PUMPED WELLS
Cut-off walls do not reach deep
impermeable stratum: dewatering
wells are needed

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EXCLUSION: VERTICAL CUT-OFF AND HORIZONTAL BARRIERS
Cut-off walls do not reach deep
impermeable stratum:
horizontal barrier is used to exclude
groundwater from base

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JOINTS AND LEAKS IN CUT-OFF WALLS
Walls installed as panels or sections
Walls installed
as line of
overlapping
columns
Walls or barriers installed
as multiple line of
overlapping columns

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ALTERNATIVE GEOMETRIES OF GROUNDWATER BARRIERS

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EXCLUSION TECHNIQUES
• Displacement barriers
– Steel sheet-piles
• Excavated barriers
– Concrete diaphragm walls
– Bored pile walls (secant pile walls and contiguous pile walls)
– Bentonite slurry walls and trenches
• Injected barriers
– Permeation grouting
– Rock grouting
– Jet grouting
– Mix-in-place methods
• Artificial ground freezing
• Compressed air (for tunnels and shafts) and full face TBMs

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STEEL SHEET-PILING
Circular sheet-
pile cofferdam
with concrete
walings

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CONCRETE DIAPHRAGM WALLS
Circular
concrete
diaphragm
wall

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CONCRETE DIAPHRAGM WALLS
Rope operated
diaphragm wall grab
Construction sequence for diaphragm walls
from Woodward (2005): An Introduction to
Geotechnical Processes
Source: Bachy Soletanche
Rockmill diaphragm
wall cutter (hydromill
or Hydrofraise)
Source: Cementation Skanska

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BORED PILE WALLS
Contiguous pile wall – concrete piles installed at a spacing of more than one pile diameter
Secant pile wall – overlapping concrete piles installed at a spacing of less than one pile diameter

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BENTONITE SLURRY WALLS
Excavation of slurry trenches can be by long
reach backhoe down to 15 to 25 m.
Deeper trenches are typically excavated by
clamshell grabs or hydromills

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BENTONITE SLURRY WALLS
Bentonite-cement slurry wall
constructed by long reach
excavator
Common European practice
Soil-bentonite slurry wall
constructed by long reach
excavator
Common North American
practice

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GROUTING
Definition:
• Grouting is the process of controlled injection of a
fluid (grout) into the pores (in soil) or fissures (in
rock) of the ground, where the grout sets and
changes the properties of the in-situ material,
typically by reducing permeability and increasing
strength

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GROUTING IN SOILS AND ROCKS
Permeation grouting
(in soils) – little or no
disturbance of
soil structure
Rock
grouting –
little or no
disturbance of
rock structure

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GROUTING
• Most grouts are suspensions of particles in water (with
other additives). Cement-based grouts are the most
common type used for groundwater control
• The penetration distance of grout into soil and rock is
controlled by the relative sizes of the grout particles
and the soil or rock openings. Distance of penetration
is often very limited, unless the soil is very porous or
the rock fissures are very open

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GROUTING
Indicative grout types
For different types of soil

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JET GROUTING
Structure of
soils or soft
rocks is
disrupted to
create
overlapping
columns of
mixed grout
and disturbed
in-situ material

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JET GROUTING
Jet grouting rig operating with jetting
head above ground level
Source: Keller Geotechnique
Jet grouting systems from
Woodward (2005): An
Introduction to
Geotechnical Processes

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ARTIFICIAL GROUND FREEZING

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ARTIFICIAL GROUND FREEZING
Artificial ground
freezing system around
a shaft
Source: British
Drilling and
Freezing Co. Ltd

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ARTIFICIAL GROUND FREEZING (BRINE)
AGF using brine circulation Portable brine freeze plant – This freeze plant is driven by a
180-kW electric motor. The output is 166 320 kcal/h when
evaporating at −37.5°C
Source: British Drilling and Freezing Co. Ltd

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ARTIFICIAL GROUND FREEZING (LN)
Schematic diagram of
liquid nitrogen (LN) freezing
system
On-site liquid nitrogen (LN) storage tank
receiving a LN delivery by road tanker

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RANGE OF APPLICATION OF METHODS
Amount of
lowering of
groundwater
level
Low permeability (silts) High permeability (gravels)
From CIRIA Report
C515 (2000):
Groundwater
Control: Design
and Practice

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RANGE OF APPLICATION OF METHODS
Low permeability (silts) High permeability (gravels)
From CIRIA Report
C515 (2000):
Groundwater
Control: Design
and Practice

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SOME TUNNELLING AND SHAFT SINKING PROBLEMS
There are some interesting problems and challenges associated
with tunnelling and shaft sinking projects:
• The tunnel as a drain
• Running sand
• Tunnelling without ‘dewatering’
• Advance dewatering of tunnels
• Cross-passage construction

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THE TUNNEL AS A DRAIN
• A tunnel being constructed
with an open face will act as a
drain and water will enter the
tunnel
• If rates of groundwater inflow
are manageable, and face
instability is not a concern (e.g.
in rock) then this can be a
viable method of ‘groundwater
control’
• Inflows can be reduced by
grouting ahead of the tunnel

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THE TUNNEL AS A DRAIN
• Geometry of the groundwater flow regime can be more
complex in long section
Example of segmentally lined tunnel with open face shield
Direction of progress
Groundwater flow
Groundwater level lowered
above working face

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RUNNING SAND
• Running sand is often mentioned
in relation to ‘bad ground’ in
tunnelling and shaft sinking
• It is not a type of material, it is a
state in which a granular material
can exist, when pore water
pressures are high and the
material strength becomes very
low
• Dewatering can lower pore water
pressures and transform material
into more stable ground
Running sand in the base of a shaft

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RUNNING SAND
s‘ = s - u
Soil shear strength
t = s’tanf’
Effective stress = total stress - pore water pressure
Groundwater
flow
Sump pumping
within an
underpinned shaft

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RUNNING SAND
Dewatering used to lower groundwater levels and
prevent running sand during shaft construction in
Glacial Sand deposits
Groundwater
flow
Underpinned shaft
with advance
dewatering by
external wells

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TUNNELLING WITHOUT DEWATERING
• On many projects shafts or vertical structures may be
dewatered, but the tunnel itself is not dewatered
directly, even where it is below groundwater level
• Tunnelling can still be carried out in a ‘shirt sleeve
environment’
• This is the result of a groundwater exclusion
approach
• This can be achieved by compressed air working or
full face TBMs (EPB or slurry)

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COMPRESSED AIR WORKING
1 bar air pressure
approximates to
10 m head of water

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COMPRESSED AIR WORKING
• Compressed air working for tunnelling was developed in the
late 19th century
• Up until the 1980s and 1990s compressed air was used
relatively widely in the UK to allow hand or mechanised
excavation below groundwater level using open face shields
• There are health risks associated with compressed air working
(decompression sickness, bone necrosis)
• Compressed air working is now largely limited to short term
use such as interventions to access the front of a TBM to
change cutters mid-drive or to deal with obstructions

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FULL FACE TBMS
TBM exposed
in cofferdam

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ADVANCE DEWATERING OF TUNNELS
• While shafts and tunnel portals are routinely dewatered, it is rare to
carry out advance dewatering for tunnel drives themselves
• This may be due to lack of surface access for wells, or because
tunnelling methods (e.g. TBMs) do not require it
• However, even if dewatering of tunnel drives is not ‘necessary’
there can be operational and efficiency advantages from
dewatering (or depressurisation) of tunnel drives:
– Improved production rates
– Reduced moisture content of spoil
– Easier (depressurised) conditions for cross-passage construction and
TBM cutter head maintenance

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ADVANCE DEWATERING OF TUNNELS
• On the Jubilee Line Extension (JLE) project in London
in the late 1990s a de-facto advance dewatering
system in the Chalk and Basal Sands was adopted
when the drawdown effect of dewatered
neighbouring shafts interacted
• This allowed the existing TBMs to operate in open
mode (rather than closed mode), thereby improving
tunnel production

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ADVANCE DEWATERING OF TUNNELS
• The Channel Tunnel Rail Link (CTRL) London Running Tunnels took this
a stage further in the early 2000s and developed a planned advance
dewatering system to depressurise the Chalk and Basal Sands.
• 39 wells at 22 locations, pumping up to 700 l/s
• This highlighted some of the challenges of advance dewatering:
– The Project Client had to purchase parcels of land on which to locate the
wells
– The local sewer network could not cope with the pumped dewatering flow
rate, so a 3.5 km long water disposal main (up to 600 mm diameter) had to
be constructed below the streets
• Some of the wells were installed to water industry standards and were
subsequently adopted by Thames Water

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CROSS-PASSAGE CONSTRUCTION
• Many transportation tunnels are twin bore, and
require cross-passages to be constructed periodically
along the route for access, maintenance and
ventilation
• Other headings or tunnel enlargements may be
needed at shafts and stations
• If the tunnels are constructed by full-face TBMs, then
the cross-passages may be the only explicit
groundwater control required for the tunnels

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CROSS-PASSAGE CONSTRUCTION
• The geometry can be difficult, short drives between
tunnels, often in poorly investigated areas
• Groundwater exclusion strategies can be attractive –
grouting, artificial ground freezing
• Groundwater depressurisation – radial wells – can also
be used. The wells will flow naturally into the tunnel, but
usually need to be pumped
• Challenges relate to drilling out through tunnel lining for
wells, including sub-horizontally and upwards

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CROSS-PASSAGE CONSTRUCTION
Plan view
Section

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CROSS-PASSAGE CONSTRUCTION
Examples of wellpoints or drains penetrating
through segmental tunnel linings

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CROSS-PASSAGE CONSTRUCTION
Wellpoint pump
For tunnel
cross-passage
dewatering system

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GROUNDWATER CONTROL TECHNIQUES FOR
TUNNELLING AND SHAFT SINKING
Dr Martin Preene
Preene Groundwater Consulting
May 2015