Groundwater Control Techniques for Tunnelling and Shaft Sinking

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GROUNDWATER CONTROL TECHNIQUES FOR TUNNELLING AND SHAFT SINKING

Dr Martin PreenePreene Groundwater ConsultingMay 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

DefinitionGroundwater 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 orGroundwater 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 sandPorous and fractured rockLow to very low permeability:Silt, clayUnfissured 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


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EJECTOR WELLS


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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


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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: dewateringwells 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 wallsfrom 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 excavatorCommon European practice

Soil-bentonite slurry wall constructed by long reach excavatorCommon 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 nodisturbance ofsoil structure

Rock grouting –little or nodisturbance ofrock 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 jettinghead above ground levelSource: 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 ofliquid nitrogen (LN) freezingsystem

On-site liquid nitrogen (LN) storage tankreceiving 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)


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RANGE OF APPLICATION OF METHODS

Low permeability (silts) High permeability (gravels)


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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 loweredabove 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 pumpingwithin anunderpinned 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 shaftwith advancedewatering byexternal 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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• 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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• 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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• 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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Plan view

Section

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Examples of wellpoints or drains penetratingthrough segmental tunnel linings

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Wellpoint pumpFor tunnel cross-passagedewatering system

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GROUNDWATER CONTROL TECHNIQUES FOR TUNNELLING AND SHAFT SINKING

Dr Martin PreenePreene Groundwater ConsultingMay 2015

Groundwater Control Techniques for Tunnelling and Shaft Sinking

Engineering

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