underground construction technology · lugeon test prior to the beginning of the test a maximum...
TRANSCRIPT
2015.05.08 1Tunnelling and Underground
Construction Technology
Underground Construction
Technology
Course Lectures
Part 4.1 – Groundwater control and
Waterproofing techniques
Dr Ákos TÓTH
2015.05.08 2Tunnelling and Underground
Construction Technology
Porosity of rocks
1. Primary porosity
Unjointed rock is porous, but usually it has a much smaller porosity
than soils. Some porosity values are:
-Magmatic and Metamorphic rocks: 2%
-Sandstones: 1-5%
-Sediment slates: 5-20%
-Soft limestone: 20-50%
The pores can be connected or unconnected. The existence of
connected pores will enable water to flow through the rock mass
2. Secondary porosity
It represents the volume of the open joints, and will provide large
channels for the water to flow through the rock mass
2015.05.08 3Tunnelling and Underground
Construction Technology
Permeability of rocks
In rock, the principle of effective stresses also applies
σ = σ’ + p
In rock, the Darcy’s law also applies, and therefore, the permeability of
the rock mass can be identified by the so-called hydraulic conductivity
K
v = K * I
In rock, most of the hydraulic conductivity is due to the secondary
porosity (joints and fractures within the rock mass), the primary
porosity playing a minor role
2015.05.08 4Tunnelling and Underground
Construction Technology
Permability measurements
In order to measure the hydraulic conductivity of the rock mass K,
different methods exist:
Lugeon tests (single or double-packer)
Pumping tests
…
2015.05.08 5Tunnelling and Underground
Construction Technology
Lugeon test (1933)
• Constant head type test
that takes place in an
isolated portion of a
borehole
• Water at constant
pressure is injected into
the rock mass through a
slotted pipe bounded by
pneumatic packers
2015.05.08 6Tunnelling and Underground
Construction Technology
Lugeon test
Prior to the beginning of the test a maximum test pressure (PMAX) is
defined. PMAX is chosen such that it does not exceed the confinement
stress (σ3) expected at the depth where the test is being conducted, thus
avoiding the development of hydraulic fracturing or hydraulic jacking. As a
rule of thumb PMAX is equal to D = minimum ground coverage – depth in
the case of a vertical boring in a flat site or minimum lateral coverage in the
case of a test conducted in a hillside.
The test is conducted in five stages, with a particular water pressure
magnitude associated with each stage. A single stage consists of keeping a
constant water pressure at the test interval for 10 minutes by pumping as
much water as required. The first stage is held at a low water pressure,
increasing the pressure in each subsequent stage until reaching PMAX.
Once PMAX is reached, pressures are decreased following the same
pressure stages used on the way up, thus describing a “pressure loop”.
2015.05.08 7Tunnelling and Underground
Construction Technology
Lugeon test
During the execution of each stage, both water pressure (P) and flow rate (q) values
are recorded every minute.
Average values for P and q are then used to compute the hydraulic conductivity for
each stage.
The hydraulic conductivity is expressed in terms of Lugeon value hydraulic
conductivity required to achieve a flow rate of 1 l/min / m of test interval under a
reference water pressure P0= 1 MPa
2015.05.08 8Tunnelling and Underground
Construction Technology
Pumping tests
2015.05.08 9Tunnelling and Underground
Construction Technology
Pumping tests
The tests consists in applying a stress to an aquifer by extracting
groundwater from a pumping well and measuring the aquifer response to that
stress by monitoring drawdown as a function of time (24-72h)
It can be applied by single or multi –wells.
These tests are carried out to determine:
- how much groundwater can be extracted (well efficiency)
- hydraulic properties of the aquifer
- spatial effects (influence radius of the well)
- suitable depth of pump
- water quality and variability with time
2015.05.08 10Tunnelling and Underground
Construction Technology
Groundwater control – Why?
Water inflows during construction
Heading inflows occur when a
water-bearing zone is
penetrated during tunnelling.
The risks are immediate
flooding and ground collapse
associated with water ingress.
Critical when the tunnel is
headed downhill or in shafts
The flow rate can be very high
(up to 1000 l/s), but usually
slows down quickly as the
water stored is depleted
2015.05.08 11Tunnelling and Underground
Construction Technology
Groundwater control – Why?
Water inflows during construction
2015.05.08 12Tunnelling and Underground
Construction Technology
Groundwater control – Why?
In some kind of rocks (limestone,
gypsum, etc) the water can dissolve
minerals of the rock.
If the water does not move, the
dissolution process stops as soon as
the saturation concentration is
reached
If water flows, the dissolution
process continues, leading to the
formation of karsts or cavities within
the rock mass
Karsts encountered
2015.05.08 13Tunnelling and Underground
Construction Technology
Groundwater control – Why?
Tunnel Durability problems
According to inspection data, the ingress of water is one of the main factors
that compromise serviceability and durability of tunnels
2015.05.08 14Tunnelling and Underground
Construction Technology
Groundwater control – Why?
Other
- Sub Sea Tunnelling (or below lakes and rivers)
- Tunnelling down slope (water collecting at the face)
- Tunnelling through settlement-sensitive areas (urban tunnelling)
- Environmental protection (depletion of natural spring, destruction of
ecosystems, etc)
- Poor ground stability reasons
- etc
2015.05.08 15Tunnelling and Underground
Construction Technology
Injections
A countermeasure to prevent water inflows when tunnelling is grouting the
water carrying joints using cement mortars with up to 10% sodium
silicate or polyurethane foams
A successful grouting may require the reduction of the water inrush by
pressure relief
In cases where the water velocity is too high, the bonding can be unable to
take place
This technique can also be used for filling karsts (cavities) and fault zones
with high permeability
2015.05.08 16Tunnelling and Underground
Construction Technology
Drainage and “umbrella solution”
Above the groundwater table, a tunnel has to be
protected against downwards percolating water.
This is achieved with a so-called « umbrella »
solution, by means of a crown + sides
waterproofing and a drainage system that
collects the water at both sides of the tunnel
Drainage affects the distribution of hydraulic head
by attracting groundwater and relieving the lining
from hydrostatic pressure. The groundwater is
then collected and discharged
Drainage must be achieved in a permanent way
and maintenance must always be possible
2015.05.08 17Tunnelling and Underground
Construction Technology
Drainage and “umbrella solution”
The drainage path
1. Groundwater penetrates the shotcrete shell through fissures and
ad hoc bored holes
2. Water reaches the interface drainage systems, placed between
shotcrete and concrete lining/waterproofing lining. Higher water
discharges can be caught with separate pipes embedded in the
shotcrete. Water infiltrated from the crown and the sides of the
tunnel is guided downwards to the longitudinal drainage pipes, at
both sides.
3. Transversal slots, which guide the groundwater from the side
pipes to the main collector underneath the carriageway
4. Longitudinal collection of water by means of a granular filter with
perforated pipe
2015.05.08 18Tunnelling and Underground
Construction Technology
Drainage and “umbrella solution”
2015.05.08 19Tunnelling and Underground
Construction Technology
Drainage and “umbrella solution”
2015.05.08 20Tunnelling and Underground
Construction Technology
Drainage and “umbrella solution”
2015.05.08 21Tunnelling and Underground
Construction Technology
Drainage and “umbrella solution”
Danger of environmental impacts:
Depletion of Natural Springs and Thermal Water Springs
2015.05.08 22Tunnelling and Underground
Construction Technology
Drainage and “umbrella solution”
Danger of impact on existing infrastructures: Settlements
2015.05.08 23Tunnelling and Underground
Construction Technology
Drainage and “umbrella solution”
Clogging problems in drainage system due to
calcareous concretions
2015.05.08 24Tunnelling and Underground
Construction Technology
Waterproofing: « Submarine Solution »
When the tunnel is below the groundwater table, groundwater is
pressurized so that an all-embracing waterproofing must be applied
- Water tight concrete: Water pressure <3 bars
- Watertight membranes: Water pressure of 3 to 15 bars
- Grouting: Used in addition, when water pressure is higher than 15 bars
2015.05.08 25Tunnelling and Underground
Construction Technology
Water tight concrete
A properly fabricated cast concrete is watertight if some
conditions are fulfilled (water to cement ratio, grain size, thickness
of shell, etc)
Even in the case of a theoretically watertight concrete, the permeability
can increase if fissures appear due to large shrinkage, hydration heat
dissipation, etc
Longitudinal reinforcement reduces the spacing and the width of fissures,
but does not avoid them.
It is important to keep short sections of concreting but also trying to
minimise the number of joints (in practice, compromise at 12-20 m)
The shotcrete and concrete must be separated with a foil
The advantage of concrete waterproofing is that leakages are easily
localised
2015.05.08 26Tunnelling and Underground
Construction Technology
Water tight membranes
Geosynthetics applied for waterproofing,
mounted directly on the rough concrete
lining, with a geotextile between them to
prevent tearing and damage of the
geomembranes. They are welded to each
other.
Polyester should not be used as it can be
destroyed by hydrolysis
PVC should not be used, as it is toxic
when burnt
2015.05.08 27Tunnelling and Underground
Construction Technology
Grouting
When very large water pressures and/or very high water inrush
velocity are expected, grouting techniques should be applied in
addition. In this way, the created « reinforced and
impermeable » vault of rock will withstand the water pressures
(Alpine tunnels)
Grouting is performed in advance, so that the tunnel stretch is
previously treated before it is excavated