Underground Construction Technology · Lugeon test Prior to the beginning of the test a maximum...

2015.05.08 1Tunnelling and Underground

Construction Technology

Underground Construction

Technology

Course Lectures

Part 4.1 – Groundwater control and

Waterproofing techniques

Dr Ákos TÓTH



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



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



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

…



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



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”.



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



Pumping tests



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



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




Water inflows during construction




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




Tunnel Durability problems

According to inspection data, the ingress of water is one of the main factors

that compromise serviceability and durability of tunnels




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



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



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

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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






Danger of environmental impacts:

Depletion of Natural Springs and Thermal Water Springs






Danger of impact on existing infrastructures: Settlements






Clogging problems in drainage system due to

calcareous concretions





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

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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





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





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



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