Prof Eivind GROV

1SINTEF Building and Infrastructure 1

General Review on Geotechnical Aspeects

of Cavern Engineering

Presented by Professor/Chief scientist Eivind Grv

NTNU/SINTEF

The importance of understanding and utilizing in-

situ rock stress

SINTEF Building and Infrastructure 2

Introduction

Annual tunnelling production in Norway since 1973, statistics

prepared by the Norwegain Tunnelling Society


Introduction

The Scandinavian host rock: igneous & metamorphic, poor to

extremely good rock.

Folding, faulting and high tectonic stresses influence the

quality of the rock

Weakness zones can exhibit great variation in quality:

extremely poor to good

The width of zones may be a few centimeters to tens of meters

Hard rock not necessarily good rock

Frequently changing rock mass conditions to be negotiated

The same conditions can be

found a lot of places and is

certainly not limited to Norway or

Scandinavia

It is maybe more a matter of

philosophy and courage to utilize

the rock mass properties


Large rock caverns: in Norway mainly used for: Hydro electric power stations number of 200++

Oil and gas storage, appr. 50 caverns

Combined sports halls and civil defense facilities

Sewage treatment plants, potable water storage

Railway stations

Underground parking, ice cream storage, waste repository

By the Way: What is a large underground rock cavern?

No clear definition found, assume something larger than a normal road and railroad tunnel, eg. greater than 12m width

But TBM's today are up to 18m diameter! Caverns?

What about height? Height can be an unfavourable parameter too which may need to be considered

Introduction


Development of large underground caverns

The hydropower development: started in the 1950es in Norway

Lack of steel then the penstock was deleted

Underground caverns became compatible in price and the technology

level evolved

Unlined head race tunnels forced its way into the design, with a maximum

water pressure to appr. 1100m

Cavern design: width 12 to 25m, straigth walls, up to 35m height,

unlined, complicated geometry


Development of large underground caverns Flexible design and

construction approach!

Layout is pre-designed on the baseline geology knowledge

But subject to modifications based on encountered conditions:

Rock type and mechanical properties (mapping and testing)

Characteristics of discontinuities (continuous mapping in tunnels)

In-situ rock stress (measured at various locations during constr.)

Groundwater conditions (monitored under ground)



Flexible design and construction approach!

Layout is subject to modifications based on

encountered conditions

Here: stress measurements in a number of locations to fix;

The location and layout of power house caverns

Air cushion chamber

Length of steel lined penstock



The mining industry: learned us numerous cases with large

span:

Were 60-80 meters wide

Were Stable

With no rock support at all

WHY? The rock mass has certain

excellent properties:

Its stress induced confinement

Its selfstanding capacity

Its impermeable nature

Its thermal capacity

Stable mining rooms

Length 70m, width 30m andheight 400 m

Feasible by:

understanding rock mechanics performing stress measurement numerical and analyticalmodeling

Stjerny

underground mine


Measuring in-situ stress

A diamond drill hole (76 mm outer diameter) is drilled to the desired

depth. Usually, this depth is 1,5 times the span of tunnel/ cavern.

The hole bottom is flattened with a special drill bit, and a concentric

hole with smaller diameter (36 mm o.d) is drilled approximately 30

cm further.

A measuring cell with strain gauges and data log unit is installed

with a special installing tool containing orienting device.

Compressed air is used to expand the cell in the hole, and the strain

gauges are fixed to the walls in the hole.

The cell is now ready to start measuring, and continously logging of

strain data is stored in the measuring cell. The installing tool is

removed and the cell is ready for overcoring.

The small hole is over cored by the larger diameter bit, thus stress

relieving the core. The corresponding strains are recorded by the

strain gauge rosettes. The core is recovered from the hole with a

special core catcher, and immediately after removal from for the

hole the recorded data is transferred to the computer. When the

elastic parameters are determined from biaxial- and laboratory test,

the stresses may be calculated.


Arch effect in roof

Pillar capacity

Water curtain

pressure

Optimized geometry!

Area for 3D

Pilar stress

horizontallyPilar stress vertically


In a virgin rock body



Increase the pressure at a speed of 0,1-0,5 MPa/sek

At critical pressure a crack is initiated and water flows into the rock mass

Water supply is cut off

Re-pressurize



Measuring hydraulic

fracturing and

jacking existing

cracks

Shut-in pressure sets

equal to 3


Tunnel Engineering Handbook

By Bickel & Kuesel:

Knowledge of the in-situ

stresses is essential to the

sound, logical design of

rock reinforcement systems,

excavation procedures and

opening layouts, and to the

interpretation of expected

rock strength and

deformational properties.

How did this happen??

Lack of knowledge of the in-situ

stress?!



Some guidelines on design: Geotechnical mapping focusing joints

and weak zones, geomechanical tests

In-situ stress measurements are required (often from surface/adjacent UG opening)

Work in the 3-D picture, pillars are subject to high stresses, corners & bends are released

Locate and align the cavern

Empirical and analytical design

Numerical modeling and analytical models to verify layout

Observation/monitoring in excavation


A particular functional requirement of certain

facilities!

To reduce the length of access tunnels

Lead to:

Shallow locations, such as

Holmlia: width 25m, rock cover is only 15m

Gjvik: width 61m, rock cover between 25 and 50m

Confinement, OR in-situ rock stress

How??

Knowledge of the in-situ

stress situation is crucial


Construction of large underground caverns

Normally excavation is split into various sections:

Top heading

Benching

Each level must be mapped and sup--ported prior to going to lower level

Benching can be by vertical blast holes

Logistic is a challenge and do not believe that work is going on as smooth as on the figure

Knowing the stress conditions may allow a different sequence leaving a horisontal pilar to be done lastly

Observe the water

curtain holes above the

caverns, typical oil and

gas caverns to work in

saturated rock mass

9SINTEF Building and Infrastructure


Stability of caverns:

Visual inspections of exposed rock, shotcrete and bolts

Measuring bolts

Extensometers (rod & tape), convergence pins

The Gjvik hall

was a complicated excavation

procedure with pilots and a

number of side stopping,

slashing, headings and

benches



Unlined Tunnelling! Permanent rock support

consists of rock bolts and

shotcrete

Primary support is approved as permanent on the condition that

it meets the material standard

Active design of support to fit the encountered geological

conditions

Water control by groutingHydrocarbon storage at

Sture

10


A cargo terminal in rock cavern?

A cargo terminal; is being planned in

Trondheim covering a huge surface area replacing the existing one.

Is there any other option?

We were looking at the possibility of locating the shunting area underground.

In large underground caverns! Of course.



Underground solutions: One single cavern with a width of 42m

and length of 700m

Two parallel caverns, each 28m wide and with a pillar of 25m width

EXIT

ENTRANCE

2 CAVERNS

SOLUTOION FOR

SHUNTING AREA

1 TUNNEL TRACK THROUGH THE SYSTEM 2500 M

11



Design input: Rock cover 125m

K-value of 1

Vertical and horizontal stress components 3,5 to 4 MPa

One major joint set

Rock bolts in a 2x2m pattern, 5m long, no other support

modelled

Notice the model ran with 42m width in 2 caverns and

25m pilar

An existing railway tunnel

through the area provided

excellent possibilities for data

acquisition



Results from the numerical analysis:

No particular indications of over-stressing in the pillar, its stable

Positive redistribution of the stresses above the cavern

roof establishing a stabilizing

arch

Some minor tensional areas in the roof

12


The worlds largest public rock cavernWhat made the Gjvik hall

feasible?? Results from earlier projects

indicate presence of sufficient high horizontal stress

In situ stress measurements were done; h=3-5MPa at a depth of 25-50m which is far more than the theoretical gravity approach (

13


Some pros and cons of in-situ stressType of facility 1 3 K = hor/vert

Rock caverns Moderate to high level can

enable an optimised geometry,

too high may lead to stability

problems

Low level may produce a too

small arch building in the

roof/lack of confinement thus

instability.

K= 1-2 is OK.

K> 3 is not OK.

K< 0,5 is not OK.

Pressurised

tunnels/caverns

Minor influence, high level may

give stability problems.

Minor principal stress

component must be higher

than the water pressure (or

the pressure from any other

confined material) as an

ultimate requirement.

No particular

requirement.

Facilities with

particular

requirements to

tightness

Moderate to high level can

provide good confinement and

stability and improved

tightness.

Critical low level gives

poor safety against leakage.

Grout design press. < 3Storage pressure < 3

No particular

requirement.

Transport-tunnels Moderate to high level is good

for the confinement and

stability. Too high stresses may

give stability-problems,

spalling.

No particular requirements,

but low stress might lead to

lack of confinement and

instability.

K appr. 1 is OK.


Conclusions

Large underground caverns have been used in Norway

for several purposes

Majority have typical dimensions 15-25m width

Unlined caverns, supported by rock bolts and sprayed

concrete

In-situ stresses are utilized to obtain confinement

Field testing is needed

Underground, presence of

in-situ rock stresses would

normally be an asset

Prof Eivind GROV

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