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Lecture at Nanyang Technical Univerity
16th January 2015
Lessons learned from international
underground projects during 30 years
Professor emeritus Einar Broch
NTNU; Norwegian University of Science and Technology
Trondheim, Norway
Geology of Norway
Two thirds: Precambrian rocks. Gneiss
dominating (granites, gabbros and quartzite).
One third: Paleozoic rocks. Gneisses,
mica-schists and greenstones + sandstones,
shales, limestones.
Typical hard rock province.
Folding, faulting and high tectonic stresses
influence the stability in tunnels and
underground caverns
TUNNELS AND UNDERGROUND WORKS FOR
HYDROPOWER PROJECTS
• The development of underground hydropower projects in Norway
• Applying the experience from air cushions to gas storage caverns
• Roof stability of large underground powerhouses.
• Lessons learned from water tunnels around the world
• Concluding remarks
Norwegian hydroelectric power capacity and
accumulated length of tunnels excavated for the period
1950 – 1990
NORWEGIAN HYDROPOWER PROJECTS
• 99% of a total annual production of 125 TWh of
electric energy is generated from hydropower
• Of the world's 600 - 700 underground powerhouses
200 are located in Norway
• More than 4000 km of hydropower tunnels.
• 2 – 4 % of the tunnels are lined with concrete or
shotcrete
Development of
the general lay-
out of
hydroelectric
plants in
Norway.
EARLY REASONS FOR GOING
UNDERGROUND
• Traditional design was to bring the water to the powerhouse through a steel penstock
• Both the penstock and the powerhouse were above ground structures.
• Four Norwegian hydropower stations with unlined pressure shafts were put into operation during the years 1919-21 because of shortage of steel after the war
• Water heads varied from 72 to 152 m.
DEVELOPMENT AFTER 1945
• After the Second World War, underground location of powerhouses was given preference based on safety considerations
• Rapid advances in rock excavation methods soon showed that this was the most economic solution.
• Underground solutions also gave freedom of layout independent of the surface topography.
• Underground location of the powerhouse is now chosen whenever sufficient rock cover is available.
Early underground hydropower stations
Modern underground hydropower station
Steelfibre
reinforced
shotcrete arch
Rock bolt
supported crane
beam.
Exposing and illuminating the gneissic rock wall at
Tafjord K5 hydropower station
Extension of the Nedre Vinstra hydropower
station
Development of
the general lay-
out of
hydroelectric
plants in
Norway.
UNLINED
PRESSURE
SHAFTS
The development of unlined pressure shafts and
tunnels in Norway
Plan and
cross section
of an
underground
hydropower
plant with
unlined
waterways.
CROSS SECTION THROUGH THE WATERWAY
LINING LINING
PLAN OF AN UNDERGROUND HYDROPOWERPLANT WITH UNLINED WATERWAY
TR
AN
SPO
RT TUNNEL & SURGE CH
AM
BE
R
TAILRACE TUNNEL
GATETURBINE
TRASHRACK
PRESSURESHAFT
CONCRETE PLUG ACCESS TUNNEL
STEEL & CONCRETE
Development of
the general lay-
out of
hydroelectric
plants in
Norway.
AIR CUSHION
SURGE
CHAMBER
Plan and
profile of
the Ulset
air
cushion
surge
chamber
53 m
plan
section A - A
A
A
unlined rock
unlined rock
headrace flow
headracepressuretunnel
air cushion surge chamber
53 m
connectiontunnel
el. 599
el. 595
air cushion
water bed
Plan and
profile of the
Torpa air
cushion
surge
chamber plug
Plan
Vertical section
cavern
connectiontunnel toheadrace
connectiontunnel
air pressure4.44 Mpa (abs.)
water bed
water-curtainborehole
water-curtainborehole
water-curtaingallery
Main data for compressed air storage
PRINCIPLES FOR STORING OF GAS IN
UNLINED ROCK CAVERNS
• Internal storage pressure must be sustained by the minimum in-situ rock stress to avoid hydraulic splitting
• The ground water pressure and the gradient of the water seepage towards the cavern provides containment
• Ground water infiltration from ‘curtains’ of drill-holes supplements the natural ground water
• The Norwegian authorities require a safety margin of 15m water head above the storage pressure
GAS STORAGE - GROUTING
• If the rock mass is too permeable, grouting is
performed to obtain the required permeability.
• As a rule, grouting shall be performed as pre-
excavation grouting of the rock mass ahead of the
cavern, around the periphery
• Post- excavation grouting is limited to supplementing
pre-excavation grouting where required
Unlined gas storage development in
Norway
• Pressurised water tunnels up to 10 MPa
• LPG storage pressure up to 1 MPa at rock
temperature
• Refrigerated LPG storage down to – 42
degrees C at atm. pressure
• Air cushion chambers up to 8 MPa
Xiaolangdi Powerhouse (China)
• Span: 26 m, Length: 250 m, Heigth: 58 m
• Flatlying sandstone, Q = 8 – 12, RMR = 59 – 66
• Rock overburden: 85 – 115 m
• Original roof support: 8 and 12 m long rock bolts, spacing 1.5 m + 20 cm reinforced shotcrete
• Added support: 345 pieces of 25 m long tendon anchors, capacity 1500 kN
Flatlying sandstone in the roof of Xiaolangdi
powerhouse
Xiaolangdi
powerhouse
in China
Xiaolangdi Powerhouse (China)
Stability analyses of the cavern show:
• Displacements in walls are greater than in roof
• A natural 5 m thick crown arch is formed
• The grouted rock bolts contribute to the reduction of
the roof subsidence.
• Only small loads within limited lengths of the bolts
• Self-supported natural crown arch is established
Xiaolangdi Powerhouse (China)
• Tensioned cables have marginal reduction effect on
roof settlement
• Tension cables may increase wall displacement
• The upward support force counteracts the forming of
an arch
• Tensioned rock anchors may have a negative
influence on the stability of the roof in a cavern
Application of high tension cables apply upward
force to the cavern roof arch, thus might not work
as expected
Gjøvik
Olympic
Mountain
Hall
Span: 61m
Gjøvik Olympic Mountain Hall
Guavio tailrace
tunnel
(Colombia)
Guavio Tailrace Tunnel
GUAVIO TAILRACE TUNNEL, COLUMBIA
• 7.Nov. 1983 water under high pressure in probe hole
25 m ahead of tunnel face
• Leakage increased rapidly to 7 l/sec.
• During following day two slides, leakages up to 40
l/sec and 70 l/sec
• 350 m3 of sand flushed into the tunnel
• Tunnel face moved 4 m
• Tunnel face was blocked by concrete
GUAVIO TAILRACE TUNNEL, COLUMBIA
• During following months several attempts to reduce
groundwater level
• Many small inflows of sand , total 5,000 m3
• Impossible to reduce pore water pressure below 20
bars (Annual rainfall 4,000-5,000 mm)
GUAVIO TAILRACE TUNNEL, COLUMBIA
• Decided to excavate 3.5 m diameter pilotunnel
and to grout a head of tunnel face.
• 15 months for excavation of 77 m pilot tunnel
• Several slides and inflows, total 15,000 m3
• Most serious water inflow 400 l/sec
Guavio Tailrace Tunnel
Guavio Tailrace Tunnel
• Final tunnel has diameter 8.5
• Radial grouting and drainage.
• Max. distance between grout holes 1.5 m
• Max. distance between drain holes 3.0 m
• All drilling through blow-out preventers
• Total amount of grout for pilot tunnel and final tunnel
was 15,000 m3
Grouting and drainage pattern for
enlargement of the Guavio tunnel
GUAVIO TAILRACE TUNNEL, COLUMBIA
• The 77 m of tailrace tunnel was excavated by
roadheader in 1 m steps
• Heavy steel ribs, spacing 1 m and shotcrete
• Final support: circular concrete lining
• Tunnelling through the 77 m difficult zone completed
after 3.5 years
Plan and longitudinal section of the Lesotho
Highland Water Project.
Upper formation: Basalt, Lower formation: Sandstone
”Dog-earing” in sandstone due to high vertical
stresses in tunnel in Lesotho
”Dog-earing” in sandstone due to high
vertical stresses in tunnel in Lesotho
• Spalling developed very slowly several weeks after
the TBM excavation
• Spalling always occurred were the ratio of the uniaxial
strength/vertical stress was lower than 2.5
• Time dependent overstressing was also observed in
areas with ratios up to 4.0
”Dog-earing” in sandstone due to high
vertical stresses in tunnel in Lesotho
Plan and longitudinal section of the Lesotho
Highland Water Project.
Upper formation: Basalt, Lower formation: Sandstone
”Crazing” due to weathering in
amygdaloidal basalts
• The 45 km long, 5.0 m diameter Transfer tunnel was
basically dry during excavation.
• After some time cracking was observed in the few wet
places
• Caused by swelling smectite minerals and zeolites
• Comprehensive system for evaluation of the rock and
the need for support
” ”Crazing” due to weathering in
amygdaloidal basalts
”Crazing” due to weathering in
amygdaloidal basalts
”Crazing” due to weathering in
amygdaloidal basalts
Glendoe HPP in Scotland
Collaps in headracetunnel Glendoe HPP
Nessies oldemor
CONCLUDING REMARKS
Hydropower tunnels are special because:
• During excavation they are filled with air
• During operation they are filled with water
Some rocks and particularly some gouge material in
weakness zones are sensitive to water.
Stability problems if not properly supported.
Worst case is full tunnel collapse, - has happened in four
projects: Norway(1989), Scotland(2009), Chile(2011),
Peru (2012).
CONCLUDING REMARKS
Many rocks are not sensitive to water.
Leaving hydropower tunnels basically unlined gives
considerable cost savings.
Minot rock falls are accepted. Rock trap needed at the
end of the unlined tunnel.
CONCLUDING REMARKS
Internal water pressure in hydropower tunnels can be counter-acted by localising the tunnels or shafts deep enough.
Considerable cost and time saving if steel lining can be avoided.
From the operation of unlined air cushions we have learned that also air and gas under high pressure can be stored underground.
Only fantasy sets limits to the utilization of the underground.
