Water Power and Dam Construction. Issue July 2010
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22 JULY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
TUNNELLING
E
XCAVATION for the bypass tunnel at the top end of
Glendoe headrace is underway to overcome a rockfall
discovered almost a year ago, barely eight months after
the 100MW hydro project began operations. UK contrac-
tor BAM Nuttall has been brought in to the project to excavate a
short access adit and the water diversion tunnel.
The owner, Scottish and Southern Energy (SSE), said that a 900m
long bypass tunnel is being bored by drill and blast but the scale of
the recovery work for the project is unlikely to see the plant supplied
and generating again before the middle of 2011 – almost two years
after the problem was discovered.
In its 2009-10 Annual Report, issued in late May, SSE said that
BAM Nuttall had been retained to drive the two tunnels and that
work was already underway. However, no specic details have been
given about the local geology, tunnel build or cause of the rockfall.
The Glendoe scheme near Fort Augustus, Scotland, has approxi-
mately 16km of tunnel network, including the 6.2km long headrace.
It was constructed under a design and build contract by the Hochtief
Glendoe JV, led by Hochtief and including Poyry as the designer. The
contract was awarded in late 2005. The client’s adviser is Jacobs.
Geology along the alignment of the headrace comprises quartz
mica schist and quartzite with some minor faults. Most of the head-
race was excavated using a 5.03m diameter open gripper TBM and
some drill and blast, while the rest of the network was bored by
drill and blast.
Depending on local ground conditions, four classes of support were
available for the headrace ranging from minimum of rock bolts plus
mesh and shotcrete, if needed, up to using all of those plus a steel set
full ring. The UCS of the rock at the top end was 30MPa-130MPa.
The successful tunnelling works were completed in early 2008.
When reporting the plant had to be shut down, SSE noted that no
equipment had been damaged in the underground powerhouse as a
consequence of the rockfall at the top of the headrace, which is fed by
a reservoir. The plant has a single, six-jet vertical Pelton turbine and
operates under a gross head of 608m with a ow of 18.62m
3
/sec.
Investigations in August 2009 revealed the rockfall to be ‘very
substantial’, said SSE. Then, it was anticipating that the plant might
remain shutdown until well into 2010 at the earliest. But by then end
of 2009 it had become clear the scale of repairs would have the plant
out of action until well into 2011. The recovery options all required
signicant programmes of work.
In a presentation to the British Tunnelling Society (BTS) during
the construction phase, SSE noted that for risk management at
Glendoe it had a degree of geotechnical risk on the project, three
geologists on site, and the contractor’s tender was based on a ref-
erence ground classication system. It worked alongside the JV
contractor, which produced its own design and was responsible
for the tunnelling.
IWP& DC
Above, top left: Inside the Glendoe tunnel works during main construction.
Now, a bypass tunnel is being excavated by drill and blast after a rockfall near
the top of the headrace following completion of the project; Above, right: TBM
before launch to drive most of Glendoe headrace; Above: View of the dam
Glendoe bypass
A bypass tunnel is to recover
Glendoe’s headrace following a
rockfall, writes Patrick Reynolds
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24 JULY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
TUNNELLING
I
N the last few years, four TBMs supplied by Italian manufac-
turer and tunnelling subcontractor Seli were used to traverse
varied, and often complex, geology in Ethiopia to construct tun-
nels for two major hydro power schemes – Beles Multipurpose
(‘Beles II’) and Gilgel Gibe II. The projects called for a range of equip-
ment, including one of the most advanced shields being used for weak
ground, as anticipated at Beles II, but it was at Gilgel Gibe II that
the most difcult challenges were faced when a double shield TBM
unexpectedly encountered an extremely difcult fault zone.
Seli has worked on a number of hydro projects among the many
sectors it has served over its 60 years of operations, which it celebrates
this year. In Ethiopia, as often elsewhere, it was able to draw upon its
experience as a tunnel subcontractor as well as innovative equipment
provider to have its crews and engineers plan ahead and address the
challenges to be faced, such as the known weak stretch of ground at
Beles II, but also work to cope with and overcome unexpected dif-
culties when met, such as on Gilgel Gibe II.
The developer on each project is the Ethiopian Electric Power Corp
(EEPCo), the main contractor was Salini Costruttori and the tunnel-
ling subcontractor – and equipment supplier – was Seli.
BELES II
Located in Amhara region in north west Ethiopia, the Beles II scheme
takes water from Lake Tana along a 12km long headrace tunnel to
an underground powerhouse of 460MW (4 x 115MW, Francis units)
installed capacity. The plant discharges to a 7.2km long tailrace
which conveys the ow to the river Jehana, a tributary of the Beles.
The underground works on the project also include a 270m long
penstock shaft and a 90m high surge shaft.
Beles II - Headrace
Seli selected a dual mode TBM to operate either as a double shield
universal (DSU) or an earth pressure balance (EPB) machine to deal
with the varied geology along the alignment of the headrace. The
majority of the drive – approximately 10km – was in basalt up to
UCS 350MPa with some local faults, but the 8.1m diameter TBM
also had to bore through more than 1.8km of loose soils, described
as lake deposits.
The cutterhead was equipped with 52 x 17” backloading and
recessed discs for the hard rock drive, and switchover to EPB exca-
vation mode took a few weeks to perform on the machine, one of
Seli’s most advanced. The TBM, equipped to deal with squeezing
ground and to undertake face treatment, cost about 15% more than
a standard DSU TBM.
The tunnel is lined with 7.2m i.d concrete rings, each 1.5m long,
formed of 300mm thick segmental concrete segments (6+key).
The TBM was launched in late 2006 and made reasonable progress,
having bored about two-thirds of the tunnel after two years. Before
then, and just after the midway point in the basalt drive, there was
a large, blocky face collapse and recovery work called for polymer
resins from Innotek to ll the void and consolidate the rock mass,
restoring the ground for the drive to continue.
By early 2009 the TBM had switched excavation mode and was
driving through the lake deposits on the nal stretch of its journey.
Boring through the lake deposits, the EPB-DSU holed through in
November last year.
Beles II - Tailrace
Geology along the tailrace was reasonably good basalt with some
stretches of agglomerates and tuftes, and overburden was up to
The Beles II and Gilgel Gibe II projects have brought varied tunnelling challenges that were
successfully overcome by TBM manufacturer and subcontractor Seli. By Patrick Reynolds
Meeting challenges in Ethiopia
Beles II – The TBM completing
the headrace drive in Nov 2009
WWW.WATERPOWERMAGAZINE.COM JULY 2010 25
TUNNELLING
120m in areas, and 250m-370m elsewhere. About 17 major discon-
tinuities, mostly vertical and obtuse to the alignment, were expected
and would present locally poorer ground conditions.
Seli used an 8.07m diameter double shield TBM for the tailrace
drive, which was bored from the outlet end of the tunnel towards the
powerhouse. Like the headrace tunnel TBM, the tailrace machine’s
cutterhead was equipped with 52 x 17” backloading and recessed
discs. The cutterhead drive had a maximum torque of 5,250kNm,
and the maximum design advance rate was 6m/h.
To test the ground ahead of the TBM, the machine is also tted for
probe drilling and 18 holes through which to extend the drills.
The tunnel lining for the tailrace was the same as for the head-
race tunnel, and the annular gap lled with 8mm-12mm peagravel
and grout.
The TBM was launched in mid-2007 and completed its drive just
less than a year later, in May 2008, and had advanced at an average
of approximately 20m/day, achieving a best day of 36m (24 rings)
and best week of 189m (126 rings).
Tunnelling operations on the tailrace were performed in 3 x 8hr
shifts, 7 days per week with scheduled daily maintenance in the morn-
ings, service extensions and probe drilling when required. In total, the
efcient tunnelling system had 51.9% of the time in excavation and
23.4% in maintenance. Where there was time lost a prime reason was
the blocking effect of the pea gravel from the quarry-derived, angular
aggregate because no local uvial sources of gravel were available.
With the headrace drive by far the more difcult compared to the
tailrace on Beles II, the Seli tunnellers found the unexpectedly greater
technical challenge came on Gilgel Gibe II, underway at the same time.
GILGEL GIBE II
Gilgel Gibe II is a 428MW (4 x 107MW, Pelton units) scheme. The
headrace tunnel is smaller diameter but, at 26km, is far longer than
that of Beles II. Geology anticipated along the alignment included ve
main rocks – tertiary volcanics, rhyolite, trachyte, basalt and some
dykes. As hard basalt was anticipated for the bores, the TBMs select-
ed for the headrace drives were two 6.98m diameter DSUs that would
bore from either end of the tunnel, on Inlet and Outlet drives.
The tunnel lining is 1.6m long concrete rings of 6.8m outside diameter
and formed of four hexagonal, 250mm thick honeycomb segments.
In the end, the TBMs encountered 24 geological difculties, or clas-
sied, ‘events’, of varying scale and nature on the headrace excava-
tions – 15 in the Inlet drive and nine in the Outlet drive. By far the
most difcult to overcome was Event 19, on the Inlet drive.
Gilgel Gibe II – Inlet Drive
The drives started well and advanced 10m-20m per day despite
the almost continuous presence of weaker rock and ravelling faces.
Probes were drilled up to 50m ahead of the TBMs, From early on,
though, it was the Inlet drive that was being more hampered by geo-
logical problems – more than three times as many events to deal with
even before Event 19 struck.
Chemical grouting was employed for local face stabilisation
and some bypass digs were required to handle the early events.
Overcutting helped to counter squeezing ground. The geological
events caused hold-ups ranging from a few days to about a month.
But when Event 19 occurred, in October 2006, about 14 months and
just over 4km into the Inlet drive, it was to take more than 20 times
as long to resolve.
The TBM had already stopped when the problem emerged that
would be classied as Event 19. There was a sudden extrusion and
collapse of the face against the TBM, the progressive creep of the rock
mass and crushing force against the shield bent and broke parts of the
TBM, pushing it back more than 600mm and displaced it laterally
by more than 400mm. Probing ahead, with some difculty, hot and
high pressure mud was found – 40 degrees Centrigrade and up to 35
bar- 40 bar pressure.
Efforts were made rst to open a small tunnel over the TBM to
start to release the shield but high rock pressures prevented success.
Next, a Back Chamber was to be opened up fully around the TBM
and also an exploratory adit was excavated to the left from which
boreholes would be drilled to probe the weak rock mass nearer the
fault. However, the mud broke into the adit and overtopped bulk-
head to reach and partly bury the TBM in the main tunnel. Two
further surges of mud soon followed, and measurements showed they
caused rapid reduction in acting pressure.
The tunnellers proceeded by opening up an adit to the right from
which exploratory galleries were formed and much longer boreholes
drilled to reach into and beyond the fault. A programme of drainage
helped to lower the pressure in the rock mass around the TBM head
and the partly constructed Back Chamber around the shield. The adit
was then turned towards the front of the TBM to create space for the
insitu recovery and repair work but could not continue due to loads
and deformation rising, as a consequence, in the Back Chamber.
Eventually, without further excavation and the Back Chamber
completed, the TBM could be fully accessed and was dismantled by
early 2008. Removed to the surface, it was refurbished, rebuilt and
re-assembled, and the cutterhead diameter was increased, to 7.07m,
to help the TBM reduce the squeezing effects from rock mass with
plastic behaviour. The shield was assembled again underground in a
new chamber, located about 400m back from the site of Event 19.
The intervening distance had been backlled with concrete.
The TBM was relaunched in August 2008 on a bypass route
around the fault, through which resin injection was used to help con-
solidate the clayey basalt and to limit squeezing. Beyond the fault the
TBM rejoined the headrace alignment.
The time and difculty in overcoming the problem of Event 19
meant that the Outlet drive did much more of the overall excava-
tion than originally planned, and the Inlet drive would only bore the
same distance again beyond the fault zone. The TBMs completed the
headrace excavation in the middle of last year.
With the four TBM bores nished on Beles II and Gilgel Gibe II, and
the machines long gone, there is one remaining challenge at the latter
project. Some months after the operations began at the hydro scheme a
blockage was found in a short section of the headrace, on what was the
Outlet drive. The investigations have revealed the cause to be a rock-
fall at a zone of undetected, weak ground above and well behind the
tunnel lining. It is believed the weak rock mass, comprising some loose
large blocks in weak soil, was agitated by hydraulic pressure variations
induced by the active headrace. Recovery work is well underway and
the repairs to the tunnel should be completed soon.
IWP& DC
Gilgel Gibe II – Breakthrough celebration
at the headrace tunnel in mid-2009
22
nd
- 23
rd
September 2010 Innsbruck, Austria
DAM SAFETY
Sustainability in a Changing Environment
ATCOLD
Organized by
http://www.IECS2010.TUGraz.at
TOPICS
'XULQJ WKH ODVW GHFDGHV VLJQL¿FDQW FKDQJHV LQ
personnel resources for design, construction, and
operation of dams occurred in many European
countries, leading to a decrease of skilled practiti-
oners. In contrast to that, the construction of new
dams undergoes a renaissance now, due to the de-
mands of sustainable water and energy management,
whereas the existing dams require increasing efforts
for their maintenance and upgrading in order to sustain
adequate safety and operability.
Therefore, strategic resource management beco-
mes more and more important for the dam industry.
One main aspect thereby is the transfer of valuable
experience and knowledge from senior practitio-
ners to their successors – an issue crucial for all
parties involved: dam owners, designers, and contrac-
tors, as well as authorities. Accordingly, sustainable
management of knowledge transfer between the ge-
nerations and between science and “day-to-day-busi-
ness” will be one main scope of the symposium.
Day 1
21
st
September 2010
Technical Excursion: Finstertal, Zillergruendl
Evening: Meeting European Working Groups
SYMPOSIUM
Day 2
22
nd
September 2010
Sypmposium
Evening Event
Day 3
23
rd
September 2010
Symposium
Evening: Meeting European Club Board
Day 4
24
th
September 2010
Technical Excursion: Finstertal, Zillergruendl
PROGRAMM
Sustainability of Know How
Public Awareness of Dams and Dam Safety
Maintenance and Rehabilitation
Regulations and Guidelines
Small Dams
Surveillance Practice
WWW.WATERPOWERMAGAZINE.COM JULY 2010 27
TUNNELLING
T
HE Jinping I hydro power project is located in Sichuan
Province in the Southwest of China, about 150km upstream
of the Jinping II hydropower scheme (Wu and Huang,
2008). A 305m high arch dam – the highest of its type in
the world – has been developed as part of the project, with a total res-
ervoir storage of 7.765Bm
3
at a normal water level, and an adjustable
storage of 4.91Bm
3
. The project construction began in November
2005, with the rst generator planned to commence in 2013 and the
whole project construction to be completed in 2015.
The power plant is designed to have a total installed capacity of
3600MW with a large underground powerhouse complex, consist-
ing of headrace tunnels, a machine hall, bussbar tunnels, a trans-
former chamber, two tailrace surge chambers and tailrace tunnels.
The layout of the powerhouse complex is shown in Figure 1, with
horizontal cover 110~300m and 180~350m vertical overburden.
The underground powerhouse caverns are set in massive marble with
faults and Lamprophyre veins crossing the caverns. The in-situ stress
in the powerhouse region is high.
The powerhouse caverns include the machine hall (276.99m
× 28.9m × 68.8m; length × width × height) with roof elevation at
1675.10m, the transformer chamber (197.10m × 19.3m × 32.7m)
wall to wall distance of 45m downstream from the machine hall
with roof elevation at 1679.20m, and surge chamber 1# (height of
80.5m, diameter of 41m and 38m upper and lower chamber, respec-
tively), surge chamber 2# (height of 80.5m, diameter of 37m and 35m
upper and lower chamber, respectively). The center to center distance
between the two surge chambers is about 95.1m. A 3D illustration of
the caverns is shown in Figure 2.
The machine hall and the transformer chamber longitudinal axes
are parallel and oriented N65°W, with angles within 6.0°~ 36.5°,
average of 16.3°, to the direction of the major principal stress, angle
of 45.0°~55.0° and 45.0°~60.0° to fault f13 and f14, respectively,
and a small angle to NWW oriented joint set. The layout of the pow-
erhouse complex is designed to be favorable to the stability of the
surrounding rock masses. However, during the excavation, severe
engineering problems were encountered such as cracks in shotcrete
at downstream springlines both in the machine hall and the trans-
former chamber. Large deformation in the rock masses in the vicinity
of the faults and overstress of support elements were monitored. As
a result, the excavation had to be paused in order to add additional
Shiyong Wu, Ziping Huang and Ge Wang present details on the engineering challenges
and solutions developed during excavation of the underground powerhouse complex at
Jinping I hydropower project in China
Excavation challenges and
solutions at Jinping I
Above, left – Figure 1:
Layout of the power-
house and a horizontal
geological profile at
EL.1665;
Right – Figure 2: 3D
illustration of the
underground power-
house complex;
Right, top – Figure
3: Topography above
the powerhouse area,
photo by Ziping Huang;
Right, bottom – Figure
4: Three dominate
faults intersecting the
powerhouse
28 JULY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
TUNNELLING
rock support to enhance the stability of the
rock masses.
DESIGN OF THE UNDERGROUND
POWERHOUSE
Engineering geology
Yalong River flows towards N25 ºE at the
dam site where the V shape deep valley is
straight and narrow and the mountain is over
1000m high above the river. The base rock
outcrops are shown in Figure 3. The rock
formation in the powerhouse area is affected
by tectonic movement with development of faults, shear zones and
joint sets. The powerhouse complex is located in marble of Triassic,
with thin layers of greenschist. Lamprophyre veins invaded
stretching straight over 1000m, with a width of about 2~3m, up to
7m, oriented N60~80°E/SE 70~80°, developing small faults in
the vicinity of the veins. The body of veins is crushed with poten-
tial instability problems. The bedding of the rock formation in the
powerhouse is oriented N50~60°E/NW 30~40° dipping towards
the valley. The faults f13, f14 and f18 are NE oriented, as shown in
Figures 1 and 4 and are signicant to the stability of the rock masses.
Fault f14 intersects the machine hall and the transformer chamber
having a broken rock zone of 0.2~3m wide and an affected zone of
3~5m in each side of the fault. Locally the affected zone is about 20m
wide, such as on the downstream wall of the machine hall at EL.1640
St. 0+103, where the rock masses are strongly weathered, as shown
in Figure 5.
The major portion of rock masses in the powerhouse is classied
as III in Chinese rock mass classication system with saturated uniax-
ial compressive strength of 60~75MPa, deformation modulus of
6~15GPa. Fault zone and affected zone, lamprophyre zone, intensive
joint zone are classied as IV-V with saturated uniaxial compressive
strength of 25~45MPa, deformation modulus of 0.4~3GPa. After
excavation of Layer VI of the machine hall and the bussbar tunnels
was completed, it was noted that the quality of revealed rock masses
is in line with what assumed during the investigation. The in-situ
stress is high in the area of the powerhouse. The major principal
in-situ stress
T1 is in a range between 20~30 MPa with a maximum
of 35.7MPa, oriented N30~50°W/20~35°. Some borehole core discs
are shown in Figure 6 taken during investigation. Other high stress
induced rock phenomena include slabbing in the adit, buckling, etc.
The measured rock strength to in-situ stress ratio is about 1.5~4.
After excavation, large deformations of rock masses were record-
ed and rupturing of rock masses, buckling of shotcrete, and large
increase of load or stress in rockbolts and prestressed cable anchors
took place.
STABILITY ANALYSIS OF POWERHOUSE CAVERNS
AND ROCK SUPPORT DESIGN
Principle of rock support for the powerhouse caverns
The major engineering geological challenges are the presence of three
faults intersecting the underground powerhouse caverns and the fact
that the rock mass strength is not very high. The following princi-
ples of rock support of the powerhouse caverns are thus considered
and implemented: Make use of rock masses to support themselves by
applying exible rock support approach, use systematic rock support
with local enhanced support; implement dynamic support approach
to adjust the rock support by systematic analysis combining excava-
tion revealed rock conditions and the monitoring data with numerical
back analysis of the rock deformation.
Stability analysis of the rock mass in the powerhouse
The layout of the underground powerhouse caverns was designed to
be favourable to the stability of the surrounding rock masses. One
joint set, set No. 4 oriented N60~70°W/NE(SW) 80~90°, is nearly
parallel to the walls of the machine hall and the transformer chamber
and thus not favorable to the stability of the walls. The rock masses
are generally stable with potential local instability. At locations with
faults and the affected zones, the lamprophyre veins and intensive
fractured zones the strength of the rock masses are low under the
condition of high in-situ stress, and there is potential local instability
of the rock masses featured by large deformations or rock failures.
Rock support design and the actual rock support
The systematic rock support includes shotcrete, pattern rockbolts,
and prestressed cable anchors. Additional rockbolts or cable anchors
with smaller spacing and longer bolts, steel ribs, weak rock replace-
ment with concrete and consolidation grouting were also applied to
enhance the local stability, as shown in Figure 18.
In the machine hall roof, rock support includes a pattern rockbolts
of
32, L=7.0m, prestressed rockbolts of 32, L=9.0m, prestressed
to 120kN, spacing 1.2m ×1.4m, wire mesh (
8@20×20cm) shot-
crete C30 and C25 20cm thick. For fault f14, above the springline
at EL.1665.5m, longer rockbolts of L=9m, prestressed rockbolt of
L=12m, spacing at 1.2×1.3m, and shotcrete steel rib were applied.
The rib is 0.5m wide and spacing 1.2m~1.4m, containing two layers
shotcrete C30 with each layer 20cm thick. Each shotcrete rib layer
has main reinforcement rebar of 3
36@20cm, and distribution rebar
22@30cm.
ENGINEERING CHALLENGES AND SOLUTIONS
Excavation progress
The machine hall is divided into 11 vertical layers for excavation.
Excavation of an adit in the roof began in May 2005. In July 2009,
excavation and rock support above Layer VI at EL1640m was com-
pleted. Excavation and rock support of the transformer chamber
commenced in May 2007 and was completed in November 2009.
In July 2009, excavation of the two surge chambers reached Layer
II at EL.1668m. Excavation and rock support of six bussbar tun-
nels between the machine hall and the transformer chamber have
been nished.
Local instability of rock masses during excavation
During excavation, rupturing (Figure 7), buckling, slabbing, slip-
ping or shear along joints (Figure 8) in rock masses took place.
Monitoring data indicated large displacements in extensometers
and that stress or load exceeded the designed limit in some rock-
bolts and prestressed cable anchors. At several locations cracks
occurred in the shotcrete, steel ribs were bent by large deformation
of the surrounding rock masses in the roof of the machine hall, as
shown in Figure 9~10.
Figure 5 (a) Layer VI in the machine hall, St0+103,
fault f14 wide around 0.8~3.5m, affected zone
of 20m wide in the hanging wall of the fault; (b)
L
amprophyre veins in the northern end wall of
the machine hall
WWW.WATERPOWERMAGAZINE.COM JULY 2010 29
TUNNELLING
MONITORING DATA AND ANALYSIS
Monitoring design and results
Monitoring of rock masses deformation and rock supports stress is
to ensure the safety of the construction and the operation, to provide
effective tools to optimize the excavation and rock support design and
excavation process. At nine cross sections intersecting the machine
hall, the transformer chamber and the surge chambers, and the three
faults, f13, f14, f18, extensometers rockbolt dynamometers and pre-
stressed cable anchor dynamometers were installed (Figure 11).
Up to 4 May 2009, the number of measurement points with displace-
ment less than 10mm is about 57.6% of the total points, the number
of points with displacement between 10~50mm is about 31.9%, while
the number of points with displacement greater than 50mm is about
10.5%. In about 72.8% of the total deformation measurement points
the displacement is less than 30mm. The maximum roof vault settlement
is recorded about 2.2mm, and the maximum displacement in the down-
stream side rib of the roof 7.2mm. At St.0+031, St.0+126 and St.0+196,
at EL.1659 and EL.1651, upstream side wall of the machine hall there
are four points with displacement larger than 20mm. At St. 0+126, in
the vicinity of fault f14 a displacement of 41.6mm was measured. In the
downstream wall of the machine hall, there are 15 points with displace-
ment larger than 20mm. There are seven points in the machine hall with
displacement at rock surface larger than 50mm, with a maximum of
96.8mm. Among ve points near the downstream springline there are
two points in the vicinity of faults f14 and f18, three points related to
joints of ssures, the deformation developed within 0~9m deep (Figures
12 and 14). Another two points with displacement more than 50mm
are at EL1650 and EL.1659, between Unit 3#~4#, in the downstream
wall where the rock is integrated, the large deformation develops up to
12m deep in the rock mass. This is probably caused by high in-situ stress
(Figures 13 and 15). Normally the deformation takes place within the
rst 5m in the rock masses.
In the transformer chamber the number of measurement points
with displacement in three categories of less than 10mm, between
10~50mm and larger than 50mm at rock surface is about 33%, 48%
and 19% of the total measurement points, respectively. The number
of points with displacement less than 30mm is about 70% of the total
measurement points. The chamber roof settlement is less than 5mm.
Most measurement points in the upstream wall show displacement
less than 12mm, large displacements up to 55.7mm were monitored
between Unit 4#~6# at EL.1668. In the downstream wall at the same
elevation, Unit 2#, 5# and 6#, displacement between 28.5~57.5mm
with a maximum value of 132.7mm at Unit 5# were monitored.
There are two and three points with displacement larger than 50mm
at rock surface in the upstream and downstream walls, respectively.
In the upstream wall, the two points were located in integrated rock
masses, large deformation develops down to 12m in the rock (Fig 16).
After excavation of Layer II in surge chambers 1# and 2# (EL.1688
to EL.1668), the doom settlement is about 0.1mm and 4.7mm,
respectively. The maximum wall displacement in surge chambers 1#
and 2# is about 11.5mm and 47.9mm both in the upstream wall at
EL.1668, respectively, where the fault f18 and lamprophyre veins
intersect the chambers.
Among 88 rockbolt dynamometers in the machine hall, with stress-
es in ranges less than 100MPa, 100MPa~200MPa, 200MPa~300MPa
and larger than 300MPa, the corresponding percentage of measure-
ment points to the total points are 54.5%, 19.3%, 8% and 18.2%,
respectively. There are six rockbolt dynamometers in the roof rib
and springlines of the machine hall, and 10 in the walls exceeding the
measurement limit (300MPa). The measurement points are within
2~6m deep in the rock.
There are 18 rockbolt dynamometers in the transformer cham-
ber, with stresses in ranges less than 100MPa, 100MPa~200MPa,
200MPa~300MPa and larger than 300MPa, the corresponding per-
centage of measurement points to the total points are 50%, 22.2%,
16.7% and 11.1%, respectively. The measurement points are within
2~4m deep in the rock.
In the surge chambers there are 18 rockbolt dynamometers,
with stresses in ranges less than 100MPa, 100MPa~200MPa,
200MPa~300MPa and larger than 300MPa, the corresponding per-
centage of points to the total measurement points are 44.4%, 16.7%,
22.2% and 16.7%, respectively.
There are 32 prestressed cable anchor dynamometers in the
machine hall, among which about 76% dynamometers the measured
load exceeding the locked value, 38.6% of the dynamometers exceed-
ing the designed capacity, with a maximum exceeding load at St.
0+42.7, EL.1662.5, downstream wall. Of the 24 cable dynamometers
in the transformer chamber, about 75% measured load exceeding
the locked value, and 33% of the dynamometers exceed the designed
capacity, at EL1664.5 (3) and EL.1661 (5) both in upstream and
downstream walls of the transformer chamber. From the load devel-
opment curve it is noted that the load tends to stable (Figure 17).
ANALYSIS ON THE LARGE DEFORMATION OF ROCK
MASSES AND OVERSTRESS IN BOLTS AND CABLES
The large deformation of the surrounding rock masses in the caverns
may be related to the engineering geological conditions and the exca-
vation and rock support performance. The saturated uniaxial com-
pressive strength of intact marble is not high while the in-situ stress is
signicant. The ratio of strength to stress is between
2~3.8, even as low as 1.7 locally. According to a
Chinese Standard, in-situ stress in the powerhouse
region belongs to high and extreme high locally.
Observations made on the behavior of the rock
masses during the excavation are in line with the
descriptions of the rock under the same geological
conditions in the Standard, including rock ruptur-
ing, slabbing and fracturing. Therefore, large defor-
mation in integrated rock masses may be expected.
In the vicinity of the faults f13, f14 and f18 in
the machine hall and the transformer chamber, the
quality of rock masses in the fault and the affected
zones is poor. Even though there is no large rock
block identied from combination of faults and
joints and ssures. Geological investigations indi-
cated that some combination of joint sets might be
Far left: Figure 6 Borehole core discs from the left bank
indicating high in-situ stress;
Left, bottom: Figure 7 (a) Rupturing of rock in the down-
stream side rib of the machine hall at St.0+150
Left, top: Figure 8 Sliping along a joint indicated by shear
movement of the blasting borehole on the upstream wall of
the machine hall at St. 0+150m and EL1654–1650
30 JULY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
TUNNELLING
unfavorable to the stability of high walls. Thus the geological struc-
tures may also lead to large deformation of the surrounding rock
masses.
The monitoring data shows that after excavation large deformation
of rock masses develops relative shallow where the geological struc-
tures presented while large deformation develops deeper in integrated
rock masses with higher in-situ stress.
Obviously the large deformation in rock masses caused the over-
stress in bolts and prestressed cable anchors. It may be necessary to
analyze a proper support pressure that allows certain deformation
of the rock masses while the stress or load built up in the bolts and
prestressed cable anchors is limited to a certain extent.
MEASURES TO CONTROL THE LARGE
DEFORMATION OF THE ROCK MASSES
Rock support measures are requested to limit the shallow deforma-
tion in the rock masses by reinforcing the loosened rock masses and
thus prevent it developing further inside the rock, creating a rein-
forced deformable rock ring to support rock inside and thus ensure
the overall stability of the surrounding rock masses.
To control the large deformation in the vicinity of faults in the roof
where large displacements were monitored, rock support measures
include systematic rockbolts, shotcrete, double layers rebar reinforced
shotcrete rib, systematic prestressed cable anchors, and fault anchors.
Figure 11 shows the concrete beam and prestressed cable anchors in
the downstream side, at springline and upper wall of the machine
hall. The monitoring data indicates that the displacements of the rock
and stress of the rockbolts and cable dynamometers tend to be stable
after the installation of the extra rock support.
At locations with large deformation in the walls of both the
machine hall and the transformer chamber, the loosened rock
blocks were removed and heavy rock support was applied such as
steel bre shotcrete, random rockbolts, prestressed rockbolts, pre-
stressed cable anchors. At some locations when the displacement
did not converge prestressed, so many cable anchors were installed
that there was no room to install any more, see Figure 18. Finally,
the rock masses are stable.
DISCUSSION
After excavation of Layer IV in the machine hall, due to large defor-
mations, the excavation of the machine hall had been paused for
about half a year in order to apply extra rock supports. An option
to avoid large deformation of walls could be that the high wall was
designed as an arch shape rather than a straight line. There might be
less deformation due to smaller loosened zone in the shallow region
and thus probably such heavy rock support might be not requested.
Much smaller displacements in the roofs of the caverns provide some
evidence on the design alternatives of the wall. Numerical modelling
could have been applied to simulate this option. Economical evalua-
tion should be carried out to compare the options on cost and exca-
vation progress. For the long term stability of the surrounding rock
masses the arch shape walls may be favorable.
Delay of the rock support after excavation may have partly contrib-
uted the large deformation. After blasting, if the rock support was not
installed in time, the joint may open and blasting induced ssures may
further develop and cause larger loosening zone. This is against the prin-
ciple of NATM to make use of the reinforced rock ring to support the
surrounding rock masses themselves. And thus large deformation took
place, with high stress built up in rockbolts and cable anchors.
With regard to rock bolting, under the conditions of low strength
stress ratio large deformation of the rock masses may be expected,
end anchored rockbolt combining grouting should have been consid-
ered, such as CT-bolt widely used in Norway, where full grouting of
the bolt is applied later. The cable anchors were probably prestressed
too much to stand for further support pressure due to further defor-
mation development. Therefore so many prestressed cable anchors
were overstressed beyond the designed capacity. The support was
probably too stiff to accommodate future displacement of the rock
masses. Numerical modeling and analytic study may be useful to
study a proper prestress load level and timing of installation of the
cable anchors combining a study on an acceptable and reasonable
displacement magnitude of the rock masses in caverns.
CONCLUSIONS
The geological challenges of the Jinping I hydro power project under-
ground powerhouse are the presence of faults and lamprophyre veins
intersecting the caverns, high in-situ stress in the region, integrated
marble and thus low strength to stress ratio. During excavation large
deformations of the rock masses occurred, high stresses built up in
some rockbolts and prestressed cable anchors in the machine hall
and the transformer chamber. The monitoring data shows that large
deformation of rock masses develops relative shallow where the geo-
logical structures presented while it develops deeper in integrated rock
masses where the high in-situ stress plays important role.
The stability of the caverns was under concern and the excava-
tion was paused for months in order to apply extra rock supports to
enhance the reinforcement of the roof and the walls of the caverns.
The additional rock supports were heavy and effective. At present the
monitoring data indicate that the displacement in rock masses and
stresses in rock supports are stable. Observations also conrm that
the surrounding rock masses are stable overall.
The additional rock supports were costly and impacted the con-
struction progress considerably. What we could learn from the
excavation and the design may be meaningful. Given that a large
deformation of the surrounding rock masses is expected, a design of
arch shape walls rather than straight high walls may lead to smaller
displacement of the walls and reduced amount of rock support, even
though the excavation work may increase. Prestress level and timing
of installation of the cable anchors could have been optimized in
order to save cost and to increase the reliability of the rock support
elements, and secure the long-term safety of the support and stabil-
ity of the powerhouse caverns during operation. Rock support of
end anchored rockbolts and shotcrete installed in time is critically
Left top and bottom: Figure 9 (a) Spalling of shotcrete at St. 0+170 down-
stream rib of the machine hall roof; (b) Spalling of shotcrete in detail;
Left, small image: Figure 10 Bending of the reinforced shotcrete rib in the roof