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Iceland's Blanda Hydroelectric Project: Monitoring of deformations, rock support and testing of rock anchors in the powerhouse cavern
PLANNING AND DEVELOPMENT
Iceland's Blanda Hydroelectric Project: Monitoring of Deformations, Rock Support and Testing of Rock Anchors in the Powerhouse Cavern Sveinn Thorgrimsson, Matthlas Loftsson and Olafur Jensson
A b s t r a c t m A s part of a national plan to harness Iceland's l~surn@mLe projet hydro-dlectrique Blanda qui est actuellement en considerable hydro energy potential, the Blanda Hydroelectric construction et qui sera dtat de plein fonctionnement ~ partir de 1992, Project is being built and will be fully operational in 1992. Site fait partie d"un plan national d'exploitation des ressources ~nerg~tiques exploration for the 150-MW, 720-GWh /yr facility have involved considerables du pays. L'exploration du ehantier pour la centrale de extensive geological and geophysical investigatione. This paper 150 MW, 720 GWh / annuelles a entra2nd des recherches gdologiqnes et discusses the rock properties of the site, results of geological gdophysiques extensives. Cet article traite des propri~t& des rochers investigations, monitoring of deformations during and after du chantier, des r~sultats des reeherehes gdologiques, du contr~le des excavation of the powerhouse cavern, and measures taken for roek d~formations au courset apr~s l'excavation de la caverne de la centrale et des mesures prises en mati~re du sout@nement. support.
Introduction celand is a volcanic island, rich in thermal as well as hydro energy. To date, only about 12% of its e n e r g y resources have been harn e s s e d . T h e B l a n d a Hydroelectric Project, whichis a 150-MW, 720-GWh/ year facility, is the most recent power scheme, will be partly in operation in 1991 and completed in 1992. It is located in northern Iceland, about 340 km from the capital city of Reykjavik. The o w n e r of t h e p r o j e c t is Landsvirkjun (the National Power Company). This paper deals with the rock properties on the Blanda site, geological havestigations, monitoring of deforma tions during and atter excavation of the powerhouse cavern, and the measures taken for rock support. A short description of a pull-out test on rock anchors in the highly fractured basaltic rock concludes the paper. Figure 1 shows the generallayout of the underground structures, with the main dimensions of the powerhouse cavern andtunnels, excavationofwhich took place from 1984 to1987. Conven-
I
Present address: Sveinn Thorgrlmsson, Resident Engineer; Matt hias Loftsson, Engineering Geologist;and OlafurJensson, Project Manager, Landsvirkjun, H~aleitisbraut 68, 103 Reykjavlk, Iceland.
tional drill-and-blast methods were used in the excavation, except for the two shales, which were raise-drilled.
Rock Conditions Site investigations preceeding excavation involved extensive geological and geophysical investigations, including 2230 m of core drilling. Preinvestigations were undertaken by the National Energy Authority of Iceland. The rock formations are six to seven million years old, and predominantly volcanic in origin. The bedrock is built up of 5- to 7-m-thick tholeiitic and porphyritic basalts, with scoriacious contact breccias and interbedding of sedimentary rocks. The tholeiitic basalts are highly fractured, with rock substance often consisting of fist-size stones. The fracturing is for the most part a result of previous tectonic stresses. In drill cores, R Q D values range between 20 and 80, with a mean value of about 45. The fractures coated with silts and clay, mostly rock flour.SmAll quantifies of the clay mineral smectite have been detected. The porphyritic basalts are less fractured than the tholeiitic basalts, even though both have been subject to the same tectonic forces. RQD-values range between 50 and 90, with a m e a n value of about 70. The reason for the difference in fracture frequency between the two basalt types is explained
Tunnelling and UndergroundSpace Technology, Vol. 6, No. 2, pp. 235-239, 1 9 9 1 . Printed in Great Britain.
0886-7798/91 $3.00 + .00 Pergamon Press pie
by their different deformation properties. Porphyritic basalts were only encountered in the tailrace t~Innel. The top and bottom part of each basalt layer is made of scoria. Because the engineering characteristics of the scoria are different from those of the more solid main part, the scoria is taken as a separate entity. The sedimentary rocks are in most cases silt- and sandstones; silt fractions up to 60% have been measured. Clay fractions vary in most cases between 2 % and 8%, only part of which is clay minerals, mostly smectite. Oedometer swell testsshow only mlnor swelling. The sedimentary layers, however, have proved to be sensitive to changes in water content. Weakening and some disintegration of layers has been observed as a result of reduced confinement, and of repeated wetting and drying. As an indication of the characteristicsofthe rock substance, Table I shows a few geotechnical properties, presented as average values (x)with standard deviations (s) of samples from several sites in Iceland.
Initial Evaluation of Deformation and Support Requirements Before excavation started, a twodimensional finite element model was
235
PRESSURE SHAFT
Lf235m, A-11m 2
(~
POWERHOUSE CAVERN
Lffi66m, W-12,5m, H "30m
(~
SURGE TUNNEL
L" 235m, A-22-37m 2
(~)
ACCESS TUNNEL
L ' 8 0 8 m , A-25m 2
(~
ENTRANCEHALL
L -28m L-215m, A-11m 2
(~)
CABLE AND ELEVATOR SHAFT
(~
TAILRACE TUNNEL
L- 1,700m, A-36m 2
(~)
TAILRACE CANAL
L" 1,200m
~_~
ACCESS TO TAILRACE
(~
CONTROL AND SWlTCHGEAR HOUSE
(~)
TRANSMISSION LINES
Figure 1. General layout of tunnels and powerhouse cavern for the Blanda hydroelectric facility. employed, mainly for initial evaluation of stability and support requirements. The calculations were based on elastic deformations of a homogeneous material. The in-situ stresses and rock properties used in the model were estimated to a great extent. The in-situ stresses used were estim a t e d on t h e b a s i s of p r e v i o u s hydrofracture measurements in boreholes. These measurements were interpreted tA~nginto consideration the influence of the local topography and
experience from other sites. At the proposed powerhouse location, the vertical and horizontal stresses were accordingly estimated to be approximately equal, or 6-7 MPa, and k = 1.0. Part of the pre~investigations involved evaluating rock cores with regard to the main characteristics that affect t - n n e l stability. Comparison of several recognized classification systems revealed t h a t the Norwegian tunnelling rock quality index (Q-value) was better suited t h a n other systems
to estimating tunnel support in the fractured basalts. However, some adjustment in the rating of the joint set n u m b e r and joint r o u g h n e s s was needed to give better correlation to the ]~rn~ted openings available. The rock in the powerhouse cavern consisted mostly of tholen'tic basalts and scoria. In mostcases, calculated Qvalues in the tholeiitic basalts were between 2 and 4, but ranged from 0.3 in fracture zones to 8.3 in the least fractured sections. The Q-values for the
Table 1. Geotechnical properties of rnain rock types from various sites in Iceland. Uniaxial Compressive Strength (MPa)
Elastic Modulus (GPa)
Poisson Ratio
Average
Standard
Average
Standard
Average
Value (x)
Deviation (s)
Value (x)
Deviation (s)
Value (x)
Standard Deviation (s)
Tholeiitic basalts
242
90
37
8.5
.123
0.25
Porphyritic basalts
168
69
25
7
.130
.007
Scoria
25
17
4.1
1.3
.180
.078
Sedimentary rocks
21
17
3.8
5.7
.066
.095
Rock Type
236 TUNN~LLn~GANDUNDERGROUNDSPACETECHNOLOGY
Volume 6, Number 2, 1991
scoria were somewhat higher, with a weighted average of about 4. The Q-values were used for estimating the elastic modulus E for the finite element model. The E-values obtained in t h ~ m A n n ( ~ " Z~_n m ~ fi'om 5 to 20 GPa, and were regarded as only a rough appre~mAtion. These values are dearly lower than the laboratory values for the rock substance presented in the table. The uncertainty of these and other values was given due consideration during the first calculations.
/
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//// oO
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o...,,
10m
Em
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12
14
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15
2O
............
22
24
DAYS
:2' 5T.35
:t,,. Monitoring of Rock Deformations
t a k e n ~ a task t h a t has continued during the construction period. Based on measured deformations, the premises for the finite element model were revised and gradually the range of values could be narrowed down. During excavation of the top heading in the powerhouse, points for measuring convergence with tape extensometers were established, and rod extensometers were installed at depths of 2 m, 6 m and 10 n~ The top heading was excavated in two parts--first the left half, followed by the right half. Figure 2 shows the status of excavation when the rod extensometers were installed, and the reaction of the roof to the excavation. During excavation of the left half, very small deformationsinthe roofwere registered. As soon as the excavation of the right part started deformations began, reaching a maximum value when the excavation passed the measuring station at 35 m, and gradually decreasing thereafter. At completion of the excavation of the top heading, deformations at the measuring station had stepped for the most part, and a new equilibrium had been obtained. After excavation of the top heading and the first 6-m bench, the results from the tape- and rod extensometer measurements were used independently to obtain the elastic modulus E and the stress ratio k by back-calculating these values from the observed deformations. Figure 3 shows the calculated values for both vertical and horizontal deformations of the top heading, with deviations due to assumed variation in the Poisson ratio. According to this calculation, the most probable values for E and k are E = 9.8 GPa and k = 1.25. These values were used to reevaluate probable deformations and establish new safely margins for the powerhouse cavern as the excavation proceeded. All finite element calculations were undertaken by VST Consulting Engineers. The excavation of the powerhouse cavern was conducted in several stages.
Volume 6, Number 2, 1991
D.
i t. ~'~ .......
Du;~Jlg excavation, extensive monit o r i n g of rock deformations was under-
2m
oo
1 2 3 4 S S 7 8 9 10 o
6m
l
5 5 7 8 S 10
12
~._o.m_.
. .......
14
16
18
20
22
24
DAYS
-"-,----~-- :- L, . . . . . . . . . . . . . . . . . ~*%~
~
~ _
2m
%
A. DISPLACEMENT MEASURED BY VERTICAL ROD EXTENSOMETERS B. OISPLACEMENT MEASURED BY 45=INCLINED ROD EXTENSOMETERS C. CROSS-SECTION D. PLAN-SECTION
I SHOWING STATUS OF EXCAVATION WHEN EXTENSOMETERS WEREINSTALLED
!
Figure 2. Measurements of displacement of top heading. -
MPa
FOR V E R T I C A L D E F O R M ~ 10.000 9.800-
5.000
.\
~
\
\
\
\ \ \
\ \ . " v ~ , L Y ~ "
/
, OR HORIZONTAL DEFORiATIONS
15.000 0,5
1,0
1,25
1,5 k
STRESS RATIO k ( (T'H/0"v )
Figure 3. Relationship between elastic modulus and stress ratio from backcalculating measured vertical and horizontal deformations (VST Consulting Engineers).
~.Lr~G
ANDU~U~.P~OUSD SPACETECHNOLOGY237
ma.s.I. 138 --
2t 3
132 --
126
--
118
--
113
--
111 --
Figure 4. Section through the powerhouse, showing typical lines for convergence measurements and stages of excavation.
The top h e a d i n g was excavated in M a y 1985. The first 6 m bench down to t h e generater floor was excavated 10 months later, in March-April 1986. The m a i n p a r t of t h e cavern down to t h e d r a i ~ u b e s and s u m p w a s excavated late in 1986, a n d excavation for drafttubes a n d sump was u n d e r t a k e n in two stages in 1987. As t h e e x c a v a t i o n c o n t i n u e d , n e w m e a s u r i n g p o i n t s were i n s t a l l e d . A typical m e a s u r i n g section a n d stages of excavation are shown on F i g u r e 4. The rock r e a c t e d v e r y closely with t h e excavation of t h e cavern. Deformations began as soon as excavation commenced, but the rate of deformation decreaeedrapidly as excavation stopped. Curves for deformations at t h e middle section of the cavern are shown on Figure 5, a n d t h e lines refer to F i g u r e 4. The g r e a t e s t deformation was measured on line designated 6-7 a t elevation 121 m, which is about 10 m above the drafttubes. Total convergence wall to wall on this line was 90 ram. Total
convergence on line 4-5, which is about 6.5 m higher (elevation 127.5 m) was 60 ram. Total convergence was 25 m m on line 2-3, and 18 m m on line 8-9. The highest r a t e of deformation occurred during excavation of t h e second bench, between elevation 126 m a n d 118 m. For this period, t h e average convergence wall-to-wall was 253"10 -2 m m / d a y o n l i n e 6-7, and82*10 -2ram/day on line 4-5. After all excavation in t h e powerhouse cavern was completed, a creep r a t e of up to 0.22 * 10 s ram/day was m e a s u r e d on lines 4-5 a n d 6-7; however, by the end of t h e y e a r 1989, no such movements could be registered. Figure 6 shows, in a very exaggera t e d way, the relative deformations of the walls and ceiling of t h e powerhouse cavern. The direct m e a s u r e m e n t s described above show somewhat g r e a t e r deformations t h a n those calculated by t h e finite element model. This is not unexpected and reflects t h e limits of t h e ability of t h e model to s i m u l a t e the actual rock mass properties. Nonetheless, the relatively simple model gave a satisfactory approximation to t h e actual conditions and proved to be of aid in the overall design. F o r establishing t h e elastic properties of t h e rock m a s s further, t h e dyn~mlc modulus E-dyn was m e a s u r e d with seismic velocity m e a s u r e m e n t s . The m e a s u r e m e n t s were carried out b y the N a t i o n a l E n e r g y A u t h o r i t y of Iceland. E-dyn was m e a s u r e d a s 36 G P a for t h e less fractured tholeiitic basalts, and about 25 G P a for heavily fractured tholeiitic b a s a l t s a n d scoria. The monitoring of deformations in the cavern have established t h a t in spite of the high fracture frequency of t h e rock and t h e presence of w e a k scoria layers, t h e rock h a s considerably b e t t e r stability t h a n can be anticipated by t h e use of t h e e s t a b l i s h e d classification systems. The raA~n explanation for t h e relatively good stability of t h e highly fract u r e d b a s a l t s is the unevenness or undulation ofjoints and fractures, t h e fact t h a t joints and fractures are not con-
1986 mm
03
i4
05
06
07
tinuous through the rock m ~ s , and the high strength of the rock substance. The rock mass is made ofirregularly stacked interloctdng blocks of otherwise i n t a c t material. The rock m a s s is, thus, able to s u s t a i n high s h e a r stresses; however, because of t h e high fracture frequency, it needs to deform substantiallyfor the shear strength of the rock mass to be mobilized. As noted above, the m a x i m u m convergence of the powerhouse walls was measured at 90 ram, i.e.,an average movement of each wall of 45 m m before a new stress equilibrium was reestablished after the excavation.
Rock Support Supporting of the powerhouse cavern was undertaken in two--and, in some places, thrcc ctages. W o r k support consisted of applying a thin layer of shotcrete on the rock surface altereach blasted round. PermAnent supporting followed, by rock bolting;and, in the top heading and cavern above the generatot floor, by application of a second shotcrete layer. In the top heading, the initial shotcrete thickness was about 3 to 5 cm on b a s a l t a n d scoria, which cover most of the crown. O n the sedimentary rock, 10-cm-thick, steel-fiber-reinforced shotrete (type EE-fibres) was applied. Rockbolts were fully grouted 25 m m reinforcement bars 4 m long in basalts and scoria and placed on a 2 m by 2 m grid. In the sedimentary rock, which is lAmlnated w i t h n e a r horizontal w e a k ness planes, 6-m-long rock bolts were placed on a 1.5 m by 1.5 m grid. The p e r m a n e n t shotcrete layer was steelfiber- reinforced a n d 10 c m t h i c k on the entire top heading. F o r work support, t h e walls of the cavern were shotcreted w i t h a layer of unreinforced shotcrete about 3 to 5 cm thicl~ Rock bolting followed, using 4-mlong bolts i n s t a l l e d on a 3 m b y 3 m grid, b u t a t closer spacing a r o u n d corners a n d openings. As p e r m a n e n t support, walls above t h e g e n e r a t o r floor were given a 5- to 1 0 - c m - t h i c k l a y e r of
1987 08
09
~
"11 1
12
01 1 02
03
04
05
06
1988 07 i 08
Og
10
11
12
01
02
03
04
05
lO
'-t"-'-~
20
_o ~-
~
F---- .L-- A-~ _ _
LINE 8 - 9
!
"~1
~
°
~
.~
....~__~.~__.LINE .2-13 _ J _ _ ~
/
40
E 0
50
--
~
60
--
70
I .....
NOT ~M~ I "'"~4SURF:t~
r L,.E !-s
80
90
~ - ~ - "
. . . .
--, . . . . .
~ .
.
.
.
f
~_L,,_e._6 : J .
_
Figure 5. Measured deformations wall-to-wall in the powerhouse.
238 TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY
Volume 6, N u m b e r 2, 1991
un~inforced shotcreto. No ~ n e n t shotcreting was done below the generator floor because that part of the cavern was later covered by the concrete walls of the powerhouse. kg/cm 20
Testing of Rock Anchors
18
I n a s t r o n g , umCractm'ed r o c k m a s s , a
load on a rock anchor is fully transferred to the rock only a few tens of centimeters from the rock surface. In highly fracturedrock, such as thebasalts at Blanda, longer bolts are required to transfer the same load to the rock. With regard to the design of rock anchors for the crane be~_m~ and col11mns in the powerhouse, a pull-out test on rock anchors was conducted. The anchors used were 25-ram bars with a yield point of 400 MPa. The anchors were tensioned to yielding, and deformations along the anchor registered by electronic strain gauges. Figure 7 shows best-fit curves for a series of five tests conducted in the tholeiitic basalts. At 2 tons of pull, the stress in the anchor is close to zero at a depth of about 1 m; but at a pull of 20 tons, approximately zero stress is reached at a depth of 2-2.5 m. These curves are for short-term equilibrium (less than 24 hrs); with long-term loading, the stress distribution will change and the loads will be transferred further into the rock mass.
16
14 12 Z
O_ 10 z ~ 8
i
0
1 DISTANCE FROM ROCK SURFACE
2
3m
Figure 7. Stress distribution along rock anchor under tension.
Acknowledgments This paper is based on reports made by the National Energy Authority of Iceland on pre-investigations for the Blanda Project and for measurement of dy~Rm~c modulus; on calculations and
[]
e v a l u a t i o n s by VST C o n s u l t i n g Engineers on finite d e m e n t analysis of t h e p o w e r h o u s e cavern; and on surveying and testing at site, performed by the staff of the Resident Engineer. The contributions of all are greatly appreciated.
ma.s.I.
L/
\
/
\
I
--
138.6
--
133.5
/
\
/.,_ I
\
\ \
\
/ / (O,STRI-~ I BUTOR I LTU__NN~
/
--I
I L-/ /
-- 127.5
--
121.0
t
\
\
\ --115.0
TAILRACE
I
]
TUNNEL
I
I
~__L
--
[
I
J
108.2
Figure 6. Relative deformations of the powerhouse cavern.
Volume 6, Number 2, 1991
Tus~.r.r,r~G ANDUNDERGROUNDSPACETECHNOLOGY239
Iceland's Blanda Hydroelectric Project: Monitoring of Deformations, Rock Support and Testing of Rock Anchors in the Powerhouse Cavern Sveinn Thorgrimsson, Matthlas Loftsson and Olafur Jensson
A b s t r a c t m A s part of a national plan to harness Iceland's l~surn@mLe projet hydro-dlectrique Blanda qui est actuellement en considerable hydro energy potential, the Blanda Hydroelectric construction et qui sera dtat de plein fonctionnement ~ partir de 1992, Project is being built and will be fully operational in 1992. Site fait partie d"un plan national d'exploitation des ressources ~nerg~tiques exploration for the 150-MW, 720-GWh /yr facility have involved considerables du pays. L'exploration du ehantier pour la centrale de extensive geological and geophysical investigatione. This paper 150 MW, 720 GWh / annuelles a entra2nd des recherches gdologiqnes et discusses the rock properties of the site, results of geological gdophysiques extensives. Cet article traite des propri~t& des rochers investigations, monitoring of deformations during and after du chantier, des r~sultats des reeherehes gdologiques, du contr~le des excavation of the powerhouse cavern, and measures taken for roek d~formations au courset apr~s l'excavation de la caverne de la centrale et des mesures prises en mati~re du sout@nement. support.
Introduction celand is a volcanic island, rich in thermal as well as hydro energy. To date, only about 12% of its e n e r g y resources have been harn e s s e d . T h e B l a n d a Hydroelectric Project, whichis a 150-MW, 720-GWh/ year facility, is the most recent power scheme, will be partly in operation in 1991 and completed in 1992. It is located in northern Iceland, about 340 km from the capital city of Reykjavik. The o w n e r of t h e p r o j e c t is Landsvirkjun (the National Power Company). This paper deals with the rock properties on the Blanda site, geological havestigations, monitoring of deforma tions during and atter excavation of the powerhouse cavern, and the measures taken for rock support. A short description of a pull-out test on rock anchors in the highly fractured basaltic rock concludes the paper. Figure 1 shows the generallayout of the underground structures, with the main dimensions of the powerhouse cavern andtunnels, excavationofwhich took place from 1984 to1987. Conven-
I
Present address: Sveinn Thorgrlmsson, Resident Engineer; Matt hias Loftsson, Engineering Geologist;and OlafurJensson, Project Manager, Landsvirkjun, H~aleitisbraut 68, 103 Reykjavlk, Iceland.
tional drill-and-blast methods were used in the excavation, except for the two shales, which were raise-drilled.
Rock Conditions Site investigations preceeding excavation involved extensive geological and geophysical investigations, including 2230 m of core drilling. Preinvestigations were undertaken by the National Energy Authority of Iceland. The rock formations are six to seven million years old, and predominantly volcanic in origin. The bedrock is built up of 5- to 7-m-thick tholeiitic and porphyritic basalts, with scoriacious contact breccias and interbedding of sedimentary rocks. The tholeiitic basalts are highly fractured, with rock substance often consisting of fist-size stones. The fracturing is for the most part a result of previous tectonic stresses. In drill cores, R Q D values range between 20 and 80, with a mean value of about 45. The fractures coated with silts and clay, mostly rock flour.SmAll quantifies of the clay mineral smectite have been detected. The porphyritic basalts are less fractured than the tholeiitic basalts, even though both have been subject to the same tectonic forces. RQD-values range between 50 and 90, with a m e a n value of about 70. The reason for the difference in fracture frequency between the two basalt types is explained
Tunnelling and UndergroundSpace Technology, Vol. 6, No. 2, pp. 235-239, 1 9 9 1 . Printed in Great Britain.
0886-7798/91 $3.00 + .00 Pergamon Press pie
by their different deformation properties. Porphyritic basalts were only encountered in the tailrace t~Innel. The top and bottom part of each basalt layer is made of scoria. Because the engineering characteristics of the scoria are different from those of the more solid main part, the scoria is taken as a separate entity. The sedimentary rocks are in most cases silt- and sandstones; silt fractions up to 60% have been measured. Clay fractions vary in most cases between 2 % and 8%, only part of which is clay minerals, mostly smectite. Oedometer swell testsshow only mlnor swelling. The sedimentary layers, however, have proved to be sensitive to changes in water content. Weakening and some disintegration of layers has been observed as a result of reduced confinement, and of repeated wetting and drying. As an indication of the characteristicsofthe rock substance, Table I shows a few geotechnical properties, presented as average values (x)with standard deviations (s) of samples from several sites in Iceland.
Initial Evaluation of Deformation and Support Requirements Before excavation started, a twodimensional finite element model was
235
PRESSURE SHAFT
Lf235m, A-11m 2
(~
POWERHOUSE CAVERN
Lffi66m, W-12,5m, H "30m
(~
SURGE TUNNEL
L" 235m, A-22-37m 2
(~)
ACCESS TUNNEL
L ' 8 0 8 m , A-25m 2
(~
ENTRANCEHALL
L -28m L-215m, A-11m 2
(~)
CABLE AND ELEVATOR SHAFT
(~
TAILRACE TUNNEL
L- 1,700m, A-36m 2
(~)
TAILRACE CANAL
L" 1,200m
~_~
ACCESS TO TAILRACE
(~
CONTROL AND SWlTCHGEAR HOUSE
(~)
TRANSMISSION LINES
Figure 1. General layout of tunnels and powerhouse cavern for the Blanda hydroelectric facility. employed, mainly for initial evaluation of stability and support requirements. The calculations were based on elastic deformations of a homogeneous material. The in-situ stresses and rock properties used in the model were estimated to a great extent. The in-situ stresses used were estim a t e d on t h e b a s i s of p r e v i o u s hydrofracture measurements in boreholes. These measurements were interpreted tA~nginto consideration the influence of the local topography and
experience from other sites. At the proposed powerhouse location, the vertical and horizontal stresses were accordingly estimated to be approximately equal, or 6-7 MPa, and k = 1.0. Part of the pre~investigations involved evaluating rock cores with regard to the main characteristics that affect t - n n e l stability. Comparison of several recognized classification systems revealed t h a t the Norwegian tunnelling rock quality index (Q-value) was better suited t h a n other systems
to estimating tunnel support in the fractured basalts. However, some adjustment in the rating of the joint set n u m b e r and joint r o u g h n e s s was needed to give better correlation to the ]~rn~ted openings available. The rock in the powerhouse cavern consisted mostly of tholen'tic basalts and scoria. In mostcases, calculated Qvalues in the tholeiitic basalts were between 2 and 4, but ranged from 0.3 in fracture zones to 8.3 in the least fractured sections. The Q-values for the
Table 1. Geotechnical properties of rnain rock types from various sites in Iceland. Uniaxial Compressive Strength (MPa)
Elastic Modulus (GPa)
Poisson Ratio
Average
Standard
Average
Standard
Average
Value (x)
Deviation (s)
Value (x)
Deviation (s)
Value (x)
Standard Deviation (s)
Tholeiitic basalts
242
90
37
8.5
.123
0.25
Porphyritic basalts
168
69
25
7
.130
.007
Scoria
25
17
4.1
1.3
.180
.078
Sedimentary rocks
21
17
3.8
5.7
.066
.095
Rock Type
236 TUNN~LLn~GANDUNDERGROUNDSPACETECHNOLOGY
Volume 6, Number 2, 1991
scoria were somewhat higher, with a weighted average of about 4. The Q-values were used for estimating the elastic modulus E for the finite element model. The E-values obtained in t h ~ m A n n ( ~ " Z~_n m ~ fi'om 5 to 20 GPa, and were regarded as only a rough appre~mAtion. These values are dearly lower than the laboratory values for the rock substance presented in the table. The uncertainty of these and other values was given due consideration during the first calculations.
/
tom
.o
o..
~-~ o~ ,','~ .o÷
//// oO
~" ,.o
o...,,
10m
Em
o~ o+
12
14
16
15
2O
............
22
24
DAYS
:2' 5T.35
:t,,. Monitoring of Rock Deformations
t a k e n ~ a task t h a t has continued during the construction period. Based on measured deformations, the premises for the finite element model were revised and gradually the range of values could be narrowed down. During excavation of the top heading in the powerhouse, points for measuring convergence with tape extensometers were established, and rod extensometers were installed at depths of 2 m, 6 m and 10 n~ The top heading was excavated in two parts--first the left half, followed by the right half. Figure 2 shows the status of excavation when the rod extensometers were installed, and the reaction of the roof to the excavation. During excavation of the left half, very small deformationsinthe roofwere registered. As soon as the excavation of the right part started deformations began, reaching a maximum value when the excavation passed the measuring station at 35 m, and gradually decreasing thereafter. At completion of the excavation of the top heading, deformations at the measuring station had stepped for the most part, and a new equilibrium had been obtained. After excavation of the top heading and the first 6-m bench, the results from the tape- and rod extensometer measurements were used independently to obtain the elastic modulus E and the stress ratio k by back-calculating these values from the observed deformations. Figure 3 shows the calculated values for both vertical and horizontal deformations of the top heading, with deviations due to assumed variation in the Poisson ratio. According to this calculation, the most probable values for E and k are E = 9.8 GPa and k = 1.25. These values were used to reevaluate probable deformations and establish new safely margins for the powerhouse cavern as the excavation proceeded. All finite element calculations were undertaken by VST Consulting Engineers. The excavation of the powerhouse cavern was conducted in several stages.
Volume 6, Number 2, 1991
D.
i t. ~'~ .......
Du;~Jlg excavation, extensive monit o r i n g of rock deformations was under-
2m
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DAYS
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A. DISPLACEMENT MEASURED BY VERTICAL ROD EXTENSOMETERS B. OISPLACEMENT MEASURED BY 45=INCLINED ROD EXTENSOMETERS C. CROSS-SECTION D. PLAN-SECTION
I SHOWING STATUS OF EXCAVATION WHEN EXTENSOMETERS WEREINSTALLED
!
Figure 2. Measurements of displacement of top heading. -
MPa
FOR V E R T I C A L D E F O R M ~ 10.000 9.800-
5.000
.\
~
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\
\
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, OR HORIZONTAL DEFORiATIONS
15.000 0,5
1,0
1,25
1,5 k
STRESS RATIO k ( (T'H/0"v )
Figure 3. Relationship between elastic modulus and stress ratio from backcalculating measured vertical and horizontal deformations (VST Consulting Engineers).
~.Lr~G
ANDU~U~.P~OUSD SPACETECHNOLOGY237
ma.s.I. 138 --
2t 3
132 --
126
--
118
--
113
--
111 --
Figure 4. Section through the powerhouse, showing typical lines for convergence measurements and stages of excavation.
The top h e a d i n g was excavated in M a y 1985. The first 6 m bench down to t h e generater floor was excavated 10 months later, in March-April 1986. The m a i n p a r t of t h e cavern down to t h e d r a i ~ u b e s and s u m p w a s excavated late in 1986, a n d excavation for drafttubes a n d sump was u n d e r t a k e n in two stages in 1987. As t h e e x c a v a t i o n c o n t i n u e d , n e w m e a s u r i n g p o i n t s were i n s t a l l e d . A typical m e a s u r i n g section a n d stages of excavation are shown on F i g u r e 4. The rock r e a c t e d v e r y closely with t h e excavation of t h e cavern. Deformations began as soon as excavation commenced, but the rate of deformation decreaeedrapidly as excavation stopped. Curves for deformations at t h e middle section of the cavern are shown on Figure 5, a n d t h e lines refer to F i g u r e 4. The g r e a t e s t deformation was measured on line designated 6-7 a t elevation 121 m, which is about 10 m above the drafttubes. Total convergence wall to wall on this line was 90 ram. Total
convergence on line 4-5, which is about 6.5 m higher (elevation 127.5 m) was 60 ram. Total convergence was 25 m m on line 2-3, and 18 m m on line 8-9. The highest r a t e of deformation occurred during excavation of t h e second bench, between elevation 126 m a n d 118 m. For this period, t h e average convergence wall-to-wall was 253"10 -2 m m / d a y o n l i n e 6-7, and82*10 -2ram/day on line 4-5. After all excavation in t h e powerhouse cavern was completed, a creep r a t e of up to 0.22 * 10 s ram/day was m e a s u r e d on lines 4-5 a n d 6-7; however, by the end of t h e y e a r 1989, no such movements could be registered. Figure 6 shows, in a very exaggera t e d way, the relative deformations of the walls and ceiling of t h e powerhouse cavern. The direct m e a s u r e m e n t s described above show somewhat g r e a t e r deformations t h a n those calculated by t h e finite element model. This is not unexpected and reflects t h e limits of t h e ability of t h e model to s i m u l a t e the actual rock mass properties. Nonetheless, the relatively simple model gave a satisfactory approximation to t h e actual conditions and proved to be of aid in the overall design. F o r establishing t h e elastic properties of t h e rock m a s s further, t h e dyn~mlc modulus E-dyn was m e a s u r e d with seismic velocity m e a s u r e m e n t s . The m e a s u r e m e n t s were carried out b y the N a t i o n a l E n e r g y A u t h o r i t y of Iceland. E-dyn was m e a s u r e d a s 36 G P a for t h e less fractured tholeiitic basalts, and about 25 G P a for heavily fractured tholeiitic b a s a l t s a n d scoria. The monitoring of deformations in the cavern have established t h a t in spite of the high fracture frequency of t h e rock and t h e presence of w e a k scoria layers, t h e rock h a s considerably b e t t e r stability t h a n can be anticipated by t h e use of t h e e s t a b l i s h e d classification systems. The raA~n explanation for t h e relatively good stability of t h e highly fract u r e d b a s a l t s is the unevenness or undulation ofjoints and fractures, t h e fact t h a t joints and fractures are not con-
1986 mm
03
i4
05
06
07
tinuous through the rock m ~ s , and the high strength of the rock substance. The rock mass is made ofirregularly stacked interloctdng blocks of otherwise i n t a c t material. The rock m a s s is, thus, able to s u s t a i n high s h e a r stresses; however, because of t h e high fracture frequency, it needs to deform substantiallyfor the shear strength of the rock mass to be mobilized. As noted above, the m a x i m u m convergence of the powerhouse walls was measured at 90 ram, i.e.,an average movement of each wall of 45 m m before a new stress equilibrium was reestablished after the excavation.
Rock Support Supporting of the powerhouse cavern was undertaken in two--and, in some places, thrcc ctages. W o r k support consisted of applying a thin layer of shotcrete on the rock surface altereach blasted round. PermAnent supporting followed, by rock bolting;and, in the top heading and cavern above the generatot floor, by application of a second shotcrete layer. In the top heading, the initial shotcrete thickness was about 3 to 5 cm on b a s a l t a n d scoria, which cover most of the crown. O n the sedimentary rock, 10-cm-thick, steel-fiber-reinforced shotrete (type EE-fibres) was applied. Rockbolts were fully grouted 25 m m reinforcement bars 4 m long in basalts and scoria and placed on a 2 m by 2 m grid. In the sedimentary rock, which is lAmlnated w i t h n e a r horizontal w e a k ness planes, 6-m-long rock bolts were placed on a 1.5 m by 1.5 m grid. The p e r m a n e n t shotcrete layer was steelfiber- reinforced a n d 10 c m t h i c k on the entire top heading. F o r work support, t h e walls of the cavern were shotcreted w i t h a layer of unreinforced shotcrete about 3 to 5 cm thicl~ Rock bolting followed, using 4-mlong bolts i n s t a l l e d on a 3 m b y 3 m grid, b u t a t closer spacing a r o u n d corners a n d openings. As p e r m a n e n t support, walls above t h e g e n e r a t o r floor were given a 5- to 1 0 - c m - t h i c k l a y e r of
1987 08
09
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Figure 5. Measured deformations wall-to-wall in the powerhouse.
238 TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY
Volume 6, N u m b e r 2, 1991
un~inforced shotcreto. No ~ n e n t shotcreting was done below the generator floor because that part of the cavern was later covered by the concrete walls of the powerhouse. kg/cm 20
Testing of Rock Anchors
18
I n a s t r o n g , umCractm'ed r o c k m a s s , a
load on a rock anchor is fully transferred to the rock only a few tens of centimeters from the rock surface. In highly fracturedrock, such as thebasalts at Blanda, longer bolts are required to transfer the same load to the rock. With regard to the design of rock anchors for the crane be~_m~ and col11mns in the powerhouse, a pull-out test on rock anchors was conducted. The anchors used were 25-ram bars with a yield point of 400 MPa. The anchors were tensioned to yielding, and deformations along the anchor registered by electronic strain gauges. Figure 7 shows best-fit curves for a series of five tests conducted in the tholeiitic basalts. At 2 tons of pull, the stress in the anchor is close to zero at a depth of about 1 m; but at a pull of 20 tons, approximately zero stress is reached at a depth of 2-2.5 m. These curves are for short-term equilibrium (less than 24 hrs); with long-term loading, the stress distribution will change and the loads will be transferred further into the rock mass.
16
14 12 Z
O_ 10 z ~ 8
i
0
1 DISTANCE FROM ROCK SURFACE
2
3m
Figure 7. Stress distribution along rock anchor under tension.
Acknowledgments This paper is based on reports made by the National Energy Authority of Iceland on pre-investigations for the Blanda Project and for measurement of dy~Rm~c modulus; on calculations and
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e v a l u a t i o n s by VST C o n s u l t i n g Engineers on finite d e m e n t analysis of t h e p o w e r h o u s e cavern; and on surveying and testing at site, performed by the staff of the Resident Engineer. The contributions of all are greatly appreciated.
ma.s.I.
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Figure 6. Relative deformations of the powerhouse cavern.
Volume 6, Number 2, 1991
Tus~.r.r,r~G ANDUNDERGROUNDSPACETECHNOLOGY239