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Correlation between observed support pressure and rock mass quality
RESEARCH
Correlation between Observed Support Pressure and Rock Mass Quality Bhawani Singh, J. L. Jethwa, A. K. Dube and B. Singh
Abstract The correlation between rock muss quality and support pressure proposed by Barton eta]. (1974) has proven useful, except in cases of squeezing ground conditions. Field data collected systematically from 20 tunnel sections indicate a clear need for correction factors to account for height of overburden and tunnel closure, which do not seem to be adequately accounted for by the stress reduction factor. As expected, the support pressure decreases rapidly with tunnel closure and then increases beyond a limiting closure. The fact that the observed wall support pressures were always close to zero except in squeezing ground conditions has been taken care of by slightly modifying wall factors for Q-wall. A criterion derived from the field data shows that squeezing ground conditions would be encountered where the height of the overburden is greater than 350 Qm. The data reported herein confirm the earlier findings of Barton et al. (1974) that the support pressure is independent of the tunnel size.
Introduction herellability of a realistic quantitative classification system for estimatingt~nnel support pressure has increasedwith the passage of time. Ever since its development, the Q-system of Barton et al. (1974) has attracted interest oft~mnel engineers, field geologists and researchers. In spite of being overly comprehensive and complicated, this classification method has now found acceptance. Jethwa et al. (1982) measured the support pressure by load cells and contact pressure cells in several steel-ribsupported t~mnel sections through both squeezing and elastic ground conditions and compared the measured values with those estlmsted after Q-system. The study brought to light significant limitations of Barton's methods for application to tunnel sections under squeezing ground conditions. For example, the support pressure is a funcT
Present address: Bhawani Singh, Professor, Dept. of Civil Enffineering, University of Roorkee, Roorkee 247 667, India; J. L. Jethwa, Asst. Director, C M R S Unit, Q/8, Laxmi Nagar, Nagpur 440 022, India; A. I~ Dube, Asst. Director, C M R S Unit, CBRI, Roorkee - 247 667, India; B. Singh,
Director, Central Minlu~Research Station, Dhanbad - 826 001, India.
tion oftlmnel closures, which, in turn, depend on the support stiffness. Furthermore, a t~mnel at a greater depth is likely to attract higher support pressure. The t~nnel closure and therefore the support pressure continues to build up for a considerable time due to creep of the failed rock mass. Empirical correlations developed in a effort to eliminate the above limitations of the Q-system are discussed herein. Because the proposed empirical correlations are based on only 24 tunnel sections, there is scope for refinement.
Recording of Field Data The
following field data w e r e
collected: a) Radius of tunnel excavation. b) Depth of tunnel section from ground level. c) Unit weight of ground overlying the tunnel section. d) Q of the rock mass around the tunnel section. e) RMR of the rock mass around the t~mnel section. i9 Hoop load in steel ribs by compression load cells. g) Radial support pressure by contact pressure cells. h) ~[Mnnel closure by the tape extensometers and closure meters.
TunneUingand UndergroundSI~ceTechnology,Vol. 7, No. 1, pp. 59-74, 1 9 9 2 . Printed in Great Britain.
P ~ s u m A - L a correspondanceentre/aqual/~ de/a masse rocheuseet/a pression de soutienpropos~par Barton et at (1974)s"est r ~ utile, sauf dans les cas de terrain entas~. Lee d o n n ~ sur le terrain r~ueillies s y ~ dans 20 sections de tunnel diff~rentesiadiquent que les facteurs correctifs doivent tenir compte de la hauteur du surchargement et de la ferme~re da tunne& lesqueUessemblont pas ~tre suffiswnment prises en comptepar les facteurs de r~duction de t~nsion. Commepr#vu, lapreseion de soutien diminue rapidement avec la fermeture du tunneler puis augmente au-del~ de la fermeture d~limitante. Le fair que les pressions de soutien de paroi obeero~es~talent constamment proches de z~ro sauf dans les cond~ns de terrain e n t a ~ a ~ corrig~grdce a une l~gare m o d ~ des facteurs de la paroi Q (Q-wall). Des c~.res d,~riv&des donn~essur le terrain indiquent que les conditions de termin entas~ seraientpr&entes aux endroits of~la hauteur de la surcharge est sup~rieure a 350 Q. I~s d o n n ~ p u b l i ~ ici confirment les d&ouvertes p~,c~dentes de Barton et at (1974)selon lesquelles la pression de soutien est indc~ndante des dimensions du tunnel.
0886-7798/92 $5.00 + .00 ~) 1992 Pergamon Press plc
Deep-seated radial displacement of the rock mass around the tunnel opening by single- and mnltipoint borehole extensometers. Q and RMR It is general practice to divide a
tunnel into several rock mass units on the basis of the variation in the geomining conditions. Each rock mass is then assigned a Q value, depending on the values of the size parameters RQD, Jn, Jr, Ja, Jw, and SRF. It has been experienced t h a t a single value of some of these six parameters is sometimes influenced by personal bias. Therefore, a range of values is assigned to these six parameters and a range of Q is obtained. The range and average values of Q obtained fromtnnnel sections are given in Tables A1 and A2 in the Appendix. The rock mass ratings RMR (after Bieniawski 1981) were also obtained. The correlation of RMR and Q provided the necessary confidence (Jethwa et al. 1981). Whenever Q value was doubtful, the doubt was reflected in a wider range of Q and the absence of a close correlation with RMR. Support Pressure Compression load ceils of 50-100 tonne capacity and contact pressure
59
cells of 5-15 kg/cm2capacity were used to measure support pressure on steel ribs. The load cells were inserted into rib joints, a vertical joint at the tlmnel crown and two horizontal joints at the spring level. No load cell was installed at the bottom. The vertical support pressure was obtained as a ratio of the arithmatic sum of the loads recorded by the two load cells installed at the spring level to the product of the excavation width and the rib spacing. Since no load cell was installed at the bottom, the horizontal support pressure was taken as a ratio of the load recorded by the crown load cell to the product of half the excavation height and the rib spacing. The coutactpressm~cellswereinstalled at the interface of the steel ribs and the bael.-Rll The load cells and the contact pressure cellswere installedcloseto the bmnel face. All ofthese instruments were protected against directhit during blast~ ing. The blast vibrations did not affect these ~ o u t s .
Tunnel Closure Diametral deformations of the tunnel sections were m e a s u r e d by t a p e extensometers, closure meters, and sometimes even by simple invar tapes. The change in tlmnel diameter was halved to obtain the radial tunnel closures.
Type of Rock Masses The instrumented tunnel sections covered both hard rock masses such as quartzites,metabasics, and dolomites; and c o m m o n l y occurring soft rock masses such as shales, clays, slates, and phyllites.
A clear line of demarcation between the elastic and the squeezing conditions can be seen. The equation forthis line has been obtained as
Theoretical Criterion Theoretically,squeezing conditions around a t-nnel opening would be encountered if ~6 > q~
H = 350 Q,3
(1)
Thus, a rock mess m a y undergo squeezing w h e n the depth of the tunnel section exceeds 350 Qm.
where ~o is the tangential stress and Cl¢ is the uniaxial crushing strength of the rock mass. In the case of a circular tunnel under hydrostatic stress field, Eq. 1 can be written as 2P > q¢
(3)
Comparison of Measured and Predicted Roof Support Pressure
(2)
Barton's correlation, given in Eq. 4 below, was used to obtain predicted values of short-term roof support pressure p~. As discussed above, measured roof support pressure values were obtained from instrumented t-nnel sections. Out of a total of 19 case histories listed in Tables A1 and A2, 16 cases have been included in this analysis. These 16 case histories involve 8 tunnel sections under non-squeezing and 8 under squeezing ground conditions. The measured roof support pressures have been compared with the predicted values shown in Figure 2. The comparison has not been shown for the wall support pressure because the number of measurements is small. For predictedroofsupport pressures, the classification methods of Terzaghi (1946), Deere et al. (1969), Protodyakouov(1963), Wickhamet al. (1974), Barton et al. (1975), and Bieniawski's RMR method (supplemented by Unal 1983) have been used. It can be seen that the predictions are unreliable in all cases except one. In the case of Barton's Q-system, the predictions are reliable for non-squeezing conditions. The predictions turned out to be unsafe for squeezing ground. ForexAmple,
in which P is the primary stress value. It follows that a tunnel section experiencing elastic conditions in a given soft rock mass can encounter squeezing conditions if the prlm~ry stress level increases due to increase in the tlmnel depth or any other reason. This explains why phyllites and shales squeeze at one place and present elastic conditions at another, as shown in Tables A1 and A2 (in the Appendix). Equations I and 2 can thus be used to predict squeezing conditions in a tunnel, provided that P and q~ are known.
Empirical Criterion Measurement ofp~rnary stress field and the in-situ crushing strength of rock masses across a tvnnel for predicting squeezing conditions is both expensive and time-cons-mlng. Therefore, an attempt was made to seek a simple criterion for predicting squeezing conditions. An empirical criterion was developed (as shown in Fig. 1) that gives a log-log plot between the tunnel depth H in metres and the logarithmic mean of the reck mass quality Q. Some of the case histories of Barton et al. (1974) have also been used in Figure 1.
Criterion for Squeezing Ground Condition Incompetent or soft rock masses characterized by low in-situ crushing strength undergo plastic failure when overstressed. Such a rock mass around a t - n n e l opening fails when the tangential stress exceeds its uniaxJal crushing strength. The failure of the rock mass is associated with volumetric expansion, which is manifested in the form ofradialinward displacement of the t - n n e l periphery called t - n n e l wall. These deformations are called tunnel closures. The t - n n e l closures can be very large (measured closures have been as large as 17% of the size of the t-nnel opening). This phenomenon is called "squeezing" of the rock mass. The squeeze can occur not only from the roof and the sides, but also from the floor. ~ m n e l closures resulting from the elastic relaxation of a t - n n e l opening, on the other hand, are smaller than 1% of the tlmnel size (see measured values in Tables A1 and A2, Appendix). 60
,® o -- MANERI BHALI PROJECT b -- SALtd. PROJECT c -TEHRI DAM PROJECT d -- SANJAY VIDYUT PARIYOJNA • - KOLAR GOLD MINES f - CHHIBRO - KHOORI T UNNEL g -- GIRl HYDEL TUNNEL h - LOKTAK HYDEL TUNNEL i -- KHARA HYDEL PROJECT
2000
I00C
-139-BARTON'S
• x
NON-SOUEEZING CONDITION SQUEEZING CONDITION
@
R O C K BURST
CASE HISTORIES
E
*, SQUEEZING d
Q: 5 0 0 x 9
x¢ / ®x ~ 142 g ~ / 141 - xo/~o / ~
Xg xh xf
x 159
xf c : y
/~s?9 ~
xg
2 O0 Xh
ei
/ /
100
•001
"01
-c
" x^ v e4a~ ' --
NON- SQUEEZING
et04
el05
eq
ec
;~
@101
•
OB
b
0.1
1
t0
100
Q
Figure1. Criteria for predicting squeezing ground condition.
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Volume 7, N u m b e r 1, 1992
WICKHA~
1. ~ m n e l depth or thickness of the overburden. 2. ~ m n e l closure. 3. Time. 4. %mnel size.
12 16 "
e~E
TERZAGHI~ I R ~
If other factors are unchanged, the t - n n e l closures depend on the support stiffness. It is difficult to estimate the support stiffness in the present case, since the stiffness of backfill has to be taken into consideration while estimating the overall stiffness of a steelrib support system. Therefore, bmnel closure has been used to replace the stiffness of a support system (Table 3).
8
12
- 8 4
cL •
O
x
xx
x
0
4
8
Probsd
1'2
0
1'6
4
kg /crn2
8
12
Influence of Overburden on Roof
SupportPressure
prObSd , ko /crn2
Barton et al. (1975) suggested the following correlations for support pressures:
12 DEERE
16
i~= 2Q~/J,
%12-
8
l~w = 2 Q i ~ / J r in w h i c h
8-
Pi~
4
%
4
x
• x
0¥ 0
Pi~ , 12
8
4
16
/a_ , u , - " * . ~x Or 4 0
pObSd kg /cm 2
x x
x
8
J 12
Probsd ko/crn2
x SQUEEZING , • NON- SQUEEZING 12'
NE u
S
A
R
T
O
12
~
e~E 8
8-
4-
x
x
./
~2"
~
I
x x
• i
i
4
S
12
]o(
0
x i 4
pob,d
, "~¢m a
xx i S
,
12
kglcm2
x SQUEEZING, • NON- SQUEEZING
Figure2. Comparison of predicted and observed roof support pressures.
the measured support pressures were 10.8 and 11.5 kg/cm2when compared to predicted values of 4.2 and 4.4 kg/cm2 for t~,n n el sections 2 and 4, respectively. Such large differences in the measured and p r e d i c t e d s u p p o r t p r e s s u r e s prompted the authors to look for possible reasons. Some of the data in Table A2 that are related to these four t-nnel sections have been shown in Table 1. It can be Volume 7, Number 1, 1992
seen from Table 1 that in the cases of sections 1 and 2, the difference in support pressure could be the result of depth, t-nnel closure, t-nnel radius, and time ofobservatious. Similarly, in t~mnel sections 3 and 4, the difference would be related to tlmnel radius and tlmnel closures. It follows that the following four factors might have influenced the measured support pressure:
short-term roof support pressure, = short-term wall support pressure, = Barton's joint roughness coefficient, short-term roof rock mass quality, short-term wall rock mass quality. =
The values of Q~ and Q~, have been taken as 5 times Qr and Q,, where Q, and Q , are Barton's rock mass quality for roof and wall rock, respectively (values of Q, and Q~ should be obtained separately for the roof and the wall rock, respectively). The short-term roof and wall support pressures were estimated from Eqs. 4 and 5. These values were used to calculate correction factor f for overburden or tunnel depth. The correction factor f is defined as a ratio of measured support pressure to the predicted support pressure. A relationship of f to t - n n e l depth is shown in Figure 3. Because the elasto-plastic theory suggests a linear relationship between the overburden pressure and the support pressure, a linear relationship has been attempted in Figure 3. According to Figure 3, the correctlon factor f can be given by f = 1 + ( H - 320)/800 > 1
(6)
in w h i c h H is t h e t h i c k n e s s of overburden or t, mnel depth in metres. The data points for squeezing ground appear to suggest that the line in Figure 3 should be much steeper to represent a natural trend. In reality, the difference between observed support
TUNNm~n~e ANDUNDERGROUNDSPACETECHNOLOGY 61
Table I. Details of tunnel sections under squeezing ground conditions (from Table A2). Support Pressure (kg/sq. cm)
Tunnel Tunnel
Depth
Radius
(m)
T y p e of Rock Mass
Q
1
Crushed red shales
0.025 to 0.10
1.5
2
-do-
-do-
Soft and plastic black clays within thrust zone
-do-
S. No.
4
Radial tunnel closure
Observation Period (months)
Predicted
Measured
(%)
280
3.3
3.1
2.8
4.5
680
4.2
10.8
1..2
0.016 to 0.03
1.5
280
4.4
3.2
4.5
26
-do -
4.5
-do-
4.4
11.2
1.7
26
pressures and proposed line is mainly the result of excessive tunnel closures, which have been taken into account by another factor, f', for squeezing ground condition. Some m a y doubt that the correlation proposed in Eq. 4 can account for the method of construction, the type of supports, the primitive stresses and tunnel closures. The instrumented tunnels were constructed by conventional means, i.e.,drillingand blasting followed by steel ribs. This practice resulted in significant damage to the rock mass. Therefore, equation 4 is on the safe side. In the case of machine t, mnelling, designers should reduce the support pressures obtained from Eq. 4 by perhaps 20%, as there will be reduced damage to the rock mass. Another valid concern is that the field data are not sufficient to prove the validity of the proposed correlations. In the opinion of the authors, the International'l~,nnellingAssociation should compile a data bank for observed supp o r t pressures from all parts of the world and should try to improve these correlations. Ratio of Wall Support Pressure to Roof Support Pressure
Barton et al. (1975) realized that the wall support pressure would be smaller than the roof support pressure and therefore suggested increasing the observed Q values for estimating the
(m)
wall support pressure, as shown in Table 2. The ratio of the wall support pressure p, to the roof support pressure p,, corresponding to Q i , ' , have also been shown in col,,mn 3 of~able 2.
The observed wall support pressures from some ofthe squeezing and the nonsqueezing case histories have been plotted in Figure 4. It can be seen that the recommendations ofBarton et al. (1975)
x2
2.5-
•
NON-SQUEEZING
x SQUEEZING
2.0-
3 • 11
1.5--
•
09
012 5x x7 x~, 511
I.O 0.8
J
61
• I0 , ,
0
~ l X6 200
xl l 400 OVERBURDEN ( H }~m
ob~ Pr
f
.... f - Pi( '
= 1+ (H-320}/SOO ~! I 600
8uO
Figure 3. Correction factor for overburden in Barton's correlation for shortterm roof support pressure under non.squeezing ground conditions.
62 TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY
Volume 7, Number 1, 1992
Table2. Wall factor Q~.. .:/ Q~for estimating wall support pressure. Authors' Recommendation
Recommendation of Barton et al. 1975 Range of Qi
Qi-w~u
Oj.wat]
1
2
3
4
5
6
<0.1
1.0
1.0
<0.1
1.0
1.0
0.1-10
2.5
0.7
0.1~
2.5
0.7
>10
5
0.6
>5
>15
0.0-0.4
are overly safe for ~. values greater than 5. The modified wall factors have therefore been recommended as shown in Figure 4 and Table 2.
f:= 1~°b~d
(7)
f- pi~ in which p ~ d = measured roof support pressure, p~ = predicted short-term roof support pressure, and f = correction factor for overburden (Eq. 6).
Correlation Between Support Pressure and Tunnel Closure in Squeezing G r o u n d C o n d i t i o n Variation of the normalized roof support pressure with the tunnel closure at the crown is shown in Figure 5. The ordinate represents f;,which isthe correction factor for tunnel closure at the crown. The correction factor f/is given as
Z
Range of Oi
I~
The data points in Figure 5 are taken from Table A2 and represent eight tunnel sections from four different tunnels. The normalized reef sup-
m
port pressures are higher for low tunnel roof closures. The roof support pressures decrease when the t~mnel closures increase and attain minimum values when the roof closures are appro~qmAtely 5%. The normAHTed roof support pressures again rise when the t~mnel roof closures exceed 5%. Such a variation is in conformity with the ground reaction curve concept. This trend is repeated in Figure 6, which shows the variation of the normRllzed wall-support pressure with the measured t~mnel-wall closures. The correction factor f~ for t, mnel-wall closure is given as
x
SQUEEZING GROUND CONDITION
•
NON-SQUEEZING GROUND CONDITION ACCORDING TO BARTON ET AL ( 1 9 7 5 )
---
SUGGESTED BY AUTHORS
Q i - w o l l : Qi
x x7
i
x3
Qi--wall = 2 . 5 Qi i
1
Qi-wall
" .5 Qi
I
1I
XI
I
k X2
,
,,I
0-001
, 0"01
,
,,I
l
0"t
i
I
1
I
|
e~L,4.,
I t0
,_z~'-~
, t00
Qi
Figure 4. Variation of ratio between wall support pressure and roof support pressure with short-term rock mass quality. Volume 7, Number 1, 1992
TuNN~.r.~ ANDUND~.R~ROUNDSPACE TECHNOLOaY 63
4 obsd PW
obsd
fPw f" Pr pObSd : w
pObSd : MEASURED ROOF SUPPORT PRESSURE
Pw Pr
3 == .=
= PRE~CTED ROOF SUPPORT PRESSURE
=
X4
MEASURED WALL SUPPORT PRESSURE PREDICTED
WALL SUPPURT PRESSURE
CORRECTION FACTOR FOR TUNNEL~WALL
f
= CORRECTION FACTOR FOR OVERBURDEN
CLOSURE
tr
: CORRECTIOI( FACTOR FOR TUNNEL CLOSURE AT TIlE CROWN
CORRECTION FACTOR FOR OVERBURDEN
o.
X4
x7 t~
$
/
.=, m =
o
// N
//X B
N "3
5X6~
/ X7
Y~ DATA POINTS PLOTTEO FROM
TABLE ].b
I
0
5
r
J
0
15
t0
OBSEBV.O TU..EL CLOSURE AT CBOWN
~.,.~
l~w where p~= f
=
Piw
=
(8)
measured wall support pressure correction factor for overburden, and predicted short-term wall support pressure.
Thus, the correction factors f'r and fw' are the same as the normalized roof and wall support pressure, respectively. The recommended values of these correction factors are given in Table 3. The validity of Table 3 for squeezing ground has been questioned, particularly with regard to highly squeezing or flowing ground. It is suggested that the application of Table 3 should be restricted to moderately squeezing ground by limiting closure to 5%, by strengthening the support system immediately. Serious construction problems may arise if this remedial measure is not followed. It is recommended that all such tlmnel sections be instrumented. Jethwa (1984) concluded that the wall support pressure may be significantly higher than the roof support pressure in the case of parallel tunnels if the clear spacingis less than the sum of the tunnel widths.
64
]
5 10 OBSE.VEO TU..E~ WALL CLOSU~(~
t5
Figure 6. Correction factor for wall closure under squeezing ground condition (H > 350 Q^1/3).
Figure 5. Correction factor for roof closure under squeezing ground condition (H > 350 Q^ I / 3).
f~ _I ~
t
0
Variation of Support Pressure with Time
port pressure, it has been possible to study the effect of time on the support pressure. Figure 7 shows the variation of the correction factors t~'over time. The correction factors f" for time are given as
After studying the influence of the overburden and the tunnel closures and incorporating these influences in the correlations between Q and sup-
Table 3. Correction factors for tunnel closures in squeezing ground conditions.
S. No.
1.
Ground Condition
Support System
Non-squeezing (H < 350 Ola) Squeezing
Tunnel Closure (%)
fworf;
<1
1,0
(H > 350 ~1/3)
Very stiff
1-2
> 1.80
3.
Moderately squeezing
Stiff
2-4
0.85
4.
-do-
Flexible
4-6
0.70
5.
Highly squeezing
Very flexible
6-8
1,15
6.
-do-
Extremely flexible
>8
1.80
2.
T I Y N N E L L I N G AND U N D E R Q R O U N D S P A C E TEcI-INOLOGY
Volume 7, Number 1,
1992
X
ROOF S U P P O R T
P R E S S U R E IN
•
ROOF S U P P O R T
PRESSURE
0
WALL
SUPPORT
SQUEEZING GROUND CONDITION
IN NON-SQUEEZING
PRESSURE
IN S Q U E E Z I N G
GROUNO C O N D I T I O N
GROUND CONDITION
.1 -
0 5 ....
. ...-.. . I I-'-t0
Combining Eqs. 9, 10, and 11, the long-term roof and wall support pressures can be given as
t
""
(12)
P. = Pl." f ' f ' ' log 9.5 t °'~
(13)
in which p, and p,, are long-term roof and wall support pressures. Barton et al. (1975) suggested that the ratio of the ultimate to the shortt e r m support pressure is about (5) ~, i.e., 1.7. Equation 11, however, suggests the following relationship:
I
"/
. ....4
f •
p, = p~' f ' f ' " log 9.5 t °'~
x,
p o~ f ' log 9.5 t °'~ i.e., •~
07
~
- 4 ~ 1 C - ~ , -~ ~
zx
i
~
x8
x3_
O~ .....-
....."-"'''-'""~
.~.
..... ,~...,. • ~
91 _f: log 9.5t 0a~
"
po f: log 9.5 t~a~ where Pl and Po are support pressures after t~ and t_ months of excavation. t ! In case ofa r ~ d hnmg, f~= fo' so that the ratio of the ultimate support pressure a i ~ r 100 years to that alter one month is given by (t o = 1 month and t~ = 100 years or 1220 months) u
t 0 2 A
,
,
'{
,
i
,
10
,1
L
100
t , TIME OF OBSERVATION IN
,
,
,
t000
.
,
•
.
MONTHS
Figure 7. Variation in observed support pressure over time.
f~_
~.,~,,a f" f" 1~
(9)
(lO)
-
f{~w
12CMRS
where
f;
= correction factor for the influence of time on the roof support pressure,
f~w
= correction factor for the influence of time on the wall support pressure,
1~b~
= measured pressure,
I~ ~
= measured wall support pressure, = correction factor for overburden (Eq. 6), and = correction factor for tlm~el closures (Table 3).
°'1E
8
roof support a - L .
f f'
All the data points from Tables A1 and A2, except those for the wall support pressures in the non-squeezing ground conditions (being negligible), have been plotted in Figure 7. According to Figure 7, the correction factor f " can be given by f" = log 9.5 t °'~ (11) in which t is the time in months after excavation of the t - n n e l opening.
Volume 7, Number 1, 1992
Q.
4
0
T"
I
|
1
0
4
8
12
pobsd , kg/crn 2 x
SQUEEZING
~
•
NON-
SQUEEZING
Figure 8. Comparison of observed roof support pressure with predicted values from authors" Eq. 12 (p, ffi p~. f . f . log 9.5 t^0.25).
TUNN~.LWOANDUNDERGROUNDSPACETECHNOLOGY 65
pz _ log 9"5t°'as - 1.75 po
log 9.5 t~25
In other words, the support pressure will increase in 100 years to 1.75 times the support pressure observed after 1 month of excavation. The corrected support pressures compare well with the observed values, as shown in Figure 8. This ratio of 1.7 between the ultimate and the short-term support pressure tallies with the (5) ~ suggested by Barton et al. (1975). However, the ultimate support pressure for t-nnels under squeezing reck conditions may be 2-3 times the short-term support pressure, according to Jethwa (1984). The ratio of the ultimate support pressure to the short-term support pressure, worked out here as 1.7, is relatively small compared to the ratio of 2 3 after Jethwa (1984), probably because the period of observations reported herein is relatively short and the number of squeezing case histories is small. Furthermore, in special cases of soluble or erodible joint fillings and where seepage is a serious problem, the long-term support pressure may be as high as the cover pressure or 6 times the short-term support pressures, whichever is smaller. This trend has been indicated from a 10-year performance study of Chhibro-Khodri under ground powerhouse complex in India (Mitra 1991). For designing a temporary support system, one m a y assume a unlforin distributionof the short-term support pressure, but a factorof safety of 1.75. However, the permanent support system should be designed for the net ul~mAte support pressures, with a factor of safety of 1.5. Effect of Excavation Support Pressure
G
Conclusions
The combined approach of field instrumentation and quantitative classification of Barton et al. has proved rewarding at this stage of development of rock mechanics. Despite limited field data, some practical trends showing the influence of overburden, tnnnel closures and time of excavation on the t , nnelling condition and the support pressures have emerged. It would perhaps be hasty to draw any definite conclusions from these trends; however, some tentative correlations have been possible. These correlations are subject to refinement as more field data is collected. The following tentative conclusions are possible from these correlations: 1. Squeezing is likely to occur in a t~mnel section where the height of overburden in metres exceeds 350 Q1/8. 2. The s h o r t - t e r m roof support pressure is given by the following correlations: 1~ =2.0 (5Q) ~3. f. f Jr in which f is the correction factor for thickness of overburden (H) in metres,
Size on
According to Terzaghi (1946) and Unal (1983), the support pressure is directly proportional to the size of a tunnel opening. O n the other hand, Barton et al. (1974), suggest that the support pressure is independent of the t - n n e l size. Colnmns 3 of Tables A1 and A2 (see Appendix) list support pressures obtained from Terzaghi's method for nonsqueezing as well as squeezing ground conditions. It may be noted that the estimated support pressure values (shown in col-ran 3) do not compare well with the observed support pressures (in co]-mnR 12 and 13). The support pressures corrected according to the proposed correlations (eol, mns 10 and 11) are in better agreement with the observed support pressures. Figure 9 shows the variation of pO~/p/f, f. f, with the diameter of
66 ~
discontinuities in a larger opening. Thus, the size effect is automatically accounted for in the est;m.te of Q. The adverse effects of deteriorating hydrogeological conditions ( J ) should also be determined, if possible, after a water tlmnel is commlssioned. Itwould therefore be unsafe to obtain Q from small drifts and use it to estimate support requirements for large excavations. For underground excavations in non-dialatant rock masses (schists, slate, etc.) with smooth planes of weakness, it is cautioned that Terzaghi's concept may still be v a h d
tunnel opening (2a). The ordinate represents the observed roof support pressure corrected for overburden, the tunnel closure and the time of excavation. It may be seen that the corrected support pressure is independent of the tunnel size. The reason for support loading in non-squeezing conditions may be related to the dead weight of the loosened rock blocks which become detached from the parent rock mass at the tunnel roof and rest on the support system. This type of support pressure is called the "loosening pressure". The loosening pressure has been attributed to poor blasting practice, gravity and delayed support in the form of steel ribs. Excessive tunnel closures under squeezing ground conditions are also considered responsible for mobihzing large loosening pressures, even in t u n n e l sections s u p p o r t e d by shotcrete immediately on excavation. The loosening pressure (dead weight of the loosened or a destressed zone) is therefore mobilized due to poor blasting practice and is likely to be independent of the tunnel size. In a recent study at the underground powerhouse complex of Lakhwar dam project in India (Q = 8-9 and H= 250 m), the observed roof support pressures were nearly the same (i.e., about 0.5 kg/cm2) for the 6-m-wide approach adit, 14-m-wide expansion surge tank and 21-m-wide powerhouse cavern, all excavated through tightlyjointed traps. These observations should erase all doubts about size effect in underground openings. It may be noted that rock mRss quality Q estimated from a larger bmnel would be smaller than that obtained from small driRsin a similar rock mass. This isdue to the possibilityofintersecting greater n u m b e r of geological
~,_
x • '
2-
*-
,,
.
.
.
.
SQUEEZING NON- SQUEEZING
.
.
.
.
.
.
.
.
.
• ~
w-=
.~_ ~ 1 "~
t x
61
-,-
~2
7
~X Q'7
"O
h-
o O
i
!
4
8
!
12
I
i
16
20
DIAMETER OF OPENING jm
Figure 9. Support pressure virtually independent of tunnel size.
ANDU~DZRGROUNDSFACZTzcm~oLoGY
Volume 7, Number 1, 1992
and f ' is t h e correction factor for t ~ n n e l closure (see Table 3, equal to 1 in nonsqueezing ground conditions). The value of t h e correction factor f i s given as f = 1+(H-320)/800
> 1
3. I n squeezing g r o u n d conditions, t h e s u p p o r t p r e s s u r e is significantly influenced by t u n n e l closures. The correction factor f' for t u n n e l closure v a r i e s from 0.7 to 1.8 in t h e case of a single t u n n e l . The m i n i m u m s u p p o r t p r e s s u r e occurs w h e n t h e t u n n e l clos u r e is about 5% of t h e t u n n e l diameter. The s u p p o r t p r e s s u r e i n c r e a s e s r a p i d l y beyond t h i s limiting closure. 4. The short-term wall support pressure m a y be o b t a i n e d from t h e above correlation b y s u b s t i t u t i n g Q~n for Q. I n general, t h e a c t u a l wall s u p p o r t p r e s s u r e for t h e non-squeezing rock conditions is likely to be negligible. The s h o r t - t e r m v a l u e s o f P i / p ~ d e p e n d on Qi (i.e., 5Q), as given below: Ptw/P.
Qt
1.0
5Q < 0.1
1.0 - 0 . 0
5 < 5 Q < 0.1
0.0
5Q > 5
5. The u l t i m a t e s u p p o r t p r e s s u r e m a y be 1.75 t i m e s t h e s h o r t - t e r m supp o r t p r e s s u r e for t l m n e l sections u n d e r non-squeezing g r o u n d conditions, except for cases of soluble a n d erodible j o i n t fillings w i t h seepage. 6. The s u p p o r t p r e s s u r e is indepond e n t of t h e t u n n e l size, provided t h a t Q is o b t a i n e d from a full-sized opening.
[] References Barton, N.; Lien, R.; and Lunde, J. 1974. Engineering Classification of Rock Masses for the Design of Tunnel
Volume 7, N u m b e r 1, 1992
Support, RockMechanics, Vol. 6, 189236. Springer-Verlag. Barton, N.; Lien, R.; and Lunde, J. 1975. Estimation of Support Requirements for Underground Excavations, Proc. Sixteenth Syrup. on Rock Mechanics, Univ. of Minnesota, Minneapolis, U.S.A., 163-177. Bieniawski, Z. T. 1981. Case Studies Prediction of Rock Masses Behavior by the Geomechanical Classification, Second Australia-New Zealand Conference on Geomechanies, Brisbane, 3641. Daemen, J. J. I~L 1975. ~ m n e l support loading caused by rock failure. Ph.D. Thesis, UniversityofMinnssota,U.S~. Deere, D. U.;Peck, R. B.;Monsees, J.E.;and Sc.hmidt,B. 1969. "Design of Tunnel Liners and Support System." Highway Research Record No. 339, U.S. Department of Transportation, Washington, D.C. Dube,~ I~ 1979. Goomechanicalevaluation of a tunnel stability under FAilingrock conditions in a Himalayan Tunnel. Ph.D. Thesis, Univ. of Roorkee, India. Dube, A. I~; Jethwa, J. L.; Singh, B.; Singh, Bhawani; and Mithal, R. S. 1982. Geoengineering evaluation of problems of a large underground cavity for Tehri Dam Project (India). ISRMSymp. Rock Mechanics: Caverns and Pressure Shafts (ed. W. Wittke), 239-244. Rotterdam: AM. Balkema. Dube, A. I~;Singh,B.;and Singh, Bhawani. 1986. Study of squeezing pressure phenomenon in a tnnnel--I and H. Tunnelling and Underground Space Technology 1:35-48. Jethwa, J. L.; Dube, A. I~; Singh, B.; Singh, Bhawani; and Viladlc,r, M. N. 1979. I n s t r u m e n t a t i o n and Design for multiple openings in f-illngrock mass. Int. Syrup. on In-Situ Testing of Soils and Rocks and Performance of Underground Structures, Roorkee, India, Dec. lg--22, 1979, Vol. 1. Jethwa, J. L.; Dube, A~ K.; Singh, B.; and Singh, Bhawani. 1981. Rock load estimation for t~mnels in squeezing
ground conditions. Proc. of the Rapid Excavation Tunneling Conference, San Francisco, Calif., May 3-7, 1981, 766783. New York: AIME. Jethwa, J. L.; Dube, A. IC; Singh, B.; Singh, Bhawani; and Mithal, R. S. 1982. Evaluation of classification system for tunnels in non-squeezing ground conditions. Proc. of lSRM Syrup. Rock Mechanics: Caverns and Pressure Shafts, ed. W. Wittke, 607-612. Rotterdam: AM. Balkema. Jethwa, J. L.; Singh, B.; and Singh, Bhawani. 1984. Estimation of ultimate rock pressure for tnnnel linings under squeezing rock Conditions--a new approach. Proc. ISRM Symposium on Design and Performance of UndergroundExcavations, Cambridge, U.K., Sept. 3-4, 1984. Mitra, S. 1991. Study of long-term behavior of underground powerhouse cavities in soft rocks. Ph.D. Thesis, University of Roorkee (under submission). Protodyakonov, N . M . 1963. Firmness coefficient for estimation of rock loads. Personal communication to Beas Design Org-ni~ation, New Delhi, India. Sharma, V.M. 1985. Prediction ofclesure and rock lo ads for tunnels in squeezing ground,. Ph.D. Thesis (p. 254), I.I.T., Delhi, India. Terzaghi, K. 1946. Rock defects and load on t~lnnel supports. In Introduction to Rock Tunnelling with Steel Support, I t V. Proctor and T. C. White (Youngstava, Ohio, U.S ~..: Commercial Shearing and Stamping Co.). Unal, E. 1983. Design guidelines and roof control standards for coal mines roofs. Ph.D. Thesis, Pennsylvania State University. Refer to p. 113 of Rock Mechanics in Mining and Tunnelling (Bieniawski, Z.T., 1984, Rotterdam: A.A. B-Ikema). Wickham, G. E.; Tiedmann, H. R.; and Skinner, E. H. 1974. Ground support prediction model--RSR concept. Proc. of North American Rapid Excavation and Tunneling Conference, San Francisco, California, Vol. 1,691-708.
TUNN~.T.n~a AND U~mE~mOUND SPACE TECHNOLOGY 6 7
¢.D ¢,D b~
l.a
r
0
<
2
S. No.
1
1.
Moderately fractured quartzite. a = 2.4 y = 2.5 RQD = 75, Q = 3.6 uh/a = 0.06, H = 225 (Jethwa et al. 1982)
from
0.3 to 0.7 (0.5)
3
kg/sq, cm
Terzaghrs Classification
Maneri-Bhati Hydro Project
Geological description
Vertical Support Pressure
15 tO 30 (21)
4
= 5Q
Qi
Shortterm Rock Mass Quality
0.5 to 0.7 (0.6)
0.1 tO 0.2 (0.4)
6
kg/sq. cm
kg/sq, cm
5
from Eq. 5
P.,
Horiz.
from Eq. 4
Vert.
Short-term Support Pressure
1.0
7
from Eq. 6
f
Overburden
f W'
1.0
8
1.0
9
from Table 4
f 1"'
Tunnel Wall Closure
Correction Factors for:
cm
cm
0.5 to 0.7 (0.6)
0.03 to 0.05 (0.04)
11
kg/sq,
kgL~l,
10
= P..,, f - f ,w
p.
Horiz.
= P.~, f . f 'r
p,
Vert.
Corrected Short-term Support Pressure:
Table A1. Comparison of predicted and observed support pressure from Q-system in non-squeezing ground conditions.
Appendix
0.6
12
cm
kg/sq,
p o=
Vert.
13
cm
kg/sq.
p.O=
Horiz.
Observed Support Pressure
14
month
Observation Period
Steel ribs stable
15
Remarks
O~ ~D
+
Q
i.a p.a ¢.0 ¢.0 bO
+
.<
2
Highly jointed dolomites. a = 6,~' = 2 . 8 RQD = 30-40 Q =1.2-1.7 H=110
5.
Grade I phyllites, massive and distinctly jointed a = 7,Y = 2.64 RQD = 75, RMR = 67 Q =5, H = 320 (Dube et al. 1982)
Khara Hydel Proiect
4.
1.7 to 5.5 (3.6)
(25)
(7)
6 to 8.5
1.5 to 16.5 (5)
0.8 to 2.6 (1.7)
Sheared matabasics. a=2,4,7 =2.5 RQD = 60 Q = 0.3-3.3 uh/a = 0.4 H = 350 (Jethwa et al. 1982)
3.
Salal Hydel Tunnel
1 7 t o 34 (24)
0.3 to 0.7 (0.5)
4
Foliated metabasics. a = 2.4;y = 2.5g RQD = 82, Q = 3.4-6.8 uh/a = 0.05, H = 550 (Jethwa et al. 1982)
3
2.
Maneri-Bhali I-Ivdel Proiect (contd.)
1
Table A1 (contd.)
0.16 to 0.45 (0.3)
1.0 to 1.2 (1.1)
0.7 to 1.8 (1.25)
0.5 to O.7 (0.6)
5
(0.13)
0.4 tO 0.5 (0.45)
0.2 to 1.2 (0.7)
0.1 to 0.20 (0.15)
6
1.0
1.0
1.04
1.29
1.0
1.0
1.0
1.0
1.0
1.0
1.0
(0.3)
1.0 to 1.2 (1.1)
0.70 to 1.9 (1.3)
0.6 to 0.9 (0.8)
10
(0.04)
0.14 to 0.2 (0.17)
0.2 tO 0.5 (0.35)
(o)
0.005 to 0.05
11
0.25 to 0.56 (0.40)
1.1
2.0
0.8
12
appears negligible
appears negligible
13
1.5
3
2
14
Steel ribs; stable
Steel ribs; stable
Steel ribs; stable
Steel ribs; stable
15
=~ ,e-
" -
~ :.--&6 =
A
" ~ -+ Lm _~ GO
[~.-' • 0
O
O
0
IN
~,,uo
v
0
O
v0
m-.
o.
v
q
b-
o.
o
O
o
o
O
q
o.
o5
O
~ 8 m---.
o
°oO~
t~ ~ V
-.:.
.+
~o
K"
_
>~.
.8
~ •-
o
0~ I-
,,
~
~ . ~ mr ~,~
O E o-Z m O z : ~ I
E'
n O
o
E
~D
el
o
E
~ o
~-
~
II e"
¢-
~ 8 m'-,
o
:D
"11"
m t:
..d
<,D
70
TUNN~,T,~G
AND UNDERGROUND SPACE TECHNOLOGY
O
v'
i,=
o5
V o l u m e 7, N u m b e r
1,
1992
~D ~D b~
2
Chalnage 389 m a = 3 , Q = 0.4 H = 150-200 uh = 22.5 mm
(ii) a = 7 (iii) a = 3
Tightly jointed basic reck. H = 250, Q = 8-9 (i) a = 10.5
--
do
do
0.35
(o)
do
6
(0.22)
do
5
++
Estimated support capacity.
Notations: a = average radius of tunnel opening in metres. RQD = rock quality designation in percent. Q = rock quality based on classification of Barton et al. (1974). 7 = unit weight or rock material in gm/cc. uh/a = tunnel wall closure in per cent. uv/a = closure at crown level in per cent. H = height of overburden above opening in metres. ( ) = average values, except in column 4, where root mean square value is given.
12.
42
do
Chalnage 204 m a = 6 . 5 , Q = 15 H = 52, uv = 5 mm
11.
Lakhwar Hvdro Proiect
75
do
4
Thinly bedded shales with calcite bands. Chalnage 761 m a = 6.5, uv = 12 mm Q=15, H=34
--
3
10.
Upper Krishna Project
9.
Khara Hydro Proiect (contd.)
1
Table A1 (contd.).
1.0
do
do
1.0
1.0
do
1.0
do
1,181
0.35
(0.22)
1.0
1.0
(0.22)
(1.7)
10
1.0
1.0
9
0.2 to 0.4 (0.3)
(o)
0.45 to 0.55
0.2
2.4
12
(0)
(1.1)
11
appears negligible
appears negligible
1.6
13
I
1.0
1.0
14
Distribution of support pressure is asymmetric
do
do
do
15
tj~ b3
-.3
< o
o
t~
2
a = 1.5, RQD =1020 Q = 0.025 -0.10 7 = 2.73 uh = 2.8 H = 280 (Jethwa et al. 1982)
OR!!==¢1
Chibro-Khodri Tunnel
1
S. NO.
Geological Description
1.8 to 3.4 (2.7)
0.125 to 0.50 (0.25)
4
2.5 to 4.2 (3.3)
5
3
1.8 to 3.0 (2.4)
6
kg/sq. cm
= 5Q
kg/sq, cm
p~,,
kg/sq, cm
P,~
Pressure
from Eq. 5
Qi
Shortterm Rock Mass Quality
S h o r t - t e r m Support
from Eq. 4
Vertical Support Pre~ure from Terzaghl's Classification
1.0
7
from Eq. 6
Overburden
(0.85)
1.8
9
from Table 4
from Table 4
8
fW'
wall
f'r
roof
T u n n e l Closure
Correction Factors for:
Table A2. Comparison of predicted and observed support pressure in squeezing ground conditions.
(2.8)
10
kg/sq, cm
f " f'r
= P~of"
P~
Vert,
(4.3)
11
kg/sq, cm
f " f*'
= P.=?
P.
Horlz.
Corrected Short-term Support Pressure:
3.1
12
kg/sq, cm
P~
1.7
13
kg/sq. cm
pw°b
Pressure
O b s e r v e d Support
14
month
Observation Period
Circular ribs stable
15
Remarks
~2
Q
b.a ~D CD ~0
2
5.
Very blocky and seamy slates, moderately squeezing. a = ~ . l , =2.5 uh/a = 7.6 H = 380 Q - 0.32-0.82 (Jethwa et al. 1982)
Giri Hydro Tunnel 0.7 to 2.3 (1.2)
do
Soft plastic black clays within thrust zone, moderately squeezing. = 2.64, RQD = 10 Q = 0.016-0.03 H -- 280 (Jethwa et al. 1982)
4.
Y
0.08 to 0.15 (0.11)
1.5 to 3.3 (2.4)
Soft plastic black clays in thrust zone, moderately squeezing. a~'5, = 2.64 RQD = 10 Q = 0.016-0.03 uh/a = 4.1 uv/a = 4.5 H = 280 (Jethwa et al. 1982)
3.
1.5 to 4.1 (2.5)
do
0.06 to 0.25 (0.12)
10.3 to 22.1 (16.2)
Crushed red shale, highly.squeezing a=415, =2.73 RQD = 10-20 Q = 0,012-0.5 uh/a = 6, H = 680 (Jethwa et al. 1982)
3
2.
Chibro-Khodri Tunnel (contd.)
1
Table A2 (contd.).
1.2 to 1.8 (1.5)
do
4.0 to 4.8 (4.4)
3.3 to 5.0 (4.2)
0.8 to 1.3 (1.0)
do
3.0 to 4.8 (3.9)
2.4 to 5.0 (3.7)
1.08
1.0
1.0
1.45
7
1.15
1.8
0.70
1.8
8
1.15
1.8
0.7
0.7
9
(1.9)
(7.9)
(3.1)
(11)
10
(1.2)
(7.0)
(2.7)
(3.8)
11
2.0
11.5
3.2
10.8
12
2.4
12.2
2.6
3++
13
12
26
26
8
14
Roof closure considered equal to wall closure as horseshoe ribs with invert struts deformed, but not as severe as in case 6.
Circular ribs of very high capacity were stable. Consequently, tunnel closures were likely to be low, approx. 0-2%.
Circular ribs were stable; compressible backfill behind ribs. Enlargement of drift to 9 m size in close proximity delayed stabilisation.
Heavy circular ribs; severe buckling due to squeezing. Little closure at crown.
15
b3
~D
0
Q
2
Crushed phyllites, highly squeezing. a = 2~1, = 2 . 3 RQD = 10-25 q = 0.124-0.32 uh/a = 12.4 uv/a = 5 H = 240 (Jethwa et al 1982)
Crushed shales, moderately squeezing. a = 2~., = 2 . 7 gm/cc RQD = 10-20 Q = 0.011-0.044 uh/a = 7 H = 300 (Jethwa et al 1982)
Highly fractured quartzites. a=23$, =2.5 RQD = 6O, Q = 0.5 uh = 190 mm H = 35O (Sharma 1985)
3
2.64
2.9 to 5.4 (4.2)
2.1 to 4.1 (3.1)
See footnotes in Table A1 for notations.
8.
Maneri Bhali Proiect
7.
L o k t a k Hydro Tunnel
6.
Giri Hydro Tunnel (contd.)
1
Table A2 (contd.).
(2.5)
0.055to 0.22 (0.11)
0.62 to 1.6 (1)
1.6
3.5 to 5.3 (4.4)
1.8 to 2.3 (2.1)
1.1
2.5 to 5.3 (4.0)
1.2 to 1.9 (1.6)
1.04
1.0
1.0
1.15
1.15
0.7
1.15
1.15
1.8
(1.9)
(5.1)
(1.5)
10
(1.3)
(4.6)
(2.9)
11
2.0
5.4++
1.7
12
5.4++
4.0
13
14
27
14
Supports buckled. Vertical and horizontal closures appeared equal.
15-cm-thick shotcrete with 4-m-long rock bolts supplemented with circular ribs. Squeezing occurred even at H = 160 m. Roof closure is considered equal to wall closures.
occurred at half total wall closures when horseshoe ribs with invert budded.
k0/sq, cm
measured support pressure of 5
Peak
15
Correlation between Observed Support Pressure and Rock Mass Quality Bhawani Singh, J. L. Jethwa, A. K. Dube and B. Singh
Abstract The correlation between rock muss quality and support pressure proposed by Barton eta]. (1974) has proven useful, except in cases of squeezing ground conditions. Field data collected systematically from 20 tunnel sections indicate a clear need for correction factors to account for height of overburden and tunnel closure, which do not seem to be adequately accounted for by the stress reduction factor. As expected, the support pressure decreases rapidly with tunnel closure and then increases beyond a limiting closure. The fact that the observed wall support pressures were always close to zero except in squeezing ground conditions has been taken care of by slightly modifying wall factors for Q-wall. A criterion derived from the field data shows that squeezing ground conditions would be encountered where the height of the overburden is greater than 350 Qm. The data reported herein confirm the earlier findings of Barton et al. (1974) that the support pressure is independent of the tunnel size.
Introduction herellability of a realistic quantitative classification system for estimatingt~nnel support pressure has increasedwith the passage of time. Ever since its development, the Q-system of Barton et al. (1974) has attracted interest oft~mnel engineers, field geologists and researchers. In spite of being overly comprehensive and complicated, this classification method has now found acceptance. Jethwa et al. (1982) measured the support pressure by load cells and contact pressure cells in several steel-ribsupported t~mnel sections through both squeezing and elastic ground conditions and compared the measured values with those estlmsted after Q-system. The study brought to light significant limitations of Barton's methods for application to tunnel sections under squeezing ground conditions. For example, the support pressure is a funcT
Present address: Bhawani Singh, Professor, Dept. of Civil Enffineering, University of Roorkee, Roorkee 247 667, India; J. L. Jethwa, Asst. Director, C M R S Unit, Q/8, Laxmi Nagar, Nagpur 440 022, India; A. I~ Dube, Asst. Director, C M R S Unit, CBRI, Roorkee - 247 667, India; B. Singh,
Director, Central Minlu~Research Station, Dhanbad - 826 001, India.
tion oftlmnel closures, which, in turn, depend on the support stiffness. Furthermore, a t~mnel at a greater depth is likely to attract higher support pressure. The t~nnel closure and therefore the support pressure continues to build up for a considerable time due to creep of the failed rock mass. Empirical correlations developed in a effort to eliminate the above limitations of the Q-system are discussed herein. Because the proposed empirical correlations are based on only 24 tunnel sections, there is scope for refinement.
Recording of Field Data The
following field data w e r e
collected: a) Radius of tunnel excavation. b) Depth of tunnel section from ground level. c) Unit weight of ground overlying the tunnel section. d) Q of the rock mass around the tunnel section. e) RMR of the rock mass around the t~mnel section. i9 Hoop load in steel ribs by compression load cells. g) Radial support pressure by contact pressure cells. h) ~[Mnnel closure by the tape extensometers and closure meters.
TunneUingand UndergroundSI~ceTechnology,Vol. 7, No. 1, pp. 59-74, 1 9 9 2 . Printed in Great Britain.
P ~ s u m A - L a correspondanceentre/aqual/~ de/a masse rocheuseet/a pression de soutienpropos~par Barton et at (1974)s"est r ~ utile, sauf dans les cas de terrain entas~. Lee d o n n ~ sur le terrain r~ueillies s y ~ dans 20 sections de tunnel diff~rentesiadiquent que les facteurs correctifs doivent tenir compte de la hauteur du surchargement et de la ferme~re da tunne& lesqueUessemblont pas ~tre suffiswnment prises en comptepar les facteurs de r~duction de t~nsion. Commepr#vu, lapreseion de soutien diminue rapidement avec la fermeture du tunneler puis augmente au-del~ de la fermeture d~limitante. Le fair que les pressions de soutien de paroi obeero~es~talent constamment proches de z~ro sauf dans les cond~ns de terrain e n t a ~ a ~ corrig~grdce a une l~gare m o d ~ des facteurs de la paroi Q (Q-wall). Des c~.res d,~riv&des donn~essur le terrain indiquent que les conditions de termin entas~ seraientpr&entes aux endroits of~la hauteur de la surcharge est sup~rieure a 350 Q. I~s d o n n ~ p u b l i ~ ici confirment les d&ouvertes p~,c~dentes de Barton et at (1974)selon lesquelles la pression de soutien est indc~ndante des dimensions du tunnel.
0886-7798/92 $5.00 + .00 ~) 1992 Pergamon Press plc
Deep-seated radial displacement of the rock mass around the tunnel opening by single- and mnltipoint borehole extensometers. Q and RMR It is general practice to divide a
tunnel into several rock mass units on the basis of the variation in the geomining conditions. Each rock mass is then assigned a Q value, depending on the values of the size parameters RQD, Jn, Jr, Ja, Jw, and SRF. It has been experienced t h a t a single value of some of these six parameters is sometimes influenced by personal bias. Therefore, a range of values is assigned to these six parameters and a range of Q is obtained. The range and average values of Q obtained fromtnnnel sections are given in Tables A1 and A2 in the Appendix. The rock mass ratings RMR (after Bieniawski 1981) were also obtained. The correlation of RMR and Q provided the necessary confidence (Jethwa et al. 1981). Whenever Q value was doubtful, the doubt was reflected in a wider range of Q and the absence of a close correlation with RMR. Support Pressure Compression load ceils of 50-100 tonne capacity and contact pressure
59
cells of 5-15 kg/cm2capacity were used to measure support pressure on steel ribs. The load cells were inserted into rib joints, a vertical joint at the tlmnel crown and two horizontal joints at the spring level. No load cell was installed at the bottom. The vertical support pressure was obtained as a ratio of the arithmatic sum of the loads recorded by the two load cells installed at the spring level to the product of the excavation width and the rib spacing. Since no load cell was installed at the bottom, the horizontal support pressure was taken as a ratio of the load recorded by the crown load cell to the product of half the excavation height and the rib spacing. The coutactpressm~cellswereinstalled at the interface of the steel ribs and the bael.-Rll The load cells and the contact pressure cellswere installedcloseto the bmnel face. All ofthese instruments were protected against directhit during blast~ ing. The blast vibrations did not affect these ~ o u t s .
Tunnel Closure Diametral deformations of the tunnel sections were m e a s u r e d by t a p e extensometers, closure meters, and sometimes even by simple invar tapes. The change in tlmnel diameter was halved to obtain the radial tunnel closures.
Type of Rock Masses The instrumented tunnel sections covered both hard rock masses such as quartzites,metabasics, and dolomites; and c o m m o n l y occurring soft rock masses such as shales, clays, slates, and phyllites.
A clear line of demarcation between the elastic and the squeezing conditions can be seen. The equation forthis line has been obtained as
Theoretical Criterion Theoretically,squeezing conditions around a t-nnel opening would be encountered if ~6 > q~
H = 350 Q,3
(1)
Thus, a rock mess m a y undergo squeezing w h e n the depth of the tunnel section exceeds 350 Qm.
where ~o is the tangential stress and Cl¢ is the uniaxial crushing strength of the rock mass. In the case of a circular tunnel under hydrostatic stress field, Eq. 1 can be written as 2P > q¢
(3)
Comparison of Measured and Predicted Roof Support Pressure
(2)
Barton's correlation, given in Eq. 4 below, was used to obtain predicted values of short-term roof support pressure p~. As discussed above, measured roof support pressure values were obtained from instrumented t-nnel sections. Out of a total of 19 case histories listed in Tables A1 and A2, 16 cases have been included in this analysis. These 16 case histories involve 8 tunnel sections under non-squeezing and 8 under squeezing ground conditions. The measured roof support pressures have been compared with the predicted values shown in Figure 2. The comparison has not been shown for the wall support pressure because the number of measurements is small. For predictedroofsupport pressures, the classification methods of Terzaghi (1946), Deere et al. (1969), Protodyakouov(1963), Wickhamet al. (1974), Barton et al. (1975), and Bieniawski's RMR method (supplemented by Unal 1983) have been used. It can be seen that the predictions are unreliable in all cases except one. In the case of Barton's Q-system, the predictions are reliable for non-squeezing conditions. The predictions turned out to be unsafe for squeezing ground. ForexAmple,
in which P is the primary stress value. It follows that a tunnel section experiencing elastic conditions in a given soft rock mass can encounter squeezing conditions if the prlm~ry stress level increases due to increase in the tlmnel depth or any other reason. This explains why phyllites and shales squeeze at one place and present elastic conditions at another, as shown in Tables A1 and A2 (in the Appendix). Equations I and 2 can thus be used to predict squeezing conditions in a tunnel, provided that P and q~ are known.
Empirical Criterion Measurement ofp~rnary stress field and the in-situ crushing strength of rock masses across a tvnnel for predicting squeezing conditions is both expensive and time-cons-mlng. Therefore, an attempt was made to seek a simple criterion for predicting squeezing conditions. An empirical criterion was developed (as shown in Fig. 1) that gives a log-log plot between the tunnel depth H in metres and the logarithmic mean of the reck mass quality Q. Some of the case histories of Barton et al. (1974) have also been used in Figure 1.
Criterion for Squeezing Ground Condition Incompetent or soft rock masses characterized by low in-situ crushing strength undergo plastic failure when overstressed. Such a rock mass around a t - n n e l opening fails when the tangential stress exceeds its uniaxJal crushing strength. The failure of the rock mass is associated with volumetric expansion, which is manifested in the form ofradialinward displacement of the t - n n e l periphery called t - n n e l wall. These deformations are called tunnel closures. The t - n n e l closures can be very large (measured closures have been as large as 17% of the size of the t-nnel opening). This phenomenon is called "squeezing" of the rock mass. The squeeze can occur not only from the roof and the sides, but also from the floor. ~ m n e l closures resulting from the elastic relaxation of a t - n n e l opening, on the other hand, are smaller than 1% of the tlmnel size (see measured values in Tables A1 and A2, Appendix). 60
,® o -- MANERI BHALI PROJECT b -- SALtd. PROJECT c -TEHRI DAM PROJECT d -- SANJAY VIDYUT PARIYOJNA • - KOLAR GOLD MINES f - CHHIBRO - KHOORI T UNNEL g -- GIRl HYDEL TUNNEL h - LOKTAK HYDEL TUNNEL i -- KHARA HYDEL PROJECT
2000
I00C
-139-BARTON'S
• x
NON-SOUEEZING CONDITION SQUEEZING CONDITION
@
R O C K BURST
CASE HISTORIES
E
*, SQUEEZING d
Q: 5 0 0 x 9
x¢ / ®x ~ 142 g ~ / 141 - xo/~o / ~
Xg xh xf
x 159
xf c : y
/~s?9 ~
xg
2 O0 Xh
ei
/ /
100
•001
"01
-c
" x^ v e4a~ ' --
NON- SQUEEZING
et04
el05
eq
ec
;~
@101
•
OB
b
0.1
1
t0
100
Q
Figure1. Criteria for predicting squeezing ground condition.
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Volume 7, N u m b e r 1, 1992
WICKHA~
1. ~ m n e l depth or thickness of the overburden. 2. ~ m n e l closure. 3. Time. 4. %mnel size.
12 16 "
e~E
TERZAGHI~ I R ~
If other factors are unchanged, the t - n n e l closures depend on the support stiffness. It is difficult to estimate the support stiffness in the present case, since the stiffness of backfill has to be taken into consideration while estimating the overall stiffness of a steelrib support system. Therefore, bmnel closure has been used to replace the stiffness of a support system (Table 3).
8
12
- 8 4
cL •
O
x
xx
x
0
4
8
Probsd
1'2
0
1'6
4
kg /crn2
8
12
Influence of Overburden on Roof
SupportPressure
prObSd , ko /crn2
Barton et al. (1975) suggested the following correlations for support pressures:
12 DEERE
16
i~= 2Q~/J,
%12-
8
l~w = 2 Q i ~ / J r in w h i c h
8-
Pi~
4
%
4
x
• x
0¥ 0
Pi~ , 12
8
4
16
/a_ , u , - " * . ~x Or 4 0
pObSd kg /cm 2
x x
x
8
J 12
Probsd ko/crn2
x SQUEEZING , • NON- SQUEEZING 12'
NE u
S
A
R
T
O
12
~
e~E 8
8-
4-
x
x
./
~2"
~
I
x x
• i
i
4
S
12
]o(
0
x i 4
pob,d
, "~¢m a
xx i S
,
12
kglcm2
x SQUEEZING, • NON- SQUEEZING
Figure2. Comparison of predicted and observed roof support pressures.
the measured support pressures were 10.8 and 11.5 kg/cm2when compared to predicted values of 4.2 and 4.4 kg/cm2 for t~,n n el sections 2 and 4, respectively. Such large differences in the measured and p r e d i c t e d s u p p o r t p r e s s u r e s prompted the authors to look for possible reasons. Some of the data in Table A2 that are related to these four t-nnel sections have been shown in Table 1. It can be Volume 7, Number 1, 1992
seen from Table 1 that in the cases of sections 1 and 2, the difference in support pressure could be the result of depth, t-nnel closure, t-nnel radius, and time ofobservatious. Similarly, in t~mnel sections 3 and 4, the difference would be related to tlmnel radius and tlmnel closures. It follows that the following four factors might have influenced the measured support pressure:
short-term roof support pressure, = short-term wall support pressure, = Barton's joint roughness coefficient, short-term roof rock mass quality, short-term wall rock mass quality. =
The values of Q~ and Q~, have been taken as 5 times Qr and Q,, where Q, and Q , are Barton's rock mass quality for roof and wall rock, respectively (values of Q, and Q~ should be obtained separately for the roof and the wall rock, respectively). The short-term roof and wall support pressures were estimated from Eqs. 4 and 5. These values were used to calculate correction factor f for overburden or tunnel depth. The correction factor f is defined as a ratio of measured support pressure to the predicted support pressure. A relationship of f to t - n n e l depth is shown in Figure 3. Because the elasto-plastic theory suggests a linear relationship between the overburden pressure and the support pressure, a linear relationship has been attempted in Figure 3. According to Figure 3, the correctlon factor f can be given by f = 1 + ( H - 320)/800 > 1
(6)
in w h i c h H is t h e t h i c k n e s s of overburden or t, mnel depth in metres. The data points for squeezing ground appear to suggest that the line in Figure 3 should be much steeper to represent a natural trend. In reality, the difference between observed support
TUNNm~n~e ANDUNDERGROUNDSPACETECHNOLOGY 61
Table I. Details of tunnel sections under squeezing ground conditions (from Table A2). Support Pressure (kg/sq. cm)
Tunnel Tunnel
Depth
Radius
(m)
T y p e of Rock Mass
Q
1
Crushed red shales
0.025 to 0.10
1.5
2
-do-
-do-
Soft and plastic black clays within thrust zone
-do-
S. No.
4
Radial tunnel closure
Observation Period (months)
Predicted
Measured
(%)
280
3.3
3.1
2.8
4.5
680
4.2
10.8
1..2
0.016 to 0.03
1.5
280
4.4
3.2
4.5
26
-do -
4.5
-do-
4.4
11.2
1.7
26
pressures and proposed line is mainly the result of excessive tunnel closures, which have been taken into account by another factor, f', for squeezing ground condition. Some m a y doubt that the correlation proposed in Eq. 4 can account for the method of construction, the type of supports, the primitive stresses and tunnel closures. The instrumented tunnels were constructed by conventional means, i.e.,drillingand blasting followed by steel ribs. This practice resulted in significant damage to the rock mass. Therefore, equation 4 is on the safe side. In the case of machine t, mnelling, designers should reduce the support pressures obtained from Eq. 4 by perhaps 20%, as there will be reduced damage to the rock mass. Another valid concern is that the field data are not sufficient to prove the validity of the proposed correlations. In the opinion of the authors, the International'l~,nnellingAssociation should compile a data bank for observed supp o r t pressures from all parts of the world and should try to improve these correlations. Ratio of Wall Support Pressure to Roof Support Pressure
Barton et al. (1975) realized that the wall support pressure would be smaller than the roof support pressure and therefore suggested increasing the observed Q values for estimating the
(m)
wall support pressure, as shown in Table 2. The ratio of the wall support pressure p, to the roof support pressure p,, corresponding to Q i , ' , have also been shown in col,,mn 3 of~able 2.
The observed wall support pressures from some ofthe squeezing and the nonsqueezing case histories have been plotted in Figure 4. It can be seen that the recommendations ofBarton et al. (1975)
x2
2.5-
•
NON-SQUEEZING
x SQUEEZING
2.0-
3 • 11
1.5--
•
09
012 5x x7 x~, 511
I.O 0.8
J
61
• I0 , ,
0
~ l X6 200
xl l 400 OVERBURDEN ( H }~m
ob~ Pr
f
.... f - Pi( '
= 1+ (H-320}/SOO ~! I 600
8uO
Figure 3. Correction factor for overburden in Barton's correlation for shortterm roof support pressure under non.squeezing ground conditions.
62 TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY
Volume 7, Number 1, 1992
Table2. Wall factor Q~.. .:/ Q~for estimating wall support pressure. Authors' Recommendation
Recommendation of Barton et al. 1975 Range of Qi
Qi-w~u
Oj.wat]
1
2
3
4
5
6
<0.1
1.0
1.0
<0.1
1.0
1.0
0.1-10
2.5
0.7
0.1~
2.5
0.7
>10
5
0.6
>5
>15
0.0-0.4
are overly safe for ~. values greater than 5. The modified wall factors have therefore been recommended as shown in Figure 4 and Table 2.
f:= 1~°b~d
(7)
f- pi~ in which p ~ d = measured roof support pressure, p~ = predicted short-term roof support pressure, and f = correction factor for overburden (Eq. 6).
Correlation Between Support Pressure and Tunnel Closure in Squeezing G r o u n d C o n d i t i o n Variation of the normalized roof support pressure with the tunnel closure at the crown is shown in Figure 5. The ordinate represents f;,which isthe correction factor for tunnel closure at the crown. The correction factor f/is given as
Z
Range of Oi
I~
The data points in Figure 5 are taken from Table A2 and represent eight tunnel sections from four different tunnels. The normalized reef sup-
m
port pressures are higher for low tunnel roof closures. The roof support pressures decrease when the t~mnel closures increase and attain minimum values when the roof closures are appro~qmAtely 5%. The normAHTed roof support pressures again rise when the t~mnel roof closures exceed 5%. Such a variation is in conformity with the ground reaction curve concept. This trend is repeated in Figure 6, which shows the variation of the normRllzed wall-support pressure with the measured t~mnel-wall closures. The correction factor f~ for t, mnel-wall closure is given as
x
SQUEEZING GROUND CONDITION
•
NON-SQUEEZING GROUND CONDITION ACCORDING TO BARTON ET AL ( 1 9 7 5 )
---
SUGGESTED BY AUTHORS
Q i - w o l l : Qi
x x7
i
x3
Qi--wall = 2 . 5 Qi i
1
Qi-wall
" .5 Qi
I
1I
XI
I
k X2
,
,,I
0-001
, 0"01
,
,,I
l
0"t
i
I
1
I
|
e~L,4.,
I t0
,_z~'-~
, t00
Qi
Figure 4. Variation of ratio between wall support pressure and roof support pressure with short-term rock mass quality. Volume 7, Number 1, 1992
TuNN~.r.~ ANDUND~.R~ROUNDSPACE TECHNOLOaY 63
4 obsd PW
obsd
fPw f" Pr pObSd : w
pObSd : MEASURED ROOF SUPPORT PRESSURE
Pw Pr
3 == .=
= PRE~CTED ROOF SUPPORT PRESSURE
=
X4
MEASURED WALL SUPPORT PRESSURE PREDICTED
WALL SUPPURT PRESSURE
CORRECTION FACTOR FOR TUNNEL~WALL
f
= CORRECTION FACTOR FOR OVERBURDEN
CLOSURE
tr
: CORRECTIOI( FACTOR FOR TUNNEL CLOSURE AT TIlE CROWN
CORRECTION FACTOR FOR OVERBURDEN
o.
X4
x7 t~
$
/
.=, m =
o
// N
//X B
N "3
5X6~
/ X7
Y~ DATA POINTS PLOTTEO FROM
TABLE ].b
I
0
5
r
J
0
15
t0
OBSEBV.O TU..EL CLOSURE AT CBOWN
~.,.~
l~w where p~= f
=
Piw
=
(8)
measured wall support pressure correction factor for overburden, and predicted short-term wall support pressure.
Thus, the correction factors f'r and fw' are the same as the normalized roof and wall support pressure, respectively. The recommended values of these correction factors are given in Table 3. The validity of Table 3 for squeezing ground has been questioned, particularly with regard to highly squeezing or flowing ground. It is suggested that the application of Table 3 should be restricted to moderately squeezing ground by limiting closure to 5%, by strengthening the support system immediately. Serious construction problems may arise if this remedial measure is not followed. It is recommended that all such tlmnel sections be instrumented. Jethwa (1984) concluded that the wall support pressure may be significantly higher than the roof support pressure in the case of parallel tunnels if the clear spacingis less than the sum of the tunnel widths.
64
]
5 10 OBSE.VEO TU..E~ WALL CLOSU~(~
t5
Figure 6. Correction factor for wall closure under squeezing ground condition (H > 350 Q^1/3).
Figure 5. Correction factor for roof closure under squeezing ground condition (H > 350 Q^ I / 3).
f~ _I ~
t
0
Variation of Support Pressure with Time
port pressure, it has been possible to study the effect of time on the support pressure. Figure 7 shows the variation of the correction factors t~'over time. The correction factors f" for time are given as
After studying the influence of the overburden and the tunnel closures and incorporating these influences in the correlations between Q and sup-
Table 3. Correction factors for tunnel closures in squeezing ground conditions.
S. No.
1.
Ground Condition
Support System
Non-squeezing (H < 350 Ola) Squeezing
Tunnel Closure (%)
fworf;
<1
1,0
(H > 350 ~1/3)
Very stiff
1-2
> 1.80
3.
Moderately squeezing
Stiff
2-4
0.85
4.
-do-
Flexible
4-6
0.70
5.
Highly squeezing
Very flexible
6-8
1,15
6.
-do-
Extremely flexible
>8
1.80
2.
T I Y N N E L L I N G AND U N D E R Q R O U N D S P A C E TEcI-INOLOGY
Volume 7, Number 1,
1992
X
ROOF S U P P O R T
P R E S S U R E IN
•
ROOF S U P P O R T
PRESSURE
0
WALL
SUPPORT
SQUEEZING GROUND CONDITION
IN NON-SQUEEZING
PRESSURE
IN S Q U E E Z I N G
GROUNO C O N D I T I O N
GROUND CONDITION
.1 -
0 5 ....
. ...-.. . I I-'-t0
Combining Eqs. 9, 10, and 11, the long-term roof and wall support pressures can be given as
t
""
(12)
P. = Pl." f ' f ' ' log 9.5 t °'~
(13)
in which p, and p,, are long-term roof and wall support pressures. Barton et al. (1975) suggested that the ratio of the ultimate to the shortt e r m support pressure is about (5) ~, i.e., 1.7. Equation 11, however, suggests the following relationship:
I
"/
. ....4
f •
p, = p~' f ' f ' " log 9.5 t °'~
x,
p o~ f ' log 9.5 t °'~ i.e., •~
07
~
- 4 ~ 1 C - ~ , -~ ~
zx
i
~
x8
x3_
O~ .....-
....."-"'''-'""~
.~.
..... ,~...,. • ~
91 _f: log 9.5t 0a~
"
po f: log 9.5 t~a~ where Pl and Po are support pressures after t~ and t_ months of excavation. t ! In case ofa r ~ d hnmg, f~= fo' so that the ratio of the ultimate support pressure a i ~ r 100 years to that alter one month is given by (t o = 1 month and t~ = 100 years or 1220 months) u
t 0 2 A
,
,
'{
,
i
,
10
,1
L
100
t , TIME OF OBSERVATION IN
,
,
,
t000
.
,
•
.
MONTHS
Figure 7. Variation in observed support pressure over time.
f~_
~.,~,,a f" f" 1~
(9)
(lO)
-
f{~w
12CMRS
where
f;
= correction factor for the influence of time on the roof support pressure,
f~w
= correction factor for the influence of time on the wall support pressure,
1~b~
= measured pressure,
I~ ~
= measured wall support pressure, = correction factor for overburden (Eq. 6), and = correction factor for tlm~el closures (Table 3).
°'1E
8
roof support a - L .
f f'
All the data points from Tables A1 and A2, except those for the wall support pressures in the non-squeezing ground conditions (being negligible), have been plotted in Figure 7. According to Figure 7, the correction factor f " can be given by f" = log 9.5 t °'~ (11) in which t is the time in months after excavation of the t - n n e l opening.
Volume 7, Number 1, 1992
Q.
4
0
T"
I
|
1
0
4
8
12
pobsd , kg/crn 2 x
SQUEEZING
~
•
NON-
SQUEEZING
Figure 8. Comparison of observed roof support pressure with predicted values from authors" Eq. 12 (p, ffi p~. f . f . log 9.5 t^0.25).
TUNN~.LWOANDUNDERGROUNDSPACETECHNOLOGY 65
pz _ log 9"5t°'as - 1.75 po
log 9.5 t~25
In other words, the support pressure will increase in 100 years to 1.75 times the support pressure observed after 1 month of excavation. The corrected support pressures compare well with the observed values, as shown in Figure 8. This ratio of 1.7 between the ultimate and the short-term support pressure tallies with the (5) ~ suggested by Barton et al. (1975). However, the ultimate support pressure for t-nnels under squeezing reck conditions may be 2-3 times the short-term support pressure, according to Jethwa (1984). The ratio of the ultimate support pressure to the short-term support pressure, worked out here as 1.7, is relatively small compared to the ratio of 2 3 after Jethwa (1984), probably because the period of observations reported herein is relatively short and the number of squeezing case histories is small. Furthermore, in special cases of soluble or erodible joint fillings and where seepage is a serious problem, the long-term support pressure may be as high as the cover pressure or 6 times the short-term support pressures, whichever is smaller. This trend has been indicated from a 10-year performance study of Chhibro-Khodri under ground powerhouse complex in India (Mitra 1991). For designing a temporary support system, one m a y assume a unlforin distributionof the short-term support pressure, but a factorof safety of 1.75. However, the permanent support system should be designed for the net ul~mAte support pressures, with a factor of safety of 1.5. Effect of Excavation Support Pressure
G
Conclusions
The combined approach of field instrumentation and quantitative classification of Barton et al. has proved rewarding at this stage of development of rock mechanics. Despite limited field data, some practical trends showing the influence of overburden, tnnnel closures and time of excavation on the t , nnelling condition and the support pressures have emerged. It would perhaps be hasty to draw any definite conclusions from these trends; however, some tentative correlations have been possible. These correlations are subject to refinement as more field data is collected. The following tentative conclusions are possible from these correlations: 1. Squeezing is likely to occur in a t~mnel section where the height of overburden in metres exceeds 350 Q1/8. 2. The s h o r t - t e r m roof support pressure is given by the following correlations: 1~ =2.0 (5Q) ~3. f. f Jr in which f is the correction factor for thickness of overburden (H) in metres,
Size on
According to Terzaghi (1946) and Unal (1983), the support pressure is directly proportional to the size of a tunnel opening. O n the other hand, Barton et al. (1974), suggest that the support pressure is independent of the t - n n e l size. Colnmns 3 of Tables A1 and A2 (see Appendix) list support pressures obtained from Terzaghi's method for nonsqueezing as well as squeezing ground conditions. It may be noted that the estimated support pressure values (shown in col-ran 3) do not compare well with the observed support pressures (in co]-mnR 12 and 13). The support pressures corrected according to the proposed correlations (eol, mns 10 and 11) are in better agreement with the observed support pressures. Figure 9 shows the variation of pO~/p/f, f. f, with the diameter of
66 ~
discontinuities in a larger opening. Thus, the size effect is automatically accounted for in the est;m.te of Q. The adverse effects of deteriorating hydrogeological conditions ( J ) should also be determined, if possible, after a water tlmnel is commlssioned. Itwould therefore be unsafe to obtain Q from small drifts and use it to estimate support requirements for large excavations. For underground excavations in non-dialatant rock masses (schists, slate, etc.) with smooth planes of weakness, it is cautioned that Terzaghi's concept may still be v a h d
tunnel opening (2a). The ordinate represents the observed roof support pressure corrected for overburden, the tunnel closure and the time of excavation. It may be seen that the corrected support pressure is independent of the tunnel size. The reason for support loading in non-squeezing conditions may be related to the dead weight of the loosened rock blocks which become detached from the parent rock mass at the tunnel roof and rest on the support system. This type of support pressure is called the "loosening pressure". The loosening pressure has been attributed to poor blasting practice, gravity and delayed support in the form of steel ribs. Excessive tunnel closures under squeezing ground conditions are also considered responsible for mobihzing large loosening pressures, even in t u n n e l sections s u p p o r t e d by shotcrete immediately on excavation. The loosening pressure (dead weight of the loosened or a destressed zone) is therefore mobilized due to poor blasting practice and is likely to be independent of the tunnel size. In a recent study at the underground powerhouse complex of Lakhwar dam project in India (Q = 8-9 and H= 250 m), the observed roof support pressures were nearly the same (i.e., about 0.5 kg/cm2) for the 6-m-wide approach adit, 14-m-wide expansion surge tank and 21-m-wide powerhouse cavern, all excavated through tightlyjointed traps. These observations should erase all doubts about size effect in underground openings. It may be noted that rock mRss quality Q estimated from a larger bmnel would be smaller than that obtained from small driRsin a similar rock mass. This isdue to the possibilityofintersecting greater n u m b e r of geological
~,_
x • '
2-
*-
,,
.
.
.
.
SQUEEZING NON- SQUEEZING
.
.
.
.
.
.
.
.
.
• ~
w-=
.~_ ~ 1 "~
t x
61
-,-
~2
7
~X Q'7
"O
h-
o O
i
!
4
8
!
12
I
i
16
20
DIAMETER OF OPENING jm
Figure 9. Support pressure virtually independent of tunnel size.
ANDU~DZRGROUNDSFACZTzcm~oLoGY
Volume 7, Number 1, 1992
and f ' is t h e correction factor for t ~ n n e l closure (see Table 3, equal to 1 in nonsqueezing ground conditions). The value of t h e correction factor f i s given as f = 1+(H-320)/800
> 1
3. I n squeezing g r o u n d conditions, t h e s u p p o r t p r e s s u r e is significantly influenced by t u n n e l closures. The correction factor f' for t u n n e l closure v a r i e s from 0.7 to 1.8 in t h e case of a single t u n n e l . The m i n i m u m s u p p o r t p r e s s u r e occurs w h e n t h e t u n n e l clos u r e is about 5% of t h e t u n n e l diameter. The s u p p o r t p r e s s u r e i n c r e a s e s r a p i d l y beyond t h i s limiting closure. 4. The short-term wall support pressure m a y be o b t a i n e d from t h e above correlation b y s u b s t i t u t i n g Q~n for Q. I n general, t h e a c t u a l wall s u p p o r t p r e s s u r e for t h e non-squeezing rock conditions is likely to be negligible. The s h o r t - t e r m v a l u e s o f P i / p ~ d e p e n d on Qi (i.e., 5Q), as given below: Ptw/P.
Qt
1.0
5Q < 0.1
1.0 - 0 . 0
5 < 5 Q < 0.1
0.0
5Q > 5
5. The u l t i m a t e s u p p o r t p r e s s u r e m a y be 1.75 t i m e s t h e s h o r t - t e r m supp o r t p r e s s u r e for t l m n e l sections u n d e r non-squeezing g r o u n d conditions, except for cases of soluble a n d erodible j o i n t fillings w i t h seepage. 6. The s u p p o r t p r e s s u r e is indepond e n t of t h e t u n n e l size, provided t h a t Q is o b t a i n e d from a full-sized opening.
[] References Barton, N.; Lien, R.; and Lunde, J. 1974. Engineering Classification of Rock Masses for the Design of Tunnel
Volume 7, N u m b e r 1, 1992
Support, RockMechanics, Vol. 6, 189236. Springer-Verlag. Barton, N.; Lien, R.; and Lunde, J. 1975. Estimation of Support Requirements for Underground Excavations, Proc. Sixteenth Syrup. on Rock Mechanics, Univ. of Minnesota, Minneapolis, U.S.A., 163-177. Bieniawski, Z. T. 1981. Case Studies Prediction of Rock Masses Behavior by the Geomechanical Classification, Second Australia-New Zealand Conference on Geomechanies, Brisbane, 3641. Daemen, J. J. I~L 1975. ~ m n e l support loading caused by rock failure. Ph.D. Thesis, UniversityofMinnssota,U.S~. Deere, D. U.;Peck, R. B.;Monsees, J.E.;and Sc.hmidt,B. 1969. "Design of Tunnel Liners and Support System." Highway Research Record No. 339, U.S. Department of Transportation, Washington, D.C. Dube,~ I~ 1979. Goomechanicalevaluation of a tunnel stability under FAilingrock conditions in a Himalayan Tunnel. Ph.D. Thesis, Univ. of Roorkee, India. Dube, A. I~; Jethwa, J. L.; Singh, B.; Singh, Bhawani; and Mithal, R. S. 1982. Geoengineering evaluation of problems of a large underground cavity for Tehri Dam Project (India). ISRMSymp. Rock Mechanics: Caverns and Pressure Shafts (ed. W. Wittke), 239-244. Rotterdam: AM. Balkema. Dube, A. I~;Singh,B.;and Singh, Bhawani. 1986. Study of squeezing pressure phenomenon in a tnnnel--I and H. Tunnelling and Underground Space Technology 1:35-48. Jethwa, J. L.; Dube, A. I~; Singh, B.; Singh, Bhawani; and Viladlc,r, M. N. 1979. I n s t r u m e n t a t i o n and Design for multiple openings in f-illngrock mass. Int. Syrup. on In-Situ Testing of Soils and Rocks and Performance of Underground Structures, Roorkee, India, Dec. lg--22, 1979, Vol. 1. Jethwa, J. L.; Dube, A~ K.; Singh, B.; and Singh, Bhawani. 1981. Rock load estimation for t~mnels in squeezing
ground conditions. Proc. of the Rapid Excavation Tunneling Conference, San Francisco, Calif., May 3-7, 1981, 766783. New York: AIME. Jethwa, J. L.; Dube, A. IC; Singh, B.; Singh, Bhawani; and Mithal, R. S. 1982. Evaluation of classification system for tunnels in non-squeezing ground conditions. Proc. of lSRM Syrup. Rock Mechanics: Caverns and Pressure Shafts, ed. W. Wittke, 607-612. Rotterdam: AM. Balkema. Jethwa, J. L.; Singh, B.; and Singh, Bhawani. 1984. Estimation of ultimate rock pressure for tnnnel linings under squeezing rock Conditions--a new approach. Proc. ISRM Symposium on Design and Performance of UndergroundExcavations, Cambridge, U.K., Sept. 3-4, 1984. Mitra, S. 1991. Study of long-term behavior of underground powerhouse cavities in soft rocks. Ph.D. Thesis, University of Roorkee (under submission). Protodyakonov, N . M . 1963. Firmness coefficient for estimation of rock loads. Personal communication to Beas Design Org-ni~ation, New Delhi, India. Sharma, V.M. 1985. Prediction ofclesure and rock lo ads for tunnels in squeezing ground,. Ph.D. Thesis (p. 254), I.I.T., Delhi, India. Terzaghi, K. 1946. Rock defects and load on t~lnnel supports. In Introduction to Rock Tunnelling with Steel Support, I t V. Proctor and T. C. White (Youngstava, Ohio, U.S ~..: Commercial Shearing and Stamping Co.). Unal, E. 1983. Design guidelines and roof control standards for coal mines roofs. Ph.D. Thesis, Pennsylvania State University. Refer to p. 113 of Rock Mechanics in Mining and Tunnelling (Bieniawski, Z.T., 1984, Rotterdam: A.A. B-Ikema). Wickham, G. E.; Tiedmann, H. R.; and Skinner, E. H. 1974. Ground support prediction model--RSR concept. Proc. of North American Rapid Excavation and Tunneling Conference, San Francisco, California, Vol. 1,691-708.
TUNN~.T.n~a AND U~mE~mOUND SPACE TECHNOLOGY 6 7
¢.D ¢,D b~
l.a
r
0
<
2
S. No.
1
1.
Moderately fractured quartzite. a = 2.4 y = 2.5 RQD = 75, Q = 3.6 uh/a = 0.06, H = 225 (Jethwa et al. 1982)
from
0.3 to 0.7 (0.5)
3
kg/sq, cm
Terzaghrs Classification
Maneri-Bhati Hydro Project
Geological description
Vertical Support Pressure
15 tO 30 (21)
4
= 5Q
Qi
Shortterm Rock Mass Quality
0.5 to 0.7 (0.6)
0.1 tO 0.2 (0.4)
6
kg/sq. cm
kg/sq, cm
5
from Eq. 5
P.,
Horiz.
from Eq. 4
Vert.
Short-term Support Pressure
1.0
7
from Eq. 6
f
Overburden
f W'
1.0
8
1.0
9
from Table 4
f 1"'
Tunnel Wall Closure
Correction Factors for:
cm
cm
0.5 to 0.7 (0.6)
0.03 to 0.05 (0.04)
11
kg/sq,
kgL~l,
10
= P..,, f - f ,w
p.
Horiz.
= P.~, f . f 'r
p,
Vert.
Corrected Short-term Support Pressure:
Table A1. Comparison of predicted and observed support pressure from Q-system in non-squeezing ground conditions.
Appendix
0.6
12
cm
kg/sq,
p o=
Vert.
13
cm
kg/sq.
p.O=
Horiz.
Observed Support Pressure
14
month
Observation Period
Steel ribs stable
15
Remarks
O~ ~D
+
Q
i.a p.a ¢.0 ¢.0 bO
+
.<
2
Highly jointed dolomites. a = 6,~' = 2 . 8 RQD = 30-40 Q =1.2-1.7 H=110
5.
Grade I phyllites, massive and distinctly jointed a = 7,Y = 2.64 RQD = 75, RMR = 67 Q =5, H = 320 (Dube et al. 1982)
Khara Hydel Proiect
4.
1.7 to 5.5 (3.6)
(25)
(7)
6 to 8.5
1.5 to 16.5 (5)
0.8 to 2.6 (1.7)
Sheared matabasics. a=2,4,7 =2.5 RQD = 60 Q = 0.3-3.3 uh/a = 0.4 H = 350 (Jethwa et al. 1982)
3.
Salal Hydel Tunnel
1 7 t o 34 (24)
0.3 to 0.7 (0.5)
4
Foliated metabasics. a = 2.4;y = 2.5g RQD = 82, Q = 3.4-6.8 uh/a = 0.05, H = 550 (Jethwa et al. 1982)
3
2.
Maneri-Bhali I-Ivdel Proiect (contd.)
1
Table A1 (contd.)
0.16 to 0.45 (0.3)
1.0 to 1.2 (1.1)
0.7 to 1.8 (1.25)
0.5 to O.7 (0.6)
5
(0.13)
0.4 tO 0.5 (0.45)
0.2 to 1.2 (0.7)
0.1 to 0.20 (0.15)
6
1.0
1.0
1.04
1.29
1.0
1.0
1.0
1.0
1.0
1.0
1.0
(0.3)
1.0 to 1.2 (1.1)
0.70 to 1.9 (1.3)
0.6 to 0.9 (0.8)
10
(0.04)
0.14 to 0.2 (0.17)
0.2 tO 0.5 (0.35)
(o)
0.005 to 0.05
11
0.25 to 0.56 (0.40)
1.1
2.0
0.8
12
appears negligible
appears negligible
13
1.5
3
2
14
Steel ribs; stable
Steel ribs; stable
Steel ribs; stable
Steel ribs; stable
15
=~ ,e-
" -
~ :.--&6 =
A
" ~ -+ Lm _~ GO
[~.-' • 0
O
O
0
IN
~,,uo
v
0
O
v0
m-.
o.
v
q
b-
o.
o
O
o
o
O
q
o.
o5
O
~ 8 m---.
o
°oO~
t~ ~ V
-.:.
.+
~o
K"
_
>~.
.8
~ •-
o
0~ I-
,,
~
~ . ~ mr ~,~
O E o-Z m O z : ~ I
E'
n O
o
E
~D
el
o
E
~ o
~-
~
II e"
¢-
~ 8 m'-,
o
:D
"11"
m t:
..d
<,D
70
TUNN~,T,~G
AND UNDERGROUND SPACE TECHNOLOGY
O
v'
i,=
o5
V o l u m e 7, N u m b e r
1,
1992
~D ~D b~
2
Chalnage 389 m a = 3 , Q = 0.4 H = 150-200 uh = 22.5 mm
(ii) a = 7 (iii) a = 3
Tightly jointed basic reck. H = 250, Q = 8-9 (i) a = 10.5
--
do
do
0.35
(o)
do
6
(0.22)
do
5
++
Estimated support capacity.
Notations: a = average radius of tunnel opening in metres. RQD = rock quality designation in percent. Q = rock quality based on classification of Barton et al. (1974). 7 = unit weight or rock material in gm/cc. uh/a = tunnel wall closure in per cent. uv/a = closure at crown level in per cent. H = height of overburden above opening in metres. ( ) = average values, except in column 4, where root mean square value is given.
12.
42
do
Chalnage 204 m a = 6 . 5 , Q = 15 H = 52, uv = 5 mm
11.
Lakhwar Hvdro Proiect
75
do
4
Thinly bedded shales with calcite bands. Chalnage 761 m a = 6.5, uv = 12 mm Q=15, H=34
--
3
10.
Upper Krishna Project
9.
Khara Hydro Proiect (contd.)
1
Table A1 (contd.).
1.0
do
do
1.0
1.0
do
1.0
do
1,181
0.35
(0.22)
1.0
1.0
(0.22)
(1.7)
10
1.0
1.0
9
0.2 to 0.4 (0.3)
(o)
0.45 to 0.55
0.2
2.4
12
(0)
(1.1)
11
appears negligible
appears negligible
1.6
13
I
1.0
1.0
14
Distribution of support pressure is asymmetric
do
do
do
15
tj~ b3
-.3
< o
o
t~
2
a = 1.5, RQD =1020 Q = 0.025 -0.10 7 = 2.73 uh = 2.8 H = 280 (Jethwa et al. 1982)
OR!!==¢1
Chibro-Khodri Tunnel
1
S. NO.
Geological Description
1.8 to 3.4 (2.7)
0.125 to 0.50 (0.25)
4
2.5 to 4.2 (3.3)
5
3
1.8 to 3.0 (2.4)
6
kg/sq. cm
= 5Q
kg/sq, cm
p~,,
kg/sq, cm
P,~
Pressure
from Eq. 5
Qi
Shortterm Rock Mass Quality
S h o r t - t e r m Support
from Eq. 4
Vertical Support Pre~ure from Terzaghl's Classification
1.0
7
from Eq. 6
Overburden
(0.85)
1.8
9
from Table 4
from Table 4
8
fW'
wall
f'r
roof
T u n n e l Closure
Correction Factors for:
Table A2. Comparison of predicted and observed support pressure in squeezing ground conditions.
(2.8)
10
kg/sq, cm
f " f'r
= P~of"
P~
Vert,
(4.3)
11
kg/sq, cm
f " f*'
= P.=?
P.
Horlz.
Corrected Short-term Support Pressure:
3.1
12
kg/sq, cm
P~
1.7
13
kg/sq. cm
pw°b
Pressure
O b s e r v e d Support
14
month
Observation Period
Circular ribs stable
15
Remarks
~2
Q
b.a ~D CD ~0
2
5.
Very blocky and seamy slates, moderately squeezing. a = ~ . l , =2.5 uh/a = 7.6 H = 380 Q - 0.32-0.82 (Jethwa et al. 1982)
Giri Hydro Tunnel 0.7 to 2.3 (1.2)
do
Soft plastic black clays within thrust zone, moderately squeezing. = 2.64, RQD = 10 Q = 0.016-0.03 H -- 280 (Jethwa et al. 1982)
4.
Y
0.08 to 0.15 (0.11)
1.5 to 3.3 (2.4)
Soft plastic black clays in thrust zone, moderately squeezing. a~'5, = 2.64 RQD = 10 Q = 0.016-0.03 uh/a = 4.1 uv/a = 4.5 H = 280 (Jethwa et al. 1982)
3.
1.5 to 4.1 (2.5)
do
0.06 to 0.25 (0.12)
10.3 to 22.1 (16.2)
Crushed red shale, highly.squeezing a=415, =2.73 RQD = 10-20 Q = 0,012-0.5 uh/a = 6, H = 680 (Jethwa et al. 1982)
3
2.
Chibro-Khodri Tunnel (contd.)
1
Table A2 (contd.).
1.2 to 1.8 (1.5)
do
4.0 to 4.8 (4.4)
3.3 to 5.0 (4.2)
0.8 to 1.3 (1.0)
do
3.0 to 4.8 (3.9)
2.4 to 5.0 (3.7)
1.08
1.0
1.0
1.45
7
1.15
1.8
0.70
1.8
8
1.15
1.8
0.7
0.7
9
(1.9)
(7.9)
(3.1)
(11)
10
(1.2)
(7.0)
(2.7)
(3.8)
11
2.0
11.5
3.2
10.8
12
2.4
12.2
2.6
3++
13
12
26
26
8
14
Roof closure considered equal to wall closure as horseshoe ribs with invert struts deformed, but not as severe as in case 6.
Circular ribs of very high capacity were stable. Consequently, tunnel closures were likely to be low, approx. 0-2%.
Circular ribs were stable; compressible backfill behind ribs. Enlargement of drift to 9 m size in close proximity delayed stabilisation.
Heavy circular ribs; severe buckling due to squeezing. Little closure at crown.
15
b3
~D
0
Q
2
Crushed phyllites, highly squeezing. a = 2~1, = 2 . 3 RQD = 10-25 q = 0.124-0.32 uh/a = 12.4 uv/a = 5 H = 240 (Jethwa et al 1982)
Crushed shales, moderately squeezing. a = 2~., = 2 . 7 gm/cc RQD = 10-20 Q = 0.011-0.044 uh/a = 7 H = 300 (Jethwa et al 1982)
Highly fractured quartzites. a=23$, =2.5 RQD = 6O, Q = 0.5 uh = 190 mm H = 35O (Sharma 1985)
3
2.64
2.9 to 5.4 (4.2)
2.1 to 4.1 (3.1)
See footnotes in Table A1 for notations.
8.
Maneri Bhali Proiect
7.
L o k t a k Hydro Tunnel
6.
Giri Hydro Tunnel (contd.)
1
Table A2 (contd.).
(2.5)
0.055to 0.22 (0.11)
0.62 to 1.6 (1)
1.6
3.5 to 5.3 (4.4)
1.8 to 2.3 (2.1)
1.1
2.5 to 5.3 (4.0)
1.2 to 1.9 (1.6)
1.04
1.0
1.0
1.15
1.15
0.7
1.15
1.15
1.8
(1.9)
(5.1)
(1.5)
10
(1.3)
(4.6)
(2.9)
11
2.0
5.4++
1.7
12
5.4++
4.0
13
14
27
14
Supports buckled. Vertical and horizontal closures appeared equal.
15-cm-thick shotcrete with 4-m-long rock bolts supplemented with circular ribs. Squeezing occurred even at H = 160 m. Roof closure is considered equal to wall closures.
occurred at half total wall closures when horseshoe ribs with invert budded.
k0/sq, cm
measured support pressure of 5
Peak
15