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.

P~sumA-La correspondance entre/a qual/~ de/a masse rocheuse et/a pression de soutien propos~ par Barton et at (1974) s" est r ~ utile, sauf dans les cas de terrain entas~. Lee donn~ sur le terrain r~ueillies s y ~ dans 20 sections de tunnel diff~rentes iadiquent que les facteurs correctifs doivent tenir compte de la hauteur du surchargement et de la ferme~re da tunne& lesqueUes semblont pas ~tre suffiswnment prises en compte par les facteurs de r~duction de t~nsion. Comme pr#vu, la preseion 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 en ta~ 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~es sur le terrain indiquent que les conditions de termin entas~ seraient pr&entes aux endroits of~ la hauteur de la surcharge est sup~rieure a 350 Q. I~s donn~ pub 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.

Introduction T herellability of a realistic quan-

titative classification system for est imatingt~nnel support pres-

sure 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 con- tact pressure cells in several steel-rib- supported t~mnel sections through both squeezing and elastic ground condi- tions and compared the measured val- ues with those est lmsted after Q-sys- tem. The study brought to light signifi- cant limitations of Barton's methods for application to tunnel sections un- der squeezing ground conditions. For example, the support pressure is a func-

Present address: Bhawani Singh, Professor, Dept. of Civil Enffineering, University of Roorkee, Roorkee 247 667, India; J. L. Jethwa, Asst. Director, CMRS Unit, Q/8, Laxmi Nagar, Nagpur 440 022, India; A. I~ Dube, Asst. Director, CMRS 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. Fur- thermore, a t~mnel at a greater depth is likely to attract higher support pres- sure. The t~nnel closure and therefore the support pressure continues to build up for a considerable t ime 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. Be- cause the proposed empirical correla- tions are based on only 24 tunnel sec- tions, there is scope for refinement.

Recording of Field Data The following field data were

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 com-

pression load cells. g) Radial support pressure by con-

tact pressure cells. h) ~[Mnnel closure by the tape ex-

tensometers and closure meters.

Deep-seated radial displacement of the rock mass around the tun- nel opening by single- and mnlti- point 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 geo- mining 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. I t has been experienced tha t a single value of some of these six parameters is sometimes influenced by personal bias. There- fore, 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 rat ings RMR (after Bieniawski 1981) were also obtained. The correlation of RMR and Q provided the necessary confidence ( Je thwa et al. 1981). Whenever Q value was doubt- ful, 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

TunneUingand UndergroundSI~ce Technology, Vol. 7, No. 1, pp. 59-74, 1 9 9 2 . 0886-7798/92 $5.00 + .00 Printed in Great Britain. ~) 1992 Pergamon Press plc 5 9

cells of 5-15 kg/cm 2 capacity 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 hori- zontal 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 coutact pressm~cellswereinstalled at the interface of the steel ribs and the bael.-Rll The load cells and the contact pressure cells were installed close to the bmnel face. All of these instruments were protected against direct hit during blast~ ing. The blast vibrations did not affect these ~ o u t s .

Tunnel Closure Diametral deformations of the tunnel

sections were measured by tape 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 commonly occurring soft rock masses such as shales, clays, slates, and phyllites.

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 -nnel opening fails when the tan- gential stress exceeds its uniaxJal crushing strength. The failure of the rock mass is associated with volumet- ric expansion, which is manifested in the form ofradialinward displacement of the t -nnel periphery called t -nnel wall. These deformations are called tunnel closures. The t -nnel closures can be very large (measured closures have been as large as 17% of the size of the t -nnel opening). This phenom- enon is called "squeezing" of the rock mass. The squeeze can occur not only from the roof and the sides, but also from the floor.

~mne l closures resulting from the elastic relaxation of a t -nnel opening, on the other hand, are smaller than 1% of the tlmnel size (see measured values in Tables A1 and A2, Appendix).

Theoretical Criterion Theoretically, squeezing conditions

around a t-nnel opening would be en- countered if

~6 > q~ (1)

where ~o is the tangential stress and Cl¢ is the uniaxial crushing strength of the rock mass.

In the case of a circular tunnel un- der hydrostatic stress field, Eq. 1 can be written as

2P > q¢ (2)

in which P is the primary stress value. It follows that a tunnel section expe-

riencing elastic conditions in a given soft rock mass can encounter squeez- ing 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 elas- tic 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 tun- nel, 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 pre- dicting squeezing conditions is both expensive and time-cons-mlng. There- fore, an attempt was made to seek a simple criterion for predicting squeez- ing 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.

A clear line of demarcation between the elastic and the squeezing condi- tions can be seen. The equation for this line has been obtained as

H = 350 Q,3 (3)

Thus, a rock mess may undergo squeezing when the depth of the tun- nel section exceeds 350 Qm.

Comparison of Measured and Predicted Roof Support Pressure

Barton's correlation, given in Eq. 4 below, was used to obtain predicted values of short-term roof support pres- sure p~. As discussed above, measured roof support pressure values were ob- tained from instrumented t -nnel sec- tions. 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 tun- nel 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 com- parison has not been shown for the wall support pressure because the num- ber of measurements is small.

For predictedroofsupport pressures, the classification methods of Terzaghi (1946), Deere et al. (1969), Proto- dyakouov(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 un- safe for squeezing ground. ForexAmple,

2 0 0 0

I00C

E

Q: 5 0 0

2 O0

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

-139-BARTON'S CASE HISTORIES

• NON-SOUEEZING CONDITION

x SQUEEZING CONDITION

,®

x 159

@ ROCK BURST

*, S Q U E E Z I N G

d

x 9 x¢ / " Xg ® x ~ x ^

142 g ~ / 141 v e4a ~ - x o / ~ o ' -- N O N - S Q U E E Z I N G

xh / xf x f ~ e t 0 4 e l 0 5

c : y -c eq

xg /~s? 9 ~ ec ;~

Xh / ei @101 OB

/ • b

0.1 1 t0 100 Q

100 • 001 "01

Figure1. Criteria for predicting squeezing ground condition.

60 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 7, Number 1, 1992

16 " TERZAGHI I R ~ ~

• x

O 0 4 8 1'2 1'6

obsd kg /crn 2 Pr

12

8

4

12

WICKHA~ x x x

0 4 8 12

pr ObSd , ko /crn 2

16

12-

DEERE

% 8-

% 4 x

• x

0¥ 0 4

8

4

, Or 8 12 16 0

/ a _ x , u , - " * . ~x x x

4 8 12

pObSd kg /cm 2 obsd ko/crn2 Pr

x SQUEEZING , • NON- SQUEEZING

N E u

12 e~ E

- 8

cL

12' 12

8 -

4 -

S A R T O ~

x x ~ I x x

. / i i

4 S 12

~ 2 " , " ~ ¢ m a

e~E 8

• ]o( x x x

i i 0 4 S 12

pob,d , 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/cm 2 when compared to predicted values of 4.2 and 4.4 kg/cm 2 for t~, n n el sections 2 and 4, respectively. Such large differences in the measured and predic ted suppor t p ressures prompted the authors to look for pos- sible 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

seen from Table 1 that in the cases of sections 1 and 2, the difference in sup- port 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 influ- enced the measured support pressure:

1. ~ m n e l depth or thickness of the overburden.

2. ~ m n e l closure. 3. Time. 4. %mnel size.

If other factors are unchanged, the t -nnel 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 esti- mating the overall stiffness of a steel- rib support system. Therefore, bmnel closure has been used to replace the stiffness of a support system (Table 3).

Influence of Overburden on Roof Support Pressure

Barton et al. (1975) suggested the following correlations for support pressures:

i ~ = 2 Q ~ / J ,

l~w = 2Qi~/Jr

in which

Pi~ =

Pi~ =

J =

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 (val- ues of Q, and Q~ should be obtained separately for the roof and the wall rock, respectively).

The short-term roof and wall sup- port pressures were estimated from Eqs. 4 and 5. These values were used to calculate correction factor f for overburden or tunnel depth. The cor- rection factor f is defined as a ratio of measured support pressure to the pre- dicted support pressure. A relation- ship of f to t -nnel 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 relation- ship has been attempted in Figure 3.

According to Figure 3, the correc- tlon factor f can be given by

f = 1 + ( H - 320)/800 > 1 (6)

in which H is the th i ckness of overburden or t, mnel depth in metres.

The data points for squeezing ground appear to suggest that the line in Fig- ure 3 should be much steeper to repre- sent a natural trend. In reality, the difference between observed support

Volume 7, Number 1, 1992 TUNNm~n~e AND UNDERGROUND SPACE TECHNOLOGY 61

Table I. Details of tunnel sections under squeezing ground conditions (from Table A2).

Type of Rock S. No. Mass

1 Crushed red shales

2 -do-

Soft and plastic black clays within thrust zone

4 -do-

Q

0.025 to 0.10

-do-

0.016 to 0.03

-do -

Tunnel Radius

(m)

1.5

4.5

1.5

4.5

Tunnel Depth

(m)

280

680

280

Support Pressure (kg/sq. cm)

Predicted

3.3

4.2

4.4

Measured

3.1

10.8

3.2

11.2

Radial tunnel

closure (%)

2.8

1..2

-do- 4.4

4.5

1.7

Observat ion Period

(months)

26

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 may doubt that the correla- tion 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 conven- tional means, i.e., drilling and 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 re- duced 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 Inter- national'l~,nnelling Association should compile a data bank for observed sup- por t pressures from all parts of the world and should try to improve these correlations.

R a t i o o f W a l l S u p p o r t P r e s s u r e to R o o f S u p p o r t P r e s s u r e

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

wall support pressure, as shown in Table 2. The ratio of the wall support pressure p, to the roof support pres- sure p,, corresponding to Q i , ' , have also been shown in col,,mn 3 of~able 2.

The observed wall support pressures from some of the squeezing and the non- squeezing case histories have been plot- ted in Figure 4. It can be seen that the recommendations of Barton et al. (1975)

2 . 5 -

2 . 0 -

1.5--

I.O

0.8 0

• NON -SQUEEZING

x SQUEEZING

x2

3

• 11 •

09 012

511 5x x7 x~,

ob~ .... f J Pr - Pi( ' 6 1

- ~ f = 1+ (H-320}/SOO xl ~ !

• I0 , , l X 6 l I

200 400 600 8uO

OVERBURDEN ( H }~m

Figure 3. Correction factor for overburden in Barton's correlation for short- term roof support pressure under non.squeezing ground conditions.

62 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 7, Number 1, 1992

Table2. Wall factor Q~.. .: / Q~ for estimating wall support pressure.

Recommendation of Barton et al. 1975 Authors' Recommendation

Range of Qi Qi-w~u I~ Range of Oi 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.

Correlation Between Support Pressure and Tunnel Closure in Squeezing Ground Cond i t i on

Variation of the normalized roof support pressure with the tunnel clo- sure at the crown is shown in Figure 5. The ordinate represents f;, which is the correction factor for tunnel closure at the crown. The correction factor f/is given as

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).

The data points in Figure 5 are taken from Table A2 and represent eight tunnel sections from four differ- ent tunnels. The normalized reef sup-

port pressures are higher for low tun- nel roof closures. The roof support pressures decrease when the t~mnel closures increase and at tain minimum values when the roof closures are ap- pro~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 nor- mRllzed wall-support pressure with the measured t~mnel-wall closures. The correction factor f~ for t, mnel-wall clo- sure is given as

Z m x SQUEEZING GROUND CONDITION

• NON-SQUEEZING GROUND CONDITION

ACCORDING TO BARTON ET AL ( 1 9 7 5 )

- - - SUGGESTED BY AUTHORS

Qi -wo l l : Qi

i x x7

x 3 Qi--wall = 2 . 5 Qi

X I

X 2

l i I 1 I

i 1 Qi -wa l l " .5 Qi I

1 I I k

e ~ L , 4 . , I ,_z~ ' -~ , , , , I , , , , I 0-001 0"01 0"t | t0 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.~ AND UND~.R~ROUND SPACE TECHNOLOaY 63

3

obsd

f" Pr

p ObSd : MEASURED ROOF SUPPORT PRESSURE

Pr = PRE~CTED ROOF SUPPORT PRESSURE

f = CORRECTION FACTOR FOR OVERBURDEN

== .=

.=, m =

N "3

tr : CORRECTIOI( FACTOR FOR TUNNEL CLOSURE AT TIlE CROWN

X4

/

/ /

//X B

5 X 6 ~ / X 7

Y~ DATA POINTS PLOTTEO FROM

TABLE ].b

I J 0 5 t0

OBSEBV.O TU..EL CLOSURE AT CBOWN ~.,.~

Figure 5. Correction factor for roof closure under squeezing ground condition (H > 350 Q^ I / 3).

15

4

o.

t~

$ o

N

r

0 0

X4

Figure 6.

pObSd : w

Pw

obsd PW

fPw

MEASURED WALL SUPPORT PRESSURE

= PREDICTED WALL SUPPURT PRESSURE

CORRECTION FACTOR FOR TUNNEL~WALL

CLOSURE

CORRECTION FACTOR FOR OVERBURDEN

x 7

t ] 5 10

OBSE.VEO TU..E~ WALL CLOSU~(~

Correction factor for wall closure under squeezing ground condition (H > 350 Q^1/3).

t5

f~ _I ~ l~w

where p ~ =

f =

Piw =

(8)

measured wall support pressure correction factor for over- burden, and predicted short- term wall support pressure.

Thus, the correction factors f' and f ' r w

are the same as the normalized roof and wall support pressure, respectively. The recommended values of these cor- rection factors are given in Table 3.

The validity of Table 3 for squeezing ground has been questioned, particu- larly with regard to highly squeezing or flowing ground. I t is suggested that the application of Table 3 should be re- stricted to moderately squeezing ground by limiting closure to 5%, by strength- ening 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 signifi- cantly higher than the roof support pressure in the case of parallel tunnels if the clear spacingis less than the sum of the tunnel widths.

Variation of Support Pressure with Time

After studying the influence of the overburden and the tunnel closures and incorporating these influences in the correlations between Q and sup-

port pressure, it has been possible to study the effect of t ime on the support pressure. Figure 7 shows the variation of the correction factors t~'over time. The correction factors f" for t ime are given as

Table 3. Correction factors for tunnel closures in squeezing ground conditions.

S. No.

1.

2.

3.

4.

5.

6.

Ground Condition

Non-squeezing (H < 350 Ola)

Squeezing (H > 350 ~1/3)

Moderately squeezing

-do-

Highly squeezing

-do-

Support System

Very stiff

Stiff

Flexible

Very flexible

Extremely flexible

Tunnel Closure

( % )

<1

1-2

2 -4

4 -6

6-8

> 8

fwor f ;

1,0

> 1.80

0.85

0.70

1,15

1.80

6 4 TIYNNELLING AND U N D E R Q R O U N D SPACE TEcI- INOLOGY Volume 7, Number 1, 1 9 9 2

X ROOF S U P P O R T P R E S S U R E IN SQUEEZING GROUND C O N D I T I O N

• ROOF SUPPORT P R E S S U R E IN N O N - S Q U E E Z I N G GROUNO CONDIT ION

0 W A L L S U P P O R T P R E S S U R E IN S Q U E E Z I N G GROUND CONDIT ION

I . 1

- 0 5 . I " / . ...-..

. ....- . . . . I - ' - t 0 4

f

• t " " x ,

• ~ 0 7 ~ . . . . . " - " ' ' ' - ' " " ~

- 4 ~ 1 C - ~ , -~ zx ~ x 8 x3_ . ~ . " ~ i O ~ ..... ,~...,. • ~

.....-

2

t 0

A , , '{ , i , , 1 L , , ,

10 1 0 0 t 0 0 0

t , T I M E OF OBSERVATION IN M O N T H S

Figure 7. Variation in observed support pressure over time.

Combining Eqs. 9, 10, and 11, the long-term roof and wall support pres- sures can be given as

p, = p~' f ' f ' " log 9.5 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 ult imate to the short- te rm support pressure is about (5) ~, i.e., 1.7. Equation 11, however, sug- gests the following relationship:

p o~ f ' log 9.5 t °'~

i.e.,

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.

u . , • . t ! In case ofa r ~ d hnmg, f~ = fo' so that the ratio of the ult imate support pres- sure a i ~ r 100 years to that alter one month is given by (t o = 1 month and t~ = 100 years or 1220 months)

f~_ ~.,~,,a (9)

f" f" 1~

where

f;

- ( l O )

f { ~ w 12-

= correct ion factor for the influence of t ime on the roof support pressure,

f~w = correct ion factor for the influence of time on the wall support pressure,

1~ b~ = m e a s u r e d roof s u p p o r t pressure,

I ~ ~ = m e a s u r e d wal l s u p p o r t pressure,

f = correction factor for over- burden (Eq. 6), and

f ' = correction factor for tlm~el closures (Table 3).

All the data points from Tables A1 and A2, except those for the wall sup- port pressures in the non-squeezing ground conditions (being negligible), have been plotted in Figure 7.

According to Figure 7, the correc- tion 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 -nne l opening.

C M R S

°'1E 8

a - L .

Q. 4

0 T " I | 1

0 4 8 12

pobsd , kg/crn 2

x S Q U E E Z I N G ~ • N O N - 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).

Volume 7, Number 1, 1992 TUNN~.LWO AND UNDERGROUND SPACE TECHNOLOGY 65

pz _ log 9"5t°'as - 1.75

po log 9.5 t~ 25

In other words, the support pres- sure will increase in 100 years to 1.75 times the support pressure observed after 1 month of excavation. The cor- rected support pressures compare well with the observed values, as shown in Figure 8.

This ratio of 1.7 between the ulti- mate and the short-term support pres- sure 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 pres- sure, worked out here as 1.7, is rela- tively small compared to the ratio of 2- 3 after Jethwa (1984), probably be- cause the period of observations re- ported 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 perfor- mance study of Chhibro-Khodri under - ground powerhouse complex in India (Mitra 1991).

For designing a temporary support system, one may assume a unlforin distribution of the short-term support pressure, but a factor of safety of 1.75. However, the permanent support sys- tem should be designed for the net ul~mAte support pressures, with a fac- tor of safety of 1.5.

tunnel opening (2a). The ordinate rep- resents the observed roof support pres- sure corrected for overburden, the tun- nel closure and the time of excavation. It may be seen that the corrected sup- port pressure is independent of the tunnel size.

The reason for support loading in non-squeezing conditions may be re- lated to the dead weight of the loosened rock blocks which become detached from the parent rock mass at the tun- nel roof and rest on the support sys- tem. 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 mobi- hzing large loosening pressures, even in tunne l sect ions suppor ted by shotcrete immediately on excavation. The loosening pressure (dead weight of the loosened or a destressed zone) is therefore mobilized due to poor blast- ing practice and is likely to be indepen- dent 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 pres- sures were nearly the same (i.e., about 0.5 kg/cm 2) 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 qual- ity Q estimated from a larger bmnel would be smaller than that obtained from small driRsin a similar rock mass. This is due to the possibility of intersect- ing greater number of geological

Ef fect of E x c a v a t i o n S i z e on S u p p o r t P r e s s u r e

According to Terzaghi (1946) and Unal (1983), the support pressure is directly proportional to the size of a tunnel opening. On the other hand, Barton et al. (1974), suggest that the

2- support pressure is independent of the ~,_ t -nnel size.

Colnmns 3 of Tables A1 and A2 (see *- Appendix) list support pressures ob- w-=

tained from Terzaghi's method for non- .~_ squeezing as well as squeezing ground ~ 1 conditions. It may be noted that the "~ estimated support pressure values (shown in col-ran 3) do not compare " O h-

well with the observed support pres- sures (in co]-mnR 12 and 13). The support pressures corrected according o 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

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 hydro- geological conditions ( J ) should also be determined, if possible, after a water tlmnel is commlssioned. Itwould there- fore 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 weak- ness, it is cautioned that Terzaghi's concept may still be vahd

C o n c l u s i o n s

The combined approach of field in- strumentation and quantitative clas- sification of Barton et al. has proved rewarding at this stage of development of rock mechanics. Despite limited field data, some practical trends show- ing the influence of overburden, tnnnel closures and time of excavation on the t , nnelling condition and the support pressures have emerged. It would per- haps be hasty to draw any definite conclusions from these trends; how- ever, some tentative correlations have been possible. These correlations are subject to refinement as more field data is collected. The following tenta- tive 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 shor t - term 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,

x SQUEEZING • ' NON- SQUEEZING

t x ~X

, , . . . . . . . . . . . . . - , -

• ~ ~2

61 7

Q'7

i ! ! I i

O 4 8 12 16 20

DIAMETER OF OPENING jm

Figure 9. Support pressure virtually independent of tunnel size.

66 ~ G AND U~DZRGROUND SFACZ Tzcm~oLoGY Volume 7, Number 1, 1992

and f ' is the correction factor for t~nnel closure (see Table 3, equal to 1 in non- squeezing ground conditions). The value of the correction factor f i s given as

f = 1 + ( H - 3 2 0 ) / 8 0 0 > 1

3. In squeezing ground condit ions, the suppor t p ressu re is s ignif icant ly inf luenced by tunne l closures. The correction factor f ' for t unne l closure var ies from 0.7 to 1.8 in the case of a single tunnel . The m i n i m u m suppor t p ressure occurs when the tunne l clo- sure is about 5% of the tunne l diam- eter. The suppor t p ressu re increases rap id ly beyond th is l imi t ing closure.

4. The shor t - term wall support pres- sure m a y be obta ined from the above corre la t ion by subs t i tu t ing Q~n for Q. In general , the ac tua l wal l suppor t p ressu re for the non-squeezing rock condit ions is l ike ly to be negligible. The sho r t - t e rm values 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 suppor t p ressure m a y be 1.75 t imes the shor t - t e rm sup- por t p re s su re for t lmne l sections unde r non-squeezing ground conditions, ex- cept for cases of soluble and erodible jo in t f i l l ings wi th seepage.

6. The suppor t p ressu re is indepon- dent of the tunne l size, provided t h a t Q is ob ta ined from a full-sized opening.

[]

References Barton, N.; Lien, R.; and Lunde, J. 1974.

Engineering Classification of Rock Masses for the Design of Tunnel

Support, RockMechanics, Vol. 6, 189- 236. 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 Con- ference on Geomechanies, Brisbane, 36- 41.

Daemen, J. J. I~L 1975. ~mnel 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 FAiling rock 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. Geo- engineering 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. Ins t rumenta t ion 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, 766- 783. 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 Under- groundExcavations, 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~lnn el 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 Tunnel ing Conference, San Francisco, California, Vol. 1,691-708.

Volume 7, N u m b e r 1, 1992 TUNN~.T.n~a AND U~mE~mOUND SPACE TECHNOLOGY 6 7

<

0 r l.a

¢.D

¢,

D

b~

App

endi

x Ta

ble

A1.

C

ompa

riso

n of

pred

icte

d an

d ob

serv

ed s

uppo

rt p

ress

ure

from

Q-s

yste

m in

non

-squ

eezi

ng g

roun

d co

nditi

ons.

Ge

olo

- g

ica

l S

. d

esc

rip

- N

o.

tio

n

Ver

tical

S

up

po

rt

Pre

ssur

e fro

m

Te

rza

gh

rs

Cla

ssifi

- ca

tio

n

Sho

rt-

term

R

ock

Mas

s Q

ua

lity

Sh

ort

-te

rm S

up

po

rt

Pre

ssu

re

Ver

t.

Hor

iz.

Co

rre

ctio

n F

act

ors

fo

r:

Ove

r-

bur-

T

un

ne

l W

all

de

n

Clo

sure

Co

rre

cte

d

Sh

ort

-te

rm S

up

po

rt

Pre

ssu

re:

Ver

t.

Ho

riz.

Obs

erve

d S

up

po

rt

Pre

ssur

e

Ver

t.

Hor

iz.

Qi

= 5Q

P

.,

f f'

f'

1"

W

p,

p

. p

o=

p.O

=

from

fr

om

from

fr

om T

able

4

Eq.

4

Eq.

5

Eq.

6

= P.~,

= P..,,

f .f

' f-

f,

r w

Ob

ser-

va

tio

n

Per

iod

kg/s

q, c

m

kg/s

q,

kg/s

q.

cm

cm

kgL~

l, kg

/sq,

kg

/sq,

kg

/sq.

cm

cm

cm

cm

m

onth

1 2

3 4

5 6

7 8

9 10

11

12

13

14

15

Man

eri-

Bha

ti H

ydro

Pro

ject

1.

Mod

erat

ely

frac

ture

d qu

artz

ite.

a =

2.4

y =

2.5

RQ

D =

75,

Q

= 3

.6

uh/a

=

0.06

, H

=

225

(Jet

hwa

et

al.

1982

)

0.3

to 0

.7

(0.5

) 15

tO 3

0 (2

1)

0.5

to

0.7

(0.6

) 0.

1 tO

0.

2 (0

.4)

1.0

1.0

1.0

0.5

to

0.7

(0.6

) 0.

03 to

0.

05

(0.0

4)

0.6

Rem

arks

Ste

el r

ibs

stab

le

.< + i.

a

p.a

¢.0

¢.0

bO

Q + O

~

~D

Tab

le A

1 (c

ontd

.)

1 2

3 4

5 6

Man

eri-

Bha

li I-

Ivde

l Pro

iect

(co

ntd.

)

2.

3.

Fol

iate

d m

etab

asic

s.

a =

2.4;

y =

2.5g

R

QD

= 8

2,

Q =

3.4

-6.8

uh

/a =

0.0

5,

H =

550

(J

ethw

a et

al.

1982

)

She

ared

m

atab

asic

s.

a=

2,4

,7

=2

.5

RQ

D =

60

Q =

0.3

-3.3

uh

/a =

0.4

H

= 3

50

(Jet

hwa

et a

l. 19

82)

0.3

to

0.7

(0.5

)

0.8

to

2.6

(1.7

)

17

to 3

4 (2

4)

1.5

to

16.5

(5)

0.5

to

O.7

(0.

6)

0.7

to

1.8

(1.2

5)

Sal

al H

ydel

Tun

nel

4.

Hig

hly

join

ted

dolo

mite

s.

a =

6,~'

=

2.8

R

QD

= 3

0-4

0

Q =

1.2-

1.7

H=

11

0

1.7

to

5.5

(3.6

) 6

to 8

.5

(7)

1.0

to

1.2

(1.1

)

Kha

ra H

ydel

Pro

iect

5.

Gra

de I

phy

llite

s,

mas

sive

and

di

stin

ctly

join

ted

a =

7,Y

=

2.64

R

QD

= 7

5,

RM

R =

67

Q =

5, H

= 3

20

(Dub

e et

al.

1982

)

(25)

0.

16 t

o 0.

45

(0.3

)

0.1

to

0.20

(0

.15)

0.2

to

1.2

(0.7

)

0.4

tO

0.5

(0.4

5)

(0.1

3)

1.29

1.04

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

10

11

12

13

14

15

0.6

to

0.9

(0.8

)

0.70

to

1.9

(1.3

)

0.00

5 to

0.0

5 (o

)

0.2

tO

0.5

(0.3

5)

0.8

2.0

2 S

teel

rib

s;

stab

le

3 S

teel

rib

s;

stab

le

1.0

to

1.2

(1.1

) 0.

14 t

o 0.

2 (0

.17)

1.1

1.5

appe

ars

negl

i-

gibl

e

Ste

el

ribs;

st

able

(0.3

) (0

.04)

0.

25 t

o 0.

56

(0.4

0)

appe

ars

negl

i-

gibl

e

Ste

el

ribs

; st

able

,e-

IN

b-

[~.-' •

0

v

0 v

v

q

-.:. .+ ~ o _

.8

• - , ,

E '

O

~ , , u o O

m-.

o.

o

q

O

~ 8 m---.

K"

~ ~ . ~ mr ~,~

..d <,D

0~ I -

I n O

o

O v '

=~ ~ :.--&6 A " - = " ~ -+ Lm _~

0

0

o.

o

o

o5

o

°oO~ t~ ~ V

>~.

O E o-Z mOz:~

o el

o :D

"11"

m t : i,=

GO

O

O

O

o.

E ~D E o ~

~ ~ - ~

II e" ¢- ~ 8 m'-, o

o5

7 0 T U N N ~ , T , ~ G AND U N D E R G R O U N D S P A C E T E C H N O L O G Y V o l u m e 7, N u m b e r 1, 1 9 9 2

~D

~D

b~

Tab

le A

1 (c

ontd

.).

1 2

3 4

5 6

Kha

ra H

ydro

Pro

iect

(co

ntd.

)

9.

Cha

lnag

e 38

9 m

a

=3

, Q

= 0

.4

H =

150

-200

uh

= 2

2.5

mm

do

do

do

Up

pe

r K

rish

na P

roje

ct

10.

11.

Thi

nly

bedd

ed

shal

es w

ith

calc

ite b

ands

. C

haln

age

761

m

a =

6.5,

uv

= 1

2 m

m

Q=

15

, H

=3

4

Cha

lnag

e 20

4 m

a

=6

.5,

Q =

15

H =

52,

uv

= 5

mm

75

do

(0.2

2)

do

(o)

do

Lakh

war

Hvd

ro P

roie

ct

12.

Tig

htly

join

ted

basi

c re

ck.

H =

250

, Q

= 8

-9

(i) a

= 1

0.5

(ii) a

= 7

(ii

i) a

= 3

--

42

0.35

--

Not

atio

ns:

a =

aver

age

radi

us o

f tun

nel

open

ing

in m

etre

s.

RQ

D =

roc

k qu

ality

des

igna

tion

in p

erce

nt.

Q =

roc

k qu

ality

bas

ed o

n cl

assi

ficat

ion

of B

arto

n et

al.

(197

4).

7 =

unit

wei

ght

or r

ock

mat

eria

l in

gm/c

c.

uh/a

= t

unne

l wal

l clo

sure

in p

er c

ent.

uv

/a =

clo

sure

at c

row

n le

vel i

n pe

r ce

nt.

H =

hei

ght o

f ove

rbur

den

abov

e op

enin

g in

met

res.

(

) =

aver

age

valu

es,

exce

pt in

col

umn

4, w

here

roo

t m

ean

squa

re v

alue

is g

iven

.

++

Est

imat

ed s

uppo

rt c

apac

ity.

1,18

1 do

1.0

do

1.0

do

1.0

do

1.0

9 10

11

12

13

1.0

1.0

1.0

1.0

(1.7

) (1

.1)

2.4

1.6

(0.2

2)

(0)

0.2

(0.2

2)

(o)

0.2

to

0.4

(0.3

)

appe

ars

negl

i-

gibl

e

appe

ars

negl

i-

gibl

e

0.35

0.

45 t

o 0.

55

I 14

1.0

1.0

15

do

do

do

Dis

trib

utio

n of

sup

port

pr

essu

re i

s as

ymm

etri

c

t~

o

Tab

le A

2.

Com

pari

son

of p

redi

cted

and

obs

erve

d su

ppor

t pre

ssur

e in

squ

eezi

ng g

roun

d co

ndit

ions

.

S. N

O.

1 2

Geo

lo-

gica

l D

escr

ip-

tion

Ver

tical

S

uppo

rt

Pre

~u

re

from

Te

rzag

hl's

C

lass

ifi-

ca

tion

Sho

rt-

term

R

ock

Mas

s Q

ualit

y

Sh

ort

-te

rm

Sup

port

P

ress

ure

Cor

rect

ion

Fact

ors

for:

Ove

r-

bur-

d

en

T

un

ne

l C

losu

re

Cor

rect

ed

Sho

rt-t

erm

Sup

port

P

ress

ure:

Ver

t,

Hor

lz.

Ob

serv

ed

S

uppo

rt

Pre

ssur

e

Obs

er-

vati

on

Per

iod

Qi

P,~

p~,,

roof

w

all

f'

f'

r W

P~

P

. P

~

pw °b

= 5Q

fr

om

from

=

P~of

" =

P.=

? fr

om

from

fr

om

Tab

le

Tab

le

Eq.

4

Eq.

5

Eq.

6

4 4

f " f

'r f

" f*'

kg/s

q, c

m

kg/s

q,

kg/s

q.

cm

cm

kg/s

q,

kg/s

q,

kg/s

q,

kg/s

q.

cm

cm

cm

cm

mon

th

3 4

5 6

7 8

9 10

11

12

13

14

15

Rem

arks

Chi

bro-

Kho

dri T

unne

l

< o -.3

tj~

b3

OR!

!==¢

1

a=

1.5

, R

QD

=10

- 20

Q

= 0

.025

-0

.10

7 =

2.73

uh

= 2

.8

H =

280

(J

ethw

a et

al

. 19

82)

1.8

to 3

.4

(2.7

) 0.

125

to

0.50

(0

.25)

2.5

to 4

.2

(3.3

) 1.

8 to

3.0

(2

.4)

1.0

(0.8

5)

1.8

(2.8

) (4

.3)

3.1

1.7

Circ

ular

rib

s st

able

b.a

~D

CD

~0

Q ~2

Tabl

e A2 (contd.).

1 2

3

Chi

bro-

Kho

dri

Tun

nel

(con

td.)

2.

3.

4.

Cru

shed

red

sha

le,

high

ly.s

quee

zing

a=

415,

=

2.73

R

QD

= 1

0-20

Q

= 0

,012

-0.5

uh

/a =

6,

H =

680

(J

ethw

a et

al.

1982

)

Sof

t pl

astic

bla

ck

clay

s in

thru

st z

one,

m

oder

atel

y sq

ueez

ing.

a~

'5,

= 2.

64

RQ

D =

10

Q =

0.0

16-0

.03

uh/a

= 4

.1

uv/a

= 4

.5

H =

280

(J

ethw

a et

al.

1982

)

Sof

t pl

astic

bla

ck

clay

s w

ithin

thru

st

zone

, m

oder

atel

y sq

ueez

ing.

Y

=

2.64

, R

QD

= 1

0 Q

= 0

.016

-0.0

3 H

-- 2

80

(Jet

hwa

et a

l. 19

82)

Gir

i H

ydro

Tun

nel

5.

Ver

y bl

ocky

and

se

amy

slat

es,

mod

erat

ely

sque

ezin

g.

a=

~.l

, =

2.5

uh/a

= 7

.6

H =

380

Q

- 0

.32-

0.82

(J

ethw

a et

al.

1982

)

10.3

to

22

.1

(16.

2)

1.5

to

3.3

(2.4

)

do

0.7

to

2.3

(1.2

)

0.06

to

0.

25

(0.1

2)

0.08

to

0.

15

(0.1

1)

do

1.5

to

4.1

(2.5

)

3.3

to

5.0

(4.2

)

4.0

to

4.8

(4.4

)

do

1.2

to

1.8

(1.5

)

2.4

to

5.0

(3.7

)

3.0

to

4.8

(3.9

)

do

0.8

to

1.3

(1.0

)

7

1.45

1.0

1.0

1.08

8 1.8

0.70

1.8

1.15

9 0.7

0.7

1.8

1.15

10

(11)

(3.1

)

(7.9

)

(1.9

)

11

(3.8

)

(2.7

)

(7.0

)

(1.2

)

12

10.8

3.2

11.5

2.0

13

3++

2.6

12.2

2.4

14

8 26

26

12

15

Hea

vy c

ircu

lar

ribs;

sev

ere

buck

ling

due

to

sque

ezin

g.

Littl

e cl

osur

e at

cro

wn.

Cir

cula

r rib

s w

ere

stab

le; c

ompr

ess-

ib

le b

ackf

ill

behi

nd r

ibs.

E

nlar

gem

ent o

f dr

ift to

9 m

siz

e in

cl

ose

prox

imity

de

laye

d st

abi-

lis

atio

n.

Circ

ular

rib

s of

ve

ry h

igh

capa

city

w

ere

stab

le.

Con

sequ

ently

, tu

nnel

clo

sure

s w

ere

likel

y to

be

low

, app

rox.

0-

2%.

Roo

f clo

sure

co

nsid

ered

equ

al

to w

all c

losu

re a

s ho

rses

hoe

ribs

w

ith in

vert

str

uts

defo

rmed

, bu

t no

t as

sev

ere

as in

ca

se 6

.

Q 0 ~D

Tabl

e A

2 (c

ontd

.).

1 2

3

Gir

i Hyd

ro T

unne

l (c

ontd

.)

6.

Cru

shed

phy

llite

s,

high

ly s

quee

zing

. a

= 2~

1,

=2

.3

RQ

D =

10-

25

q =

0.12

4-0.

32

uh/a

= 1

2.4

uv/a

= 5

H

= 2

40

(Jet

hwa

et a

l 19

82)

2.1

to

4.1

(3.1

) 0.

62 t

o 1.

6 (1

) 1.

8 to

2.

3 (2

.1)

1.2

to

1.9

(1.6

)

Lo

kta

k H

ydro

Tun

nel

7.

Cru

shed

sha

les,

m

oder

atel

y sq

ueez

ing.

a

= 2~

., =

2.7

gm

/cc

RQ

D =

10-

20

Q =

0.0

11-0

.044

uh

/a =

7

H =

300

(J

ethw

a et

al

1982

)

2.9

to

5.4

(4.2

) 0

.05

5to

0.

22

(0.1

1)

3.5

to

5.3

(4.4

) 2.

5 to

5.

3 (4

.0)

Man

eri

Bha

li Pr

oiec

t

8.

Hig

hly

frac

ture

d qu

artz

ites.

a=

23$,

=

2.5

R

QD

= 6

O, Q

= 0

.5

uh =

190

mm

H

= 3

5O

(Sha

rma

1985

)

2.64

(2

.5)

1.6

1.1

See

foo

tnot

es in

Tab

le A

1 fo

r no

tatio

ns.

1.0

1.0

1.04

0.7

1.15

1.15

1.8

1.15

1.15

10

(1.5

)

(5.1

)

(1.9

)

11

(2.9

)

(4.6

)

(1.3

)

12

1.7

5.4+

+

2.0

13

4.0

5.4+

+

14

27

14

15

Peak

m

easu

red

supp

ort

pres

sure

of

5 k0

/sq, c

m

occu

rred

at

half

tota

l wal

l cl

osur

es w

hen

hors

esho

e rib

s w

ith in

vert

bu

dded

.

15-c

m-t

hick

sh

otcr

ete

with

4-

m-l

ong

rock

bo

lts s

uppl

e-

men

ted

with

ci

rcul

ar r

ibs.

S

quee

zing

oc

curr

ed e

ven

at H

= 1

60 m

. R

oof c

losu

re is

co

nsid

ered

eq

ual t

o w

all

clos

ures

.

Sup

port

s bu

ckle

d.

Ver

tical

and

ho

rizo

ntal

cl

osur

es

appe

ared

eq

ual.

b3