s. Shunsuke Sakurai and Yasutoshi Kitamura - osmre.gov · ed grouted steel dowels - Dr. P.G....

Mining Industry

,dgements

ors would like to acknowledge the fina ncial sponsorship of the uded in this paper by several Australian Mining Companies, or through the Australian Mineral Industries Research

td. ~!embers staff of the companies have provided bvaluable in all projects. ors would like to express their sincere thanks to all members he Division with whom they have worked collaborati vely on the cts:

ed grouted steel dowels - Dr. P .G. Fuller , Mr. R.H.r. Cox.

tion of crown pillars

pe investigation

- Mr. G. worotnicki, ~~- A. Spathis.

- Mr. L,G . Alexander , Mr . s. Mathews .

le and high-wall stability - Dr. P .G. Fuller, Mr. R.H.T. Cox.

es

by , O. R. - "Techniques Applicable in Mining Industry Investigations" RO, Division of Applied Geomechanics , Technical Report, 1974.

O..H . - "A Laboratory Investigation of a High Modulus Borehole ;~ Gauge for the Measurement of Rock Stress" 4th Symposium on !t Mec hanics, Pennsylvania State University, 1961.

<i, G. and Walton, R.J. - "Triaxial Hollow Inclusion Gauges for ~rmination of Rock Stresses in-situ". Inve§tigation of Stress ~ock, Advances in Stress Measurement, I.S .R.M. Conference, 1ey, 1976 .

ci, G., Enever, J .R. and Spathis, A. - "A Pressiometer for ~.rming Deformation Modul us at Rock in- situ" . Symposium, >i tu Testing for Design Parameters, Aust. Goomechanics iety, Victoria Group, Melbourne, 1975.

:i, G., Alexander , L .G., Willoughby, O.R. a nd Ashcroft, J.F. -·ormation and Behaviour of High Rise Filled Stopcs at CSA Mino, u: , N.S.W." Proc . , 2nd Aust. - N.Z. Conf. on Geomcchanics, ;bane, 1975.

'·

VIBRATION OF TUNNEL DUE TO ADJACENT BLASTING OPERATION

Shunsuke Sakurai1 and Yasutoshi Kitamura2

I. Introduction

61

In recent years, the construction of utility tunnels has been quite active bcc3use of the effec tive usc of land and the settlement of environmentAl problems. And the demand will increase more in the future. Under these circumstances, it is quite often necessary for a newly constructed tunnel to crO!s thrcc·di· mensionally with an existing old tunnel in the underground medium. When the construction work$ u e conducted by blasting, much attention must be paid to the blasting operation in order not to cause any serious damage on the existing tunnel.

Dynamic behaviors of underground structures like tunnels due to blasting have not yet been clea rly understood. Therefore, there are difficulties In establishing controlled blasting techniques. Determinations of charge weight, its pattern and design criterion for vibration of structure are based on previous experiences.

In order to establish controlled blasting techniques to avoid damage in an adjacent tunnel , the dynamic behavior of the tunnel under blast excitation must be first understood. for this purpose, some field works have been conducted. This paper is concerned with the results of these field experiments. This consists of two parts, the first is concerned with the dynamic behavior of two unlined tun.nels which three-dimensional ly cross each other in the underground medium, the second part is for the dynamic behavior of the concrete lining of tunnels due to an adjacent bl3!1, and much attention is paid to discus.slng the relation be tween the particle velocity and the strain in the lining.

2. Dynamic Behnviors of Two Unlined Tunnel.s

2.1 Test Site The test site where the experimento.l works have been conducted is shown in Fig. 1. There are two

tunnels embedded in tltis site; one is designed as a water supply tunnel (called Tunnel A), and the other is for an underground subway (called Tunnel B). Both tunnels are under construction, so that no liners arc installed as yet. Tite crossing angle of these two tunnels is about 78° in a level plane. The clearance between the two tunnels is 29.7m at the excavation of a pilot tunnel , which is located at tte bottom of Tunnel n, and 24.5m at the excavation of the upper part of Tunnel B. The underground formation around the tunnels consists of granite, and the propagation velocity of the longitudinal w:ave is approximately 3.0- 3.7km/s.

r Professor of CivU Engineering, Kobe Univenity, Kobe, Japan • Research Associate, Kobe University, Kobe, Japan

International Symposium on Field Measurements in Rock Mechanics- Zurich, Apri/4·6, 1977

62 Dynamic Behaviour

7fl'lltiNT J ,.......,....,_. M

!i ... el A

S,qm

~ ~

-·--.. -----·------·----Pilot Tunnel

Tuno 9.0m

Fig. I Section View of Two Tuonels

2.2 Instrumentation

e "' ""

The particle velocities on the surface of the tunnels were measurecj by electromag,nctlc·typc velocity gages (Geo·space: na\u.ral frequency fo • 4.5Hz and 8Hz). The data are-, recorded on an elec\romognetic oscillog111ph through a direct current amplifier. The velocity gages were ·directly mounted on me bottom rock surface of both tunnel A and 8 with cement mortar. The blasting operation was done only at the face of tunnel 8 and all measurements were taken at the same time for each bias\.

2:3 Results and Discussions (I) Comparison of Particle Velocity in Tunnel A and Tunnel B

Particle velocities measured at Tunnel A are compared whit those at Tunnel B, u shown in Fig. 2. These measurements ~ve been obtained for blasting operations at tllree different positions of the face of Tunnel B, but the measuring points in both tuMels are located at tile same clistance from the bhut zone. Tlllle-delay shots are used for blasting. The results are plotted agaiast each shot.

Fig. 2 (a) shows the ratio of the vertical component In Tunnel A ~gainst the horizontal component in tloe direction cf tlle axis of Tunnel 8 , where the horizontal compor,ent Usually gives the maximum \':llue. On tlle other hud, Fig. 2 (b) shows the ratio of the vertical compo~ent in Tunnel A against tlle vertical in Tunnel B. It can be seen from Fig. 2 (b) that the magnitude of the particle velocity in Tunnel A may become more than ten times greater th~n in Tunnel B, and the ' 'ertical component of Tunnel A gives a 2- 7 times lll'eater magnitude than the horizontal component, depending on the location of blast zone. The maximum value of thiJ ratio iJ obtained when the face of the tunnel approaches to within 30m of the crossing point, and th is ratio tends to decrease as the tunnel fa~ approaches the crossing point and passes through it.

....

8

:[ s ~ 4

2

Effects of Blasting 63

'=~ -lA)

( ) · ··•(Bl

h;"' ....... / \,..· '· ,.

... :;:.·· .... .... •·; : __ __.'!,_ ·- · ........ ' w• • / '.._...· - .... _..... . .....

, 2 3 4 5 6 7 8 9 \0 1\ 12 1J Shot No.

V4 : Vert I cat Component in Tunnel A

Va: Ver t ical Component on Tunnel B

Ho: Horizontal Compon~nt in Tunne l a

" s., >

8

6

2

(bf)

- ,_,\_.,_ \ / '~.......... ... .... \. .. /

...... ,.. ;'\ ··--·- / '~./'*'.'..,..,.,.· \

2 3 4 5 6 7 8 !) \() 1\ \2 \ 3 Shot No

Fig. 2 R~tio of Pnrticle Velocities

These results give valuable informa\ion as in the following C3se. When the 13ce of the new tunnel approaches ~n existing tunnel , knowledge about the magnitude of vibration of the existing tunnel is of Importance to avoid any damage in it, prior to the blasting operation. But the mocnting of any vibration measuring devices is often prevented by the dllily use of the tunnel. so that estimation must be done without direct measurement. Therefore, it is necessary that the dynamic behavior of the existing tunnel must b~ presumed by means of monitoring the vibration of the newly constructed tunnel.

The above mentioned results c:~n be used for tJoiJ estirmtlon of the vibration of the oxistlng tunnel . The results show that the magnitude of vibration in the existing tunnel is greater than that in the new one, even thou&h the measuring poir.ts are at the same distance from the blast.

Fig. 3 showing the p>rticlc motion in both tunnels, indicates that the bottom of Tunnel A mainly vibrates in the vertical direction, but the bottom corner of Tunnel B seems to vibrate in a line throuah the centroid of the tunnel cross section. Titerefore, It Is noted th~t the vertical components uf menurement are strongly influenced by the place where the measurements are done. If the measurements arc taken at the center of the bottom, the magnitude would be greater than those obtained ot a cornor or side wall . (2) Orientation of EnefllY Pro?a&ation from the Blur Zone

It has been seen from Fig. 2 t~t the greater magnl\udes of particle \•elocity ore melsurod in Tunnel A as the face of Tunnel 8 approa: hes the crossing point, compared with those obtai1cd ,. the focc leaves beyond the crossing. To show this more clearly, the measurements are continuouS:y oaken •t Tunnel A for each blast proa:ressing the ftce of the upper p31t or Tunnel 8 .

By ouuming tJoat the partlc.c velocity is proportional to ltl'll ( ltl: charge weight), the particle velocity for a unit charge Is expressed io Fig. 4 as a function of the location of the bllst . It appears from the figure that the magnitude of particle velocity becomes a maximum when the fxe of Tunnel B arrives at a little before the crossing poino, not exactly at the crossing where the dlsunoe bot~<•een the measuring point and blast zone becomes the shorteSt.

It is also clear that there seems to be a non·uniformity In the orientation of the travelling wave, prl!pagated from the blast zone. The energy is propagated mainly in the direction of tunnel progression, rather than in the backw~rd direction. This non·uniformity may be due to the free boundary of the tunne l face.

64

"' i' ' ~ c ~ >

•tO' 8

7

6

5

3 I

T

.. . . ... ~ i' : ,

LD l)

Dynamic Behaviour

y (a)Tunne l B

v (b) Tunnel A

0 I X

L

• Measuring Point

v

l

x Position ot Blas t

Fig. 3 Particle Motion

Cross ing Point . . ·[. .

. ! . I· ., I ... .. .: . : : .

•• • • 0

• g I=· 8 ~ ofo • t o I • ,· .a, ··t·''~ .. . , ' :~ ' t ••

• : Shot No. t

•: Shot No.~-15

Tunnol A

~easurlng Poin t

~ Tunno t 9

~:~:,t Loot . . -Direct i on of Tunnel

Progression

. D ~

0 a •

. •• .a ,a 2D 1D D 10 2D 3D LO 50 6D 70

-Oi roc t l on of Tunnol Progrossion X (ml

Rg. 4 Particle Velocity In Tunnel A due to Progression of Tunnel n

'


In Fig. 5, particle velocity magnitudes in both tunnels are plotted against the distunces between the blut zone and the measuring points. It Is obvious that the regression lines in these ligures art quite different from each other. Vibration of Tunnel A decreases more rapidly with distmce than Tunnel ll. These differences come from the different wave paths from the blast zone. Assumin£ that thero is similar geometrical relation between the case u shown in Fig. S (b) and the case where the tunnel fa~e approoches a messuring point, as shown In Fig. 4, the following relation is valid among parti:lc velocity v (kine), charge weight IV (gr) and distance r (m),

v:CIII"hr- >.7 (II

where C is a constant , depending on the method of bla.stlng, nature of rock Jnd type of cil3J SC· It is seen that the constant C is a factor giving the magnitude of energy tr:Jnsmitted int~ the medium. Tire results calculated show that the constant C decruses as the blast zone appro~ches rhc me:,surins point. Tire results are plotted as a function of the angle between the tunnel uxls and the directhlll to J h~ mcusurin~ point, as shown in Fig. 6.

It may be concluded that the energy is not uniformly propagoted in aU directiou, but mu.;h of the energy is transmitted In the direction of tunnel progression.

:~ Tumel A

5.01- • a .. Tunnel B ~ MJIQsu rtng

0 Points <o:g •"o Blast 8

ol ol I I

" Tunnol8 :68 Moosuring Points 0 (i) 4:=::=

So o Blast _1,D 0 yt• u 'i>' 8 8 ~0.5 ~ a 111. I "' v. ·~ g I ii\• v.c.,-" ];. 0,5 0 0 0

:s lo ~ ·g :Bo.2 .. . IB ... > 0 > 4> Oo . 't\ ~0. 1 :li .. 0

~ 0,1 '8> t • ~D.DSI . " . • O.C6 .

10 20 5D 1DO 2D LO 70 100 Distance im) OJStance(m)

(a) Tunnel B (b) Tunnel A

Fig. S Regression Unes of Particle Velocity (Vertical Component\

3. Effects of Blasling on Tunnel Lining

3.1 Test Sites Field experiments have been conducted at two different sites having tunnels wit'l concrete linings .

One is a utility tunnel embedded beneath building bnd (coiled Site 1). Tl1e other is a bypnss tunnel at a dam construction site (called Site 2). Configurations of the blast holes. chorge wei&ht u well as the geometrical relation or both tunnels ue shown in ~1g. 7 (a) and {b). Tl1e types or ch•1ce were dynamite and AN·FO. T11e charges were fored from a farther point. so as not to damage the ground th rough which wave tr3velled. Rocks around the tunnel are granite 31 Site I and liparltc at Site :!. For both sites. there are no surface soil layers. The prop~gation velocities or the longitudin~l wJvc 31 Site I and 2 a1e in the range or 2- 3km/s and 2.5- 4.4km/s, respectively.

66

1301

E.L 120

;,.

160

140 (.)

120

100

80

60

40

20

~~ ~0

g 0 0

a.

0

: 0 . ,\ . 0 8 ~ as • • • ·f·. ·ro r

30

Dynamic Behaviour

()Tunnel A

~easurlng Point

'"' -~~=...;;;~~~--Blast

I . 8 0 0

0 8 0 °o •"§~. • : ~~' •• §fa:.·,,. • i A l~-4--o Blf: • ; ·I i'' .. r:a; ••

60 90 120 ISO 180 ((I)

Fig. 6 Orientation of Energy l'ropagation from Blast Zone

0 tO 20 30m

(a) Si t e <D

0 10 20m

(b) Site (2)

Charge We igh t lor Each Hole

Dynaml te : 0. 5 kg AN- FO : 2.0kg

Cha rg9 We ight fo r Each Hole Dynamite : 0. 1 kg A N-FO : 0.75kg

Fig. 7 Sccdon View of Test Sites

.....

Effects of Blastir1Q 67

3.2 Instrumentation The measuring devices used in these field works were, eiectromagrretic·lypc velvcily gages (Gccrspa,e:

natural frequency fo u 4.Sih), mini-accelerometers (Kyowa: natural frequency fo = iSOIIz, max. SOG) and SR-4 strain gages (O.L. = 67mm). Tirese measuring devices were mounted on the internal surface of the concrete lining, and the gage anays arc shown in Fig. 8. The vertical and horizontal components of p31ticlc velocity and acceleration were measured. The horizontal components were all in a direction perpendicu13r to the tunnel axis. For strain, circumferential components along the internal surface of the lining were obtained, and !he radial compcnents within the lining were measured by molded SR-4 strain &ages.

tunrt: rrJ lo} Site (j)

1YJ : Velocity Go ge {Verllco l Component)

!EJ : Velocity Gage IHorizontol Component)

® :S train Gcge

lbl Silo~

® : Accelerometer IVe rt ico I Comp onent I

® : Accelerome·ter (Horizontal Compontn tl

Fig. 8 Location of Musuring Points

3.3 Results and Discuuions (I) Seismogram of Particle Velocity and Propagation Equation

Some of the seismograms 'f particle. velocity measured at the ground sur face and on the internal surface of the lining, arc shown in Fig. 9. The data for the lining arc obtained at the measuring poinl facing the blast zone. It is obvious that particle l'eiocity on the &round surface is wongly influenced by the free bounduy of the ground. The data for both Site I and 2 :rc ploaccl In Fig. 10, to detennine propagarion equations relating th~ magnitude of peak particle velocity 10 distance. In t!rc;e Ogurcs the magnitudes of the initial peak and the maximum after the initial are ln~ludcd only in rile frgure showi ng the ground surface measurement.

The empirical propagation rquation may be written in the followin& gc11eral ft,rm,

v• CII' II, - • 121

where v is particle velocity, r Is distance between blast zone and measuring point, Ill is ch•rgc weighl, and C, ll, «, are all positive coefficients.

It is nolcd that the regre!slon lines drown In Fig. 10 are similar, so that !he decay exponent a can be assumed to be 'a.s 2 for both cases of the ground surface and the lining, I.e. the n:asnilude c:rl' p3rticlc velocity decreases with square of the disrance. (2) l'llrticle Velocity Distributions on Uning

Distributions of particle velocity on the Uning for Sites I and 2 ore shown in Fig. I I and 12. respccrlvcly. Fig. II is the result of the blasting operation at a distsnce of 24m from the lining. and f- ig. 12 is for I Sen. The~ figures are obtained by rlotting rhe pa.rricle velocity at each meanrring poilu. as a vecH>r, orisinMing from the undeformed lining axis. It is obvious thor the maximum particle velocity rrn the lining may occur at a point facing the blast zone or at hs crown.


Distance from Blast ~

19m ~~ .J\ /\.._,.... "'

Distance from Blast ~

l:i

~

.~ 14m a ]~

0 0.02soc 0 O.Ohtc ..__..

Lining (Horizontal Component} Lining (Vertical Component)

l 20m ]~

·!

18m ];,

10m

!! ~

)~

Ground Surface (Vertical Component) Ground Surface (Vertical Component) (aJ Site 'G) . (b) Site <6)

Fig. 9 Sel!lllograrru of Particle Veloclly

Lining Ground Surface Mtosur ing Point

\"' "·'1: ::::··: .~:~.,. .• , s.or., 0:1 • :2

t.D est No.2( • lnltio l Ptolc • : 3 4 Hox. otht Jftll lol .. I 0 : ' ... .: I : 5 .5 0

>< . • :6 >< .... . A ; 7 -

\ ,., t.D • :8 ,., -u Su F1t. lto1 u 0 0 .. .. > > 0.1 .. .. '\ u u • .. .. A 4 ~A 0 Cl

f .. 0. 0.1 0.

W:2.5kg W=O 5kg • 00

O.DSI

0

O.Dl .. I ' 0.02

20 50 100 200 5 10 50 100 Dis t ance lm) Olslonco (m)

(a ) Site CD

Fig. 10 (a) Regres;slon llnes of Vcrrlcal Component of l'lllllcle Velocily

I!

1~ '1 0

Effects of Blasting

Lining ' nl 0 ."leasurlng Po• tr,. o : 1

', • : 2 c; \ A; J C

c; 1• • : ' >< c \ 0 5 ->< \ · • : 5 ,., ,., \ e • : 78 "

\ • ' a tbt 0 o },\ $u F•<;. 0

0 \ > - • .. ru ~ ~\ u

\ ·-.. \ .. .!! ~ ~\ tt 0.05

' . \ "1 . I ··'·'"' W:0.8S kg f

0.0110 20 50 0.0110 20

Distance (ml Distance lm)

(b)Sitea>

Fig. 10 {b) Regression Unes of Vertical Componenr of Parliclc Velo:lty

(3) Rela tion between l'llrticle Velocity and Strain on Surface of Uning

69

Strain is not assoclated with rigid motion but panicle velocity cont~ios a component of rigid motion. The particle velocity essentially differs from the strain. Therefore, the maximum strain does nut necessarily occur at the same place as where the particle velocity becomes a maximum. Some of the seismograms of circumferential strain on the surfact of lining are shown in Fig. 13. The distributions of particle velocity and strain on the internal surface of the lining for Site 2 arc given in Fig. 14. · It Is obvious from the experimental rosults shown In Fig. 14 (a) that the radlnl coonponent of particle velocity give.s a maximum \'aluc at t = 90°, I.e. at the arch crown. On the other hand, the maximum value of circumferential strain is obtained u roughly 0 = 45° or 150°. It is noted from these retulls that the point giving the maximum panicle velocity docs not coincide with one civing 1he mlxlrnum nroln. This Is also obvious from the theoretical rcsulu shown In Fig. 14 (b). 11tese theoretical solurions Jre for the me of sinusoidal harmonic motion. But It has been verified that the difference between sinusoidal harmonic solution ond one for a single sinusoidal pulse becomes negligibly small if the wave length is moll' than rhree times larger than tunnel diamcter.ll

Experimental results for the rad~l component of pQrticle velocity give the maximum value at the crown (0 • 90"), but they differ from the theoretical solutions in which rhe maximum •alue uccurs at the point facing the blast :tone (0 = ss•). Titls discrepancy may arise as follows: a) 111ere usually exists an opening between the arch crown ard the surrounding medium so that the ratllal porridc velocity at the crown becomes extremely luge. b) There Is much loosening in the medium behind t~e arch crown.

70

-Undeformed ·····Sms -·-10ms -··- ISms

~ h.cldtnct

Dynamic Behaviour

- - Undetormed ···-20ms -·-2Sms

Flg. I I Particle Velocity Dist ributions in Concrete Uning (Si te (I))

r • i:' ,.

-2ms · ··S ms -·- Bms --toms

Flg. 12 Particle Velocity Distributions in Concrete Uning (Site (2))

t,

3--v' ]~ ~

4 )~

5~ ]~ ~

6 J~ 0....._,0.0\sec distance: 137m

Fig. 13 Seismogram of Circumferential Strain (Site (2) l

~ .

•' ,.

•••·· 12ms --ISms - 20ms

'

0 ., 0 a:

,g

~ . -e 0 , ·- ... ., ~

0-a:u

H

(~I PJOOOtfJ

(OUj)IJ ~1!00 \ 0A t\0\ P Od

a

~

Effects of Blasting

(01 PJOO OI!J

V!O.ItS tO! .UIJtiWn,JIJ

(Z I'll PJOOtlj I U!DJJS IDIIUt JI~wn~JI:l

:il 1

8~

~~ 'E~ ~~ !!0_ e e r-M --§' ..

"' ~ "! 0 0-;; i c5

. ~ ~ 6~~---~~~~~-----------~ 0 0

(0 1 pJOOIIj \

(OU!)CI ~1\00)011 t)OIIJDd

Ill c 0

::> 0 Vl

0 u -.. ~

0 .. .c. 1-

..0

6) ~ (/) -.. c <» E L .. 0. )(

UJ

:E

= ... eli ., j;

~

] ~ .. :g J! ._ 0

.~ ;; :!!

~ ... .0 u:

71


ee .. ..

Vl Vl

:.· ..

4

.. c :.:

... 9::

u 0

~ .. u

~

"' 0 Q.

E ::> E

" 0 ::E ~

-~~------~3~-------g~--------~~~-------~~------~·

U!DJIS WOWJXDW

00

0

OJ

8

0 ~ 0

0 0

U!OJIS

ee .. .. Vl Vl

:.,· 0

oo

t .,. "~. 0.,

0 <tl 4 4

8 1lo t '6.,

..

.. c ~

5! ... u 0 .. > .. u

11'1-

i

>. ·g ~ .5

" ..!!::J .g_ ~ " .. c c.. " E ~ .§ s ~ " ::o -e 5Vi ., E ~ " .8 .5 "~ o·~ .e. .!!.., .. " c::"'

"' ~

>.

j u .. >.;;;

~:3 .E-;; £a ~~ "' " ~ 0 .Sl" c: 'i! 0-., II)

.!!.., .. c :>:"' .,.,

~


Fig. IS shows the relation between particle veloclly and str.lin, both of which are obuined at the same time and at the same measuring point. From this figure, II Is clear that the strain cannot uniquely be determined by particle velocity because of their different distributions along the linlns. Plotting the maximum particle velocity against the maximum Strain, even though they occur at the different points, gives Fig. 16, which shows a linear relation between those two masnltudes. The ploce whe re the measure· ments are obtained is of fundarrental importance In discussing tilt relation between m ain and particle velocity.

The strain < is often estimated through particle velocity 11 by the following equation:

. .. ,,c 13)

where C is usually chosen to be constant. But, according to the theoretical solution, the constJnt depends on the geometrical relation of the tunnel, relative rigidity of lining ond medium, And frequency of vibration, as shown In the followln& form.

I Goki2 1Sir)l a• c " 2rr[k o1Ho1211ktrl l .£211•

where, S (r) • II J'i{k1.r )/K2 + JJ~\k1 r) ·I 1/K 2 - I) K a ,/2 (1-v)!( I -2•) • : Poisson's ratio of medium ko 2•ffVp

f Frequency Vp Propagation velocity of longitudinal wave k~ = K•k1

Go : Shear modulus of medium E2 : Young's modulus of lining o• : Concentration factor of stress 11 • : Concentration factor of particle velocity JJ~!J: Nth order Hankel function of 2nd kind r Distance from blast zone

(41

The linear relation between the maximum P.artlcle veloclly and the maximum strain, shown In Fig. 16, gives thJt the particle velocity for f:illure of the lining becomes approximately 35 kine (cm/s) on an assumption tlut tensile stress for fadure of concrete is 20kg/cm1 • In fact, it has been reported that crack initiation occurred on a concrete tunnel lining at the pnticle velocity of 33.8 kinc.2i (4) Radial Strain Within Unlng

Tite strain discussed In the ~revious section was that on the surface of lining. 'The nex t question is what about strain within the linin!. A result of the measurement is shown in Table I, which shows the magnitude of the initial peak when the wave arrives at the lining. These data ore those for the measuring point facing the blast zone.

Table I Strains within Lining

Tangential component on the surface Radio! component within lining

Distance from intern~Jm ~rface o.o• o.o•• 2.S 17.S 32.5 4S.S

Strain (x 1 o:'l 38.2 32.8 -17.0 - 9.4 -1.9 0.0 ----

• Tunnel axis direction (- : Compression, +: T: nsion) • • Circumferential direction


It is nottd from the result that the radial strains within the lining are ail compressive for the initial peak, and thdr magnitudes incrtase at measuring points near to thl Internal frte surface of the linlns. On the other han~. strains at tltc surfac-e of the lining arc tensile, the magnitudes of which are larger than those of compression within the lining. It is concluded, therefore, that the maximum tensile strain occurs at rhe surface.

4. Conc.lusions

Tite results obtained herein ore summariud as follows: (I) Even though the distances from the blast zone are the same, a large vibration may occur In an existing tunnel (Tunnel A), rather than in a newly constructed tunnel (TuMd B) where the blasting operation is conducted. Therefore, If the measurements obtained in Tunntl 8 are used for estimating the vibration of Tunnd A, care in estimation Is necessary, because it is not on the safe side.

(2) When the face of Tunnel B approaches Tunnel A, blast control is of much importance, so as not to damage Tunnel A, in comparison with when the face puses through or leaves the crossing point.

(3) The slopes cf regression lines for panicle velocity versus dlstanc: sre approximately the same for both measurements obtained on the surface of the concrete tunnel lining and on the ground surface.

(4) The ver1ical component of particle velocity at the arch crown of the lining gives the maximum value. This may be due to an openlng or loostnin& of medium bthind the arch crown. Apar1 from the arch crown, the particle velocity in any position ' facing the bbst zone is also remarkably high.

(5) 1ltc place where the maximum strain occurs is not necessarily the same as where the par1iclc velocity gives the ma ximum value.

(6) The maximuo strain seems to be propor1ional to the maximum particle velocity, and Its propor1ionaUty constant may be a function of the geometrical relation, relath·e rigidity of lining and ground medium, and frequency, even though they occur at different places on the lining.

(7) 1lte radial strain within the lining facina the blan zone is con:pressive at the initial peak and its magnitude is always small, compared with the tensile strain obtained at the surface of the lining.

5. Acknowledgments

The authors ~cknowledge Kobe and Takarazuka Municipal Offices for their financial support for this study. 1lte authors also wish to express their sincere tl13nks to Mr. K. Miyagawa, Mr. K. Yoshida and Mr. I. K.itoda for tnaiyzing the fie.id data. Spec ill! thanks are also due to Mr. S. Kasal for solving the analytical problems.

Rdercnces

(I) Y. Niwa, S. Kobayashi and T. Matsumoto; Transient Stresses Produced around Tunnels by Traveling Waves, Journal of the Sociery on Materi:ds Science, Japan Vol. 23, No. 248, 1974, pp. 361- 367.

(2) H. Okazawa, et al.; The Effect of Vibration in Nearby Tunnels Caused by Blasting and Its Preventive Measures - The Case of Koi·Tunnel on Sanyo·Shlnkansen, Tunnels and Underground, Vol. 6, No. 11, 1975, pp. 789 -797.

(3) S. Kasal; Theoretical Study on Dynamic Behaviors of Tunnel Uning, M.S. Thesis, Kobe University, 1974.

STUDY OF UNDERGROUND STRUCTURAL STASI LITY USING NEAR-SURFACE AND DOWN-HOLE MICROSEISMIC TECHNIQUES

H. Reginald Hardy, Jr. 1 and Gary L. Mowrey1

1 . Introduction

75

When stressed , most solids emit bursts of microlevel acoust i c energy. This phenomenon is commcnly termed microseismic ~ctivity, although synonymous terms such as acoustic e mission , rock noise, and seismo-ncoustic activity a rc often utilized. In geologic materials, relatively little is known in regard to the basic mechanisms responsible for microseisttic activity. Such activity, however, is known to be associated wi th mechanical instabili ty within the materiul; and wlth suitable instrumen tation , it is possible to locate the source of the instability and t o evalullte its intensity. Microseismic activity therefore, provides the reseGrch worker and the engineer with an indirect means of monitoring the internal stability of a field structure, and as s uch, forms the basis for one of t he most useful tools presently available in rock mechanics.

In recent yeQrs extensive use has been made of microscismic techniques for evaluating the stability of such geologic structures as mines, rock and soil slopes, tunnels, earth filled dams, and more recently underground storage facilities. A more detGiled discussion of the microseismic concept and details of a variety of applications is available in a number of re~.ent publications [ for example, Hardy (1971 , 1972, 1973, 1975) and Hardy and Leighton (1977) ),

}licroseismic activi:y at a field site is monitored by installing a suitable transducer, or usuall y a number of transducers (an array), in locations where they can detect any microseismic activity which may be

1chairman Ccomechanics Section, and Director, Rock Mechanics :.aboratory, Department of }Uneral Engineering, College of Earth lind Mine ral Sciences, The Pennsylvania State Universi ty, University Park, Pennsylvania, U.S.A.

2Research Assistant, Geomechanics Section, Department of Minc:31 Engineering, College of Earth and Hineral Sciences, The Pennsylvania State University, University Park , Pennsylvania, U.S .A.

International Symposium on Field Measurements in Rock Mechanics- Zurich, Apri/4·6. 1977

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