Blasting-induced vibration in tunnelling

0886-7798(94)E0007-6

Blasting-induced Vibration in Tunnelling Giorgio Berta

Abstract--This paper identifies the parameters that influence vibration generated by rock blasting, and specifically considers blast design relative to large-section tunnels. The paper discusses ways of modifying round blasting so as to reduce the initial vibration level. The second part of the paper describes some cases of tunnels excavated by explosives close to structures under particular types of hazards.

Introduction ock blasting is the rock excavation technique most widely Fadopted in the various branches of the mining and construction industry because it is economical, reliable, and safe. It is widely used in mining and quarrying, excavations, trenches, tunnels and large underground works. The job of the explosive---which is loaded in boreholes in the rock and blasted according to a prearranged sequence (round)--is to fracture, fragment and displace from its natural position a well-defined portion of the rock. A m o n g the secondary effectsof the explosion round, rock vibration at excavation contour generatedbythe shock wave following the explosion deserves special attention. This phenomenon is usually harmless: vibrationlastsa very short time (a few tens of milliseconds), after which the rock reverts to its initial conditions. However, the situation differs,and a few problems arise,in the presence of important structures likely to be particularly susceptible to dynamic

R

Present address: G. Berta, Italesplosivi, via Turati, 25, 20121 Milan, Italy. This article originally appeared in Gallerie, Nos. 39 and 40 (1993), and is reprinted herein with permission of the author and the SocietAItaliana Gallerie. Wearegrateful to Mr. A. Motta, editor of Gallerie, and to Mr. Berta for their help in acquiring and preparing the manuscript for publication in T&UST.

Tunnellingand UndergroundSpace Technology,VoL Elsevier Science Ltd Printed in Great Britain 0886-7798/94 $6.00 ÷ .00

RdsumL,--L'article d~crit les param~tres qui influcnt sur les vibrations gdn~r~espar les tits de mine, etplus particuli~ rement les plans de tir dans les tunnels de grande section. I1 examine les moyens permettant de modifier les cycles de tir de faqon r~duire le niveau initial de vibration et d~crit quelques cas de tunnels creusds l'explosif pr~s d'ouvrages sensibles.

stress (buildings,bridges, dsms, tunnels,etc.)because the vibration can be transmitted to t h e m through the ground. In such cases it is necessary to check, during blast design, whether the vibration-induced stressm a y compromise structure integrity; and, where danger might be expected, it is necessary to reconsider blast design in order to reduce the induced vibration to acceptable levels. In analyzing the behavior of structures subjected to dynamic stress and in blast design, a paper recently published on this subject (Piovano discusses two f u n d a m e n t a l phases t h a t facilitate the understanding and solution of the problems arising from blasting close to structures sensitive to vibration. The present paper is limited to blast design for large-sectiontunnels.

1992)

Part h Parameters Influencing Vibration Generated by Rock Blasting 1.1 Blasting-induced Vibration Danger from vibration isevaluated on the basis of its peak particlevelocity value related to the corresponding frequency; or, in particular cases, according to the m a x i m u m acceleration value. The vibration velocityderiving from an explosion is directlyproportional to the energy developed during the explosive reaction (and, therefore, to the amount of explosive used); and itis inversely proportional to the distance from the blasting point. The value is also influenced by the w a y the explosive is used and by the features

of the ground that lies between the blasting spot and the place where vibration is recorded. In short: (1

v=k'~

(1)

where v = vibration velocity Q = weight of the explosive charge R = distance k',c~~ are factors,depending on the type of round and ground It must be pointed out that Q does not refer to the totalquantity of explosivesblasted in a round, but only to the quantity primed, within one round, with detonators having the same delay number. In fact, in the ground the seismic wave lasts only a few tens of milliseconds, whereas the typical delay time of the detonators used in tunnels is 250 m s (or,m u c h more seldom, 100 ms). As a consequence, the vibrationsgenerated by the shotholesprimed by N delay time fade out before the occurrence of the vibrations generated by the blast of the shotholes primed by delay time N + 1 (see Fig. 1). The relationship(1) between vibrationvelocityand round parameters has a general validity. In order to study a particularsituation it is necessary to carry out a certain number of tests in the ground, measuring vibration velocity at different distances from blast point. In practice, the study is based on formula (1), modified as follows: v = k (__R_RR)-m

(2)

9, No. 2, pp. 175-187, 1994

(~

Pergamon

175

should probably be modified if the need arises to keep round-induced vibration within determined levels.

ftJ. ....

I Wr''' I'

''

I[Ir+

IrIIr ''--- IFr-'1

' ' '

'

I

" ' '

'

I

''~

'l

''

|

,11, 250 ms

I

I

~

I

I

.~

Figure 1. Recording of vibration induced by shots primed with 250-ms delay detonators. The values of vibration velocity (v) and the scaled distance (pdV/-Q) are respectively reported on the ordinates and on the abscissae ofa bilogarithmic diagram. The value of factor k and m (defining features of the shot and of the ground, respectively) is graphically obtained by the interpolation line (see Fig. 2). By expressing Q in (kg) and R in (m), v is defined in (ram/s).

1.2 Tunnelling In rock blasting, every shothole must be parallel to a free surface in order to work correctly. The distance between the shothole and the free surface, which is called burden, is calculated on the basis of the characteristics of both the shothole and the rock to be blasted. The rounds consist of a certain number ofshotholes blasted in a p r e d e t e r m i n e d s e q u e n c e : the

Vmm~ looo

. v=k

_

/ R ~+rn;

loo

lo

m,zg

~

ol

; lo

1oo R

-

lOOO m -

,5 -~+°t Figure 2. Interpolation line of the peak particle velocity values (v) with the scaled distance (RIv/Q).

shotholes next to the free surface are blasted first and thus create new free surfaces, which are exploited by the shotholes blasted subsequently. The initial free surface usually coincides with excavation face. In the specific case of tunnels, because the excavation face is the only surface where the holes can be drilled, it cannot function as a free surface. It is therefore necessary to have recourse to an alternative free surface, obtained at the beginning of the round by blasting a g r o u p of s h o t h o l e s (V-cut shotholes). If the narrow space available permits, these holes are drilled with a 60 ° inclination with respect to excavation face. Because of the lack of an adequate free surface, the V-cut shots generate more intense vibrations than the other shots, given the same charge; therefore, if vibration is a problem, they require especially careful study. The other shotholes (production shotholes), blasted in a regular sequence, enlarge the initial opening almost up to design dimensions. In the V-cut and the production shotholes, charge diameter tends to coincide with hole diameter. The shotholes determining the walls and the top of the tunnel (contour holes) are blasted last. Their dimensions depend on the need to avoid excessive overbreak and damage to the contour rock. In particular: • The spacing is smaller than that of the other round shotholes. • The hole diameter to charge diameter ratio is substantially greater than 1; often it is ~bf= 2 ¢c" • The charges consist of connectable rigid plastic tubes provided with spiders which facilitate centering in the hole. • They are primed simultaneously, if possible. The round pattern and priming sequence typical in the excavation of a large-section tunnel are shown in Figure 3. A pattern of this type, which is quite valid for an efficient blasting,

1 7 6 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY

1.3 Round Features In large-section tunnels, the holes are drilled by jumbos with two or more booms, and their diameter is nearly always 51 mm. Hole length--and, therefore, round length (advance)--generally ranges from 4 m to 5 m. These lengths are sometimes incompatible with the particular field situation (e.g., poor rock type, small rock covering), and advances consequently have to be reduced. The number of holes substantially depends on face dimension and rock characteristics. The face may range from 75 m 2to 100 m2; production shots spacing and burden, from 0.80 to 1.20 m; and contour shots spacing, from 0.40 to 0.80 m. Consequently, 90 to 130 shots per round are needed. The excavation face generally coincides with tunnel section (full-section excavation). In any case, it is possible to carry out excavation in partial sections, i.e.: • Top heading followed by benching (see Fig. 4). • Pilot tunnel followed by full section enlargement (see Fig. 5). The adoption of the particular method selected has a considerable influence on field organization because the times for the two different phases (top heading and benching; pilot tunnel and enlargement) must be quite distinct. In general, no special problems are associated with the planning of the round. In fact: • Blasting of the top heading does not greatly differ from that of full section, except for the reduced dimensions of the face. Consequently, the round simply requires fewer shotholes. • The blast design for the benching is facilitated by the existence of the free surface created by the top heading. • The round pattern for pilot tunnel enlargement largely coincides with the pattern for full-section excavation, except that V-cut shots are not necessary in this case. The explosives adopted must be approved for underground use, and it is advisable that they be water-resistant and have a high strength. Excellent explosives may be chosen from among gels, slurries and emulsions. The diameter of the explosives (based on the hole disameter, which is nearly always 51 mm) is usually 40 nun for V-cut and production holes, and 25 mm for contour holes. The blasting sequence is generally

Volume 9, N u m b e r 2,

1994

obtained by electric detonators with 250 ms delay (or,lessfrequently and in special cases,with non-electricdetonators with 100 ms delays). Electricdetonators provide 22 delays numbers (0 to 18 and 20-22-24). Non-El detonators provide 24 delays numbers. The detonator is always inserted in the bottom charge.

5 @

9

1.4 Procedures for Reducing Vibration

@5

The goal of blast design is to attain the expected technical target (advance and good contour) at an economical cost. In practice,optimization ofthe costs of the following items is desired: 5e e4

• Explosives and detonators. • Drilling. • Loading and h a u l a g e of the blasted rock.

5e

o4

e5 C"

•¢

e3

04

e5

03

e2

e4 03 02 ,1 eO ,=4 e3 02

t2"' 6

•3

~2 1

e2

I

el eO

e~

• 4 .3 .2

e6

•~

e4

.3

e2

e7

e6

e5

04

e3

3

~3

~ eO

I i2 I

e3

e2

3e

e4 e3

~Se

•

40

Oo 10 20 30 40

5o

Oe le 2e 3e 40

5e

Oo 1e 2e 3e 4e

5e

6,

9~

3e

40

5e

60

7e

4~

50

6e

7e

8e

structures close to the tunnel course dictates a reduction of the stresses transmitted to the environment, it is necessary to reconsider the round--in particular, by modifying those parameters (shown in relationship 2) which influence vibration velocity values. It is advisable in such cases to analyze carefully all of the changes to be made to the initial round pattern, gradually determining the possible benefits and the expense that the changes imply. Figure 3 represents a typical round pattern for the excavation of a largesection tunnel, with regard to:

I

2

I

0

0

1

3

12.5 m

Figure 3. Drilling pattern and priming sequence of a typical full-section tunnel round. The detail evidences the decoupling between the hole and charge in contour shots.

J

-Excavation section - N u m b e r of holes -Advance -Explosive quantity - N u m b e r of detonators - D e l a y detonator - N u m b e r of delays

90 m s 118 4m 445 kg 118 250 ms 10 (0-9)

Table 1 shows, for each N delay time, the number of n shotholes and

~1~~ i1~ I~ ~ .~'~I~

Figure 4. Top heading followed by benching.

Volume 9, Number 2, 1994

Time.

The vibration problem is not taken into consideration. However, when the presence of

Figure g. Pilot tunnel boring followed by full section enlargement. TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY177

Table 1. Number of shot holes and quantity of explosive primed by detonators of the same delay number, for each delay time.

Type of Shots

A V cut shots

Delay Time (N)

No. of Shotholes (n)

Quantity of Explosive

0

6

30 kg

6 12 16 19 21 6 4 2

30 kg 48 kg 64 kg 76 kg 84 kg 24 kg 16 kg 8 kg

26

65 kg

118

445 kg

B Production shots

C Contour shots

9

Totals

the Q q u a n t i t y of explosive p r i m e d by d e t o n a t o r s of the s a m e d e l a y number. Each V-cut hole (A) is l o a d e d with 5 k g of explosive (~ = 40 ram); each production hole (B), w i t h 4 k g (¢ = 40 ram); a n d each contour hole (C), w i t h 2.5 k g (~ = 25 ram). W i t h reference to r e l a t i o n s h i p (2), the s i t u a t i o n a s s u m e d t a k e s into cons i d e r a t i o n a type of g r o u n d characterized b y t h e factor m = 1.5. The factor indicative ofshothole feat u r e s (different for each t y p e of shot) is a s s u m e d to be: • k^ = 250 for V-cut shots. • k s = 200 for production shots. • k c = 150 for contour shots. The k factor is g r e a t e r for V-cut shots t h a n for production shots because of the lack of a n a d e q u a t e free surface; t h e k factor for t h e contour shots is smaller, as a r e s u l t of t h e considerable decoupling b e t w e e n hole and charge diameter. Let us also consider b l a s t i n g a r o u n d a t a d i s t a n c e 50 m from a b u i l d i n g (R = 50 m). I t is possible to m a k e a prelimin a r y e v a l u a t i o n of the s t r e s s e s induced in the b u i l d i n g b y the v a r i o u s groups of shots, calculating on t h e basis of relationship (2) the m a x i m u m values t h a t can be e n v i s a g e d for v i b r a t i o n velocity. Table 1 shows that: • F o r t h e V-cut shots: Q=~ = 3 0 k g • For t h e production shots: Q=~ = 84 kg (delay n. 5) • F o r t h e contour shots: Q~ =65kg S u b s t i t u t i n g the respective values for the symbols k, m, Q a n d R in relationship (2), the r e s u l t s are:

• F o r t h e V-cut shots: ( 50 y15 Vmax= 250 ( ~ } = 9.1mn4s • F o r t h e production shots: Vm~ = 2 0 0 ( ~ 50 ~-1.5 ) = 15.7ram/s • For t h e contour shots: Vm~ = 150

= 9.7 mr~s

Therefore, i t can be p r e d i c t e d t h a t a r o u n d t h u s p l a n n e d m a y induce vibration o f v e l o c i t y u p to v = 15.7 mm/s. For the t i m e being, l e t us n o t discuss w h e t h e r t h i s value is acceptable or not, b u t i n s t e a d consider all t h e w a y s by which i t can be decreased.

1. D e c r e a s e t h e Qm~ r e l e v a n t to t h e p r o d u c t i o n shots, r a t i o n a l i z ing and balancing the distribution of the delays. There are 86 shotholes a n d 8 delay times (1 to 8). Eight groups, including 10 or 11 holes each, c a n b e formed, with a Qm~ = 11 x 4 = 44 kg of explosive. The pea]¢ particle velocity t h a t can be predicted is: ( 50 ~-,5 Vm~ = 200 t~444) = 9.7mm/s

Relevant benefit: t h e r o u n d v=. x is d e c r e a s e d by 38% (from 15.7 to 9.7 ram/s). Charge for the field: Need for g r e a t e r care w h e n placing the detonators. 2. Again, a d j u s t t h e Qmaffio f a l l the shots, using a greater number of delays. Also, change k of the V cut shots by decreasing t h e i r burden. This is implem e n t e d by drilling b a b y - c u t shots (see Fig. 6). It can therefore be a s s u m e d t h a t the V-cut shots factor k s u b s t a n tially corresponds to t h a t of t h e production shots (k A = 200). Each of the 6 baby-cut shots is loaded w i t h 1.5 k g of explosive. Using all the 22 delay n u m b e r s available, the p r i m i n g p a t t e r n can be arr a n g e d as i n d i c a t e d in Table 2. The r e s u l t i n g p e a k particle velocity is:

( 50 1-I'5

Vm =2001~ ] =6.1mIn/s Relevant benefit: v=.~ decreases by 61% with r e s p e c t to the i n i t i a l value (from 15.7 to 6.1 minis). Charge for the field: The drilling of the 6 baby-cut holes a n d i n c r e a s e in consumption by 6 electric d e t o n a t o r s a n d 9 kg of explosive m e a n s m a n a g i n g a g r e a t e r n u m b e r of delays. production shots

~

V cut shots

baby cutshots

Figure 6. Pattern of baby cut shots, V cut shots and production shots.

178 TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY

Volume 9, N u m b e r 2, 1994

Table 2. Priming pattern, using all of the 22 delay numbers available. Delay Time (N)

No. of Shotholes (n)

Maximum Quantity of Explosive (Q max)

0

6

9 kg

A V cut shots

1-3

2

10 kg

B Production shots

4-18

5-6

24 kg

20-22-24

8-9

22.5 kg

Type of Shots A' Baby cut shots

C Contour shots

3. T h e r e a r e t w o w a y s t o furt h e r d e c r e a s e t h e Q~a: • Decrease the advance, continuing to m a k e full-section rounds. • W o r k with partial sections of the excavation face,blasting two or more rounds in a sequence, each with a smaller number of shots, while keeping the advance constant. F r o m an operational viewpoint, itis preferable to reduce the advance and keep the round section constant. However, in particularly difficultcases and afterhaving reduced to a m i n i m u m the advance, itm a y be necessary (and convenient) to work with partial excavation sections and with rounds with a number of shots equal to or less than the number of delays available. By operating in this manner, it will be possible to prime each shot with a different delay number. B y reducing the advance from 4 to 2 m, the Q _ ~ of the round decreases to 10.5 kg ofexplosive, based on a charge of 1.75 kg for the production shots. Moreover, thanks to the substantialreduction in the advance, a smaller shothole factor can be assumed, e.g., k = 150. The following peak particle velocity then results:

A considerable reduction in the k factor can be assumed by further reducingthe advance (k = 100). Calculation of the expected vm~ gives: f 50 ~-1.5 Vmu = 150 - = 2.5 mm/s

Relevant benefit: v . reduced by 95% with respect to im~al conditions (from 15.7 to 0.7 ram/s). Charge for the field: The same as evidenced in no. 3, with an increase in the waiting times. 1.5 Remarks To fully appreciate the efficiency of the proposed changes, it may be interesting to determine the compatible distance of a building from a round such as that outlined in Figure 3, and from a round planned according to the criteria expressed in no. 4, above. Assuming v = 10 mm/s as the acceptable peak particle velocity value for a building, by applying relationship (2),the value of the distance R is:

For the round shown in Figure 3 (Qm~ = 84 kg and k = 200): __1_

[ 50 ~-l.s = 0.7 mm/s

Vmax = 150 [---~-~)

Relevant benefit: v

reduced by 8 4 % with respect to ini~t~alconditions (from 15.7 to 2.5 ram/s). Charge for the field: Need for a greater number of detonators, compensated by a smaller specific explosive consumption; a greater incidence of waiting times resulting from the reduction in the advance. 4. T h e n e e d t o k e e p t h e a d v a n c e reduced may go as far as boring h o l e s c o m p a t i b l e w i t h a s i n g l e cart r i d g e (i.e., 40 mm in diameter, 400 m m l o n g and weight ranging from 0.58 kg for slurries and emulsions to 0.730 kg for gels). Operations can be carried on with round advance reduced to I m, with Qm~ approx. 3.5 kg (production shots).

V o l u m e 9, N u m b e r 2, 1994

For the modified round (Qm~ = 3.5 kg and k = 100): __!_

It can therefore be stated that when a round is to be blasted, the safe distance of a structure can be considerably reduced by an appropriate blast design without compromising safety. Although the measures explained above sometimes entail a certain increase in costs, the cost of rock excavation by blasting is usually much lower, even in situations involving considerable risk, than the cost of excavation by different means. In actual practice, theoretical validity of the above calculation must be

constantly checked and the round-induced vibration controlled by means of the appropriate instruments. To explain such procedures and document the possibility of operating safely, even in very difficult conditions, Part II of this paper describes some examples of tunnels that have been excavated with explosives, where the tunnels are located near structures at particularrisk.

Part I1: Vibration Induced by Rounds Blasted During Tunnel Driving The remainder of this paper deals with vibration induced by rounds blasted during tunnel driving. It concludes with some examples of cases considered both significant and interesting, in terms of the need to safeguard the soundness of, respectively: • A residential area. • A bridge and a railway tuimel. • A rock face under unstable conditions. • Ground previously treated with resin and cement. • A natural cave.

2.1 Vibration Control Usually vibration control measures are taken when either"constructions ~ (buildings, bridges, tunnels, towers, dams, etc.) or natural structures that are particularly sensitive (sliding slopes, unstable rocks, pseudo cohesive grounds, springs, etc.) are located close to the front of the excavation. Because the fields of constructions and natural structures are very different, the respective problems associated with both are discussed below.

2.1.1. Common constructions The term "construction" refers to man-made structures, the parts, connections and material features ofwhich are known, and which are connected to the ground where blasting occurs. To analyze their behavior under seismic conditions, it is necessary to refer to construction theory and to structural engineering. The parameters usually adopted to define safety condi-

TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY1 7 9

tions are vibration velocity and frequency, as measured by triaxialgeophones. Various countries have adopted fairly concordant, but different,regulations, which are compulsory in the respective states and represent a reference for those countries where there isno specificregulation. In Italy,reference is usually given to D I N Standard 4150 (1986). 2.1.2 Natural structures The natural structures whose behavior it is necessary to study in relation to the dynamic stress induced by blastingm a y be eithersuperficial(sliding slopes,rock masses detached along a slope,etc.)or underground (aquifers where disaggregation is feared, caves in which stalactitesor stalagndtes are present, etc.). The reference discipline for these studies is geotechnology. The parameters usually adopted to evaluate the acceptability of blastinduced vibration are: • Vibration velocity (measured by triaxialgeophones), when disaggregation of pseudo-cohesive ground is possible. • Vibration acceleration (which is measured by accelerometers, either singleor triaxial,depending on the needs), in the case of unstable masses of rocks. Because the problems that m a y be encountered are varied and complex, every situationmust be evaluated carefully to obtain a reliable definitionof the acceptabilitylimits. There are no rules facilitatingsolutions,but only bibliographicaldata that m a y be consulted. 2.2 E x a m p l e s

The cases discussed in bibliographies usually referto constructions,in particular to residential buildings. Thus far,explosivesfirms have mostly dealt with the study of vibration in residences--both because such buildings are frequently encountered above or next to tunnels being driven, and because reservations and complaints submitted by building dwellers, sometimes even before the works begin, need to be promptly answered. However, some lessfrequent occurrences-e.g., constructions other than buildings and natural structures---arealso discussed in this paper. The decision toinclude such structureswas dictated both by the interestinherent in these problems and by the need to find solutionsboth simple and reliable,although inevitably approximate. The instruments used in nearly all of the vibration recordings mentioned in this paper are of the type Instantel D S 477. This unit was expressly de-

"51.5

B

•

112.o

+ 4o.o

]-

•

160.0

-I

] \

+0.0

; 60.0

.162.5

D Figure 7. Location o f advancement face with respect to the areas where vibration is recorded.

signed to measure and record vibrationinduced by blasting. For the single case in which different instruments were used, they are specificallymentioned.The recordingsare always made along three axes perpendicular to one another. 2.2.1 Vibration control in buildings in a residential area In the largest number of cases, vibremetric control is made in buildin_gs. It is, in fact, in residential areas that recording needs to be continuous, and the data constantly interpreted. Furthermore, there must be coordination between the tunnel face and the ever changing situations occurring outside the tunnel. Because of the complexity ofgronnd structure, it often is nearly impossible to define a law of the type v-- k ( ~ - having general validity. In this case, vibration should be controlled systematically, by measuring its velocity in the structures that are gradually subjected to a risk as excavation proceeds. In particular, the vibration recording of the entire round must display separately the effects of every group of shotholes primed with detonators with the same delay number, thereby allowing the group ofshotholes causing the greatest stress to be detected. It will thus be possible to redetermine the balance of the round and prevent dangerous situations. One example is the case of a road tunnel being driven underneath a residential area, at very close distance. The tunnel is being driven full-section (85 m 2) in rock consisting of schists. The initial round pattern envisaged the following parameters: • Number of holes: 120-130 • Opening with V-cut holes • Advancements: 3.00 m • Explosive used (Tutagex 810 + Emuldin A + Profil X): 350 kg

180TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY

• Priming with electricdetonators (250 m s delay) availablein numbers from 0 to 24. The results given below are relative to a typical situation, as shown in Figure 7, along with the measures taken to keep seismic stress within an acceptable limit. As blasting proceeded, round advancements were planned on the basis of the values ofv recorded in the houses closer to the tunnel face (see Table 3). Reference is made to the same lhnit values set forth by DIN Standard 4150, relating to buildings under norms] conditions (v < 5 mm/s for f< 10 Hz; v = 515 mm/s for f = 10-50 Hz; v = 15-20 mm/s forf= 50-100 Hz; v = 20 mm/s for f > 100 Hz). R o u n d n + 000.0: The check in building A records a v = 5.4 mm/s against a limit ofv = 20 minis. R o u n d n + 49.0: As distance decreases, velocity in A increases (v = 7.7 minis). According to the previous recording, the correlation among v, Q and R is determined as: v = 40 D.S. °s° Even when distance is greater with respect to A, the velocity measured in B is much higher (v = 12.2 ram/s). This value is anomalous with respect to the law studied for A, and it is due to the bedding continuity linking advancement face and building foundations (see Fig. 8). In order to avoid coming too close to the limit value, it was decided to reduce round advancement by adopting Q = 25 kginstead ofQ = 30 kg. R o u n d n + 51.5: Velocity decreases in A (from 7.7 to 6.7 ram/s), whereas it further increases in B. Because the values in the V-cut holes were much higher than the values in the other shotholes (see Fig. 9), it was decided to bore baby cuts. R o u n d n+ 60.0: The usefulness of the baby cuts is checked. Now all of the vibration recordings are balanced (see Fig. 10). A decrease of velocity in B is obtained. The valuesin A always agree with a v = 40D.S. -°'8°

Volume 9, Number 2, 1994

Table 3. Values o f recorded in houses closer to the tunnel face.

PosUtlon of the Rounds (m)

Buildings

Q (kg)

R (m)

n + 000.0

A

30

67

12.2

>100

5.4

5.4

n + 49.0

A

30

43

7.8

>100

7.7

7.7

n + 49.0

B

30

48

8.8

>100

12.2

7.0

n + 51.5

A

25

42

8.4

>100

6.7

7.3

n + 51.5

B

25

45

9.0

>100

13.5

6.9

n + 60.0

A

25

44

8.8

>100

6.4

7.0

n + 60.0

B

25

43

8.6

>100

9.1

7.1

n + 62.5

B

25

43

8.6

>100

10.0

7.1

n + 112.0

B

25

60

12.0

>100

4.5

5.5

n + 160.0

C

30

70

12.8

>100

5.2

5.2

n + 160.0

D

30

90

16.4

>100

4.3

4.3

n + 162.5

C

3O

70

12.8

>100

5.3

5.2

n + 162.5

D

30

90

16.4

>100

4.4

4.3

O.S.

v* = expected value (calculated by v = 40 D.S. u °

R o u n d n + 62.5: Velocityin B is still within acceptable levels (v = 10.0 ram/s). R o u n d n + 112.0: A s the tunnel face moves away, there is no longer bedding continuity with B, and velocity now agrees with v = 40 D.S. ~.s° R o u n d n + 160.0: Vibration velocity is checked in two more buildings (C and D). The resulting values are quite acceptable and in agreement with v = 40 D.S. ~'8° R o u n d n + 162.5: Results of the previous round are confirmed.

2.2.2 Vibrometric control of a railway bridge and tunnel The course of a road tunnel being driven was to approach some structures of a railway line, namely an arch bridge in reinforced concrete (see Fig. 11) and a concrete-lined tunnel (Fig. 12), located at a minimum distance of 70 m and 60 m, respectively, from the tunnel. The ground consisted of alternating marl and limestone, in layers of variable thickness and inclinations. The problem arose when the round pattern necessary to drive the tunnel had to be defined, because the exist-

Volume 9, Number 2, 1994

(m//kg )

f(Hz)

v

(mm's)

v" (mm/,)

v ffi actual v a l u e

\

Figure 8. Location o f the tunnel being driven with respect to the houses outside.

ence of the railway bridge and t-nnel would entail limitations in the use of explosive. The problem received the attention of the Railway Organization, ANAS and the contractor, all of whom agreed that in the liningofthe railway tunnel,

vibration velocitywas not to exceed the following limits: • 30 ram/s, for frequencies ~ 60 Hz; • 40 rnm/s, for frequencies > 60 Hz. The same limits were adopted for the bridge as well. The measure was

TUN~LLINGANDUNDERGROUNDSPACETECHNOLOGY181

kk.

Jlk

i Figure 9. The vibrograph shows values much higher for the V cut holes.

d i c t a t e d by previous works of t h e Railw a y s (Dilena a n d Kajon 1986) a n d b y the Swiss regulation S N 640 312 (1978). The l a t t e r r e g u l a t i o n sets forth, for t u n n e l s a n d bridges w h e n v i b r a t i o n is g e n e r a t e d by blasting, a limit of: • 30 mnds for 10-60 Hz frequencies; • 3 0 - 4 0 mm/s for 60-90 Hz frequencies. Of g r e a t e s t concern was the r a i l w a y t u n n e l , which would be more exposed t h a n t h e bridge to the s t r e s s i n d u c e d b y t h e rounds b l a s t e d in the neighboring r o a d tunnel. However, the l i m i t

Figure 10. When baby cuts are drilled (first trace), the V cut shots (second trace) generate vibration equal to that from the other shotholes.

a s s u m e d was j u d g e d to be quite safe, as it was k n o w n t h a t micro-fractures begin to a p p e a r in concrete a t a velocity of a p p r o x i m a t e l y 5 0 0 m n d s (Oriard 1980). E v e n a r o u g h calculation can demo n s t r a t e t h e full r e l i a b i l i t y of the l i m i t s a g r e e d upon. In fact, t u n n e l l i n i n g should be considered a s a r i n g subj e c t e d to stress p e r p e n d i c u l a r to i t s axis, a n d therefore deformed. A t t h e m a x i m u m deformation ~ in d i a m e t e r , ~ corresponds the m a x i m u m strain o=

w h e r e E = elasticity m o d u l u s in concrete. The m a x i m u m v i b r a t i o n velocity acceptable m a y t h e n be calculated according to the following procedure. (*) By the above formula, calculate the m a x i m u m d e f o r m a t i o n acceptable on the b a s i s of a t u n n e l d i a m e t e r ~p= 8 m a n d of a concrete w i t h k n o w n resistance to compression ~c = 200 kg/cm 2 and elasticity m o d u l u s E = 2.5 105 kg/ cm2: 4-r~ 0/2 11= . . . . Oc 2 E

2E I] ( 4 - n)- ¢/2 =

4 - ~ 4000 • 200 2 2.5.105

-

-

-

-

--~

1.373 ram

(*) B y v = 2 u - r I" f (f being t h e v i b r a t i o n frequency, for which i t is wise to a s s u m e a very low value a t t h e b e g i n n i n g of work), calculate t h e maxim u m vibration velocity compatible w i t h the l i m i t I] _< 1.373 ram: v < 2 u ' 1.373 • 20 < 172 minis.

®

Figure 11. Location of the tunnel being driven with respect to the railway bridge.

Figure 12. Location of the tunnel being driven with respect to the nearby railway tunnel.

182 TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY

It should be noted t h a t t h e a s s u m e d value f = 20 Hz is c o n s i d e r a b l y s m a l l e r t h a n t h a t m e a s u r e d d u r i n g tests in tunnel, w h e r e f was a l w a y s > 70 Hz. Therefore, even a t t h e b e g i n n i n g of excavation i t was possible to rely on a considerable safety m a r g i n , o b t a i n e d by the difference b e t w e e n t h e calcul a t e d v a l u e (172 ram/s) a n d t h e l i m i t value a s s u m e d (30-40 mnds). D u r i n g operations, the a m o u n t of v i b r a t i o n was a l w a y s v e r y low. A m o n g t h e r e s u l t s obtained b y t h e n u m e r o u s vibrometric recordings, some a p p e a r p a r t i c u l a r l y i n t e r e s t i n g - - f o r example, the different b e h a v i o u r of t h e two struct u r e s (bridge a n d r a i l w a y tunnel) w h e n subjected to stress from two subsequent rounds, the second of which envisaged a g r e a t e r a d v a n c e t h a n t h e first (see Table 4). Both s t r u c t u r e s were located 85 m from t u n n e l face. The r e s u l t s of the two s u b s e q u e n t vibrometric recordings a r e shown in Table 5. The geophones were placed: • Into a recess o b t a i n e d i n the tunnel lining (see Fig. 12). • A t the top of one of the bridge arches (see Fig. 11). The two s t r u c t u r e s v i b r a t e in differe n t ways: • In t h e tunnel, t h e recording of the v i b r a t i o n i n d u c e d by t h e dif-

Volume 9, N u m b e r 2, 1994

Table 4. Comparison of two subsequent rounds in tunnel driving near a bridge and a railway tunnel.

Face section

Rrst Round

Second Round

100 sq. meters

100 sq. meters

Number of holes

118

118

Length of round

2.40 m

3.60 m

Explosive (Gelatina 2 +

260 kg

400 kg

Electric Detonators

1 -15

1 -15

Max. weight per delay (Q)

25 kg

35 kg

W h e n vibrometric control was started, it was noticed that when blasting occurred, already detached minor rocks slid along the discontinuity surfaces with an average inclination of about 40 °. Moreover, it was noted that the relevant vibration velocity values were always greater than 60 minis. On the basis of this experimental data, and after noting that the inclination of the naturally detached surfaces was never greater than 40 °, it was decided to change the round design so that the vibration velocity value resulting at the face of the slope would never be greater than 30-40 muds. When data recordings were started, the round pattern was as follows: - Face section: m 280 - Number of holes: 100-110 - Advancement: 3.2 m -Explosive: Tutagex 210 + Profil X - Explosive quantity: 240(Tutagex)+ 35 (Profil X) = 275 kg - Rock blasted in-situ: 256 m ~ - No. of detonators (250 ms): 100-110 - D e l a y number: 9 (from 0 to 8) - Maximum quantity of explosive primed with the same delay number (Off: 44 kg

Tutagex 210 + Profil x)

Table 5. Results of the two subsequent vibrometric recordings.

First Round

Recording of

Units

Max. vibr. velocity

Second Round

Tunnel

Bridge

Tunnel

Bridge

(mm/s)

4.0

1.1

5.2

1.6

Max. acceleration

(g)

0.25

0.07

0.32

0.06

Frequency

(Hz)

85

15

75

15

ferent groups ofshotholes shows a clear separation in time (see Fig. 13). On the bridge, the first stresses generate a resonance where the vibration frequency is the same as the bridge's own frequency. The vibration persists without interruption for the complete duration of round blasting (about 4 sec.) and later fades away very slowly (see Fig. 14).

2.2.3 Vibrometric control for an unstable rock face ~ J s case involved the driving of a close-to-surface tunnel designed to replace the original (surface) road, which was excessively narrow for presentday traffic and dangerously exposed to rock fall. The excavation was carried out full-face in bedded limestone (see Fig. 15). Because the face was approaching the outside surface, it was feared that the narrow distance would cause the seismic stress deriving from the rounds to disturb the already compromised static balance of the rock face overhanging the outside road, which was still being used for traffic. A study was therefore started, substantially based on vibrometric recordings, aimed at defining the best method to follow in order to ensure safety conditions suffi-

Volume 9, Number 2, 1994

With this type of round, at a distance R = 28 m from the face, the ms~dmum vibration velocity recorded on the outside face was v = 66 mm/s. The round was modified as follows: • Advancement was reduced from 3.2 m to 2.2 m. * Delay time was increased from number 9 to 24. • Abandonment of instantaneous detonators and primiug of the V cut shots by microdelay detonators (30 ms), with different delay numbers.

cient to carry out the most difficult phase of the excavation work. The rock face, shaped and eroded by the weather, presented such a high degree of instability that safety nets and boulder guard frames for the protection of the road beneath had been installed long before. Given this situation, it was fairly difficult to state what kinds of operating conditions would satisfy the requirement for sufficient safety.

I

÷ if.-

,.

.;;k:

.,'~'-. ,~~-."~='

~|.

~

~ ..,,_

,.~.

,dr ~

Ji . L

IpI •

,..: . . . .

I~111"" l~M,h,11p~-'-

;.

,]

. ld . . . . .

.,t.

il z.m

.....

L.

....

2,~0

Figure 13 (top). Graph of the vibration recorded in the lining of the railway tunnel. Figure 14 (bottom). Graph of the vibration recorded on the railway bridge.

TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY183

3. Immediately after the round, and after taking the appropriate safety measures, the tunnel was checked to determine whether the gas had invaded the tunnel. 4. After a certain number of trial rounds, after it had been ascertained that no exit of gas had occurred, it was agreed that the seismic levels adopted were acceptable, as the rock structure had not been compromised. 5. Excavation was continued and was completed by constantly checking the round-induced vibration and the possible exit of gas. Excavation was carried out by dividing the face in sections blasted in the following sequence (see Fig. 16):

_

_

I

\

I k

Excavation of the top heading: -section 45 m 2 --advance 1m -hole diameter 38 m m -explosive 45 kg of Gelatine 2 in 25-mm-dia.

cartridges -number of holes 100-110 -priming el. det. with 30-ms and 500-ms delays

Figure 15. Location of the tunnel with respect to the outside face. Consequently, at a distance R = 26 m and with Q reduced from 44 kg to 8 kg, the vibration velocity recorded was 18 m m / s - - a value which did not produce any rock fall on the outside face. Table 6 gives the data for the two rounds. These data, together with findings f r o m the two subsequent rounds, allowed the attribution of the values to the p a r a m e t e r s of relationship (2), considered in P a r t I of this paper, in formula: ++

This relationshipwas subsequently used to calculate,aftereach round (as a function of the R distance of the face from the outside slope),the m A x l m u m value of Q compatible with a value ofv on the outside face,kept within the 40 mm/s limit.

2.2.4 Vibrometric control in rock, previously waterproofed with resin, subject to toxic gas under pressure In order to drive two twin tunnels through a series of grounds with

Table 6. Data for the two blasting rounds for an unstable rock face.

microfractures and in the presence of toxic gas (H2S) under pressure, the rock was treated with injections of mortar of cement and acrylic resin, in the following sequence (see Fig. 16): 1. Injection o f c e m e n t i n Zone 1 in order to fill in the rock breakages. 2. Injections of resin in Zone 2 in order to seal the microfractures. Before the blasting was to begin, the problem arose of evaluating the possibility t h a t the round-induced stress might disaggregate the rock (especially the resin-treated rock); and, as a consequence, that the gas, no longer blocked, might invade the tunnel. In order to avoid this possibility, the following procedure was adopted: 1. Bibliographic research permitted determination of the range of vibration velocity values that might produce the disaggregation of semi-cohesive grounds or originate micro-fractures in the rock (see Table 7). 2. At the m o m e n t ofround blasting, vibrometric recordings were taken, by positioning a few geophones in the treated rock at the tunnel contour inside probe holes (see Fig. 16).

i:

r -

....

_ ....

~/,~ 'N.," . . . .. . . ... . . ... . . ~ ','_~:

,

~

~:",

',~I, .,__~

~ ,i

Figure 16. Location of the advancement face with respect to the cement and resin-treated areas. 1[: Top heading. II: Benching. III: Side walls. 1 and 3: Cement-treated zone. 2: Resin-treated zone. GC: GR:

Geophones in cement-treated rock. Geophones in resin-treated rock.

Table 7. Range of vibration velocity values that might produce the disaggregation of semi-cohesive grounds or originate microfractures in the rock.

Data for:

Standard Round

Modified Round

Q (kg)

44

8

80 - 120

Disaggregation of cemented sands (Galati~ 1975)

R (m)

28

26

150 - 450

Disaggregation of clay (Galati

v (mrn/s)

66

18

450

Vibration Velocity (mm/s)

184 TUNNELLING ANDUNDERGROUND SPACETECHNOLOGY

Relative Effect

1975)

Micro-fissures in the rock (Dowding 1985)

Volume 9, Number 2, 1994

Table 8. Maximum values of vibration velocity recorded before and after modification of the priming pattern adopted for the top heading rounds. Round Primed Symmetrically

v (mm/s) v* (mm/s)

GR (R = 2m)

GC (R = 4m)

GR (R = 2m)

GC (R = 4m)

1000 452

150 269

320 452

80 269

GR) Geophone in the resin-treated rock GC) Geophone in the cement-treated rock R) Distance geophone/nearest shot

-no. of delays

Round Primed Asymmetrically

6 (30-ms type) + 12 (500-ms type)

E x c a v a t i o n o f t h e bench: -section 40 m s --advance 2.3 m -hole diameter 38 mm -explosive 35 kg of Gelatine 2 in 25-mm-dia. cartridges -no. of holes 40-45 -priming el. detonator with 500-ms delays -no. of delays 12

Excavation to place the side walls: -rounds with few shotholes to blast limited volumes of rock. Before b l a s t i n g b e g a n , some vibrometric recordings were made in rock not treated, in order to obtain the variation law of v (vibration velocity) as a function of the scaled distance D.S. (D.S. = pfv/-Q ). The resulting law was:

The following interesting phenomena were evidenced by the analysis of the firstrecordings in treated rock: • The behaviour of the resintreated rock was anomalous with respect to that ofrockin general. A very slow decrease in vibration was evident. Therefore there was continuity in the vibration generated by explosions with 500m s intervals. • The values ofvinthe area treated with resin were greater than expected. • In the outer,cement-treated zone, vibration velocity was smaller than expected. The above phenomena were explained as follows: • The small decrease in vibration in the resin-treated rock was interpreted as though the rock there behaved as an elasticspring

Volume 9, Number 2, 1994

v) Actual value v*) Expected value

between two stiff supports (the two cement-treated areas). • Because the groups of shotholes blasted in subsequent delays were arranged in a symmetrical pattern with respect to tunnel axis, the increase in vibration velocity in the resin-treated rock could be related to resonance induced by the symmetry of stress. • The smaller vibration intensity in the outer area treated was explained by the enormous absorption of energy in the form of elastic deformation due to the resin-treated rock. The assumption that the anomalous vibration velocity values in the resin-treated rock were caused by resonance led to a modification of the symmetrical priming pattern initially adopted for the top heading rounds. The resulting asymmetric pattern featured even-number delay detonators all on one side of the face, and odd numbers all on the other side. Table 8 reports the m a x i m u m values (v) of vibration velocity recorded before and after this modification: the substantial reduction that resulted is

evident. The m a x i m u m charge per delay time was always, within close approximation, 3.6 kg. The table also reports the v values calculated on the basis of the equation 470 ( W~/-Q )-0.75,which had been V verified in the rock before the treatment and which was used in the definitionofthe Q charge in the initialrounds. A level v =320 m m / s was deemed acceptable. It was decided that every time the recordings approached this value, the round would be modified. The m a x i m u m vibration velocity values obtained in the excavation of the bench were substantially smaller--on average, about 80-100 mm/s in the resin-treated rock, and 40-50 m m / s in the cement-treated rock. N o problems were experienced in the excavation to place the side walls. Vibrometric recordings were m a d e using geophones Sensor S M 6 with their own frequency, 4.5 Hz.

2.2.5 Vibrometric Control in a Cave A karst-origin cave---rich in stalactites,stalagmites, and crystals of rare beauty--was the object of a careful survey intended to determine whether the soundness ofthis natural structure would be compromised by the mining activitythat was being carried out at a distance of a few hundred meters (see Fig. 17). In fact, the cave is located within a mining permit area, and was discovered by chance w h e n the mining tunnels had already developed in alldirections. It was therefore necessary to study the behaviour of the weakest elements of the cave at the m o m e n t they were subjected to the stress of blasting-induced vibration. The closest mining zones were located more than 250 m from the cave. However, in the past, before the cave

Figure 1Z Location of the stoping operation with respect to the cave. A = Accelerometers. G = Geophones.

TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY185

was discovered, a research tunnel had been driven only a few meters away, and the scaled distance of the shotholes blasted at the time must have been much more risky than is the case for present-day shotholes. The cave exhibits two types of damage: 1. Many stalactites and stalagmites, especially the slimmer ones, have lost their apexes, which are lying on the floor, broken and re-cemented. 2. Many of the columns and concretions stretching from the ground to the ceiling show evident horizontal fractures. Mining is carried out by sublayers, with shothole spread in a fan pattern parallel to the blasting face, and a burden of 3-4 m per hole, diam. = 64 ram, pneumatically filled with bulk AnFo. In this type of blasting, the use of"long~ delay detonators is not strictly necessary (100 or 250 ms). In fact, the microdelays (30 ms) are quite convenientbecause they produce blasted material with better rock dimensions. The data for three subsequent rounds blasted in limestone with Pb and Zn sulphide minerals are given in Table 9. The following units were used for recording: 1. Instantel DS 477 with triaxial geophones. 2. Monoaxial accelerometers Bruel & Kjaer. The units were positioned inside the cave in the following different spots (see Fig. 17): GI: geophones on the ground. G2: geophones on a squat column at 1.70 m from the ground. AI: accelerometer on a column 7 m high, at 1.50 m from the ground. A2: accelerometer on the same column at 3.50 m from the ground. The three rounds, blasted at a distance of 270 m from the cave, produced vibration, for which the

Table 9. Data for three rounds blasted in limestone with Pb and Zn sulphide minerals.

Round Data for:

First

Second

Third

11

2

6

14

14

14

535

75

185

0-6

0

0-5

155 (*)

75 (^)

40 (.)

Number of holes Average length of holes (m) Total charge of explosive (kg) Number of delays (30 ms) Max. quantity of explosive primed by same delay (kg)

J

(*) 3 shots charge (^) 2 shots charge (o) 1 shot charge

velocity and acceleration values are given in Table 10. These data, added to others, allowed the extrapolation of the law ruling vibration velocity variation v as a function of Q and R -in

-1.16

This relationship was used at the end of the survey to plan a reduction in Q with respect to the value obtained in the first round (Q = 155 kg). The vibration velocity v = 7.1 muds related to Q = 155 kg was, in fact, judged to be at the limit of tolerance in consideration of the weakness of the natural structures present in the cave. The damage detected in stalactites, stalagmites and the columns stretching from ground to top are at least partly the result of rounds where the Q charge was excessive with respect to its R distance from the cave. This damage probably occurred while driving the research tunnel, when the existence of the caves was not known; therefore, it is necessary to ensure that this will not happen again, either now

or in the future. This aim may be fulfilled only by keeping the seismic level of the round within control. The most interesting findings, made possible by the use of accelerometers, concern the slimmer structures (stalactites, etc.). These structures usually behave similarly to the ground or squat bodies, but undergo a resonance phenomenon when their own frequency coincides with the stress frequency. The damage and breakage observed in such structures are probably linked to induced resonance and fatigue phenomena. In the first case, breakage occurs when the limit of resistance to flexion is exceeded in a particular section. In the latter case, breakage occurs when, because ofincreasing concentric breakage, the area of the resisting section decreases below a minimum value, depending on the dimensions, mass and material of the structure. Because it was not possible to analyze each single element of the cave, the study could not reach general conclusions regarding the interpretation of and solution to the problem. It was judged advisable to propose guidelines

Table 10. Velocity and acceleration values for thre rounds blasted at a distance of 270 rn from the cave

Round

a ~ (g)

v (mm/s)

Recording Unit

v (ram/s)

G1

7.1

4.5

G2

7.1

4.4

A1 A2.

Third

Second

First

0.294

186 TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY

a o (g) m

v (mm/s)

a o (g)

3.1 3.2

0.073

0.049

0.114

0.090

Volume 9, Number 2, 1994

Table 11. Characterisitcs of explosives mentioned in this paper.

Density (kg/m s )

Velocity of Detonation (m/s)

Energy (MJ/kg)

Gelatine 1

1450

6550

4.52

Gelatine 2

1420

6100

4.44

Tutagex 810

1250

4200

3.55

Tutagex 210

1150

4200

3.52

Emuldin 1A

1160

4900

3.84

ANFO

800

2300

3.66

Profil X

1200

3240

2.66

Type of Explosive

to continue mining under acceptable conditions, taking into account both the conservation of the cave and the economy of mining. Upon consideration that w h e n research was being conducted close to the cave, the value of v must have approached 15-20 mm/s (*);and having checked that during tests, V was always kept within _<7.1 ram/s, a proposal was m a d e to the mine management to blast every shothole with a microdelay electric detonator with a different delay number. This arrangement would increase neither the cost nor the organization of rounds. By fixing Q = 40 kg (while R remained 270 because mining was proceeding away from the cave), it could be calculated that v would be kept within the value v<241 270

= 3.1 mngs

and itwas judged that that level would have sufficiently protected the cave structures. Table 11 lists the characteristics of explosives m e n t i o n e d i n this paper.

2.3 Conclusions By giving examples of some rules to follow, this paper has attempted to illustratethat problems relatedtoblasting-induced vibration can nearly always be solved. In fact,the vibrations induced by tunnel rounds have some positive features:

• They last just a few seconds---a

short time compared to vibration from mechanical means. • Their intensity m a y be adjusted in order to satisfy any need by simply dimensioning rounds appropriately. • They occur at 8- to 10-hourintervals; therefore, fatigue problems concerning persons or structures under risk should not arise. • They can be generated w h e n the conditions to withstand them are best and prior warning is possible. • The structure is subjected to risk for a limited time, i.e.,the time necessary for the tunnel face to move away from the structure itself. Because the advancement face moves fairly quickly and new situations always occur, it is advisable to tackle the problems related to seismic impact when the tunnel route is being decided; and to set forth--possibly in agreement with the owner ofthe structure or the person responsible for its protection--the respective stress limit values. It might be convenient at that stage to accept limit values far below those calculated to be tolerable, especially w h e n this does not raise problems with respect to excavation. However, it is necessary to always bear in mind that vibration is not an exclusive effect of blasting;,and that people, as well as their environment, constantly

produce vibration and, equally constantly,are subjected to vibration (with smaller or greater annoyance). Given this context, it seems absurd to argue whether 3 or 4 m m / s is a dangerous vibration velocity for plaster in buildings, w h e n the same plaster, because of the thermic difference between day and night, is daily subjected to stress corresponding to a vibration level of 30-40 mm/s (U.S. Bureau of Mines 1988). It is therefore advisable that everyone in the whole area surrounding the operation be promptly informed and reassured that the round-induced vibration is under control and is not dangerous for structures; and that the annoyance it produces, although undoubtedly unpleasant, will be shorter in duration the fewer the obstacles that are placed in the w a y of the development of the excavation. Experience shows that ifthe relationship with outsiders is based on reciprocal trust and cordiality,the majority ofthe problems connected with vibration can be solved at the beginning of the project. In allcases, itisadvisable to record, before blasting, the damage already existing in those structures that are of interest because of their proximity to the works being undertaken. This is the best way to avoid charges, albeit made in good faith, that such damage can be attributed to blasting. []

References Atlas Powder Company. 1987. Explosives and Rock Blasting. Dallas: Atlas Powder Co. Berta, G. 1989. L'esplosivo strumento di lavoro. Italesplosivi Milano. Berta, G. 1990. Explosives: An Engineering Tool. Italesplosivi Milano. Dowding, C. H. 1985. Blast Vibration Monitoring and Control. Englewood Cliffs, N.J.: Prentice-Hall. Hutchison, E. C. and Smith, G. 1981. Effective construction blasting damage control. In Proceedings of the Seventh

Conference on Explosives and Blasting Techniques. Society of Explosives Engineers. Konya, C. J. and Walter, E.J. 1985. Rock Blasting. U.S. Department of Transportation, Federal Highway Administration.

Piovano, G. 1992. Controllodellevibrasioni nell'usodell'esplosivo.Galleriee Grandi Opere Sotterranee (38).

(*) The values 15-25 mm/s were calculated after a s c e r t a i n i n g t h a t i n the r e s e a r c h t u n n e l , the r o u n d s h a d a

m a x i m u m charge per delay time ranging from 15 to 20 kg; and that the m i n i m u m distance from the single natural structures had been 30-40 m.

Volume 9, N u m b e r 2, 1994

TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 187