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On the influence of rock mass quality on the quality of blasting work in tunnel driving
PII:80886-7798(98) 0002;7-3
On the Influence of Rock Mass Quality on the Quality of Blasting Work in Tunnel Driving N. Innaurato, R. Mancini, and M. Cardu A b s t r a c t - - A f t e r defining the parameters describing the quality of blasts in tunnel d r i v i n g - - the pull efficiency, the overbreak and the H C F factor (the ratio o f thee observe length o f h a l f cast on the tunnel wall to the total drilled length o f the contour holes) - - the paper examines the influence o f the quality o f the rock mass on said parameters. A s indicators of the quality o f the rock, the R M R (Rock Mass Rating) according to Bieniawski and the rock mass strength according to Hoek a n d Brown criterion are assumed. On the basis o f observations in more titan 15 stretches o f tunnels in different geological conditions a n d excavated with different techniques, it has been possible to outline t~:e relationships between R M R and rock mass strength and the blasting efficiency, the overbreak and the HCF. It is concluded that the rock mass structure has a not negligible influence when a quality threshold has to be set for a tunnel blasting operation; the threshold values as suggested by observation are indicated. Moreover, it has been found that the use of sophisticated drilling systems, using expensive equipment, does not provide advantages in cases of rock masses of poor quality. Finally, it has been f o u n d that H C F is a sensitive indicator o f the quality of blasting, and that the same factor increases in an evident manner as the rock mass quality improves.
© 1998Elsevier ScienceLtd
Introduction xplosives have been used for centuries to excavate all types of tunnels, whether mining or civil. Since the 1960s they have, in many cases, been replaced by mechanical methods (TBMs in particular), for several reasons, including: 1) quicker advanceraent in favourable conditions; 2) less effect on the environment; and 3) controlled overbreak or "extra profile," which is less than that caused by explosives. However, mechanized systems also have some negative characteristics, such as: 1) the shape of the tunnel must be circular; 2) the utilisation coefficient of the machines is reduced in cases of poor tunnel stability;
E
Present addresses: Prof. N. Innaurato, Dipartimento Georisorse e Territorio, Politecnico, Torino 10129, Italy; Prof. R. Mancini, Centro Studi per la Fisica delle Roccee le Geotecnologie(CSFR) - CNR and Dipartimento Georisorse e Territorio, Politecnico, Torino 10129, Italy; Dr. M. Cardu, Dipartimento Georisorse e Territorio, Politecnico, Torino 10129, Italy.
Tunnelling and Underground Spc~'.eTechnology, VoL 13, No. 1, pp. 81-89, 1998 © 1998 Elsevier Science Ltd Printed in Great Britain, All rights reserved 0886-7798/98 $19.00 +0.00
- - Apr~s avoir ddfini les param~tres qui r~glent la qualitd de tirs d l'explosif pour l'excavation des tunnels, c'est d dire: l'efficacitd du tir, l'hors-profil, I'HCF ( le rapport entre la longueur totale des demi-trous visibles s ur le contour apr~s le tir et la longueur totale des trous fords sur le contour m$me), on a dtudid 12nfluence de la structure de l'arnas rocheux sur ces param~tres. Comme indicateur de la qualitd de la roche on a choisi d'une cotd le R M R (Rock Mass Rating) de Bieniawski et de l'autre cord la rdsistance de l'amas ~ la rupture selon Hoek et Brown. A la suite d' observations faites s u r p l u s que 15 tronfons de tunnels dans des diffdrentes conditions gdologiques et techniques il a dtd possible mettre en dvidence les relations entre le facteur R M R et la rdsistance de l'amas avec l'efficacitd du tir, l'hors-profil et I'HCF. On a donc p u conclure que l'influence de la structure de l'amas rocheux ne peut pas ~tre ndgligd quand on veut fixer un seuil de la qualitd du travail d l'explosif" on a pu dgalement donner la valeur de ce seuil selon les observations faites. Une autre dvidence est posde p a r le manque d'avantage de conduire la perforation des trous avec des dquipements co?~teux et tr~s sophistiquds dans des massifs rocheux de tr~s mauvaise qualitd. Enfin on a mis en dvidence que H C F est un indicateur sensible d la qualitd du tir et qui augmente de fafon dvidente avec la qualitd de la roche. Rdsumd
3) the high investment costs of the machines and the long setting up and dismantling times hinder their economic use in short tunnels; 4) in large-diameter tunnels, TBM cannot achieve the same rate of advancement as in small or medium diameter tunnels; 5) in poor quality ground, the overbreak caused by the machine is of the same order of magnitude as that caused by explosives. All these factors contribute to making excavation with explosives still the most widely used method in non-circular or large-section tunnels, or in those, such as mining tunnels, where great flexibility of use is required. The existing and future effectiveness of these mining technologies greatly depends on the progress that has been made over recent years and that can still be made. To this end, the CSFR (Center for the Study of Rock Mechanics and Geotechnology) working group of the CNR (National Council for Research), which deals with excavation techniques for underground works, is carrying out research into the possibility of introducing suitable verification methods for controlling current blasting techniques. The object of the research is to establish quality parameters for blasting work (by checking the underground void surroundings and their stability) by defining an acceptable mean quality
Pergamon
threshold and possible causes of deviations from such a threshold. In blasting, as well as in other excavation methods, we can distinguish between: • the "ideal result," which means that the geometrical features of the cavity exactly match the design, and the surrounding rock is undisturbed; • the "best attainable result," which is poorer than the ideal result because neither a "perfect rock" exists, nor a "perfect technology" has yet been developed; and • the "actual result," which is still poorer. While the discrepancies between the "ideal" and the "best attainable result" stimulate the development of new technologies, the discrepancies between the "best attainable" and the "actual" result should stimulate the optimal use of existing technologies, in order to reduce the actual gap. To do this, reliable and simple quantitative indicators of the "quality" of the blast results, based on post-blast surveys, have to be employed in order to evaluate and compare blast practices, as explained in a previous paper (Mancini et al. 1996). To establish what should be accepted as '`best attainable result," however, implies the evaluation of the role or the rock features in determining the quality of the blast results, irrespective of the merits of the drilling and blasting system; this paper deals with the side of that problem.
C o n c e p t of Controlled Blasting "Controlled blasting" is a loosely defined concept stemming from the time-honoured aphorism, "blasting is not bombing." A general overview of the "controlled blasting" technological research subjects and ofthe related motivations is shown in Table 1; the particular activities and aspects considered in the research underway are underlined. For tunnels destined for civil purposes, it is of the utmost importance to keep the cross-profile of the tunnel as close as possible to that of the project. This results in beneficial effects on, amongst others, limiting of the cost of the linings to within those of the project. In practice, however, this is not really completely possible. Some countries have issued special standards which regulate the deviation of the real profile from that of the project (extra-profile) and which also regulate the call for tenders. For example the Swiss Society of Engineers and Architects limits the extra-profile to a figure lower than 0.07 qA, where A is the tunnel cross-section area, with a maximum limit of 0.4 m. It is clear from the above that the objective of controlled blasting is to reduce the extra profile (or better still, the over-profile, which is the deviation beyond the theoretical
profile) to a minimum which, apart from the previously mentioned advantages of saving on the supports, also offers the advantage of reducing the fracturing of the rock around the tunnel and, therefore, of increasing the stability of the face and wall (the quality of the rock mass being equal). Obtaining a smooth profile is accomplished through various techniques that cannot all be mentioned here in detail and which tend to limit the energetic effects of the explosives, contained in the contour boreholes, to a minimum; at the same time, the contour boreholes must be as close as possible to the theoretical profile. While the first requirement influences the way the explosive works, the second relates to the way the drilling is performed. One technique that is frequently used today to limit the energetic effects is carrying out the so-called de-coupling between the charge and the borehole - - that is, to introduce much lower-diameter cartridges (about half) than the boreholes themselves into the contour boreholes. The coincidence between the ideal line that unites the contour boreholes and the theoretical profile is impossible since, even without taking the precision errors in the positioning and orientation of the boreholes themselves into account, a minimum divergence of these boreholes from the theoretical direction of the tunnel axis is unavoidable because of the way the drilling is performed. Recent progress, such as the introduction of the use of computerized jumbos, which permit a noteworthy drilling precision, can result in a drastic reduction of the extra-profile to within the so-called "physiological" values (Mancini et al. 1996).
Blasting Quality C o n c e p t The factors that can be used to verify the quality of blasting are: 1) the ratio between the real and the theoretical pull of the round (q); 2) the geometry of the contour of the cross-profile; 3) the powder factor of the explosive (c); and 4) the size distribution of the rock fragments produced by the round and the muckpile profile. The first two parameters furnish information on the efficiency of the blasting pattern to obtain the maximum advancement per cycle that is compatible with the crosssection area of the tunnel. The third is an indicator of the overall unit (volumetric) cost of the excavation; specific drilling (hole meters per cubic meter) could be employed to the same aim; indeed, specific explosive consumption and specific drilling are interrelated in tunnel excavation. The fourth parameter gives an indication of the possibility of carrying out mucking work quickly and easily. The muckpile profile also can affect mucking time, and some-
Table 1: Reasons that make suitable controlled blasting.
Geometry control MINING (Tunnels and production) CIVIL TUNNELS
Selectivity
Surrounding rock fracturing control Exploitation recovery Ore dilution Vnid.~ .~t~hility
Voids st~bilitv
Safety Dimension stone
Vnid.~ .~t~hilitv Lininas cost
Void.~ .~t~bilitv Dangerous material stockage
v
82 TUNNELLING ANDUNDERGROUND SPACETECHNOLOGY
Blasted rock breakage control Ore dressing Pop shooting Mucking out
Fines discarding
Mucking out
Flyrock control
Vibrations control
Equipment damage Mucking out Safety
Structure damage Human discomfort
Equipment damage Mucking out Safety
Structure damage Human discomfort
Volume 13, Number I, 1998
times is considered an indica1,2tor of blasting efficiency under Cost ratio )tal cost this aspect. Another muckpile feature affecting muck removal 1 ................................. Support cost is the '"oulking factor," i.e., the ratio of the muckpile volume to 0,8 the in-situ volume of the blasted Excavation cost ! rock. Blasting pattern and timing can affect both features; i.e., 0,6t the parallel holes round tend to produce a "flatter" muckpile, and firing the lifter holes at the latest stage of the round is rec0'4 t ommended by m a n y blasters in 0,2 o r d e r to o b t a i n a " s o f t e r " muckpile; however, quantita0 tive data on this subject are 0,71 0,5 0,38 0,24 lacking. In any case, muck-pile shape and volume r a n k hierarDc/Dh chically lower than fragmentation in determining ,~he diffiFigure 1. Cost ratio in function of the ratio: diameter of the cartridge/borehole culty of mucking work. diameter (modified from Holmberg 1984). The adequacy of the profile geometry after blasting to the (1987), the damage can be correlated with the detectable theoretical one can be evaluated from two parameters: peak velocity in the rock; for mining works, a velocity a) the value of the overbreak (OB) or the extra-profile threshold in the range 200-600 mm/s is considered in rock (OB: the ratio of the difference of the theoretical and masses of poor quality and 600-2000 mm/s in rocks of good real areas of the cross sections to the perimeter of the quality. These values obviously must be considered too high tunnel cross section, excluding the floor); and in comparison to those that are tolerable for concrete strucb) the ratio between the length of the half-cast holes tures that make up linings of civil tunnels. The potential present in the contour after blasting and the total instability caused by the explosion should be taken into length of the contour boreholes (HCF). consideration during rock characterisation for the calculaThe OB value defilaed as in a) is expressed as "average tion of the support structures. For example, in some classithickness" (in meters or feet) of the overbreak. Other fications, established with reference to mining with exploauthors prefer to express the OB through a dimensionless sives, it is normal to assign the rock to the class immediately factor, as the ratio ( or the percent ratio) of the extra-profile above the reference class when the excavation is carried out cross-section to the de:sign cross section areas. This method with smooth profile techniques or with machines. is preferable when the overall economic impact of the The importance of minimising the fracturing around overbreak has to be evaluated. However, when the intrinthe tunnel by means of suitable devices (controlled blastsic merits of a drilling and blasting system are to be judged, ing) is emphasized, for example, by Holmberg (1994). The or compared to the ones of another system, it does not lead diagram of the trend of the total cost (supports and excavato a fair evaluation. Indeed, it can be easily seen that to tion) in function of the control grade of the blasting work, stay within a 10% (surface area) overbreak threshold represented on the abscissa by the ratio between the value, when driving a large tunnel, e.g., a tunnel 10-m diameter of the cartridge and the borehole diameter (called wide, requires a much lower drilling and blasting accuracy de-coupling ratio), is given in Figure 1: while the cost of the than to maintain the same percent overbreak in a small, supports diminishes, the cost of the drilling increases (the e.g., 3-m-wide, tunnel~. boreholes must be closer as the de-coupling ratio grows). Parameters ~, OB and HCF are indicators of geometriThis creates an optimal condition which corresponds to a cal perfection: OB = 0 and h = 1 correspond to a round which minimal total cost. respects the design profile and which takes complete adPast research into the quality of blasting dealt not only vantage of the perforated length. HCF = 1 indicates that with the aforementioned technological problems, but also the action of the explosive t h a t can be seen on the profile with a series of problems t h a t refer, in an indirect way, to does not extend beyond the surface on which the perimeter the mechanics of the rock, which showed that the quality boreholes lie, hence t h a t the blast, when considered as an of the blasting, defined in the previous sections, and the excavation device, did not remove more rock than desired; quality of the rock were in some way connected. As a moreover, HCF is an indirect indicator of a low unblasted consequence, the hypothesis t h a t the entity of the extrarock disturbance. Indeed, in order to obtain a high HCF profile could be influenced by the same parameters that value, the tensile stress occurring during the blast in the influence the stability was evaluated, e.g., depth of the inter-holes rock bridges should not greatly exceed the underground void, rock mass resistance characteristics, tensile strength of the rock. This is a condition for a correct frequency and orientation of the discontinuities, rough"fracture guidance" by contour holes, implying a comparaness level, grade of alteration of the discontinuity surface, tively low stress level on the contour surface, and obviwater flow, etc. ously, beyond the contour surface in the rock that stays in The research carried out has confirmed, at least in a place. general set-up that must be further defined, the relationships between the technological parameters and the quality of the rock mass, as is discussed below. The Role of the IVlechanical Characteristics of
the Rock on the Quality of Blasting The energy released by the explosive to the outside of the volume to be blasted not only creates an extra-profile, but also diminishes the stability of the rock mass as it causes fracturing around the void. For example, according to Page
Volume 13, Number 1, 1998
Research Methodology The research that was carried out basically examined the checking criteria of the quality of the round based on the following parameters:
TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY83
1. the ratio of the real to the theoretical pull of the round (~); 2. the value of the overbreak (OB); and 3. the ratio of the length of the half casts of the holes present in the contour after blasting to the total length of the contour boreholes (HCF). These parameters were determined from a certain number of tunnels that had different geomechanical characteristics, typology and drilling-and-blasting techniques conditions. The quality of the rock mass was also established through the use of suitable geomechanical classifications. The entity of the extra-profile was, in particular, determined by measuring the real profile after blasting: • in the case oflarge tunnels, using infrared distometers mounted on electronic theodolites, or, alternatively, using ultrasonic distometers; and • in the cross-sections of the middle sized tunnels (2025 m2), using telescopic stadias; and then calculating the difference between the measured values and the theoretical profile. To obtain a suitable mean value, two determinations of the profile were carried out in two sections per round. The HCF was calculated simply by measuring the length of the half casts of the holes observed around the tunnel walls after blasting and calculating the ratio between this value and the total length of the contour boreholes. The value of ~ was determined by measuring the distance between the mean positions of the face before and after blasting and mucking, and then calculating the ratio between this value and the theoretical one.
Operative and Geomechanical Conditions The operating conditions of the sites where the data were collected and measured varied from site to site: 13 out of 17 cases were mining tunnels, opened for the development of mining; the other cases were civil tunnels. In 1 case, an explorative pilot tunnel was excavated with TBM and then enlarged to full section with the use of explosives. All the tunnels belong to the category of medium or large section area (from 20 to 90 m2). This can hardly be considered a limitation because in small section tunnels the problems created by overbreak are very limited. In 11 cases, the cut of the round was with parallel boreholes with the central boreholes unloaded; in 5 cases, it was with convergent boreholes. In 11 cases, the smooth blasting technique was used, and in 5 cases the drilling was done by computerised jumbo. The term "computerised jumbo" actually encompasses two kinds of machines: those where the design drilling pattern and the actual drill positions are simply displayed to the operator, who is charged with manually setting the drills at the correct places (referred to as a "semi-computerized" machine in Table 3); and those where all operations are automatically performed according to the program stored in the machine. Computerised drilling warrants centimetric accuracy; but accuracy has a cost, apart from the higher cost of the machine: something has to be paid in terms of drill set time. The latter drawback still hinders the development of computerised machines in mine tunnel driving, where accuracy is not rated as highly as speed. Nonetheless, some of the cases investigated (cases A/l/, C/2/,D/2/,E/2/, in Tables 2 and 3) refer to mine tunnels driven by computerised jumbo. Analogously, the nature of the rock was different for each tunnel, both in terms oflithology (marbles, micaschists, gneiss and calcareous schists) and in terms of position of the bedding and/or the discontinuity. The laboratory uniaxial compressive strength registered in the various cases varied between 65 and 200 MPa, and Rock Mass Rating (RMR) according Bieniawski was between from 30
84 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
and 86, which cover a large range ofgeologicaYgeotechnical in-situ conditions. As far as the possible correlation between the rock characteristics and the quality of the mining work is concerned, an attempt was made to verify any possible dependence between the technological parameters (~, c, OB and HCF) and the quality of the rock. The Rock Mass Rating value, according to Bieniawski's classification, was assumed as the index of the quality of the rock mass because it is widely used in tunnel design. Moreover it was directly available in some cases, as it is quoted successively: this was not the case for Q factor, another common parameter in tunnelling. The values of RMR were obtained in the various examined cases: 1) from the determinations carried out by the contractors (in this case the rock mass class, relative to the investigation area, was furnished); and 2) from direct observations by the CSFR researchers. In the first case, the central RMR value relative to the class to which the rock pertains was assumed for the elaboration. In the second case, because the Rock Quality Designation (RQD) value was not always available, the geostructural data according to Wickham's classification (Wickham et al. 1974) was first used, and then the RMR value was obtained using the relation RMR = (RSR - 12.4)/0,77 (Bieniawski, 1989). Analogously, the evaluation of the resistance to the laboratory uniaxial compression was carried out using: 1) the data obtained from the contractors from the various sites; 2) the data obtained directly in the laboratory by the research group; and 3) the measurement of the Point-Load-Test resistance index on shapeless pieces. In the first case, it was unce~ain how the tests were carried out; however, it was believed that they were carried out according to ISRM recommendations. In the third case, data were s tandardised to that of Is0and then the compressive strength was calculated using the multiplicative coefficient 24.
Research Results The research has shown the possibility of controlling blasting quality through the measuring ofOB, HCF and q, and has also permitted the identification of some correlation between these parameters. In particular, the relationship between OB and HCF can be noted (Fig. 2). The points are distributed in two well separated families: on the left, the points that refer to blasts that are close to technical perfection (high HCF and low OB); and on the right, those that represent less perfect results (high OB and average HCF). The plot of~ vs. RMR data (Fig. 3) shows a wide scatter. This is expected, since different r<)und types and excavation cross-sections are involved, in addition to RMR and drilling accuracy variability from one case to another. Apparently,higher RMR values adversely affect ~, which simply means that tough rock is usually harder to blast than soft rock. Computerised drilling tends to assure higher efficiency; when the faces are inspected upon blast, it can be seen that with computerised drilling, the hole bottoms always exactly lie in the same plane, orthogonal to the tunnel axis, which seldom occurs with manually operated jumbos. This is probably the main reason for the very good efficiency observed, Regarding the powder factor, it was not possible to show any particular correlation, even of a generic type, with the quality of the rock. This could be due to the fac.t that c is dependent, to a great extent, on the tunnel section ( Fig. 4,
Volume 13, Number 1, 1998
Table 2. Details of the geotechnical parameters of interest for the cases investigated. Co: Uniaxial Compressive Strength (Tunnel N: from Point Load Tests) S: Area of the tunnel cross-section. R=: Rock Mass Compressive strength.
Tunnel (Ref.)
Rock
Rock mass Rating
R am
S
CO
m2
MPa
56 56 60
65 65 65
58
2
60
65
50
1
60 60 60
65 65 65
45
0.7
Porphyritic gneiss
76.8
200
70
38
i/1 /
Chloritic Micaschist
93
//1 /
Phyllitic Micaschist
93
100
30
2
M/3/
Calcschist
76
40-50 30
1
22.2
90
55
7.4
22.2
185
86
85
22.2
90
64
12
22.2
185
86
85
22.2
185
62
22
22.2
90
64
12
A/1/ B/1/
C/2/ D/2/
Marble
E/2/ F/2/ G/2/
H/1 /
N/4/ Ch.337348 m 348-372 m 372-387 m 387-415 m 41 5-448 m 448-531 m
Micaschist
N o t e s o n T a b l e 2: Reference numbers (see References)
/1/Castano, Grad. Thesis. /2/Blengini, Grad. Thesis. /3/Salvaia, Grad. Thesis. /4/Magro, Grad. Thesis.
Volume 13, Number 1, 1998
Condition of rock and joints surface
(MPa).
30
Undulating, slightly altered smooth, moderately altered ;Smooth, altered
Smooth or slightly undulating withouth filling
Smooth, altered
Smooth, withouth filling
Moderately foliated, slighly altered Massive, good conditions Slightly foliated, slighly altered Massive, good conditions Moderately fractured, altered Slightly foliated, slightly altered
Tunnels are characterized by: • Low or medium overburden. • A - G: tunnels pertain to an underground quarry that exploits a white marble for chemical calcium carbonate; marble is generally homogeneous, gently stratified. * H - M : road tunnels; in particular, tunnel I was excavated by blasting after the construction of a pilot hole (dia. 3.9 m) with a T B M . • case N: pertains to a talc mine; tunnel excavated in micaschist barren rock. • Explosives used were: Gelatina 1 (a gelatine-dynamite) for cut holes; Gelatina i and Tutagex (a type of Water-Gel) for the production holes; and Profilx or Emuldin (slim charges) for the contour holes.
TUNNELLINGAND UNDERGROUNDSPACETECHNOLOGY85
Table 3. Details of the technical parameters of interest for the cases investigated. OB: Overbreak. HCF: Half Cast Factor (see text), q : see text. c : powder factor. Comp. : computerised jumbo.
Tunnel (Ref.)
OB
HCF
vI
m
%
%
kg/m 3
0.23 0.16 0.17 0.17 0.24 0.28 0.54
31 43 27 18 13.5
9
100 85 98 99 87 96 87
0.08
50
I/1/
0.27
L/1/
M/3/
A/1/ B/1/ C/2/ D/2/ E/2/ F/2/ G/2/
H/1 /
N/4/ Ch.337348 m 348-372 m 372-387 m 387-415 m 415-448 m 448-537 m
Drill. Machine
Controlled blasting (smooth blasting)
2.5 2.25 1.85 1.85 1.85 1.85 1.85
Comp. Man. Comp. Comp.
N Y N N N
Man. Man.
N N
!II // /I //
94.2
1.27
Comp.
Y
V
42
100
1.1
Man.
!Y
0.75
35
91.1
1.65
Man.
Y
IV
0.27
38
91.7
1.4
Semicomp.
Y
V
Y
// //
8
89.7
2.41
0.13
24
85
2.41
Y
//
0.26
27
88.7
2.41
Y
//
0.16
21
85
2.41
Y
//
0.37
10
90
2.41
Y
//
0.24
25
88.7
2.41
Y
//
Reference numbers (see References) /2/Blengini, Grad. Thesis. /3/Salvaia, Grad. Thesis. /4/Magro, Grad. Thesis.
Man.
!// V //
0.4
N o t e s o n T a b l e 3:
/1] Castano, Grad. Thesis.
Comp.
Type of cut
Tunnels are characterized by: * L o w or m e d i u m overburden. • A - G: tunnels pertain to an underground quarry that exploits a white marble for chemical calcium carbonate; marble is generally homogeneous, gently stratified. • H - M : road tunnels; in particular,tunnel I was excavated by blasting after the construction of a pilothole (dia. 3.9 m) with a T B M . • case N: pertains to a talc mine; tunnel excavated in micaschist barren rock. • Explosives used were: Gelatina 1 (a gelatine-dynamite) for cut holes; Gelatina 1 and Tutagex (a type of Water-Gel) for the production holes; and Profilx or Emuldin (slim charges) for the contour holes.
86 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Volume 13, N u m b e r 1, 1998
Symbols: 50 45
[] Cases in which smooth blasting and manually controlled jumbo were used;
HCF ( % } o
40
Q
35
• Cases in which smooth blasting was not used; computerised jumbo;
30 n [3
25
Cases in which smooth blasting and computerised jumbo were used.
(3
2015-
[]
10-
o
5 0 0
I
I
I
t
I
I
I
0,1
0,2
0,3
0.4
0,5
0,6
0.7
0,8
O B (m}
Figure 2. Trend of the OB values in function of HCF. 1 O0 98
-q
Symbols:
u
[] Cases in which smooth blasting and manually controlled jumbo were used;
96 94
Cases in which smooth blasting was not used; computerised jumbo;
92 90
[]
o
Cases in which smooth blasting and computerised jumbo were used.
88 86 []
84
I
10
--~
20
t
I
30
40
I
I
F
I
50
60
70
80
90
RMR
Figure 3. Trend of ~ values in function of the RMR. after Mancini et al. 1.995) and to the fact that the operative factors (explosive type, round type and type of profiling of the contour) were different for each case. OB and HCF turn out to be greatly dependent on the rock class, notwithstanding the role of the different parameters. (Figs. 5 and 6). As an alternative to the use of RMR as the rock characterising factor, the rock mass strength according to Hoek and Brown's criterion (Hoek and Brown 1980) was referred to. In this research it was used as a indicator parameter (i.e., one that is able to furnish the resistance level of tile mass), rather than as a characteristic of the rock in conformity with the way of working of the explosive. Rock mass strength ( R ) can be calculated by Hoek and Brown criterion:
Volume 13, Number l, 1998
6.00 ] o • 5.00
• Parallel holes cut o Inclined holes cut
...
-60
~ 2.00
~
•
•
.oo 0.00
. /
0
0
I
I
t
I
I
I
I
I
20
40
60
80
100
120
140
160
S (-,^2)
Figure 4. Specific consumptions of the explosives (hg / m 3) as a function of the area of tunnel section (S) (from Mancini et al. 1995).
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY87
0,8OB(m)
o
Symbols:
0,7"
[] Cases in which smooth
blasting and manually controlled jumbo were used;
0,60,5-
Cases in which smooth blasting was not used; computerised jumbo;
0,40,3-
[]
o
•
Cases in which smooth blasting and computerised jumbo were used.
8
0,20,1 0 0
I
I
I
I
I
t
10
20
30
40
50
60
t
7O
80
90
RMR
Figure 5. Trend of the total extra-profile in function of RMR.
50 HCF(%}
Symbols:
45" []
Cases in which smooth blasting and manually controlled jumbo were used;
40Q
35 30
Cases in which smooth blasting was not used; computerised jumbo;
25" 20
Cases in which smooth blasting and computerised jumbo were used.
15, 105 O
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
RMR
Figure 6. Trend of the HCF in function of RMR. ~1 =%+ (mCo a3+ s Co2)1~ (1) where a 1 and ~ are, respectively the major and minor principal stress at failure; Co is the uniaxial compressive strength of the intact rock material; m and s are constants dependent on the properties of the rock and the extent to which it is fractured. The resistance of the mass, in the absence of confining actions (in this case neglected, as the examined tunnels were not deep) can be written as: R = C J s , where s = exp (RMR- 100)/6, in the case of the contour damaged by blasting (non-controlled blasting), and s = exp (RMR - 100)/9 in the case of controlled blasting; CO is the laboratory uniaxial compression strength (see also Bieniawski 1989). In practice, using the Hoek and Brown criterion the explosive action in the rock is dominated by a failure criterion that takes into account both the rock quality (expressed by RMR in s formulas) and UCS of the intact rock (C). Moreover the added weight of C allows us to show better the sensitivity of OB to the laboratory and in-mtu quality of the rock. Finally, the operative condition of using or smooth blasting or not (as well as the result of a low or high rock wall damage) is taken into account in the equation expressing the parameter s. o
o
88 TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY
.
Unfortunately the range of this research is not extended to deep tunnels, but it is possible to state that the Hoek and Brown criterion can be used where a confining action caused by a high border plays a role that can be taken into account through the parameter m for the rock mass in Eq. (1). Figure 7 shows the trend of the values of the extra-profile in function of the Rock Mass strength ( R ) .
Conclusions Through the analysis of various cases of tunnels driven in different rock masses, of different types (civil and mining) and in diverse operative conditions, the research has permitted some preliminary conclusions. The parameters that can evaluate the quality of mining work with explosives have been defined from a technological point of view. The adequacy of these same parameters (TI, HCF and OB) has been evaluated as positive. The research has indicated the basis for the definition of limit values from which it would be unlikely to stray in order to maintain the correctness of excavation work, and the optimal operative conditions capable of keeping the parameters within the assigned limits.
V o l u m e 13, N u m b e r 1, 1998
0,8"
o
OB (m)
0,7 "
Symbols: [] Cases in which smooth blasting and manually controlled jumbo were used;
0,60,5
• Cases in which smooth blasting was not used; computerised jumbo;
0,40,3-
• Cases in which smooth blasting and computerised jumbo were used.
0,2' 0,1 0 0
I
I
I
10
20
30
I
L
40 50 Rock Mass Strength
I
I
I
60
70
80
90
Figure 7. Trend of the extra-profile in function of the rock mass strength. At the same time, the influence of the rock mass characteristics (RMR) on the values of the technological parameters has been verified. The dependence ofT1, HCF and OB on the RMR is seen in the diagrams given in the previous sections. In some cases, the dispersion of the values is noteworthy, but this can be substantially attributed to the differences in the opera~ive factors (e.g., the drilling method used. In some cases, using the same operative factor (e.g., making the boreholes with the computerised jumbo), the correlation between the parameters appears to be promising, even though it is n.ot yet possible to make a j u d g m e n t founded on a large statistical basis. The possibility of using the rock mass resistance (R_), as defined by Hoek and Brown s criterion, was tested with good results from among the alternatives to the use of the RMR factor to qualify the rock in comparing the blasting works with explosives. It therefore results t h a t the influence of the rock quality (however defined) carLnot be neglected when fixing the blasting work quality threshold. The results obtained usually allow one to assert t h a t the better the rock, the better the quality of the blasting. The data analysed ';uggest that: 1) the placement of the boreholes in the correct way and as close to the project as possible improves q and OB; 2) the technically acceptable value of q should not fall below 0.9, even in the case of best quality rock; 3) the technically acceptable extra-profile m u s t not exceed 0.4 m in rocks with RMR above 50. Although this is an independent result, it is in a g r e e m e n t with w h a t has been established by the previously indicated standards; 4) the use of sophisticated technology (such as the use of computerised j u m b o or even smooth blasting) in rocks of poor gecmechanical quality can be fruitless; 5) the lower limit of OB for good quality rocks with optimal drilling and charging can be reduced to the mere look-out of the contour holes; 6) HCF is a sensitive index of the drilling quality and is positively linked[ with the quality of the rock. To establish the "best attainable result" as a function of the rock mass quality :requires a far larger statistical basis t h a n is currently available. Using a population of 17 cases and examining the two extreme cases - - the one in which the best technology is used with the best rock (i.e., the case ,
Volume 13, Number 1, 1998
.
.
.
tall
H/l/ in Tables 2 and 3: computerised drilling, profile charges in the contour holes, and RMR 70), and the best technology used with the worst rock (i.e., the case M/3/: computerised drilling, profile charges and RMR 30) - researchers found t h a t in case I-I/l/the quality indicators are HCF= 50, OB = 0.08, while in case M / 3 / t h e quality indicators are HCF = 38, OB = 0.27. If these two cases can be considered typical, the mere RMR improvement from 30 to 70 apparently accounts for a 30% increase in H C F and a 340% decrease in OB when the supposed "best available technology" is employed. Acknowledgments
This research is a part of the Strategic Proj ect"Tunnels," and has been carried out with the contribution of the C.N.R. (National Council for Research). References Bieniawski, Z. T. 1989. Engineering Rock Mass Classifications, 251. New York: Wiley and Sons. Blengini, G. A. 1994. Evoluzione del concetto di "Abbattimento controllato" nell'ingegneria mineraria e civile. Grad, Thesis, Politecnico di Torino. Castano, D. 1992. Analisi della precisione di abbattimento in galleria. Grad. Thesis, Politecnico di Torino. Hoek, E. and Brown, E. T.1980. Underground Excavation, 527. London: IMM. Holmberg, R., Larsson, B., and Sjoberg, C. 1984. Improved stability through optimized rock blasting. Proc. lOth Conf. on Explosives and Blasting Techniques, Orlando, U.S.A., 166-171. Magro, A. 1995. Analisi di fattori influenti sulla qualit~ del risultato hello scavo di gallerie con esplosivo. Grad. Thesis, Politecnico di Torino. Mancini, R., Gaj, F., and Cardu, M, 1995. Atlas of blasting rounds for tunnel driving. Internal Report, Dip. Georisorse e Territorio del Politecnico, Torino. Mancini, R. et al. 1996. Technical and economic aspects of tunnel blasting accuracy control. Tunnelling and Underground Space Technology 11(4), 455--463, Page, C. H. 1987. Controlled blasting for underground mining. Proc. 13th Conf. on Explosives and Blasting Techniques, SEE, Miami, U.S.A., 33-48. Salvaia, E. 1992. Controllo del profilo nello scavo sotterraneo con esplosivi. Grad. Thesis, Politecnico di Torino. Wickham, G. E., Tiedemann,-H. R., and Skinner, E. H. 1974. Ground support prediction model, RSR Concept. Proc. Rapid Excavation Tunnelling Conference, AIME, 691-707.
TUNNELLINGAND UNDERGROUNDSPACETECHNOLOGY89
On the Influence of Rock Mass Quality on the Quality of Blasting Work in Tunnel Driving N. Innaurato, R. Mancini, and M. Cardu A b s t r a c t - - A f t e r defining the parameters describing the quality of blasts in tunnel d r i v i n g - - the pull efficiency, the overbreak and the H C F factor (the ratio o f thee observe length o f h a l f cast on the tunnel wall to the total drilled length o f the contour holes) - - the paper examines the influence o f the quality o f the rock mass on said parameters. A s indicators of the quality o f the rock, the R M R (Rock Mass Rating) according to Bieniawski and the rock mass strength according to Hoek a n d Brown criterion are assumed. On the basis o f observations in more titan 15 stretches o f tunnels in different geological conditions a n d excavated with different techniques, it has been possible to outline t~:e relationships between R M R and rock mass strength and the blasting efficiency, the overbreak and the HCF. It is concluded that the rock mass structure has a not negligible influence when a quality threshold has to be set for a tunnel blasting operation; the threshold values as suggested by observation are indicated. Moreover, it has been found that the use of sophisticated drilling systems, using expensive equipment, does not provide advantages in cases of rock masses of poor quality. Finally, it has been f o u n d that H C F is a sensitive indicator o f the quality of blasting, and that the same factor increases in an evident manner as the rock mass quality improves.
© 1998Elsevier ScienceLtd
Introduction xplosives have been used for centuries to excavate all types of tunnels, whether mining or civil. Since the 1960s they have, in many cases, been replaced by mechanical methods (TBMs in particular), for several reasons, including: 1) quicker advanceraent in favourable conditions; 2) less effect on the environment; and 3) controlled overbreak or "extra profile," which is less than that caused by explosives. However, mechanized systems also have some negative characteristics, such as: 1) the shape of the tunnel must be circular; 2) the utilisation coefficient of the machines is reduced in cases of poor tunnel stability;
E
Present addresses: Prof. N. Innaurato, Dipartimento Georisorse e Territorio, Politecnico, Torino 10129, Italy; Prof. R. Mancini, Centro Studi per la Fisica delle Roccee le Geotecnologie(CSFR) - CNR and Dipartimento Georisorse e Territorio, Politecnico, Torino 10129, Italy; Dr. M. Cardu, Dipartimento Georisorse e Territorio, Politecnico, Torino 10129, Italy.
Tunnelling and Underground Spc~'.eTechnology, VoL 13, No. 1, pp. 81-89, 1998 © 1998 Elsevier Science Ltd Printed in Great Britain, All rights reserved 0886-7798/98 $19.00 +0.00
- - Apr~s avoir ddfini les param~tres qui r~glent la qualitd de tirs d l'explosif pour l'excavation des tunnels, c'est d dire: l'efficacitd du tir, l'hors-profil, I'HCF ( le rapport entre la longueur totale des demi-trous visibles s ur le contour apr~s le tir et la longueur totale des trous fords sur le contour m$me), on a dtudid 12nfluence de la structure de l'arnas rocheux sur ces param~tres. Comme indicateur de la qualitd de la roche on a choisi d'une cotd le R M R (Rock Mass Rating) de Bieniawski et de l'autre cord la rdsistance de l'amas ~ la rupture selon Hoek et Brown. A la suite d' observations faites s u r p l u s que 15 tronfons de tunnels dans des diffdrentes conditions gdologiques et techniques il a dtd possible mettre en dvidence les relations entre le facteur R M R et la rdsistance de l'amas avec l'efficacitd du tir, l'hors-profil et I'HCF. On a donc p u conclure que l'influence de la structure de l'amas rocheux ne peut pas ~tre ndgligd quand on veut fixer un seuil de la qualitd du travail d l'explosif" on a pu dgalement donner la valeur de ce seuil selon les observations faites. Une autre dvidence est posde p a r le manque d'avantage de conduire la perforation des trous avec des dquipements co?~teux et tr~s sophistiquds dans des massifs rocheux de tr~s mauvaise qualitd. Enfin on a mis en dvidence que H C F est un indicateur sensible d la qualitd du tir et qui augmente de fafon dvidente avec la qualitd de la roche. Rdsumd
3) the high investment costs of the machines and the long setting up and dismantling times hinder their economic use in short tunnels; 4) in large-diameter tunnels, TBM cannot achieve the same rate of advancement as in small or medium diameter tunnels; 5) in poor quality ground, the overbreak caused by the machine is of the same order of magnitude as that caused by explosives. All these factors contribute to making excavation with explosives still the most widely used method in non-circular or large-section tunnels, or in those, such as mining tunnels, where great flexibility of use is required. The existing and future effectiveness of these mining technologies greatly depends on the progress that has been made over recent years and that can still be made. To this end, the CSFR (Center for the Study of Rock Mechanics and Geotechnology) working group of the CNR (National Council for Research), which deals with excavation techniques for underground works, is carrying out research into the possibility of introducing suitable verification methods for controlling current blasting techniques. The object of the research is to establish quality parameters for blasting work (by checking the underground void surroundings and their stability) by defining an acceptable mean quality
Pergamon
threshold and possible causes of deviations from such a threshold. In blasting, as well as in other excavation methods, we can distinguish between: • the "ideal result," which means that the geometrical features of the cavity exactly match the design, and the surrounding rock is undisturbed; • the "best attainable result," which is poorer than the ideal result because neither a "perfect rock" exists, nor a "perfect technology" has yet been developed; and • the "actual result," which is still poorer. While the discrepancies between the "ideal" and the "best attainable result" stimulate the development of new technologies, the discrepancies between the "best attainable" and the "actual" result should stimulate the optimal use of existing technologies, in order to reduce the actual gap. To do this, reliable and simple quantitative indicators of the "quality" of the blast results, based on post-blast surveys, have to be employed in order to evaluate and compare blast practices, as explained in a previous paper (Mancini et al. 1996). To establish what should be accepted as '`best attainable result," however, implies the evaluation of the role or the rock features in determining the quality of the blast results, irrespective of the merits of the drilling and blasting system; this paper deals with the side of that problem.
C o n c e p t of Controlled Blasting "Controlled blasting" is a loosely defined concept stemming from the time-honoured aphorism, "blasting is not bombing." A general overview of the "controlled blasting" technological research subjects and ofthe related motivations is shown in Table 1; the particular activities and aspects considered in the research underway are underlined. For tunnels destined for civil purposes, it is of the utmost importance to keep the cross-profile of the tunnel as close as possible to that of the project. This results in beneficial effects on, amongst others, limiting of the cost of the linings to within those of the project. In practice, however, this is not really completely possible. Some countries have issued special standards which regulate the deviation of the real profile from that of the project (extra-profile) and which also regulate the call for tenders. For example the Swiss Society of Engineers and Architects limits the extra-profile to a figure lower than 0.07 qA, where A is the tunnel cross-section area, with a maximum limit of 0.4 m. It is clear from the above that the objective of controlled blasting is to reduce the extra profile (or better still, the over-profile, which is the deviation beyond the theoretical
profile) to a minimum which, apart from the previously mentioned advantages of saving on the supports, also offers the advantage of reducing the fracturing of the rock around the tunnel and, therefore, of increasing the stability of the face and wall (the quality of the rock mass being equal). Obtaining a smooth profile is accomplished through various techniques that cannot all be mentioned here in detail and which tend to limit the energetic effects of the explosives, contained in the contour boreholes, to a minimum; at the same time, the contour boreholes must be as close as possible to the theoretical profile. While the first requirement influences the way the explosive works, the second relates to the way the drilling is performed. One technique that is frequently used today to limit the energetic effects is carrying out the so-called de-coupling between the charge and the borehole - - that is, to introduce much lower-diameter cartridges (about half) than the boreholes themselves into the contour boreholes. The coincidence between the ideal line that unites the contour boreholes and the theoretical profile is impossible since, even without taking the precision errors in the positioning and orientation of the boreholes themselves into account, a minimum divergence of these boreholes from the theoretical direction of the tunnel axis is unavoidable because of the way the drilling is performed. Recent progress, such as the introduction of the use of computerized jumbos, which permit a noteworthy drilling precision, can result in a drastic reduction of the extra-profile to within the so-called "physiological" values (Mancini et al. 1996).
Blasting Quality C o n c e p t The factors that can be used to verify the quality of blasting are: 1) the ratio between the real and the theoretical pull of the round (q); 2) the geometry of the contour of the cross-profile; 3) the powder factor of the explosive (c); and 4) the size distribution of the rock fragments produced by the round and the muckpile profile. The first two parameters furnish information on the efficiency of the blasting pattern to obtain the maximum advancement per cycle that is compatible with the crosssection area of the tunnel. The third is an indicator of the overall unit (volumetric) cost of the excavation; specific drilling (hole meters per cubic meter) could be employed to the same aim; indeed, specific explosive consumption and specific drilling are interrelated in tunnel excavation. The fourth parameter gives an indication of the possibility of carrying out mucking work quickly and easily. The muckpile profile also can affect mucking time, and some-
Table 1: Reasons that make suitable controlled blasting.
Geometry control MINING (Tunnels and production) CIVIL TUNNELS
Selectivity
Surrounding rock fracturing control Exploitation recovery Ore dilution Vnid.~ .~t~hility
Voids st~bilitv
Safety Dimension stone
Vnid.~ .~t~hilitv Lininas cost
Void.~ .~t~bilitv Dangerous material stockage
v
82 TUNNELLING ANDUNDERGROUND SPACETECHNOLOGY
Blasted rock breakage control Ore dressing Pop shooting Mucking out
Fines discarding
Mucking out
Flyrock control
Vibrations control
Equipment damage Mucking out Safety
Structure damage Human discomfort
Equipment damage Mucking out Safety
Structure damage Human discomfort
Volume 13, Number I, 1998
times is considered an indica1,2tor of blasting efficiency under Cost ratio )tal cost this aspect. Another muckpile feature affecting muck removal 1 ................................. Support cost is the '"oulking factor," i.e., the ratio of the muckpile volume to 0,8 the in-situ volume of the blasted Excavation cost ! rock. Blasting pattern and timing can affect both features; i.e., 0,6t the parallel holes round tend to produce a "flatter" muckpile, and firing the lifter holes at the latest stage of the round is rec0'4 t ommended by m a n y blasters in 0,2 o r d e r to o b t a i n a " s o f t e r " muckpile; however, quantita0 tive data on this subject are 0,71 0,5 0,38 0,24 lacking. In any case, muck-pile shape and volume r a n k hierarDc/Dh chically lower than fragmentation in determining ,~he diffiFigure 1. Cost ratio in function of the ratio: diameter of the cartridge/borehole culty of mucking work. diameter (modified from Holmberg 1984). The adequacy of the profile geometry after blasting to the (1987), the damage can be correlated with the detectable theoretical one can be evaluated from two parameters: peak velocity in the rock; for mining works, a velocity a) the value of the overbreak (OB) or the extra-profile threshold in the range 200-600 mm/s is considered in rock (OB: the ratio of the difference of the theoretical and masses of poor quality and 600-2000 mm/s in rocks of good real areas of the cross sections to the perimeter of the quality. These values obviously must be considered too high tunnel cross section, excluding the floor); and in comparison to those that are tolerable for concrete strucb) the ratio between the length of the half-cast holes tures that make up linings of civil tunnels. The potential present in the contour after blasting and the total instability caused by the explosion should be taken into length of the contour boreholes (HCF). consideration during rock characterisation for the calculaThe OB value defilaed as in a) is expressed as "average tion of the support structures. For example, in some classithickness" (in meters or feet) of the overbreak. Other fications, established with reference to mining with exploauthors prefer to express the OB through a dimensionless sives, it is normal to assign the rock to the class immediately factor, as the ratio ( or the percent ratio) of the extra-profile above the reference class when the excavation is carried out cross-section to the de:sign cross section areas. This method with smooth profile techniques or with machines. is preferable when the overall economic impact of the The importance of minimising the fracturing around overbreak has to be evaluated. However, when the intrinthe tunnel by means of suitable devices (controlled blastsic merits of a drilling and blasting system are to be judged, ing) is emphasized, for example, by Holmberg (1994). The or compared to the ones of another system, it does not lead diagram of the trend of the total cost (supports and excavato a fair evaluation. Indeed, it can be easily seen that to tion) in function of the control grade of the blasting work, stay within a 10% (surface area) overbreak threshold represented on the abscissa by the ratio between the value, when driving a large tunnel, e.g., a tunnel 10-m diameter of the cartridge and the borehole diameter (called wide, requires a much lower drilling and blasting accuracy de-coupling ratio), is given in Figure 1: while the cost of the than to maintain the same percent overbreak in a small, supports diminishes, the cost of the drilling increases (the e.g., 3-m-wide, tunnel~. boreholes must be closer as the de-coupling ratio grows). Parameters ~, OB and HCF are indicators of geometriThis creates an optimal condition which corresponds to a cal perfection: OB = 0 and h = 1 correspond to a round which minimal total cost. respects the design profile and which takes complete adPast research into the quality of blasting dealt not only vantage of the perforated length. HCF = 1 indicates that with the aforementioned technological problems, but also the action of the explosive t h a t can be seen on the profile with a series of problems t h a t refer, in an indirect way, to does not extend beyond the surface on which the perimeter the mechanics of the rock, which showed that the quality boreholes lie, hence t h a t the blast, when considered as an of the blasting, defined in the previous sections, and the excavation device, did not remove more rock than desired; quality of the rock were in some way connected. As a moreover, HCF is an indirect indicator of a low unblasted consequence, the hypothesis t h a t the entity of the extrarock disturbance. Indeed, in order to obtain a high HCF profile could be influenced by the same parameters that value, the tensile stress occurring during the blast in the influence the stability was evaluated, e.g., depth of the inter-holes rock bridges should not greatly exceed the underground void, rock mass resistance characteristics, tensile strength of the rock. This is a condition for a correct frequency and orientation of the discontinuities, rough"fracture guidance" by contour holes, implying a comparaness level, grade of alteration of the discontinuity surface, tively low stress level on the contour surface, and obviwater flow, etc. ously, beyond the contour surface in the rock that stays in The research carried out has confirmed, at least in a place. general set-up that must be further defined, the relationships between the technological parameters and the quality of the rock mass, as is discussed below. The Role of the IVlechanical Characteristics of
the Rock on the Quality of Blasting The energy released by the explosive to the outside of the volume to be blasted not only creates an extra-profile, but also diminishes the stability of the rock mass as it causes fracturing around the void. For example, according to Page
Volume 13, Number 1, 1998
Research Methodology The research that was carried out basically examined the checking criteria of the quality of the round based on the following parameters:
TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY83
1. the ratio of the real to the theoretical pull of the round (~); 2. the value of the overbreak (OB); and 3. the ratio of the length of the half casts of the holes present in the contour after blasting to the total length of the contour boreholes (HCF). These parameters were determined from a certain number of tunnels that had different geomechanical characteristics, typology and drilling-and-blasting techniques conditions. The quality of the rock mass was also established through the use of suitable geomechanical classifications. The entity of the extra-profile was, in particular, determined by measuring the real profile after blasting: • in the case oflarge tunnels, using infrared distometers mounted on electronic theodolites, or, alternatively, using ultrasonic distometers; and • in the cross-sections of the middle sized tunnels (2025 m2), using telescopic stadias; and then calculating the difference between the measured values and the theoretical profile. To obtain a suitable mean value, two determinations of the profile were carried out in two sections per round. The HCF was calculated simply by measuring the length of the half casts of the holes observed around the tunnel walls after blasting and calculating the ratio between this value and the total length of the contour boreholes. The value of ~ was determined by measuring the distance between the mean positions of the face before and after blasting and mucking, and then calculating the ratio between this value and the theoretical one.
Operative and Geomechanical Conditions The operating conditions of the sites where the data were collected and measured varied from site to site: 13 out of 17 cases were mining tunnels, opened for the development of mining; the other cases were civil tunnels. In 1 case, an explorative pilot tunnel was excavated with TBM and then enlarged to full section with the use of explosives. All the tunnels belong to the category of medium or large section area (from 20 to 90 m2). This can hardly be considered a limitation because in small section tunnels the problems created by overbreak are very limited. In 11 cases, the cut of the round was with parallel boreholes with the central boreholes unloaded; in 5 cases, it was with convergent boreholes. In 11 cases, the smooth blasting technique was used, and in 5 cases the drilling was done by computerised jumbo. The term "computerised jumbo" actually encompasses two kinds of machines: those where the design drilling pattern and the actual drill positions are simply displayed to the operator, who is charged with manually setting the drills at the correct places (referred to as a "semi-computerized" machine in Table 3); and those where all operations are automatically performed according to the program stored in the machine. Computerised drilling warrants centimetric accuracy; but accuracy has a cost, apart from the higher cost of the machine: something has to be paid in terms of drill set time. The latter drawback still hinders the development of computerised machines in mine tunnel driving, where accuracy is not rated as highly as speed. Nonetheless, some of the cases investigated (cases A/l/, C/2/,D/2/,E/2/, in Tables 2 and 3) refer to mine tunnels driven by computerised jumbo. Analogously, the nature of the rock was different for each tunnel, both in terms oflithology (marbles, micaschists, gneiss and calcareous schists) and in terms of position of the bedding and/or the discontinuity. The laboratory uniaxial compressive strength registered in the various cases varied between 65 and 200 MPa, and Rock Mass Rating (RMR) according Bieniawski was between from 30
84 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
and 86, which cover a large range ofgeologicaYgeotechnical in-situ conditions. As far as the possible correlation between the rock characteristics and the quality of the mining work is concerned, an attempt was made to verify any possible dependence between the technological parameters (~, c, OB and HCF) and the quality of the rock. The Rock Mass Rating value, according to Bieniawski's classification, was assumed as the index of the quality of the rock mass because it is widely used in tunnel design. Moreover it was directly available in some cases, as it is quoted successively: this was not the case for Q factor, another common parameter in tunnelling. The values of RMR were obtained in the various examined cases: 1) from the determinations carried out by the contractors (in this case the rock mass class, relative to the investigation area, was furnished); and 2) from direct observations by the CSFR researchers. In the first case, the central RMR value relative to the class to which the rock pertains was assumed for the elaboration. In the second case, because the Rock Quality Designation (RQD) value was not always available, the geostructural data according to Wickham's classification (Wickham et al. 1974) was first used, and then the RMR value was obtained using the relation RMR = (RSR - 12.4)/0,77 (Bieniawski, 1989). Analogously, the evaluation of the resistance to the laboratory uniaxial compression was carried out using: 1) the data obtained from the contractors from the various sites; 2) the data obtained directly in the laboratory by the research group; and 3) the measurement of the Point-Load-Test resistance index on shapeless pieces. In the first case, it was unce~ain how the tests were carried out; however, it was believed that they were carried out according to ISRM recommendations. In the third case, data were s tandardised to that of Is0and then the compressive strength was calculated using the multiplicative coefficient 24.
Research Results The research has shown the possibility of controlling blasting quality through the measuring ofOB, HCF and q, and has also permitted the identification of some correlation between these parameters. In particular, the relationship between OB and HCF can be noted (Fig. 2). The points are distributed in two well separated families: on the left, the points that refer to blasts that are close to technical perfection (high HCF and low OB); and on the right, those that represent less perfect results (high OB and average HCF). The plot of~ vs. RMR data (Fig. 3) shows a wide scatter. This is expected, since different r<)und types and excavation cross-sections are involved, in addition to RMR and drilling accuracy variability from one case to another. Apparently,higher RMR values adversely affect ~, which simply means that tough rock is usually harder to blast than soft rock. Computerised drilling tends to assure higher efficiency; when the faces are inspected upon blast, it can be seen that with computerised drilling, the hole bottoms always exactly lie in the same plane, orthogonal to the tunnel axis, which seldom occurs with manually operated jumbos. This is probably the main reason for the very good efficiency observed, Regarding the powder factor, it was not possible to show any particular correlation, even of a generic type, with the quality of the rock. This could be due to the fac.t that c is dependent, to a great extent, on the tunnel section ( Fig. 4,
Volume 13, Number 1, 1998
Table 2. Details of the geotechnical parameters of interest for the cases investigated. Co: Uniaxial Compressive Strength (Tunnel N: from Point Load Tests) S: Area of the tunnel cross-section. R=: Rock Mass Compressive strength.
Tunnel (Ref.)
Rock
Rock mass Rating
R am
S
CO
m2
MPa
56 56 60
65 65 65
58
2
60
65
50
1
60 60 60
65 65 65
45
0.7
Porphyritic gneiss
76.8
200
70
38
i/1 /
Chloritic Micaschist
93
//1 /
Phyllitic Micaschist
93
100
30
2
M/3/
Calcschist
76
40-50 30
1
22.2
90
55
7.4
22.2
185
86
85
22.2
90
64
12
22.2
185
86
85
22.2
185
62
22
22.2
90
64
12
A/1/ B/1/
C/2/ D/2/
Marble
E/2/ F/2/ G/2/
H/1 /
N/4/ Ch.337348 m 348-372 m 372-387 m 387-415 m 41 5-448 m 448-531 m
Micaschist
N o t e s o n T a b l e 2: Reference numbers (see References)
/1/Castano, Grad. Thesis. /2/Blengini, Grad. Thesis. /3/Salvaia, Grad. Thesis. /4/Magro, Grad. Thesis.
Volume 13, Number 1, 1998
Condition of rock and joints surface
(MPa).
30
Undulating, slightly altered smooth, moderately altered ;Smooth, altered
Smooth or slightly undulating withouth filling
Smooth, altered
Smooth, withouth filling
Moderately foliated, slighly altered Massive, good conditions Slightly foliated, slighly altered Massive, good conditions Moderately fractured, altered Slightly foliated, slightly altered
Tunnels are characterized by: • Low or medium overburden. • A - G: tunnels pertain to an underground quarry that exploits a white marble for chemical calcium carbonate; marble is generally homogeneous, gently stratified. * H - M : road tunnels; in particular, tunnel I was excavated by blasting after the construction of a pilot hole (dia. 3.9 m) with a T B M . • case N: pertains to a talc mine; tunnel excavated in micaschist barren rock. • Explosives used were: Gelatina 1 (a gelatine-dynamite) for cut holes; Gelatina i and Tutagex (a type of Water-Gel) for the production holes; and Profilx or Emuldin (slim charges) for the contour holes.
TUNNELLINGAND UNDERGROUNDSPACETECHNOLOGY85
Table 3. Details of the technical parameters of interest for the cases investigated. OB: Overbreak. HCF: Half Cast Factor (see text), q : see text. c : powder factor. Comp. : computerised jumbo.
Tunnel (Ref.)
OB
HCF
vI
m
%
%
kg/m 3
0.23 0.16 0.17 0.17 0.24 0.28 0.54
31 43 27 18 13.5
9
100 85 98 99 87 96 87
0.08
50
I/1/
0.27
L/1/
M/3/
A/1/ B/1/ C/2/ D/2/ E/2/ F/2/ G/2/
H/1 /
N/4/ Ch.337348 m 348-372 m 372-387 m 387-415 m 415-448 m 448-537 m
Drill. Machine
Controlled blasting (smooth blasting)
2.5 2.25 1.85 1.85 1.85 1.85 1.85
Comp. Man. Comp. Comp.
N Y N N N
Man. Man.
N N
!II // /I //
94.2
1.27
Comp.
Y
V
42
100
1.1
Man.
!Y
0.75
35
91.1
1.65
Man.
Y
IV
0.27
38
91.7
1.4
Semicomp.
Y
V
Y
// //
8
89.7
2.41
0.13
24
85
2.41
Y
//
0.26
27
88.7
2.41
Y
//
0.16
21
85
2.41
Y
//
0.37
10
90
2.41
Y
//
0.24
25
88.7
2.41
Y
//
Reference numbers (see References) /2/Blengini, Grad. Thesis. /3/Salvaia, Grad. Thesis. /4/Magro, Grad. Thesis.
Man.
!// V //
0.4
N o t e s o n T a b l e 3:
/1] Castano, Grad. Thesis.
Comp.
Type of cut
Tunnels are characterized by: * L o w or m e d i u m overburden. • A - G: tunnels pertain to an underground quarry that exploits a white marble for chemical calcium carbonate; marble is generally homogeneous, gently stratified. • H - M : road tunnels; in particular,tunnel I was excavated by blasting after the construction of a pilothole (dia. 3.9 m) with a T B M . • case N: pertains to a talc mine; tunnel excavated in micaschist barren rock. • Explosives used were: Gelatina 1 (a gelatine-dynamite) for cut holes; Gelatina 1 and Tutagex (a type of Water-Gel) for the production holes; and Profilx or Emuldin (slim charges) for the contour holes.
86 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Volume 13, N u m b e r 1, 1998
Symbols: 50 45
[] Cases in which smooth blasting and manually controlled jumbo were used;
HCF ( % } o
40
Q
35
• Cases in which smooth blasting was not used; computerised jumbo;
30 n [3
25
Cases in which smooth blasting and computerised jumbo were used.
(3
2015-
[]
10-
o
5 0 0
I
I
I
t
I
I
I
0,1
0,2
0,3
0.4
0,5
0,6
0.7
0,8
O B (m}
Figure 2. Trend of the OB values in function of HCF. 1 O0 98
-q
Symbols:
u
[] Cases in which smooth blasting and manually controlled jumbo were used;
96 94
Cases in which smooth blasting was not used; computerised jumbo;
92 90
[]
o
Cases in which smooth blasting and computerised jumbo were used.
88 86 []
84
I
10
--~
20
t
I
30
40
I
I
F
I
50
60
70
80
90
RMR
Figure 3. Trend of ~ values in function of the RMR. after Mancini et al. 1.995) and to the fact that the operative factors (explosive type, round type and type of profiling of the contour) were different for each case. OB and HCF turn out to be greatly dependent on the rock class, notwithstanding the role of the different parameters. (Figs. 5 and 6). As an alternative to the use of RMR as the rock characterising factor, the rock mass strength according to Hoek and Brown's criterion (Hoek and Brown 1980) was referred to. In this research it was used as a indicator parameter (i.e., one that is able to furnish the resistance level of tile mass), rather than as a characteristic of the rock in conformity with the way of working of the explosive. Rock mass strength ( R ) can be calculated by Hoek and Brown criterion:
Volume 13, Number l, 1998
6.00 ] o • 5.00
• Parallel holes cut o Inclined holes cut
...
-60
~ 2.00
~
•
•
.oo 0.00
. /
0
0
I
I
t
I
I
I
I
I
20
40
60
80
100
120
140
160
S (-,^2)
Figure 4. Specific consumptions of the explosives (hg / m 3) as a function of the area of tunnel section (S) (from Mancini et al. 1995).
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY87
0,8OB(m)
o
Symbols:
0,7"
[] Cases in which smooth
blasting and manually controlled jumbo were used;
0,60,5-
Cases in which smooth blasting was not used; computerised jumbo;
0,40,3-
[]
o
•
Cases in which smooth blasting and computerised jumbo were used.
8
0,20,1 0 0
I
I
I
I
I
t
10
20
30
40
50
60
t
7O
80
90
RMR
Figure 5. Trend of the total extra-profile in function of RMR.
50 HCF(%}
Symbols:
45" []
Cases in which smooth blasting and manually controlled jumbo were used;
40Q
35 30
Cases in which smooth blasting was not used; computerised jumbo;
25" 20
Cases in which smooth blasting and computerised jumbo were used.
15, 105 O
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
RMR
Figure 6. Trend of the HCF in function of RMR. ~1 =%+ (mCo a3+ s Co2)1~ (1) where a 1 and ~ are, respectively the major and minor principal stress at failure; Co is the uniaxial compressive strength of the intact rock material; m and s are constants dependent on the properties of the rock and the extent to which it is fractured. The resistance of the mass, in the absence of confining actions (in this case neglected, as the examined tunnels were not deep) can be written as: R = C J s , where s = exp (RMR- 100)/6, in the case of the contour damaged by blasting (non-controlled blasting), and s = exp (RMR - 100)/9 in the case of controlled blasting; CO is the laboratory uniaxial compression strength (see also Bieniawski 1989). In practice, using the Hoek and Brown criterion the explosive action in the rock is dominated by a failure criterion that takes into account both the rock quality (expressed by RMR in s formulas) and UCS of the intact rock (C). Moreover the added weight of C allows us to show better the sensitivity of OB to the laboratory and in-mtu quality of the rock. Finally, the operative condition of using or smooth blasting or not (as well as the result of a low or high rock wall damage) is taken into account in the equation expressing the parameter s. o
o
88 TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY
.
Unfortunately the range of this research is not extended to deep tunnels, but it is possible to state that the Hoek and Brown criterion can be used where a confining action caused by a high border plays a role that can be taken into account through the parameter m for the rock mass in Eq. (1). Figure 7 shows the trend of the values of the extra-profile in function of the Rock Mass strength ( R ) .
Conclusions Through the analysis of various cases of tunnels driven in different rock masses, of different types (civil and mining) and in diverse operative conditions, the research has permitted some preliminary conclusions. The parameters that can evaluate the quality of mining work with explosives have been defined from a technological point of view. The adequacy of these same parameters (TI, HCF and OB) has been evaluated as positive. The research has indicated the basis for the definition of limit values from which it would be unlikely to stray in order to maintain the correctness of excavation work, and the optimal operative conditions capable of keeping the parameters within the assigned limits.
V o l u m e 13, N u m b e r 1, 1998
0,8"
o
OB (m)
0,7 "
Symbols: [] Cases in which smooth blasting and manually controlled jumbo were used;
0,60,5
• Cases in which smooth blasting was not used; computerised jumbo;
0,40,3-
• Cases in which smooth blasting and computerised jumbo were used.
0,2' 0,1 0 0
I
I
I
10
20
30
I
L
40 50 Rock Mass Strength
I
I
I
60
70
80
90
Figure 7. Trend of the extra-profile in function of the rock mass strength. At the same time, the influence of the rock mass characteristics (RMR) on the values of the technological parameters has been verified. The dependence ofT1, HCF and OB on the RMR is seen in the diagrams given in the previous sections. In some cases, the dispersion of the values is noteworthy, but this can be substantially attributed to the differences in the opera~ive factors (e.g., the drilling method used. In some cases, using the same operative factor (e.g., making the boreholes with the computerised jumbo), the correlation between the parameters appears to be promising, even though it is n.ot yet possible to make a j u d g m e n t founded on a large statistical basis. The possibility of using the rock mass resistance (R_), as defined by Hoek and Brown s criterion, was tested with good results from among the alternatives to the use of the RMR factor to qualify the rock in comparing the blasting works with explosives. It therefore results t h a t the influence of the rock quality (however defined) carLnot be neglected when fixing the blasting work quality threshold. The results obtained usually allow one to assert t h a t the better the rock, the better the quality of the blasting. The data analysed ';uggest that: 1) the placement of the boreholes in the correct way and as close to the project as possible improves q and OB; 2) the technically acceptable value of q should not fall below 0.9, even in the case of best quality rock; 3) the technically acceptable extra-profile m u s t not exceed 0.4 m in rocks with RMR above 50. Although this is an independent result, it is in a g r e e m e n t with w h a t has been established by the previously indicated standards; 4) the use of sophisticated technology (such as the use of computerised j u m b o or even smooth blasting) in rocks of poor gecmechanical quality can be fruitless; 5) the lower limit of OB for good quality rocks with optimal drilling and charging can be reduced to the mere look-out of the contour holes; 6) HCF is a sensitive index of the drilling quality and is positively linked[ with the quality of the rock. To establish the "best attainable result" as a function of the rock mass quality :requires a far larger statistical basis t h a n is currently available. Using a population of 17 cases and examining the two extreme cases - - the one in which the best technology is used with the best rock (i.e., the case ,
Volume 13, Number 1, 1998
.
.
.
tall
H/l/ in Tables 2 and 3: computerised drilling, profile charges in the contour holes, and RMR 70), and the best technology used with the worst rock (i.e., the case M/3/: computerised drilling, profile charges and RMR 30) - researchers found t h a t in case I-I/l/the quality indicators are HCF= 50, OB = 0.08, while in case M / 3 / t h e quality indicators are HCF = 38, OB = 0.27. If these two cases can be considered typical, the mere RMR improvement from 30 to 70 apparently accounts for a 30% increase in H C F and a 340% decrease in OB when the supposed "best available technology" is employed. Acknowledgments
This research is a part of the Strategic Proj ect"Tunnels," and has been carried out with the contribution of the C.N.R. (National Council for Research). References Bieniawski, Z. T. 1989. Engineering Rock Mass Classifications, 251. New York: Wiley and Sons. Blengini, G. A. 1994. Evoluzione del concetto di "Abbattimento controllato" nell'ingegneria mineraria e civile. Grad, Thesis, Politecnico di Torino. Castano, D. 1992. Analisi della precisione di abbattimento in galleria. Grad. Thesis, Politecnico di Torino. Hoek, E. and Brown, E. T.1980. Underground Excavation, 527. London: IMM. Holmberg, R., Larsson, B., and Sjoberg, C. 1984. Improved stability through optimized rock blasting. Proc. lOth Conf. on Explosives and Blasting Techniques, Orlando, U.S.A., 166-171. Magro, A. 1995. Analisi di fattori influenti sulla qualit~ del risultato hello scavo di gallerie con esplosivo. Grad. Thesis, Politecnico di Torino. Mancini, R., Gaj, F., and Cardu, M, 1995. Atlas of blasting rounds for tunnel driving. Internal Report, Dip. Georisorse e Territorio del Politecnico, Torino. Mancini, R. et al. 1996. Technical and economic aspects of tunnel blasting accuracy control. Tunnelling and Underground Space Technology 11(4), 455--463, Page, C. H. 1987. Controlled blasting for underground mining. Proc. 13th Conf. on Explosives and Blasting Techniques, SEE, Miami, U.S.A., 33-48. Salvaia, E. 1992. Controllo del profilo nello scavo sotterraneo con esplosivi. Grad. Thesis, Politecnico di Torino. Wickham, G. E., Tiedemann,-H. R., and Skinner, E. H. 1974. Ground support prediction model, RSR Concept. Proc. Rapid Excavation Tunnelling Conference, AIME, 691-707.
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