CRITERION AND

FOR ASSESSING

A METHOD

THE FRACTURABILITIES

OF F I S S U R E D

ROCKS

FOR ITS DETERMINATION

O. S. M e c h i k o v , V. F. M a k a r e v i c h , a n d M. D. S e d l o v

U D C 622.83

The fracmrabilities of rocks during blasting are determined not only by the Protod'yakonov hardness,but also by the degree of development of primary fissUration. Attempts to express this degree by a quantitative index usually reduce to finding the fissure frequency in a particular direction or the mean distances between the fissures. The mean distance between the fissures, or the m e a n cleavage, have been used by many investigators as a criterion of the natural fissuration [1]. However, experimental work has not confirmed the presence of a consistent and u n a m biguous relation between this criterion and the fracturability of a rock during blasting. This is due to the fact that the mean cleavage only comprises the number of fissures in a particular plane of exposure and does not clearly typify the characters of these fissures or the lengths to which they run into the rock as a whole. The intensity of d e velopment of fissuration in the in situ rock can be expressed indirectly by the thicknesses of the fissures in a plan of exposure [2]. However the inaccuracy of fissure thickness measurement technique leads to widely differing e x perimental resultsand makes them unreliable. In studying the relation between natural fissurationand the results of rock fracturing by blasting, we noted that for the same fissure frequency per unit area of the exposures the results of primary fracturing of these rocks were different; this was an indication of the marked influence of fissure character. This was also noted by Turuta and Bruyakin [3]; these investigatorsstate that the fissures'screening capacities, which prevent stresspropagation in the rock during blasting, are the main factor influencing the fractionation of fissured rock. This postulate is rather categorical, but it is correct in the sense that the fractionation of rock during blasting depends markedly on the screening of the stress waves by the natural fissures. The screening action of fissuration on an explosion, but a l so by their character and the degree to which they run into the in situ rock. These factors can be expressed by a common index, which we c a l l the disturbance of the in situ rock by different fissure systems. The disturbance of the solid rock by primary fissuration is the o v e r - a l l area of the visible fissures running through a unit volume of the rock. This area is composed of individual elements l o c a t e d in the delineation planes of the natural structural blocks of the rock. In the ideal case of maximum development of the plane of weakening of the rock mass, the unknown area would be equal to half the sum of the lateral surfaces of the structural blocks forming the mass. However, the structural blocks are not isolated individual bodies, but connected to a considerable degree of monolithic sectors, extending from one block to another. The distinct macroscopic fissures form only part of theplanes of fissuration. They only "outline" the structural blocks and thus form directions of weakening, along which cleavage and fracr~onation occur during blasting. The degree of weakening of a particular fissure system can be expressed by the proportion of the area due to distinct fissures of specific orientation out of the whole area for the given direction. The mean value of such an index for a l l the principal fissure systems characterizes the degree of "definedness " of the structural blocks within the rock mass; we c a l l this index K o the structuring coefficient of the rock mass. The distinct fissures, formed when blasting is begun, appear on the surfaces of the lateral faces of the rock fragments in the form of sectors covered with a layer of weathering products, formed at the points where the fissures become wider. The structuring coefficient can,therefore,be found by measuring the visuaUy discernible weathering zones on the lateral faces of the rock fragments and comparing their values with the o v e r - a l l lateral surfaces o f these fragments. The product obtained by multiplying the structuring coefficient by half the sum of the lateral surfaces of a l l the structural b l o c k forming a single unit volume is the index of the disturbance of the work by primary fissuration. The index of disturbance D n is the effective area of the macroscopic fissures in a unit volume oi~rock. Expressing VNIITsvetmet, Ust'-Kamenogorsk, Integrated Lead Enterprises~ Zyryanovsk. Translated from F i z i k o - T e k h nicheskie Problemy Razrabotki Poleznykh Iskopaemykh, No. 1, pp. 118-122, January-February, 1967. Original article submitted January 12, 1966. 93

e,X

5c t 20

~.=~ r 4v~2~s,s-6 9

the number of existing fissures per unit volume of the rock mass, this index is the concentrated quantitative characteristic of the degree of development of fissuration, the generalized criterion of the mass's primary fissuration. It can be introduced into the formulas for calculating the main drilling and blasting parameters.

The degree of disturbance of a rock mass by fissures is the result of the action on the rocks t o tectonic processes, displacement of the rocks by underground workings, the circulation of subsurface water, and weathering. The greater the subjection of the rock to the influence of these factors and the. greater its disturbance by primary fissuration, the lower will be its m e It! # chanical strength not only in large volumes of rock composed of structural Categories blocks, but also in individual specimens whose structures are monolithic and without visible fissures. Therefore, the index of disturbance is inversely Coefficient of rock fracturability C, related to the Protod'yakonov hardness; it gives a more accurate picture of plotted versus r, the yield of the course a rock's m e c h a n i c a l strength in the in situ state by taking into account the fraction obtained by an explosion, for fissured structure, whose influence on blasting performances is undisputed. conditions at the Zyryanovsk quarry. For the most accurate assessment of the yield of a rock mass to the crushing effect of an explosion, we must combine the hardness coefficient f and the index of disturbance D n into a single criterion of rock fracturability C:

I0 ~ .

~=5

C --

Dn /

(1)

It w i l l be seen from (1) that the scale of C should be the inverse of f, so that the index of disturbance must be expressed by a dn-nensioniessvalue, without changing its physical seine. For this purpose, i t is sufficient to express the number of fissures by the number of structural blocks per unit volume of the rock mass, If the mean volume of the rocks composing the rock masses is v (mS), their number per unit volume (for example, per 1000 m s of the rock mass) w i l l be n = lO00/v. Equation (1) thus becomes: C ~-~

I000 Kc

(2)

~f

where C is the fracmrabiliry of the rocks in 1 / k g / c m z, and Kc is the structuring coefficient of the rock mass. To determine the value of v in the block being blasted, we select the zone of the bench slope where the plane of exposure coincides with that of one of the main fissure systems. The spatial orientation of the structural blocks composing the rock mass,is established by visual observations. For p r a c t i c a l purposes, it is sufficient to distinguish in the bench slope merely one linear element of the structural blocks because the other two elements can be d e terrnined from the m e a n ratio of B e linear dimensions of the given rocks fragments [4]. A scale rod is then installed on the upper or lower lip of the bench. The angle of the bench slope is measured and stereoscopic photographs taken of this sector. After pairs of photographs have been processed, a 1 x I m m scale grid is marked out on one of them in such a way that its lines are p a r a l l e l to the main direction of the fissuration. The stereopair is then placed under a stereoscope and the o v e r - a l l length of the fissures of different orientation calculated. By dividing the count area by the sum of the lengths of the vertical and horizontal fissures, we find the m e a n horizontal and vertical dimensions of the structural blocks, We then use the ratio found for the dimensions of the blocks' faces, and the data on the blocks' orientations in the rock mass, to calculate the mean volume of the structural block. To determine Ire, the structurization coefficient of the rock mass, it is sufficient to analyze and measure the surfaces of the large rock fragments formed by blasting the block. From the surface layer of the rock brought down by the explosion were select fragments whose sizes are commensurate with the structural blocks. The minimum size of a fragment should be two to three times smaller than the mean size of a structural block, and the maximum size two to three times larger than a block. Bearing in mind that the crushability is usually assessed from the yield of oversized rock, in practice we must select such fragments. From the measured linear dimensions of these fragments we c a l c u l a t e the areas of their lateral surfaces and the areas of their lateral faces covered with ~i layer of weathering products and differing markedly from the zones of fresh cleavage formed during the explosion.

94

Fissuration Characteristics and Crushing Pattern of the a a i n Rocks of the Zyryanovsk Quarry Mean dimensions I of the s uctural ]blocks, m

~=.,o ~,

I

I 1"1 oO

o

o-=

I'C~

''"'

Rock t:~da 9.~

Unweathered porphyroids with average fissuration Weathered porphyroids with distinct fissuration Unweathered porphyrites with average fissuration Clay shales with distinct fissuration

..x;

0

[

~

~

~

-..-,

v, vI

~~ o [ ";

~r

~. 0

.

I I t

I

C:

C =

~

--"

I

I I.~

1.4a a.oo

o.as5 I o . ~ o

Io.a~ I ~1

~,.~

"C~

Io.o 1 l

,-

[ o

'ability

~,-'a .I

.

Cr,tor,on offractur-[~

..~

0

I a~.a

/

I

~

By this method, we analyzed the degree of fissuration of the main rock types of the Zyryanovsk quarry and compared the results with those of crushing by an e x p l o s i o n - a l l the drilling and blasting factors being kept constant (cf. the table). Crashing was assessed from the yield of + l m fragments, using the stereophotograph-count method [5]. The graph plotting these results (cf, the figure) indicates a definite relation between the coefficient of fracturability C and the percentage content of the coarse fraction in the spoil heap. This relation, established from the principle of least squares, is described by the equation of a hyperbola: r = 4.32 + 45,5

I_. C

(3)

The confidence limit O of this equation was found to be 1.78; this is much less than the lower l i m i t of a c curacy, equal to 5.12 [6], and therefore shows good agreement between the theoretical curve and the experimental data. Graphs of the criterior~.of fracturability, plotted for different drilling and blasting parameters, can be used for forecasting the yield of the course fraction during large-scale blasting in quarries. As an objective numerical c h a r acteristic of the fissuration and hardness of rocks, the value of C can be introduced into formulas for calculating rock breaking by blasting, so as to control the results of the explosion and obtain the required degree of crushing of the rock in the spoil. By numerically characterizing the yield of the rock ot the crashing action of the explosion, this criterion merits recognition as the principal measure for classifying rocks according to the ease of blasting. The graph plotting fragment size versus fracmrability (cf. the figure) indicates that when the rock being worked has C greater than 10,the yield of the coarse fraction is relatively small and does not vary greatly. Therefore, the control of the hole network parameters in such rocks should be governed not so much by the degree of crushing the rock mass, but by the way the floor of the bench behaves during blasting. In this connection the structure of the rock mass shows the feasibility of increasing the drilling and blasting parameters. On the other hand, with C less than 5, the emphasis must be on the size of the m a t e r i a l in the spoil. This conclusion agrees closely with results for the bal~ting of high benches with heightened blasting parameters at the Zyryanovsk quarry, where two experimental blocks of volume 53 and 72 thousand m s were worked. CONCLUSIONS 1. The criterion of fracturability C is a reliable numerical characteristic, of the yield of a rock mass to the crushing action of an explosion. 2. The criterion of fracmrability can greatly increase the efficiency of intelligent control of the main blasting parameters during the worldng of fissured rock. 3. This method of assessing the fissurability of rock provides a reliable quantitative characteristic of the rock's structure in the in sire state and can be used for improving d r i ~ n g and blasting in quarries and for solving other problems associated with the mining of fissured rock.

95

LITERATURE CITED 1. 2. 3.

4. 5.

6.

96

V.K. Rubtsov, The granulometric composition of rock masses worked by blasting. Paper from: Blasting, No. 57/15, The Development of Blasting in Mining [in Russian], Moscow, Nedra (1965). O.S. Mechikov, Photogrammetric method of assessing blasting performances and the influence of natural fissuration [in Russian], Nauchn. dokl. yysshei shkoly, Gornoe delo, No. 3, Moscow, Sovetskaya nauka (1958). N.S. Turuta and A. V. Bruyakin, Fracturing of fissured rocks by an explosion with different values of t h e changes. Paper from: Blasting, No. 57/14, The Development of Blasting in Mining.[in Russian], Moscow, Nedra (1965). L.I. Baron, Fragment Size and its Determination [in .Russian], MOscow, Izd. AN SSSR (1960). O.S. Mechikov, A. K. Bakhtin, and V. P. Kurlyantsev, The Stereophotographic-Count Method of Determining the Content of Oversize Fragments in a Rock Spoil [in Russian], Trudy AGMNII AN KazSSP,, 15_, Alma-Ata, Izd. AN KazSSR (1963). A. Dlin, Mathematical Statistics in Technique [in Russian], Moscow, Izd. Sovetskaya Nauka (1958).