06 nuevo método para obtener un criterio de diseño Óptimo de pre-corte considerando las...

8/10/2019 06 Nuevo Mtodo Para Obtener Un Criterio de Diseo ptimo de Pre-corte Considerando Las Propiedades Del Macizo Rocoso - A. Gonzlez, C. Muoz & a

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New method to obtain optimum pre-splitting design criteria consideringthe rock mass properties.

lvaro Gonzlez, Carlos Muoz1, lvaro Andrades

BBS & Technology Management, Orica Mining Services-LATAM

RESUMEN

The pre-split practices is the most used technique in the named controlled blasting. The appropriateestimations of the pre-split blasting design impact directly in the stability and economics results of themine operations. Current methods for pre-split blasting design are based only on intact rocks propertieswithout consider the structural and geotechnical properties of the rock mass, for that reason theapplicability is quite restricted to rock mass of good quality. In this work a new method, called dynamic

method is presented. The dynamic method utilized a direct measurement to evaluate the real conditionof the rock mass through the in-situ p-waves velocity in the same direction of the pre-split holes line. Thevalidation of the method proposed was done in 03 open pit mines located in northern Chile. The dynamicmethod clearly shows how the fractures frequency and geotechnical quality of rock mass control theefficiency of pre-splitting and therefore the level of damage induced over slopes and benches.

1. INTRODUCTION

The pre-split blasting is the most used technique in order to control the induced blast damage into slopesand benches. This blasting practice is an important element in the slope creation process and thecalculation of the optimal configuration impact directly in the stability and economics results of the mineoperations. Pre-split design obtained with the current methods often move away enough of the practical

reality and it is finally worked on the basis of the technical-economical parameters and the experience ofthe professional responsible for the mine planning.

Basically the pre-split practice consist of detonates two simultaneous de-coupled explosives charge spacedto a S distance, with the objectives of to obtain a pre-split plane that permit to reduce and control theseismic vibrations from the rock blasting. The detonation of the explosive charges produces a stress that itis transmitted to the rock massif in form of shock wave and gas pressure. The last one is considered as theresponsible for the creation of the pre-split planes. Figure 1

1Correspondingautor

Emailaddress:[email protected](CarlosMuozL).


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Figure 1 Pre-split plane creation process obtained by detonates two simultaneous (A and B) de-coupledexplosives charge spaced to a S distance into a rock mass.

The most used formula for to obtain the S distance between two de-coupled charges at which the pre-splitplane can be obtained is based on Sanden (1974)

Where;

borehole spacing (m)borehole radius (m)borehole pressure (Mpa)tensile strength of intact rock (Mpa)

The borehole pressure can be obtained according to the Chiappetta (2008) (Canadian Method) consideringthat gases are expanding adiabatically at the moment of the detonation

Where;

borehole pressure (Mpa)explosive density (gr/cm3)velocity of detonation (m/s)explosive charge diameter (mm)borehole diameter (mm)

C Percentage of the total explosive column loaded, expressed as a decimal.

Equation 1

Equation 2


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Recently Hustrulid (2007) considered that gases are expanding isothermally and propose the followingexpression according to Utah/Niosh Method;

Where and a can be obtained by;

And Pe, ae,


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In this work, a new method to estimate the spacing of the pre-split holes is presented. This new methodwhich has been called Dynamic method, was developed based on two important changes applied to thecurrent methods used in the pre-split design. The dynamic method to pre-split design follows very closelythe work of Hino (1959), which was discovered only recently, to calculate the borehole pressure inisothermal conditions based upon the co-volume concept introduced by Cook (1958).The way how the

induced stress decay through the rock mass is described by semi-empirical method derived fromtheoretical (hydrodynamic) modeling propose by Liu and Katsabanis (1995) and Onederra et al (2004),which consider that the decay of the borehole pressure depend of a decay factor which relates theexplosives properties, p-wave velocity and dynamic elastic modulus of the rock mass. Hereby structuraland geotechnical properties of the rock mass can be considered. Practical example of the new proposedmethods is included.

2. DEVELOPMENT OF DYNAMIC METHOD FOR PRE-SPLITING BLASTING

The dynamic method to pre-split design follows the work developed for Liu and Katsabanis (1993) andOnederra et al (2004). The way how the induced stress decay through the rock mass is described by semi-

empirical method derived from theoretical (hydrodynamic) modeling which consider that the boreholepressure decay depend of a decay factor which relates the explosives properties, p-wave velocity anddynamic elastic modulus of the rock mass, in this form the borehole pressure not only decays with thesquare distance as is considered in the current methods.

The pressure decay proposed by Liu and Katsabanis is expressed in the following expression as tangentialstress induced by the borehole pressure related to detonation process

Where;

tangential stress transmitted to rock mass (Mpa)borehole pressure (Mpa)

r distance at point of evaluation (m)borehole diameter (m)decay pressure factor

As described above the pressure decay factor (Onederra et al., 2004) is a function of rock mass andexplosive properties which is given by:

Where;

decay pressure factorDynamic Youngs modulus (GPa)P-wave velocity (m/s)velocity of detonation (m/s)

Equation 9

Equation 10


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The total force acting (product of the action of the energy produced in the explosive detonation) can beobtained by integrating the tangential induced stress equation with respect to distance r and thenevaluating the integral for r extending from radius a to infinity.

Or

The decay factor is a negative number, with a simple substitution it can be written as:

Evaluating between ro y :

Therefore, the total tangential induced stress of a pre-split hole can be written as:

In pre-splitting, a series of closely spaced blast holes are drilled along the pre-split line. The separationbetween the centers of the blast holes (the hole spacing) is S. The initiation of the explosives is planned so

that all holes detonate as closely as possible to the same instant in time. This means that all of the blastholes would be concurrently pressurized to Pb. The result would be that the tangential stress fields inducedby adjacent holes would overlap.

Due to there are two holes detonating at the same time, the total tensile force, is given by:

Equation 11

Equation.12

Equation 13


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On the other hand, the resisting force of a rock mass subjected to a stress field just before that pre-splitplane is created is given by:

Where;

resisting forceborehole spacing (m)borehole diameter (m)tensile strength of intact rock (Mpa)

At equilibrium, the driving force will just be matched by the resisting force. Hence,

As = - , we obtain the following expression for the new Dynamic Method proposed

Where;

borehole spacing (m)borehole diameter (m)borehole pressure (Mpa)tensile strength of intact rock (Mpa)decay pressure factor

Equation 14

Equation 15


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3. IN-SITU P-WAVE VELOCITY AND DYNAMIC YOUNGS MODULUS AS ROCK MASSPROPERTIES IN THE DYNAMIC METHOD

According to Eq.15 the spacing between two pre-split holes depend on the pressure decay factor that is

controlled by P-wave velocity (Vp) and Dynamic Youngs modulus (Ed) as rock mass properties. Crossholes techniques is used to evaluate the real condition of the rock mass through the propagation of the Vpin the same direction of the pre-split holes line. This parameter is used to determinate the Ed according to:

Where;

dynamics young modulus (Gpa)rock density (gr/cm3)P-wave velocity (m/s)

S-wave velocity (m/s)

As Vp can correlated with RQD (%) and therefore with fractures frequency (Zhang, 2005), it is possible toget an estimation of the degree of fracturing of the rock massif and to evaluated the influence of theintensity of discontinuities on the pre-splitting design criteria and results. For example Deere et al., 1967proposed the following relation:

Where;in-situ P-wave velocity (m/s)

Laboratory P-wave velocity (m/s)

The Fig.4 shows the influence that the Vp, for the same rock but with different RQD, have in thedetermination of the spacing between pre-split blast holes with 3 different method. It can be appreciatedthat the Vp is not considerate in the Canadian and Utah/NIOSH methods, due to the values are constantfor any Vp. The Graphic also shows that for high values of Vp (high RQD values) the Dynamic Methodreduce the spacing between blastholes due to the concept of attenuation induced strength, while for lowVp (low RQD values) the spacing is reduced too due to higher fracture frequency that affect the decayfactor. Figure 2

Equation 16

Equation 17


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Figure 2 Vp influence in the pre-split holes spacing determination with Dynamic Method.

4. PRACTICAL APPLICATION OF THE PROPOSED DYNAMIC METHOD

The proposed Dynamic Method was validated in three open pit mines located in northern Chile (Romeral,Los Colorados and Candelaria) by means of designing accomplishment. The results obtained were quiteencouraging and show that the dynamic method gives a pre-split spacing that allows being applied inpractical reality. The validation realized demonstrated that the pre-split spacing used which gave goodresults and the calculated ones with this method were quite close.

In this work is presented a practical application of the method carried out in Los Colorados Mine

The parameters considered are (Table 1, 2, 3):

Table 1 P-wave velocities obtained for the cross hole technique at the study area.


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Table 2 Explosives and rock properties considered.

Considering the equations Eq.1, Eq.2, Eq.3, Eq.10 and Eq.15 we obtain;

Table 3 Pre-split spacing evaluated by Canadian, Utah/Niosh and Dynamic methods.

Figure 3 Pre-split spacing determination with Dynamic Method.

As show in Table 3 current methodologies give values of 1.2 m and 0.9 m, which are too far of the realvalue utilized (1.8 m) which gives good result (Image 1). The dynamic methodology gives us spacing

between pre-split holes of 1.9 m, which gave good result by means of designing accomplishment andstability achieved. If the spacing obtained by the Canadian and Utah/Niosh methods were considered,damage related to fractures induced and reopening of pre-existent structures can be achieved. Moreoverimportant additional expenses can be related.Image 1


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Image 1 Pre-spitting results at the study area with a spacing S=1.8 m.

CONCLUSIONS

For pre-split design the current methods are based only on intact rocks properties without consider thestructural and geotechnical properties of the rock mass, for that reason the applicability is quite restrictedto rock masses of good quality. Moreover current methods assume a constant decay of borehole pressurewith the only dependence of square distance.

The new method proposed considers the in-situ P-wave velocity (Vp) and Dynamic Youngs modulus(Ed) as a relevant variables into the borehole pressure decay factor. In this way the local properties of rockmass are evaluated by cross hole techniques, therefore the spacing of pre-split holes can be obtained fordifferent geotechnical domains.

The validation realized demonstrated that the pre-split spacing used currently in open pit mines were quiteclose that ones calculated by dynamic methods. Therefore the new proposed methods are quite close withthe operative practical reality.


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REFERENCES

Chiappetta, F. (2008). Efficient and Practical Solutions in Drilling and Blasting .Orica Chile 2008-Presplitting.Technical Presentation.

Deere, D. U., Hendron, A. J., Patton, F. D. and Cording, E. J. (1967). Design of surface and near surfaceconstruction in rock. Failure and Breakage of Rock, Proc. 8th U.S. Symp. Rock Mech., Ed: C. Fairhurst, 237-302.

Esen, S.(2004). Statistical Approach to Predict the Effect of Confinement on the Detonation Velocity of CommercialExplosives.Rock Mechanics and Rock Engineering, 37 (4), 317-330.

Esen, S., Souers, P.C.,Vitello, P. (2005). Prediction of the non-ideal detonation performance of commercialexplosives using the DeNE and JWL++ codes. International Journal for numerical methods in engineering, 64,18891914.

Hino, K. (1959). Theory and Practice of Blasting, Nippon Kayaku Co., Ltd. 189 pp.

Cook, M.A. (1958). The Science of High Explosives, Reinhold Publishing Corporation, New York. 440 pp.Hustrulid, W. (2007). A Practical, Yet Technically Sound, Design Procedure for Pre-Split Blasts. In:Proceeding ofthe annual conference on explosives and blasting technique, 01, 1-34.

Liu, Q., Katsabanis, P.D.(1993). A theoretical approach to the stress waves around a borehole and their effect onrockcrushing. In: Proceedings of the Fourth InternationalSymposium on Rock Fragmentation by Blasting Fragblast-4, 1993, 916.

Liu,Q., Tidman, J.P.(1995).Estimation of the Dynamic Pressure around a Fully Loaded Blasthole. CANMET-MRL1995

Mockoviakov, A., Pandula, B (2003) Study of the relation between the static and dynamic moduli of rocks. METALURGIJA 42(1), 37-39.

Onederra, I., Esen, S. , Bilgin, H.A. (2003) Modelling the size of the crushed zone around a blasthole. InternationalJournal of Rock Mechanics and Mining Science 40 (4), 485-495.

Onederra, I., Esen, S., Jankovic, A. (2004). Estimation of fines generated by blasting applications for the miningand quarrying industries.Trans. Inst. Min. Metall. A , 113 4: A237-A247.

Sanden, B.H. (1974). Pre-Split Blasting, unpublished MSc Thesis-Mining Engeneering Department, QueensUniversity. 122 pages.

Zhang, L. (2005).Engineering Properties of Rocks. Elsiever, 308 pages.

06 nuevo método para obtener un criterio de diseño Óptimo de pre-corte considerando las...

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