AU J.T. 14(1): 56-60 (Jul. 2010)

Drillability and Strength Characteristics of Selected Rocks in Nigeria

Babatunde Adebayo*, Zacheus Olaniyan Opafunso and Jide Milliu Akande

Department of Mining Engineering, Federal University of Technology Akure, Nigeria

E-mail:

Abstract

This study examines drillability and strength characteristics of selected rocks in Nigeria. Eight rock samples were selected from different locations in the country and were tested in the laboratory for uniaxial compressive strength, tensile strength, hardness, and Drilling Rate Index (DRI) using 1,100 kN compression machine, Schmidt hammer, and miniature drill. The results of uniaxial compressive strength for the eight selected rocks vary from 89.90 - 124.71 MPa and were classified as very high compressive strength characteristics and the rock characteristics from Porphyritic Hornblende-Granite to Medium Biotite-Granite on the basis of point load strength index range from high to very high strength index. The hardness classification shows that the rocks characteristics vary from very strong to extremely strong rocks. The result of correlation showed that strong relationship exist between bit life and rebounds hardness Value with coefficient of correlation of R2 = 0.926. The drillability characteristics of the rocks vary from extremely low to low drillability based on the Drilling Rate Index (DRI) and drill bit life range from very low to extremely low life span.

Keywords: drillability, strength, characterization, rocks, hardness, schmidt hammer.

Introduction

Drillability is the resistance of rock to penetration by a drilling technique (Kramadibrata et al. 2001) and it is a term used to describe the influence of numbers of parameters on the drilling rate (drilling velocity) and the tools wear of the drilling rig. Penetration of quarriable rocks is influenced by rock properties as well as machine parameters. Also, two key parameters have proved to be most valuable: the (net) drilling rate in metres per minute (the drilling performance, derived from the time of drilling one single borehole); and the bit life span in metres per drilling bit that can be drilled in a homogeneous tunnel section (Thuro 1997).

However, the Strength of a rock has an appreciable influence on drilling force required. Therefore, to cause rock to break during

drilling is a matter of applying sufficient force with a tool to exceed the strength of the rock (Hartman and Mutmansky 2002). The behaviour of rock material under compression is important as the uniaxial compressive strength of intact rock is a basic parameter for rock classification and rock mass criteria and rock classification. Therefore, the strength characteristics of rocks are usually considered to be necessary for design of rock structures, stability of rock excavations as well as influence rock fragmentation in quarry and working of mine rocks (Ojo and Olaleye 2002). In addition Size of fragments may be influenced drillabilty of rocks (Plinninger et al. 2002). Generally, coarse-grained rock can be drilled more quickly than fine-grained varieties or those in which the grain size is variable (Liu et al. 2002) and it is a measure of brittleness (Thuro and Spaun 1996).

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Materials and Methods Uniaxial Compressive Strength

The test procedure was in accordance with ISRM (1981) and ASTM (2001) D 2938. The uniaxial compressive strength was determined using Eq. (1) as presented in Table 1: Co = P/A = P/(W.D), (1) where: Co = Uniaxal compressive strength (MPa); P = the applied peak load, (kN); W = Width of the sample (mm); D = Height of the sample (mm). Point Load Strength Index

Five rock samples were prepared for each rock type to the standard suggested by the International Society of Rock Mechanics (ISRM 1981) and the American Society for Testing and Materials International (ASTM 2001) D5731. The point load index was determined using Eqs. (2)-(3) for blocks as presented in Table 1: IS = F/(W.D), (2) where: IS = Point load Strength Index (MPa); W = Width of the sample (mm); F = Applied load at Failure (kN).

Equation (3) was proposed by Brook (1993) to obtain IS value for a standard diameter of 50 mm IS(50): IS(50) = f[F/(4W.D)]. (3)

The value of size correction factor was obtained from Eq. (4): f = (De/50)0.45. (4) Schmidt Rebounds Hardness Values

The rebounds hardness of the was done in accordance with ISRM (1981) and the results are presented in Table 1. Determination of Drilling Rate Index (DRI)

The brittleness test is basically an aggregate impact test and this was conducted in accordance with Norwegian soil and Rock Engineering Association (Borg 1988). The Siever J-Value test is a miniature drill test and

this penetration in 1/10 mm after 200 revolutions of the drill and S20 is defined as the percentage of material that passes through 11.2-mm sieve after 20 drops of 14-kg weight and Siever J-Value (SJ) as presented in Table 2. Field Measurement of Drill Bit Life

The bit life was determined on field by monitoring the total depth drilled before the bits were replace from different quarries in Nigeria and the results are presented in Table 5.

Results and Discussion

Table 1 presents the summary of strength properties and hardness of the selected rocks. Also, bit life was correlated with uniaxial compressive strength and the result of the relationship is shown in Fig. 1. It could be observed that polynomial relationship exist between bit life and uniaxial compressive strength expressed by Eq. (5) and weak relationship exist between them with correlation coefficient of R2 = 0.586: BL = -0.012Co3 + 4.051Co2 - 432.1Co + 15435,

(5) where: BL is bit life (m/bit); Co is uniaxial compressive strength (MPa).

Figure 2 shows correlation between bit life and point load strength index, the result showed that there is no correlation between bit life and point load strength index with coefficient of correlation of R2 = 0.001. Also, Fig. 3 presents the relationship between bit life and rebounds hardness value. The result obtained showed that a strong linear relationship exists between them with coefficient of correlation of R2 = 0.926 and the equation of their relationship is expressed in Eq. (6): BL = -7.313RH + 566.9, (6) where: BL is bit life (m/bit); RH is the rebounds hardness value.

Figure 4 shows that the Drilling Rate Index (DRI), brittleness value (S20), uniaxial compressive strength and point load strength Index vary from one rock to another, it could

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therefore be concluded that rock properties varies widely.

Strength Characterization of Selected Rocks

The selected Nigeria rocks uniaxial compressive strength and point load strength index as a measure of tensile strength were characterized based on ISRM (1981) and Bell (1992), respectively. Table 3 shows that the uniaxial compressive strength of all the selected rocks have very high strength. The point load strength index of the selected rocks varies from high strength index for Porphyritic Hornblende-Granite to very high strength index for Coarse Biotite-Granite as presented in Table 4.

Fig. 2. Plot of bit life against point load strength index.

Table 1. Summary of strength parameters and rebounds hardness value of selected rocks.

S/N

Rock code

Uniaxial comp-ressive strength (MPa)

Point load

strength index (MPa)

Re-bounds hard-ness value

1 AD01 91.48 8.85 52.00 2 IK02 89.90 7.22 51.10 3 OR03 121.54 10.50 47.10

4 OS054 94.16 9.44 52.60

5 IF05 90.65 7.59 48.00 6 IB06 119.48 10.88 49.40 7 DE07 124.71 13.54 51.00 8 IJ08 90.60 8.19 49.00

Fig. 3. Plot of bit life against rebounds hardness value.

Fig 4. Plots of variation of some selected rock properties.

Fig. 1. Plot of bit life against unaxial compressive strength.

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Table 2. Brittleness value (S20) and drilling rate index (DRI) of rock samples for drillability characteristics.

Rock code Name of rock

Brittleness value (S20)

%

Siever J-Value

(SJ)

Drilling rate index

(DRI)

Drillability characterization

term AD01 Porphyritic Hornblende-Granite 46.0 0.7 30 Very low IK02 Micro Feldspar-Granite 50.0 1.0 39 Low OR03 Micro Granite 50.0 0.5 30 Very low OS04 Medium Feldspar-Granite 42.0 0.5 22 Extremely low IF05 Coarse Muscovite-Granite 50.0 1.0 40 Low IB06 Biotite- Hornblende Granite 42.0 0.5 22 Extremely low DE07 Coarse Biotite-Granite 48.0 0.5 30 Very low IJ08 Medium Biotite-Granite 42.0 0.5 22 Extremely low Table 3. Uniaxial compressive strength classification of selected Nigeria rocks.

S/N Rock code Name of rock Uniaxial compressive strength (MPa) Strength

classification 1 AD01 Porphyritic hornblende-granite 91.47 - 91.54 Very high 2 IK02 Micro feldspar-granite 89.90 - 90.03 Very high 3 OR03 Micro granite 120.86 -123.13 Very high 4 OS054 Medium feldspar-granite 94.09 - 94.30 Very high 5 IF05 Coarse muscovite-granite 90.51 - 90.86 Very high 6 IB06 Biotite-hornblende granite 119.40 -119.60 Very high 7 DE07 Coarse biotite-granite 124.55 -124.71 Very high 8 IJ08 Medium biotite-granite 90.13 - 90.60 Very high

Table 4. Point load strength classification of selected Nigeria rocks.

S/N Rock Code Name of rock Point load Strength (MPa) Strength

Classification 1 AD01 Porphyritic hornblende-granite 8.85 - 8.59 High 2 IK02 Micro feldspar-granite 7.10 - 7.34 High 3 OR03 Micro granite 10.43 - 10.57 Very high 4 OS04 Medium feldspar-granite 9.39 - 9.54 High 5 IF05 Coarse muscovite-granite 7.52 - 7.75 High 6 IB06 Biotite- hornblende granite 10.58 - 10.88 Very high 7 DE07 Coarse biotite-granite 13.03 - 13.54 Very high 8 IJ08 Medium biotite-granite 8.04 - 8.25 High

Table 5. Classification of bit wear rate and drill bit lifetime of selected Nigeria rocks.

Rock code Name of rock

Bit wear rate/crown life value (m/bit)

Wear rate term Drill bit life time term

AD01 Porphyritic hornblende-granite 192 Extremely High Extremely low IK02 Micro feldspar-granite 192 Extremely High Extremely low OR03 Micro granite 225 Very High Very low OS04 Medium feldspar-granite 180 Extremely High Extremely low IF05 Coarse muscovite-granite 210 Very High Very low IB06 Biotite- hornblende granite 210 Very High Very low DE07 Coarse biotite-granite 190 Extremely High Extremely low IJ08 Medium biotite-granite 210 Very High Very low

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Wear Classification of Selected Rocks

The bit wear and life of selected South Western Nigeria rocks was base on classification suggested by (Thuro 1997). The results show that the bit wear rate varies between Very high to extremely high wear rate; the drill bit life range from extremely low to very low life span as presented in Table 5.

Conclusion

The results of the laboratory and field measurements carried out on the selected rocks from eight different locations in Nigeria have shown that the properties of rock vary from one location to another. Strength characterization of the selected rocks shows that uniaxial compressive strength range from 89.90 - 90.03 MPa for Micro Feldspar-Granite to 124.55 - 124.71 MPa for Coarse Biotite-Granite and are all classified to have very high uniaxial compressive. The bit life exhibit weak correlation with uniaxial compressive strength and strong correlation with rebounds hardness value. Ultimately, understanding the drillability and strength characteristics will give quarry/mine operators the likely response of rock to drilling and excavation.

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