ROCK ABRASIVITY Advanced mechanization in excavation and particularly the growing use made of tunnel boring machines, requires knowledge regarding rock abrasivity. This factor dictates indeed primarily the tool efficiency and replacement rate. As a consequence, abrasivity may in fact result in extra costs. Two types of test, i.e. Cerchar test and LCPC test, allow for a quantitative characterization of rocks abrasivity. Cerchar abrasiveness test The Cerchar test (which was developed by the Centre d’Etudes et de Recherches des Charbonnages de France) is an abrasivity test that is widely used in France in the coal mining industry. In the Cerchar test, a sharp steel indenter (hardness of 200 kg/mm2) of 90°cone angle is applied to the surface of a rock specimen with a static force of 70 N. The steel point is then slowly moved on 10 mm. This procedure is repeated five times in various directions on the rock surface, always using a fresh steel tip. The abrasivity of the rock is obtained by measuring with a microscope the resulting wear flat on the steel cone. The unit of abrasivity is defined as a wear flat of 0.1 mm diameter.
70 N
1
10 mm 2
1
2
Schematic of the test picture of the apparatus
The abrasivity scale is given in the table below: ICA value <0.3 0.3 - 0.5 0.5 - 1.0 1.0 - 2.0 2.0 - 4.0 4.0 - 6.0 6.0 - 7.0
Abrasivity scale not abrasive not very abrasive medium abrasive abrasive very high abrasive extremely abrasive quartz
Abrasivity_ eng/ jfmathier EPFL-ENAC-Laboratoire de mécanique des roches
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LCPC abrasivity coefficient The abrasivity test of the Laboratoire des Ponts et Chaussées (normalised according to AFNOR P18-579) consists in measuring the weight loss of a steel plate rotating at 4500 rpm for 5 minutes in 500g of rock, which was previously crushed to pieces of 4-6.3 mm diameter. The metal plate (25x20x5 mm) presents a Rockwell hardness B 60-75.
Schematic of the test
picture of the apparatus LCPC
State of the plate before and after test
The abrasivity coefficient, ABR, is defined as the ratio of the plate’s weight loss to the mass of tested material. The index is given in grams per ton and varies between 0 and over 2000 depending on whether the rock is respectively little or highly abrasive. P0 − P G0 P0 = weight of metal plate before test [g] P = weight of metal plate after test [g] G0 = weight of sample [t] ABR =
ABR [g/t] 0-500 500-1000 1000-1500 1500-2000 >2000
Abrasivity_ eng/ jfmathier EPFL-ENAC-Laboratoire de mécanique des roches
Scale very small small average high very high
Example of rock schists, limestones molassic sandstone gneiss granites quartzites
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Using about sixty samples from different origins, two studies [3,4] outlined some correlation between both types of abrasivity indices. (fig 1). An analysis of Cerchar results showed that 1 ICA index point is approximately equivalent to a 300g/t LCPC coefficient.
1800
LCPC [g/ton]
1500
1200
900
600
300
0 0
1
2
3
4
5
6
ICA Granodiorit
Biotit-Serizit Gneis
Granit
igneous rocks
metamorphic rocks
sandstone
Olivine basalt
Porphyritic basalt
Tholeitic basalt
Fig. 1 Correlation between ICA and LCPC abrasivity indexes
References [1] West G. Rock abrasiveness testing for tunnelling, technical note. International Journal of Rock Mechanics & Mining Sciences 26, pp 151-160, 1989. [2] Plinninger R., Kässling H., Thuro K., Spaun G. Testing conditions and geomechanical properties influencing the CERCHAR abrasiveness index (CAI) value. International Journal of Rock Mechanics & Mining Sciences 40, pp 259263, 2003. [3] Büchi E., Mathier J.-F., Wyss, Ch. Rock abrasivity – a significant cost factor for mechanical tunnelling in loose and hard rock. Tunnel no 5, pp 38-43, 1995. [4] Mathier J.-F., Gisiger J.-P. Abrasivity of Icelandic basalts. ISRM 2003–Technology roadmap for rock mechanics, South African Institute of Mining and Metallurgy, 2003
Abrasivity_ eng/ jfmathier EPFL-ENAC-Laboratoire de mécanique des roches
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