Drill & Blast is a commonly used excavation method for the construction of underground openings (tunnels, caverns etc.) in hard rock conditions worldwide. Wear of tools may take place duringC different stages of the working cycle, affecting a wide range of machinery and materials.

Although excavators, dump trucks or conveyor belts are permanently exposed to the excavated rock mass and therefore undergo geologically influenced wear, the wear on the rock cutting tools (i.e. drilling bits, excavator chisels, picks) is the most expensive wear phenomenon.

Common blast hole diameters range from 38 to 48mm and are typically drilled by use of hydraulic rotary percussive drilling hammers. In most geological conditions button bits are predominantly used, which consist of a number of cemented carbide buttons inserted and/or soldered into holes of a steel body. The properties of the button bit can be adjusted effectively to the local circumstances by variation of the amount of inserted buttons, button composition, button geometry, soldering and steel quality or the bit’s flushing system.

Tool wear will occur under certain loads and temperatures just as one result of a complex tribological system. The material removal is caused by microscopic and macroscopic processes such as abrasion, adhesion, material fatigue or brittle failure of tool materials. Mode and rate of these processes are controlled by a vast variety of factors coming from the main fields of geology, tools and logistics. Engineering geologists, material scientists, mining and civil engineers have been researching these effects through the last decades at universities, research institutes and machine manufacturers to identify and quantify main influencing factors. Rock properties, joint features, weathering/alteration of rock, water situation, composition of inhomogenous rock masses and underground stress situation have been identified as major geological factors. Tool characteristics, flushing, feed, rotating velocity, temperatures, tool handling and rock supporting methods represent some main factors from the fields of tool and logistics.

Classification of bit wear

The wear type and the wear rate can be used as parameters describing the effect of the wear process. The wear type describes the specific form of wear observed on the tool. It can be described qualitatively by use of a wear classification system (Table 1). The wear rate describes the velocity of material removal from the tool. This term is normally expressed in drilled meters per bit [m/bit], also entitled the “drill bit lifetime”. The wear rate is a basic factor for the calculation of tool consumption and wear costs. It can be obtained on site from measurements on single tools or calculations based on stock lists and delivery notes.

The bit wear type can be used as a “fingerprint” of the wear process. From its classification, valuable information can be obtained about the typical processes taking place and the geological and machinery causes for the encountered wear forms. It is evident, that transitions and mixes of the presented types are possible.

Normal wear (BB-A1) may be observed when tool body and hard metal inserts are more or less evenly undergoing abrasive wear. This wear type is typical for abrasive rocks with high compressive strength, for example quartzite, gneiss, granite or quartzitic sandstones. The even wear distribution can be explained by the low penetration of the bit in such rock types, so that mainly the hard metal inserts get in contact with the rock and are therefore worn with low wear rates – even by minerals that can be classified as “non abrasive” to cemented carbides.

Predominant wear of the tool body (BB-A2) with possible breaking out of buttons (BB-A3) is a typical phenomenon for drilling weak rock types. It can often be observed in poorly cemented or weathered sandstone, sandy marlstone or weathered granite or gneiss. In this rock the bit penetrates deep into the rock mass and produces a lot of debris, so that both steel body and buttons are more or less evenly exposed to rock and rock debris. Since at the same mineral hardness steel and cemented carbide are undergoing different wear rates, the tool body wears faster than the inserted buttons. Buttons are preparated out of their steel bedding and begin to fall out or are broken out of the bit when embedding is insufficient (BB-A3).

Wear of the diameter (BB-A4, BB-A5) is typical for unstable or highly stressed abrasive rock mass when the drilled hole deforms during drilling. Abrasive rock material is forced onto the bit from the walls. First, the peripheral buttons begin to show wear on the outer side (BB-A4), later the tool body itself is affected, the hole diameter of the bit is reduced and peripherical buttons break out (BB-A5).

Macroscopic failure of buttons (BB-F1, BB-F2) is mostly independent from the rock’s abrasivity and mineral content. It is mostly influenced by the rock´s strength and fabric, properties of the rock mass, machinery, tools and support method. Dynamic impact is the main cause for this material failure. Type BB-F2 (total button removal) can easily be caused by no or bad soldering of the buttons into the steel body. These main factors have been recognised to cause button failure:

  • inhomogeneous rock masses with rock of high strength in combination with open joints or joints filled with soft rock;

  • inhomogeneous rock types with very hard components exceeding diameters of about 20mm, like conglomerates, fanglomerates or breccias;

  • drilling through already installed steel support – like forepoling through lattice arches or anchoring through reinforced shotcrete.

Failure of the bit shaft (BB-F3) is mainly a result of manufacturing problems or bad handling. In these cases, no conclusions may be drawn on geological circumstances.

The occurrence of thermic wear (BB-T) of button bits depends on the effectiveness of the flushing system. Under normal circumstances button bits are cooled very effectively by their water flushing system so that tool temperatures normally don’t exceed 40oC. If no, or insufficient, flushing is available thermal wear may occur. Wear types equal those of abrasive wear and wear due to material failures since high tool temperatures only increase those wear mechanisms. When heated above 200oC specific temper colours should show on the steel body and may be used diagnostically to estimate the maximum tool temperature.

Two special wear types occur mostly independently from geological circumstances:

  • total wear down (BB-Sp1) is stated, when the bit is worn down to or below the buttons. In such cases one may not be able to definitely recognise the predominant wear process;

  • widening of flushing holes (BB-Sp2) and flutes is a phenomenon, which in most cases is caused by aggressive flushing fluids or suspended abrasive particles in the flushing. It may even be caused by cavitation alone which means material loss out of the tool surface due to forming and implosion of microscopic vapour bubbles under high velocities of flow.

Predicting bit wear rates

Table 2 gives a system for the description of the drill bit lifetime for button bits with a diameter of 43-48mm. These terms have proved suitable on numerous projects since their introduction by Thuro in 1996.

Prediction of tool wear rates can be based on a large variety of testing procedures and standards. Procedures cover a wide span of scale, ranging from on-site real-scale drilling tests to model tests with simplified tools and further on to microscopic and chemical analysis of rocks and minerals.

Depending on their scale and parameters they are able to take different factors into account whilst disregarding others. The following chapters present results of case studies, derived from 12 projects in Germany and Austria from 1989 to 2001.

Real-scale drilling tests, using the original drilling tools and machinery, performed on representative outcrops or samples of rock are a reliable testing method to obtain data for tool wear and drilling performance. Depending on the condition and size of the testing area or sample, nearly all influencing factors are taken into account. Unfortunately, the procedures are rather expensive with respect to personnel and material costs and therefore carried out most seldomely.

The Bit Wear Index, BWI is – besides the Drilling Rate Index (DRI) and Cutter Life Index (CLI) – part of a testing procedure used for performance prediction of hard rock excavation methods. The factors developed at the NTNU Trondheim are based on a impact crushing test (“Brittleness test”), a model boring test (“Siever´s Miniature Drill Test”) and a model abrasion test (“Abrasion Value”). There are currently no diagrams available to estimate bit lifetime from a given BWI. Bruland (1998) reports that “… the BWI has been found to have some weakness… We are currently working to replace the BWI with VHNR…”.

In western Europe the Cerchar scratch test (Cerchar, 1986) is one of most commonly used testing methods when it comes to the laboratory investigation of a rock sample’s abrasivity. The test is performed on a rock sample of hand specimen size and features a steel needle of defined geometry being scratched over the rock surface under static load. The CAI is then calculated from the diameter of the needle wear flat. The 15 data sets plotted in Figure 2 show only bad correlation between CAI and encountered drill bit lifetimes. Judging from own and Bruland’s experience one may conclude, that model tests may be used for a quick and cheap estimation of a rock´s abrasivity, but appear to be not suitable for a more precise calculation of drill bit lifetime and drill costs.

Geotechnical wear indices

Geotechnical wear indices are calculated from standard laboratory parameters and so should be available from every standard hard rock investigation program. SiO2- and Al2O3-contents derived from X-Ray Fluorescence Analysis have shown only bad correlation with bit wear rates (Plinninger, 2001). One explanation may be found in the fact, that the SiO2- and Al2O3 element oxides are only calculated from the analysed Si- and Al- contents of the rock sample so that non-abrasive clay minerals and mica are included as well as quartz and corundum.

AMC, VHNR and EQC are very similar geotechnical parameters using petrographical thin section analysis. They are calculated by multiplying the content of a mineral with a specific hardness value and then adding the values up. The indices vary in the use of different hardness values (Mohs scratch hardness, Vickers indentation hardness or Rosiwal grindig hardness). VHNR and EQC have proven to be suitable for drill lifetime calculation, prediction diagrams are given in Johannessen et al (1995, Figure 3), Thuro (1996) or Plinninger (2001).

The Schimatzek wear index was developed in the 1970s in order to estimate roadheader pick consumption in german coal mining operations. Besides the content of abrasive minerals (calculated similar as the EQC) the factor uses the brazilian tensile strength (BTS) and the grain size. Primarily defined for clastic quartz-rich sediments only, Ewendts investigations (1980) lead to a modified Schimatzek wear index, adapted to all kinds of rocks and using the Point-Load-Strength I50 instead of the BTS. This modified index has proven to be a more reliable factor for bit lifetime prediction than the EQC. A prediction diagram is available from Plinninger (2001).

The Rock Abrasivity Index and RAI prediction procedure (geotechnical index)

The Rock Abrasivity Index (RAI) is a new geotechnical wear index, part of a prediction procedure for drill bit wear rate. This procedure suggests an investigation program taking into account the hole range of scale from rock mass to mineral scale (Table 3). Based on the “mineral scale” and “rock scale” investigations, the RAI is calculated for relevant rock types by multipliing the rock´s unconfined compressive strength and equivalent quarz content (Figure 4). Rock mass scale information is then taken into account by use of “positive” and “negative” factors, that can either increase or decrease the drill bit lifetimes derived from the RAI prediction diagram.

Related Files
Figure 4: Drill bit lifetime plotted against RAI
Figure 2: Drill bit lifetime plotted against CAI
Figure 1: The main characteristics of a button bit.