Cerchar Abrasivity Index Value Research Articles

Water saturation effects on rock abrasivity and TBM tunneling efficiency

Tunnel boring machine (TBM) tunneling efficiency is influenced by surrounding rocks, TBM design, and interaction between the TBM cutter and rocks. Using a case study from a water conveyance tunnel project located in northwestern China and involving five different lithologies (metamorphic andesite, sandstone, granite, quartz schist, and quartz diorite), this work investigates the water saturation effects on Cerchar abrasivity index (CAI) and its correlations with other physical and mechanical rock properties and TBM disc cutter and energy consumption. The results show that water saturation reduces CAI for all examined rocks except for the quartz diorite at the 90 % confidence level. CAI of saturated rocks (CAIsat) has statistically better correlations with weighted Mohs hardness value of rock minerals (Ha), equivalent quartz content (EQC), and uniaxial compressive strength (UCS), than CAI of dry rocks (CAIdry). The in-situ construction data analyses suggest that the average disc cutter consumption increases exponentially with the rise of CAI values in both dry and saturated conditions, and the water saturation effects significantly reduce the disc cutter consumption within medium-hard and hard surrounding rocks. Both disc cutter and specific energy consumption have a quadratic relationship with the ratio of the maximum cutter spacing to penetration depth (Smax/P), and water saturation can significantly shift the quadratic relationships. Optimal Smax/P ranges for minimizing the disc cutter and energy consumption are recommended for different rocks under dry and saturated conditions. The findings from this study can support estimating cutter and energy consumption with considerations of groundwater effects.

Effect of water saturation on Cerchar abrasivity index (CAI) of clay-rich rocks at different scratch lengths

Predicting the cutter consumption rate of mechanical excavators using the Cerchar abrasivity index (CAI) results is possible. However, changing operational parameters in the field may cause differences between the estimated cutter consumption rate based on CAI test results and the real cutter consumption rate observed in the field. One of the essential parameters in the field that affects the cutter consumption rate is the increase in the water content of the excavated rocks. In theory, an increase in the water content of rocks is expected to reduce their abrasivity. In addition, the prediction of cutter consumption rate is based solely on CAI test results conducted on dry surfaces. However, especially during the excavation of low-strength clay-rich rocks, the excavated rock material can adhere to the cutters and block them, leading to a higher cutter consumption rate than initially predicted. This study aims to analyze the rock-pin interaction in CAI tests conducted on wet surfaces of clay-rich rocks and investigate the effect of water saturation on CAI at different scratch lengths. CAI tests were conducted on dry and wet surfaces of 20 clay-rich rocks with low strength at 10, 30, 50, and 70 mm scratch lengths, respectively. The relationships between the CAI test results obtained on dry and saturated rock surfaces were statistically analyzed for each scratch length. A positive linear correlation was found between CAI values obtained on dry (CAIDry) and saturated surfaces (CAISat) at each scratch length. The percentage variations in CAI values at each scratch length were analyzed in detail. It was found that 90 % of samples have higher CAIDry than CAISat at 10 mm scratch length. However, it was observed that as the scratch length increased, the relationship between the CAI values obtained in dry and saturated conditions changed, and the proportion of samples with CAIDry higher than CAISat decreased to 65 %, 55 %, and 25 % at scratch lengths of 30 mm, 50 mm, and 70 mm, respectively. In CAI tests conducted on saturated surfaces, it was observed that the excavated rock material adhered to the test pins, and as the scratch length increased, the increase in CAISat values was considered to be due to this situation. This study revealed the importance of considering the adhesion potential of rocks to cutters at water-saturated conditions for the accurate prediction of cutter consumption in the excavation of clay-rich rocks.

Evaluation of Cutting Performance of a TBM Disc Cutter and Cerchar Abrasivity Index Based on the Brittleness and Properties of Rock

The brittleness of rock is known to be an important property that affects the fragmentation characteristics of rock in mechanized rock cutting. As the interaction between the cutting tool and the rock (i.e., cutter forces, cutting efficiency, s/p ratio, and abrasivity) during mechanical rock cutting is strongly influenced by the characteristics of rock fragmentation, the cutting tools (i.e., disc cutter and pick cutter) experience different cutting behaviors depending on the rock brittleness. In this study, the relationships between the rock brittleness and the abrasivity of rock, and the cutting efficiency of a Tunnel Boring Machine (TBM) disc cutter were investigated for Korean rock types. The brittleness was calculated by the mathematical relations between the uniaxial compressive and Brazilian tensile strengths of the rock. The cutting efficiency and abrasivity were evaluated by the cutter forces and specific energy from the linear cutting machine (LCM) test and the Cerchar abrasivity index (CAI) test, respectively. The results show that rock brittleness is significantly correlated with cutting efficiency and CAI values. Consequently, some prediction models for cutter forces, specific energy, and the CAI were proposed as functions of the rock brittleness.

Investigation on the relationship among the Cerchar abrasivity index, drilling parameters and physical and mechanical properties of the rock

Rock abrasivity is an important factor that affects tool wear and tool parameter design, construction efficiency, and cost budgeting during rock excavation. The Cerchar abrasivity index (CAI) is used as a standard parameter to characterize the abrasiveness of rocks. It can be obtained through the Cerchar abrasivity test in a laboratory. However, conventional laboratory Cerchar abrasivity tests and petrophysical and mechanical tests are associated with certain problems, such as long cycles, high costs, and measurement delays, which cannot serve the determination of rock abrasiveness in field. Digital drilling technology is an efficient in situ test method that can effectively link drilling parameters with rock properties. Based on the test results for the CAI and physical and mechanical properties of 13 groups of rock samples collected from southwestern China, this paper focuses on the correlations of the CAI value with rock strength, petrographic characteristics and drilling parameters. By adopting a flat bottom diamond bit that mainly relies on grinding, according to the principle of mechanical balance and energy conservation during rock drilling, the grinding energy per unit volume of rock (ηe) is defined and derived. The univariate regression analysis results show that the uniaxial compressive strength (UCS) and equivalent quartz content (EQC) are the most important factors for explaining CAI and that the CAI can be well estimated using the ηe. Additionally, two optimal regression models for predicting CAI were established using stepwise multiple regression analysis. UCS and ηe were introduced into the model, and the use of both can predict approximately 96% of the variance in the CAI. In addition, a performance evaluation of the models proposed in this paper and previously published CAI prediction models that the prediction models established in this paper are verify statistically more reasonable and reliable than the previously proposed models. The research methods and results provide a new method for the rapid and accurate determination of rock abrasiveness in engineering field.

Temperature-dependent abrasivity of Bukit Timah granite and implications for drill bit wear in thermo-mechanical drilling

Thermo-mechanical drilling combines the advantages of flame thermal treatment and rotary head drilling to achieve a high penetration rate in granitic rocks and a low wear rate of drill bits. This drilling strategy has been recognized as a potential technique to develop deep geothermal energy, but further improvement is still needed to optimize the drilling efficiency, such as the reduction in drill bit wear in thermally treated rocks. We use the Cerchar abrasivity test to investigate the abrasivity characteristics of Bukit Timah granite after the thermal treatment at different high temperatures. The Cerchar abrasivity index (CAI) decreases from 3.5 at 25 °C to 3.0 at 400 °C, remains constant between 400 and 600 °C and subsequently increases to 4.1 at 800 °C. Based on the meso-scale and micro-scale observations using polarizing microscope, 3D scanner and scanning electron microscope, the reduction in CAI value at lower temperatures is due to the abrasive wear of stylus tip, while the mechanical interaction between the stylus cone and abrasive minerals causes the increase in CAI value at higher temperatures. The Cerchar abrasivity test results indicate that overheating rocks may result in the exposure of drill bits to more abrasive minerals and enhance drill bit wear.

Effect of Rock Abrasiveness on Wear of Shield Tunnelling in Bukit Timah Granite

The abrasiveness of rocks significantly influences the wear endured by excavation tools in tunnel boring machines (TBMs). In slurry shield TBMs, wear occurs not only in excavation tools but also in discharge pipes and pumps of slurry transport systems. To explore this phenomenon, this study investigated the effect of rock abrasiveness on the wear of both disc cutters and slurry discharge pipes in slurry shield TBMs. The radial wear of disc cutters and the thickness of slurry discharge pipes were measured and monitored at a TBM site in Singapore. The relationship between the weathering grade of Bukit Timah granite and the wear coefficient of disc cutters was analysed, and a correlation between the Cerchar Abrasivity Index values and the wear coefficient of 19 inch disc cutters was suggested. Additionally, the average wear rates of slurry discharge pipes used for Bukit Timah granite were determined based on the pipe thickness measurements. The wear rates of weathering grades G(I) to G(IV) of Bukit Timah granite were observed to be highly similar to each other, whereas the wear rates of G(V) grade and mixed ground were 1.55 times higher than those of the G(I) to G(IV) grades. The correlation between the slurry discharge velocity and the wear rate in slurry discharge pipes was thus derived. The accurate assessment of the effect of rock abrasiveness on both the wear of disc cutters and the wear rate in slurry discharge pipes, investigated in this paper, will highlight the need for efficient maintenance of the excavation tools in TBMs and encourage the development of various equipment wear prevention measures in order to minimize overall construction costs and time.

Assessment of cerchar abrasivity test in anisotropic rocks

There have been developed a number of methods to assess the abrasivity of rock materials with the increased use of mechanized rock excavation. These methods range from determination of abrasive and hard mineral content using petrographic thin section analysis to weight loss or development of wear flat on a specified cutting tool. The Cerchar abrasivity index (CAI) test has been widely accepted for the assessment of rock abrasiveness. This test has been considered to provide a reliable indication of rock abrasiveness for isotropic rocks. However, a great amount of rocks in nature are anisotropic. Hence, viability assessment of Cerchar abrasivity test for the anisotropic rocks is investigated in this research. The relationship between CAI value and quartz content for the isotropic rocks is well known in literature. However, a correlation between EQ, F-Schimazek value, Rock Abrasivity Index (RAI) and CAI of anisotropic rocks such as phyllite was done first time in literature with this research. The results obtained with this research show F-Schimazek values and RAI values should be considered when determination of the abrasivity of anisotropic rocks instead of just using Cerchar scratch test.

Investigating the effects of various testing parameters on Cerchar abrasivity index and its repeatability

Cerchar abrasivity index (CAI) test is widely used to estimate the tool life and wear in various excavation equipment in mining and tunneling projects. The aim of this study is to analyze the effect of various factors related to the CAI testing procedure. Particularly, sensitivity of CAI results is investigated due to variation of certain testing parameters, including the type of apparatus used, wear flat measurement methods, specimen surface conditions and stylus hardness. Relationship between CAI and certain testing parameters, and its repeatability are also investigated through statistical analyses of a series of CAI tests results obtained from twenty-one selected rock specimens. Resultsshow little variation between the type of apparatus used and between the selected wear flat measurement methods. Nevertheless, there is a significant large variation in CAI values according to the changes in specimen surface conditions and the hardness of stylus used. The results show that with an increase in the hardness of the steel stylus the CAI values decrease differently in accordance with the rock type tested.

Estimation of Cerchar abrasivity index of andesitic rocks in Ecuador from chemical compounds and petrographical properties using regression analyses

An important issue in any rock engineering project is the adequate prediction of tool consumption. Excavation tools are subjected to wear, and repair/replacement of those tools is usually an important expense on any excavation budget. The key factor that affects wear of excavation tools is rock abrasivity. In mining and civil engineering, rock abrasivity is typically measured by the Cerchar abrasivity index (CAI), which is obtained in laboratory from a Cerchar abrasivity test. This paper studied the relation between CAI and the chemical compounds and petrographical properties of andesitic rocks from the central area of Ecuador. A series of regression analyses are performed to study the influence of the different chemical compounds and petrographical properties on the CAI value. Results show that it is possible to make a good estimation of CAI from the plagioclase grain size and/or the content of SiO2, FeO, MgO, CaO, Na2O and K2O compounds.

Effects of rock water content on CERCHAR Abrasivity Index

Cerchar Abrasivity Index (CAI) has been commonly used to represent rock abrasion for estimation of tool wear in rock excavation applications. This test is a simple and effective measure of rock abrasion but operating parameters can impact its results. This paper focuses on the influence of water saturation on CAI values based on laboratory testing of 33 sedimentary rock units collected from different regions of Pakistan. The tests include CAI measurements as well as engineering rock properties on both dry and saturated rocks. It was found that overall 79% of CERCHAR abrasivity index values measured on saturated rock surfaces “CAIsat” were less than the CERCHAR abrasivity index values measured on dry rock surfaces “CAIdry”. Meanwhile, 52% of CAIsat values showed significant reduction from CAIdry values in the statistical analysis of CAI results with significance level of 15% (α=0.15). Regression analysis was used to correlate CAIsat values with CAIdry, normally measured in CERCHAR testing. The proposed formula can be used to estimate CAIsat from result of laboratory testing where CAI reported is often CAIdry. This could lead into more accurate estimation of tool wear and related adjustments when CAI is used for estimation of tool wear when excavation is done under wet conditions. Multiple regression was also used to establish a predictive model of CAIsat with petrographical, wear indices and geotechnical rock properties of saturated rock samples. In addition, the validity of correlations of CAI with rock properties that are already published was evaluated.

Investigation into effect of scratch length and surface condition on Cerchar abrasivity index

The Cerchar abrasivity index (CAI) obtained from Cerchar abrasivity tests is an indicator of abrasiveness of rocks, and widely used for the estimation of bit/cutting tool life and wear rates in various mining and tunnelling applications. The effect of scratch length on CAI is investigated in this study by use of West apparatus and 6 different steel styluses with Rockwell Hardness of HRC40-42, HRC44-46, HRC48-50, HRC50-52, HRC54-56, and HRC58-60. The tests are carried out on 15 different rock samples (sedimentary, igneous, and metamorphic) on which rough and sawn cut surfaces with scratch lengths varying between 2mm and 20mm with an increment of 2mm, thus resulting in total 27,000 scratches. It is observed that the CAI values between 85% and 93% are reached at the sliding distance of 10mm while about the final CAI value of 99% is reached at 15mm. It is also observed that the CAI values on rough surfaces are about 18% higher than those on sawn cut surfaces. Besides, it is determined that the most suitable surface condition in CAI test is sawn cut surfaces according to the coefficient of variation of CAI values in measurement depending on the stylus hardness and the measurement surface condition.

Correlation of physico-mechanical properties of granitic rocks with Cerchar Abrasivity Index in Turkey

During the last decades, the rapid developments in mining operations and tunnel construction have lead to a rapid increase in the number of excavation machines. In order to achieve the expected benefits of mechanical excavation machines, these machines should be selected in accordance with the characteristics of rocks. Tool abrasion is an important factor in hard rock tunnelling, mining, and it is highly affected by rock abrasivity. There are several methods to identify the rock abrasivity. One of the commonly used abrasion test in rock is the Cerchar abrasivity index (CAI). Before selection and implementation of excavating machines, physical and mechanical properties of the rocks should be determined. It’s known that, physical-mechanical properties of granitic rocks are generally better than those of many rock types although they cause some difficulties (tool wear, lost time, etc.) in excavation and increase the cutter costs. The purpose of the present study is to determine empirical relationships between CAI and physical-mechanical properties of different granitic rocks using regression method. In this study, some laboratory experiments were conducted on samples collected from granite quarries in different parts of Turkey, particularly from the Marmara Region. Firstly, petrographical, mineralogical and physical-mechanical characteristics of the collected granitic rocks were determined. Then, empirical relationships between these properties and CAI were determined using method of regression analysis. According to the results obtained, a strong correlation is found between CAI value, quartz content and quartz size of the granitic rocks. In addition, the uniaxial compressive strength and indirect tensile strength of the studied granitic rocks increase as CAI increases. Surface roughness, waviness and peak number of granitic rocks lead to an increase in CAI. On the contrary, Bohme abrasion resistance increases while CAI decreases.

Cuttability assessment using the Drilling Rate Index (DRI)

For given geological conditions, cuttability or performance prediction can be expressed by specific cutting force (SFC), specific normal force (SFN), specific energy (SE), and pick wear rate. These parameters are difficult to determine. For this reason researchers try to assess these parameters indirectly by rock mechanical parameters. The Drilling Rate Index (DRI) is the most important input parameter of a commonly used performance prediction model for tunnel boring machines. However, little research has been seen in the literature about assessing cuttability indirectly by DRI. In this study, different types of rock and ore samples were subjected to a small-scale rock cutting test, the Cerchar abrasivity test, the Sievers J miniature drill test, and the brittleness (S 20) test. With the aid of the Sievers and brittleness test results, the DRI parameter was determined. SFC, SFN, and SE, which are the output parameters of small scale rock cutting tests, and the Cerchar abrasivity index values were correlated with Sievers J (S j ) value and DRI. It was seen that SFC, SFN, SE and CAI parameters slightly decreased with increasing Sievers J value and DRI. With these results, it can be stated that DRI and S j could be an alternative to other mechanical rock parameters for assessing cuttability.

Cerchar abrasivity index (CAI) and its relation to strength and abrasion test methods for marble stones

The Cerchar abrasivity index (CAI) is the most widely known test method for identification of rock abrasivity. The CAI value is related directly to cutter life in the field. On account of this, it is significant for building and constructions materials especially for the dimension stones. This paper presents strength tests, which are uniaxial compressive (UCS) and direct shear (DSS) influencing the CAI value. Additionally, the relationship between CAI and abrasion test methods that are Bohme abrasion (BA) and Wide-Wheel (WW) values are investigated. In the study tests were performed on 15 different marbles stone specimens. Results show that a strong correlation exists between CAI value, strength and abrasion test values. Based on this information, two converter charts were developed using multi regression analysis method.

A study of rock abrasivity and tool wear in Coal Measures Rocks

This paper examines the relations between Cerchar abrasivity index (CAI) and petrographic properties of Coal Measures Rocks in the Zonguldak Coal Basin, Turkey. In this study, petrographic properties and abrasivity indices of 29 sedimentary rock samples were determined and the values of CAI and petrographic analysis results were compared. A good linear relationship between CAI and average quartz grain size parameters was found. The test results also showed that abrasivity was affected by the mineralogical composition, cement type, degree of cementation, quartz content, and average quartz grain size. It was found that abrasivity of rock was decreased by the low grade of cementation between abrasive minerals. CAI values and petrographic properties were used to estimate tool consumption in underground works in the Zonguldak Coal Basin. It was observed that when CAI values increased, tool consumption increased.

Stress dependency of the Cerchar abrasivity index (CAI) and its effects on wear of selected rock cutting tools

The abrasiveness of rocks is often described by the Cerchar abrasivity index CAI. However, the state of stress of the rock sample with the standard CAI test in the laboratory is different from the state of stress of rock to be excavated at the face of any underground opening. Until this work, no attempt has been made to estimate if and how the CAI is effected by in situ stresses. This paper demonstrates that the CAI is definitely stress-dependent. A novel test procedure in which the CAI tests is conducted on samples in a triaxial cell shows the stress dependency for various rock types by higher CAI values upon confining pressure. Additionally, the states of stress at the face of different underground openings are discussed and application of the elevated CAI values for estimating wear of roadheader picks and TBM discs is given.