WEDA Cerchar Abrasivity Prieto Final

Proceedings, WEDA XXXII Technical Conference & TAMU 43 Dredging Seminar

THE CERCHAR ABRASIVITY INDEX’S APPLICABILITY TO DREDGING ROCK

L. A. Prieto1

ABSTRACT This paper proposes the application of one of the most useful rock testing procedures, the Cerchar abrasivity test to predict the rate of wear of cutter-head and excavator bucket teeth in dredging, especially in rock. These predictions will permit the optimal selection of teeth, replacement rates and scheduling maintenance. The Cerchar Abrasivity Index (CAI) has been in use for over a decade in the field of tunneling. It has good record predicting the wear of cutter-teeth on tunnel boring machines (TBM). The CAI has become increasingly popular over other rock testing methods, including the unconfined compressive test (UCS) due to its simplicity, speed, low cost and the possibility of using much smaller samples. This paper provides an overview of the procedures of the CAI oriented towards dredging compared to other rock testing methods, and includes CAI data obtained by the author on Miami grainstones (sandstone and limestone) with the intent of optimizing the dredging of the Port of Miami, which is scheduled to start in 2013. Keywords: Cerchar abrasivity index, dredging rock, cutter-teeth wear, rock abrasivity, CAI of Miami grainstones.

TOOL WEAR The wear of cutter-head and excavator bucket teeth needs to be better quantified as a function of the material to be dredged. This knowledge could in turn, improve productivity and reduce dredging costs. The modes of wear are of interest to equipment manufacturers, and are controlled by (1) petrology (rock strength, joint features, weathering, mineral composition, in-situ stresses, grain-size, shape/elongation, anisotropy, porosity), and (2) tribology, the wear due to friction, feed-force, rotating velocity and wear due to microscopic processes (abrasion, adhesion, material fatigue or brittle failure of the tool metal), reported in Plinninger (2002); and (3) the rate of ware is the velocity of metal removal from the tool, or “drill tooth lifetime” expressed in excavated cubic meters per tooth (m3 excavated per bit). Table 1 shows the rate of wear used calculate the tool consumption.

Classification Tooth lifetime m 3 exc. / bit Lifetime

very low > 2,000 very highlow 1,500 to 2,000 high

moderate 1,000 to 1,500 moderatehigh 500 to 1,000 low

very high 200 to 500 very lowextremely high < 200 extremely low

Table 1: Classification of Drill Tooth Wear Rate.

Table 1: Classification of wear rates versus drilling teeth lifetime (m3 excavated per tooth). The type of wear is of interest to contractors, and is, (1) primary wear, which is the expected wear from normal excavation, permitting replacement at appropriate intervals, and (2) secondary wear, which is unplanned and usually occurs when the primary wear is excessive or unexpected and may lead to sudden failure. However, up to now, there has not been a generally accepted method for estimating wear except through anecdotal references.

1 CEO, Piedroba Marine Construction, 3250 Grand Avenue, Suite 201, Miami, FL 33133, USA, C: 305-972-2779,

Email: [email protected] / Professor, Department of Civil and Environmental Engineering, School of Engineering and Computer Sciences, Florida International University, Miami, FL 33176.


There are about a dozen tests that have attempted to predict tool wear in excavations. The two important methods today are the Cerchar Abrasivity Index (1986) and the Rock Abrasivity Index (RAI). Readers interested in other abrasivity testing methods used in the past are directed to search the literature for rocks: (1) the Vickers Hardness Number VHN, (2) the Moh Hardness, (3) the LCPC Abrasimeter test (ABR), and (4) the NTNU Abrasion test (AV/AVS); and for soils, (5) the Los Angeles Abrasion test, (6) the Nordic Ball Mill test, (7) Dorry’s Abrasion test, (8) the Miller test ASTM G75-01, and (9) the new NTNU Soil Abrasion test (SAT), all listed in Ozdemir (1999).

DREDGING IN HARD ROCK Cutter-head dredges and bucket excavators remove consolidated materials through shear. Shear can remove almost all dredgeable materials, including soft rock (limestone, sandstone, etc). The shearing is performed mechanically through the teeth on a bucket, or hydraulically through the teeth/pick on the cutter-head. However, harder rocks cannot be dredged through shear alone. Turner (2004) proposed that the cementation of the rock will require for it to first be shattered by high impact forces and then the small pieces removed by the suction pipe. In order for a cutter-head to have sufficient kinetic energy to shatter rock, the velocity of the teeth must be at least 5 m/s (16.4 ft/s), and the cutter be in excess of 670 kN (150 kips). In order to attain the required kinetic energy, a cutter-head would require over 1,500 kW (2,000 hp). Bucket excavators have low velocities of about 0.3 m/s (1 ft/s) that their kinetic energy is two orders of magnitude smaller (1%) and will not shatter hard rock strata. For example, Dutra Dredging Company of San Rafael, CA was unable to excavate the Port of Miami’s freshwater limestone stratum during the 1995 to 1997 period, using one of the largest barge-mounted excavators in the US. In spite of the huge crowding force of the excavator of over 900 kN (200 kips), the teeth on the bucket were incapable of shearing the rock (Prieto, 2003), due to the very low velocity of the bucket entering the limestone stratum. Currently, the most cost-effective way of shattering rock is blasting. The alternative to blasting is the use of percussive tools, with typically impact power of 15 to 20 kW per bit (20 to 27 hp). However, the dredging industry has not developed specialized percussive cutter-heads, and until newer shattering tools are developed, contractors will attempt to dredge rock through shear, by increasing the power at the cutter-head and providing stronger pick/teeth. The prediction of the wear of these teeth is an important factor in estimating the cost of dredging.

THE ROCK ABRASIVITY INDEX (RAI)

There are two practical methods of correlating the wear of excavating tools to rock properties: the Cerchar and the Rock abrasivity tests. The Rock Abrasivity Index, RAI is a new geotechnical wear index that attempts to predict the tooth wear rate while excavating in different rocks. The RAI is calculated by multiplying the rock´s Unconfined Compressive Strength (UCS) by its Equivalent Quartz Content (EQC) and plotted in Figure 1, adapted from Plinninger (2002). The data shows good correlation within the two-sigma envelope. However, the RAI is an expensive test because, (1) it requires the recovery of several good-sized samples for the UCS test, and (2) requires a determination of the percentage of very hard minerals (quartz, garnet, etc.) in the rock, usually through a petrographical thin section, X-ray diffraction (XRD) and differential thermal analysis (DTA). These analyses are not expensive if the clay and silt content are high.

THE CERCHAR TEST - ASTM D7625

The Cerchar scratch test (1986) was developed in France in the mid-1980s to help predict the wear of cutters on the tunnel boring machines (TBM). These TBMs were used to bore hundreds of high-speed rail tunnels built primarily in France and Spain during the 1980 to 2010 period. The constant wear on the cutters of a TBM, force the contractor to stop excavation, back the TBM away from the face-cut, and introduce men into the cavity to hand-remove and replace the cutters. In hard rock, the inspection is done daily because access to the cutter-head is relatively easy. In soft ground with a high water table (like Miami, Florida), the cutter-head is flooded with slurry. As a consequence, cutter-head interventions are very complicated, expensive and time consuming. Predicting the primary wear of the cutters is essential, and that is where the Cerchar test becomes advantageous.


Figure 1: The Rock Abrasivity Index (RAI) versus drill bit lifetime (m3 excavated / bit).

The advantages of the Cerchar test have permitted the prediction of cutter wear in every type of rock and soil, and the scheduling of maintenance during off-peak production periods. The TBM manufacturers have incorporated the Cerchar test as an industry standard, and accumulated an extensive database. The test is performed on a small freshly broken rock sample, requiring less than 25 mm in size (1 inch). The sample is scratched by a hardened sharp heat-treated alloy steel needle of a defined geometry over a length of 10 mm (0.4 inches) in 1 second, under a static load of 70 N (15.7 lbs), as shown in Figure 2 from Nilsen (2006). Another advantage of the Cerchar is that it can be performed on irregular rock samples. The CAI is then calculated as the average measured worn-flat diameters (in tenths of mm) on the testing needle, as shown in Figure 3. Table 2 provides a classification of the Cerchar, and compares it with a new abrasivity index: the Schimazek F-value.


Figure 2: The Cerchar Abrasivity machine and its 10 mm long scratches on rock samples.

Figure 3: The Cerchar index is obtained from a steel needle’s “sharp point” (left); the test produces a conic blunt surface where its new diameter is measured in integers of 0.1 mm with a scale going from 0 to 6.

Classification CAI Type of rock Schimazek F-value (N/mm)

Not abrasive < 0.5 young limestone < 0.5Very slightly abrasive < 1.2 limestone < 0.5

Slightly abrasive 1.2 to 2.5 young sandstone 0.5 to 2Medium abrasive 2.5 to 3.5 weathered granite/dolerite 2 to 4

Moderately abrasive 3.5 to 4.0 sandstone 4 to 5Abrasive 4.0 to 4.2 granite/schist/pyroxenite 5 to 8

Highly abrasive 4.2 to 4.5 amphibolite 8 to 11Extremely abrasive > 4.5 quartzite/gneiss/pegmatite > 11

Table 2: CAI versus Schimazek F-value.

Table 2: The Cerchar Abrasivity Index (CAI) versus the Schimazek F-value. There are two testing devices in use: the original Cerchar (1986) and the “West apparatus” (1989). The Cerchar is performed in 1 second, whereas the West is done in 10 seconds. West surmised that a slower test would be more


accurate but the velocity does not seem to alter the results, which are practically the same for both. The steel pin’s flattened-tip diameter is read with a 50x reflected-light magnification microscope with a measuring ocular. The steel tip wears proportionally to the quartz content of a rock (or soil) sample, which thus becomes the principal geo-mechanical parameter that influences the CAI, as shown in Figure 3 adapted from Plinninger (2002).

Figure 3: CAI versus the equivalent quartz content EQC (left) and versus Young’s Modulus (right).

The good correlation between Young’s Elastic Modulus E and the EQC with the CAI indicates that the rock’s abrasiveness is mainly influenced by both its deformability and the percentage of abrasive minerals in its matrix. The Rock Abrasivity Index (RAI) appears to have fair to good logarithmic correlation to the Cerchar, as seen in Figure 4, also adapted from Plinninger (2002).

Figure 4: Logarithmic correlation between the RAI and CAI.


The good correlation between the CAI and the predicted TBM cutter life (m3 excavated per cutter disc) has lead to the popularity of the Cerchar among tunneling contractors and TBM equipment manufacturers, and is shown in Figure 5, which has been adapted and modified from Maidl (2001).

Figure 5: Correlation between CAI and cutter wear rates for different rock types.

Other geotechnical indices under development, such as the Schimazek F-value (given on Table 2), the Equivalent Quartz Content (EQC) and the Rock Abrasivity Index (RAI) may improve in time to offer cheaper and more accurate alternatives to the CAI. However, the CAI remains the most cost effective method to predict tool wear in rock excavations.

MIAMI GRAINSTONES The author performed Cerchar tests on 32 rock samples at Florida International University’s Geotechnical Laboratory in 2006, in order to ascertain the abrasivity of sandstone and limestone samples taken from borings performed by PSI, the geotechnical consultant, in the Port of Miami. The borings were requested FDOT in support of their preliminary design of the new tunnel for the Port of Miami. The Cerchar tests were performed to predict the cutter wear for a TBM being considered by one of the RFP respondents using of a Carl Zeiss AxioCam 50x microscope with an image analyzer. The summary of those tests is given on Table 3 alongside the geotechnical profile for the Port of Miami rock and soil strata. The upper limestone and sandstone formations in the South Florida area are largely composed of oolitic carbonate grains with variable amounts of sand grains. The oolite itself is CaCO3 surrounding tiny sand grains in the shape of eggs (“òoion” is the Hellenic for egg). The proportion of each (CaCO3 versus SiO2) determines how a geologist


would classify them as either limestone or sandstone. A better classification is to call them “grainstones”, since their only difference is the percentage of calcium carbonate cementing the sand grains. Notice how the higher proportion of sand in both the sandstone and limestone yields higher CAI values, and hence higher wear on bits and teeth.

Formation Geological Description Depth g solids Porosity n CAI UCS Tensile Poisson ELoose quartz fine + 5 ft

Pamlico aolian SAND + 3 ft 165 pcf 55%Water Table 167 pcf 37% 1.2 198 ksf 27 ksf 505 Msf

Tan to whiteMiami porous to very porous 166 pcf 43% 1.3 242 ksf 32 ksf 0.23 521 Msf

soft to mediumoolitic LIMESTONE 167 pcf 45% 1.3 152 ksf 26 ksf 468 Msf

Large voids / vugs > 3 feet -20 ft 168 pcf 55 - 200% 1.5 25 ksf 3 ksf NALight grey

loose to medium 164 pcf 46%fine quartz SAND - 35 ftTan, fossiliferous 159 pcf 46% 1.9 156 ksf 27 ksf 0.24 842 Msfmedium to hard

sandy LIMESTONE - 40 ft 158 pcf 40% 2.1 189 ksf 32 ksf 755 MsfBrown, hard to very hard 168 pcf 22% 2.2 808 ksf 126 ksf 0.28 1,120 Msf

Fort Thompson fine-grainedfreshwater LIMESTONE - 45 ft 170 pcf 17% 2.3 1,785 ksf 260 ksf 1,250 Msf

Tan 144 pcf 25% 3.1 565 ksf 97 ksf 0.14 655 Msfmedium to hard

shelly SANDSTONE - 50 ft 158 pcf 12% 3.3 720 ksf 118 ksf 842 MsfTan

loose to medium 165 pcf 42%quartz SAND - 80 ft

Grey 159 pcf 25% 3.4 625 ksf 108 ksf 0.13 705 Msfmedium hardporous quartz 161 pcf 18% 3.7 855 ksf 140 ksf 856 Msf

shelly SANDSTONE - 95 ftAnastasia Sandy coquina -130 ft NA NA NA NA NA NA NAKey Largo Coralline limestone - 160 ft NA NA NA NA NA NA NA

Caloosahatchee Not found in this area - NA NA NA NA NA NA NATamiami Silty, highly fossiliferous limestone - 300 ft NA NA NA NA NA NA NA

Table 3: Generalized Geotechnical Profile at Watson Island for the Port of Miami TunnelFIU Geotechnical Laboratory tests performed on 36 of PSI's rock samples - August 2006 - L.A. Prieto

Table 3: Cerchar and other tests performed by the author at FIU on Port of Miami rocks.

CONCLUSIONS

The abrasiveness of rocks/soils may result in project completion delays if, (1) the designer fails to include abrasion tests in the geotechnical investigation phase, (2) the owner lacks knowledge of the effects of abrasion on the equipment, and (3) the contractor neglects to provide scheduled cutter changes, add the time required, the costs and consequent documentation. The Cerchar has become the most common geotechnical parameter that describes a rock’s abrasivity. The Cerchar tests have generated extensive data around the world that permits a rough correlation between tool wear rates, tooth-bit lifetime predictions, and point-attack pick consumption versus the CAI indices. Furthermore, numerous research institutions are actively improving the predictability of tool wear using the CAI. This paper proposes that dredging contractors (and designers, such as the USACE) adopt the Cerchar Abrasivity Index as a means to predict cutter-head and bucket excavator performance. The future requirements for deeper draft navigational channels and port basins will require dredges with cutter-head power in excess of 1,500 kW (2,000 hp) to be able to remove the deeper and less porous rock sea beds. An essential component will also be stronger teeth that will be capable of imparting shattering (impact) forces along with shearing forces on the stronger materials.

REFERENCES


- Prieto, L.A., “A Tabulation of the Engineering Parameters and the Cerchar Abrasivity Indices of Miami Grainstones”, FIU Geotechnical Laboratory report for Dragados-USA, September 2006. - Nilsen, B., Dahl, F., Holzhauser, J., Raleigh, P, “Abrasivity Testing for Rocks and Soils”, Tunnels and Tunneling International, April 2006. - Turner, T.M., “The Secret to Dredging Rock”, WEDA XXIV, Orlando, Florida, 2004. - Prieto, L.A., “Dredging Freshwater Limestone in the Port of Miami”, Technical Report submitted to Miami-Dade County, Miami, Florida, 2003. - Plinninger, R.J., Käsling, H., Spaun, G., Thuro, K., “Versuchstechnische und geologische Einflußfaktoren beim Cerchar-Abrasivitätstest,” Geotechnik, 25, 2, Essen, 2002. - Ozdemir, L., Nilsen, B., “Recommended Laboratory Rock Testing for TBM Projects”, AUS News, 14, 2, 1999. - West, G., “Technical Note: Rock Abrasivity Testing for Tunneling”, International Journal of Rock Mechanics, Mineral Sciences and Geomechanics, 26, 2, 1989. - Cerchar, Centre d’Etudes et Recherches de Charbonnages de France, 1986.

CITATION

Prieto, L.A., “The Cerchar Abrasivity Index’s Applicability to Dredging Rock,” Proceedings of the Western Dredging Association (WEDA XXXII) Technical Conference and Texas A&M University (TAMU 43) Dredging Seminar, San Antonio, Texas, June 10-13, 2012.

WEDA Cerchar Abrasivity Prieto Final

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