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Proceedings, WEDA XXXII Technical Conference & TAMU 43 Dredging Seminar

THE CERCHAR ABRASIVITY INDEXSAPPLICABILITY 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 topredict 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.

Table 1: Classification of wear rates versus drilling teeth lifetime (m3excavated 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.

1CEO, 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.

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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) Dorrys 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 sfreshwaterlimestone 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 theRock 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 rocks 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.

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Figure 1. The Rock Abrasivity Index (RAI) versus drill bit lifetime (m

3

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.

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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 needlessharp 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.

Table 2. The Cerchar Abrasivity Index (CAI) versus the Schimazek F-value.

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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 pins

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 Youngs Modulus (right).

The good correlation between Youngs Elastic Modulus E and the EQC with the CAI indicates that the rocks

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.

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The good correlation between the CAI and the predicted TBM cutter life (m 3 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 boringsperformed 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 CaCO3surrounding tiny sand grains in the shape of

eggs (oion is the Hellenic for egg). The proportion of each (CaCO3 versus SiO2) determines how a geologist

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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.

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

rocks 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.

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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., Ksling, H., Spaun, G., Thuro, K., Versuchstechnische und geologische Einflufaktoren beim

Cerchar-Abrasivittstest, 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 Mechani cs,

Mineral Sciences and Geomechanics, 26, 2, 1989.

Cerchar, Centre dEtudes et Recherches de Charbonnages de France, 1986.

CITATION

Prieto, L.A., The Cerchar Abrasivity Indexs 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.

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