prieto portar 10-17-12 cerchar for dredging12 prieto
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PIANC-USA / COPRI-ASCE / Dredging12
The Application of the Cerchar Abrasivity Indexs
to Dredging Hard Rock
by Luis A. Prieto-Portar PhD, PE, SE
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Some machines have replaceable teeth.
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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.
The mining and tunneling industry define the rate of wear as the velocity of metal removal from the tool, or drill tooth lifetime expressed in excavated
cubic meters per tooth (m3 excavated per bit). The table shows the rate of wear used calculate the tool consumption.
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Excessive wear on the cutter-head of a slurry TBM 11.7 m
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The Unconfined Compressive Strength Test (UCS) on rock core specimens is performed following ASTM D 2983 or AASHTO T 226. The test yields the
unconfined compressive strength of an intact piece of rock core sample, which may be used to estimate allowable bearing capacity values for bedrock. A rock
specimen must be intact, and at least measure 2f by 4 long.
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Piedroba Marine Construction Sketch of a Cerchar machine.
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Photo of a typical Cerchar machine set-up.
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Ten Cerchar tests on two samples.
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The Cerchar index is obtained from a sharpened steel needles sharp point (shown at left). The test produces a conic blunt surface (shown at right) where its new diameter is measured in integers of 0.1 mm with a scale going from 0
for minimum wear to 6 for maximum wear.
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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 provides a classification of the Cerchar, and compares it with a newly proposed abrasivity index: the Schimazek F-value.
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Mohs Hardness Scale Mineral Common testing tool
Softest 1 Talc
2 Gypsum 2.5 = most fingernails
3 Calcite 3.0 = copper penny
4 Fluorite
5 Apatite 5.5 = pen knife
6 Orthoclase 6.0 = plate glass
7 Quartz
8 Topaz
9 Corundum
Hardest 10 Diamond
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Ooids from beach sand in the Bahamas.
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Oolitic limestone of the Miami Formation
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Freshwater limestone of the Fort Thompson in Miami, may have strength that exceeds 5,000 psi. Notice the lack of joints and large voids that would permit easy
excavation using a toothed bucket.
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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
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Conclusions: 1. The abrasivity of soils and rocks must be part of the geotechnical study of any dredging project. 2. The CAI is a cheap and reliable predictor of abrasivity.
Questions?
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