PIANC-USA / COPRI-ASCE / Dredging12

The Application of the Cerchar Abrasivity Index’s

to Dredging Hard Rock

byLuis A. Prieto-Portar PhD, PE, SE

Some “machines” have replaceable teeth.

Classification Tooth lifetime m 3 exc. / bit Lifetime

very low > 2,000 very high

low 1,500 to 2,000 high

moderate 1,000 to 1,500 moderate

high 500 to 1,000 low

very high 200 to 500 very low

extremely 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.

Excessive wear on the cutter-head of a slurry TBM Ø 11.7 m

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 2”f by 4” long.

Piedroba Marine ConstructionSketch of a Cerchar machine.

Ten Cerchar tests on two samples.

The Cerchar index is obtained from a sharpened steel needle’s “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.

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

Not abrasive < 0.5 young limestone < 0.5

Very slightly abrasive < 1.2 limestone < 0.5

Slightly abrasive 1.2 to 2.5 young sandstone 0.5 to 2

Medium abrasive 2.5 to 3.5 weathered granite/dolerite 2 to 4

Moderately abrasive 3.5 to 4.0 sandstone 4 to 5

Abrasive 4.0 to 4.2 granite/schist/pyroxenite 5 to 8

Highly abrasive 4.2 to 4.5 amphibolite 8 to 11

Extremely 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.

Ooids from beach sand in the Bahamas.

Oolitic limestone of the Miami Formation

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.

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 NA

Key Largo Coralline limestone - 160 ft NA NA NA NA NA NA NA

Caloosahatchee Not found in this area - NA NA NA NA NA NA NA

Tamiami 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

Conclusions:

1. The abrasivity of soils and rocks must be part of the geotechnical study of any dredging project to schedule maintenance and control costs. 2. The CAI is a cheap and reliable predictor of abrasivity.

Questions?