PIANC-USA / COPRI-ASCE / Dredging12 The Application of the Cerchar Abrasivity Index’s to Dredging Hard Rock by Luis A. Prieto-Portar PhD, PE, SE

Some “machines” have replaceable teeth.

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.

Sketch of a Cerchar machine.

Photo of a typical Cerchar machine set-up.

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.

Table 2 provides a classification of the Cerchar, and compares it with a newly proposed abrasivity index: the Schimazek F-value.

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

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.

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?