The Delft Sand, Clay & Rock Cutting Model

Introduction.

This book gives an overview of cutting theories for the cutting of sand, clay and rock as applied in dredging engineering. In dredging engineering in general sand, clay and rock are excavated with buckets of bucket ladder dredges, cutter heads of cutter suction dredges, dredging wheels of wheel dredges, drag heads of trailing suction hopper dredges, clamshells, backhoes and other devices. Usually the blades have a width much larger than the layer thickness of the cut (2D process) and the blade angles of these devices are not too large in the range of 30°- 60°. Although clamshells and backhoes may have blade angles around 90° when they start cutting. Other devices like drill bits of oil drilling devices, blades of tunnel boring machines, ice berg scour and the bull dozer effect in front of a drag head may have cutting angles larger than 90°. In such a case a different cutting mechanism is encountered, the so called wedge mechanism.

The book starts with some basic soil mechanics, the Mohr circle and active and passive soil failure in Chapter 2: Basic Soil Mechanics. These topics can also be found in any good soil mechanics book, but covering this makes the reader familiar with the use of the many trigonometrically equations and derivations as applied in the cutting theories.

A generic cutting theory for small blade angles is derived in Chapter 3: The General Cutting Process. This generic cutting theory assumes a 2D plane strain cutting process, where the failure lines are considered to be straight lines. The generic cutting theory takes all the possible forces into account. One can distinguish normal and friction forces, cohesive and adhesive forces, gravitational and inertial forces and pore vacuum pressure forces.

Six types of cutting mechanisms are distinguished; the Shear Type, the Flow Type, the Curling Type, the Tear Type, the Crushed Type and the Chip Type. The Shear Type, the Flow Type and the Crushed Type are mathematically equivalent. The Chip Type is a mix of the Shear Type and the Tear Type.

The generic theory also contains a chapter on the so called snow plough effect, a blade not perpendicular to the direction of the cutting velocity like a snow plough. Finally the methods for determining the shear plane angle and the specific energy are discussed.