The Delft Sand, Clay & Rock Cutting Model, 2019a

Which Cutting Mechanism for Which Kind of Soil?
The forces K1 and K2 on the blade, chisel or pick point are now:
K
K
1
2
W2
sin( ) W1
sin( )
Gsin( )
sin( )
I cos( ) Ccos( ) A cos ( )
sin( )
W2
sin( ) W1
sin( )
G sin( )
sin( )
I cos( )
Ccos(
) A cos( )
sin( )
(4-7)
(4-8)
The normal forces N1 on the shear plane and N2 on the blade are:
N 1 K1
cos and N 2 K2
cos
(4-9)
The horizontal and vertical forces on the blade, chisel or pick point are:
Fh W2
sin( ) K2
sin( )
Aco s( )
(4-10)
F W cos( ) K cos( )
Asi n( )
(4-11)
v 2
2
The equilibrium of moments around the blade tip is:
N1 W1 R1 GR 3 N2 W2 R2
(4-12)
Analyzing these equations results in the following conclusions:
The pore pressure forces W1 and W2 are limited by the occurrence of cavitation.
All the terms are positive, resulting in positive forces on the blade and also positive normal forces.
In the non-cavitating case the pore pressure forces are related to the (mobilized) blade height or the length of
the shear plane. In the cavitating case the pore pressure forces are proportional to the (mobilized) blade height
or the length of the shear plane. Theoretically the Curling Type and the Tear Type may occur. This has
however never been observed with in dredging normal blade heights and layer thicknesses.
When the argument of the sine in the denominator gets close to 180 degrees, the forces become very large. If
the argument is greater than 180 degrees, the forces would become negative. Since both conditions will not
happen in nature, nature will find another cutting mechanism, the wedge mechanism.
4.3. Cutting Clay.
In clay the cutting processes are dominated by cohesion and adhesion (internal and external shear strength).
Because of the φ=0 concept, the internal and external friction angles are set to 0. Gravity, inertial forces and pore
pressures are also neglected. This simplifies the cutting equations. Clay however is subject to strengthening,
meaning that the internal and external shear strength increase with an increasing strain rate.
The reverse of strengthening is creep, meaning that under a constant load the material will continue deforming
with a certain strain rate. Under normal circumstances clay will be cut with the Flow Type mechanism, but under
certain circumstances the Curling Type or the Tear Type may occur. The Curling Type will occur when the
blade height is large with respect to the layer thickness, hb/hi, the adhesion is high compared to the cohesion a/c
and the blade angle α is relatively big. The Tear Type will occur when the blade height is small with respect to
the layer thickness, hb/hi, the adhesion is small compared to the cohesion a/c and the blade angle α is relatively
small.
Clay cutting is dominated by cohesive (internal shear strength) and adhesive (external shear strength) forces. The
basic cutting mechanism is the Flow Type. Cutting a thin layer, combined with a high adhesive force may result
in the Curling Type mechanism. Cutting a thick layer combined with a small adhesive force and a low tensile
strength may result in the Tear Type mechanism. This is covered in Chapter 7: Clay Cutting.
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