Cohesion is the force by which particles of clay are bound together. This force is the result of
ionic attraction among individual particles, and is a function of the type of mineral, particle
spacing, salt concentration in the fluid, ionic valence, and hydration and swelling properties
of the constituent minerals. Cohesive soils are composed of silts and clay. Their size
classification is given in Table 3.1. In the unified soil classification system, silt and clay soils
have more than 50 percent by weight of particles passing the 0.075 mm sieve opening.
However, cohesive soils are not classified by grain size but by their degree of plasticity,
which is measured by Atterberg limits.
Clays are alumino-silicate crystals composed of two basic building sheets, the tetrahedral
silicate sheet and the octahedral hydrous aluminum oxide sheet. Various types of clays
result from different configurations of these sheets. The two main types of clays are kaolinite
and montmorillonite. Kaolinite crystals are large (70 to 100 layers thick), held together by
strong hydrogen bonds, and are not readily dispersible in water. Montmorillonite crystals are
small (3 layers thick) held together by weak bonds between adjacent oxygen layers and are
readily dispersible in water into extremely small particles.
Several laboratory and field measurement techniques are available for determining the
magnitude of cohesion, or shear strength, of clays. Among these, the vane shear test, which
is performed in the field is one of the simplest. The vane is forced into the ground and then
the torque required to rotate the vane is measured. The shear strength is determined from
the torque required to shear the soil along the vertical and horizontal edges of the vane.
Briaud et al. (1999) describe equipment and methods developed to determine the erosion
rate of cohesive soils. Erosion rate is defined as the vertical distance scoured per unit of
time. The erosion rate, expressed as mm/hr, is related to shear stress, given in N/m2,
imposed at the soil water interface using an erosion function apparatus (EFA). The EFA
develops an erosion rate-shear stress curve for a soil sample taken with an ASTM standard
Shelby tube with a 3.0 inches (76.2) mm outside diameter. The erosion rate-shear stress
curve for samples taken at a site can then be used to determine the scour depth as a
function of time.
3.2.8 Angle of Repose
The angle of repose is the maximum slope angle upon which non-cohesive material will
reside without moving. It is a measure of the intergranular friction of the material. Simons
developed Figure 3.4 for the angle of repose for dumped granular material.
3.3 FLOW IN SANDBED CHANNELS
Most larger streams flow on sandbeds for the greater part of their length. Thus, there are
potentially many more opportunities for highway crossings or encroachments on sandbed
streams than in cohesive or gravel streams. In sandbed rivers, the sand material is easily
eroded and is continually being moved and shaped by the flow. The mobility of the sandbed
creates problems for the safety of any structure placed in or over the stream, for the
protection of private property along these streams, and in the preservation and enhancement
of the stream environment.