Soil Structure

There are 2 main approaches to the discussion of soil structure:

            structure of the particles

            structure of the pores

            We will look mainly at the particles for now, and come back later to look at the pores (check the class schedule). Particle structure mainly affects porosity and strength. Pore structure mainly affects fluid interactions - flow, drainage, and the like.

            How does structure arise? Order arises out of chaos through the input of energy. In the case of soil, the energy comes in the forms of adding and subtracting water, heat, and living organisms.

3 broad kinds of particle structure: Single-grain, massive, and aggregate.

Single grain, e.g. a sand pile

Angle of repose of sand depends on shape and roughness of particles, and can be altered by input of energy (e.g. vibration). Adding energy to a sandpile will result in a lower angle of repose, and likely a denser packing.

So if adding energy compacts the sand pile, why does adding energy to sand on the beach (stepping on moist sand) expand the sand? Hint: See Hillel’s box on soil physics at the beach.

Angle of repose would also differ depending on whether the sand is dry, moist, or saturated (for example, completely under water). Dry versus saturated differences are due to density and viscosity differences (like Stokes settling). The moist case, though, is due to tension of the water films under unsaturated conditions, which we’ll explore later in the semester.

Other things being equal, porosity is lower for a poly-disperse sand than for a mono-disperse sand. Or, in English, a wider distribution of particle sizes can be packed more densely than a narrow distribution (why?). Gupta and Larson (1979) developed a random- and maximum-packing model that shows this, among other things. I’m trying to get hold of this as a demo.

Notice that a cubic packing has particle coordination 6, and pore coordination 6. But there is no simple pattern here: for example, the tetrahedral packing has particle coordination 12, and pore coordination in the range of 6 (4 for the tetrahedra, and 8 for the unavoidable octahedra).

Massive, e.g. a puddled soil

Like a single-grained soil, this is in some ways a lack of structure: everything sticking to everything else. We generally think of a massive structure being characteristic of fine-textured soils, but it can also simply result from mis-management, like plowing a field that’s too wet (more on this next week).

Aggregate structure

We made a list of inputs and processes that contribute to, and detract from, the aggregation of the soil. Can you reconstruct the list?