LECTURE 6

1. LECTURE 6 Soil Physical (Mechanical) Properties – Bulk density, porosity, strength, consistency

2. Definitions… • Atterberg limits (H. Matengu) • Soil strength (L. Olver) • Soil dynamics (N. Davenport) • Soil micromorphology (A. Pietersen)

3. Physical properties: • “Characteristics of soil which can be measured by physical means and expressed in physical terms, such as colour, density, porosity, hydraulic conductivity, structure, texture and depth” – van der Watt & van Rooyen, Soil Science Society of South Africa • Mechanical properties: • “An expression of the materials which make up the soil…” – Pitty (1978)

4. Bulk Density… • Definition: “The mass of dry soil per unit bulk volume. Values range roughly from 1000 – 1800 kg.m-3, although higher values may be found in compacted soils.” – van der Watt & van Rooyen, Soil Science Society of South Africa • Is an expression of the proportions of solid and void in the soil matrix. • Strongly influenced by texture, sorting and organic matter content. • Why? How? • Why is bulk density important? • Influences permeability, drainage rate and penetration by roots and burrowing animals.

5. Porosity… • Definition: “The percentage volume of the soil occupied by pores and pore space.” • Effective pore space = part of the pore system through which fluids can move freely. • Strongly related to bulk density • Lower bulk density = higher total pore space. • Pore size • Can be divided into macropores (larger than 0.08mm), and micropores (smaller than 0.08mm).

6. Soil strength… • Definition: “A general term referring to the ability of a soil to resist deformation by applied forces, which could be any one of several types.” • Can be described in terms of cohesion, shear strength and adhesion.

7. Cohesion: • “The attraction of a substance for itself; the mutual attraction among molecules or particles comprising a substance that allows it to cling together as a continuous mass” • Changes as soil dries out and bulk density increases. • Maximum bond when soils are wet with an extremely restricted amount of water (water retreats into micropores and creates a “suction” force). • Commonly measured as soil’s resistance to penetration (use of a penetrometer). • Strongly related to soil erodibility. • NB in engineering.

8. Shear strength: “The maximum resistance to shearing stresses which a specimen or element of soil can withstand before failure occurs” • Depends on internal friction and cohesion

9. Adhesion: “Refers to a molecular attraction which holds two dissimilar substances in contact, such as water and soil particles” • Increases as texture becomes finer. • Increases with higher soil organic matter content. • Adhesion between soil and a foreign object can be attributed to the water film between the 2 surfaces. • Soil “stickiness” occurs when cohesion is less than adhesion (this is dependent on moisture content).

10. Consistency… • The greater the amount of moisture in a soil, the more it can behave like a liquid • Less interaction between adjacent particles. • As water is added to dry soil, it passes through phases: • Solid • Semi-solid • Plastic • Liquid

11. Between each of these states, there is a boundary/limit. • These known as the Atterberg limits • Shrinkage limit: • Limit between solid and semi-solid states. • Above this limit, semi-solid state is reached. • Plastic limit: • Limit between semi-solid & plastic states. • Soil becomes plastic and can be moulded into shapes. • Limit defined as the minimum moisture content at which the soil can be rolled into a thread of 3mm diameter without breaking up. • Liquid limit: • Limit between plastic and liquid states. • At this moisture content, the soil will flow under its own weight.