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

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

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)

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.

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).

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.

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.

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

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).

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

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.