Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It is concerned with the behaviour of soil mass under force. Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils.

This lesson discuss the elements of soil mechanics, only the basic considerations. The purpose is to provide an overview of the subject with an emphasis on those concepts related to geologic investigation and analysis. The following topics ar covered in this overview: index properties of soils, soil structure, compaction, effective stress, shear strength, stress distribution, and consodilation.

Key points

What is soil texture? soil structure? and soil fabric?

Cohesive vs Cohesionless soil?

Well-graded vs Poorly-graded vs Gap-graded?

What type of soil gradation is easy to be compacted?

What are different Atterberg limits (liquid limit, plastic limit, shrinkage limit)?

Distinguish total stress and effective stress in soil.

What is pore pressure/neutral stress in soil?

What is soil consolidation?

What types of test to determine the compaction behavior of soil?

What are different states of cohesive soil? Which index is used to determine the soil state/consistency?

What are two factors of soil shear strength?

What are the controlling variables of the compaction of soil?

Given a gradation curve, student must determine the D10, D30, D60 and two parameters, i.e. Cu and Cc.

Given a soil cross-section, student must be able calculate the stresses at any depth.

Technical terms

Soil texture

The appearance or feel of soil determined by its partcle size, shape, and gradation

Cohesive soil

Soils that particles stick together even without confinement, they contain clay mineral and pocess property of plasticity.

Cohesionless soil

Soils that contains particles that do not stick together, they have no plasticity.

Grain size distribution/gradation

A list of values that defines the relative amount, typically by mass, of particles present according to size

Well graded

even distribution of particles sizes over a wide range

Poorly graded

grain size is concentrated in a narrow range, grading curve is steep.

Gap graded

the gradation lacks of certain sizes.

Uniformity coefficient

Coefficient of curvature

Void ratio

Porosity

Degree of saturation

Water content

quantity of water contained in a material , defined as the ratio between mass of water and mass of solid in soil

Density

the relation between the mass and the volume of a soil

Saturated density

the density of soil when pores in soil are fully occupied by water

dry density

the ratio of mass of solids to the total volume of the soil

submerged density

defined as the mass of soil minus the mass of water displaced by it upon submergence, divided by the volume.

Relative density

the ratio of the difference between the void ratios of a cohesionless soil in its loosest state and existing natural state to the difference between its void ratio in the loosest and densest states.

Soil consistency

the relative ease with which a soil can be deformed, depends greatly on the nature of soil minerals and on the water content.

Atterberg limits

the limits of moisture content that define soil behavior, proposed by Atterberg

Liquid limit

lowest water content at which soil will behave as a viscous liquid

Plastic limit

lowest water content at which soil will behave as plastic material

Plasticity index

the range in water content over which soils behave plastically.

Shringkage limit

water content that below which soil do not decrease further in volume.

Liquidity index

the index that measures the sensivity of soil

Soil structure

the arrangement of the soil particles and the interparticle forces acting between them.

Fabric

the arrangement of the soil particles

Stress in soil

Stresses at a point in a soil layer are caused by: 1‐Self weight of the soil layers (Geostatic Stresses) 2‐Added load (Such as buildings, bridges, dams). When a load is applied to soil, it is carried by the water in the pores as well as the solid grains.

Total stress

The total vertical stress acting at a point below the ground surface is due to the weight of everything lying above: soil, water, and surface loading. Total stresses are calculated from the unit weight of the soil.

Pore water pressure

The pressure within porewater is called pore pressure (u).

Effective stress

The difference between the total stress and the pore pressure is called the effective stress

neutral stress

the other name of pore pressure or pore water pressure

Shear strength

Maximum shear stress that soil can sustain

Cohesion

the force that holds together molecules or like particles within a soil. Cohesion, c, is usually determined in the laboratory from the Direct Shear Test.

Internal friction

internal force that resists the movement between the particles of a soil

internal friction angle

Its tangent (S/N) is the coefficient of sliding friction. Its value is determined experimentally

Compaction

the densification of soil through the application of mechanical energy, mainly by the rearrangement of particles..

Compactive effort

amout of mechanical energy applied to the soil during compaction.

Maximum density

Optimum moisture content

Consolidation

the reduction in volume of clays under external loading as water drains from the pore, it is time dependent phenomenon

Specific gravity

the ratio of the unit weight of solid particles to the unit weight of water