WEEK 7A: Soil Mechanics Origins and properties of soil What is soil? • Uncemented aggregates of minerals, and sometimes decayed organic matter, with liquid and

gas filling the space (voids) between solid particles Origins • Soil is formed from weathering of rock • Weathering: the process of breaking down rocks by mechanical and chemical means into

smaller pieces • Three main types of rock

o Igneous: solidification of molten magma or lava

o Sedimentary: cemented deposits of soil

o Metamorphic: created from other types of rock through changes in compositions and

texture by heat or pressure

Properties of soil • Important to be able to quickly identify and describe properties of soil that can be related to

their engineering properties Particle shapes • Three main categories for shapes

o Bulky (rounded, angular, sub-rounded, sub-angular) o Flaky (mostly found in clays)

o Needle (very rare) Particle size • Soils categorised into two main groups based on particle size: 1. Coarse grained soil: D>0.075mm 2. Fine grained soil: D<0.075mm

Where D = diameter • Coarse grained soil separated into:

o Gravel o Sand

• Fine grained soil separated to o Silt o Clay

• Diameter ranges can help categorise them

Sieve analysis • Consists of shaking a soil sample through a set of sieves with progressively smaller openings,

distribution of particle sizes is plotted afterwards Particle size distribution • Represents the distribution of sizes in a soil sample on a logarithmic scale • Curve indicated whether a soil sample has a large range of particle sizes or a smaller range of

sizes

• Poorly graded (I): most of soil particles are of the same size (most particle sizes are of one size,

no smaller particles to fill in void spaces, however it does allow for drainage) • Well graded (II): soil particles sizes are distributed over a wide range (stronger/denser) • Gap graded (III): soil sample has particles of two or more similar sizes (two poorly graded)

Clays • Have flaky particle shapes • Develop plasticity (the ability to undergo deformation without cracking/crumbling) when

mixed with limited amounts of water (moulding) Soil consistency • Clays develop plasticity with limited amounts of water, this will change a soil's consistency

• The limits tell us the ranges of plasticity with water content

Plasticity index • The measure of the plasticity of soil is given by:

PI = (liquid limit)LL - (plastic limit)PL

Weight and volume relationship • Soils are made of three phases:

o Soil (soil particles) o Liquid (usually water) o Gas (air)

• Phase diagram: used to represent the three phases of soil mass

Basic definition

• Weight of air is close to 0 so it is ignored

• (Don’t have to memorise) • Void ratio: how much empty space there is to total volume • Degree of saturation: how much water is taking up of the total empty space • Unit weight: total weight over volume • Dry unit weight: excludes water • Saturated unit weight: if degree of saturation is 1 (if voids are full of water)

Stresses in soil

• The total vertical stress at a point in a soil mass is given by

o Y is the unit weight of soil, which could be the dry, moist or saturated unit weight, depending on the state of the soil

o H is the depth of the soil from the ground surface to the point of interest

• Pore water pressure

o Yw is the unit weight of water, use Yw=9.8kN/m^3 o Hw is the depth from the top of the ground water surface to the point of interest

• For layered soils, suppose that we have n layers of soil above the point of interest, the total

vertical stress is given by:

o The total stress delta in a soil is divided into two parts: 1. Portion carried by the water 2. Remaining portion carried by soil solids at points of contact = effective stress

• Effective stress: the sum of the intergranular forces over the cross sectional area of the soil

mass (governs strength and failure)

o Sigma prime = total stress - pore water pressure

• For a layered soil:

• When effective stress becomes zero, the soil loses its strength (liquefaction)

Shear strength of soil • Soil will slide on one another (effective stresses is not high enough to prevent grains from

sliding against one another) • Direct shear test

o Increase T until shear failure (soil slides on top and bottom)

• Sliding of the grains

• Dense sand requires higher sheer stress

Shear strength of soil

• Mohr Caulomb failure criteria

• Anything below the line is safe and doesn't fail • On and above the line fails

Types of foundations

Shallow foundations

a. Spread footing b. Mat/raft foundation

Deep foundations

c. Pile foundation d. Drilled shaft

Bearing capacity • Strip footing

Terzaghi's bearing capacity equation • For a strip footing:

• To find Nc, Nq, Ny we use table • For a square footing:

• For a circular footing:

Factors of safety for bearing capacity Factors of safety (FS)≥3 Allowable bearing capacity

• Reasons for factor of safety: 1. Soil is neither homogenous or isotropic 2. Uncertainty in determining shear strength parameters

• (only a is relevant)