WEEK 7A: Soil Mechanics Origins and properties of soil ...
Transcript of WEEK 7A: Soil Mechanics Origins and properties of soil ...
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: