[L9] Soil composition & classification 1

Engineering examples where soil analysis is important

• earth dams (strong, low permeability, minimise settlement)

• backfilling / pipeline trenching

• offshore pipelines

• retaining walls

• road embankments

Nature

= essential characteristics or basic qualities of soil

= assessed via intrinsic (= essential / natural) parameters

Examples: quartz sand is always made of quartz

• Particle sizes

• Particle shapes

• Mineralogy (plasticity)

• Plastic limit

• Liquid limit

• Plasticity index

Grain sizes

PSD methods

1. Sample preparation

2. Riffling (= machine) or Quartering (=dividing circle)

3. Sieving methods → COARSE or Sedimentation methods → FINE

Particle size boundaries

fine-grained soils < 0.06mm < coarse grained soils

Sieving methods

Dry sieving or Wet sieving

• aperture size

• mass retained

• % passing

Sedimentation methods

Hydrometer or Pipette

• sedimentation

• settling velocity

• dispersing agent

PSD coarse fraction → dry sieving or wet sieving?

Dry sieving ➞ only suitable for soils w/ insignificant quantities of silt & clay, mainly used for coarse-grained soils

Wet sieving ➞ significant fines

Sieving

• pre-wash & break clumps (matrix suction) before to prevent sticking

• class what goes through sieve at each aperture size

• source of error: shape of grains (not all square)

Example: m>75mm = particles bigger than mesh size of 75mm don’t pass

Aperture size

size of holes in sieve

Sedimentation techniques

for fine particles <425 microns

• pipette

• hydrometer

• laser diffraction

Settling velocity

The speed with which particles of a certain diameter settle out from a fluid

Dispersing agent

Added to soil/water solution to prevent particles from sticking together (laser diffraction)

Hydrometer

= device that measures density of a fluid

• graduated stem + glass bulb

• placed in sample to be tested

• water level read off the scale (bottom of meniscus)

Hydrometer → Stoke’s law

Applied to:

• smooth spherical particles

• laminar flow

• not applicable to particles <0.2 microns in diameter

→ describes settling velocity

Real soils:

• range of roughness

• non-spherical

• <0.2 microns

➞ causes particles to move faster than w/ Stoke’s law

Brownian motion = random movement of microscopic particles suspended in a fluid caused by collisions w/ surrounding molecules

PSD plot

x - Particle size in log10 scale (mm)

y - Percentage passing in linear scale

Sieve range

0.075mm to 4.75mm = fine, medium & coarse sand

Hydrometer range

0.001mm to 0.075mm = clay & silt (fine-grained)

PSD Grading descriptions

• Uniform (all similar size)

• Gap graded

• Well-graded

• Poorly-graded

d50

particle size such that 50% of the particles are smaller than d50 (mm)

PSD intrinsic parameters

Coefficient of uniformity Cu

C_u=d₆₀/d₁₀

C_u < 4 = poorly-graded

➞ describes range of sizes

Coefficient of curvature Cz

C_z=(d₃₀)²/d₆₀*d₁₀

1 < C_z < 3 = well-graded

other = gap-graded

State

= physical conditions in which soil exists

= quantified via state variables

• depends on its depositional and stress histories

Examples

• Water content

• Void ratio

• Liquidity index LI

• Clay structure

Example: dense or loose soil

Three phases in soils

SOLID

1. Soil particles: minerals, organic matter

VOID

Liquids:

2. Water

Gas:

2. Air

Soil matrix

= interconnected structure = solid particles + voids

Soil volume S

V=V_v+V_s

with

V_v=V_a+V_w

Soil mass S

M=M_w+M_s

(M_a=0)

Density of water N

1000 kg/m3

Density of solids N

Ms/Vs kg/m3

Specific gravity N

density solid / density water

Water content S

= ratio of the weight of water to the weight of solids in percentage (%)

w = Ww/Ws = Mw/Ms

Void ratio S

= ratio of the volume of voids to the volume of solids (dimensionless)

e = Vv/Vs = (Vw+Va)/Vs

Min e and max e ➞ determined w/ given test methods

Packing density S

• based on void ratio

Dense ➞ e = 0.2

Loose ➞ e = 1.45

Density of solids N

= mass per unit of volume in kg/m³

ρ = M/V

Degree of saturation S

= volume of water over volume of voids in percentage (%)

S = Vw/Vv = (w*Gs)/e

Relative density S

Same soil can have different arrangements ➞ max and min densities soil can have

• tells us how dense the soil is relative to its limits

Dr = (emax-e)/(emax-emin)

(for sands)

Unit weight S

𝛾 = ρ*g = M/V*g (bulk density)

𝛾_w = 10 kN/m³

Bulk density S

ρ = Mtot/Vtot

Specific gravity N

= ratio of the density of solids to density of water (dimensionless)

• typically between 2.6-2.7

Gs = 𝛾_s /𝛾_w = ρ_s / ρ_w = Ws/(Vs*𝛾_w)

Bulk unit weight S

= a portion of voids is filled by water & other by air (partial saturation)

Dry unit weight S

= all the volume of voids is filled by air

Saturated unit weight S

= all the volume of voids is filled by water (S=1)

Effective unit weight S

= weight of soil solids in a submerged soil per unit volume = weight of soil - buoyancy force exerted on it

𝛾_d ? 𝛾 ? 𝛾_sat

𝛾d < 𝛾 < 𝛾sat

Min void ratio N

emin

Densest soil packing that can be determined with a given testing method

Max void ratio N

emax

Loosest soil packing that can be determined with a given testing method

Soil life cycle

Natural state (in situ)

1. Deposition

• Genesis

• Formation of fabric

2. Burial

• Overburden

• Development of bonds

• Consolidation

3. Erosion

• Unloading

• Swelling

• Over-consolidation

State structure

arrangement & connections of soil components

Fabric + bonding

Undisturbed soil

Soil sample in its natural state

Remoulded soil

Soil sample that has been disturbed from its natural state with some or most of its natural structure removed due to mechanical means

Reconstituted soil

Soil sample that has all of its natural structure removed due to mechanical mixing e.g. at ~1.5 to 2 times its liquid limit

Intact soil

somewhat disturbed, some structure lost

Atterberg limits

• classifying ‘fines’ fraction of soil

Plasticity

= soil’s ability to undergo irrecoverable deformation without cracking/crumbling

Plastic limit (wp or PL) N

= lowest water content at which the soil remains deformable without cracking/crumbling

= empirically established water content at which the soil becomes too dry to be plastic

Test:

Soil shears both longitudinally and transversely when rolled to about 3 mm in diameter after 5 – 10 hand movements

• subjective results

Liquid limit (wL or LL) N

= lowest water content at which the soil cannot maintain its shape

Tests:

• cone penetrometer

• Casagrande cup

Cone penetrometer

• simple apparatus

• less affected by operator

• less subjective

→ more repeatable/reproducible results

Liquid limit = water content corresponding to a cone penetration of 20mm

Minimum of 4 evenly distributed readings ranging from 15 to 25 mm penetration

Casagrande cup

• simple apparatus

• operator dependant

• subjective

A-line chart

Ip =wL-wP

Plasticity index = liquid water limit - plastic water limit

A-line: IP​=0.73(wL​−20)

• does not always work (offshore sediments, fly ash etc)

Liquidity index S

I_L = (w - w_p) / (w_L - w_p)

Activity

A = I_P / C

C = clay fraction

I_P = plasticity index

Clay fraction

= % (by dry mass) of particles < 2 micrometers

Nature and state examples for clay vs sands

Clays

Nature

• Mineral composition

• Plastic limit

• Liquid limit

• Plasticity index

State

• Water content

• Void ratio

• Liquidity index LI

• Clay structure

Sands

Nature

• Mineral composition

• Particle size distribution

• Min void ratio e_min

• Max void ratio e_max

State

• Grain structure

• Void ratio

• Relative density

• State parameter

I_L=0?

at plastic limit

I_L=1

at liquid limit

Solid ? Plastic ? Liquid ?