Revision: Soil = function (climate, organisms, relief
(landscape), parent material, time) Hans Jenny (1941) Regolith =
all surficial materials above fresh bedrock (Ollier & Pain
1996) Definition of a soil varies according to the science Pedology
- soil profile A horizon, B horizon, C horizon soil physics, soil
chemistry, soil biology Agriculture soil as a resource soil
fertility, soil health topsoil, subsoil Geomorphology soil series,
transported soil, residual soil
Slide 3
Engineering soil: a natural aggregate of mineral grains that
can be separated by such gentle mechanical means as agitation in
water. Rock on the other hand, is a natural aggregation of minerals
connected by strong and permanent cohesive forces the boundary
between rock and soil is necessarily an arbitrary one. (Karl
Terzaghi) Engineering soil: Loose, aggregated, low strength,
inelastic. Rock: Compact, solid, high strength, some elasticity
When is a soil a rock? When is a rock a soil?
Slide 4
Mechanical properties of soils = function (soil mineralogy,
particle size, particle shape, moisture, fabric, organic content)
Soil mineralogy: clay minerals have the most influence. Kaolin
clays - 1:1 clays (1 silica : 1 gibbsite) - Low CEC - Low water
absorption - Low plasticity Illite clays - 2:1 sheets -
Intermediate CEC - Intermediate water absorption - Intermediate
plasticity Montmorillonite clays - 2:1 sheets - High CEC - High
water absorption - High plasticity
Slide 5
Slide 6
Origins: Residual soil = product of in situ weathering of the
underlying parent rock Transported soil = transported from their
place of origin to where they are now Engineering soil description
and classification
Slide 7
ParticleSize Boulder>200 mm Cobble60 mm 200 mm Gravel2 mm 60
mm Coarse20 mm 60 mm Medium6 mm 20 mm Fine2 mm 6 mm Sand 2 mm 60 m
Coarse 2 mm 600 m Medium 600 m 200 m Fine 200 m 60 m Silt 60 m 2 m
Clay < 2 m Fine grained Coarse grained
Slide 8
Particle Size Distribution In coarse grain soils ... By sieve
analysis Determination of particle size distribution: In fine grain
soils ... By hydrometer analysis or particle sizer or sedimentation
(pipette) Sieve Analysis Hydrometer Analysis soil/water suspension
hydrometer stack of sieves sieve shaker
Hydrometer Laser particle sizer Fine grained soils Engineering
soil classification
Slide 11
Grain Size Distribution Curve can find % of gravels, sands,
fines define D 10, D 30, D 60.. as above.
Slide 12
Engineering soil classification GravelSandSiltClay Gravel, well
graded Sand, poorly graded (uniform) Sand, gap graded
Slide 13
Engineering soil classification Behaviour: Cohesive = sticks
together. Usually contains clay minerals. Non-cohesive = friable,
flows like sand. Can stick together when wet (apparent cohesion)
Colour: - Relatively unimportant as it is not an mechanical
property - Stick to primary colours (red-brown clay, pale yellow
sand, grey silt, etc.) - Can be useful to indicate: Organic matter
Saturation Filled ground (imported soils) Deleterious
materials
Slide 14
card house fabric Soil structure Soil fabric Engineering soil
classification
Slide 15
Consistency: Cohesive soils TermAbbreviationUCS (kPa)Tactile
test Very softVS
Engineering soil classification Consistency: Non-cohesive soils
TermAbbreviationRelative density (%)Tactile test Very looseVL<
15Ravelling LooseL15 35Easy shovelling Medium denseMD35 - 65Hard
shovelling DenseD65 - 85Picking Very denseVD> 85Hard picking
Relative density = relative to maximum dry density achieved in a
standard soil compaction test dealt with in later lectures
Slide 17
Engineering soil classification Sensitivity: TermUCS in situ :
UCS remoulded Insensitive< 2 Medium sensitive2 4 Sensitive4 8
Very sensitive8 16 Quick> 16 Thixotropy: Looses strength under
vibration.
Slide 18
Slaking soils: Dispersive soils: Engineering soil
classification Behaviour when saturated:
Slide 19
TermAbbreviationTactile DryDLooks and feels dry MoistMFeels and
looks moist, but no fee water on hands when remoulding WetWFeels
and looks wet, free water on hands when remoulding Engineering soil
classification Moisture: Moisture content is given by: Where: % is
the percentage moisture Wt is the initial weight of the soil Wd is
the dry weight of the soil
Slide 20
Engineering soil classification Consistency indices: Atterberg
Limits Liquid limit the above which the soil behaves as a liquid
Plastic limit the below which the soil ceases behaving as plastic
Shrinkage limit the below which no further volume change occurs
Sticky limit the below which the soil looses its adhesive quality
Cohesion limit the below which the soil particles no longer stick
together Liquid Limit (LL) - Plastic Limit (PL) = Plasticity index
(PI) Linear shrinkage Free swell
An international unified soil classification (USC) was
developed to for use in airfield construction (Casagrande 1948) and
dam construction (1952). The USC provides a common terminology for
soils based on their mechanical behaviour. Engineering soil
classification ComponentSymbolQualifierSymbol GravelGWell gradedW
SandSPoorly gradedP SiltMSiltyM ClayCClayeyC OrganicOLow
plasticityL PeatPtHigh plasticityH The primary division is made
between: Coarse grained soils >50% of the soil is >60 m Fine
grained soils >50% of the soil is
Engineering soil classification For coarse grained soils: If 5%
is 12%