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Soil Types
 Soil – all unconsolidated material in the earth’s crust
 Soil includes –
 Mineral particles – sand and clay
 Organic Materials – found in topsoil and marsh
 Air
 Water
Mineral Soils
 Result from weathering of rock that forms the solid
crust of the earth
 Physical weathering – due to the action of frost, water,
wind, glaciers, landslides, plant and animal life, and
other weathering agents – that break particles away
from the bedrock
 Particles are often transported by wind, water , or ice
Rounds them and further reduces their size
 Soils created this way are referred granular soils
Grains or particles are similar to the original bedrock
Mineral Soils
 Chemical weathering – occurs when water flows
through rocks and leaches out some mineral
 New soil particles are formed from these mineral
 Called clays
 Clay particles are mineral crystals that have very
different properties from those of the original bedrock
Types of Mineral Soils
 Gravel
 Sand
 Silt
 Clay
 Course grained soils – gravel and sand
 Fine grained – silt and clay
 Cobbles – over 75mm or 3 in
 Boulders – over 200mm or 8 in
 Clays are cohesive soils – bonded to each other
Gravels and Sand
 Composed mainly of rounded or cubical grains that
are supported by adjacent grains
 Can carry significant loads
 Loads are spread across many particles through friction
 Fairly easy to compact
 Excellent soils for construction
Clays and Silts
Clays are softer
Do not carry loads very well
Clay grains are small size and flat plate like shape
The mass of the grain as a force is negligible when compared to
the forces resulting from the surface properties of the grain
 Clays have charges on surface – figure 1-1 page 3
 Result of these charges is clay can hold a lot of water
Surface charges attract water molecules
Clays absorb or hold water – permanently unless conditions change
May dry out due to evaporation – or squeeze water out when load is
applied –
Will absorb moisture quickly when re applied
 The plates of clay are attracted to opposites charges
Field Test to Identify Soil types
 Large grains (sand and gravel) are easily to identified
 Organics are also easy
 Silts and clays – are not as easy cause grains are not
 Page 5 table 1-2 differences between silt and clay
Mass-Volume Relations
 Soil sample contains
 Mineral – possibly organic particles
 Water
 Air
 Mass and volume of each phase is usually calculated
 Va = Volume of Air
 Vw = Volume of water
 Vv = Volume of voids (=Va +Vw)
 Vs = Volume of dry soil solids
 V = Total volume (=Va +Vw +Vs)
 Ma = Mass of air (=0 by definition)
 Mw = Mass of water
 Ms = Mass of dry soil solids
 M = Total mass (=Mw +Ms)
Mass and Volume
 Soil sample consisting of 10cm3 of air, 25cm3 of water
(mass = 25g) and 65cm3 of soil solids (mass = 175g)
 Vv = 35cm3
 Va = 10cm3
 Vw = 25cm3
 Vs =65cm3
 V=100cm3
 Mw =25g
 Ms=175g
 M=200g
Mass and Volume
 Relationship between the mass and volume
 Water
Pw = Mw/Vw
Where Pw = density of water
Density of water is 1g/cm3 or 1000kg/m3
Example 1-1 Pw = 25g/25cm3 = 1g/cm3
 Soil solids
Psoil solids = Ms/Vs
Where Psoil solids = density fo the dry soil solids
 Ratio between soil solids and density of water is the relative density
of the solids or specific gravity Gs
RD (relative density) = Psoil solids/Pw = Ms/(Vs*Pw)
Or RD =Ms/(Vs x Pw)
Example 1-1 RD=175g/(65cm3 x 1g/cm3) = 2.69
 Most soils – RD is between 2.6 and 2.8
Properties Calculations
 Density (P)
 Dry Density (Pd)
 Water content (W)
 Void ratio (E)
 Degree of saturation (S)
 Porosity(n)
 Problem 1-1 on page 8
e = Vv/Vs
Classification Tests
 Two Tests
 Grain size – to measure grain sizes
Sieve analysis used for sands and gravels
 Grain size distribution graph
 Example 1-9 page 17
Hydrometer used for silts and clays
 Sedimentation test
 Rate at which particles settle
 Strokes law states –that particles in a suspension settle out at a
rate that varies with their size
 Plasticity – to measure grain types
Grain Size Distribution Curve
 Used to help describe and classify a soil
 Shape –
Uniform soil –curve a on page 19
Well graded – curve b on page 19
 Effective size
 10% size is considered effective size – page 19 – sample b
 Uniformity coefficient –
 Value gives some indication of the shape of the curve
 Cu=D60/D10
 Coefficient of curvature
 Cc=(d30)2/(d60xD10)
Textural Classification
 American Society for Testing and Materials
 Gravel – larger than 4.75 (no. 4)
 Sand 4.75mm to 0.075mm (no. 4 to No. 200)
 Silt .075mm to .005mm (No. 200 to .005mm)
 Clay – smaller than .005mm
Plasticity Test
 Measures the amount of water that a soil adsorbs
 Plastic limit – soil is roll into a thread(Wp)
 Liquid limit (WL)
 Index of plasticity – range of water contents over which
this soil is plastic
 Atterberg Limits test
Soil Water
 Type of water found in soil
 Free water or gravitational water – found below
groundwater – free to flow under the laws of gravity
 Capillary water – brought up through the soil pores –
above the groundwater table
 Attached water or held water – moisture film around
soil grains
 The rate of water flow – or permeability of the soil
Darcy’s law
Q=k (h/l)a – page 33
Problem page 34
Soil Strength and Settlement
 Two type of soil failure
 Failures due to shear – grains slide with respect to other grains
 Settlement failures – where a layer of soil is compressed and
becomes thinner under loading
 Forces on soil
Forces acting perpendicular to the plane are normal forces
Forces acting parallel are shear
 Shear strength in most clays is due to cohesion
 T=c were t is sheering pressure and c = cohesion
 Shear strength in granular soils is due to friction
T=o tan o/