Topic 2: Fine soil classification Flashcards
What soils are classed as ‘fine soils’?
Clay and Silt.
What are the three main properties of clay?
- Extremely flaky shape
- Small particle size
- Negative electrical charge on surface
Finish the statement:’ The smaller and flakier a particle is, the _______ will be its _______ ____.
(1) greater
(2) surface
(3) area
What is the symbol for Specific Surface?
S subscript s.
What is the equation for specific surface?
Specific surface = surface area (m^2)/ mass (g)
What is the specific surface equation for a 6-sided cube?
Specific surface of cube = 0.006/length x density of cube
What is the highest value for the specific surface of clay?
up to 350 m^2g^-1
Under what category of minerals does clay fall under?
Silicates, the most common group of minerals.
What do all silicates have in common?
They all contain the SiO4 tetrahedron.
What does a silicate tetrahedron contain?
It contains one Si4+ at the centre and four oxygen ions O^-2 at the corners.
What does a tetrahedral sheet consist of?
Multiple tetrahedrons placed next to each other.
What does a Octahedron consist of?
1 Aluminium/Magnesium ion surrounded by 6 oxygen ions.
Why do sheet silicates form flaky particles?
Weak bonds exist between the sheets where no oxygen ions are shared (lack of ionic bonding).
What does a 1:1 structure (Kaolin) consist of?
1 tetrahedron and one octahedron sheet.
How are the the tetrahedron sheet and octahedron sheet bonded?
By the strong bonds of shared oxygen-ions.
What does a 1:2 structure (TOT) consist of?
One octahedron sheet sandwiched between two tetrahedron sheets.
What is the process of clay swelling?
Water enters the space between the TOT sandwiches causing the clay to swell or shrink.
What happens to the surface area of an object if it is sliced up into smaller pieces?
Its surface area increases.
What happens when water molecules enter the gaps between the TOT sanwiches?
The cations (+) are attracted to the negatively charged oxygen ions on the surface creating a positively charged layer of ions. This occurs on both sides of the clay particle.
How can the size of the clay mineral be determined?
By the amount of charge/cations on the surface.
What do the mechanical properties of clay depend on?
The specific surface area.
What is the classification of clay based on?
Plasticity.
What creates a plastic material?
The fine flaky particles and the binding of water to the surface.
What type of soil is sand?
A non-plastic soil.
What happens when you mix sand with lots of water?
It will create a layer of sand at the bottom of the jar.
What type of soil is clay?
A plastic soil.
What can happen to high plasticity clay?
It can become a pourable liquid when mixed with enough water.
What has a greater influence on the engineering properties: the shape or the size of the particle?
The size.
What are consistency limits?
The water contents at which the consistency changes from one state to the next.
What are consistencty limits also known as?
Atterberg limits.
What are the four consistencies of soil?
Solid, semi-solid, plastic, and liquid.
What is the liquid limit?
The change of consistency from plastic to liquid.
What is the plastic limit?
The change of consistency from plastic to crumbly.
What is the shrinkage limit?
The boundary between semi-solid and solid.
How is the shrinkage limit measured?
By placing moist clay into an oven until it’s dry. Then measure the volume change.
