Soil physical properties and testing Flashcards
What is soil?
an aggregate of grain and void spaces
How is soil formed?
accumulation of organic material and/or rock weathering (physical or chemical)
What is the cementation of soil like?
non or weakly cemented
What is the compressive strength of unconfined soil?
Will fail/ collapse after 1 Mpa
What will all soils be derived from?
A parent rock
What do the mechanical properties of soil depend on?
Grain size
Minerology
Water content
Ability to move water
What methods are there for finding soil particle size?
Hydrometer method (stokes law - particle settling)
Laser particle analysis (sieves and finer particles decided using laser)
What is a 2 phase and 3 phase system with soil?
2 phase saturated soil which is solid and water
3 phase partially saturated so water, air and soild
What arethe spaces in soils known as?
voids or pores
What can pores be filled with?
Pore fluid: air, water, or both
What is pore size controlled by?
Size of pores depends on density of grain packing
What is the moisture content of soil?
measure of the amount of water present in a soil and is typically expressed as a percentage of materials dry weight
What is the outcome of the moisture test?
Ratio of mass of water contained in pore spaces of soil to soild mass of particles in that material
What is bulk density?
mass of a material per unit volume, including the material itself and the void spaces between the particle. (kg/m³ or g/m³)
What is the equation for bulk density?
Mass of material / Total volume
What is bulk unit weight (unit weight)?
Weight of material per unit volume, which includes the weight of the material and the voids. (N/m³)
What is the equation for bulk unit weight?
𝐵𝑢𝑙𝑘 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑥 𝑔𝑟𝑎𝑣𝑖𝑡𝑦 (𝑎𝑝𝑝𝑟𝑜𝑥. 9.81 𝑚/𝑠2)
What are the different unit weights for dry soil, saturated soil and water?
𝜸𝒅= dry soil unit weight = (14 - 20 KN/m³)
𝜸𝒈= saturated soil unit weight = (18 - 23 KN/m³)
𝜸𝒘= unit weight of water = (9.81 KN/m³)
What is the void ratio?
ratio of the volume of empty space to the volume of
solid particles in a soil mass
What is the formula for the void ratio?
Volume of Voids/Volume of Solids
e = (Vv / Vs)
What does the void ratio have a significant impact on?
soil behaviour such as volume change tendency, hydraulic conductivity, particle movement
What is degree of saturation?
Ratio of the volume of water to the total volume of void space. The Degree of saturation ranges from 0% (completely dry to 100% (fully saturated)
What is the formula for the degree of saturation?
Volume of Water / Volume of Voids
Sr = (Vw / Vv) x 100
What is specific gravity of soil?
ratio of the soil unit weight to the unit
weight of water
What is the formula for specific gravity?
𝑮𝒔 = 𝝆𝒔/𝝆𝒘
What is the solid limit of soil (SL)?
Minimum water content
Water content (%) soil is
saturated
Change from semi solid to solid
What is the plastic limit of soil (PL)?
Water content (%) soil
changes from plastic to
semi-solid state
What is the liquid limit of water (LL)?
Water content
(%) soil changes
from liquid to
plastic state
How is plastic limit tested?
The lowest water content at which the soil can
be remoulded into a 3.175 mm diameter
thread (ASTM, 2010)
What happens as plastic limit increases?
potential for swelling and shrinkage
increases
What are 2 tests for liquid limit and how is it done?
Cassgrande method - amount of bands for cut walls to meet when water added and banged (LL = 25 bangs)
Drop cone pentrometer - how far cone penetrates soil with diiferent amounts of liquid (20mm =LL)
How is the plasticity index derived?
Liquid limit – Plastic limit
What is the typical content of a soil with a high plasticity index?
High clay content
How can a soils stability be tested?
Using activity - capacity to hold water
What soil will have the highest acitivity?
clays that contain smectite
How does acticity affect plastic behaviour?
More activity = more plasticity
What is the difference between drained and undrained soil?
Drained - water can flow in/ out
Undrained - water cant flow
What is the problem with drained and undrained soils?
Drained might compress under load
Undrained water pressure increased
What will surface area be like for smaller particle size and how does it affect water content?
Soils with smaller particles have larger overall
surface areas
Therefore, clay- (and silt-) dominated partially saturated soils will tend to hold more water than coarser soils
What will hold more water clay or coarser materials?
clay as it has lots of micropores and generally have high porosities
What is permeability?
Ease of fluid flow
Smaller grains/ pores reduce flow (clay has lower permeability)
What are the formulas for total stress? (normal, with weight on top and layered soil)
Y = unit weight Z= depth
𝝈𝒗 = 𝜸. 𝒛
𝝈𝒗 = 𝜸. 𝒛 + 𝜸𝒘.𝒛𝒘
𝛾𝑤 = 𝑢𝑛𝑖𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟, 𝑧𝑤 = 𝑑𝑒𝑝𝑡ℎ 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟
𝝈𝒗 = 𝜸𝟏𝒅𝟏 + 𝜸𝟐𝒅𝟐 + 𝜸𝟑(𝒛 − 𝒅𝟏 − 𝒅𝟐)
