Soil physical properties and testing

Question 1

What is soil?

Answer 1

an aggregate of grain and void spaces

Question 2

How is soil formed?

Answer 2

accumulation of organic material and/or rock weathering (physical or chemical)

Question 3

What is the cementation of soil like?

Answer 3

non or weakly cemented

Question 4

What is the compressive strength of unconfined soil?

Answer 4

Will fail/ collapse after 1 Mpa

Question 5

What will all soils be derived from?

Answer 5

A parent rock

Question 6

What do the mechanical properties of soil depend on?

Answer 6

Grain size
Minerology
Water content
Ability to move water

Question 7

What methods are there for finding soil particle size?

Answer 7

Hydrometer method (stokes law - particle settling)
Laser particle analysis (sieves and finer particles decided using laser)

Question 8

What is a 2 phase and 3 phase system with soil?

Answer 8

2 phase saturated soil which is solid and water
3 phase partially saturated so water, air and soild

Question 9

What arethe spaces in soils known as?

Answer 9

voids or pores

Question 10

What can pores be filled with?

Answer 10

Pore fluid: air, water, or both

Question 11

What is pore size controlled by?

Answer 11

Size of pores depends on density of grain packing

Question 12

What is the moisture content of soil?

Answer 12

measure of the amount of water present in a soil and is typically expressed as a percentage of materials dry weight

Question 13

What is the outcome of the moisture test?

Answer 13

Ratio of mass of water contained in pore spaces of soil to soild mass of particles in that material

Question 14

What is bulk density?

Answer 14

mass of a material per unit volume, including the material itself and the void spaces between the particle. (kg/m³ or g/m³)

Question 15

What is the equation for bulk density?

Answer 15

Mass of material / Total volume

Question 16

What is bulk unit weight (unit weight)?

Answer 16

Weight of material per unit volume, which includes the weight of the material and the voids. (N/m³)

Question 17

What is the equation for bulk unit weight?

Answer 17

𝐵𝑢𝑙𝑘 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑥 𝑔𝑟𝑎𝑣𝑖𝑡𝑦 (𝑎𝑝𝑝𝑟𝑜𝑥. 9.81 𝑚/𝑠2)

Question 18

What are the different unit weights for dry soil, saturated soil and water?

Answer 18

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)

Question 19

What is the void ratio?

Answer 19

ratio of the volume of empty space to the volume of
solid particles in a soil mass

Question 20

What is the formula for the void ratio?

Answer 20

Volume of Voids/Volume of Solids
e = (Vv / Vs)

Question 21

What does the void ratio have a significant impact on?

Answer 21

soil behaviour such as volume change tendency, hydraulic conductivity, particle movement

Question 22

What is degree of saturation?

Answer 22

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)

Question 23

What is the formula for the degree of saturation?

Answer 23

Volume of Water / Volume of Voids
Sr = (Vw / Vv)

Question 24

What is specific gravity of soil?

Answer 24

ratio of the soil unit weight to the unit
weight of water

Question 25

What is the formula for specific gravity?

Answer 25

𝑮𝒔 = 𝝆𝒔/𝝆𝒘

Question 26

What is the solid limit of water (SL)?

Answer 26

Minimum water content
Water content (%) soil is
saturated

Question 27

What is the plastic limit of soil (PL)?

Answer 27

Water content (%) soil
changes from plastic to
semi-solid state

Question 28

What is the liquid limit of water (LL)?

Answer 28

Water content
(%) soil changes
from liquid to
plastic state

Question 29

How is plastic limit tested?

Answer 29

The lowest water content at which the soil can
be remoulded into a 3.175 mm diameter
thread (ASTM, 2010)

Question 30

What happens as plastic limit increases?

Answer 30

potential for swelling and shrinkage
increases

Question 31

What are 2 tests for liquid limit and how is it done?

Answer 31

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)

Question 32

How is the plasticity index derived?

Answer 32

Liquid limit – Plastic limit

Question 33

What is the typical content of a soil with a high plasticity index?

Answer 33

High clay content

Question 34

How can a soils stability be tested?

Answer 34

Using activity - capacity to hold water

Question 35

What soil will have the highest acitivity?

Answer 35

clays that contain smectite

Question 36

How does acticity affect plastic behaviour?

Answer 36

More activity = more plasticity

Question 37

What is the difference between drained and undrained soil?

Answer 37

Drained - water can flow in/ out
Undrained - water cant flow

Question 38

What is the problem with drained and undrained soils?

Answer 38

Drained might compress under load
Undrained water pressure increased

Question 39

What will surface area be like for smaller particle size and how does it affect water content?

Answer 39

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

Question 40

What will hold more water clay or coarser materials?

Answer 40

clay as it has lots of micropores and generally have high porosities

Question 41

What is permeability?

Answer 41

Ease of fluid flow
Smaller grains/ pores reduce flow (clay has lower permeability)

Question 42

What are the formulas for total stress? (normal, with weight on top and layered soil)

Answer 42

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)