Soil composition and classification - particle sizes & plasticity Flashcards
1
Q
Why do we need to know how to describe or classify soils from a geotechnical engineering standpoint?
A
To properly analyse, design and predict the performance of infrastructure (i.e. engineering properties of soils depend on their nature and state).
2
Q
What properties do earth dams need to have?
A
- Need to be strong
- Low permeability
- Minimise settlement
3
Q
Why do they use clay in the centre of earth dams?
A
They use clay as it is very compacted and has small voids. This means its harder for water to flow through.
4
Q
Why is pipeline trenching/backfilling relevant?
A
Relevant to the transport of natural gas or energy
5
Q
Why is choosing the material for a retaining wall important?
A
Compact it to increase strength and reduce settlement.
6
Q
Why’s must engineers understand soils?
A
Engineers must understand soils to propose technically sound and safe design solutions.
7
Q
What is the nature of a soil?
A
- Essential characteristics or basic qualities of soils
- Assessed through intrinsic parameters
8
Q
What is the state of a soil?
A
- Will change over time
- Physical conditions in which the soil exists
- Quantified via state variables
9
Q
What are 3 examples of characteristics of the nature of soil?
A
- Particle size
- Particle shape
- Mineralogy/plasticity
10
Q
What’s involved with particle size distribution?
A
- Sample preparation
- Riffling and quartering
- Sieving and/ Sedimentation
11
Q
What are the key aspects of the sieving method?
A
- Can be dry or wet
- Aperture size
- Mass retained
- Percentage passing
12
Q
What are the key aspects of sedimentation?
A
- Hydrometer/pipette
- Sedimentation
- Settling velocity
- Dispersing agent
13
Q
For soil composition and classification
What does the method choice depend on?
A
The size of the material
14
Q
What size are sands?
A
0.06-2 mm
15
Q
What size is gravel?
A
2-60mm
16
Q
What are the subcategories of soils?
A
- Fine
- Medium
- Coarse
17
Q
Why do we need to use microscopes after soils become 60 µm?
A
The physics starts to change
18
Q
What is dry sieving suitable for?
A
For soils containing significant quantities of silt and clay
19
Q
What is wet sieving mainly used for?
A
Wet sieving is mainly used for coarse-grained soils.
20
Q
What is the pitch?
A
The aperture size plus the wire diameter.
21
Q
What’s the limit of the number of sieves in a row?
A
7
22
Q
What is the smallest possible sieve aperture?
A
63µm
23
Q
What are the possible sources of error in the sieve method?
A
Sieve holes are square so the grains tested could be more rounded and may not pass depending on size.
24
Q
What are classified as fine-grained soils?
A
Clays and silts
25
What are the 3 methods of analysing sedimentation?
1. Hydrometer
2. Pippette - a bit tricky
3. Laser diffraction → better method
26
What are the two main principles to measure the size of materials in sedimentation?
- Settling velocity
- Dispersing agent
27
What is the settling velocity?
The speed with which particles of a certain diameter settle out from a fluid.
28
What is a dispersing agent?
Added to soil/water solution to prevent particles from sticking together.
29
What is a hydrometer used for?
- To see the distribution of grain size.
- A device that measures the density of a fluid.
30
What is involved in using a hydrometer?
- Consists of the graduated stem (marks) and glass bulb with mercury and will float in the fluid
- Container with slurry - Shake well so homogeneous soil settles
- Place hydrometer in the fluid which is to be measured and read the water level off the scale on the stem
31
What equation needs to be taken into account when analysing particles during sedimentation?
Stokes' Law
32
What does laser diffraction do?
Uses a light source and camera to capture soil particles and grains that fall
33
What's are the issues associated with laser diffraction?
- Expensive
- Not available everywhere
34
What makes a "real" soil?
- A range of particle roughness
- Non-spherical particles
35
How can we analyse PSD from a plot?
- Positive slope - material in that range
- Gap graded - horizontal line → that size not within the material
- Well graded - linear line increasing
- Uniformly graded - vertical line
36
What is some grading description?
- Well-graded
- Poorly Graded
- Uniformly Graded
- Gap graded
37
What does d_50 mean?
50% of the material is going to have a diameter of N or smaller
38
What is C_u?
Coefficient of uniformity
39
What is the min value of C_u?
1 - vertical line
40
What values are usually obtained for C_u?
Values between 2 and 10
41
What is the formula for C_u?
d_60/d_10
42
What does C_z mean?
Coefficient of curvature
43
What is classified as well graded in the coefficient of curvature?
Well graded: 1
44
What is the formula for C_z?
(d_30)^2/d_60*d_10
45
What can be said about soils and their state?
Soils are not continuous, homogenous - three-phase material
46
What materials are within soils?
Have solids (minerals and organic matter) and liquids(water and air)
47
What is the equation for volume of a soil?
V = V_a + V_w + V_s
48
What is the void volume of a soil?
V_v=V_a+V_w
49
What is the mass of a soil?
M = M_w+M_s
50
What characteristics depend on the nature?
G_s = P_s/P_w - specific gravity
- density
51
What values for specific gravity are usually obtained for soils?
2.6-2.7 -> due to similar mineral composition
52
What characteristics depend on the state of a material?
- Bulk density → P = M/V
- Void ratio → e=V_v/V_s
- Water content → w=M_w/M_s
53
What is the water ratio?
Ratio of the weight of water to the weight of solids
54
What is the formula for water content?
w=W_W/W_s=M_W/M_S
55
What is void ratio?
Ratio of volume of voids to the volume of solids
56
What is the min and max void ratio?
- Minimum void ratio = 0
- Maximum void ratio = infinity → no solids at
57
What is the formula for specific volume?
1+e = V/V_s
58
What is the min and max for specific volume?
- Max value = infinity
- Min value = 1
59
What is porosity?
The ratio of the volume of voids to the total volume
60
What is the formula for porosity?
n = e/1+e
61
What is the degree of saturation?
The volume of water over the volume of voids
62
What are the units for degree of saturation?
%
63
What is the formula for degree of saturation?
S=V_w/V_v = wG_s/e
64
What is saturated conditions a limited state?
As much water as the voids allow it to be filled
65
What is unit weight?
Weight per unit of volume
66
What are the 3 different types of unit weight?
- Bulk unit weight
- Dry unit weight
- Saturated unit weight
67
What's the formula for bulk unit weight?
𝛾= 𝛾/1+w = (Sr*e+G_s/1+e)𝛾_w
68
What is the formula for dry unit weight?
𝛾_d = W_s/V_t= (G_s /1+e)𝛾_w
69
What is the formula for saturated unit weight?
𝛾_s=(G_s+e/1+e)𝛾_w
70
What is the effective unit weight?
Is the weight of the soil solids in a submerged soil per unit volume
71
What's a a key thing to note about the submerged unit weight?
Submerged unit weight<<
72
Whats the formula for effective unit weight?
𝛾'=𝛾_s - 𝛾_w
73
What can a min void ratio suggest?
Densest soil packing that can be determined with a given testing method
74
What does a max void ratio suggest?
Loosest soil packing that can be determined with a given testing method
75
What is relative density in soils?
The measure of the compactness of cohesionless soil
76
What does the fabric of a soil suggest?
The same soil can have different arrangements - Thus, there is a maximum and minimum density a soil can have.
77
What is the formula for relative density?
D_r = (e_max -e)/(e_max-e_min)
78
What are 3 characteristics included in soil plasticity?
- Deposition
- Burial
- Erosion
79
What happens in burial?
- Overburden
- Development of bonds
- Consolidation
80
What are the two categories in the "state" of a soil?
- Fabric
- Bonding
81
What is fabric?
Physical arrangement and organisation of particles(e.g. sizes, shapes, orientation, layering, packing)
82
What is bonding?
Soil particles connections and their stability (e.g cementation, bonding nature and durability)
83
What is structure?
Structure = fabric + bonding
84
What is undisturbed soil?
Underground and not affected by human action - natural state
85
What is remoulded?
Soil sample disturbed - change structure and retain some of it
86
What is reconsitituted?
All of its natural structure removed due to mechanical mixing
87
What is intact?
Some structure lost - as close to an undisturbed state
88
What does the current state of a soil depend on?
The current state of a soil depends on its depositional and stress histories.
89
What is liquidity index (clay) formula?
I_L=w-w_p/w_L-w_p
90
What is liquidity index?
The liquidity index (LI) is used for scaling the natural water content of a soil sample to the limits.
91
What is the formula for relative density in sands?
D_r=e_h-e/e_h-e_l
92
What are the atterberg limits?
- Plasticity
- Liquid limit (w_L or LL)
- Plastic limit (w_p or PL)
93
What is plasticity?
Soil’s ability to undergo irrecoverable deformation without cracking or crumbling.
94
What is the liquid limit?
The lowest water content at which the soil cannot maintain its shape [~ thick slurry to suspension].
95
What is the plastic limit?
The lowest water content at which the soil remains plastic/deformable without cracking or crumbling [firm and mouldable to soft and sticky].
96
What is the plasticity index formula?
I_p = w_L − w_P
97
How can you find the plastic limit?
Get smaller specimens of clay and start rolling and find water content at which you can do 3mm in diameter and barely starts to crumble
98
What are sources of error for the method for finding the plastic limit?
- Its user dependent
99
What are the two methods to find liquid limit?
- Cone penetormeter
- Casagrande cup
100
How does the cone penetrometer work?
- Mix soil to smooth paste and fill metal cup with soil don’t leave any air trapped inside and huge air bubble will make it easier to penetrate but wont be accurate measure
- Strike of excess soil so smooth surface
- Lower tip of cone to surface - initial reading
- Lower stem of dial gauge to contact cone shaft - 5s - final position (cone penetration measurement)
101
What are the positives of the cone penetrometer?
- Simple apparatus
- less affected by the operator
- less subjective
102
What are the negatives of a Casagrande cup?
- Operator dependant
- Subjective
103
What does a A-line chart do?
Splits clays from silts
104
What are on the axes of a A-line chart?
- Liquid Limit in H axis
- Plasticity Index in V axis
105
Why can't you plot offshore sediments on an A-line chart?
Offshore sediments - not going to work - **weird shape so behave differently → Internal pores and the weird shape**
106
What is the activity?
The ratio of plasticity index to the clay size fraction
107
What is the formula for activity?
A = I_p/C
108
What are the nature characteristics of clay?
- Mineral composition
- Plastic limit
- Liquid limit
- Plasticity index
109
What are the state characteristics of clay?
- Water content
- Void ratio
- Liquidity index
- Clay structure
110
What are the nature characteristics of sand?
- Mineral composition
- PSD
- Min. void ratio
- Max. void ratio
111
What are the state characteristics of sand?
- Grain structure
- Void ratio
- Relative density
- State parameter
112
What are clay minerals?
Decomposition products of feldspar and volcanic ash
