Revision Flashcards
1
Q
List all 7 main tasks in geotechnics
A
- Desk Study
- Site reconnaissance
- Ground Investigation
- Ground Model
- Rock Unit Characteristics
- Analysis/Modelling
- Design of Structures
2
Q
What is the only strength of newly deposited soil?
A
Friction
3
Q
What does grain binding and cementation lead to the development of?
A
Tensile and cohesive strength
4
Q
How does cementation and recrystallisation occur?
A
From pore fluids and plastic migration from highly stressed grain contacts
5
Q
How many grades of rock weathering are there?
A
6
6
Q
What are the 6 grades of rock weathering?
A
- Fresh Rock
- Slightly Weathered
- Moderately Weathered
- Highly weathered
- Completely weathered
- Soil
7
Q
Give the name of and describe Grade 1 weathered rock
A
Fresh rock, clean rock
8
Q
Give the name of and describe Grade 2 weathered rock
A
Slightly weathered, increased fractures and mineral straining
9
Q
Give the name of and describe Grade 3 weathered rock
A
Moderately weathered, partly changed to soil, rock>soil
10
Q
Give the name of and describe Grade 4 weathered rock
A
Highly weathered, partly changed from rock to soil, soil>rock
11
Q
Give the name of and describe Grade 5 weathered rock
A
Completely weathered, decomposed soil with some remaining structure
12
Q
Give the name of and describe Grade 6 weathered rock
A
Soil, some organic content, no original structure
13
Q
Name 3 ways soil samples are recovered
A
Boreholes, excavations and in situ measurements
14
Q
What does the reliability of soil descriptions depend on? and why?
A
The quality of sampling, as soils can be delicate
15
Q
What are the two broad types of soil?
A
Cohesive and non-cohesive/granular
16
Q
What gives a soil its apparent cohesion?
A
Pore water suction in fine grain soils that are unable to drain water quickly
17
Q
List the 3 overarching elements of a soil description following BS5930:2015
A
a) Mass Characteristics (state and structure)
b) Material characteristics (nature and state)
c) Stratum name, GEOLOGICAL FORMATION, age and type of deposit
18
Q
In order, list the 3 things that should be included in the Mass Characteristics section of a soil description
A
a) Mass characteristics
1. relative density/consistency
2. discontinuities
3. Bedding
19
Q
In order, list the 4 things that should be included in the Material Characteristics section of a soil description
A
b) Material characteristics
1. colour
2. composite soil types: particle grading and composition: shape and size
3. tertiary constituents either before or after the principal soil type as appropriate
4. PRINCIPLE SOIL TYPE (SAND etc)
20
Q
List the 3 overarching elements of a rock description following BS5930:2015
A
a) Material characteristics
b) General information
c) Mass Characteristics
21
Q
In order, list the 6 things that should be included in the Material Characteristics section of a Rock description
A
a) material characteristics
1. strength
2. structure
3. colour
4. texture
5. grain size
6. ROCK NAME
22
Q
In order, list the 2 things that should be included in the General Information section of a Rock description
A
b) General Information
1) additional information and minor constituents
2) geological formation
23
Q
In order, list the 3 things that should be included in the Mass Characteristics section of a Rock description
A
c) Mass characteristics
1) state of weathering
2) discontinuities
3) fracture state
24
Q
Define extremely weak rock
A
Can be indented by a thumbnail, gravel sized lumps crush between finger and thumb, <1MPa
25
Define very weak rock
Crumbles under firm blows with point of geological hammer, can be peeled with a pocket knife, 1-5MPa
26
Define weak rock
Can be peeled with a pocket knife with difficulty, shallow indentations made by with a firm blow of the geological hammer, 5-25 MPa
27
Define medium strong rock
Cannot be scraped with pocket knife. Can be fractured with a single firm blow of geological hammer, 25-50MPa
28
Define strong rock
Require more than one blow of geological hammer to fracture, 50-100MPa
29
Define very strong rock
requires many blows of geological hammer to fracture, 100-250MPa
30
Define extremely strong rock
Can only be chipped by a geological hammer, >250MPa
31
Name 4 uses for Airr Photograph Interpretation
1. Identifying features (man made and geological)
2. Assessing land usage change
3. Assessing changes in the physical landscape
4. Identifying potential hazards
32
Name 7 variables that can be useful when looking at an aerial photo
1. Shape
2. Pattern
3. Size
4. Tone/Colour
5. Shadow
6. Texture
7. Time
33
What is the main limitation on photo interpretation?
It only provides a view of the surface with no information on the subsoil or lithological details
34
Why is looking at the texture important in an aerial photo?
Disturbed ground often looks rough or torn
35
Why is looking at time important in an aerial photo?
It can be helpful to determine the historical change of an area
36
Why is looking at colour important in an aerial photo?
Can hint at features or conditions of the ground (e.g. greenery suggests abundant water)
37
When would you use shallow foundations?
With small buildings with good quality ground
38
When would you use end bearing piles?
Larger buildings where good quality ground is deeper
39
When would you use skin friction piles
When there is no good quality ground within economical depths
40
Why do ground properties improve with depth?
Confinement increases strength
41
How is the MPa of a rock determined?
Using an unconfined compressive strength test (UCS)
42
What is the bearing pressure?
The load applied to the ground over the area of the foundation
43
What is the Ultimate Bearing Pressure (UBP)?
The maximum load the ground is able to sustain
44
What is the Safe Bearing Pressure (SBP)?
The maximum permitted bearing pressure under a particular design code. Defined as UBP/safety factor (normally 3)
45
What is the Acceptable Bearing Pressure (ABP)?
SBP lowered by further reduction factor to satisfy specific structural requirements. Usually 1 with rock but can be significant for soils
46
What is the typical cost of ground investigations as a percentage of total cost of roads in the UK?
0.2-1.5%
47
How do soil and rock landslides differ?
Soil failures occur through 'intact' material whereas rock landslide fail along pre-existing discontinuities
48
What are the 3 purposes of Engineering Geology in a site investigation
1. Confirm the suitability of the site for the proposed project
2. Documenting the strengths, behavioural characteristics and engineering properties of rocks and soils present (soils and drift geology)
3. Recognising potentially hazardous ground (much of engineering practice involves Quaternary deposits
49
What is the difference between a ground investigation and a site investigation?
Ground investigation looks to assess ground conditions
| Site investigations are holistic and include legal and environmental aspects
50
Identify 9 parts of a thorough site investigation
1. Surface and near surface soils and rocks, lithology, weathering, digenesis
2. Rock structure and discontinuities
3. Climatic and sea-level changes
4. Resulting, active and relic erosional fluvial and marine features
5. Hydrogeological features
6. Man-made impacts (brownfield sites)
7. Mass movement on slopes
8. Volcanic and seismic activity
9. Potential environmental issues
51
How is a site investigation holistic?
It look to understand the importance of the world and the interdependence of its parts
52
Describe an engineering geological model
An approximation of the geological conditions at varying scales created to solve an engineering problem. It is a constantly evolving model. They can be as simple or as complex as the project dictates
53
What are the 3 broad types of engineering models?
1. Geological models
2. Ground models
3. Geotechnical models
54
How do geological models compare with ground models?
Geological models are based largely on geological knowledge whereas ground models are embedded with engineering parameters
55
How does a geotechnical model differ from ground and geological models?
Geotechnical models include mathematical or physical analysis
56
What are the two approaches to engineering geological models?
1. Conceptual approach
| 2. Observational approach
57
Describe the conceptual approach to geological/ground models
Based on concepts formulated from the previous knowledge and experience and are not related to real 3D space and time
58
Describe the observational approach to geological/ground models
Based on the observed and measured distribution of engineering geological units and processes. Data is related to actual space and time and are constrained by surface or subsurface observations
59
What percentage of building projects are delayed by unforeseen ground conditions that add cost?
~30%
60
What percentage of road projects that are over budget are due to inadequate ground conditions?
~50%
61
What steps are included in the initial stage of a site investigation? (3)
1. Desk Study of available data
2. Site visit and visual assessment (walkover survey)
3. Preliminary report and fieldwork plan
62
What are the steps included in the main stage of a site investigation? (3)
1. Fieldwork
2. Lab testing
3. Final report
63
What are 3 areas of fieldwork that could be carried out in a site investigation?
1. Geological mapping if necessary and possible
2. Geophysical survey if appropriate
3. Trial pits, trenches, boreholes and in situ testing
64
What are the aims of a desk study? (4)
1. To locate, collect, and interpret existing available data
2. To limit costs
3. Aid in the design process
4. Highlight problems early
65
What are the benefits of a desk study? (2)
1. Low cost & cost effective
| 2. Provides information which would otherwise be difficult to obtain
66
What are the 4 broad categories of data used in a desk study?
1. Maps
2. Archive
3. Specialist Surveys
4. Observational
67
What are the aims of a walkover survey?
Recognise potential difficulties and true ground conditions, whether site matches desk study
68
What do you check for in a walkover survey? (6)
1. Truth of air photos
2. Land Use/Past Land Use
3. Physical Features (geology/drift)
4. Groundwater conditions
5. Talk to locals
6. Examine existing structures
69
What are the objectives of geomorphological mapping?
1. Mapping distribution of drift/unconsolidated deposits
2. Mapping of geometry of deposits
3. Mapping surfaces features-runoff, drainage, patterns, slope angles etc.
4. Identifying potential hazards
70
How is geomorphological mapping carried out?
By surveying, field surveying, air photo or satellite
71
What are the 5 inclusions in geological mapping?
1. Type of Rock
2. Continuity of Rock
3. Fracture evaluation
4. Geometry Rock
5. Weathering
72
What are 5 common methods of geophysical surveys?
1. Electric & Electromagnetic
2. Gravity
3. Magnetic
4. Seismic
5. Ground Penetrating Radar
73
What are the common applications of geophysical surveys?
1. Reconnaissance tool for locating boreholes or test pits
2. Interpolating geological information between boreholes
3. Searching for hidden features
4. Rockhead profiling
5. Estimating material properties
74
List 2 types of intrusive investigations
1. Trial Pits & Trenches
| 2. Boreholes
75
What are the 3 applications of trial pits and trenches?
1. Assess 3D nature of drift and rock
2. Obtain samples for testing
3. Conduct in situ testing
76
List 2 benefits and 2 limitations of trenches and trial pits?
+ Cheap
+ Especially useful in variable man made fills
- Limited depth
- Poor spatial coverage
77
What are the 3 main groups of drilling rigs that are available?
1. Light Percussion
2. Rotary Coring
3. Rock Probing
78
When drilling boreholes for a building, what is the typical spacing between holes?
10-30m apart
79
When drilling boreholes for a road, what is the typical spacing between holes?
30-300m apart
80
What is the typical depth of a borehole?
1.5x foundation width below founding depth + 10m control beyond this, 3m below rock head, 3-10m to locate cavities
81
What determines the drilling method used when drilling a borehole? (4)
1. Ground conditions
2. The information required from the ground for the design and construction methods
3. What if any or laboratory testing may be required
4. The size of hole or core required
82
Why would you use percussion drilling?
Small or large, limited sample recovery, cheap
83
Why would you use rotary drilling?
Lots of sample recovery but expensive
84
How deep can the typical cable percussion rig drill?
15-40m depth
85
With what rock would you use cable percussion drilling?
Soils and weak/soft rock and sediments: Clays, silts and sands
86
How deep can a typical rotary drill reach?
>100m depth
87
Why are air, bentonite or water important in borehole drilling?
They are used by rotary drills to flush, lubricate and wash chippings to the surface. Bentonite stabilise the walls of the hole
88
What are the 4 commonly recorded values taken from rotary cored logs?
1. TCR - Total Core Recovered
2. SCR - Solid Core Recovered
3. FI (If) - Fracture Index
4. RQD - Rock Quality Designation
89
Define TCR
Total Core Recovered = length of core recovered/total core run
90
Define SCR
Solid Core Recovered = length with one full diameter/length of core run
91
Define FI (If)
Fracture Index (using BS5930 terms for fracture, calculated per meter)
92
Define RQD
Rock Quality Designation = length of solid core pieces >10cm recovered/total core run
93
What are the 4 main in situ tests?
1. Point load tests
2. Schmidt Hammer
3. Cone Penetration Tests (CPT)
4. Standard Penetration Tests (SPT)
94
What are the 5 main laboratory tests?
1. Point load tests
2. Brazilian Tests
3. Box and Ring Shear Tests
4. Triaxial and Unconfined Compressive Strength Testing (UCS)
5. Soil contamination testing
95
Describe SPT tests, draw diagram.
1. Reference beam with 6 marks 75mm apart inserted into borehole
2. Standard weight is dropped from a standard height
3. First 2 marks disregarded as seating blows
3. Number of blows recorded for marks 3, 4, 5 & 6.
96
For UK rock logging, what are the 4 steps?
1. Scale the logging sheet
2. Describe lithology to BS5930 & BS EN-ISO-14689-1:2003
3. Draw a graphical column giving location and orientation of any discontinuities and drilling induced fracturing. Describe any discontinuities
4. Determine fracture indices for assessing rock excavatability / rock mass quaility
97
What are 3 pitfalls to avoid when carrying out a site investigation?
1. Poor interpretation based on over reliance on sparse collection of boreholes or trial pits
2. Examining parameters that are not relevant to the problems at hand
3. Providing the client with information they neither requested or require (and haven't paid for)
98
What are four advantages of CPT?
1. Fast & continuous profiling
2. Repeatable and reliable data (not operator-dependent)
3. Economical and productive
4. Strong theoretical basis for interpretation
99
What are four disadvantages of CPT?
1. Relatively high capital investment
2. Requires skilled operators
3. No soil sample, during a CPT
4. Penetration can be restricted in gravel/cemented layers
100
Describe the methods of a CPT
A cone on the end of a series of rods is pushed into the ground at a constant rate and continuous measurements are made of the resistance to the penetration of the cone and of a surface sleeve. Pore pressure can also be measured
101
List some additional sensors that have been added to cones of CPTs.
1. Temperature
2. Geophones (seismic activity)
3. Pressuremeter
4. Camera (Visible Light)
5. Radioisotope (gamma/neutron)
6. pH
102
Give 6 reasons why underground infrastructure is necessary
1. Lack of space above ground/Landscape Topography reasons
2. Stable environmental conditions (temperature, humidity)
3. Safer space from natural hazards
4. Environmental Restrictions
5. Cost efficiency
6. Social benefits
103
Describe gravitational stress
```
A stress (σ) on a plane (P) equals force (F) per unit area (A):
σ = ΔF/ΔA
```
104
What is the unit for forces?
Newtons
105
What is the unit for stresses (pressure)?
Pascals (newtons per m^2)
106
Define strength in terms of stress
Stress level when engineering failure or yield occurs
107
Define stiffness in terms of rock strength, how can it be inferred on stress/strain curves. Give a measure of stiffness.
Relationship between stress and strain before a rock yields. Is represented as the initial slope of stress/strain curves. Young's Modulus (E)
108
Define ductility in terms of rock strength
Relationship between stress and strain after yield
109
Define elasticity in terms of rocks
Elasticity is the ratio of stress to strain
110
Define stress in terms of rock strength
Stress is the amount of force per unit area
111
Define strain in terms of rock strength. What unit is used?
Strain is the displacement/initial length of material. No unit is used
112
Describe a UCS Test
Uniaxial Compression Strength Tests compress a cylindrical sample on a single axis with no compression until it shears. This is UCS value and is measured in Pascals
113
What is Young's Modulus?
Young's Modulus = E and is a measure of stiffness. Therefore it is the slope of the stress strain curve taken at 50% of the UCS value. Can also be given as σ/v, where εv is vertical strain.
114
What is Poisson's Ratio?
Poisson's ratio describes the relationship between horizontal and vertical strain:
υ = εH/εV
115
How is horizontal strain calculated?
εH = ΔW/W. Change in width/initial width
116
How is vertical strain calculated?
εV = ΔH/H. Change in height/initial width
117
Draw/Describe the initial slopes of stiff and soft rocks on stress/strain curve
Stiff is steep, higher ratio of stress:strain; soft is shallow, lower ratio of stress:strain
118
Describe the Young's Modulus of a soft rock
Low E
119
Describe the Young's Modulus of a stiff rock
High E
120
Draw/Describe the stress/strain curves of a weak rock and a strong rock
Weak rock has a low peak; strong rock has a high peak
121
Draw/Describe the stress/strain curve of a weak soft rock that is ductile
Shallow initial gradient with a low peak, flat secondary gradient
122
Draw/Describe the stress/strain curve of a strong stiff rock that is brittle
Step initial gradient with a high peak, steep secondary gradient
123
Describe elastic deformation in terms of rock strength
Completely recoverable upon unloading, occurs before rock yields (stiff/soft)
124
Describe peak strength in terms of rock strength
Maximum load carrying capacity of a sample before a rock yields (weak/strong)
125
Describe residual strength in terms of rock strength
Post-peak strength of sample after large deformation, occurs after yield (brittle/ductile)
126
Draw/Describe the stress/strain curve of rocks that are brittle and ductile respectively
Secondary curve is steep for brittle rocks and flatter for ductile rocks
127
What does the strength, stiffness and ductility of a rock depend on
Geology of the rock: minerals with the rock and how they are bonded to one another.
128
Define anisotropy in terms of rocks
An anisotropic rock has different strengths in different loading directions
129
What causes anisotropy in rocks?
Foliations, closely spaced planes of weakness, cleavage or schistosity
130
Describe triaxial testing of rocks
Compression testing when a cylindrical sample of rock is confined and compressed. σ1 is vertical compression while σ2 and σ3 are lateral confinement
131
How does confinement affect the strength of a rock? Give an example of the use of this
Strength is increased with confinement. When mine pillars start to collapse, the first action is to confine them.
132
How do tensile strengths and compressive strengths of a material compare?
Tensile strengths are always lower than compressive strengths
133
What are the two components of shear strength?
Cohesion and Friction
134
In regards to shear strength what are cohesion and friction dependent on?
Friction is fully dependent on normal pressure while cohesion is not dependent on normal pressure
135
How is cohesion displayed on a Mohr Diagram or a stress/strain curve?
Where the tangent crosses the y axis (c)
136
Define shear strength
The ability of a rock to with stand offset compression, depends on friction and cohesion
137
Where are σ1 and σ3 plotted on a Mohr Diagram?
On the x axis
138
Define the term friction angle of a rock
The slope angle of the line defining failure on a stress-strain curve or in Mohr-Coloumb space
139
What does σ3 equal in a UCS Test?
σ3=0 as no compression
140
What does σ2 equal in a UCS Test?
σ2=0 as no compression
141
What are the axis on a Mohr Diagram?
X axis = normal stress (σn);
| Y axis = shear stress (τ)
142
Where is the failure envelop in Mohr Coloumb space?
above the tangent (friction angle line). Where Shear strength < shear stress
143
Where is the stable envelope in Mohr Coloumb space?
Below the tangent (friction angle line) where shear strength > shear stress
144
What does the addition of water do in terms of normal stress?
It reduces 'effective normal stress' and so can cause shear failure.
145
What is the Mohr-Coloumb failure criterion? Define Terms
τ=c+σntanθ, where θ = friction angle, σn = normal stress, τ = shear stress, c = cohesion
146
What quality is needed in a soil for it to fail in a planar manner?
It needs to be truly granular or have a clear discontinuity
147
Give 3 examples of discontinuities in soils
1. Weathering fronts
2. Relic shear surfaces
3. Root zone boundaries
148
What are the 3 main areas of a location's geological history that are important for engineering?
1. Genesis - intact rock and inherent structures
2. Tectonics and deformation - rock mass structure and joint conditions
3. Paleogeographical evolution - weathering and final fabric
149
How does one quantify a geological model (and turn it into a geotechnical model)? (2)
1. Define parameters using in-situ testing, lab testing and rock mass classification systems
2. Carry out numerical analysis
150
What are 7 parameters that need to be defined in a geotechnical model?
1. Strength of rock mass
2. Elasticity (Poisson's Ratio, Young's Modulus
3. Texture
4. Porosity
5. Permeability
6. Density
7. Strength of Discontinuities
151
Name 5 types of rock lab testing
1. Uniaxial Compression (UCS)
2. Point load testing
3. Triaxial Compression Testing
4. Tensile Testing
5. Direct Shear Testing
152
What does the strength of a rock depend on? (3)
1. Strength of rock
2. State of weathering
3. Pore water pressures
153
What does the strength of discontinuities depend on? (3)
1. Strength of joints
2. Total and effective stress
3. Joint roughness and joint wall strength
154
What properties do we need to assess the strength of a discontinuity? (9)
1. Strength
2. Strike and Dip (orientation)
3. Roughness
4. Aperture
5. Persistence
6. Separation/Spacing
7. Opening
8. Filling material
9. Water content
155
Why is the fracture frequency important and give an example?
May need to know direction of maximum or minimum fracture frequency, for example to make a borehole the most production
156
Define RQD
Rock Quality Designation: percentage of intact core pieces longer than 100mm in a 54mm+ core
157
List an advantage and a disadvantage of using RQD
+ Implicitly accounts for discontinuities and weak materials by using core loss as an analogue for fractures and weak layers.
- Difficult for engineers to assess why core is lost or fractured, could be high denisty of fractures or weak layer in stratigraphy
158
Write the slope stability, factor of safety formula, define all terms.
See notes
159
Define persistence
Persistence implies the areal extent or size of discontinuity within a plane. (whether the discontinuities are long or short)
160
What are 3 typical profiles of a discontinuity in terms of roughness?
1. Stepped
2. Undulating
3. Planar
161
How is the joint wall strength of a discontinuity measured?
With a Schmidt Hammer
162
Define aperture in terms of a rock discontinuity
The perpendicular distance separating adjacent rock walls of an open discontinuity
163
What are the 3 broad groups of discontinuity apertures?
1. Closed <0.5mm
2. Gapped 0.5-10mm
3. Open 10-1000mm
164
Why is the filling of a discontinuity important when assessing strength (* things affecting behaviour)
Behaviour depends on:
1. Mineralogy of filler
2. grading of particle size
3. Over-consolidation ratio
4. Water content/permeability
5. Previous shear displacement
6. Wall roughness
7. Width
8. Fracturing of wall rock
165
What is JRC and JCS?
Joint Roughness Coefficient
| Joint Compressive Strength
166
What does the strength of a rock mass depend on? (7)
1. Intact rock strength
2. Fracture Density/Drill Core Quality
3. Joint Persistence
4. Joint Spacing
5. Joint Contour, Aperture and Surface Condition
6. Groundwater
7. Joint Orientation
167
Define RMR and explain the 6 factors that are incorporated into RMR
1. Uniaxial Strength of intact rock material
2. RQD
3. Spacing of discontinuities
4. Condition of discontinuities
5. Groundwater conditions
6. Orientation of discontinuities
168
When would you use Mohr-Coulomb criterion and Hoek-Brown criterion
Mohr-Coulomb is used for intact rock, Hoek-Brown is used for highly fractured rock
169
Describe a continuum and and what dominates the strength
A continuum represents a mass that acts as a single body and is dependent on rock mass strength
170
Describe a discontinuum
A discontinuum represents a mass that is dominated by the properties of discontinuities
171
Give a real world example of a structure that acts as a continuum
Volcanoes, mountains etc
172
What are the two types of failure that occur in a discontinuum?
Planar, wedge failure
173
What are the properties of a material that are relevant to rock slope stability? (3)
1. Angle of friction
2. Cohesive strength
3. Unit weight (density of rock)
174
At what angles are discontinuities in a stable slope?
Horizontal flat or vertical
175
How are discontinuities arranged in unstable slopes?
Inclined out of the face of the slope and daylight on the face
176
Describe planar slope failure
Single discontinuity striking parallel to the slope face, dips out of the slope face at an angle steeper than the angle of friction
177
Describe wedge failure
Two discontinuities strike obliquely across the slope, line of intersection dips out of the face at an angle greater than the angle of friction
178
Describe toppling failure
involves rotation of column or block about a fixed base and is also controlled by discontinuites
179
When will a block topple? Define terms
Ψ
180
When will a block slide? Define terms
```
Ψ>Ф and Δx/y>tanΨ
Ψ = slope angle
Ф = friction angle
Δx= width of block
y = height of block
```
181
When is a block stable? Define terms
Ψ>Ф and Δx/y
182
When will a block topple and slide? Define terms
Ψ>Ф and Δx/y
183
What are the main sources of discontinuities in rock? (4)
1. Inherent structures such as bedding
2. Deformation of the rock
3. Weathering effects
4. Volcanic rocks may also exhibit cooling joints after thermal contraction
184
How do compression joints appear in rock?
Joints develop in conjugate shear directions (~60° and ~120°) making the lower angle with the major principle stress direction
185
How do tension joints appear in rock?
Cracks perpendicular to extension direction
186
How do orthogonal sets form?
Through burial and diagenesis, sets form orthogonal (90° to each other) to σ2 (intermediate) and σ1 (primary) tress directions
187
Draw a rough kinematic overlay and define sections (4 labels). Define unstable zones (3)
Daylight envelope, friction circle, slope great circle, toppling envelope. Unstable zone for toppling, wedge and planar
188
Does larger daylight envelope imply a steeper or flatter slope?
Steeper as all but very steep discontinuities will daylight
189
How big is the daylight envelope for shallow slopes?
Smaller than when compared to steeper slopes
190
What the toppling envelope for a steep face be smaller or larger than one for a shallow face?
Toppling envelope for steep face is larger than for shallow face
191
What are the 6 main ways rock faces can be stabilised?
1. Reinforced concrete shear keys
2. Tensioned rock anchors
3. Tied-back wall to prevent sliding on fault zone
4. Shotcrete to prevent raveling of zone of fractured rock
5. Drain hole, oriented to intersect with water bearing joints
8. Concrete buttress to support rock above
192
Define raveling
Gradual erosion, particle by particle, block by block; rock face crumbling away
193
Where are shear keys located and what is their purpose?
Near surface to prevent sliding
194
Why would you use a tied back wall over rock anchors in some situations?
When the rock is heavily fractured, there may be no surface for the rock anchors to 'push' on and so a tied back wall is needed
195
What is shotcrete?
Fine grain cement sprayed over rock to stop raveling
196
Define a landslide
The mass movement of material downwards and outwards driven by gravity.
197
What are the 4 ways material can move in a landslide?
1. Falling
2. Bounding
3. Sliding
4. Flowing
198
Does a landslide involve rock or soil?
Either, on their own or a combination of the two
199
What defines a landslide from creep?
Landslides need to be moderately rapid
200
What is common in 90% of landslides?
The fact that water plays a pivotal role
201
When do landslides occur?
When the stresses acting on a slope exceed the strength of the materials forming a slope
202
Give an example of an endogenic process that could start a landslide
A burst pipe
203
Give an example of an exogenic process that could start a landslide
Heavy rainfall
204
What are the 7 named classifications of landslides?
1. Rotational landslides
2. Translational landslides
3. Flows
4. Rock Falls
5. Topples
6. Avalanches
7. Lateral spreads
205
What is the most common type of landslide?
Rotational landslides
206
Describe rotational landslides
1. Common in cohesive uniform material (clays, silts, sands) with either superficial or deepseated failure
2. After failure, the mass continues to move, rotate and break up in to several blocks.
3. The blocks often become back tilted and dip towards crown of landslide
207
Draw a rotational landslide and label:
1. Crown
2. Main Scarp
3. Head
4. Toe
5. Foot
1. Crown is top edge of landslide
2. Main scarp is the topmost exposed surface
3. Head is the topmost surface of the moved material
4. Toe is the bottom edge of the landlside
5. Foot is the extension of moved material past the toe of surface of rupture
208
Where do tension cracks occur on a landslide?
At the top near the crown of the landslide
209
Where do transverse cracks and ridges tend to form on landslides?
Past the toe of surface of rupture, where material is being squashed together
210
Where do radial cracks occur in a landslide?
In the toe of the landslide
211
Where is the surface of separation in a landslide?
Between the moved material and the unmoved 'bed' material
212
What is the cause of most translational landslides?
Preexisting discontinuities, bedding etc.
213
What are 6 types of flow in terms of landslides? What is a large driving force for 5 of them
1. Mud flow
2. Debris flow
3. Sand blow*
4. Debris avalanche flow
5. Creep
6. Solifluction
All are driven largely by water apart from *sand blow
214
What is a common cause of solifluction in the UK?
Thawing of frozen, saturated soil, moves downslope during summer, then refreezes every winter
215
Define a debris flow
Mass movement loose, unconsolidated material (with a significant coarse proportion) that often have a high water content
216
What separates a mudflow from a debris flow?
Particle size, debris flows are pebble, cobble and boulder size, while mud is a lot smaller
217
Why do materials flow?
The material acts as a fluid undergoing continuous deformation, high water content decreases 'strength' of material and the particles in material are effectively floating so not strength due to particle interaction
218
Define a lateral spread
Movement of coherent blocks resting on a soft deformable underlying layer
2. Moves very slowly on shallow gradients due to loss of strength of underlying material due to the weight of over lying material
219
Give an example of a debris avalanche event
Frank Slide, Alberta, Canada, 1903
220
How fast do extremely rapid avalanches flow and what velocity class is this?
5m/sec, 7
221
How fast do rapid avalanches flow and what velocity class is this?
1.8m/hr, 5
222
How fast do slow avalanches flow and what velocity class is this?
13m/month, 3
223
Typically what is the origin of the biggest landslides?
Volcanic activity
224
What is a key indicator that tension cracks have formed recently?
Vegetation will be stretched over the crack
225
Give example features that indicate recent landslide activity
1. Landslide features are fresh, sharp and clear and there is evidence of exposed rock or soil
2. Vegetation is sparse or dominated by rapid growth species
3. Trees may be bent or kinked
4. Drainage is disordered and not fully integrated with the landscape
5. There is recent 'clean' structural damage
226
Why are landslides starting to become taken much more seriously?
They were previously seen as 'Acts of God' and not insurable against. As available space decreases and wealth increases, there is growing demand for protection and compensation against natural hazards
227
Why have landslide fatalities stayed constant over the last few decades? (2)
1. Mitigation methods are improving but more people moving into riskier areas
2. Climate change is increasing precipitation and increased water means increased landslides
228
Name 3 primary hazards of earthquakes
1. Fault Rupture
2. Shaking
3. Tectonic Subsidence/Lifting
229
Name 3 secondary hazards of earthquakes
1. Landslides and rockfalls
2. Liquefaction
3. Tsunami
230
List 6 areas engineering geologists contribute to mitigating earthquake hazards
1. Evaluation of seismic and geological conditions for engeneering works
2. Assessment of seismic hazard and effects
3. Calculation of dynamic ground properties to seismic activity
4. Geological and seismic criteria for seismic resistant design
5. Preparation of microzonation maps for urban planning
6. Vulnerability analysis of buildings and infrastructures
231
What can be put into concrete to aid
| in earthquakes and why?
Steel reinforcement, it helps it take up strain (but does not make it stronger)
232
Define the hypocentre of an earthquake
Exact earthquake location in x, y and z (at depth, not on surface)
233
Define the epicentre of an earthquake
The projection of the hypocentre onto the surface
234
Define the focal depth of an earthquake
the depth at which the hypocentre of the earthquake occurs
235
Define the epicentral distance in an earthquake
The distance between the epicentre of an earthquake and some site of interest
236
What is earthquake magnitude proportional to?
Magnitude is proportional to the area of rupture and coseismic displacement (the larger the area of displacement, the larger the energy)
237
Why is the a slight increase in UK earthquakes with magnitudes 4-5?
Mining on land and oil and gas production in the North Sea
238
What are the 3 (4) earthquake magnitude scales?
1. The Richter Local Magnitude (effective for <6.5M events within 600km
2. The Moment Magnitude (best for large scale earthquakes)
3. The Surface Wave Magnitude and Body Wave Magnitude (based on earthquake waves)
239
What largely determines the recurrence rate of an earthquake?
Slip velocity of the fault (higher slip rates accumulate more stress over shorter time scales so have shorter seismic cycles)
240
How can slip rates be determined in areas where data is non existent?
Through the use of tectonic geomorphology using satellite, aerial photos, historic maps etc.
241
What is earthquake design usually based on?
The return period of an earthquake, for civil engineering this is usually 500 years, infrastructure such as dams is 1000 years and high security installations is 10,000 years
242
Do engineers design to the magnitude of the earthquake?
No, the design to the peak acceleration or shaking. A slow accelerating, high magnitude quake would be less damaging than a fast accelerating, low magnitude quake
243
What are the 5 ways movement can be described in quakes?
1. Acceleration
2. Velocity
3. Displacement
4. Frequency (Hz or period)
5. Duration (loading cycles for liquefaction)
244
How is acceleration usually measured?
1. in 3D (N/S, E/W and up/down)
| 2. in ms^-2 or in G (0.5g etc.)
245
What is PGA in terms of earthquakes?
Peak horizontal ground acceleration, which is the peak of all horizontal accelerations (ignore vertical accelerations)
246
Give an example of a vulnerable area to earthquakes, Why?
Mexico City, for example September 1985 8.2 quake 350km away killed 5000. Mexico City is built on very old lake bed with volcanic silty clay overlain with sand and gravel in layers. The ash from surrounding volcanoes makes the soil very compressible and very variable.
247
Why does resonance frequency play a key role in earthquakes?
If the resonance frequency and frequency of quake waves are similar, there is greater amplification of acceleration
248
What determines the resonant frequency of a particular soil (2) site?
1. Type of soil
| 2. Thickness of soil
249
What do X shaped cracks on building indicate?
The building has twisted excessively due to shear, bending, compressional and torsional forces. The X indicates the energy has dissipated in shear walls
250
What causes liquefaction?
An increase in pore pressures due to quakes inducing dynamic loading on the soil causing a loss of shear strength
251
Define dynamic loading of soil and identify what it can lead to?
Dynamic loading is the loading of soil in a cyclic manner due to the passing of cyclic waves. This causes changes in the pore space without time for the pore fluids to drain. This causes a complete reduction in shear strength and causes liquefaction
252
What conditions are need for liquefaction to occur? (7)
1. Magnitude >5.5 with associated PGAs of >0.2g
2. Maximum thickness of liquefaction is around 15m
3. High water table, within 3m of the surface
4. 100% saturation
4. Medium sand to coarse silt (D50 of 0.05-1mm)
5. <10% of <0.06mm fines
6. Low density/low compaction with SPT N values of less than 10
7. A coefficient of uniformity of <15 (d60/D10)
253
How would you assess liquefaction risk? (4)
1. Standard penetration test
2. Cone Penetrometer Test
3. Cross borehole hole seismic survey
4. Groundwater observation based on piezometer records
254
How is liquefaction and earthquakes mitigated against?
1. Densification of ground by surcharging and vibrocompaction
2. Dewatering through stone or gravel columns and drainage
3. Increase strength of ground by using geotextiles and reinforced earth
4. Flexible foundations
