study guide for an exam (geotechnical)
jackalhajri
1StructuralGeology.pptGeotechnical Engineering
1
Structural Geology
Dr Martin Pritchard
Contents
1. Introduction
1.1 Drift maps
1.2 Solid maps
1.3 Key to geological shading
2. Bedding Planes
2.1 Direction and angle of dip
2.2 Structure contours
2.3 Outcropping patterns
2.4 Inliers and outliers
2.5 Unconformities
3. Three point problem
4. Folding
4.1 Fold classification
4.2 Direction of structure contours in folded strata
5. Faulting
5.1 Normal faults
5.2 Reverse faults
5.3 Tear faults
5.4 Other type of faults
6. Tutorial
6.1 Written questions
6.2 Mapping questions
1. Introduction
- A plan or map is a means by which the relative positions of surface features may be shown to a scale in the horizontal plane.
- In the production of a plan of a part of the earth’s surface all differences of altitude are eliminated by reducing linear measurements to their horizontal equivalents (e.g. a photograph taken vertically down from an aeroplane).
- If the difference in altitude between points is required, a cross section can be drawn which will show the surface profile of the slice of ground.
- In order to achieve this, the plan requires a series of spot heights or contours. This is illustrated in the example given below (on the next page).
Example 1 From the contour plan shown, below, draw the cross section A-B
100
150
200
250
100
150
200
250
Vertical scale exaggeration
- Drift maps show the distribution of rocks and engineering soil at the surface.
- They include the more recent deposits of alluvium, peat, terrace gravels, marine and estuarine deposits, glacial deposits (boulder clay, etc.), made ground (where large areas exist) and areas of landslips.
- In addition, the outcropping rock is also shown.
- These maps are particularly useful as they provide information to the Engineer on soils likely to be encountered in shallow excavations.
- They are, therefore, used to assist in the design of site investigations, and route surveys for new roads, airports etc.
1.1 Drift maps
- Geological survey maps are topographical maps overprinted with geological data
- These maps record the distribution of rocks and deposits at the surface.
- There are two types of geological map, these being known as the solid or drift editions.
1.2 Solid maps
- These maps only show the solid geology, stripped of all superficial deposits.
- Whilst they are reasonably accurate where the solid geology outcrops, they tend to be less accurate where they are covered by a thickness of drift.
- It should be appreciated that these maps are produced by Geologists, and not Engineers.
- Therefore, the geological sequence has been categorised in terms of geological time, generally using fossil evidence, and not necessarily rock type.
- This can lead to difficulties of interpretation for engineering purposes.
- For example, a strata shown on the map as a single colour, suggesting a single rock type, may comprise mudstones varying from massively bedded to shaley
1.1 Drift map
1.2 Solid map
696.unknown
1.3 Key to geological shading
703.unknown
2. Bedding planes
- Sediments which are laid down under water have a regular arrangement of rock fragments, these are placed layer upon layer.
- This structure is known as stratification, which is often clearly visible within the rock.
- In some cases there is a significant change within the deposit, which separates one bed of rock from another.
- These planes are known as bedding planes.
- Normally, stratification and bedding planes are parallel and would be approximately horizontal at the time of deposition.
Figure 1: Bedding planes
Bedding Planes
Stratum or Bed
<1cm
upper
lower
In the most simple of geological maps the bedding would be horizontal, as illustrated in the example given below:
Assuming that the bedding planes are horizontal, complete the geological outcrops over the whole map and draw a section along the line A - B. How thick is the bed of shale?
Example 2:
100m
Sandstone
Limestone
Shale
Sandstone
Limestone
Shale
704.unknown
705.unknown
- Whilst horizontal bedding is not common, most people have seen pictures of the Grand Canyon in Arizona, where horizontal beds of differing strength have been eroded by the Colorado River.
Grand Canyon in Arizona
Ref: Microsoft Encarta
- In most cases, the bedding has been distorted by movements of the Earth.
- The most extreme case would be vertical bedding, which can be seen, for example, at Alum Bay on the Isle of Wight.
Alum Bay on the Isle of Wight
Ref:www.forteamtimes.com and www.stacey.peak-media.co.uk
Assuming that the bedding planes are vertical, complete the geological outcrops over the whole map and draw a section along the line A- B. How thick is the bed of shale?
Example 3:
Sandstone
Limestone
Shale
Thickness
709.unknown
710.unknown
711.unknown
- In most cases the strata will be dipping at some angle between the vertical and horizontal.
- There are two aspects to the dip of a plane, which are:
Direction of dip
Angle of dip
- Direction of dip – the direction that water would flow if poured onto the surface, measured using a compass
- Angle of dip – from 0o for horizontal bedding to 90o from vertical bedding, measured using clinometer
To record the dip of a plane two numbers recording direction and angle of dip are used. Hence, 140/38 is a plane that dips at 38o in the direction 140oN
2.1 Direction and angle of dip
Direction of dip
Horizontal
Angle of dip
712.unknown
Figure 3
True dip – maximum angle the bed makes with the horizontal
Strike line – Line at rights to the true dip (all points on a strike line are the same distance above Ordnance Datum, thus are also known as structure contours)
Apparent dip – a smaller slope values in other directions as the direction moves round towards the strike direction.
Apparent dip
Inclined bedding planes
Strike
True dip
Strike
True dip
Apparent dip
Apparent dip
A cutting made through geological strata would unlikely be in the same direction as the true dip hence apparent obtained
Cutting = Apparent dip
2.2 Structure contours
- The height of a geological boundary is known where it crosses a topographical contour line.
- Therefore, a series of structure contours can be drawn, which will show the direction of the strike.
- If the strata forms a simple inclined plane, the structure contours will be parallel and equally spaced.
- The true dip of the strata is at right angles to the strike lines, so its direction can be assessed and the angle of dip may be calculated using simple trigonometry.
Figure 4:
Strike lines or structure contours
150m
100m
50m
True dip
714.unknown
- Estimate the direction and angle of dip for the strata shown on the following map.
- How thick is the bed of shale?
Example 4
Sh-S 150
L-Sh 200
Sh-S 200
L-Sh 250
140m
L-Sh 150
Sh-S 250
Dip
Direction of Dip =
Angle of Dip
227o
L-Sh 150
L-Sh 200
NB to calculate the angle dip use contour lines of the same bed
Shale (Sh)
140
Limestone
Shale
Sandstone
Dip
North
Direction of dip
L-Sh200-L-Sh150 = 50
715.unknown
716.unknown
717.unknown
718.unknown
719.unknown
b) How thick is the bed of shale?
Example 4
Thickness of shale
50m
L-Sh 200
Shale (Sh)
Limestone (L)
Limestone
Shale
Sandstone
140
Sh-L 150
S-Sh 150
Sh-L 200
S-Sh 200
Sh-L 250
S-Sh 250
t
50m
Sh-S 150
720.unknown
721.unknown
722.unknown
723.unknown
To draw a cross section use the following steps:
Example 4
A
B
Limestone
Shale
Sandstone
140
L-Sh 150
Sh-S 150
L-Sh 200
Sh-S 200
L-Sh 250
Sh-S 250
Corner A
Corner B
100
150
200
250
50
100
150
200
250
300
0
How to Draw section A-B
Line to show ground profile
Limestone
Shale
Sandstone
140
L-Sh 150
Sh-S 150
L-Sh 200
Sh-S 200
L-Sh 250
Sh-S 250
Corner A
Corner B
100
150
200
250
50
100
150
200
250
300
0
How to Draw section A-B – Limestone –Shale boundary
L-Sh 150
L-Sh 200
L-Sh 250
Limestone
Shale
Sandstone
140
L-Sh 150
Sh-S 150
L-Sh 200
Sh-S 200
L-Sh 250
Sh-S 250
Corner A
Corner B
50
100
150
200
250
300
0
How to Draw section A-B – Shale – Sandstone boundary
L-Sh 150
L-Sh 200
L-Sh 250
Sh-S 150
Sh-S 200
Sh-S 250
- Estimate the direction and angle of dip for the strata shown on the following map.
- How thick is the bed of shale?
Example 4.1 (BEng only)
Sh-L 150
S-Sh 200
Sh-L 200
S-Sh 250
430m
Dip
Direction of Dip =
227o
Angle of Dip
Shale (Sh)
Sandstone (S)
Limestone
Shale
Sandstone
430
NB to calculate the angle dip use contour lines of the same bed
Sh-L 150
Sh-L 200
Dip
North
Direction of dip
50
735.unknown
736.unknown
737.unknown
738.unknown
739.unknown
740.unknown
741.unknown
b) How thick is the bed of shale?
Example 4.1
Thickness of shale
x
= 148.66
= 148.66
=19.65-6.6
=13.05o
Sh-L 150
Sh-L 200
Limestone
Shale
Sandstone
Sh-L 150
S-Sh 200
Sh-L 200
S-Sh 250
Limestone (L)
Shale (Sh
Sandstone (S)
50m
t
140
S-Sh 200
50
Thickness - use the contour line of the bed above
140m
a
140m
742.unknown
743.unknown
744.unknown
745.unknown
746.unknown
747.unknown
748.unknown
749.unknown
750.unknown
Draw cross section A-B:
Example 4.1
A
B
Limestone
Shale
Sandstone
Sh-L 150
S-Sh 200
Sh-L 200
S-Sh 250
Corner A
Corner B
100
150
200
250
50
100
150
200
250
300
0
How to Draw section A-B
Line to show ground profile
Limestone
Shale
Sandstone
Sh-L 150
S-Sh 200
Sh-L 200
S-Sh 250
Corner A
Corner B
100
150
200
250
50
100
150
200
250
300
0
How to Draw section A-B – Limestone –Shale boundary
L-Sh 150
L-Sh 200
Limestone
Shale
Sandstone
Sh-L 150
S-Sh 200
Sh-L 200
S-Sh 250
Corner A
Corner B
50
100
150
200
250
300
0
How to Draw section A-B – Shale – Sandstone boundary
L-Sh 150
L-Sh 200
L-Sh 250
Sh-S 200
Sh-S 250
2.3 Outcropping Patterns
- The outcrop pattern is the configuration of the various types of rock seen at the surface.
- The outcrop width is not the same as the thickness of the bed, unless the strata has a vertical bedding plane.
Figure 5
Figure 6
Dipping Strata – Horizontal Surface
Dipping Strata – Slopping Surface
w = width of outcrop
t = thickness of bed
t
t
t
w
w
w
t
w
w
t
w
Figure 7
Dip and Scarp Slopes
Sandstone
Limestone
Shale
- The law of superposition states that for an undisturbed series of beds the oldest bed (i.e. first formed) lies at the bottom and younger beds lie upon it.
- This fundamental law helps distinguish between inliers and outliers.
2.4. Inliers and Outliers
- Outlier = younger rock surrounded by progressively older rock
- Inlier = older rock surrounded by progressively younger rock
Figure 8
Eroded
inlier
Outlier younger
- It is difficult to define an unconformity in one concise sentence as it is a complex concept. There are three major aspects to an unconformity, which are:
- Time
- An unconformity develops during a period of time in which no sediment is deposited. In this case the unconformity represents unrecorded time.
- Deposition:
- Any interruption of deposition, whether large or small in extent, is an unconformity. This aspect of discontinuity pre-supposes a standard ‘scale’ of deposition, which is complete.
- Structure:
- Structurally, unconformity may be regarded as planar structures separating older rock below from younger rocks above. A plane of unconformity may be a surface of weathering, erosion or denudation.
- The major types of unconformity are given below.
2.5. Unconformities
Figure 9
Plane of Unconformity
Unconformity: represents a period of time during which strata are not laid down. During this period, strata already formed may be uplifted and titled by earth-movements.
Figure 10
Figure 11
Figure 12
1
2
6
Gap in Sequence
Parallel Unconformity
Overlap Unconformity
Younger
Older
Older
Overstep Unconformity
Younger
Older
Older
- Where the height of a uniformly dipping bed is known at three locations it is possible to find the direction of strike and the angle of dip - this is best illustrated by the following example.
3. Three point problem
Example 5
i) Deduce the dip and strike of the coal seam which is seen to outcrop at points A, B and C.
ii) At what depth would the coal seam be encountered in a borehole sunk at point D?
iii) Complete the outcrop of the seam.
763.unknown
300
400
500
600
200
700
100
1) Join with a straight line the highest point on the coal seam (A = 700) to the lowest point on the coal seam (B = 300)
2) Divide this line into equal parts 700-300 = 400, 400/4 = 100m drop between strike lines.
3) Construct the first strike line e.g. Point C=600 and point 600 on line A-B
4) Construct other strike lines at the spacing determined in step 3 parallel to the first strike line
i) Deduce the dip and strike of the seam.
Dip
Direction of Dip =
120o
Angle of Dip
Dip
North
Direction of dip
400
600
500
100
605m
764.unknown
300
400
500
600
200
700
100
ii) At what depth would the coal seam be encountered in a borehole sunk at point D?
Approx. 180m
iii) Complete the outcrop of the seam.
400
600
500
769.unknown
(a) The map below shows the locations of three vertical boreholes drilled in order to assess the geological conditions in the area shown. The thicknesses of the various strata encountered in these boreholes are as indicated. By constructing strike lines (stratum contours) for the geological junctions and assuming that the beds are evenly dipping, of constant sedimentary thickness and are neither folded nor faulted, derive the surface geology so far as it may be safely predicted.
(b) Show the topographic profile for line XY on vertical profile and utilising the strike lines constructed on the map, draw a vertical geological cross-section for line XY.
Example 5.1 (BEng only):
770.unknown
771.unknown
772.unknown
a) derive the surface geology so far as it may be safely predicted. [10]
210-15=195
210-30=180
220-15=210
220-25=195
200
195
180
Given in question - assuming that the beds are evenly dipping, of constant sedimentary thickness and are neither folded nor faulted
165
L/M180
Sh/L195
Sh/L190
Sh/L185
Sh/L180
Sh/200
Sh/L205
Sh/L210
Sh/L215
Sh/L220
Sh/L225
M/M195
M/M190
M/M185
M/M180
M/M185
M/M180
M/M175
M/M170
M/M165
M/M160
Sa/Si200 Si/Sh195
Sa/Si260 Si/Sh255
Sa/Si255 Si/Sh250
Sa/Si250 Si/Sh245
Sa/Si245 Si/Sh240
Sa/Si240 Si/Sh235
Sa/Si235 Si/Sh230
Sa/Si230 Si/Sh225
Sa/Si225 Si/Sh220
Sa/Si220 Si/Sh215
Sa/Si215 Si/Sh210
Sa/Si210 Si/Sh205
Sa/Si205 Si/Sh200
Sh/L230
Sh/L235
Sh/L240
M/M200
M/M205
M/M210
3 point problem
15
15
165
210
180
170
200
180
185
190
195
175
205
L/M175
L/M190
L/M165
L/M185
L/M170
L/M195
L/M200
L/M205
L/M210
L/M215
L/M220
L/M225
773.unknown
774.unknown
Sandstone
Siltstone
Shale
Limestone
Mudstone
Marl
b)Show the topographic profile for line XY on vertical profile, Fig. 04(b) attached and,utiising the strike lines constructed on the map, draw a vertical geological cross-section for line XY. [6]
L/M175
L/M190
L/M165
L/M185
L/M170
L/M195
L/M200
L/M205
L/M210
L/M215
L/M220
L/M225
L/M180
Sh/L195
Sh/L190
Sh/L185
Sh/L180
Sh/200
Sh/L205
Sh/L210
Sh/L215
Sh/L220
Sh/L225
M/M195
M/M190
M/M185
M/M180
M/M175
M/M170
M/M165
M/M160
M/M155
M/M150
Sa/Si200 Si/Sh195
Sa/Si260 Si/Sh255
Sa/Si255 Si/Sh250
Sa/Si250 Si/Sh245
Sa/Si245 Si/Sh240
Sa/Si240 Si/Sh235
Sa/Si235 Si/Sh230
Sa/Si230 Si/Sh225
Sa/Si225 Si/Sh220
Sa/Si220 Si/Sh215
Sa/Si215 Si/Sh210
Sa/Si210 Si/Sh205
Sa/Si205 Si/Sh200
Sh/L230
Sh/L235
Sh/L240
M/M200
M/M205
M/M210
X
Y
215
210
210
215
215
210
205
200
L/M
Sh/L
M/M
Sa/Si
Si/Sh
195
210
180
230
235
190
205
175
225
230
185
200
170
220
225
Mudstone
Limestone
Shale
Siltstone
Sandstone
210
215
215
210
X
Y
215
210
205
200
776.unknown
4. Folding
- We have seen that strata are frequently inclined (or dipping) – but over a wider area the dipping strata may not be constant.
- When rocks bend they create folds, and when they fracture they can produce faults.
- Folding of strata represents a shortening of the earth’s crust and results in compressive forces.
- This type of folding is known as tectonic folds and are deep seated.
Section Showing Various Types of Folding
Figure13
Figure 14
Anticline
(beds are bent upwards)
Crest Axis
Limb
(beds on the side of the fold)
Limb
Axis
Axis
Syncline
(Beds are bowed downwards)
Trough
Upright fold whenever the axial plane is vertical
These folded sedimentary layers form an anticlinal fold. The convergence of the plates creates enormous compressional forces, which cause the ground to fold and sometimes rupture.
Ref: Microsoft ® Encarta ® Premium Suite 2003.
© Microsoft Corporation. All Rights Reserved.
Folding in Rocks
Microsoft ® Encarta ® Premium Suite 2003. © 1993-2002 Microsoft Corporation. All rights reserved.
Ref: Microsoft ® Encarta ® Premium Suite 2003.
4.1 Fold Classification
Symmetrical Fold
Asymmetrical Fold
Crest Axis
vertical
Axis
Crest
Overturned Fold
Axis
Inverted sequence of strata can be formed
1
2
3
Recumbent Fold
1
2
3
Plane of weakness
Isoclinal Folding
Inliers and Outliers from Folded Strata
Monoclinal Folding
Gently dipping limb
Gently dipping limb
Steep
Limb
Eroded
inlier
Outlier younger
Older
Special case of over folding in which the limbs of a fold both dip in the same direction at the same angle
4.2 Direction of structure contours in folded strata
- A structure contour is drawn by joining points at geological boundary surface) or bedding planes) is at the same height.
- This surface is the same height along the whole length of that structure contour.
- If points X and Y are joined, a structure contour is constructed.
- Also, the bedding plane is at the same height along W and Z.
- However, although points W and X are at the same height, the bedding plane is not at the same height along the line W-X therefore no structure contour can be drawn.
- X-Y is folding downwards into a syncline.
- N.B. if you attempt to draw a structure contour pattern that proves to be incorrect, the correct position would be approx. at right angles.
W
Z
X
Y
Example 6:
Draw structure contours for the upper and lower surfaces of the shaded bed of shale. Indicate on the map the position of an anticlinal and synclinal axis.
Draw a section along the line X - Y.
C-Sh 700
C-Sh 600
C-Sh 400
Sh-S 600
Sh-S 500
Sh-S 400
C-Sh 500
Sh-S 700
S-Sh 300
Sh-S 300
Sh-C 600
S-Sh 400
Sh-C 400
S-Sh 600
S-Sh 500
Sh-C 500
C-Sh 500
C-Sh 400
Sh-C 600
C-Sh 300
Sh-C 500
C-Sh 200
Sh-C 400
Sh-C 300
600
500
400
400
400
400
450
Anticline
Syncline
600
500
400
400
400
400
450
100
200
300
400
500
600
5. Faults
When rocks bend they create folds, and when they fracture they can produce faults.
Faults are factures within the upper layers of the Earth’s crust along which there has been movement. There are three major types of faults:
- Normal or tensional faults (tensional forces)
- Reverse or thrust faults (compressive forces)
- Tear or wrench faults (compressive or lateral displacement)
CD-ROM Presentation
Up throw side
Down throw side
Bed
Ground surface
Heave
Throw
Figure 15
Normal Faults
Rock mass resting on the plane which is known as a hanging wall
The mass beneath the plane is the footwall
781.unknown
Figure 16
Figure 17
Heave
Reverse Fault
Up Throw Side
Throw
Down Throw Side
Tear Fault
Up Throw
Down Throw
S = Strike Slip Component
D = Dip Slip Component
D
S
Throw
Heave
Figure 18
Figure 19
Eroded ground level
Step Faults
Two faults throwing towards each other
Graben (Trough)
Horst (Ridge)
Uplift
Uplift
The San Andreas Fault, unlike most faults that stay below the ocean, emerges from the Pacific Ocean and traverses hundreds of kilometres of land. It runs through California for about 970 km (600 mi) from the Imperial Valley to Point Arena. The fault marks the boundary between the North American and Pacific tectonic plates which, as they slide together, cause earthquakes.
Ref: Microsoft ® Encarta ® Premium Suite 2003.
Example 7:
A road tunnel is to be driven from Point A to Point B. A ground investigation has revealed the following succession for the area.
Rock Thickness
Limestone unknown
Mudstone 25m
Shale 25m
Siltstone 25m
Sandstone unknown
(a) Calculate the amount and direction of dip of the rock.
(b) Complete the geology of the area.
(c) Determine the drownthrow of the fault.
(d) Draw a cross-section along the line of the tunnel.
270m
L-M 225
Dip
Direction of Dip =
Angle of Dip
10o
L-M250
(a) Calculate the amount and direction of dip of the rock.
L-M 275
L-M 250
L-M 225
L-M 175
L-M 200
250-225=25
270
Dip
North
787.unknown
(b) Complete the geology of the area.
L-M 275
L-M 250
L-M 225
L-M 175
M-Sh 150
Rock Thickness
Limestone unknown
Mudstone 25m
Shale 25m
Siltstone 25m
Sandstone unknown
M-Sh 200
M-Sh 225
M-Sh 250
Sh-Si 125
Si-Sa 100
Sh-Si 175
Si-Sa 150
L-M 200
M-Sh 175
Sh-Si 150
Si-Sa 125
Sh-Si 200
Si-Sa 175
Sh-Si255
Si-Sa 200
Limestone
Mudstone
Shale
Sandstone
Siltstone
Limestone
Mudstone
Shale
Siltstone
Sandstone
Geological boundary = strike line and contour line of same height crosses
(c) Determine the drownthrow of the fault. [2]
Difference in height e.g. L-M225 – L-M200 = 25m
Downthrow of fault = 25m
Up Throw Side
Down Throw Side
Bed
Ground surface
Heave
Throw
Normal Faults
L-M 275
L-M 250
L-M 225
L-M 175
M-Sh 150
M-Sh 200
M-Sh 225
M-Sh 250
Sh-Si 125
Si-Sa 100
Sh-Si 175
Si-Sa 150
L-M 200
Sh-Si 200
Si-Sa 175
Limestone
Mudstone
Shale
Sandstone
Siltstone
Limestone
Mudstone
Shale
Siltstone
Sandstone
M-Sh 175
Sh-Si 150
Si-Sa 125
Sh-Si255
Si-Sa 200
(d) Draw a cross-section along the line of the tunnel.
A
B
L-M 275
L-M 250
L-M 225
L-M 175
M-Sh 150
M-Sh 200
M-Sh 225
M-Sh 250
Sh-Si 125
Si-Sa 100
Sh-Si 175
Si-Sa 150
L-M 200
Sh-Si 200
Si-Sa 175
Limestone
Mudstone
Shale
Sandstone
Siltstone
Limestone
Mudstone
Shale
Siltstone
Sandstone
M-Sh 175
Sh-Si 150
Si-Sa 125
Sh-Si255
Si-Sa 200
200
225
250
275
300
300
275
250
225
200
F
D
D
Sandstone
Siltstone
Shale
Mudstone
Limestone
Fault
Basalt Dyke
Tunnel
200
225
250
275
300
300
275
250
225
F
D
D
A
B
100
125
150
175
200
225
250
275
300
L-M 200
M-Sh 175
Sh-Si 150
Si-Sa 125
L-M 250
M-Sh 225
Sh-Si 200
Si-Sa 175
L-M 275
M-Sh 250
Sh-Si255
Si-Sa 200
25m
a
227/20
Dip
\=
50
tan
140
a
=
19.65
a
\=
o
ADJ
Cos
HYP
a
=
19.6550
Cost
\´=
47
tm
\=
a
b
a
50
tan
430
a
=
6.6
a
\=
o
227/7
Dip
\=
tan
OPP
ADJ
q
=
19.65
q
\=
o
50
tan
140
q
=
22
14050
x
=+
bqa
\=-
OPP
Sin
HYP
b
=
148.6613.05
tSin
\=´
33.6
tm
\=
q
100
tan
605
a
=
9.4
a
\=
o
120/9
\=
Dip
angleofheave
q
=
5.3
a
\=
o
00
10/5
Dip
\=
25
tan
270
a
=
0
5
orDip
=
