Engineer and Engineering Geologist

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Transcript Engineer and Engineering Geologist

Engineer and Engineering
Geologist
• What should you be able
to do as an engineer who
works together with an
engineering geologist?
 You need to be able to
communicate!!!
 Read Geological Maps
 Predict the geological
conditions at depth from
map and drill hole logs
 Construct a cross section
of the subsurface geology
Engineer and Engineering
Geologist
What should you be able to
do as an engineer who
works together with an
engineering geologist?
understand
• geological processes
• natural hazards
• weathering and products
• evaluate positive and
negative conditions
• relate to planners and
politicians
• economic implications
Can we choose where it is best to
build?
Hazardous areas?
• earthquakes are
common?
• volcanic eruptions are
common?
• landslides are
common?
etc.
Don’ let anyone live in Japan
or San Francisco
It is too hazardous from a
geological point of view!!
Your job as Engineer
• Recognize problems
• Propose an engineering solution
• If you want to build (X) then you must build
in the following way (Y)
There is an engineering solution to any
geological/constructional problem.
You- the engineer - must be able to recognize the
risk or problem
If you know the problem then you can suggest an
engineering solution
If you do not recognize the geological problems
the engineering construction can be at great
risk
Freedom of “choosing” the
solution
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Political decisions govern
Existing construct
Economic restrictions
Dialogue would be advantageous
Specific design developed
design based upon geological conditions
• rock and soil availability for foundations and
construction materials
• physical properties of the available material;
strength, permeability etc etc
Investigations for a project
• pre investigations
– maps of all types
• previous reports
• complimentary field studies
– drillings and drill hole logging
– geophysics
– detailed mapping
• active design – as the work goes on changes are
made due to the geological conditions
Source of maps
• Geological Survey of the country – all countries
have one
• Geological maps of
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bed rock distribution
soil types and distribution (glacial & post glacial)
hydrological conditions
geophysical (magnetic, electric, radiactivity)
many other types (engineering, rock quality)
Methods and scale
• Aerial photo interpretation – several are in
the book for your review
• Field mapping at a scale
– 1/10,000 for highly populated areas
– 1/250,000 for sparsely populated areas
– others -
Example of the unconsolidated
sediments maps 1:10 000
• Genetic
• Grain Size & Sorting
• Mapping depth 50 cm
(under soil zone)
Example of the hydrological maps
1:10 000
• unconsolidated
sediment map (base)
• wells
– location and depth
– production
– chemistry of water
• groundwater table and
isochrones
• flow direction
Example of the
bedrock maps
1:10 000
• rock outcrops
identified during
“soils” mapping
• stratigraphic age
• Rock type
Cross sections of maps
• cross sections
– stratigraphic
relationship
• oldest at bottom
• youngest at top
– predicted subsurface
continuation of the
rocks and soils
Cross sections of maps
Geological maps of other areas
Deserts / Non-Glacial areas / Tropics
How would these differ from those in
Sweden?
Sweden’s geology
• Precambrian shield
• bedrock is covered by
(predominately gneiss and
– glacial soils (till,
glaciofluvium, glacial clay)
granite)
– post glacial soils (post
• few sedimentary rocks
glacial clay, wave reworked
• extensive hiatus between
sand, wind sand, gyttja, peat,
rocks and unconsolidated
alluvium)
sediments
• Thickness varies
Contrast with other countries
• 70% of the earth’s land area is composed of
sedimentary rocks
– very few in Sweden
Contrast with other countries
• Tropics (warm and moist climate) enhances
chemical weathering
 decomposes the rocks forming soil
“regolith” or “saprolite” overlying the fresh
rock, up to 500 m deep
present day Sweden – very little chemical
weathering
Sweden has been located at the equator thus
we have remnants of this weathering
Contrast with other countries
• Non-glaciated areas lack extensive
colluvium (unconsolidated sediments),
– bedrock outcrops predominate
– alluvium in valleys, thin
Contrast with other countries
• Desert areas
– sand seas can be extensive
– bedrock usually crops out at surface
Aerial photos – remote sensing • satellite images
Aerial photos – remote sensing • aerial photos cover all of Sweden
• photos from several different years
• photos of different types
– black white
– infra red
stereo view
Aerial photos – remote sensing interpretation
• aerial photos
• satellite photos
• require interpretation and field checking
Aerial photos – remote sensing interpretation
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slope stability study
large area viewed in little time
location of slides mapped
location compared to geology and
geological structures
• conclusions can be drawn as to which
positions along the slope are more
susceptible to sliding
slope failure
aerial photography
• who does it? Governmental agency!
• you can not order a private study for a given
project
• photos are available
– you can visit the survey office to view them
– purchase of photos
aerial photography
– (note that quality varies from year to
year so it can be good to have visited the
office to see first that the year you order
is not taken when there were lots of
clouds)
What can we determine from
aerial photos?
• landforms
– landslides
– avalanches
– river meanders (old
abandoned channels)
– high water table conditions
– sinkholes
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faults
volcanic flows extent
layering of rocks
strike and dip of rocks
joints
magmatic intrusion
extent
– alluvial deposits extent
What can we determine from
aerial photos?
• landforms
– landslides
– avalanches
– river meanders (old
abandoned channels)
– high water table conditions
– sinkholes
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faults
volcanic flows extent
layering of rocks
strike and dip of rocks
joints
magmatic intrusion
extent
– alluvial deposits extent
Map suitability for engineering
projects
Engineering projects are often at a very small
scale compared to geological maps.
Geological maps are not to be “trusted” to
show the detail that is needed for the
project. These are constructed to show the
geological trends.
Map suitability for engineering
projects
Example: A glacial soils map may show that there is
a bedrock outcrop in a field of glacial clay.
WHY???
The map suggests that the outcrop is 2500 m2
(1mm2 on the map is the smallest area that
can be shown on the map at the scale
1:50,000).
In reality the outcrop may only be 5m2. But
the map maker has chosen to include the
outcrop instead of exclude it from the map.
information about the depth to
bedrock
What would be the difference with – or
without this outcrop on the map??
With - without
With the outcrop we make the interpretation
that there is not much clay covering the
bedrock.
Without the outcrop we would have
interpreted the map to indicate that the clay
thickness to be quite extensive.
When could this be important??
Map suitability for engineering
projects
Scale of the geological maps are seldom
sufficient for engineering projects
good for a overview and reconnaissance but
complimenting with detailed mapping is often
necessary
Maps are of the surface
- how do we determine the subsurface
geology?
- information on maps
- understanding of the stratigraphy
- correct model of the geological history
Principles of structural geology can be
employed to predict the extension of the
surface geology
Further investigations!
 trenches and holes
geophysical methods
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seismic
electric
magnetic
gravity
radar
– seismic
geophysical methods
 geophysical methods
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seismic
electric
magnetic
gravity
radar
– seismic
seismic in drill holes holes
geophysical
methods
 geophysical methods
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seismic
electric
magnetic
gravity
radar
electric – resistivity of
electric currents
magnetic
• rocks with different Fe
contents
gravity
• rocks with different
densities
radar
• profile of reflectors
• needs interpetation
Further investigations!
 logging the cores
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core recovery
rock quality, RQD
oriented cores
index test
logging the cores
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core recovery
rock quality, RQD
oriented cores
index test
logging drill holes
• drill holes
• logging drill holes
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heat
caliper
geophysical
water-pressure test
borehole photography
logging drill holes
• drill holes
• logging drill holes
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heat
caliper
geophysical
water-pressure test
borehole photography
logging drill holes
• drill holes
• logging drill holes
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heat
caliper
geophysical
water-pressure test
borehole photography
logging drill holes
• drill holes
• logging drill holes
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heat
caliper
geophysical
water-pressure test
borehole photography
trenches
• a scratch at the surface
Further investigations!
-
trenches
geophysics
drilling
logging drill holes
logging cores
– Complimentary tests
are expensive
– students seem to
believe geophysics is
the answer to
everything
– NO
– Interpretation is
needed
Further investigations!
- Maps are
relatively
inexpensive
- Complimentary
tests are costly
Further investigations!
- Maps are
relatively
inexpensive
- Complimentary
tests are costly
•drill holes are isolated points
•geophysic profiles = one line
What information do we get from
maps compared to complimentary
tests?
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amount of information
accuracy of information
cost of obtaining information
cost of having to change our project if the
geology is unsuitable for the design
What information do we get from
maps compared to complimentary
tests?
• maps
• Principles of Structural geology can be
employed to predict the subsurface
extension of the surface geology
What information do we get from
maps compared to complimentary
tests?
• geophysics
• can be applied to areas or linear
traverses. They, however, require
interpretation, which in turn requires
concrete information from drill holes
What information do we get from
maps compared to complimentary
tests?
• drill holes
• isolated points that show the relationships
downward at that point
• information from the drilling is not 100% exact
• what comes up is often bits and pieces of rock
Important questions
 where do we need to know more about the subsurface
geology
 what geophysical methods can be used
 where, how frequent, do we need to drill
 what is our budget
 what is the required “level” need for the project
what is the required “level” need
for the project
Dignity of the engineering project dictates the
need for pre-investigations
coupled to
– construction cost
– risk due to failure
EXAMPLES?
dignity of engineering projects
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Sport area for running, football etc
Private home
Road / Railroad construction
High-rise housing project
Rock quarry or gravel pit
Dam
Land fill
Mine
Tunnels
• Construction of an atomic energy plant
dignity of engineering projects
• Would there be any general
difference for pre-investigations
required in Sweden compared
to other countries?
• Past results
•
tradition
Geological situation in
Sweden
Geological situation in
another country
Geological processes
Character of rock mass
Sweden
no large earthquakes
– last volcanic eruption
was millions of years
ago
– landslides occur
– floods occur
– hard crystalline rocks
predominate
Other countries
“Data base”
• basis for decisions
concerning preinvestigations
correctness of pre- investigations
• pre-investigations are
the basis for bids
• project commissioned
– operation and
maintenance phase
– geology evaluated –
real compared to
predicted
• interactive construction
– real geological conditions
documented
– evaluation of design versus
the real conditions
– possible change in design
• information concerning the
geology needs to be added to
the “data base” – so future
projects can gain