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Engineering Geology

A revision Prezi for A2 Environmental Geology; Engineering Geology
by

Jack Woodroffe

on 8 June 2015

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Transcript of Engineering Geology

Environmental Geology Revision
Landslips and Slumping hazards
What is engineering geology?
Engineering geology is the use of geology in engineering to make sure that the geological factors of a site and the maintenance of structures on the site are geologically sound.
Mining
Engineering Geology
Aggregate
Short Answer Questions
Tunnels
Natural Aggregate
Natural aggregate is granite and sandstone (etc.) mainly extracted from Quaternary Rivers and Glaciofluvial and Shallow marine deposits. These can be obtained by dredging the gravel. Mortar can be made using cement and sand, this is commonly used for holding bricks together.
Crushed Aggregate
Hard rocks such as gneiss and greywacke are crushed up by drilling or blown up by blasting with explosives. Limestone can be used for cement (CaCO3+H2O).
Aggregate Uses
Natural aggregates: used for materials in construction, such as concrete and bituminous road materials.
Crushed aggregate is used for rip-rap, railroad ballast, and filter stone. It can also be used with a 'binder' (glue) in things like concrete, tarmac and asphalt (etc.).
AMD
AMD or Acid Mine Drainage, is the solution created when rainwater is contaminated by a spoil heap, the water runs off and percolates through the heap collecting toxic minerals and flows into the ground and pollutes the ground and surface water supplies.
Coal Mining
Opencast mining - mining above ground in large pits where coal seams are found; requires trucks, diggers and sometimes explosives are needed against hard rocks.
Deep Mining - mining deep underground or into mountain sides; requires drills, special machinery and conveyer belts.
Problems with opencast mining;
-AMD/Acid Mine Drainage
-Noise and dust pollution
-Unsightly/Scars the land
Types of deep/underground mining;
-Pillar and Stall mining - put in pillars as you go, a network of tunnels with left over coal.
-Long Wall Mining- long wall retreat mining: allowing the wall to collapse.
Problems with underground mining;
-AMD from spoil heaps
-Subsidence at the surface after mine is closed
-Methane gas, Carbon Dioxide and Carbon Monoxide
-Cave ins
Geological Mining Problems:
Faults – displace coal seams
Folds and Dipping seams – >5° = too steep for machinery to work safely and effectively
Wash out – channel sandstones cut coal seam(s)
Seam Splitting – coal seams split off into thinner separate seams
Seam Thinning – coal seams become too thin to mine economically
Dangers
Water – flooding tunnels and could cause electrocution if water reaches equipment or wires, e.g. April 6th 2010 in N. China, 33 were trapped underground and were not rescued in time, the other 115 survived after being rescued from the mine
Gases – Methane (CH4), Carbon Dioxide (CO2) and Carbon Monoxide (CO) can cause suffocation and explosions, e.g. April 5th 2010, West Virginia, 29 died after gas caused an explosion in the mine
Problems in Urban Areas;
- Need
ing to avoid other tunnels.
- Need
s to avoid pipelines, gas, water, oil, military fuel, etc.
- Tunnel boring machines (
huge pieces of equipment
) are used to make tunnels.
Tunnel Collapse Prevention
- Concrete segments,
- Shotcrete (Sprayable concrete, acts as cement to hold rocks in place),
-Rock bolts to hold loose segments in place,
- Grouting- injection grouting (drill a hole into the Rock and inject concrete into it), or jet grouting (for pre-existing fractures).
1. List the important geological considerations in any engineering project.
If the area is prone to seismic activity, as this could mean tremors or even earthquakes may occur, which would be especially bad in road or mining tunnels, this could be counteracted with special structural features such as steel rods in concrete to strengthen the overall structure, or with crisscrossing structures to take impact from the earthquake. Also, if there are any faults or joints in the rock surround a structure such as a landfill site or mining operation, as this could cause leachate to leak through the joints and infiltrate/contaminate the ground water below. Another would be whether the ground rock is suitable for the job, such as landfill sites, which require impermeable ground rocks with low porosity to stop any harmful substances from leaking into the ground water below (e.g. sandstone would allow the substances to go through, whereas shale wouldn't).
2. What are the potential hazards resulting from waste disposal in landfill sites?
The main hazard is that rain water will percolate through the landfill and pick up toxic chemicals and minerals and will create leachate, which if not counteracted will contaminate the ground water. It could also cause high levels of methane as the rubbish decays and lets off gas.
3. Define: Mass movement, Landslip, and Slump
- Mass Movement; is the movement of soil or rock down a slope due to gravity,
- Landslip; when large pieces of the land on a cliff face becomes unstable and collapses,
- Slump; when the cliff face becomes too eroded at the base and falls/slumps down towards the sea.
4. The 3 natural factors that cause landslips and slumping are rock type, slope angle, and water. Describe the key points for each one.
- Rock type will affect how a rock absorbs water, if the rock type is loose, porous and permeable, like mudstone, then it will absorb more water than a rock like limestone or shale with is much less permeable and porous, and as the weight increase, the rock will be dragged down the cliff face.
- Slope angle will affect it as if the rock is at a steep gradient, it will be more prone to landslips and slumping than a steadier gradient.
- Water can affect the rock by increasing weight if it is absorbed and by eroding the rock over time, cutting under a cliff face will cause it to become unstable and lead to it slumping onto the beach.
5. List the 8 methods used to stabilise slopes.
- Slope modification
- Vegetation
- Rock armour (to prevent erosion)
- Drains
- Putting in pipelines to steady the movement of the rock
- Adding wire netting to prevent fast collapse
- Shotcrete
- Rock bolts
Causes
- Heavy Rainfall
- Good porosity and permeability in the rock
- Loose ground
Keywords;
Mass movement
- the movement of soil or rock down a slope due to gravity.
Slump
- a rotational downwards and outwards movement of incompetent rock and material.
A Landslip or Landslide
- the rapid down slope movement of rock along a slip plane.
Landslides
= rock falls, slides, and flows (e.g. Debris flows/mudflows/mudslides).
How do these cause landslides?
Heavy rainfall can cause the lubrication of joints and faults and will increase the weight of the land as it is absorbed into the rock and soil.
Mass movement can be slow, (mm per year) or quick and disastrous, as is the case with debris flows.
Case Studies
Holbeck Hall Hotel:
Major landslip occurred in June 1993, 60m of the cliff was lost overnight. Occurred following a number of drier than average summers, followed by the first few months of 1993 being very wet. The top layer of the cliff became saturated and moved down-slope
Mt. Saint Helens eruption
in 1980 was the biggest landslide recorded by humans, so big that it changed the skyline. It moved 2.8 cubic kilometres of rock from the mountain-side.
Natural factors affecting landslips and slumping
Rock type- strength, porosity, permeability.
Slope angle
Water
Earthquakes/Tremours
Avalanches
Strength
- strong competent rocks are less likely to fail than incompetent.
Competent
fail by slipping
Incompetent
fail by slumping
Porosity and permeability
- addition of water to rock increases the chances of slope failure as it increases the weight of the hanging rock
Geological structures which are planes of weakness include -
bedding planes
,
joints
,
faults
;

rock will fail along these, especially if dipped bedding planes dip in towards a valley
Weathering
-
weakens

the

rocks

and

produces small fragments

=

increases

the

risk of mass movement

(
mechanical weathering

such as

freeze thaw can

cause landslides by

expand
ing the

land

and
then
as
the
ice melts

it

will

lubricates

the now loose

land surface
)
Important is red
Technical terms are yellow-green
Semi-important words in salmon pink
Slope angle
- the
steeper
the
slope
angle is the
more likely
it is to
fail
The
angle of repose
dictates the steepness at which a material can be arranged before it will move down the slope; well rounded sand will move at a gentle slope, whereas angular sand will move at a steeper angle, 40 degrees
Fine

Sand
- slumps at
35 degrees
Coarse Sand
- slumps at
40 degrees
Gravel
- slumps at
45 degrees
Case Study
The Aberfan disaster resulted in 144 deaths, 116 children and 28 adults. A slag tip from a colliery nearby the small Welsh village had been building up for some time and although the workers thought it was safe and stable, it wasn't, and it collapsed on the 21st of October, 1966. After 50 years of spoil build up, the tip had grown immensely in size and after some water build up in the rock and shale, it slumped and began sliding downhill as a type of viscous slurry. In minutes, 40,000 cubic metres of debris covered parts of the village, including Pantglas Junior school where children and teachers were likely killed by impact or suffocation.
Same Place Before (above) and after (right) the slag heap slumped
This was an especially terrible disaster as if the collapse had occurred a few minutes earlier, the children wouldn't have been in the classrooms, and a few hours later they would have broken up for half-term. A few lives were saved by rescue workers and miners, however the public were trying to save whoever they could and got in the way of many of the rescue workers and blocked the miners causing them to arrive later than was possible otherwise.
Removal of material at the base of slopes for building or road cuttings
Leakage from water mains and sewage pipes
Heavy traffic, machinery or blasting causing vibrations
Tarmac/impermeable surfaces increasing run off
Deforestation
Human Factors affecting landslips;
Also known as undercutting, when material is removed from the bottom of a slope it is like taking tin cans from under a stack, eventually the cans fall. The way that undercutting occurs naturally is commonly by erosion at the bottom of a shear cliff face, once enough material is removed the cliff collapses. In this case the slope will slide down as one big mass of land, like an avalanche.
Leakage from within a slope could cause the material to become saturated and after enough water has been accumulated the slope will start sliding and a landslide will occur.
Vibrations traveling through the ground could cause it to vibrate and as the individual grains vibrate the material will shift slightly, once enough of the sediment is vibrating the entire slope could shift and if too much weight is dragging down then this could cause collapse.
If roads or other impermeable surfaces are built above a slope then rainfall won't be able to be absorbed by the underlying sediment. The water will then run-off and run down the slope, this could pick up sediment along the way, weakening the structure of the slope, or it could be absorbed by the slope material, this would increase the weight of the material and therefore cause it to slump or a landslide may occur if there is enough material.
Deforestation, caused by people can also increase the amount of slumping and landslides as the roots below the ground are often holding the ground together and in place if it is loose. Also they take in water, meaning there is less in the sediment below, and trees block some of the rain fall with their leaves and branches which also means less water in the sediment and so it weighs less and the forces of gravity will be less likely to pull it off. If the trees are cut down and nothing is left to prevent water percolating into a slope, then a landslide will more then likely occur.
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