Economic and engineering geology Flashcards
Tunnels and dams
What are some examples of hard crystalline rocks?
Any igneous rock and most metamorphic rocks.
Granite, Diorite, Marble, Metaquartzite
What is the approach to tunnelling with hard crystalline rocks?
Drilling + blasting but slow and expensive.
Use of explosives calculated, otherwise there may be overbreak or underbreak. At depth, high confining pressure (risk of rock bursts). Doesn’t often require support
What are some examples of soft rocks?
Some sandstones, limestone and chalk. Ideal materials
What is the approach to tunnelling for soft rocks?
Cheap + relatively easy. Requires lining with concrete (shotcrete) or steel ribs. Specially designed tunnel boring machine.
What are some examples of soft rocks?
Weak cleavage or pre-existing joints.
Shale (mudstone), clay, unconsolidated material
What is the approach to tunnelling with weak rocks?
Prone to collapse and leakage. Requires support and de-watering techniques
What would changes in rock type (lateral variation) pose problems when tunnelling?
Multiple different techniques, different strengths. Every bedding plane is a plane of weakness. Different parts may have permeability changes. Some may need support.
What type of beds are easier and safer to construct tunnels in?
Horizontal beds are safer as equipment and supports are uniform and consistent. Choosing a bed is possible. Dipping beds may be prone to collapsing and slippage especially with water presence
What are faults?
Fractures where there has been observable displacement
How do faults cause problems when tunnelling?
Planes of weakness.
Zones of permeability that allow flooding.
May be different rock types on either side.
Fault movement may cause the tunnel to collapse
What are joints?
A fracture where there has been no observable movement
How can joints cause problems when tunnelling?
Zones of weakness and permeability. Often more closely spaced than faults. Loose blocks of rock between joints may fall out of tunnel roof
What are bedding planes?
Beds in sedimentary rocks, foliation in met.
Mark a break between beds (break in sedimentation). A change in composition, grain size or change in colour of sediment. May be clay present between beds
How might bedding planes cause problems when tunnelling?
Planes of weakness. May allow slippage or leakage of water
What are folds?
Caused by compressive forces on incompetent rocks
How can folds cause problems with tunnelling?
These have changing angles of dip, slippage may occur on fold limbs.
If gentle syncline, tunnelling can follow the dip and stay in one bed - like the channel tunnel
What may tunnels below the water table be affected by water?
Become flooded, require dewatering and draining. Require shotcrete.
Increased hydrostatic pressure. Lubricated bedding planes or fractures
How may spoil from tunnelling be recycled or reused?
Millions of tonnes excavated from tunnels sometimes used to build embankments elsewhere.
What did Crossrail do with their spoil from tunnelling?
Shipped spoil to the Essex coast to make new salt marsh nature reserve (Wallesey Island)
How can tunnels be made safer to prevent collapse?
Lining with concrete (shotcrete) or steel ribs. Using rock bolts to secure loose blocks
How can tunnels be made safer to prevent flooding?
Pumping of water sometimes required. Grouting of the surrounding rocks. Using rock drains
When did the crossrail tunnel begin?
2018
Where is the crossrail railway tunnel?
From Reading to Shenfield and Abbey Wood (East London)
What measures were carried out before the Crossrail railway tunnel?
> 1000 boreholes mostly around 50 metres. Looked at rock types, groundwater and faulting
How does the crossrail railway tunnel prevent collapse or flooding?
Shotcrete. 200,000 bolted concrete segment linings
How did faults cause issues for the crossrail railway tunnel?
Provided pathway for groundwater. Breaking up the ground. Secondary permeability and decreased stability
What are the general conditions for constructing dams?
Impermeable rock.
Valley topography is cheaper.
Geologically stable (no seismicity, faulting or folding).
Low sediment load in river.
No toxic metals - especially when used for drinking
What conditions does underlying rocks need for dam construction?
Impermeable, competent, low porosity and strong. High load bearing strength. Uniform
Are horizontal beds stable for dams?
Yes
Are beds dipping downstream stable for dams?
No
Are beds dipping upstream stable for dams?
Yes
Are syncline folds stable for dams?
Yes (depending on rock type, some layers may be permeable but this can be managed)
Are anticline folds stable for dams?
No
What is an arch dam?
Curves upstream. Thinner than other dams. Suitable for narrow gauges with steep sides of strong rock in remote areas
What happens to the hydrostatic pressure at arch dams?
Hydrostatic pressure compresses against the valley walls. Pressure/force is exerted out into valley sides
What is a gravity dam?
Held in place by gravity due to immense mass. When on impermeable, high load bearing foundations, this is often best dam type. Sometimes hollow, more economic to construct. May be supported further downstream by a series of buttress dams
What happens to the pressure at gravity dams?
It exerts downwards
What are arch-gravity dams?
Combines the strength of the arch with the force of gravity. Doesn’t have to be massive. Useful in areas with high flow of water, but limited material for dam building. Wide base, curved upstream
What is an embankment or earth dam?
Impermeable clay or concrete core, held in place by piles of rock. Material binds together by friction, making cementing unnecessary. Broad, shallow valley. require large quantities of fill material
What are the three ground improvement methods for dams?
Clay or plastic lining,
Grouting (shotcrete),
Cut-off curtain
How can clay or plastic lining be used for ground improvement?
Before filling reservoir, line with impermeable material e.g. clay or plastic. Prevents leakage. Clay is good and is a local supply + cheap
How is grouting used for ground improvement?
Holes drilled into rock and liquid cement pumped in cement fills pores, joins and fissures. This reduces permeability and increasing rock strength
How are cut-off curtains used for ground improvements?
An impermeable barrier, usually concrete, prevents leakage, especially on synclines. Also strengthens foundations and prevents slippage of beds dipping downstream
What are the environmental impacts of dams? (6)
Migration barriers, lack of nutrients downstream, create microclimates, habitat loss/change upstream, buildup of pollutants, habitat loss downstream
What are the social impacts of dams? (5)
Seismic activity, migration barriers, displacement of people, water store for agriculture, increased energy security (HEP)
What is reservoir-induced seismicity?
Earthquakes/seismicity triggered by the filling of large reservoirs behind dams.
What causes reservoir-induced seismicity?
The weight of the water exerts pressure which causes stress changes.
What is the Tibet example of reservoir-induced seismicity?
killed 126. China building a dam. 6.8 magnitude
What is the Konya example of reservoir-induced seismicity?
West India in 1976. Magnitude 6.7. around 200 deaths
Why is it difficult to prove reservoir-induced seismicity?
Little research. Historical records are inadequate for monitoring. Each dam is different (geology). Some have longer lag time (earthquakes in years later)
What recommendations are in place to reduce reservoir-induced seismicity?
More research and monitoring.
No more high dams until research complete.
Monitor seismicity before and after.
Built environment should be earthquake proof too
What are the main causes of contaminated land?
Disposal of domestic and commercial waste (landfill).
Agricultural waste.
Saltwater encroachment.
Extraction + pollutants.
Natural sources.
Resource extraction *
How does resource extraction cause contaminated land?
Acid mine drainage.
Most is inert (unreactive) waste. Spoil heaps are left after mining
How does coal and metal ore waste cause contaminated land?
Contain sulphides.
React with oxygen and water to make sulphuric acid with metals in solution.
How do natural sources cause contaminated land?
Arsenic and heavy metals can be present in certain geological formations. Potential soil contamination.
How does oil and gas extraction cause contaminated land?
Contamination by organic pollutants.
Fracking has raised environmental concerns. Fracking fluid contains toxic metals. In the US, they leave this solution
How do landfill sites cause contaminated land?
Older landfills have less management and regulations (possible leachates). Brownfield sites may contain pollutants.
How does agricultural waste cause contaminated land?
Organic and hazardous chemicals (fertilisers and pesticides)
What is bioavailability?
Potential for bioaccumulation. The build up of things inside a living organism. If a pollutant can get into a living thing.
How can metal ions become more mobile?
Acidic conditions = more metals going into solution.
Metals exposed to oxygen, so more mobile in solution. When oxygen removed, it precipitates
What is the only example of a metal that precipitates when oxidised?
Iron (rust)
How can treatment of superficial deposits be used for remediation?
Can be treated in situ or dug up and treated elsewhere. Addition of a chemical.
If acidic (making mobile metals), can add alkali to neutralise, e.g. limestone slurry. Keeping water table high = anaerobic conditions= less metal mobility
How can phytoremediation be used as a remediation method?
Use of plants.
When plants take in metals in solution, stored in leaf tissue. Then remove plant and dispose it. If metal is of value, plant is burned to get metal (phytomining)
How can stabilisation and solidification be used as a remediation method?
Stabilise pollutant, make solid, make immobile.
Reduce solubility and isolate the solid.
Treatments involve mixing range of binding reagents, something to chemically react with pollutant to make it solid. Commonly include cement, lime, limestone, fly ash, slag, gypsum and phosphate mixtures
How can groundwater treatments be used as a remediation method?
Diverting contaminated water to treatment plant. Clay used as adsorbent of heavy metals, because it has a charge. Organic pollutants are treated by ion exchange, exchanging toxic ions for safer ones. Adjusting the pH, neutralising
What is meant by adsorbent?
Sticks to the surface of
Why are clays so good at immobilising some heavy metals?
Clay particles are flat, platy and high surface area (quicker reactions = faster removal). 1:1 or 2:1 with Si tet and Al oct.
Can substitute toxic metal for si4+ or Al3+ which is less harmful. Isomorphous substitution
What is isomorphous substitution?
Substituting toxic metal ions with Si4+ or Al3+ ions which are safer
What are some examples of metals clay can remove?
Lead, copper, cadmium, arsenic, nickel, chromium, mercury
