Geology A2 - Engineering Geology 2 (Waste disposal in land (Environmental…
Geology A2 - Engineering Geology 2
Geological factors affecting the construction of tunnels
Attitude of the strata
Flat lying, competent, uniform strata are best for tunnelling.
If beds dip, then different rock types may be encountered along the tunnel.
Slippage along bedding planes may lead to rock falls into the tunnel.
Presence of structures create some big challenges during tunnelling
Zones of weakness and permeability.
Often more closely spaced than faults, can be even more problematic.
Loose blocks of rock between joints may fall out the tunnel roof.
Other linear structures:
Such as bedding planes and foliation.
Zones of weakness and may allow slippage or leakage of water along them.
Zones of weakness as they may have breccias and fault gouge clay along them.
Zones of permeability increasing flood risk.
May be different rock types either side of the fault and, in the event of an earthquake, cause the tunnel to collapse.
Folded rock sequences:
Make tunnelling difficult due to dip angle changes and risk of slippage on fold limbs.
If the fold is a gentle syncline, possible to follow the dip of the fold and stay in one bed.
Tunnelling may be through hard rock, soft rock or unconsolidated material. All require different approaches.
, which is slow and expensive.
Amount of explosives used must be carefully calculated to avoid
At depth in hard rock tunnels, high confining pressure could cause dangerous rock bursts.
Cheap and relatively easy to tunnel through but require lining with concrete or steel ribs.
Specially designed tunnel boring machine achieves tunnelling rates of 30 metres a day in soft rock.
Ideal rock types are sandstone, limestone and chalk.
Very difficult to tunnel through.
Prone to collapse and leakage.
Include weak rocks such as clay and shale and unconsolidated materials such as sand and gravels.
Porous and permeable rocks allow water seepage into tunnels and risk flooding.
Lateral variation and changes in rock type make tunnelling difficult. as rocks may have different strengths.
Weathering weakens rock and variations in compaction and cementation also cause problems.
If the tunnel is below the water table, flooding may occur:
Water may be free-flowing through unconsolidated sediments.
Strong flows may occur along joints in limestone.
Sandstones can develop high pore fluid pressures.
Saturation of clays can lead to mobilisation and failure by slumping.
Ground improvement methods to prevent collapse and flooding of tunnels
Strategies to prevent tunnel collapse include:
Lining with concrete segments or steel ribs.
Use of rock bolts to secure loose blocks.
Strategies to prevent tunnel flooding include:
Grouting the surrounding rocks.
Using rock drains.
Coastal erosion, flooding and defences
Coastal erosion and flooding
Coastlines are one of the most varied landforms. Erosion, transport and deposition processes are finely balanced and any disturbance results in rapid changes until equilibrium is restored.
- water that washes onshore as a wave breaks.
- water that moves back down the beach due to gravity.
- a process that moves sediment along the coast due to waves breaking at an oblique angle.
Geological factors affecting coastal erosion
Strength and hardness of rock influences erosion rates.
Unconsolidated material offers little resistances to wave attack.
Strong, competent rocks offer the most resistance.
Along rocky coastlines, rock type determines the cliff profile.
Weak, incompetent rocks form gently sloping cliffs, with marine erosion at the cliff foot and weathering at the cliff face.
Strong, competent rocks form vertical cliffs dominated by marine erosion at the cliff face.
Attitude of the strata
The dip of a rock changes the cliff formation.
The steepest cliffs form where rocks are horizontal or dip away from the sea.
The gentlest cliffs form where rocks dip in towards the sea. These tend to undergo landslips and slumping.
Where rocks of alternating resistance strike at an angle to the coastline:
More resistant rocks form headlands.
Less resistant rocks form sheltered bays in between.
Rocks of alternating resistance may also strike parallel to the coastline
If the sea breaks through a layer of resistant rock, it'll scour the less resistant rock to form a bay with a narrow entrance.
Faults, joints and bedding planes.
Weaknesses in the rock that can be exploited by wave action.
Selective erosion of headlands alone these planes of weakness forms sea caves, blowholes, arches and stacks.
Strategies to reduce coastal erosion and flooding
Sea walls and banks
Dual function of protecting against both coastal erosion and flooding.
Built close to the high water mark and reflect wave energy.
Usually made of concrete and may be sloped, vertical or curved.
Effective in the short term but may be subjected to scouring, undercutting and increased erosion on the seaward side.
Expensive to build, costing up to £1000 pet metre.
Cheaper banks and mounds may be built, mainly to reduce flood risk.
Rock buttresses, revetments and rip rap
Relatively cheap way of protecting against coastal erosion.
Large blocks of hard rocks piled up in front of cliffs or sea walls to reduce wave action.
Spaces between blocks effective at absorbing wave energy.
Rocks need to be imported to the area so tend to look unsightly and out of place.
Wood groynes are a familiar sight on many beaches and are one of the most effective methods of reducing sediment loss by longshore drift.
Usually extend out to sea at right-angles to the coast.
Sediment builds up o the up-drift side of the groynes so the beach is retained.
These days some groynes are made of large blocks of rock rather than wood.
One of the most popular 'soft engineering' strategies for coastal management.
Imported sand used to build up beaches.
Expensive strategy and needs regular maintenance to remain effective.
Texture of the imported sand must match existing sand.
When sand is pumped onto the offshore part of the beach, it can bury plants and animals, block out light and disturb ecosystems.
Can be used to stabilise cliff faces and reduce impact of weathering and erosion on coastal areas.
High costs mean they are only used to protect built-up areas.
Slope modification - Slope reduced to low angles to increase stability.
Retaining wall - Constructed of concrete, used to support road cutting sides.
Gabions - Wire mesh boxes filled with rocks, placed as lateral toe support at the bottom of slopes to prevent failure by slumping.
Rock bolts - Steel rods, several metres long, drilled/cemented into rock faces. Pin lose rock blocks to sound rock behind, preventing rock falls. Can only be used in competent rocks.
Rock drains - Drains of broken rock remove water and reduce pore fluid pressure.
Wire netting - Fix surfaces in place and catch small rock falls.
Shotcrete - Concrete sprayed at high pressure on rock surfaces increasing strength, reduces permeability and protects from surface weathering.
Vegetation - Plants fix soil in place reducing water infiltration. Effective stabilisation strategy for incompetent rocks.
Waste disposal in land
Landfill waste disposal
Waste disposal choices:
Isolation by burying the waste.
Incinerating, diluting or spreading out the waste.
Recycling paper, glass, metals and some plastics.
Method:- First, rubbish is compacted by heavy machinery.
Then covered with a soil layer at least 15cm thick to isolate it.
Landfill then sealed with final layer of compacted soil 50cm thick.
Surface graded is water runs off.
Environmental consequences of waste disposal in landfill
Short term noise, dust, smells, wind-blown litter and vermin infestations.
Percolation of rainwater dissolves soluble chemicals and collects microbial contaminants producing leachate.
Surrounding soils and groundwater in underlying aquifers vulnerable to contamination by leachate, which forms pollution plumes that spread out laterally in response to groundwater flow.
Once complete, settling and subsidence of landfill when biodegradable waste starts to decompose.
Large volumes of methane gas generated by anaerobic microorganisms that thrive on waste.
Little danger of explosions because there is no oxygen present but the gas must be vented off to prevent dangerous levels building up.
Increasingly, methane is recovered and used as a viable fuel.
Geological factors affecting landfill sites
Geological structures and attitude of the strata
Allow downward leakage of leachate.
Tilted or folded beds:
Allow down-dip and lateral movement of leachate, which can migrate some distance away from the landfill site through permeable beds.
Increase permeability of rocks.
Provide escape routes for leachate.
May have tension joints at their crests.
If the water table is high then there is less distance for the leachate to travel to reach underlying groundwater.
The level of the water table may also vary in aquifers.
Fine-grained impermeable rocks most suitable.
Thick, uniform, flat lying beds are best.
Porous and permeable rocks allow flow of leachate.
Limestone may be dissolved by acidic leachate leading to the formation of solution cavities, which will destabilise the site.
Cementations acts as a barrier to leachate flow, but weathering increases permeability making leachate more likely.
Crystalline igneous and metamorphic rocks may be suitable, but can be affected by jointing.
Ground improvement methods to prevent leakage of leachate
Grouting the surrounding rocks.
Laying an impermeable clay or geomembrane (plastic) lining.
Draining and collecting the leachate, which can then be treated or safely stored.
Nuclear waste disposal and environmental geochemistry
Radon gas pollution
One of the decay products of radioactive elements is radon gas.
Radon is also radioactive.
Radon gas that seeps from rocks and soils can build up to dangerous levels in houses, posing a serious health risk.
If inhaled into the lungs, alpha particles bombard and damage cells causing an increased risk of cancer.
Geochemical surveys have been used to highlight areas of the British Isles with high natural risk from radon pollution.
Limestone is a particular problem because leakage of radon is concentrated along fractures.
House-to-house surveys are carried out in high risk areas and the problem can usually be eliminated by improved ventilation in affected buildings
Nuclear waste disposal
A number of possible nuclear waste disposal options have been proposed including:
Launching the waste into space in rockets.
Burying it in the sea floor sediments close to subduction zones.
Placing it in secure containers on the ice sheets of Greenland or Antarctica.
Burying it in an underground geological repository.
Safe disposal of high level radioactive waste must meet the following criteria:
Isolation for at least 250,000 years.
Secure from accidental or deliberate entry.
Safe from natural disasters such as floods, hurricanes and earthquakes.
No chances of leakage into the surrounding environment.
Nuclear waste is classified as low level, intermediate level, high level or transuranic waste.
Low level radioactive waste is usually disposed of in secure landfill sites.
High level and transuranic wastes emit high levels of radiation, are often thermally hot, and have long half-lives.
In the UK, high level waste is stored for at least 50 years, allowing it to 'cool' prior to solidification and disposal.
All of these options have their merits, but also have serious drawbacks.
Burial in an underground geological repository is probably the least problematic of the choices. An underground geological repository for nuclear waste would need to be:
In a tectonically stable area.
Within dry, impermeable rocks with a low water table.
Free from the effects of potential natural hazards.
Evaporites have been suggested as a suitable rock type, as salt is dry and a good conductor of heat.
Unfortunately, some hydrated evaporate minerals give out water when heated so pools of saline water could form.
These would corrode storage containers and allow leakage of the radioactive waste.
Best option is storage in dry, competent rocks such as granite or volcanic rocks.
Granite contains naturally high levels of radioactive elements making it less attractive.
Best choice would be burial in crystalline basement rocks below younger sedimentary cover rocks.
Suggested for disposal of nuclear waste from the Sellafield installation in Cumbria but has proved very unpopular amongst local people.
Heavy metal contamination of soils
Toxic, heavy metals can accumulate naturally in soils and as a result of human activities.
Number of pathways which heavy metal elements can enter the human body:
Can be present in surface and groundwater supplies.
Can be taken up by plants through their roots.
Can be present in plants and soil ingested by grazing animals.
If we then eat affected plants and animals, the heavy metals can enter our bodies.
Heavy metals are dangerous because they bio-accumulate.
This means their concentration in the body increases over time because they are not easily broken down or excreted.
British Geological Survey has been carrying out regional geochemical surveys both in the British Isles and abroad for over 40 years.
Identify areas of contaminated land.
Improve our understanding of the links between geology and health.
Study geochemical factors that affect habitats and biodiversity.
Allow sustainable development of natural resources and management of waste disposal.
In the UK, data gained from analysis of stream sediment samples taken every 1-2km2, supplemented by soil samples taken from urban areas.
The stream sediment and soil samples are analysed for 48 elements and the data is used to compile a series of geochemical atlases.