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Engineering Geohazards (Radon Gas (Produced by radioactive decay of U238,…
Engineering Geohazards
Radon Gas
Produced by radioactive decay of U238, common in granite
Colourless, odourless and tasteless gas
Easily detected with electronic equipment
In open air, it is quickly diluted to safe levels, it can reach dangerous levels if it becomes concentrated in unventilated buildings, mines, caves
Concentration in buildings may occur in several ways (1) Radon gas is able to move freely through fractured and permeable rock and can entre from below. Ground seepage is strong during periods of low atmospheric pressure, (2) Soluble in water and may escape from water extracted from a well, (3) May be present in building construction materials, such as granite, gypsum plater and Portland cement
In UK estimated that 100,000 houses have levels above normal background concentrations
UK 1,800 deaths per year
Areas at greatest rick= Cornwall, Devon, Northern Scotland, Cumbria, Wales, Northamptonshire
Remedial work to reduce build up involves following steps- (1) Improve ventilation, (2) seal base of building to prevent seepage, (3) Half-life= 3.8 days, so quickly decays into less hazardous daughter. If principal source is water supply, settling pools can be built as part of the water system to ensure radioactive decay before entering buildings
Rock Mass Strength
Defined as the amount of stress needed to promote failure
Rock fails because amount of stress exceeds strength of slope
Depends on factors- Intact Rock Strength= strength depends on- (1) strength of minerals (2) strength of bonds between grains (3) frictional resistance between mineral grains
Intact Strength measured in different ways- (1) Unconfined Compressive Strength (2) Tensile Strength (3) Shear Strength
Important concept= friction angle= measure of the frictional resistance of a rock and approximates to the inclination of a shear plane that forms when a rock fails under compressive stress, depends on fractional strength of rock, Crystalline rocks >40, loosely packed grains 20-35
Groundwater
Aquifer
Water bearing rock
Water Table
Upper level of saturation
Porosity
Amount of water a rock can store, expressed as %
Permeability
Rate of groundwater flow within rock, expressed as velocity
Factors that affect porosity
Well sorted sediments have bigger pore spaces
Presence of cement reduces porosity
Sediments with rounded grains have bigger pore spaces, Angular grains interlock and reduce pore spaces
Stacked grains have bigger pore spaces, interlocking grains have smaller pore spaces
Stability of a Rock mass
Rock Fractures
Natural cracks or plane of weakness
Bedding planes
Joints
Faults
Cleavage
Movement occurs when stresses produced by weight > strength of rock mass
Key factor controlling stability= dip extent and nature of fractures
Highly fractured= less stable
Fractured dipping towards open face= unstable
Risk increased if fractures are inclined at angles > friction angle= Daylighting
Weathering
Fractured rock= vulnerable to weathering (water moves through fractures)
Increases instability in 3 ways- (1) Chemical weathering weakens bonding between materials and reduce cohesive strength (2) Causes fractures to widen which reduces overall frictional strength (3) Feldspar weathers into clay which can act as lubricant and reduce frictional strength
Groundwater
Below water table groundwater is under pressure and acts in opposition to confining pressure of rock weight
Presence of pressurised water within fracture reduces frictional resistance and causes failure
Can also dramatically weaken cohesive strength of clay rich sedimentary rocks, weakly cemented sandstone and limestone
Slope Angle
Greater angle of slope= more unstable due to increase in gravitational forces acting on slope face
Dams and Engineering Geology
Constructed to crate artificial lakes (reservoirs), which are used for water supply, flood control, HEP and irrigation
Estimated that 66% of worlds rivers are partially controlled by dams
Choice of location based on
Size of reservoir (maximise potential storage capacity)
Rock Strength (strong enough to cope with load of dam)
Ground leakage (Not built on unconsolidated, permeable rock such as sandstone, limestone etc as piping can occur and dam collapses as result)
Keppel Cove Dam, Lake District
1891 built
29 Oct 1927 burst during heavy storm, 24m wide gap in dam
Foundations weakened by piping as was built on weak glacial sediments
Geological hazards (avoid active and ancient fault zones, unstable slopes pose an issue= landslides)
Engineering design
Arch dams
Concrete, curved shape, narrow mountain valleys, stress generated by reservoir is transmitted into valley side rock
Thin, load concentrated over small area of rock at base
Constructed on strong bedrock
Gravity Dams
Wide, massive structures, rely on weight to hold in position, since load is spread over wide area they can be built on weak rock such as shale
Built across broard shallo valleys
Coposed of impermeable clay core with concrete surround and rock/ soil cover
Include embankments, buttress dams
Vaiont dam disaster
2034 died, Alps, 266m high, 4-23m thick, slide on 9 Oct 1963, 270m^3 moved 400m at 20-30m/s, 100m high wave over toppled dam, villages downstream destroyed
Daylighting limestone beds with prehistoric slide evidence, groundwater pressure, calculations wrong for estimated slip
Rock Excavations
Cuttings
Many engineering projects necessitate cuttings or excavating into a slope to create a level platform for construction. Such practices destabilise the slope because the removal of rock and soil weakens the strength of the slope, and if the stresses acting on the slope exceed its strength, the slope will fail usually as a landslide or rockfall
Engineering Solutions
Slope Modification
Grouting
Rock Bolts
Fabrics
Anchors
Gabions
Retaining wall
Drains
Vegetation
Tunnels
Few engineering projects where the feasibility, cost, design and danger of accidents during excavation are dependent on geology. The line of a tunnel is usually determined by favourable geological conditions, while the cost of the tunnel and rate of construction progress reflects ease of rock extraction and stability of rock. 3 Main tunnelling methods
Drill and Blast
Using a Roadheader
Using a Tunnel-boring machine
Difficult ground conditions caused by
Faults
Groundwater
Overbreak
Rockbursts
Squeezing
Swelling Ground
Compressive strength reduction
Geothermal Gradient
Gas
Tunnel Support Methods
Arch support
Ring support
Composed of concrete, or by spraying concrete onto rock
Channel Tunnel
Construction 1986-1994
Built through syncline, soft, impermeable rock