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

Movement occurs when stresses produced by weight > strength of rock mass

Key factor controlling stability= dip extent and nature of fractures

Risk increased if fractures are inclined at angles > friction angle= Daylighting

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

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

Greater angle of slope= more unstable due to increase in gravitational forces acting on slope face

Water bearing rock

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

Produced by radioactive decay of U238, common in granite

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

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