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Mass Movement (Slope monitoring (Hazard mapping, Surveying, Groundwater…
Mass Movement
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Mam tor Landslide
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1,000m length, 450m width, 12 degrees slope
Slope failure is result of geological instability in Mam Tor hill where inclined Carboniferous Sandstone are underlain by Edale Shales
Landslide is complex feature with 3 distinct zones=
- Upper= Rotational slide of massive blocks of sandstone up to 30m thick
- Transition= Slide blocks
- Lower= debris up to 18m thick moved by intermittent flow processes
C-14 dating of tree root shows failure occurred 3,600 years ago, records of movement over last 100 yrs indicate movements up to 0.5m occurs in transition zone if winter rainfall exceeds 200mm
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Following slips in 1974 and 1977, the A625 was closed, since initial failure toe of landslide has moved 320m downhill
Aberfan Tip disaster
21st Oct 1966, buried school killing 116 children and 28 adults
15,000m2 slide, school buried by 12m of debris
7 spoil tips composed of unconsolidated mine waste were built upon hillside above the town, location of tips at this location was unsuitable due to presence of natural springs along base of Brithdir Sandstone
Tips 4 and 5 were also placed on springs previously failed, tip saturation made worse by several days of heavy rain
an Inquiry into disaster noted following=
(1) No geological investigation was undertaken prior to dumping of mine waste, spring presence below tip 7 was unknown, natural springs at base of Brithdir Sandstone were known at other locations
(2) Site managers failed to act on signs on increasingly instability in hours prior to disaster, upper flank had subsidence by 3-6m
(3) Managers ignored local concerns over stability of site and causes of 1944 and 1956 tip failures
4 geological factors contributing to 1966 failure to tip 7=
(1) built on spring
(2) 35 degree slope too steep
(3) heavy rain saturated ground
(4) Tremors/ vibrations from railway
Rockfall
Wedge Failure
Slide and fall of rock over steeply inclined fractures dipping towards open face, often triggered by heavy rain as build-up of groundwater pressure reduces friction between layers of rock
Slab failure
Collapse of rock face due to under-cutting and removal of supporting rock at base of cliff, common on coastal cliffs following removal of cliff base by wave erosion
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Slide
Slow or rapid movement of coherent block or rock and soil along defined slip plane, 2 types=
- Planar slides= slip plane is flat
- Rotational slides= slip plane is curved, slumps as moves downslope
Debris flow
In-cohesive mass or rock, soil and water which rapidly flows downslope as saturated slurry, occur in sediments and weakly cemented rock, triggered by build-up of pore water pressure following heavy rain, also called mudflows
Soil creep
Slow transfer of soil downslope due to repeated annual cycle winter expansion and summer contraction, process leads to formation of small elongated ridges called Terracettes, seen by bent tress and poles