Geomechanics 1: Week 8
Geology of Melbourne
** Check history of Jolimont Valley in week 8 lecture slides**
Silurian Siltstone (Melbourne Mudstone)
§ Material Properties
– Low to medium strength rock.
– Sandstone (brittle)
– Siltstone (ductile)
§ Mass Properties
– Open folds.
– Highly jointed in places, regular joint spacing.
– Highly persistent bedding planes
– Faults (all types) some with fault gouge
– Intruded by dykes
§ Geomorphological Properties
– Deep chemical weathering (up to 50 m depth)
– Sediments become clay minerals – colour black when fresh, white when
weathered and grades in between.
– Iron staining gives rock an orange brown colour where weathered.
– Extensive mechanical weathering, highly variable surface level.
§ When Loaded
– Deformation influenced by mass, material and environmental properties.
Moderate stiffness.
§ When Unloaded
– Some heave, block movement, increase in permeability.
§ Change Water Pressure
– Can lead to block movement.
Older Volcanics
§ Material Properties
– Very high strength rock.
– Some layers of tuff and ash (hard soils)
§ Mass Properties
– Very highly jointed (10 mm spacing).
– Vesicles and amygdules.
§ Geomorphological Properties
– Extensive chemical weathering, can be through full thickness. Produces red,
highly plastic clays.
– Extensive chemical weathering on rock joint surfaces.
– Extensive mechanical weathering, sporadic occurrence.
§ When Loaded
– Can have high deformation where weathered or highly fractured.
§ When Unloaded.
, – Some heave and increase in permeability. Block movement.
§ Change Water Pressure
– Can lead to change in strength
Brighton Group
§ Material Properties
– Dense to very dense sands, some, cemented.
– Stiff to hard clays (overconsolidated)
§ Mass Properties
– Generally massive.
– Some fissures in clay.
– Uneven bedding planes, lenses and buried channels.
§ Geomorphological Properties
– Some chemical weathering – produces red staining, iron concretions and
cementation.
– Localised mechanical erosion, especially around drainage courses.
§ When Loaded
– Can have high deformation particularly if highly plastic clays have softened.
§ When Unloaded.
– Sand can lose density and become loose.
– Some heave.
– Can have block failure along fissures.
– Can have very high strength layers.
§ Change Water Pressure
– Change in effective stress can cause lowering of strength of sand leading to
erosion.
Newer Volcanics
§ Material Properties
– Very high strength basalt (bluestone).
– Some scoria and tuff.
§ Mass Properties
– Vertical joints most prominent.
– Joints open.
– Clay seams, boulders within clay matrix.
§ Geomorphological Properties
– Chemical weathering to shallow depth (3 m – 4 m). Produces highly plastic
(reactive) clays.
– Weathers from joints outward to produce core stones (floaters).
– Some mechanical weathering around water courses.
§ When Loaded
– Deformation of rock very small.
– Can have high deformation of clay.
– Rock likely to fail on joints, clay can fail in shear.
§ When Unloaded.
– Block failure can occur along rock joints or fissures in clay.
§ Change Water Pressure
Geology of Melbourne
** Check history of Jolimont Valley in week 8 lecture slides**
Silurian Siltstone (Melbourne Mudstone)
§ Material Properties
– Low to medium strength rock.
– Sandstone (brittle)
– Siltstone (ductile)
§ Mass Properties
– Open folds.
– Highly jointed in places, regular joint spacing.
– Highly persistent bedding planes
– Faults (all types) some with fault gouge
– Intruded by dykes
§ Geomorphological Properties
– Deep chemical weathering (up to 50 m depth)
– Sediments become clay minerals – colour black when fresh, white when
weathered and grades in between.
– Iron staining gives rock an orange brown colour where weathered.
– Extensive mechanical weathering, highly variable surface level.
§ When Loaded
– Deformation influenced by mass, material and environmental properties.
Moderate stiffness.
§ When Unloaded
– Some heave, block movement, increase in permeability.
§ Change Water Pressure
– Can lead to block movement.
Older Volcanics
§ Material Properties
– Very high strength rock.
– Some layers of tuff and ash (hard soils)
§ Mass Properties
– Very highly jointed (10 mm spacing).
– Vesicles and amygdules.
§ Geomorphological Properties
– Extensive chemical weathering, can be through full thickness. Produces red,
highly plastic clays.
– Extensive chemical weathering on rock joint surfaces.
– Extensive mechanical weathering, sporadic occurrence.
§ When Loaded
– Can have high deformation where weathered or highly fractured.
§ When Unloaded.
, – Some heave and increase in permeability. Block movement.
§ Change Water Pressure
– Can lead to change in strength
Brighton Group
§ Material Properties
– Dense to very dense sands, some, cemented.
– Stiff to hard clays (overconsolidated)
§ Mass Properties
– Generally massive.
– Some fissures in clay.
– Uneven bedding planes, lenses and buried channels.
§ Geomorphological Properties
– Some chemical weathering – produces red staining, iron concretions and
cementation.
– Localised mechanical erosion, especially around drainage courses.
§ When Loaded
– Can have high deformation particularly if highly plastic clays have softened.
§ When Unloaded.
– Sand can lose density and become loose.
– Some heave.
– Can have block failure along fissures.
– Can have very high strength layers.
§ Change Water Pressure
– Change in effective stress can cause lowering of strength of sand leading to
erosion.
Newer Volcanics
§ Material Properties
– Very high strength basalt (bluestone).
– Some scoria and tuff.
§ Mass Properties
– Vertical joints most prominent.
– Joints open.
– Clay seams, boulders within clay matrix.
§ Geomorphological Properties
– Chemical weathering to shallow depth (3 m – 4 m). Produces highly plastic
(reactive) clays.
– Weathers from joints outward to produce core stones (floaters).
– Some mechanical weathering around water courses.
§ When Loaded
– Deformation of rock very small.
– Can have high deformation of clay.
– Rock likely to fail on joints, clay can fail in shear.
§ When Unloaded.
– Block failure can occur along rock joints or fissures in clay.
§ Change Water Pressure
