20060815 Landmovement Presentation compressed

Land Movement Risks within Armadale City Hills: 

Land Movement Risks within Armadale City Hills P. Mutton BSc.Geophysics (Hons) Consultant Geophysicist Southern Geoscience Consultants

Geological Foundation: 

Geological Foundation Armadale city split by major (Continental scale) Darling Fault West is young sediments of Perth Basin (relatively!) East is ancient igneous Yilgarn Craton. Topography Gravity

Slide3: 

Geological Foundation Typical Hills Environment Typical Perth Basin Environment Sediments to approximately 10km depth Basement 0m-50m below clays and Laterite crust

Major factors contributing to Land Movement : 

Major factors contributing to Land Movement Slope (degrees of strain, water velocity) Water flow (abundance and rainfall patterns) Geology (minerals, contacts, orientation) Land Cover (vegetation, water bodies, man made disturbance, exposed areas) Flow Accumulation

#1 Factor - Slope: 

#1 Factor - Slope LOT 62 Location Armadale Topography (Blue = high areas) Armadale Slope (Black = very high slopes) CRITICAL AREAS ARE THE LOWER SLOPES OF THE LARGE RIVER VALLEYS

Common Landslide Types: 

Common Landslide Types SLUMP TOPPLE FLOW SLIDE CREEP FALL

Detailed Geology – Lower slopes of Canning Valley: 

Detailed Geology – Lower slopes of Canning Valley GRANITE DOLERITE CLAY SOILS RIVER Steep Slopes (andgt;25% common) Multiple contacts – conduits for water High rainfall area with high volume subsoil water movement Deep clay soils Vegetation clearance varies – Lot 62 in area of proposed resort is 90% cleared High flow accumulation (concave morphology) Unfavorable Geology – weathering produces expansive clay minerals = HIGH RISK OF LAND MOVEMENT

Detailed Geology – Lower slopes of Canning Valley: 

Detailed Geology – Lower slopes of Canning Valley Steep Slopes (andgt;25% common) Multiple contacts – conduits for water High rainfall area with high volume subsoil water movement Deep clay soils Vegetation clearance varies – Lot 62 in area of proposed resort is 90% cleared High flow accumulation (concave morphology) Unfavorable Geology – weathering produces expansive clay minerals = HIGH RISK OF LAND MOVEMENT

Lot 62 Location : 

Lot 62 Location Many of these land movements can be observed on Lot 62 Landslides Land Creep High subsoil water flows Debris flow Debris Flow from gullies

Land Creep in Valley (opposite Lot62): 

Land Creep in Valley (opposite Lot62) NEW FENCELINE OLD FENCELINE LEANING FENCE SHOWS GROUND SLIDING DOWN HILL

Land Movement at Lot 62: 

Land Movement at Lot 62

Water flow is critical to landslide risk: 

Water flow is critical to landslide risk Water is critical to landslides. Water content critically changes the properties of clays. The worst are expansive clays which can change in volume by 30% and vary from rocklike when dry to a slurry-like when wet. Lot62 and this area contain expansive clays. Thredbo was caused by a small leaky pipe.

Subsoil Erosion Lot 62 : 

Subsoil Erosion Lot 62 Lot62 is covered with holes where surface has collapsed into subterranean cavities eroded away by high water flow

Debris Flows on Lot 62 : 

Debris Flows on Lot 62 Eventually Gullies form with large debris flow at base (and flow with clay into river) Debris Flows often become rapidly obscured by grasses but remain unstable and may move when saturated

Small Landslide on lot 62: 

Small Landslide on lot 62 Here is a possible small landslide in the centre of the resort development In geological processes, small scale structures mirror larger ones Landslides have been observed on Lot 62 during heavy rainfall

Pretty rocks but what do they show?: 

Pretty rocks but what do they show? Contacts between rock types act as water conduits High erosional regime with history of landslides

EPA’s sound advice: 

EPA’s sound advice If this advice is to be followed the site would be incompatible for any medium-high density development

Collateral damage: massive costs to local governments: 

Collateral damage: massive costs to local governments Thredbo disaster: at last, the facts By Natasha Wallace December 3, 2004 Lost it all ... Tricia Hecher and her ex-husband, Josef. Photo: Jon Reid Finally, after seven years, the cause of the devastating Thredbo landslide has been established - a leaky mains pipe and a road built on a vulnerable slope of debris. A Supreme Court judgement handed down yesterday, which points the finger at government inaction at a looming disaster, will now enable the last remaining civil case, a multimillion-dollar personal injury claim, to proceed. Since 3500 tonnes of debris came crashing down a slope about midnight on July 31, 1997, wiping out two ski lodges and killing 18 people, the State Government has spent $40 million in out-of-court settlements with 91 businesses and individuals.

Conclusions & Recommendations : 

Conclusions andamp; Recommendations Lower slopes of Armadale hills are prone to land movement. Land movement in these hills results in damage and may result in failure of buildings. Lower slopes should only be zoned for very low land use capability and should not be cleared. Lot 62 is not capable of literally supporting any medium-high density development. Existing 'Geotechnical report' is superficial and inaccurate. Rezoning would only be courting future problems.

Subsoil Erosion Due to Human Disturbance : 

Subsoil Erosion Due to Human Disturbance Collins Rd – Road Cutting Collins Rd – Road Cutting Collins Rd – Ongoing erosion From drain outlet Video of subsoil water flow from below ground