Dr Jamie Pringle, Keele University, [email protected]C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence C-Change in GEES Ground Subsidence and Slope Stability Session One Session One: Introduction to Ground Subsidence
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Dr Jamie Pringle, Keele University, [email protected] C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence.
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C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence
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C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence
Subsidence in Clays (2): Settlement
• Clays consolidated by imposed structural load
• All clays cause some degree of settlement
• Water squeezed out by applied stress
• Degree depends on water % and stress applied– Lab assessment by consolidation test
• Remedy to avoid clay loading or wait for settlement to stop
• Modest settlement beneath buildings may fracture drains, leakage from drains removes soil & secondary settlement ensues
• Differential settlement more serious– Due to: uneven load, lateral change in silt content or uncontrolled drainage– Accelerated by tall structures– Eg. Transcona grain elevator, Canada, tilted 27º in 1 day in 1912
• Clays under raft base unevenly compacted over rockhead, sheared & laterally displaced structure which contained 875,000 bushels of wheat
• Note uneven ground movement– Some areas raised by 5 feet
C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence
Causes of Regional Subsidence
Groundwater abstraction exceeds natural recharge• Leads to water table lowering
Pumping from sand• Small but recoverable sand compaction (unlike clay!)
– Usually recovery leaves ~10% compacted
Clay compaction• Occurs as groundwater pressures equal between sand & clay• Time delay due to low clay permeability• Subsidence:head loss varies with clay type
– 1:6 on young, unconsolidated Mexico City Montmorillonite
– 1:250 on old, consolidated, London Clay Illite
– Subsidence stops if water tables recover due to pore water pressure support
C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence
US southwest problem
• Map sub-divides US subsidence problems
• Note most States have more than one type of subsidence problem
• Southwest region has a particularly high incidence of subsidence problems: mining, sinkholes, underground fluid withdrawal, hydrocompaction and drainage of organic soils
C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence
• If water table goes down (pumping or drought), cavity collapses & forms sinkholes
• Sinkholes often water-filled, terrain characterised by string of circular lakes • "Karst" topography (after Kars area in Montenegro/Albania) where common• Sinkholes range from meters to 100s of meters in diameter & a few 10s of
C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence
Mexico City (2)
• Soils moisture content 650%, Liquid limit (LL) = 500%, Plasticity Index (PI) of 350%– Normally consolidated but thixotropic (with time, stiffness & strength increases)– Very high angle of repose (Φ 35-45º), probably due to presence of angular
diatoms within soils
• Groundwater extraction problem– pumping started ~1850
• By 1974, 3,000 shallow & 200 deep wells• Estimated groundwater extraction 12m3/sec
• Between 1891-1973, estimated subsidence on Montmorillonite clays inter-bedded with over-pumped sands ~8.7m– Max. 460mm in 1950– Estimated may be 20m due to clay compaction
C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence
Pisa underlying geology
• Horizontally layered sands & clays
• Water table at about 1-2m
• Vertical settling occurred because of underling Pancone Clay plasticity & compressibility. Land surface difference now ~2m between N & S
• Main settlement due to compaction & deformation of soft clay at 11-22m
• Pisa tower imposes 500 kPa on clay with ABP ~50 kPa
• Differential movement probably started due to clay variation within silt layer; now accentuated by eccentric loadingWaltham, T., (2009) Foundations of
C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence
• Rotational weakness due to rotational creep in upper sands NOT the clay due to a weaker shear strength.
• Get a ‘racking’ effect as the tower expands & contracts during the heat/cool of the day.
• Made worse by saturation of the ground by shallow water table. Fluctuations in water table during storms cause northern side of the tower to slightly rise & fall more than southern side.
• The overturning forces of the tower are greater than resisting capacity of the sands.
• The tower is TOO TALL
• The different periods of building have actually helped as it compressed the sands & clays over time making them stronger. This is why the tower hasn't fall down (yet)
C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence
Other Settings (1): Isostatic rebound
• ‘See-saw’ affect
• Ice Age end (11,000 years bp) Scotland had 3 km thick ice sheet
• Glaciers retreated (melted): removal of weight leads to uplift– Documented varying uplift
rates, initially ‘rapid’ (7.5 cm/yr) then slowed to today (1 cm/yr)
– Will continue for 10,000 years
A model of present-day surface elevation change due to post-glacial rebound. Red areas are rising due to the removal of the ice sheets. Blue areas are falling due to
the re-filling of the ocean basins when the ice sheets melted and because of the
• Causes surface subsidence & structure settlement with imposed load or drained water loss– Greatest on thick clay, high smectite %, low silt %, young with no
over-consolidation
• Clays consolidated by imposed structural load
• Differential settlement– Due to uneven load, lateral change in silt content or uncontrolled
drainage– Accelerated by tall structures
• Regional subsidence caused by groundwater abstraction, pumping from sand, and clay consolidation
C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence
• Venice Case Study– Thick sediment pile– Over-extraction of water caused clay consolidation, now
stabilised
• Mexico City– High water content of Montmorillonite Clay– Over-extraction by pumping consolidated clays– Lots of settlement problems– Latino Americana Tower overcame this by buoyant foundations
& piles
• Leaning Tower of Pisa– Uneven settlement due to loading of clay layers– Eventually tilt reduced to stable (5º) angle by latest engineering
Hatheway, A.W. and Reeves, G.M. (1997) ‘Status of engineering geology in North America and Europe’ Engineering Geology 47(3): 191-215
Hatheway, A.W. and Reeves, G.M. (1999) ‘A second review of the international status of engineering geology: encompassing hydrogeology, environmental geology and applied geosciences’ Engineering Geology 53(3): 259-296
Jongmans D., Demanet D., C. Horrent, Campillo M., Sanchez-Sesma F.J. (1996) ‘Dynamic soil parameters determination by geophysical prospecting in Mexico City : implication for site effect modeling’, Soil Dynamics and Earthquake Engineering, 15(8): 549-559.
Pringle, J.K., Stimpson, I.G., Toon, S.M., Caunt, S., Lane, V.S., Husband, C.R., Jones, G.M., Cassidy, N.J. and Styles, P. (2008). Geophysical characterisation of derelict coalmine workings and mineshaft detection: a case study from Shrewsbury, UK. Near Surface Geophysics, 6(3), 185-194.
Waltham, T. (2009) ‘Foundations of Engineering Geology’ (3rd Edition), Spon Press. pages 56-7
Waltham, T. (2009) ‘Sinkhole Geohazards’. Geology Today, 25(3): 112-116
C-Change in GEES: Ground Subsidence and Slope Stability – Introduction to Ground Subsidence
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