dr jamie pringle, keele university, [email protected] c-change in gees: ground subsidence...

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 OneSession One: Introduction to Ground Subsidence



How to use the teaching slides These slides are not intended to form a complete lecture

on the session topic. These resources are designed to suggest a framework to

help tutors develop their own lecture material The resource slides comprise where appropriate; key

points, case studies, images, references and further resources.

These resources may be used for educational purposes only, for other uses please contact the author

These slides were last updated in November 2009



Disclaimer Links within this presentation may lead to other sites. These are

provided for convenience only. We do not sponsor, endorse or otherwise approve of any information or statements appearing in those sites. The author is not responsible for the availability of, or the content located on or through, any such external site.

While every effort and care has been taken in preparing the content of this presentation, the author disclaims all warranties, expressed or implied, as to the accuracy of the information in any of the content. It also (to the extent permitted by law) shall not be liable for any losses or damages arising from the use of, or reliance on, the information. It is also not liable for any losses or damages arising from the use of, or reliance on sites linked to this site, or the internet generally.

Pictures, photographs and diagrams within this presentation have been produced by the author unless otherwise stipulated

No content within this resource is knowingly an infringement of copyright. Any infringement can be immediately rectified on notification of the author of the resource



Session Outline

• Introduce the main types of subsidence

• Explain why clays are susceptible to subsidence

• Assess differential settlement issues

• Discuss regional subsidence & man-made problems

• Case studies:• Venice• Leaning Tower of Pisa• Mexico City

Subsidence causing structural problems for this building in Gothenberg – notice the

bottom left window

Stuart Chalmers (flickr)



Three main induced subsidence types:

• Ground-water withdrawal

• Karst & Evaporites (soluble rocks dissolved by carbonic acid & collapsed)

• Mining (past or present removal of subsurface material)

Other types include: isostatic rebound, sediment loading & natural coal ignition

Types of Subsidence

Roger g1 (flickr)



Groundwater withdrawal

• Exacerbated due to water over-extracting

• Groundwater pumped from pore spaces between sand/gravel grains

• Then slow drainage from clay/silt beds if present

• Reduced water pressure = loss of support for clay/silt beds

• Clay/silt beds compact & lowers ground surface

• Permanent!

The regolith has 20 to 50 times the water storage capacity of the bedrock

(USGS, 1983)

USGS



Subsidence in Clays

• Clays have high porosity with deformable grains of minerals, so high potential of compaction

• Compaction = volume decrease = consolidation

• Causes surface subsidence and structure settlement with imposed load or drained water loss

– Greatest on thick clay, high smectite %, low silt %, no over-consolidation

• Bearing Capacity of clays = 50-750 kPa, related to water content

• Older clays (shales & mudstones) stronger/less compressible

– Self boring pressure (SBP) 2000kPa

Exposed clay at Rio de Janeiro, Brazil(Alex Rio Brazil: wikimedia)



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



Subsidence in Clays (3): Monitoring

Source: USGS



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



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

• Associated with high damage costs

Source: USGS



Karst and Evaporites

• Dissolution of limestone/carbonate rocks

• Similar process happens to Evaporites

– e.g. rock salt & gypsum

• Water & CO2 combine to form carbonic acid

• Acid water dissolves rock, leading to cave "basement“ removal, so "roof" tumbles down

• A cave formation mechanism

– Takes 10,000 years to create continuous passage, 1,000,000 years for fully developed system

• Cavities typically water-filled, which supports surface load

– Collapse when water is removed

(above) Karst landscape – Combe laval, France (Berrucomons wikimedia)

(below) Sinkhole in North Yorkshire (unknown source)



Karst TopographyCollapse

sinkhole with fill Subsidence

sinkhole (over fissure or faulting)

Subsidence sinkhole (over

buried sinkhole)

Preferential dissolution along bedding planes

Preferential dissolution along fracture/fault planes

Preferential dissolution along water table

Collapse sinkhole with

fill Subsidence sinkhole (over

fissure or faulting)

Subsidence sinkhole (over

buried sinkhole)

Preferential dissolution along bedding planes

Preferential dissolution along fracture/fault planes

Preferential dissolution along water table

Source: USGS



• 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

meters deep (some in China are > 200 m deep).

Natural Karstic Environments: Topography & Sinkholes

Source: Jamie Pringle

Sequence showing cavity creation (from loss of water from the water table) and collapse of overlying soil



Identification of Evaporite Karstic Environments

• Most common are gypsum (or anhydrite) & salt • Readily dissolved forming typical karst features • Where outcrops (or gypsum <30 m & salt <250 m) may

partly or wholly be dissolved by unsaturated water• Outcrops typically contain sinkholes, caves,

disappearing streams & springs• Other evidence: surface collapse features, saline

springs, & saline plumes • Many deeper evaporites show paleokarst features

– dissolution breccias, breccia pipes, slumped beds & collapse structures



Gypsum Karst Evaporitic Subsidence example – Ripon, UK

• Town subsidence caused by natural dissolution of Gypsum bedrock

• Upwardly migrating crown holes causing significant structural damage & even total collapse

Image (left): Ripon Cathedral (j-pundt: flickr.com)

(right): Ripon, North Yorkshire



Groundwater extraction fissures

M.C. Carpenter/USGS

S.R. Anderson/USGS

• Fissuring often associated with over-extraction of groundwater• 3.5 km longest documented in USA, 100’s m typical• Often enlarged by erosion

Sign warning motorists of subsidence hazard caused by earth

fissure damage (S.R. Anderson/USGS)

Earth fissures near Picacho, Arizona



Case Study: Venice, Italy

• Floods on around 100 high tides/year– Alla Rampa bar regulars wear wellies!

• Has over 1 km thick sediments underlying the city

• Top 350 m has six sand aquifers interbedded with clays (illite/chlorite)

• Historically wood piling used to penetrate to harder clay layers– Modern (including stone) buildings all

based on these

• Clay soils have low plasticity & moisture content below plastic limit (water content at which soil starts to exhibit plastic behaviour)(Dimitry B: flickr)



Venice, Italy Case Study (2)• Subsidence experienced due to:

– Natural (weight) (0.4 mm/yr), groundwater lowering by pumping & eustatic changes (~1.27 mm/yr)

• Subsidence mostly due to groundwater pumping (now controlled)– In 1900, artesian head 6m

above sea level– By 1930 20 m below sea level– Controlled in 1970s & has

recovered– Problem of Non-recoverable

clay compaction (~1 m)

• Rising sea levels now demand new barriers & raised perimeter frontage

Waltham, T., (2009) Foundations of Engineering Geology. 3rd edition, Spon:

London.

Acqua alta: exceptional tide peaks that occur periodically in the northern Adriatic Sea



Venice, Italy (3)

• Spring High Tides 90 years ago not an issue, they are today– St. Mark’s Square flooded 79

times in 1997• In 1984, Italian Govt. started £2Bn

scheme– Installation of mobile barriers– Reinforced coastal defences– Improved lagoonal ecosystem– 3 lagoonal inlets, mobile flap

gates allow shipping passage, only 2m difference between lagoon & sea

• Scheme now faces axe due to funding shortfall

St Mark’s Square, Venice

Source: radiowood2000 (flickr.com)



Case Study: Mexico CityLies within a basin, 2250 m above sea level surrounded by volcanic rocks

Divided into 3 geotechnical zones:1. 10-100m soft lacustrine clays

• 10% organic content2. Transition zone with sand/silt alluvium3. Hill zone with volcanic ash flows/tuffs

Reproduced with the permission of Canada's International Development Research Centre (www.idrc.ca)



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



Mexico City (3)

Ground settlement consequences– Well casings now protrude in the

streets– Surface infrastructure disruption– Water supply loss from

dislocated pumps– Piled buildings now raised from

surrounds– In 1951, had to obtain water

from outside sources, implement sewerage system plan & control groundwater abstraction

– Rafted foundation buildings (e.g. Palace of Fine Arts) settlement

Aerial view of Mexico City(Merrick Brown: flickr)



Mexico City (4)

• Palace of Fine Arts had to be built on a massive concrete raft

– Imposed 110 kPa load that caused 3 m of settlement

– Heavy rafts cause their own subsidence bowls & damage to adjacent buildings

• Stable foundations should be piled to more stable material

– In this area, these are the sand layers– Latino Americana Tower buoyant

foundations (+basement) to reduce imposed load

– Also piles driven to uppermost sand units– Settlement by compaction of lower clay =

ground subsidence as less upper clay– Note subsidence:head loss is 1:6– London Clay is 1:250

• Old, consolidated & illite London ClayDiagram: copied from Waltham (2002)Photos: (left) mdanys; (right) Kevin Hutchinson (flickr.com)



Other clay soil subsidence areas

• Bangkok– Fastest subsiding city at >

10 cm/year

• Santa Clara Valley, California– 4 m ground subsidence

since water table lowering– Now stopped as reduced

extraction

Photos: (above) Bangkok (Swami Stream: flickr) (below) Santa Clara Valley (the tahoe guy: flickr)



Case Study: Leaning Tower of Pisa

• Building started in 1173

• Started to lean to north in 1178 when only 4 stories high (~0.6º)

• 1270s increased to seven stories using tapered masonry on southern side – made matters worse.

• Bell tower added in 1370 at angle to visually correct for the tilt, now (~1.6º)

• 58m high & weighs 14,000 Tn

• 1817 – first true tilt recording gave 4.6º

• Current tilt 5.5º or 5.5 m out of plumb with estimated movement of 1.2 mm per year

• Tower not settling now but rotating about an axis point near the first floor

Photos: Argenberg (flickr.com)



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

Engineering Geology. 3rd edition, Spon: London.



• 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)

Leaning Tower of Pisa (3)



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

collapse of the glacial forebulge.

Source: NASA



Other settings (2): Sediment Loading

• Mississippi-Missouri River system collects eroded debris from central half of US (3.3 km2)

• Reaches Gulf of Mexico, slows & deposits (~2.4b kg/yr)

• Modern delta prograding 100-150 m/yr)

• Delta subsiding 2.5 cm/yr– Some debate on causes:

sediment compaction, ground-water withdrawal & sea-level rise alternative theories

(above) Map of Mississippi Basin (US EPA); (below) Aerial photograph of Mississippi Delta

(NASA/USGS)



Other Settings (3): Natural Coal IgnitionBurning Mountain, NSW, Australia:

– takes its name from a naturally combusting coal seam running underground through the sandstone

– Documented since 1828 (white man’s arrival)

– Part of Permian (235Ma) Greta Series coalfields

– Sites moves 1m South each yr. – Whole area subsiding– All attempts to put out have

failed (trenching/drilling/watering etc)Photograph at the summit of Burning

Mountain, NSW (unknown author)



Session Summary

• Clay compaction = volume decrease = consolidation

• 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



• 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

phase

Session Summary (2)



Baracos, A. (1957) ‘The Foundation Failure of the Transcona Grain Elevator.’ Engineering Journal 40(7): 1-6

Barnes G. E. (2000) ‘Soil Mechanics: Principles & Practice’ (2nd Edition), Palgrave MacMillan. pages 155-158

Bell, F.G. (1998) ‘Environmental geology’, Blackwell Science Publications, pages 415-441.

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

References



This resource was created by the University of Keele and released as an open educational resource through the 'C-change in GEES' project exploring the open licensing of climate change and sustainability resources in the Geography, Earth and

Environmental Sciences. The C-change in GEES project was funded by HEFCE as part of the JISC/HE Academy UKOER programme and coordinated by the GEES Subject Centre.

This resource is licensed under the terms of the Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales license (http://creativecommons.org/licenses/by-nc-sa/2.0/uk/).

However the resource, where specified below, contains other 3rd party materials under their own licenses. The licenses and attributions are outlined below:

1. The name of the University of Keele and its logos are registered trade marks of the University. The University reserves all rights to these items beyond their inclusion in these CC resources.

2. The JISC logo, the C-change logo and the logo of the Higher Education Academy Subject Centre for the Geography, Earth and Environmental Sciences are licensed under the terms of the Creative Commons Attribution -non-commercial-No Derivative Works 2.0 UK England & Wales license. All reproductions must comply with the terms of that license



Author Dr Jamie Pringle

Stephen Whitfield

Institute – Owner Keele University, School of Physical and Geographical Sciences

Title Ground Subsidence and Slope Stability 1

Date Created November 2009

Description Introduction to Ground Subsidence Powerpoint

Educational Level 3

Keywords (Primary keywords – UKOER & GEESOER)

UKOER, GEESOER

Creative Commons License Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales

Item Metadata

dr jamie pringle, keele university, [email protected] c-change in gees: ground subsidence...

Documents