subsidence - varsity field · fluid withdrawal: ... right: subsidence in arizona due to extraction...

GLY 162 4 September, 2013

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Subsidence

Ms. Nelda Breedt

GLY 162 Environmental Geology

GLY 162 – 2013 GLY 162 – 2013

Subsidence

Subsidence is:

A sinking or collapse of a portion of the land surface

Movement essentially vertical

Sudden or an imperceptible lowering

Natural or aggravated by human activities

Biggest cause of subsidence worldwide is the extraction of subsurface fluids

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Causes of Subsidence

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Natural:

Volcanic activity

Tectonics

Solution (karst,

dolomite)

Vibration settlement

(earthquakes)

Piping

Thawing

Anthropogenic:

Fluid extraction

Hydrocompaction (wetting of loose soils)

Compaction

Drainage of peat and subsequent oxidation

Mining

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Consolidation Natural compression of geological materials

under the influence of static loads Dissipation of pore water - decrease in void

ratio Usually in silty or clayey soils with low

permeabilities Water flows slowly with resultant slow surface

depression Two stages:

Primary consolidation Secondary consolidation

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Consolidation

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Primary consolidation:

Pore water in voids are placed under stress and is squeezed out with increased pressure

Load is transferred from the water and soil to the soil particles

Secondary consolidation:

Decrease in void ratio of soil mass due to adjustment of internal structure

After most of the primary consolidation has taken place

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Consolidation

Consolidation is typical of:

Low density residual dolomite

Recent sands with clay layers (KwaZulu-Natal coastal areas)

Prevention

Preloading

Drainage systems in clay layers

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Consolidation

Fluid withdrawal:

Either naturally where trees with deep root systems deplete soil moisture or man-induced where groundwater, oil, gas and brines are withdrawn

Subsidence is gradual over a large area (regional)

Subsidence is caused primarily by compaction - decrease in the thickness of a layer of sediment or rock

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Consolidation

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Amount of subsidence is

proportional to the

amount of fluid

withdrawn and the

thickness of the

compressible layers

Right: subsidence in Arizona due to extraction of groundwater

© Skinner (2004): Fig 15.13

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Consolidation

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Tower of Pisa: Built on fine-grained

floodplain sediments

Began tilting when construction started in 1174

Groundwater extraction increased tilting

Prevent toppling: restrict extraction

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Consolidation Groundwater is pumped out of an aquifer

creates depression in groundwater table surface/ decrease in confined stresses

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Consolidation

The surface subsidence may be reversed by pumping

water into the aquifer to raise the fluid pressure

This has no effect when the aquifer compacted to

such an extent that it passed from the elastic to

inelastic condition

This phenomenon is not common in South Africa due

to our relatively deep water table with competent

rocks between the surface and the aquifers

As groundwater extraction increases we may see

more of this problem

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Consolidation Venice (Italy):

Built 3 km from the continent edge as protection against the Huns

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Consolidation

Venice (Italy):

Built on a number of islands in a saltwater lagoon

Underneath the city are >1 000m thick Quaternary sediments of an old deltaic deposit (unconsolidated sand, silt and clay)

Artesian water was extracted from the top 300 m

2 000 boreholes provided the city with water for its industries

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Consolidation

Venice (Italy):

Started to sink at a rate of 0.5 cm/y

Pumping was stopped in 1969 and subsidence also stopped

But aquifers were already beyond their elastic conditions and the subsidence was permanent

Periodic flooding now occurs due to storm surges and high tides

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Mining-related Subsidence

First man-induced subsidence phenomenon

Affected surface corresponds to undermined area

Tensional forces around edges & compressional forces in central area

South Africa: Gold mining areas (Gauteng & Free State) Coal mining areas (KwaZulu-Natal &

Mpumalanga)

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Gold mining:

Elastic behaviour influences deformation

Total subsidence at surface will be equal to total stope thickness

Stresses are withstood for long times until the rock fails, which causes crack zones to form

Subsidence of the hanging wall near outcrop areas

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Mining-related

Subsidence

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Gold mining:

Subsidence of the hanging wall near outcrop areas

Right: hanging wall collapse; Central Johannesburg

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Coal mining:

Mining methods: Bord and pillar

Pillar extraction Longwall mining Strip mining Open cast

Two types of subsidence: Continuous Discontinuous

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Coal mining:

Discontinuous

Typically over bord & pillar mining

Sinkholes

Unpredictable and potentially dangerous

Coal mining:

Continuous

Over longwall mining areas

Predictable

Not catastrophic

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Bord and Pillar

Longwall

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Underground

fires cause

pillars to

decrease in

size, which

cause

caving in of

the roof

rocks with

resulting

surface

cratering

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Loss of productive farmland and land for industrial uses are the main problem

Groundwater and underground drainage systems are also adversely affected

AMD (Acid Mine Drainage)

Very little can now be done to old mined-out areas

Backfilling and or additional support systems

Support for hanging wall

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Karst-related Subsidence

Karst

Complex collection of landforms associated with subsurface features due to the solution of soluble rocks (limestone, dolomite)

Usually covered by layer of surficial material (regolith)

Disturbance in surface drainage or groundwater extraction leads to slow subsidence (doline) or sudden collapse (sinkhole)

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Primary a result of dissolution:

Chemical weathering; process by which minerals pass directly into solution

The rate of dissolution exceeds the rate of erosion

Leads to cave formation below the groundwater table and grike (vertical joint enlargement) above the groundwater table

Occurs in the dolomite rocks of the Far West Rand and south of Pretoria

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Dissolution mechanism: Rain + CO2 form a weak carbonic acid –H2CO3

Carbonate minerals are readily dissolved in acid

When water table is lowered (naturally or by man) caves below the groundwater table are exposed

These cavities serve as receptacles for erosion material washed down by percolating rain water and a cavity starts to open in the residuum

When this cavity is eroded to the surface a sinkhole appears

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Dissolution mechanism:

Dissolution (of CO2) and ionization (of H2CO3):

H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-

Dissolution and hydrolysis of calcite

CaCO3 + H2CO3 Ca2+ + 2(HCO3)-

Calcite Bicarbonate

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Sinkholes on the Far West Rand formed due to

groundwater lowering by gold mines

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More than 30 people have lost their lives in sinkholes

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Useful Links

http://www.bgs.ac.uk/science/landUseAnd

Development/shallow_geohazards/sinkHoles

.html

https://www.youtube.com/watch?v=6jWyHE

3X9Uc

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http://www.bgs.ac.uk/science/landUseAndDevelopment/shallow_geohazards/sinkHoles.html




https://www.youtube.com/watch?v=6jWyHE3X9Uc

https://www.youtube.com/watch?v=6jWyHE3X9Uc

subsidence - varsity field · fluid withdrawal: ... right: subsidence in arizona due to extraction...

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