Sinkholes, Land Subsidence, and Swelling Soils

Ground Settling in Permafrost

-Arctic climates have underground temperatures below freezing year round permafrost -If ice thaws, water escapes pore spaces, ground settles -Surface-most layer thaws annually -Forest fires, removal of vegetation can lead to permafrost thawing -Building structures on -permafrost must take into account possibility of thaw and ground surface deformation

Drying of Clays

-Clay soils prone to subsidence when they dry out -Especially true of randomly oriented marine clays with high porosity (quick clays) -Leaning Tower of Pisa sits atop marine clay which was insufficient to support the weight of the building -Leda clay causes landslides throughout St. Lawrence River valley in Quebec, Canada

solution example: florida

-Ex. Central Florida, have been produced slowly over time by the solution process. Shallow limestone surface. They tend to be round -Solutional. Surface-downward origin, solution starting from surface, often in fractures. Lower slope more conducive, jointed limestones more susceptible, high vegetation cover increases susceptibility. EXAMPLE LOCATION: FLORIDA

What human activities cause land subsidence?

-Extraction of groundwater or hydrocarbons -Drainage of organic or clay-rich soils -Melting of permafrost

types of sinkholes

Solution, subsidence, collapse

sinkholes

overlying ground collapses into underground cavities

soluble rocks in water

rocks that can melt in water in subsurface conditions, acidic conditions Salt, gypsum, limestone, other carbonates (melts easily)

Land subsidence

sediment becomes more closely packed, through groundwater or petroleum extraction or earthquake shaking, and land sinks broadly or locally

Retsof Mine Collapse

-Geneseo, NY -Salt mine had been active since 1880s, shallow mine where the salt was not at the surface. -1994: section of shale roof rock collapsed into excavation due to mining practice -Hole allowed surface water and groundwater to sink into mine, filling its cavities, flooding mine -Sinkholes up to 60 m opened across region New mine began few years later

Subsidence from Groundwater Extract

-Rainwater infiltrates into ground and reaches aquifers, from which groundwater can be pumped as source of fresh water for many communities -Produced by rainwater infiltrating the surface- if you pump it out faster than it comes down than sediment can start to collapse (using up the water faster than it is naturally being produced) this process starts slow and is not easily recognizable and generally a permanent problem -If pumpage rate exceeds recharge rate mining groundwater -Withdrawal of water allows sand and gravel to pack more tightly, take up less space -Once pore space has collapsed, doesn't expand again most ground subsidence is permanent -California, Arizona (in order to agriculture, water is pumped from the ground), Texas, Venice

Swelling Soils

-Smectite clays expand when water soaks into interlayer spaces of mineral structure -Places where material swells up when water is added into them. -Expansion and contraction cause cracking of foundations, walls, chimneys, driveways -Variable rates of swelling occur when some areas get wetter than others

Amplifying Sinkhole Potential

-Storm drains and leaky water mains in urban areas -Construction leading to sinkhole development by increasing load on ground surface, de-watering foundations, drilling wells -Underground mining of salt or gypsum creates artificial cavities that can be enlarged by groundwater dissolution -Adding weight to a surface can lead to landslides, like drilling wells and removing water from the subsurface can add to the potential for sinkholes to develop

Differential Expansion: Denver, Colorado

-Underlain by flat-lying sedimentary rocks including Pierre Shale, smectite-rich clay -Clay layers swell when wet, creating broad waves that twist, crack and bend structures -Solution has been excavation and homogenization of clay layers and non-clay-bearing layers to depth of at least 3 m

solution

-acidic groundwater seeps through soil, dissolves underlying limestone along fractures, which widen and create holes -A gradual process, dissolving of limestone which widens and deepens over time. There is rock at the surface and over time portions of it expand outwards, dissolving away slowly. -High vegetation produces acid and coers increases as the lower slope becomes more conducive.

Collapse

-if deep, impermeable (clay) sediment sits atop limestone, soil cavities can grow very large, become unstable and collapse suddenly into steep-sided sinkhole - most dangerous -Happens very rapidly, and with little warning -Material on top, limestone underneath- when it dissolves the whole thing collapses into it.

Processes That Cause Sinkholes

-some common sedimentary rocks are soluble in water -Areas with underlying limestone can have caves, springs, streams that sink into ground, sinkholes -Caverns form where carbonate rocks near water table dissolve in groundwater -Roof of cavern collapses if left unsupported where water table has dropped

Subsidence

-where deep, below the surface, permeable sediment sits atop limestone, numerous gradual sinkholes form -The surface gradually drops, the support of the subsurface is lost. -Subsidence sinkhole formed by spasmodic subsidence or more continuous piping and subsidence. which forms conical or bowl-shaped hollow rather than the steep collapse shape. -Winter Park, Florida

Slowly dissolving carbonate rocks underlies more than how much of the humid eastern U.S.

40%

Sinkholes

A collapse of the ground, can vary in size and location Ground may suddenly collapse into sinkholes tens to hundreds of meters across Can damage houses and roads Can drain streams, lakes and wetlands Can channel contaminants directly into underground aquifers

Venice, Italy

Built at sea level on subsided Brenta River delta Subsidence mostly caused by extraction of groundwater from delta sediments

when do marine clays collapse?

Can collapse when jarred by earthquake or heavy equipment, or flushed with fresh water

Subsidence from Oil Withdrawal

Excessive pumping of petroleum produces same kind of sediment collapse as does water pumping Significant problem in LA, Texas, Venezuela, elsewhere

Subsidence mostly caused by extraction of groundwater from delta sediments

Load of buildings squeezes water out of sediments Groundwater is pumped to surface for domestic and industrial uses Collapsed pore spaces in sediments cause ground to subside

Which areas experience Sinkholes

Locations where there are shallow (within a few 10s of meters in surface) carbonate rocks -Fluctuations of groundwater level can lead to sinkhole formation (much lower groundwater level during last ice age) -Greatest potential for sinkholes where surface water percolates into ground, recharging aquifers -Least potential for sinkholes is where water is being discharged to surface

Land Subsidence

Occurs when ground settles as result of changes in fluid levels underground Over large regions, so commonly not obvious, happens gradually Not obvious to people living there until floods or high tides occur Small faults and fissures may develop Some areas dropped closer to sea level so more vulnerable to flooding

What Happens Chemically: Formation of Calcium Bicarbonate

Rain water and carbon dioxide dissolves in the rain water that comes down, which is why the water that comes down in rain has a lower ph than 7. When they come together it forms carbonic acid. When the chemical bonds in calcium carbonate break down that creates bicarbonate. H2O (rain water) + CO2 (carbon dioxide) = H2CO3 (carbonic acid in water) Carbonic acid reacts with limestone to form calcium and bicarbonate: H2CO3 + CaCO3 = [Ca++ + 2HCO3-]

Drainage of Organic Soils

When groundwater levels drop: Organic-rich soils (such as peat) are exposed to aerobic water instead of anaerobic water Allows bacteria to oxidize organic matter decomposes mostly to carbon dioxide Problems in California's Central Valley, Florida Everglades, Mississippi River delta

Swelling soils:

from alteration (weathering) of minerals to clays Cause of soil creep, but create problems away from slopes Land that swells upwards esp. when wet

Limestone dissolves where?

in slightly acidic rainwater, at rate of millimeters per thousand years Slightly faster rate in humid areas sinkholes and caverns more common in tropical climates, but also occur in dry areas

Landscape of limestone dissolution:

karst

Highest rate of dissolution occurs at what level

water table