lecture 15: subsidence - western washington...
TRANSCRIPT
Lecture 15: Subsidence
Key Questions1. How does removal of groundwater cause subsidence on a regional scale?
2. Under what conditions does a building sink into sediment?
3. Why do clays consolidate more the sands?
4. Why is the Mississippi delta sinking?
5. What “rapid” subsidence issues does Bellingham face?
6. What is solution leaching? Sink hole near Daisetta, Texas: May 9, 2008
Subsidence – ground surface settlement due to the consolidation or collapse of the underlying geologic material
http://mentalfloss.cachefly.net/wp-content/uploads/2007/11/crooked-house.jpg
Subsidence – ground surface settlement due to the consolidation or collapse of the underlying geologic material
Causes include the following:
1. Over pumping of large aquifer systems
2. Loading effects of overlying structures (i.e., buildings)
3. Loading due to sediment build up near delta systems
4. Collapse of mines or caves created by dissolution (e.g., karst)
5. Liquefaction of saturated sands and silts due to earthquake ground shaking
Groundwater is water located beneath the ground surface in pore spaces and fractures of geologic formations.
An aquifer is a geologic unit that can store and transmit groundwater at rates fast enough to supply reasonable amounts to wells.
new water table
old water table
1. Subsidence due to over pumping of aquifers
Groundwater “overdraft” mainly due to agricultural irrigation
Central Valley
Central Valley
Subsidence in the San Joaquin Valley, CA
~9.0 meters
saturated sand
confining layer
overburden layer
Aquifer Conceptual Picture
saturated sand
Grain-to-grain interaction is reduced in water because the grains are “effectively” lighter due to the buoyancy force
saturated sand
cubic packing (loosest possible packing)
high void space
=water tower
When the sediment is dewatered, the buoyancy force is removed.
=water tower
When the sediment is dewatered, the sediment grains are heavier
When water is removed, loose sand will attain a tighter packing due to the “extra” weight of the overburden.
Reduction of void space
rhombohedron packing (tightest possible packing)cubic packing
original ground surface
tighter packing
ground surface settlement
consolidation
Interferogram showing deformation in the Los Angeles Basin, April 1998 - May 1999 (USGS, 2005)
http://www.connectedwaters.unsw.edu.au/resources/articles/subsidence.html
InSAR is being used to quantify land-surface subsidence
http://water.usgs.gov/ogw/pubs/fs00165/
http://www.stanford.edu/group/radar/vegas_3d_big.gif
Land subsidence has been occurring across Arizona since the 1950s
Mexico City is sitting on soft saturated sediment and is sinking
2. Subsidence due to building loads
Saturated “clay” rich material
“clay” has a high percentage of void space
50 to 70 % of the total volume can be void space
Because it has a high % of void space, it can consolidate a large amount when loaded.
weight
The weight of the structure increases the water pressure in a bulbous region in the saturated clay
high water pressure low water
pressurelow water pressure
high water pressure
Water flows from high pressure to low pressure.
high water pressure low water
pressurelow water pressure
Water moves extremely slow through clay rich material
so the consolidation process is very slow, decades to 100s of years
As the high water pressure dissipates back to hydrostatic conditions, the clay grains take on more of load and hence, consolidate.
low water pressure
low water pressure
grains consolidate
building settles
low water pressure
http://mentalfloss.cachefly.net/wp-content/uploads/2007/11/crooked-house.jpg
The old “Shrimp Shack” on Holly Street
Why is the tower of Pisa leaning?
The same physical process happens in wet sands too!
But the effect is almost instantaneous and less dramatic.
saturated sand
confining layer
cubic packing (loosest possible packing)
high void space
saturated sand
Water is in part supporting the overburden weight
overburden layer
high water pressure
water migrates out of the sand pore spaces much quicker
overburden layer
high water pressure
When water is removed, loose sand will attain a tighter packing due to the weight of the overburden.
loss of void space
rhombohedron packing (tightest possible packing)cubic packing
original ground surface
tighter packing, but the pores spaces remain saturated
ground surface settlement
consolidation
more settlement
less settlement
clays have more initial void space – more settlement
sands are more conductive so settlement happens fast
Peat also consolidates when loaded or drained
Miller Hall’s concrete slab basement is resting on a peat deposit, hence is sinking about 0.25 inches per year. Some areas have sunk over 1.5 feet.
The main structure is resting on concrete pilings sitting on sandstone bedrock. It isn’t sinking. peat
http://water.usgs.gov/ogw/pubs/fs00165/
3. Subsidence due to sediment loads
The sediment that builds up near deltas is very loosely packed
Mississippi River Delta
Each year a new layer is added and
the lower layers are compacted due to the added weight
modern layer
Older layers compact due to overlying weight – hence the surface subsides.
Layer 1
Layer 2 consolidates layer 1
Layer 3 consolidates layers 1 & 2
Layer 4 consolidates layers 1, 2 & 3
Layer 5 consolidates layers 1, 2, 3 & 4
Layer 6 consolidates layers 1, 2, 3, 4 & 5
Layer 7 consolidates layers 1, 2, 3, 4, 5 & 6
Layer 8 consolidates layers 1, 2, 3, 4, 5, 6 & 7
And so on …..
The delta package continues to consolidate and settle
The old delta deposits extend landward and are settling
Mississippi River Delta
When a river floods, it deposits a layer of sediment
The Mississippi River is channelized with a levee system and is not allowed to flood naturally
The delta is settling, but not being rebuilt with natural flood deposits.
http://earthobservatory.nasa.gov/IOTD/view.php?id=6623
4. Rapid subsidence due to mine or cave collapses
Winter Park ,Florida sinkhole swallowed a swimming pool, two businesses, a house and three cars.
Karst topography is shaped by the dissolution of soluble rock by acidic groundwater, usually limestone.
Carbonates (limestone and dolomite) rocks and evaporites (salt, gypsum, and anhydrite) and are susceptible to dissolution by acidic groundwater also know as solution leaching.
http://water.usgs.gov/ogw/pubs/fs00165/
The collapse (rapid subsidence) of karst caves creates sink holes.
Underground salt dome collapsed—about 600 feet by 525 feet and 150 feet deep sink hole near Daisetta, Texas: May 9, 2008
http://yosemite.epa.gov/R10/CLEANUP.NSF/9f3c21896330b4898825687b007a0f33/e11dc590780784ed8825677e00641cd1/$FILE/20030131_Bellingham.pdf
Birchwood Mine (Bellingham No. 1 Mine)
Level 1 north
Level 2 north
Level 3 north
surface subsidence
Exploration bore hole
1932 court case
Birchwood Mine siteOctober, 2004
Railroad & Holly, 2004
1875
historic subsidence
site
1859
1853
mine portals
Railroad & Holly, 2004
Depth to mine workings estimated at 300 feet.
83 feet77 feet
1997 drill holes encountered 14’ high water-filled tunnels: