Lecture 15: Subsidence Key Questions 1. 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.
1. Subsidence due to over pumping of aquifers old water table new water table
Central Valley Groundwater overdraft mainly due to agricultural irrigation
Central Valley
~9.0 meters Subsidence in the San Joaquin Valley, CA
Aquifer Conceptual Picture overburden layer saturated sand confining layer
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 saturated sand
= 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. cubic packing rhombohedron packing (tightest possible packing) Reduction of void space
ground surface settlement original ground surface consolidation tighter packing
http://water.usgs.gov/ogw/pubs/fs00165/
InSAR is being used to quantify land-surface subsidence Interferogram showing deformation in the Los Angeles Basin, April 1998 - May 1999 (USGS, 2005) http://www.connectedwaters.unsw.edu.au/resources/articles/subsidence.html
http://water.usgs.gov/ogw/pubs/fs00165/ http://www.stanford.edu/group/radar/vegas_3d_big.gif
Houston region in the state of Texas
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
weight Because it has a high % of void space, it can consolidate a large amount when loaded.
The weight of the structure increases the water pressure in a bulbous region in the saturated clay high water pressure low water pressure high water pressure low water pressure
Water flows from high pressure to low pressure. low water pressure high water pressure low water pressure
Water moves extremely slow through clay rich material so the consolidation process is very slow, decades to 100s of years
building settles As the high water pressure dissipates back to hydrostatic conditions, the clay grains take on more of load and hence, consolidate. low water pressure grains consolidate 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.
cubic packing (loosest possible packing) high void space saturated sand confining layer
high water pressure overburden layer saturated sand Water is in part supporting the overburden weight
high water pressure overburden layer water migrates out of the sand pore spaces much quicker
When water is removed, loose sand will attain a tighter packing due to the weight of the overburden. cubic packing rhombohedron packing (tightest possible packing) loss of void space
ground surface settlement original ground surface consolidation tighter packing, but the pores spaces remain saturated
more settlement clays have more initial void space more settlement less settlement sands are more conductive so settlement happens fast
Peat also consolidates when loaded or drained http://water.usgs.gov/ogw/pubs/fs00165/ 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
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
1994 Land Loss Maps
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.
http://water.usgs.gov/ogw/pubs/fs00165/ Carbonates (limestone and dolomite) rocks and evaporites (salt, gypsum, and anhydrite) and are susceptible to dissolution by acidic groundwater also know as solution leaching.
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
Exploration bore hole surface subsidence 1932 court case
Birchwood Mine site October, 2004
Railroad & Holly, 2004 1853 historic subsidence site 1859 1875 mine portals
Depth to mine workings estimated at 300 feet. 1997 drill holes encountered 14 high water-filled tunnels: 83 feet 77 feet Railroad & Holly, 2004