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

http://water.usgs.gov/ogw/pubs/fs00165/

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

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

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

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.

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: