Little Known Reasons for Ground Subsidence in California

Fortunately, most sinkholes in California are man-made phenomena. A water main break causes a sinkhole that partially swallows a parked car. Photo © 2010 Jay Mallin

Can a street really eat a road and swallow a car?

It happened in Richmond, California in 2010. And, it could happen again. Watch the video.

Sometimes, the ground we stand on is not as solid as we think. As part of a site inventory, landscape architects investigate a site’s geologic substrate to determine if hidden dangers are lurking under foot.

Ground subsidence is a problem for California landscapes than landscape architects should be aware of when conducting a site inventory.

What is Ground Subsidence?

Subsidence is the downward motion of the ground compared to a reference point (i.e. sea level). Unlike a sinkhole, where a limited and well-defined area of ground fails and collapses, ground subsidence occurs of larger areas and moves downward more gradually.

Causes of Subsidence

Here are a few reasons that cause the solid ground to become not so solid.

  • Soluble Substrate (i.e. sinkholes)
  • Abandoned Mines
  • Hydrocarbon Extraction
  • Groundwater Pumping
Causes of Subsidence

Causes and Effects of Subsidence

Soluble Geologic Substrates – Sinkholes

Sinkhole Map shows regions with geology susceptible to sink holes.

Map shows regions with geology susceptible to sink holes. Not that only scattered areas of soluble geology exist in California.

Not all geologic substrates are made equal. Some rocks and soils contain high amounts of water-soluble chemicals like limestone. Over time, water dissolves the minerals and the underlying geology becomes riddled with voids. Over time, the voids can become so large that the weaken the geologic substrate and sink holes are created.

California Murphys Mercer Cave

California Murphy's Mercer Cave. Soluble limestone rock was dissolved by water to create a cave over time.

For example, imagine that you are standing in a dark cave dripping water from the ceiling. The water, over a long period of time, dissolved the rock and formed the void that is now the cave.

Sinkholes caused by soluble geology are uncommon in California and seldom pose a threat to development like they do in some other states, such as,  Florida. However, Bridgeport, Anza Borego, and Palm Springs have areas of karst geology that are susceptible to being dissolved by water. Fortunately, there is not much history of catastrophic sinkholes in California caused by soluble geology.

Sub-Surface Mining

The ’49ers got it started in California, but the ground is still suffering from the affects of mining in some areas of the state.

Map of abandoned mines in California.

Abandoned mines are located throughout the California.

“…after more than 100 years of heavy-mining operations, from the ’49ers onward, California’s easily-accessed gold and silver have been replaced by at least 40,000 abandoned mines, and many more ancillary mine structures like tunnels and shafts.” (Mental Floss)

Ground subsidence can be caused by old mine tunnels collapsing which causes the surface around the tunnels to sink. Modern mining operations understand the causes of mine-based subsidence and work to manage the outcomes with stakeholders (usually).

The biggest threat for sub-surface mine-causes subsidence is from abandoned mine operations. Abandoned mines continue to pose a threat to buildings and landscapes in California.

“The hillside below a 5-bedroom home…gave way, exposing a tunnel snaking under the house and extending deep into the hill. The tunnel may have been part of the legendary Pacific Mine which operated in town beginning in the 1850s.” ( Sacramento Bee, 8/12/01)

While nobody was killed or injured, the geological failure caused by mining activity could have resulted in the loss of life.

Mines can be found throughout California. Identifying possible development hazards caused by mines is prudent if you suspect that abandoned mines are located on or near your client’s site.

Natural Gas and Hydrocarbon Extraction

Los Angeles Oil Field

Pumping oil and natural gas in parts of California contributed to dramatic subsidence.

Extracting natural gas and oil is another possible cause of subsidence. Oil and gas occupy pores in the underlying geology. When hydrocarbons are extracted, the pores begin to collapse and the land above subsides.

Some areas of central and southern California have been notably affected by subsidence caused by oil extraction. For example, the Wilmington oil field in Long Beach started to subside in 1940 due to hydrocarbon extraction. Thirty square miles of land subsided. At the center of the subsidence, the surface elevation fell by 29 feet. Eventually, the subsidence was stabilized by re-introducing fluids to support the pore space in the rock and some of the elevation was recovered at great expense.

Subsidence can affect drainage patterns by changing the land’s elevation and moving soil can strain utilities that are damaged by soil movement.

Groundwater Extraction

Image showing historic elevation levels and current elevation of the San Joaquin Valley.

Historic image showing subsidence in the San Joaquin Valley caused by groundwater pumping.

Pumping groundwater for human or agricultural is another cause of ground subsidence in California. Extracting groundwater reduces the fluid pressure on the underlying rock and permits compaction. California’s central valley suffered the effects of ground subsidence due to pumping ground water for irrigating crops.

California Regions Affected by Subsidence

Many areas of the state are suffering from soil subsidence.

  • Lancaster, California has lost up to 6 feet of elevation due to ground water caused subsidence.
  • Long Beach, California has lost up to 27 feet of elevation due to hydrocarbon extraction.
  • Davis, California has lost 4 feet of elevation from groundwater pumping for agriculture.
  • The Santa Clarita Valley has lost 12 feet of elevation from groundwater loss.
  • Ventura, California has subsided 2 feet.

Subsidence Is Here to Stay

Subsidence is another factor affecting California landscape and development projects. Understanding the causes and impacts of subsidence on site development is one factor that may be assessed on the California Supplemental Exam (CSE) for landscape architects.

About

John is a landscape architect who is currently preparing to take the California Supplemental Exam to become licensed in California. He is currently a licensed professional landscape architect in Georgia and Florida. John graduated from California State University, Pomona with a BSLA degree in landscape architecture in 1997 and has extensive practice experience in residential and commercial landscape design.

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