Chapter 8 Subsidence and Soils

Learning Objectives Know what a soil is and the processes that form and maintain soils Understand the causes and effects of subsidence and volume changes in the soil Know the geographic regions at risk for subsidence and volume changes in the soil Understand the hazards associated with karst regions

Learning Objectives, cont. Recognize linkages between subsidence, soil expansion and contraction, and other hazards, as well as natural service functions of karst Understand how humans interact with subsidence and soil hazards Know what can be done to minimize the hazard from subsidence and volume changes in the soil.

Soil and Hazards Soil Solid earth material that has been altered such that it can support rooted plant life Any solid earth material that can be removed without blasting Soil is related to hazards (i.e., soil types and landslides) Soil is produced through weathering physical and chemical breakdown of rocks Changed by residual or transported activity of soil organisms Soils can stay in place or be transported to new locations

Soil and Hazards, cont. Soil development depends on: Climate Topography Parent material (the rock or alluvium from which the soil is formed) Time (age of the soil) Organic processes (activity of soil organisms) In an area, soils form distinct layers Soil Profile

Soil Horizons Layers in a profile are soil horizons O horizon entirely of plant litter and other organic material A horizon both organic and mineral material E horizon, zone of leaching, light-colored layer leached of iron-bearing components B horizon, zone of accumulation, materials moved downward top layers Bt horizon enriched in clay minerals Bk horizon calcium carbonate K horizon carbonate fills the pore spaces and the carbonate often forms in layers parallel to the surface Caliche is irregular accumulation or layers of calcium carbonate

Soil Horizons, cont. C horizon parent material partially altered by weathering processes R horizon, unaltered parent material, consolidated bedrock Hardpan, hard (compacted) soil horizon composed of compacted and/or cemented clay with calcium carbonate, iron oxide, or silica Impermeable

Figure 8.1

Soil Color Can be an important diagnostic tool for analyzing a soil profile, but can be misleading O and A horizons are dark E horizon is white B horizon varies from yellow-brown to light red-brown to dark red K horizon may be almost white Soil color can also indicate drainage

Soil Texture Depends on proportions of sand-, silt-, and clay-sized particles Clay diameter < 0.004 mm Silt 0.004 mm > diameter > 0.074 mm Sand 0.074 > diameter > 2.0 mm Gravel, cobbles or boulders > 2.0 mm Figure 8.2

Relative Soil Profile Development Soil chronosequence: series of soils from youngest to oldest to give information about the recent history of an area Weakly developed soil: A directly over a C horizon (without B) Few hundred to several thousand years old Moderately developed soil: A overlying an argillic Bt that overlies the C horizon More than 10,000 years old (at least Pleistocene) Well-developed soil: B is more red, more translocation of clay to the Bt horizon, and stronger structure Between 40,000 and several hundred thousand years and older

Water in Soils Saturated – all the pore spaces in a block of soil are completely filled with water Unsaturated otherwise Moisture content – amount of water in a soil Important to strength of soil and potential to shrink and swell Water flow Saturated flow if all the pores are filled with water Unsaturated flow otherwise (more common)

Soil Classification Soil taxonomy Engineering classification of soils By soil scientists Based on physical and chemical properties of soil profile Useful for agricultural and land use Engineering classification of soils Used by engineers (and for hazards) Based on particle size or the abundance of organic material

Table 8.1

Table 8.2

Introduction to Subsidence and Soil Volume Change Subsidence is ground failure characterized by sinking or vertical deformation of land associated with Dissolution of rocks beneath the surface Results in karst topography Thawing of frozen ground Compaction of sediment Earthquakes and drainage of magma Soil volume change result from natural processes Changes in water content of soil Frost heaving These are probably not life threatening, but is one of the most widespread and costly natural hazards

Karst Common type of landscape associated with subsidence Rocks are dissolved by surface or groundwater Evaporites, rock salt and gypsum, dissolved by water Carbonates, limestone and dolostone and marble, dissolved by slightly acidic water Acid comes from carbon dioxide from plant and animal decay Common in humid climates Figure 8.8a

Karst Topography, cont. Rocks are dissolved and groundwater level drops, leaving behind caverns and sinkholes Pits in that are near surface Sinkholes in large numbers form a karst plain Figure 8.8b, c

Sinkholes Solutional sinkholes Collapse sinkholes Acidic groundwater becomes concentrated in holes in joints and fractures in the rock Water is drawn into a cone above the hole in the limestone Collapse sinkholes develop by the collapse of material into an underground cavern

Figure 8.9

Cave Systems Cave systems are formed when dissolution produces a series of caves Related to fluctuating groundwater table Groundwater seepage causes flowstone, stalagmites, stalactites Figure 8.10

Tower Karst, Disappearing Streams and Springs Tower karst is created in highly eroded karst regions Disappearing streams are streams that disappear into cave openings Springs are places where groundwater naturally flows into at the surface Figure 8.11

Thermokarst In polar or high altitude regions, permafrost exists Soil or sediment cemented with ice for at least 2 years When permafrost thaws it can create land subsidence Extensive thawing creates uneven soil called Thermokarst

Sediment and Soil Compaction Fine sediment Sediment collapses when water is removed Common on river deltas Flooding replenishes sediment, thwarting collapse Collapsible soils Dust deposits, loess, and stream deposits in arid regions are bound with clay or water soluble minerals Water weakens bonds causing soil to collapse Organic soils Wetland soils contain large amounts of organic matter and water When water is drained or soil is decomposed, these soils collapse

Earthquakes In subduction boundaries, when fault is locked, land can become uplifted After an earthquake, the land subsides Figure 8.12

Underground Drainage of Magma Magma uplifts the land during an eruption, afterwards land subsides Lava tubes form when molten lava drains out from underneath cooled surface lava

Expansive Soils These soils expand during wet periods and shrink during dry periods Common in clay, shale and clay-rich soil containing smectite After expansion, soils can have cracks and popcorn-like texture Often will produce wavy bumps in surfaces causing tilting and cracking of sidewalks, foundations, utility poles and headstones

Figure 8.13

Frost-Susceptible Soils Soils containing water expand when frozen moving the soil upward Frost heaving Figure 8.16

Regions at Risk for Subsidence and Soil Volume Change Landscapes underlain by soluble rocks, permafrost, or easily compacted soil and sediment Soils that contain large amounts of smectite clay are susceptible to shrinking and swelling soils Soils containing silt are susceptible to frost heaving

Regions at Risk for Subsidence and Soil Volume Change, cont. Climate controls the amount and timing of rainfall and duration of freezing temperatures Sinkholes are common in humid climates Expansive soils are common in areas with wet and dry seasons Collapsible soils are found in arid and semiarid climates Areas with extensive below freezing temperatures can host frost heaving

Figure 8.17

Effects of Subsidence and Soil Volume Change Sinkhole Formation Common in Florida and Pennsylvania Damage highways, homes, sewage facilities, etc. Probably triggered by fluctuations in water table High groundwater enlarges underground caverns and fills with water. Removal of water causes roof to collapse. Figure 8.18

Effects of Subsidence and Soil Volume Change, cont. 1 Groundwater conditions Caves create direct access between surface and groundwater This access can make water vulnerable to pollution Especially during drought and when sinkholes are used as landfills Figure 8.19

Effects of Subsidence and Soil Volume Change, cont. 2 Melting of permafrost has caused roads to cave in, airport runways to fracture, railroad tracks to buckle, and buildings to crack, tilt, or collapse Figure 8.20

Effects of Subsidence and Soil Volume Change, cont. 3 Coastal flooding and loss of wetlands Along the Mississippi delta this has contributed to sinking of New Orleans Wetlands that protect the city from surges are eroding Soil volume change Responsible for billions of dollars of damage annually to highways, buildings, and structures Frost action on roads costs $2 billion each year Damage caused by soil volume change exceeds the cost of all other natural hazards combined

Figure 8.22

Links to Other Natural Hazards Can be an effect of earthquakes, volcanoes, and climate change Climate change adds to the drying of soils and altering of groundwater table May cause flooding and mass wasting Frost heaving and swelling soils cause creep Areas subsiding due to groundwater mining are most susceptible to flooding

Natural Service Functions Water supply Karst regions contain the world’s most abundant water supply Aesthetic and scientific resources Caves and karst landscapes are beautiful Caves attract visitors Caves and karst provide research for scientists Unique ecosystems Many species of animals can live only in caves Caves also provide shelter for other animals

Human Interaction Withdrawal of fluids Pumping fluids such as oil, natural gas, water, groundwater, etc. decreases fluid pressure causing rocks to subside Figure 8.25b

Figure 8.26

Human Interaction, cont. 1 Underground mining Coal mine structures have collapsed Water is used to dissolve and pump out salt leaving behind cavities Flooding in salt mines can also cause sinkholes Melting permafrost Global warming and building practices Restricting deltaic sedimentation Construction of dams, levees, etc.

Human Interaction, cont. 2 Altering surface drainage Draining soils for agriculture Draining wetland soils for development Adding water for irrigation Poor landscaping practices Adding or removing plants changes water levels contributing to shrinking and swelling soils

Minimizing Subsidence and Soil Volume Change Artificial fluid withdrawal – groundwater mining Restricting oil and water pumping Injection wells add water when oil is pumped Regulating mining Prevention of damage from thawing permafrost New engineering of buildings and pipelines on permafrost Reducing damage from deltaic subsidence Controlled flooding could rebuild marshes

Minimizing Subsidence and Soil Volume Change, cont. Managing drainage of organic and collapsible soils Limit irrigation and modify land surface Prevention of damage from expansive soils Design of subsurface drains, rain gutters, and reinforced foundations Construct buildings on compacted fill

Perception and Adjustment to Hazard Few people are aware of subsidence and soil volume change hazard People who live in dramatically affected areas are more aware than others Best solution is to avoid building in vulnerable areas through: Geologic and soil mapping Surface features Subsurface surveys

Subsidence and Soils Chapter 8 End Subsidence and Soils Chapter 8