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Soil & Foundation Health

Soil is a critical component when it comes to the health of a building's foundation. If the wrong footing is built for the soil or waterproofing solutions aren't provided for certain problem soils then the foundation can be subject to structural problems.

It's important to conduct a soil survey prior to construction so that the appropriate foundation is designed and identify potentially problematic soils.


Expansive Soils

Expansive SoilsExpansive or reactive clay soils can have a negative effect on a structure's foundation. The American Society of Civil Engineers estimates that at least half of the homes in the United States are built on expanding smectite clays, and of those at least half have or will sustain structural damage.

Expansive soils react to changes in moisture. When water is introduced the soils expand, while the soils contract when the moisture is removed. This will result in differential Expansive soils are responsible for more home damage every year than floods, tornadoes and hurricanes.movement and structural problems.

If moisture content varies dependent upon the location surrounding the structure, then localized or non-uniform settlement can occur. Evidence of this includes cracks in the foundation, uneven doorways and slab cracks.

Additionally, expansive soil can also result in damage to foundation walls. This is commonly evidenced in basements. The expansion of the soil will increase lateral pressure against the foundation wall. If the wall isn't strong enough to resist the pressure then cracks and leaks can form. Wall failure can occur in these instances.

Hydrocompaction

HydrocompactionSoils that have large amounts of silt can experience hydrocompaction. This typically occurs during period of extended drought and the sudden introduction of moisture.

When the moisture quickly evaporates from the top layers the silt and clay act as a bonding agent, holding these layers together. However space is left between the layers, and when the soil is exposed to water the bond is dissolved and the soil particles sink and move closer together.

Factors for hydrocompaction include deep watering of plants, pipe leaks, and puddles. The weight of a house or other structure increases the risk of hydrocompaction. And unlike settling, hydrocompaction can mimic earth fissures with the exception that they are restricted to a small area and typically circular.

Karst Landscapes

Karst LandscapesIn Virginia, West Virginia and Maryland karst landscape is isolated to the Blueridge mountains. Karst landscaping is prevalent in ares where carbonate, limestone and other soluble rock have dissolved.

Karst is not unique to limestone but it is the most common identifier due to the higher rate of dissolution and sinkholes. Limestone is made up of more than 50% carbonate minerals which are more likely to dissolve. However it's important to note that the quantity of water and porosity of the limestone play a critical role in the dissolution process.

The development of sink holes is the largest structural danger associated with karst landscapes. Sinkholes are classified into three categories: solution, collapse and subsidence.

Solution sinkholes are caused by the dissolution of bedrock. While collapse sinkholes are the result subsurface dissolution which resulted in the collapse. Subsidence sinkholes are caused by a change in the environment such as loading or changes in ground water.

Liquefaction

Soil LiquefactionFine textured, uncompacted, loose and saturated soils in low-lying areas near bodies of water can be subject to liquefaction. This can occur during an earthquake and is dependent upon the magnitude and susceptible soil's location to the center or even rapid loading.

The shaking results in a pressure change in the water within the soil. It dramatically increases the water pressure which results in the decreased strength of the soil. This can result in settlement or landslides.

References:

i. Virginia Department of Mines Minerals and Energy. Expansive Soils and Frost Heave. Retrieved from www.dmme.virginia.gov/DMR3/expansivesoils.shtml

ii. U.S. Inspect. Expansive Clay Soil & The Impact on Real Property. Retrieved from www.usinspect.com/resources-for-you/house-facts/environmental-concerns-home/expansive-soils

iii. Arizona Geology. Problem Soils. Retrieved from http://www.azgs.az.gov/HomeOwners-OCR/HG4_problemsoils.pdf

iv. USDA. Understanding Soil Risks and Hazards. Retrieved from ftp://ftp-fc.sc.egov.usda.gov/NSSC/Soil_Risks/risk_low_res.pdf

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