Another problem that can cause problems for a foundation subject to weak soil characteristics is "subsidence" (Shabha, Kuhwald, 1995). According to Shabha & Kuhwald (1995), "Subsidence can be defined as a downward movement or a soil on which buildings stand from causes unconnected with loading from the building. Examples are underground mining, clay shrinkage (especially due to the action of tree roots) and erosion due to water passing through the subsoil, but excluding the compaction of made-up ground or infill
." (Shabha, Kuhwald, 1995)
Subsidence is in part a natural process but yet is also in part a man-made process. Throughout millennia, the process of water creating soil erosion has changed the landscape of particles that comprise the rocky granular landscape, such as silica and including clay. The man-made shafts created for the mining process further exacerbates the problem of using clay as a foundation material as there are many mine shafts and other man-made entrances into the earth as well as other entrances created either through natural or animal forces, which reflect the problem of subsidence and can facilitate a foundation instability causing structural collapse.
According to Shabha & Kuhwald (1995), "Clay soils are particularly prone to movement as they can change their volume in response to seasonal weather, or the action of vegetation extracting moisture from the soil. Any reduction of the soil moisture content that leads to subsidence is known as desiccation. At the other extreme, when clay soil holds huge amounts of moisture, this causes the soil to expand and heave." (Shabha, Kuhwald, 1995) As mentioned earlier in the literature review, desiccation is a problem with clay foundations as it causes an increasing level of instability. Desiccation is the process of creating the deviations or striations in the clay soil due to a reduction of the soil moisture content.
The cause of clay shrinkage, according to Shabha & Kuhwald (1995), "The reason for clay shrinkage is water bonded to the surface of the clay particles, rather than filling up the pores between particles. The proportion of clay minerals in the soil has a direct relationship with the potential for shrinkage or expansion. More commonly, a combination of hot weather and vegetation cause the greatest amount of subsidence damage. There are three basic classifications of movement, which involve interaction between trees, vegetation and clay soil. Seasonal. Trees need to obtain a greater amount of moisture during summer months, when precipitation is at its least. The extraction of the water from the soil generates large-pore water suction near the surface, resulting in desiccation, shrinkage and subsidence. The reverse occurs in the winter months, when trees or vegetation will cause the desiccated area to increase, leading to shrinkage and subsidence around the affected area." (Shabha, Kuhwald, 1995)
According to Drazga (1998), "Floors crack, walls crumble, entire houses fall to the ground. A natural disaster? In a sense, it is. All this can be caused by reactive soil. Nationwide, structural damage caused by reactive soil costs $6 million to $10 million annually, according to information provided by Phil Weinert Engineering in Colorodo Springs. The most common type of reactive soil is expansive clay. This clay expands and contracts due to changes in its moisture level. Some expansive clay can swell up to 15 times its original size, causing stress on the surrounding environment. Buildings with foundations anchored in this soil will shift and bend, causing anything from wall cracks to rolling floors, depending on the amount of movement." (Drazga, 1998)
Further evidence of subsidence is evidenced by Douglas Mcleod, vice president of Engineers Incorporated. According to Haywood (2005), "Mcleod said he thinks there could be a water leak under the concrete slab beneath the hallway. He said if water is leaking onto clay-filled soil, the soil will expand, pushing that portion of the concrete foundation up and dragging the walls along with it, which would cause the walls to crack." (Haywood, 2005) However, Mcleod nor Haywood refer to the process as subsidence, the evidence of this phenomena is present and consistent with the description of the process of how subsidence works.
According to Bombardieri (1999), "Marlboro clay is an expansive soil known to cause landslides, and...
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