Most human built structures do rely on the ground for stability but what if the ground suddenly becomes unstable What if subsidence occurs This paper aims to answer these questions by discussing the nature of subsidence and its effects on civil works. I will be providing definitions and possible causes of subsidence. For the civil works, I will be enumerating the effects along precautionary and remedial measures that can mitigate its effects.
"Ground subsidence" is legally defined as the means or process that is characterized by the downward displacement of surface material caused by natural phenomena such as removal of underground fluids, natural consolidation, or dissolution of underground minerals, or by man-made phenomena such as underground mining. (Colorado Geologic Survey)
Subsidence may occur abruptly-virtually instantly-or gradually over many years. It may occur uniformly in a small, confined area as shown in Figure 1a or may occur over a wide area as local depressions as shown in Figure 1b. Subsidence is commonly associated with the dissolution of soluble rocks, such as limestone, beneath the surface while those with crystalline rocks in which most metals are mined have greater strength and are less likely to settle or collapse. The resultant landscape has closed depressions and is known as karst topography. Note that the depressions do not necessarily result to holes in the ground as shown in Figure 2. ...
ed subsidence occur as a result of withdrawal of fluids from subsurface reservoirs as shown in Figure 4, from the collapse of soil and rock over subsurface holes, such as those left by underground mining and from the draining of wetlands. (Waltham and Cushaw, 2004)
FIGURE 1a. SMALL SINKHOLE. Not all sinkholes are large subsidence features. Small collapse sinkholes, such as this one in Boyle County, Kentucky, are common. (Kentucky Geological Survey)
FIGURE 1.b LARGE COLLAPSE SINKHOLE. This sinkhole near Montevallo in central Alabama was dubbed the "December Giant" after it measured close
to 120 m (400 ft.) in diameter and 45 m (150 ft.) in depth. (U.S. Geological Survey)
FIGURE 2. KARST TOPOGRAPHY. This rolling landscape of the Mitchell Plateau in southern Indiana is typical of karst topography in a humid temperate climate. (Samuel S. Frushour, Indiana Geological Survey)
Subsidence can also occur due to expansive soils. There are clay-rich soils which shrink significantly during dry periods and expand or swell during wet periods. The swelling is caused by the chemical attraction of water molecules to the surface of very fine particles of clay. Swelling can also be caused by the chemical attraction of water molecules to layers within the crystal structure of some clay minerals. Figure 3a shows the mechanism by which expansion of soil can occur while Figure 3b shows water consumption by trees as another cause.
FIGURE 3. EXPANSIVE SOILS (a) Smectite clay expands as water molecules are added onto and within the clay particles. (b) Effects of soil's shrinking and swelling at a home site. (After Mathewson, C. C., and J. P. Castleberry, II. Expansive soils: Their engineering geology. Texas A&M University)
FIGURE 4. PROCESS OF SUBSIDENCE DUE TO WATER