Monday, March 29, 2010

How sinkholes form

Everyone knows, or at least should know, that Florida is prone to sinkholes and that sinkholes can have a very negative impact of property value. There are reasons why there is so much sinkhole activity in the state. This post will explain those reasons.

Most sinkholes occur in areas underlain by limestone, gypsum or salt. These three minerals are all easily dissolved by water, which in wet areas leads to underground cavity formation. When the roof of the cavity can no longer support the weight of the surface material above it the surface collapses into the cavity as a sinkhole. Since the hydraulic pressure of underground water also helps support whatever is above it, removal of large amounts of ground water can also lead to settling of the surface and sinkholes.

Salt and gypsum are dissolved in normal water and quite porous, so any wet climate will lead to sinkholes where these two materials are the base. Limestone is quite hard and made up of 80% to 90% calcium carbonate, usually formed by the compaction of marine life such as coral and shells. It is also prone to fissures which allow the easy passage of water. Organic material in water can form carbonic acid which dissolves the limestone.

Development activity can often lead to the collapse of otherwise stable underground cavities, thus forming sinkholes. Placing more weight on the surface above a cavity, such as settling ponds and large buildings, can cause collapse. The vibration caused by heavy highway or railroad traffic can weaken the roof of hidden cavities and lead to sinkhole formation. Removal of high volumes of ground water by farming irrigation or other commercial activities can also lead to cavity collapse or just general ground subsidence.

Any form of surface subsidence can cause major damage to any structure, roads or rails in the area. This damage can often be repaired, but repairs are usually quite expensive. The repairs generally involve mechanically jacking the structure back to its original position and then pumping the sinkhole full of concrete or other solid fill material to once again support the surface. Unless very well built, any affected structures are often either not salvageable or require extensive and expensive repairs.

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