Babsfontein Sinkhole EarthCache

EXERCISE EXTREME CAUTION AT ALL TIMES WHEN NEAR THE SINKHOLES

Babsfontein Sinkhole

There are several sinkholes in the Babsfontein area and the listed waypoint will take you to one that is well known next to the road which is now being closed off to traffic. This Sinkhole was discovered in January 2004. This Sinkhole was formed due to the lowering of groundwater levels and caused a loss of support for the soft material in the rock spaces. Today this sinkhole is holding water due to the soft soil that gradually caved into the hole, which blocked the drain hole.

There is another Sinkhole at S25 59.904 E028 25.708, but please note the following for safety reasons; this second sinkhole is located near a informal settlement and the soil and surrounding area are very unstable, but will provide a good view of the inside and cave-ins of the surrounded areas.

Babsfontein Sinkhole Feb 2004	Babsfontein Sinkhole Mar 2004	Babsfontein Sinkhole Aug 2008

What is a sinkhole?

A sinkhole is a natural depression or hole in the surface caused by the removal of soil or bedrock, often both, by water. Sinkholes may vary in size from less than a meter to several hundred meters both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms. They may be formed gradually or suddenly, and are found worldwide. These features are also common around Johannesburg and Pretoria that is underlined by dolomite.

Sinkholes are all about water.

• Water dissolved minerals in the rock, leaving residue and open spaces within the rock. (This is called “weathering”.)
• Water washes away the soil and residue from the voids in the rock.
• Lowering of groundwater levels can cause a loss of support for the soft material in the rock spaces that can lead to collapse.
• Changing groundwater gradients (due to removing or introducing water to the system) can cause loose material to flush out quicker from the voids and the surface to collapse in response.
• Any change to the hydrologic system (putting more water in or taking it out) causes the system to become at least temporarily unstable and can lead to sinkholes.
• Sinkholes can result from seasonal changes in the groundwater table, freeze and thaw of the ground, and extremes in precipitation (drought versus heavy rain).

Typical activities that can lead to sinkholes are:

• Decline of water levels – drought, groundwater pumping (wells, quarries, mines)
• Disturbance of the soil  - digging through soil layers, soil removal, drilling
• Point-source of water – leaking water/sewer pipes, injection of water
• Concentration of water flow – storm water drains, floods, etc.
• Water impoundments – basins, ponds, dams
• Heavy loads on the surface – structures, equipment
• Vibration – traffic, blasting or drilling

To get credit for the find, answer the following questions in a mail to the cache owner (No need to cross the fence. The sinkhole can be observed from the roadside.):

1. Estimate how wide is the sinkhole in meters?
2. Estimate the distance from the water level to the top edge of the sinkhole?
3. What methods were done on site to prevent passersby to disappear down this sinkhole?

EXERCISE EXTREME CAUTION AT ALL TIMES WHEN NEAR THE SINKHOLES