Karst… Like a journey in anthill!

Karst” is causing significant problem to engineering structures in the world. Especially collapse of the roof over subsurface voids causing fatalities and high cost. Therefore determining karst problem correctly gives you a way to designate mitigation measures.

Before karst giving you a trouble, estimating the underlying karst problem is very hard, only you can make rough estimations depending the regional geological conditions (hydro geological settings, rock type, weathering, tectonics and distinguishable karst features) of karst. In addition to this choosing the investigation method on karst is very important and hard as well.

I had a chance to perform ground investigations for some karstic problems to engineering structures in the past years. Furthermore, I am working as geohazard specialist on construction of linear engineering structure which is crossing ~100km gypsum karst and about ~30km limestone karst sections. I am struggling with investing the karst sections and mitigation to prevent problems due to karstic features.

In this article series, I will summarise literature information about karst classification and features, than focus on interpretation and investigation of the karst features. 

Karst; Description, Classification and Features

Description & Classification
Karst is landscape which is formed with dissolution of soluble rocks. Carbonate rocks and evaporates are classified as a soluble rocks. With presence of water this rocks dissolves. Gypsum and salt are about 150 and 7500 times more soluble than limestone respectively. 

Karst ground conditions have been divided in five classes in the engineering classification. The five classes provide the basis of an engineering classification that characterises karst in terms of the complexities and difficulties to be encountered by the foundation engineer.

Diagrammatic representations of the five classes in the engineering classification of karst, with various sinkholes and solution dolines forming one of the components of all karst terrains. From Waltham and Fookes (2003).

Juvenile Karst occurs mainly in desert and periglacial zones or on impure carbonates.Dissolution is so restricted that solution sinkholes have barely developed beyond the opening of fissures where small streams sink. Few caves are more than a metre wire, so collapse processes have not been initiated.

Youthful Karst has the minimum scale of dissolutional features normally encountered in temperate regions. Suffosion and dropout sinkholes are common. Scattered larger buried sinkholes can occur as in the juvenile karst.

Mature Karst is the normal carbonate rocks in temperate and Mediterranean climates. In many regions, solution sinkholes are up to 100m across, and may be so frequent that they create a polygonal system within a doline karst terrain. Suffosion and dropout sinkholes typical. Collapse and buried sinkholes may both be significant, and the presence of large caves may allow caprock sinkholes to develop in suitable geological situations.

Complex Karst is more typical of the tropical regions, but can be developed in some more temperate climates. Large solution dolines may be the dominant landforms, with individual features up to 1 km across. Solution and collapse sinkholes can occur in their floors. Subsidence sinkholes are numerous in any soil cover. Buried sinkholes may be numerous and large as past climates are also likely to have been warm and wet.

Extreme Karst has the largest of all dissolutional features, including ground cavities, and is only found in the wet tropics. The landscape has cone and tower hills interspersed with solution dolines and karst basins kilometres across with all type of large active sinkholes in their floors. Buried sinkholes are so large and deep that soil compaction within them, either naturally, enhanced by dewatering or under imposed load, may cause differential surface subsidence that is significant to engineering.

Morphological Features
Morphological features of karst is classified as;

  • Surface micro features : karren & microkarren, solution pits, pans, heel-prints, shafts or wells, cavernous weathering, dissolution channels
  • Surface macro features  : dolines, poljes, poligonal karst, cones, towers, dry valleys
  • Subsoil features             : pinnacled rock heads
  • Caves
  • Sinkholes 

Sinkholes are most problematic karst morphological feature to engineering structures. Dissolution of bedrocks and rock heads causes the formation of sinkholes.

The term sinkhole described as a “Water sinks into the ground of a closed depression (to prevent it becoming a lake), sediment sink into the ground where it is washed down by water, the ground surface is lowered (and effectively sinks) by slow dissolution of its soluble rock head, and the ground may subside (or sink) catastrophically in a collapse event. (Waltham, T., Bell, F., Culshaw, F., (2005), Sinkholes and Subsidence, Karst and Cavernous Rocks in Engineering and Construction, Praxis Publishing UK, pp.26).”

Sinkholes classified in a six main type.

Classification of Sinholes (A.C.Waltham and P.G.Fookes / Speleogenesis and Evolution of Karst Aquifers, 2005, 3 (1), p.6)

Solution Sinkhole: is formed by dissolutional lowering of surface. They are normal features of karst terrain that evolved over geological times. Can be up to 1000m across and 100m deep. Fissures and cave drains must exist beneath floor.

Collapse Sinkhole: is formed by instant or progressive rock roof failure into underlying cave, can be up to 100m across and 100m deep. Causes failure of  the floor by loading the cave roof.

Dropout Sinkhole:  is  formed in cohesive soil collapse into soil void formed over bedrock fissure. Overlying soil must be cohesive. Can be up to 50m across and 10m deep. Cause a sudden failure in soil-covered karst.

Buried Sinkhole: is soil filled sinkhole after environmental change. Can be up to 300m across and 100m deep.

Caprock Sinkhole: is failure of insoluble rock into cave in soluble rock below. Can be up to 300m across and 100m deep. Causes failure of the flow by loading cave roof.

Suffosion Sinkhole: is formed down washing of non-cohesive soil into fissures in bedrock. Can be up to 50m across and 10m deep.