Karstbase Bibliography Database
Karstbase Bibliography Database
The thesis aims to review the nature of karstic limestone terrains and the implications for engineering practices as a result of the uniquely difficult ground conditions they present. Case studies are included to highlight two very different, yet apparently common, engineering problems on karst. This abstract deals only with Linhay Hill Quarry in Ashburton, Devon where pinnacled rockhead and clay infilled dissolution pipes present problems in the extraction and processing of the limestone for use as aggregate.
The quarry has been in existence for over a hundred years and the current owners are drilling and blasting the Devonian limestone and processing it for a variety of purposes; namely aggregates for concrete, macadam and unbound applications. In the quarry, the rock is fairly evenly bedded and dipping towards the east. Near the ground surface it is extensively solution weathered to form a karst surface, which is now buried by more recent deposits. The extensive karst topography gives considerable problems, currently on the north side, where the intimate mixture of solution weathered limestone and later infilling clays and sandy sediments makes drilling and blasting difficult and contaminates the limestone material.
On the basis of the work carried out, the following summary of findings is presented:
Using published engineering classification schemes; the Chercombe Bridge Limestone in and around Linhay Hill Quarry has been classified as Class III to IV Karst ('Mature' to 'Complex' Karst, Waltham, 1999).
• The origin of the karst is proposed to be the result of a combination of subtropical climate and localised valley conditions in the early Tertiary. Weathering and erosion of the Dartmoor granite and adjacent Cretaceous rocks provided fluvial sediment to subsequently infill the solution channels and cavities in the limestone.
• The physical effects of weathering have been shown to reduce the strength and density of the limestone whilst increasing the water absorption. This has implications for the quality of aggregate produced in the quarry.
• The chemical effects of dolomitisation and solutional weathering have been shown to produce a highly variable material in the quarry.
• Residual insoluble minerals were found to be randomly distributed and exhibited typically high densities, high absorptions and high clay and iron oxide/hydroxide contents.
• The nature of the infilled karst together with the effects of weathering mentioned above has significantly affected the workings of the quarry with considerable cost implications. They are listed (in no particular order) as follows:
Overburden stripping extremely time consuming and costly.
Increased drilling times through clay infilled fissures/cavities.
Enforced blast hole surveying techniques due to variable ground.
Enforced blast charge restriction resulting in reduced primary fragmentation.
Induced dolines in the surrounding farmland.
Costs of washing/scrubbing of clay coated 'contaminated' rock.
Clay materials not always removed resulting in reduced efficiency of processing plant.
Quality of aggregates impaired by variable rock properties and presence of clay.
Implications for concrete and mortar include potentially reduced workability strength and durability