Engineering and construction
Engineering and construction
Karst regions may provide particular problems for all kinds of engineering projects, from house construction to major roads and bridges and also civil engineering works such as mining, canals and reservoirs. These problems mainly arise from:
- the unpredictable occurrence, extent and depth of underground cavities which may lead to inadequate foundation support;
- the great variability in the depth of subsoil, leading to unpredictability in the type and depth of excavation required and the likely extent of drilling or blasting.
The implications of karst for engineering problems may be studied using a risk assessment approach with the level of acceptable risk increasing with decreasing sensitivity of the structure. For example, the level of risk due to karst in the construction of a hay-barn is much less than that for the development of a housing estate, and the subsequent investigation and site works should be designed accordingly. The first step in the investigation of every site where karst is a potential problem should be a desk study. This will prove to be an economic and a profitable exercise. The desk study should include:
a site walkover;
a study of the geological maps of the area;
reference to the Karst Database at the Geological Survey of Ireland;
a study of present and historical Ordnance Survey maps;
examination of any nearby site investigation records or references to past events or works;
examination of aerial photographs;
a study of local drainage patterns with particular emphasis on seasonal and historical variations.
The information obtained from the desk study will allow the design of a site specific ground investigation. The ground investigation could include geophysics, trial pitting, rotary percussive boreholes, rotary cored boreholes, down the hole logging and in-situ testing.
Engineering problems - foundations
The presence of karst on a site can result in additional requirements in the design and construction of foundations. Foundations can be designed to span over a potential karst feature as an alternative to grouting. This might consist of a reinforced concrete strip footing, or raft, or a high strength plastic geogrid. Engineering problems - drainage The understanding and control of drainage is of critical importance in karst areas. Inadequately controlled drainage can trigger previously dormant karst activity. Good drainage design in karst areas should include redirection of natural drainage away from the structure, sealing of all drains and surface areas, and avoidance of soakpits and distribution systems.
This collapse only became apparent when the topsoil was removed (Donal Daly)
Engineering problems - mining
Problems have been encountered in mining due to the presence of deep karst. One recent mining development in the Irish midlands encountered peat filled karst solution features at a depth of about 60 m.
Flooding at Gort (Donal Daly)
Engineering problems - reservoirs and canals
Obviously, it is very difficult to construct water-retaining reservoirs in karst areas, and there are several examples (from abroad) of such reservoirs which failed to hold any water. Ireland has one particular example of a canal, the Cong Canal in Co. Mayo, which failed because the water disappeared through the fissures in the limestone along its course.
Engineering problems - radon
The accumulation of radon gas in basements and beneath ground floors of buildings can be harmful to humans. Radon is a potential health hazard in karst areas, particularly those adjacent to rock types which are relatively rich in radon and where open fissures exist. An example is the Galway area where the limestone is closely associated with a granite body. However, radon problems can be ameliorated by ventilation.