7 Geology and Hydrogeology
7.5. Unstable Ground
Karst
Solution Features
Some of the Carboniferous
limestone formations occurring in the study area are reported
to have potential karst solution features. The formations
most associated with solution features are known as Ballyadams
and Clogrenan and occur in a broad land between Bennettsbridge
and Carlow Town. The most exposed areas to Karst are those
where the limestone is present near the surface or lies near
a catchment area. However, areas of deep Karst collapse structures
do occur, for example at Ballyellin, County Carlow and Hollymount,
Co. Laois (Reference 7).
The Karst database
held by the Geological Survey of Ireland (GSI) was consulted.
This database holds records of locations and types of reported
Karst features. The location and features of the reported
Karst areas are plotted on the geotechnical constraint maps
and details are provided in Table 7.5.
The occurrence
of sinkholes under a roadway can lead to depressions or collapse
in the pavement. Specific areas prone to Karst should be identified
at the aerial photograph interpretation and site investigation
stages. Depending on the risks, it may be prudent to adopt
design measures to control water discharge, which can re-activate
solution features, and to incorporate measures which will
protect the roadway from potential collapse.
Slope Stability
In order to assess
the stability of rock cuttings, it is important to identify
the orientation of discontinuities, their condition and also
the extent of weathering present, during the detailed site
investigation. Steep ground should also be identified. A survey
of any previous cases of slope failure in the area of study
would prove very useful. Areas of instability may be revealed
by examination of local aerial photographs along the proposed
routes.
The rocks associated
with the Silurian period in particular, may be susceptible
to cutting instability because of the complex geological structure
characterising these rocks (i.e. folds, faults and variable
dip). Cutting stability in Limestone rocks would need careful
consideration where Karst weathering is present and also in
the granites where deep weathering can occur.
Mining Areas
(Castlecomer Coal Field)
The Geological
Survey of Ireland (GSI) minerals inventory database records
twelve coal mines, fourteen former pits and nine natural outcrops
of coal in the area of Castlecomer. Locations identified on
the GSI Maps are reproduced on the geotechnical constraints
maps. Exploitation of the coalmines has generally ceased and
there is only one mining lease issued in the area at present,
at Coolbaun three kilometres northeast of Castlecomer.
If routes are
proposed in these areas, a through search of mining records
should be carried out.
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