7.
Geology and Hydrogeology
7.5
Unstable Ground
7.5.1
Karst Solution Features
Some of the Carboniferous limestone formations occurring in
the constraints 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 band between Bennettsbridge and Carlow
Town. The massive reef limestone (Waulsortian Formation) is
also prone to karstification. The most exposed areas to Karst
are those where the limestone is present near the surface
or lies near a catchment area. The area near Kilmacow appears
to be exposed to Karst because of the water discharge in this
area coming from the surrounding hills.
The Karst database held by the 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
presented in Figures 5A and 5B with details provided in Table
1 of Appendix A
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.
7.5.2
Slope Stability
In order to assess the stability of rock cuttings, it is important
to identify the orientation of present discontinuities, their
condition and also the extent of weathering. Steep ground
should also be identified during move detailed assessment
of route options. A survey of any previous cases of slope
failure in the area of study would prove very useful. Areas
of instability along potential route options may be revealed
by examination of aerial photographs.
The
rocks associated with the Ordovician period may be susceptible
to cutting instability because of the complex geological structure
characterising these rocks (i.e. folds, faults and variable
dip). This may also be due to the presence of frequent mudstone
horizons, which can lead to low strength zones (i.e. residual
strength) following the bedding pattern due to strain slip
during folding. Cutting stability in Limestone rocks would
need careful consideration where Karst weathering is present
and also in the granites where deep weathering can occur.
7.5.3 Mining Areas
The GSI minerals inventory database records were also consulted
and indicated that no recorded coal mines, former pits or
natural outcrops of coal are present within in the study area.
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