7 Geology and Hydrogeology
7.3 Glacial Soils
Gravels
Thick gravel
deposits occur within the study area mainly along the major
river crossings and also around Kildare (Mid-Kildare gravels).
The main gravel areas identified in the study area are as
follows:
- Mid-Kildare
Gravel: The depth of the gravel layer within the study area
is between 5 m to 75 m (Reference 4). The major gravel aquifer
is designated G-1 on the geotechnical constraints mapping.
- Gravel deposits
along the River Barrow north and south of Carlow. The major
gravel aquifer within these deposits is designated G-2.
- The thickness
of the gravel deposits reaches a maximum of 25m in the low-lying
land of the Barrow Valley (Reference 8).
- Gravel deposits
in the Burren River Valley. The thickness of the gravel
deposits ranges between 10m to 15m (Reference 8).
- Gravel deposits
along the River Nore north of Kilkenny City. The major gravel
aquifer within this area is designated G-3.
These materials
do not present problems for road construction, provided the
road alignment is kept above the water table. Generally, gravels
provide good formation for pavement construction and are generally
suitable for reuse. Water bearing sand and silt layers, where
encountered, can be problematic.
Clayey Till
Glacial till
dominate much of the study area. These glacial deposits are
mainly deposits of the Midlandian ice sheet with the coarse
fraction dominated by cobbles and boulders of limestone with
lesser proportions of sandstone, slates and shales.
The depth of
the till occurring within the study area is not known precisely
and is likely to vary considerably.
The geotechnical
properties of Irish glacial tills are well-documented (Reference
6). These soils are generally well graded, variable with gravel
lenses, with quite an absence of clay minerals. The clay fraction
(rock flour) typically amounts to about 15% and the fines
fraction (clay and silt) is about 30 to 40%. The glacial tills
are generally over-consolidated, and therefore are of a low
compressibility. These soils are usually firm to stiff, however
due to their low plasticity, they are very susceptible to
softening and deterioration in wet weather, especially if
heavily trafficked. When the clayey tills are kept dry, they
present relatively little difficulty to road construction.
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