7 Geology and Hydrogeology
7.4 Soft Ground
The main soft
soils occurring within the study area are as follows:
Peat deposits
Blanket peat
(P-1): occurs to the west and south-west of Carlow Town. The
depth of the peat ranges from 0.5 m to 2 m. The peat is considered
to be highly humified and uniform throughout this section
(Reference 2).
Extensive areas
of raised bog between Athy and Kildare (P-2). Kilberry bog
near Athy was historically used by Bord na Mona for industrial
purposes.
The peat deposits
are quite extensive in the north and central sections of the
study area.
Peat soils are
considered to be problematic for construction work owing to
their high organic content, high compressibility and low shear
strength. When loaded, these soils usually exhibit large immediate
and primary settlement followed by a very pronounced long-term
creep settlement. These soils are also known to exhibit a
differential settlement due to their high variability within
the same location. Peat soils could present problems for embankment
stability as a result of the shear failure of the subsoil
during construction. Special measures for embankment construction
are usually required including piling, excavate-and-replace
and preload with vertical drains, etc.
Alluvium
These alluvial
materials are deposited by river action or the action of melting
glaciers. The following major alluvial tracts cross the study
area:
Along the River
Barrow from Monasterevin to Carlow Town (AL-1). The alluvium
typically consists of dark greyish brown clay layer with clay
content up to 60% with a total almost 90 to 100% silt plus
clay. The subsoil horizons have a much higher proportion of
silt than the top soil. Some thin peat layers can occur between
the alluvium and the lake deposits.
Along the basin
of the River Barrow from Carlow Town to as far south as Goresbridge
(AL-2). These soils are poorly drained and of slightly peaty
clay loam texture. The organic content can reach up to 20%.
The depth of the alluvium is usually greater than 1m. The
depth of the alluvial sands/gravels occurring in river valleys
can typically be about 9m to 12m.
Along the River
Nore (AL-3), River Greese (AL-4), Len River (AL-5) and Liffey
River (AL-6).
These soils
are normally consolidated (i.e. they have not been previously
compressed by glaciers) and thus are moderately to highly
compressible though not as severe as peat soils. Special measures
for embankment construction over alluvial materials include
preloading, with or without vertical drains, and piling.
Table 7.2 presents
a summary of the properties of the soils discussed in above.
|
