This chapter examines and
assesses the geological, geotechnical and hydrogeological impact of the route options
on the study area. Each of the route
options are divided into sections. This allows them to be compared to adjacent
sections of other routes. Each of the
route sections have been examined and the major constraints and construction
features identified and assessed.
The assessment falls
broadly into four categories:
Geotechnical: the lengths
and heights/depths of embankments and cuttings, the presence of soft ground and
depths of peat bog and marsh areas. The
presence of other features within the route sections are also considered.
Geological: the
composition of the underlying geology particularly the depth to rock in
cuttings and the composition and thickness of the overburden geology.
Material Balance: the
cut/fill balance of the route sections, the volume of rock excavation and the
volume of unsuitable material.
Hydrogeological: the
presence and vulnerability of aquifers and groundwater users.
The assessment of the
constraints and construction features allow each route section to be given an
overall impact rating based on the effect it would have on the environment, the
difficulty of the engineering solution and the resultant cost implications.
Each route section is
discussed in detail later in this chapter and an impact assessment made. In assessing the impacts the following
significance criteria have been used:
|
Impact |
Significance Criteria |
|
Severe adverse impact |
Deep cutting or high
embankment for the majority of route.
Significant rock excavation. Large
quantity of other suitable/unsuitable material excavated. Large quantity of material exported and
imported. Majority of route crosses
peat or soft ground. Permanent impact on
major aquifer. Significant impact on
groundwater flow/quality. |
|
Major adverse impact |
Deep cutting or high
embankment for significant sections of route. Major rock excavation.
Major amount of other suitable/ unsuitable material to be excavated. Major quantity of material exported and
imported. Major parts of route cross
peat or soft ground. Temporary impact on
major aquifer. Major impact on
groundwater flow/quality. |
|
Moderate adverse
impact |
Deep cuttings or high
embankments at a number of locations along route. Moderate excavation of rock.
Moderate amount of other suitable/ unsuitable material to be
excavated. Moderate quantity of
material to be exported and imported.
Localised sections of route cross peat or soft ground. Permanent effect on
poor/minor which is locally productive. Moderate impact on groundwater
flow/quality. |
|
Minor adverse impact |
Deep cuttings or high
embankments at a few locations. Rock
excavations in localised areas. Small
amount of other suitable/ unsuitable material to be imported and exported. Short sections of route cross peat or soft
ground. Temporary effect on
poor/minor aquifer which is locally productive. Adverse Minor impact
on groundwater flow/quality. |
|
No impact |
No significant effect |
|
Minor beneficial
impact |
Creation of new sites of
geological/geomorphological interest.
Improvement to local resources through introduction of pollution
control or flow balancing measures. |
Sources of information for
this assessment are:
· N22 Ballyvourney – Macroom – Ballincollig Constraints
Study Report, October 2001
· Bedrock geology mapping for South Cork
· Bedrock geology mapping for Kerry-Cork
· Six-inch drift geology field maps
· National Wells Database
· GSI interim aquifer classification scheme
· Vulnerability mapping
· Karst database
· Water Quality in Ireland 1995 – 1997 (EPA, 1999)
· Shanakill Landfill location
· Ballincollig Bypass Site Investigation Report
· Aerial Photography
The geological mapping and
well information was obtained from the Geological Survey of Ireland (GSI). The Shanakill landfill location, the
Ballincollig Bypass site investigation report and details of groundwater source
locations were obtained from Cork County Council.
No site investigation has
been undertaken as part of the route selection assessment. Currently the Ballincollig Bypass and
Macroom Bridge site investigation reports provide the only confirmed ground
condition information for the study area.
A ground investigation will be undertaken on the preferred route once it
has been confirmed.
The bedrock geology
mapping has been compiled from the ‘Geology of South Cork’ and the ‘Geology of
Kerry-Cork’.
The study area is
underlain by Devonian Old Red Sandstone and Carboniferous Limestone. The Old Red Sandstone and Limestone comprise
various geological units or formations.
These are given for reference in Table 7.1 below.
Table 7.1: Geological
Units Found Within Study Area
|
Geological Unit |
Description |
|
|
O L D R E D S A N D S T O N E |
Bird Hill Formation (BH) |
Purple siltstone and
fine grained sandstone |
|
Ballytrasna Formation (BS) |
Purple mudstone and pale
red fine to medium grained sandstone |
|
|
Castlehaven Formation (CE) |
Purple mudstones and siltstones
with interbedded sandstone units |
|
|
Caha Mountain Formation
(CH) |
Purple and green
sandstone and siltstone |
|
|
Gortanimill Formation (GM) |
Green fine grained
sandstone and siltstone |
|
|
Gun Point Formation (GM) |
Purple and green medium and
coarse grained sandstones with thin interbedded purple siltstones |
|
|
Gyleen Formation (GY) |
Green to grey and purple
mudstones and sandstones |
|
|
Kinsale Formation (KNcu) |
Flaser-bedded sandstone
and mudstone |
|
|
Old Head Sandstone
Formation (OH) |
Grey sandstones and
heterolithic bedded sandstones and mudstones |
|
|
Toe Head Formation (TH) |
Cross bedded sandstones
with minor mudstones |
|
|
L I M E S T O N E |
Ballysteen Formation (BA) |
Dark grey well-bedded
fossiliferous muddy limestone |
|
Little Island Formation
(LI) |
Massive and crinoidal
limestone and wackestone |
|
|
Waulsortian Limestones (WA) |
Massive calcareous
mudstones, wackestones and packstones |
|
The various formations of the
Old Red Sandstone underlie much of the study area outcropping as the Southern
Derrynasaggart Mountain succession west of Macroom and the Central Cork
succession to the east.
Interspersing the Old Red
Sandstone within the study area are two limestone outcrops. These are associated with two synclines; the
Macroom – Blarney Syncline to the north and the Cork Syncline to the south.
The Macroom – Blarney
Syncline trends east-northeast to west-southwest across the study area
underlying Coachford and terminating at the floodplain area of the River Lee
south of Macroom. Rock type varies
along the syncline with Waulsortian Limestones underlying the floodplain area
and the Gyleen and Kinsale formations of the Old Red Sandstone forming the
syncline further to the east.
The Cork Syncline also
plunges east-northeast to west-southwest through the eastern end of the study
area underlying Ovens and Crookstown.
The Ballysteen and Little Island formations and the Waulsortian
Limestones are present along the syncline. Solution features may be associated with the Waulsortian Limestone
which is classified as karstic on the Groundwater Protection Scheme map
produced by the GSI. Generally the low lying ground within the study area is
underlain by limestone with the remainder of the area and higher ground
underlain by the Old Red Sandstone.
The overburden or drift
geology information within the study area has been obtained from six-inch field
maps for County Cork (circa 1850), aerial photographs and a visual inspection
of the study area. No quaternary mapping is available for the study area and
there is very little information as to the depth of overburden.
The drift deposits across
the study area are mainly glacial in origin consisting of sandy gravel with
many cobbles, boulders and larger particles.
Alluvial deposits are also present along and adjacent to the rivers that
pass through the study area. These
deposits are likely to consist of soft compressible clay, silts, sands and
gravels. Localised bogs and marshes are
recorded on the six-inch field maps in low lying areas adjacent to many of the
rivers and some areas of poorly drained high ground west of Macroom.
The depths of the drift
deposits within the study area are related to the topography. The areas of high ground and steep slopes,
which cover much of the study area, consist predominantly of rock outcrops at
or close to the surface, with generally very little overburden. The depth of overburden tends to increase as
the elevation of the ground decreases and the gentler slopes to the hills of
the lower areas indicate a greater thickness of drift deposit. The greatest thickness of overburden is in
the low lying areas, generally coinciding with areas underlain by limestone.
The majority of the study area west of Macroom consists of rock outcrops and
shallow overburden. Whilst the area to
the east has the greatest depths of overburden.
Shanakill landfill is
situated approximately 3 km north-east of Macroom near the River Laney. The
facility closed in 1996 but when operating accepted approximately 4500 tonnes
of municipal and inert waste per annum.
The landfill covers an
area of approximately 0.35 hectares (3500 m2), however, it should be
considered that potentially a much larger area could be affected by leachate
and/or landfill gas.
From the information
available Shanakill landfill appears to be the only area of contaminated land
within the study area.
Aquifer details were obtained
from the Geological Survey of Ireland (GSI) who have an interim aquifer
classification scheme. This indicates generally unproductive zones in the west,
moderately productive zones in the east and west, and areas of good development
potential and fissure flow associated with limestone bedrock. The more productive zones are a greater
constraint to potential development. A
plan showing the location of the zones is included in the Constraints Study
Report.
A review of the “Major
Aquifers of Ireland” (1982) map shows areas of Minor/Complex Sand and Gravel
aquifers which appeared to be associated with the Sullane River between
Ballyvourney and Chransaigh and the Bride River between Crookstown and Ovens.
Vulnerability mapping is
used to identify the likelihood of contamination reaching an aquifer as a
result of contamination at or near the surface. This depends upon the type and thickness of soils and upon the
presence of karst or other highly permeable features. GSI have interim vulnerability mapping for the area, based on
outcrops recorded on field sheets, which adopts a classification of extreme
vulnerability where there are outcrops and high to low vulnerability
elsewhere. Areas of extreme
vulnerability occur frequently across the Study Area, as there is generally
less than 5m of overburden.
The presence of karst can
increase vulnerability through rapid movement of water. The GSI karst database was examined for
karst features. Two were identified;
Coleman Cave at Barnagore and a swallow hole at Ballygroman. These are in areas of Waulsortian limestone
where there is a likelihood of karst occurrence.
Information on groundwater
quality was obtained from Water Quality in Ireland (1995-1997.EPA 1999). The data was limited but indicated quality
to be good and to meet drinking water standards (see Table 4.2.1 in the
Constraint Report). It is likely that
many domestic well supplies are untreated.
Groundwater wells installed in areas where rock outcrop is shallow (e.g.
< 2m to surface) are likely to be susceptible to pollution.
The depth to bedrock
varies across the study area. West of
Macroom the rock is generally close to the ground surface and there are
frequent rock outcrops interspersing areas of shallow overburden. It is likely,
therefore, that rock will be encountered in most cuttings between Ballyvourney
and Macroom and be absent only in the shallowest cuttings. It is also likely that rock will be the
founding strata for any structures in this section. East of Macroom there is generally a greater depth of material
overlying the bedrock and although dependent on location the depth to rock in
cuttings will therefore also be greater.
Where rock is exposed it
should, after removal of weathered or loose material, prove to be an adequate
construction material either in cuttings or as a founding strata for
structures. Given the proximity of rock
to the ground surface west of Macroom it is probable that there will be a
greater depth of weathered material present and removal of rock in these areas
should be possible by mechanical means.
Generally throughout the route however, where there are long or deep
cuttings, the volume of material to be removed makes blasting a more economical
option.
The majority of rock
encountered will consist of various formations of the Old Red Sandstone,
however limestone may be encountered in the Bride Valley area between
Crookstown and Ovens. Solution features
may be present in the limestone, the depth below ground of which varies across
the valley. Should a route be chosen
that crosses this area detailed ground investigation will be needed to negate
the possibility of cavities beneath the proposed construction.
The glacial and alluvial
deposits overlying the solid strata will generally be suitable for use in the
new road construction as a fill or foundation material for embankments. The glacial deposits should also be suitable
as a founding stratum for structural foundations.
A reservoir crossing is
required on each of the route options.
The length of crossing will vary depending on the route chosen but it is
probable that piled foundations passing through the glacial and alluvial
deposits and anchoring in intact bedrock will be required for this structure.
Localised areas of soft
ground, bog and marsh exist throughout the study area. West of Macroom these soft ground areas may
be associated with poor drainage due to the shallow depth of the underlying
bedrock and are therefore likely to be of limited depth.
Although no confirmation
of the depths of the soft deposits are available at this time, it is likely
that the greatest thickness will occur in the Bride Valley where there is an
extensive bog area. It is envisaged that
soft deposits, where encountered, will be removed and replaced with a suitable
fill material. However, depending on
the volume of material involved, alternate forms of ground treatment, including
leaving the soft ground in-situ, may become more economical options.
Potential impacts on
groundwater during the construction phase include pollution through
contaminated run-off or spillage; alteration of the groundwater flow regime by
construction of cut-offs, de-watering or groundwater abstraction for
construction purposes; impact on groundwater users and impact on ecology (for
example where river baseflow or wetlands depend on groundwater).
During operation the
potential impacts are pollution; permanent alteration of the flow regime where
cuttings or cut-off walls intercept groundwater flow; and effects on
groundwater users and ecology through lowering of the water table.
The following assessment
concentrates mainly on aquifer status.
To avoid the risk of pollution of and impacts on aquifers, the preferred
route (hydrogeologically), would cross less productive and less vulnerable
aquifers.
There are other issues
such as disposal of road drainage, impacts on wetlands and river base flow, and
impacts on domestic water supply from wells.
Each of these is assessed either directly or indirectly in other
sections as follows:
Surface Water: Road
drainage. The majority of the routes
cross or pass near to rivers, therefore it is probable that road drainage would
be designed to discharge principally to surface water rather than groundwater. Sections where the proposed routes do not
cross rivers include: Yellow route nodes 1 to 2, Yellow route nodes 6 to 7, and
Link between nodes 12 and 12a.
Ecology: Impacts on
wetlands and river base flows.
Ecological features are identified in the ecology section so are not
included again in the hydrogeological assessment. Some features may be dependent on groundwater.
Water supply: Information
was obtained from the National Wells Database and well locations were provided
with varying degrees of accuracy. This
data was not sufficiently accurate to define the number and depth of supply
wells that may be affected by the proposed routes. Further investigation will be undertaken at preliminary design
stage to ascertain possible affects and mitigation measures.
There are two main
mitigation measures that could be used to reduce the geotechnical impact of the
construction of the road.
Minimising the heights and
depths of these features would reduce their visual impact on surrounding areas
and reduce the landtake required for them. Minimising the heights of the
embankments would also reduce the volume of fill material required along the
route.
Reducing the depths of
cuttings would reduce the amount of rock requiring removal and reduce the
effect of the cutting on the surrounding groundwater regime.
By having a balanced cut
and fill design the requirement for importing additional or removing excess
material would be negated representing a cost saving to the overall scheme and
reducing the overall environmental impact of the road.
The criterion for a
balanced cut and fill design should be applied to individual logical sections
rather than the complete route as a whole. This will ensure that transport of
materials remains localised within these sections as opposed to the possibility
of having to be hauled over long distances in order to achieve a good cut and
fill balance, a situation which would be undesirable and uneconomical.
There are measures that
can be taken to reduce the impact of road construction adjacent to the closed
landfill. These include a suitably
detailed ground investigation in the area to determine the extents of the
landfill, the installation of monitoring boreholes and a gas-monitoring regime
to determine whether landfill gas is present at the location of the proposed
road construction. Depending on the
results of the site investigation and monitoring a suitable earthworks design
would be produced incorporating measures to deal with contaminated land and/or
landfill gas.
To reduce the impact on
aquifers the amount of soil removed from the site would be minimised so that
the thickness of protective soils is retained where possible and aquifer
vulnerability is not increased. To
minimise the effects of cuts on the groundwater regime they would be kept to a
minimum depth. The potential impact of
road drainage on groundwater quality would be minimised by adopting drainage
and pollution control measures. These
could include positive drainage measures such as oil interceptors, silt traps
and spillage containment.
Definitions of the
classifications of height of cuttings and depths of embankments used in the
route assessments are given below:
|
Classification |
Height/Depth Description |
|
Low Embankment |
Less than 2 m high |
|
Medium Embankment |
2 m to 10 m high |
|
High Embankment |
Greater than 10 m high |
|
Shallow Cutting |
Less than 2 m deep |
|
Medium Cutting |
2 m to 10 m deep |
|
Deep Cutting |
Greater than 10 m deep |
Two route options exist
between nodes 1 and 3:
Moderate Adverse
Geological Impact. This route comprises mainly low and medium
embankment and low cutting. There is a short length of high embankment (maximum
height 12 m) and some medium depth cutting (up to 9 m deep). There are no peat
or soft ground areas. The shallow depth of overburden in this area means that
the majority of cutting is in rock. There is a moderate excess of fill with
rock comprising the majority of the excavated material.
Minor Adverse
Hydrogeological Impact. The aquifer is defined as moderately
productive locally.
Moderate Adverse
Geological Impact. This route comprises mainly low and medium
embankment and medium cutting with a short length at grade. There are no high
embankments or deep cuttings (maximum embankment height is 9.5 m and maximum
depth of cutting is 8 m). There are no peat or soft ground areas. The majority
of cutting is in rock and this route has a good material balance.
Minor Adverse
Hydrogeological Impact. The aquifer is defined as moderately
productive locally.
One route option exists
between nodes 3 and 6:
Major Adverse
Geological Impact. The majority of this route is on low and
medium embankment with a short length of high embankment (up to 12.5 m height).
Maximum depth of cutting is 22 m. The
majority of cutting is in rock and almost three-quarters of excavated material
comprises rock. There is a moderate
length of peat along the route, mainly present under embankment. It is anticipated that the depth of peat
would be fairly shallow. Primarily the depth of cutting raises this route to a
major adverse impact on the area although the presence of the peat deposits
contributes to this assessment.
Minor Adverse
Hydrogeological Impact. Aquifer is moderately productive locally to
the west and generally unproductive to the east.
Two route options exist
between nodes 3 and 7a:
Major Adverse
Geological Impact. The majority of this route is on low and
medium embankment with a short length of high embankment (maximum height 21
m). Maximum depth of cutting along the
route is 14 m with approximately one-third of cutting in rock. A short length of peat is present along the
route, mainly under embankment.
Minor Adverse
Hydrogeological Impact. Aquifer is moderately productive locally to
the west and generally unproductive to the east.
Major Adverse
Geological Impact. This route comprises mainly low and medium
embankment with a short length of high embankment (maximum height 21 m). Maximum depth of cutting along the route is
22 m. The majority of cutting is in
hard strata with approximately two-thirds of excavated material comprising
rock. There is a moderate length of
peat present along the route, nearly all under embankment. It is anticipated that the depth of these
soft deposits would be fairly shallow.
The high embankments and deep cuttings give this route a major adverse
impact on the surrounding area. The
peat deposits are also a contributory factor to this assessment.
Minor Adverse
Hydrogeological Impact. Aquifer is moderately productive locally to
the west and generally unproductive to the east.
Two route options exist
between nodes 6 and 14:
Moderate Adverse
Geological Impact. The majority of the route is in shallow and
medium cutting. There are no deep
cuttings. Maximum embankment height
along the route is 15 m. Approximately one-third of cutting is in rock and
there is a short length of peat along the route. The route passes within one hundred and twenty metres of a closed
landfill in section G/05 which poses a constraint to any development, including
road construction, within its vicinity.
Mitigation measures, however, will reduce the impact of any problems
associated with the landfill.
Minor Adverse
Hydrogeological Impact. The route between nodes 6 and 14 is
generally moderately productive aquifer with some locally productive areas.
Major Adverse
Geological Impact. The majority of the route is on medium
embankment or in medium cutting. There
is a short length of high embankment (maximum height 23.5m). There is no deep cutting along the
route. Approximately one-third of
cutting is in rock. There are no peat
areas along the route. This route
shares section G/05 of the Green route and therefore also passes close to the
closed landfill. The route generally has a moderate adverse impact, however,
the maximum embankment height increases the assessment to major adverse impact.
Minor Adverse
Hydrogeological Impact. The route between nodes 6 and 14 is
generally moderately productive aquifer with some locally productive areas.
One route option exists
between nodes 7 and 14:
Major Adverse
Geological Impact. The majority of the route is on medium embankment
or in medium cutting. There is a short
length of high embankment (maximum height 23.5m). There is no deep cutting along the route. Approximately one-third of cutting is in
hard strata and over half the excavated material comprises rock. There are no peat areas along the route. If the embankment heights in this section
could be reduced the impact assessment would improve.
Minor Adverse
Hydrogeological Impact. The route between nodes 6 and 14 is
generally moderately productive aquifer with some locally productive areas.
Two route options exist
between nodes 7a and 11:
Major Adverse
Geological Impact. The majority of the route is on medium
embankment or in medium cutting. There
is a short length of high embankment (maximum height 15 m) and a short length
of deep cutting (maximum depth 25m).
Approximately half the length of cutting is in rock. There is a negligible length of peat along
the route. There is also an imbalance
in cut and fill which contributes to the impact assessment of this section.
Minor Adverse
Hydrogeological Impact. The aquifer is generally unproductive in the
northern part; crosses the limestone aquifer to the south of Macroom and is
then generally moderately productive.
Major Adverse
Geological Impact. The majority of the route is on low and
medium embankment. There is no high
embankment. There is a short length of
deep cutting (maximum depth 17 m). Half
the length of cutting is in rock. There
is a negligible length of peat along the route. There is a small deficit of fill material for this route which
contributes to the impact assessment.
If cutting depths and/or the fill deficit were reduced the impact
assessment of this section would improve.
Minor Adverse
Hydrogeological Impact. The aquifer is generally unproductive in the
northern part; crosses the limestone aquifer to the south of Macroom and is
then generally moderately productive.
One route option exists
between node 7b and 11:
Moderate Adverse
Geological Impact. The majority of the route is on low and
medium embankment. There is a short
length of high embankment (maximum height 13 m) and a short length of deep
cutting (maximum depth 14.5 m).
Two-thirds of the length of cutting is in rock. There are no peat or soft ground areas
present along the route. Although this
route contains sections of high embankment and deep cutting, the heights/depths
and lengths of these features do not greatly exacerbate the impact of this
section.
Minor Adverse
Hydrogeological Impact. The aquifer is generally unproductive in the
north and moderately productive in the south.
Three route options exist
between node 11 and 15:
Moderate to Major
Adverse Geological Impact. The majority of the route is on low and
medium embankment (maximum height 8 m - this is a localised area of fill to an
existing quarry, finished level proposed to approximately 1m above the
surrounding ground level). There is no
high embankment or deep cutting along the route (maximum depth of cutting 4.5
m). A short length of peat or soft
ground is present along the route. The
depth of soft deposits may be several metres but it is envisaged that these
will be removed and replaced with a suitable fill rather than a more elaborate
form of ground treatment being required.
This section has a significant fill material deficit.
Moderate Adverse
Hydrogeological Impact. The proposed route crosses a moderately
productive aquifer to the west and limestone aquifer to the east.
Major Adverse
Geological Impact. The majority of the route is on low
embankment. There is no high embankment
or deep cutting along the route; maximum height of embankment is 4.5 m and
maximum depth of cutting is 3 m. There
is a moderate amount of peat along this route, almost one-sixth of the length
of the route being in or over soft ground.
The soft deposits may be of such depth that their removal and
replacement may be an uneconomical solution and an alternative form of ground
treatment required.
Major Adverse
Hydrogeological Impact. Almost the entire section crosses the
limestone aquifer with potential for karst and fissure flow.
Major Adverse
Geological Impact. The majority of the route is on low and
medium embankment. There is no high
embankment or deep cutting along the route.
One-fifth of the route is at grade.
A short length of peat is present along the route. A deficit of fill material is the major
contributor to the impact assessment, which could be improved if the deficit
were to be reduced.
Moderate Adverse
Hydrogeological Impact. The proposed route crosses a moderately
productive aquifer to the west and limestone aquifer to the east.
The preferred route
hydrogeologically is Green linked to Yellow then Red.
Between nodes 1 and 3 the
Yellow route is the preferred option.
The main advantages of this option are the absence of any high
embankments, a short length of which is present on the Green route. The Yellow route also has a very good
overall cut and fill balance compared with the Green route, which has a large
material surplus.
Between nodes 3 and 6/7a
there are three route options; Green, Yellow and Green/Link1/ Yellow. All three
of these options are assessed as having major adverse impacts. Comparatively, however, the least favourable
option is the Green/Link1/Yellow route which incorporates the worst features of
both the alternatives by having the greatest length of high embankment and deep
cutting. This route also crosses the greatest length of soft ground. The preferred option for this section is the
Yellow route which has shallower cuttings, crosses less soft ground and has a
better cut and fill balance than the Green route. The only disadvantage of this
route is that it has more high embankment compared to the Green route and
maximum embankment height is also greater.
East of Macroom there are
the northern (Green and Red) routes and the southern (Yellow and Blue)
routes. For the northern routes between
nodes 6/7 and 14 there are three possible options; Green, Red and
Green/Link3/Red. The preferred route
option for this section is the Green route due to its shorter length of high
embankment and the 15 m maximum height of the embankment. Although this route passes close to the
closed landfill at Shanakill this does not dominate the impact rating. Mitigation measures should ensure that construction
near the landfill would cause few problems.
The Red and Green/Link3/Red route options were less preferred because of
the 23.5 m maximum embankment height and longer lengths of high embankment.
For the southern routes
between nodes 7a/7b and 11 there are three route options; Yellow, Blue and
Yellow/Link4/Blue. The Blue route is
the preferred option because of its superior cut and fill balance and the
absence of any soft ground. The
Yellow/Link4/Blue route option is slightly less favourable due to a short
length of soft ground and a cut and fill balance resulting in a slight deficit
of material. The Yellow route is the
least preferred option because of the presence of a length of very high
embankment and a cut and fill balance resulting in a large fill material
surplus.
For the southern routes
between nodes 11 and 15 the Blue route is the least preferred option due to the
length of soft ground encountered but it has the best cut and fill
balance. Conversely the Yellow and
Yellow/Link6/Blue route options are both equally preferable except for poor cut
and fill balances resulting in a substantial deficit of fill material. In this situation the cut and fill balance
of the routes is the more important engineering problem to be solved and so
takes precedent over the length of soft ground likely to be encountered
therefore the Blue route is the preferred option.
East of Macroom the
overall preferred route is the Green (northern) route. Although the Green (northern) and Blue
(southern) routes have similar impact assessments the advantages of this route
are that it doesn’t have any lengths of deep cutting and it crosses a shorter
length of soft ground.
Embankment and cutting
slopes of 1 vertical to 2 horizontal (1V:2H) have nominally been assumed for
the route options. For the preferred
route it is likely that there will be some deviation from this assumption
particularly in rock cuttings. Where
the cutting is in intact rock, slopes considerably steeper than 1V:2H can be
used. For cuttings where the rock is
overlain by a depth of overburden, two-part slopes will most likely be used
with the upper (soil) part of the slope at 1V:2H and the lower (rock) part at a
steeper angle.
The majority of
embankments will have side slopes of 1V:2H, however, for high embankments or
embankments over soft ground shallower slopes will be required.