Construction Method Statement PDF
Construction Method Statement PDF
Construction Method Statement PDF
KM
March 2017
General Description
It is proposed to construct and operate a small-scale hydroelectric scheme on the Allt Garaidh
Ghualaich burn on hillside to the south west of Loch Garry, near Invergarry.
The scheme will consist of an intake weir built on the main burn, and second intake on the
tributary, Allt Lon Glas Bheinn. Water will be carried from these intakes to the powerhouse
through approximately 4.7km of buried pipeline.
The plant will be run-of-river status (i.e. will not involve storage) and the electricity generated will
be exported to the nearby grid.
The construction of a run-of-river hydroelectric generating scheme comprises the following, but
not necessarily sequential, stages:
Construction of site compounds, lay-down areas and other initial preparatory works;
Delivery of equipment on to site;
Initial construction works - Permanent and temporary forest roads, bridges etc.;
Main construction works - Intake weirs, pipeline, turbine house and tailrace; and
Reinstatement.
The construction period is expected to last approximately 18 months, although much of the
main construction works will be achieved in a shorter period of about 12-14 months.
This is possible by progressing key stages concurrently, which not only achieves cost-effective
construction but also minimises disturbance-type environmental impacts and interruptions to
activities on the site. However, it is likely that periods of poor weather, such as heavy snowfall
or periods of high rainfall, will lead to temporary cessation of some aspects of construction, such
as pipe laying and pouring concrete. Heavy rain would also interrupt intake weir construction.
The Construction Method Statement ( CMS), with the description of works for the main elements
is presented below and follows up the recommendations of separate reports carried out by
independent professional consultants expert in Archaeology, Fisheries, Habitat ( Flora),
Protected Mammals and Birds; as well as expert assessments on Access & Traffic, Landscape
Visual Hydrology, Morphology and Noise.
There may be some changes to the method described as the project proceeds and as more
knowledge of ground conditions is gained. However, the final detailed design is not expected to
differ significantly from that described and any differences are not expected to have a material
impact on the findings of this ES In addition, the detailed design and the construction contract
will ensure that the mitigation measures identified in this ES, or measures achieving equivalent
performance, are incorporated into the detailed design, construction and operation of the
scheme.
This CMS looks at the construction of the forest roads, the pipelines, the intake weirs, the
powerhouses and temporary construction areas. It considers how the construction of these may
be made with as little disturbance to the environment as possible.
Contents
Control and prevention of silt run- off and construction pollution into adjacent water
courses by means of cut-off drains, silt traps and attenuation ponds, all to be
removed and restored at completion.
Monitor turves and water during dry spells to ensure that they remain viable and do
not desiccate.
5.3 Pipeline
A pipeline trench will be dug in order to bury the pipeline and the stripping and storing of
materials will conform to the method outlined in Section 5.2. All the pipes will be delivered
to the laydown area. The pipe- laying contractor will be responsible for moving the pipes
from these areas to the pipeline route and for laying out the pipe ready for jointing / fusion
welding. It is expected that the pipe can be moved from the delivery point to appropriate
points along the pipe route with a tractor and trailer or similar.
Trial pits must be dug along the pipeline route to confirm depth of peat, soil and
rock.
Suitable material to grade must be available either from the excavation of the
pipeline itself or from the excavation of the powerhouse.
The access road must be close to the pipe route so that pipes can be handled and
graded material delivered to the pipe trench.
Deep peat should be avoided.
The route selected should minimise the need for rock excavation.
The various steps in this process including reinstatement are outlined in Drawing No. P101
60002. In summary, turf should be stripped off and kept separately so that it is not covered
over and can continue to grow. Peat should be kept separately from mineral subsoil. The
pipe will be bedded and surrounded by graded material and the trench will be filled in and
compacted with mineral subsoil. The stripped area will be re-covered with peat and turf;
boulders will be left strewn on the surface. The end result should fit into the landscape and
should not be harrowed or reseeded but left rough to allow the original vegetation to re-
establish itself.
Access to the Powerhouse and Intakes are required for the following reasons:
Best practice guidelines will be followed for the restoration of the margins of the forest
roads and extending over the disturbed area of the pipeline. The general principles will
apply:
In most cases and where a cross slope is not steep, the corridor width will be kept to
the minimum. Where roads are to be cut into steep cross slopes or widening at
corners is required, consideration will be given to widening the construction corridor
to avoid side slopes that are too steep to allow soiling and re-establishment of
vegetation and to allow integration of slopes into the adjoining landform.
Where roads will be permanent, restoration of the edges will be carried out
progressively as part of the construction; this will consist of ensuring that excavated
mineral soil is buried on top of mineral soil and covered over with vegetative/ organic
soil and covered with turf. The bulk of the restoration of verges and cuts should be
undertaken consecutive to the initial road formation to avoid prolonged storage of
stripped materials and allow assimilation of the road to commence as soon as
possible.
Where permanent roads are to be narrowed post construction, the downhill side of
the road surface will be broken up to relieve compaction and top soil / top peat
material stockpiled adjacent to the roads will be used to narrow the road to the
agreed width. Soil will be placed to a rough uneven profile. For less steep sections
of the roads, a thin covering of soil will be placed on the centre of the road to
encourage the quick establishment of a central vegetated strip.
Temporary roads will be removed by breaking up the hard core of the road to relieve
compaction and replacing the excavated topsoil and turves stockpiled adjacent to
the road and pipeline. Temporary culverts will be removed and intercepting ditches
will be filled in to restore the natural ground profile and maintain the original ground
hydrology.
Where rock is exposed, every effort is to be made to break up the profile; to round
off the top edge; to create ledges and gullies where soil can be placed.
Monitoring and management over a three- year post construction period to ensure
establishment takes place with remedial work carried out if required.
It is important to note that reinstatement of peat needs to consider the localised hydrology
to ensure that the peat will regenerate. With reference to Scottish Renewable’ s ‘ Guidance
on the assessment of peat volumes, reuse of peat and minimisation of waste’, the following
guidelines for the management of peat will be followed:
Peat re-use will be limited to the areas already disturbed during construction and
that peat should not be spread on intact vegetated areas as this will smother
vegetation.
Surplus acrotelmic peat will be reused for road verge reinstatement where possible.
The height of the deposited peat should not exceed 1m and the slope angle should
not exceed 15° to ensure slope stability. Turves will be placed on top of the peat
wherever possible to prevent desiccation. For roads near to watercourses, the peat
will only be reused on the topside of roads.
Should any surplus unconsolidated, saturated peat ( catotelmic) arise during
excavation, it can be used for the restoration of quarries. Surplus peat may be
deposited at a maximum depth of 1m. Peat will only be used for quarry restoration if
it can be placed on mineral substrate of a low drainage capacity to prevent
degradation and maintain hydrology. Turves will be replaced on top, wherever
possible to prevent peat desiccation. Alternatively, reseeding could be done, if
appropriate and in consultation with an Ecologist, to help maintain structure.
Surplus and excavated catotelmic peat will be kept well away from any
watercourses.
Peat may be utilised as backfill material for cable installations, although electrical
and thermal design criteria will need to be considered. It is important that only peat
uncontaminated with debris is used for this application ( i.e. no large stones or tree
stumps, etc.).
Peat should be temporarily stored at areas where the mineral substrate is of a low
drainage capacity.
Where any GWDTE may be intersected by the pipeline or roads, the habitat should
be marked to prevent unplanned disturbance and the exact route through the habitat
should be clearly defined.
Approach flushes from either side using boards to protect the vegetation.
Drains will be avoided as far as possible but, where necessary, will be dug around
the contour rather than downslope.
When laying pipeline, turves will be cut leaving the vegetation intact and replaced as
soon as possible, ideally on the same day.
Vegetation turves will be cut in an irregular pattern to prevent surface lines which
may channel flow.
If construction takes place during dry weather ( or in the event of any delays to
construction) water the turves to keep them wet.
Excavate the pipe trench, taking care to store topsoil and sub-soil separately.
Install the pipeline and, if necessary, place bunds at more regular intervals to
prevent the pipeline trench becoming a preferential conduit of water and to maintain
groundwater flow through the wetland.
To prevent the pipeline trench from channelling the water along it, the peat within
the trench will be returned to as near natural structure as possible. As much material
will be returned to the trench as possible and tamped down very firmly to evict air
and close any gaps caused by cutting, without causing compaction.
Excavated material will be replaced in the order that it was removed.
Take care to replicate the original soil profile and, in the case of flushes, recreate
any slight depression that channelled the flush across the peat.
Replace the turves, with any flush turves replaced along the original line of the flush.
5.8 Intakes
Construction of the intake weirs should be carried out during a period of low to moderate
flow between 1st June and 30th September and will require the temporary diversion of water.
This will involve the construction of a temporary cofferdam across one half of the riverbed
using sand bags or material available on site. Part of the weir is built downstream of the
cofferdam. The temporary cofferdam is then moved over to the other half of the river. The
river then passes through the scour valve that is capable of passing a flow equivalent to Q 20
and cast into the first half of each of weir while the remaining part is built. This work may
last for about three to four weeks.
Concrete will be placed by direct placement, the first pour being for the base slab. The weir
wall and the intake part of the structure will then be placed in two lifts to simplify the
shuttering requirements.
5.8.5 Fishery protection measures for upstream migration and habitat in the depleted
reach
Compensation flow pipe to deliver a Hands- Off flow of Q95 at all times during
operation
Compensation notch to deliver Q80 when the river upstream of the intake is at Q30
All operatives made aware of the need to prevent the watercourse from being
contaminated.
Regular monitoring of water quality downstream of these should be carried out.
5.10 Powerhouse
The powerhouse will house one turbine, one synchronous generator and control equipment.
It will be constructed as portal frame structures on a reinforced concrete floor with timber
clad walls and a profiled metal clad roof.
5.10.3 Tailrace
The flow from each turbine will discharge into a tailrace pit, which will return the water via a
partially covered open channel directly to the Allt Garaidh Ghualaich. Attention will be given
to the design so that water velocities do not lead to any scouring of the river bank or act as
an attractant flow for fish. This could include widening and deepening the outfall of the
tailrace structure to decelerate flow velocities.
Provision has been made for the exclusion of mammals and any migrating fish. Particular
attention will be given to the visual appearance, i.e. by minimising exposed concrete where
possible.
As the road and cable traverses the hillside, it is required to cross a number of small
streams and ditches. Intercepting ditches and cut off drains will be re-established on the
topside of the road; this will not serve to transfer water horizontally for long distances or to
concentrate run off where it did not exist before. Where a topside drain or cut through a
bank on the top side of the road is made, the peat and turf should be re-used within a few
hours to cover over the change in land form.
The type of water crossing employed is dependent on the size of the watercourse, width of
the channel and the depth of the river banks. The proposed number, type and locations of
watercourse crossings will be dependent on further topographic survey that will be
undertaken nearer to the time of construction.
The existing normal watercourse channel width will be maintained and if multiple barrels are
required, channel width will be maintained across all barrels.
Culverts will be placed in deep enough and at an angle so as to avoid scouring at the
outfall. The exit points of culverts should not be on to peaty soils in order to avoid scouring
and peat erosion. Where possible, the culvert will be laid below bed level to attempt to
retain material and replicate a natural bed. The calculated culvert capacity will take this into
account, in addition to flow and flood levels ( Please refer to Drawing No. P101 60010).
5.13.4 Construction Sequence for Pipe Crossings below Original Ground Level
Make a small diversion upstream of the construction area and install silt traps
downstream of the construction area.
Pump any excess water into settlement ponds so that the construction area is kept
dry.
Excavate riverbed for pipe trench.
Bed the hydro pipe in rock- free material and then backfill with river bed material so
that the natural channel width and depth are restored.
It may be necessary to lay a geotextile liner above the pipe bedding material, with a
reno/ gabion mattress on top, to prevent and erosion of the river bed once reinstated.
Remove diversion so that the river flow continues down the natural stream channel.
5.14 Quarries
There is a requirement for some material both for back filling the pipeline trench and the
creation and upgrading of forest roads. This will primarily be sourced new quarries identified
in Drawing No. P730- 1 10106. Material can also be won from small mounds along the
pipeline or forest road excavation route or from the existing FCS quarry.
If the pit is within glacial till the back wall will, where feasible, be graded back and
benched to a slope capable of retaining top peat / top soil. The upper edge will be
rounded off and integrated into the surrounding topography. Vegetative peat will be
spread over the exposed till to achieve a minimum of 300mm cover. Peat will only
be used for quarry restoration if it can be placed on mineral substrate of a low
drainage capacity to prevent degradation.
If the quarry exposes bed rock, every effort is to be made to break up the profile and
naturalise the exposed rock face; to round off the top edge; to create ledges and
gullies where soil / top peat can be placed.
On completion the floor of the quarry will be broken up and re-graded to an
undulating, rough profile, ramping fill material up against the base of the face of the
excavation. Surplus rock material in the vicinity of the quarry will be removed into
the quarry. The base of the quarry, and back face where feasible, will be fully
covered with reclaimed top peat spread to a minimum of 300mm or to match the top
peat depths in the adjacent area.
As the new quarry may be in use for a number of months, the stripped turves that
will be used for restoration may be degraded. Supplementary turves or seeding may
be required to successfully restore the vegetation.
5.17.1 Guidelines
Throughout the construction phase, the SEPA guidelines “ Prevention of Pollution from Civil
Engineering Contracts: Special Requirements” and the “ Pollution Prevention Guidelines
Relevant PPG’s include 2, 5, 11, 21 & 26)” must be followed.
These guidelines will be implemented and issued to the contractors prior to work
commencing. In addition, SEPA will be advised of the start of the construction works before
works commence and will be asked to advise on a pollution response scheme to be
implemented in the event of any incidents.
Reference to and consideration of SNH’s “Constructed Tracks in the Scottish Uplands” has
been made throughout the writing of this Construction Method Statement.
5.17.2 Communications
Close contact will be established and maintained with the contractor, the estate, local
farmers and neighbours who will be advised of the time tabling of the different construction
elements.
Discoverer of incident to alert Site Foreman of nature and magnitude of the incident.
Site Foreman to report incident to SEPA and Water Supply Users immediately in
order to inform of potential hazards and to take advice on how to proceed.
Site Foreman and Team to attempt to prevent situation from getting any worse ( i.e.
stop pollution source if possible).
If not possible to stop pollution source, Site Foreman and Team to try to contain
situation and minimise damage.
Work not to be recommenced until pollution incident is resolved and all mitigation
measures ( EPMs, etc.) have been checked and reinstated.
Following incident, Foreman to review site EPMs and working practices with SEPA
officer to ensure good practice as per WAT- SG- 29 Temporary Construction
Methods.