Introduction
The satisfactory long-term performance of a subsurface drainage system depends on
careful construction and maintenance as well as proper design. The plans and
specifications should include specific requirements with respect to construction
activities to ensure that the completed drainage system will operate as intended. In
addition, the design should incorporate features which will facilitate the full range of
maintenance operations likely to be required during the life of the system. The designer
should refer to pertinent standards and specifications.
Precautions during
Construction
The following measures are recommended to ensure satisfactory performance of the
drainage system during its design life.
If it is determined during
construction that the soil and/or water conditions differ from those indicated by the
field investigation consult the design engineer.
Both the natural foundation
material and the embankment should be well compacted prior to placement of the pavement
structure, otherwise uneven settlement is likely to produce sags in the grade that may
trap water in local pockets, thereby weakening the subgrade. To prevent the formation of
water pockets, the grading operations must be completed to true line and grade.
Prior to installation, your
LICO contractor will inspect collector pipes to ensure they are undamaged and
unobstructed. Filter-wrapped plastic subdrain pipes should be stored indoors if not used
immediately, since the geotextile and tubing tend to deteriorate with exposure to
ultraviolet light.
During installation, your LICO
contractor will take care not to damage the pipes or obstruct them with soil or debris.
The longitudinal gradient of the pipes will be checked continuously by laser.
- Perforated steel pipes should be laid with the
perforations at the bottom. Outlet pipes should be non-perforated.
- Topsoil or other landscaping materials must not be
placed over subdrain outlets, ends of drainage layers, or granular blankets on slopes.
- Back filling of drainage trenches should
immediately follow the placement of collector pipes. The backfill material should be
placed in a manner which will minimize segregation and prevent disruption of the pipes and
filter cloth (if used).
- Compaction of the trench backfill material should
be sufficient to prevent settlement of the overlying pavement or shoulder or rupture of
the impervious trench capping. Care should be taken not to damage the pipe during
compaction.
- Pipe outlets should be clearly identified with
suitable markers as soon as the installation is completed.
- The contamination of material to be used in the
pavement structure or subdrain trench by mud or other deleterious material should be
avoided.
Subdrain Installation
Trenching Method
A LICO contractor is familiar with the
sequence of operations for the installation of a subsurface drainage system in a new
roadway by trenching. The sequence is as follows.
Prepare subgrade and/or
foundation.
Excavate collector and outlet
pipe trenches.
Place bedding material and
install perforated pipe collector trenches.
Install outlet pipes in
appropriate trenches (bedding material not required).
Place and compact collector and
outlet trench backfill.
Place and compact base drainage
layer with underlying filter aggregate or filter fabric as necessary.
Install outlet appurtenances
and markers.
Construct pavement and
shoulders.
Seed right-of-way and ditches.
Ploughing Method
The following steps outline the
procedure for installing pavement edge drains in existing and new highways by the
ploughing method. The figure illustrates a layout for a typical edge drain system.
(1) Existing Highway
(a) Excavate at the edge of
pavement to establish the top of the road subgrade elevation.
(b) Plough pipe into shoulder in
a continuous line, approximately 0.3 m from the pavement edge.
(c) Place plastic plug in pipe at
beginning of each continuous line.
(d) Recompact and grade disturbed
shoulder material to final cross-section.
(e) Excavate lateral trenches
from edge of pavement through shoulder at predetermined locations.
(f) Cut main line subdrain and
insert T connectors.
(g) Attach a length of
non-perforated plastic pipe to the T and insert the other end into a 1.5 m length of 150
mm corrugated steel (CSP) pipe at the outlet end of the lateral. The CSP should be
equipped with a rodent gate.
(h) Wrap the joint between the
plastic tubing and the CSP outlet with a strip of geotextile, plastic sheeting, etc. to
prevent infiltration of fines.
(I) Backfill lateral trench with
excavated material and compact.
(j) Mark outlet location with a
stake placed close to the edge of the right-of-way.
(2) New Construction
On new roads LICO contractors
install edge drains during construction. Ploughing should be done as one of the last
operations in building the roadway. On projects involving the construction of paved
shoulders, the drains should be placed just prior to laying the final pavement course and
shoulders. The procedure is similar to that outlined above. Extra care is needed to
maintain the outlets until all construction operations such as seeding and mulching are
completed.
Maintenance considerations
Regular and proper maintenance is important for the continued effectiveness of the
subsurface drainage system and for a long pavement life. A program of regular inspection
together with preventive and remedial maintenance must be implemented.
Collector pipes
If sediment may be deposited in collector pipes due to an inadequate pipe gradient,
uneven settlement of the system and/or a heavy sediment load, clean-out boxes or risers
should be constructed at suitable locations within the pipe network. It may be desirable
to flush out the collector pipes through the clean-out boxes with clean water. If
clean-out facilities have not been provided, removing the sediment may require
back-flushing and possibly 'snaking' through the outlet pipes.
If crushing or damage of the pipe
is suspected, boreholes or local excavations may be required to identify and correct the
problem .
Outlets
Outlets are the most critical element of a subsurface drainage system because they are
susceptible to events which can impede the free flow of water. Concerns are blockages due
to weed growth, siltation of the adjacent ditch, debris from the roadway or slope, and the
activities of animals or man. Flap gates installed on outlets to prevent back-flow from
ditches sometimes become stuck because of some of the aforementioned causes or corrosion
of the hinges. Outlets and outlet markers should be inspected and repaired (if necessary)
as part of routine maintenance at least once a year.
The following general maintenance
considerations are important for satisfactory performance of the subsurface drainage
system.
Do not damage outlets during
mowing operations. If the likelihood of such an occurrence is high, erosion control aprons
or chemical weed control could be utilized in lieu of mowing.
Efficient collection and
removal of surface water aids subsurface drainage, since there is less water to infiltrate
the pavement structure. Surface drainage structures such as catch-basins should therefore
be kept free of debris. Any damage should be repaired promptly.
Periodic checks should be made
to ensure that ditches are kept free of obstructions.
The proper sealing of pavement
cracks and joints is important to reduce infiltration and the transportation of fines into
the pavement structure.
Care must be exercised when
spreading topsoil along roadside slopes so that the flow of water from base courses is not
impaired.
Water flowing or standing in
shallow ditches can soften the shoulders and subgrade. This problem is usually associated
with older roads having narrow rights-of-way, but occasionally results from pavement
widening in cut sections.
- Unpaved shoulders and medians provide a means for
surface water to enter the pavement structure. Paving shoulders and portions of medians
helps to reduce the infiltration of surface water.
Outlets
Drains outletting into ditches should consist of a length of continuous steel or
equivalent non-perforated outlet pipe. Rodent grates should be provided; grate openings
should not exceed 25 mm, and the design should allow for the removal of debris. Outlet
pipe inverts should be at least 0.3 m above normal water level or at least 0.5 m if rapid
siltation of the ditch is expected. Erosion should be controlled by grass sod, riprap or
other means where necessary.
Agricultural Subdrainage Considerations
Prior to road construction in rural and semi-rural areas, the survey should specifically
ask that the location, size and type of agricultural subsurface drainage outlets be
recorded. In areas of uncertainty, property owners should be consulted. The quality of
drainage within the right-of-way should be equal to that in the adjoining lands and not
lower than that prior to construction
The excavation required for a new road or for the
modification of an existing road may cut a series of lateral drains. It is recommended
that a header drain roughly parallel to the right-of-way be constructed in order to
minimize the number of crossings or outlets. The upstream ends of severed drains should be
capped tightly with end plugs to keep out soil and debris. Drains which are intercepted
and are to outlet into a roadside ditch should be provided with a non-perforated pipe end
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