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FACTSHEET


No. 6:  HIGHWAY SUBDRAINAGE

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.

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. Perforated steel pipes should be laid with the perforations at the bottom. Outlet pipes should be non-perforated.
  6. Topsoil or other landscaping materials must not be placed over subdrain outlets, ends of drainage layers, or granular blankets on slopes.
  7. 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).
  8. 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.
  9. Pipe outlets should be clearly identified with suitable markers as soon as the installation is completed.
  10. 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.

  1. Prepare subgrade and/or foundation.

  2. Excavate collector and outlet pipe trenches.

  3. Place bedding material and install perforated pipe collector trenches.

  4. Install outlet pipes in appropriate trenches (bedding material not required).

  5. Place and compact collector and outlet trench backfill.

  6. Place and compact base drainage layer with underlying filter aggregate or filter fabric as necessary.

  7. Install outlet appurtenances and markers.

  8. Construct pavement and shoulders.

  9. 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.

  1. 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.

  2. 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.

  3. Periodic checks should be made to ensure that ditches are kept free of obstructions.

  4. The proper sealing of pavement cracks and joints is important to reduce infiltration and the transportation of fines into the pavement structure.

  5. Care must be exercised when spreading topsoil along roadside slopes so that the flow of water from base courses is not impaired.

  6. 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.

  7. 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 sections.


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