Highway Subdrainage 1

Introduction The satisfactory long-term performance of a subsurface drainage system depends on careful construction and maintenance as well as p roper design. The plans and specifications should include specific requirements with respect to construction activities to ensure that the completed drainage system s ystem 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.

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If it is determined during construction that the soil and/or water conditions differ from those indicated by the field investigation con sult 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 dama ge 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 o n 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 roadwa y by trenching. The sequence is as follows. 1.

Prepare subgrade and/or foundation.

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Excavate collector and outlet pipe trenches.

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Place bedding material and install perforated pipe collector trenches.

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

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Place and compact collector and outlet trench backfill.

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

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Install outlet appurtenances and markers.

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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. ( considerations Regular and proper maintenance is important for the continued effectiveness of the subsurface drainage system and for a long pav ement 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 heav y 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 cle an-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 ma y be required to identify and correct the problem .