Rules Guiding the Choice of Drainage Facilities for Pavement Structures according to CD 501

Hydraulic structures are very important in pavement construction in order to prevent the sub-base or subgrade of the pavement from getting saturated. This is more onerous as the axle loads of the pavement due to modern traffic increases. If there is not adequate drainage for the pavement, surface water can enter the pavement construction though:

i. The porous surface

ii. Cracking as the pavement ages and

iii. At the edge of the carriageway if it cannot enter gullies grips or edge drainage easily.

Wherever possible, drainage through grips to ditches, kerbs and channels or continuous channel drainage should be provided. Over the edge drainage or combined surface water/sub-soil drainage is not recommended as it softens the carriageway edge and clogs with time.

Besides the surface water, there is risk of subgrade saturation from ground water due to the following circumstances enumerated below. Cut-off sub-soil drainage should be provided if these circumstances are encountered on the site.

Situations where there is Risk of Saturation of Subgrade from Groundwater

1. Where the winter height of the water table is within 600 mm of formation level;
2. Where the sub-soil is unstable because of being waterlogged;
3. Where there is a likelihood of water running from or out of adjacent ground;
4. Where springs, land drains or watercourses are present;
5. Where the finished road is below existing ground level, regardless of the water table;
6. Where the sub-grade is likely to be altered due to groundwater.

Effect of Water Saturation on the Strength of Pavement Subgrade

1. Reduction in strength of the subgrade, capping layer and unbound sub-base as pore water pressures are generated and particle interlock is lost.
2. Movement of fines within the capping layer and unbound sub-base leading to further loss of aggregate interlock, loss of strength and possible risk of frost damage.
3. Degradation of unbound aggregate generating even more fine material.
4. Friction between the sub-base and structural layers is reduced, lowering the strength of the total construction.
5. The base of the asphalt layer may be subjected to scouring by water stripping the bitumen, creating voids and reducing strength. Water can also be forced into micro-cracks leading to rapid failure.

Important Points to Note in the Drainage of Pavement Layers

1. If the water table is well below the formation level and both the capping layer material and sub-base have adequate permeability to carry away surface water, it is possible that sub-surface drainage is not necessary.
2. For unbound sub-base, it is necessary to ensure that the sub-base cannot become saturated, by installing sub-base drainage.
3. For ‘sub-base only’ options on sub-grade CBR less than 3%, as an additional safeguard, it is recommended that a non-woven geotextile separation layer is inserted to prevent fines migration into the sub-base should drainage become ineffective with time. This should be designed based upon the grading of the sub-grade.
4. For bound sub-base, it is necessary to ensure that the interface between the sub-base and asphalt road base is drained so that friction is maintained at the interface.

Drainage Facilities in Pavement

Highway drainage system may consist of;

1. An open or closed rectangular, triangular or trapezoidal drainage path adjoining road sections and discharging into catchpits or to a natural water body.
2. A piped system vested or to be vested to a water company.
3. A piped highway drainage system (minimum pipe size 225 mm) running to a public sewer vested in a water company or outfall to a ditch or watercourse.
4. Infiltration systems, flow attenuation (reduction) or retention systems (including oversized pipes) where piped system is not feasible. Infiltration systems is only proper where adjacent soils have an infiltration rate greater than 10 mm/hr and where the soakaway can be located at least 1.0 m above the highest recorded groundwater level considering seasonal variances. Infiltration systems must be located outside of the carriageway and a minimum of 5 m from buildings.

Where a piped system discharges into an existing ditch or watercourse, the pipe invert (bottom of the inside of the pipe) must not be lower than the level of the base flow in the ditch or watercourse and it should always be at least 150 mm above the ditch or watercourse invert. You must direct the end of the pipe so it discharges at an angle less than 60 degrees to the direction of flow in the ditch or watercourse. The end of the pipe must have a headwall and apron which supports the bank above and adjacent to the pipe and prevents any scouring underneath the pipe. You must protect the banks of the ditch or watercourse from scouring.

Recommended Discharge Points of Road Drainage

Road drainage shall be discharged to the following destinations in order of preference:

1) ground;

2) surface water body;

3) surface water sewer; and

4) combined sewer.

Highway runoff must outfall into the ground except where one or more of the following criteria can be demonstrated:

i. The rate of surface runoff is greater than the rate at which water can infiltrate into the ground. In this case as much of the water as reasonably practicable must be discharged by infiltration to ground; There is an unacceptable risk of ground instability or subsidence;

ii. There is an unacceptable risk of pollution from mobilising existing contaminants on the site;

iii. Infiltration is not compliant with the water quality requirements;

iv. There is an unacceptable risk of groundwater flooding.

Highway runoff that cannot discharge into ground must always outfall to a fluvial surface water body except where any of the following can be demonstrated:

i. It is not reasonably practicable to convey the runoff to a surface water body;

ii. Pumping of the surface water runoff, either on site or further downstream, would be required and there is a reasonably practicable alternative;

iii. Discharge would result in an unacceptable risk of flooding from the surface water body.

Highway runoff that cannot outfall into the ground or to a fluvial surface water body must outfall to a surface water sewer or local highway drain, except where it can be demonstrated that it is not reasonably practicable to do so.

Surface runoff that cannot outfall into the ground, a fluvial surface water body or a surface water sewer or local highway drain must outfall to a public combined sewer, not a foul sewer.

Important Specification Limits for Different Features of Highway Drainage

A. Minimum pipe size
The minimum pipe diameter for adoptable highway drains, other than gully connections, is 225 mm. The minimum size for a road gully connection is 150 mm.

B. Use of combined kerb and drainage systems
Combined kerb and drainage system must be considered where the minimum longitudinal carriageway gradient is less than 1 in 100 for flexible surfaces and less than 1 in 80 for rigid/block paved surfaces.

C. Approving drainage structures
Any drain, pipe or box culvert, sewer or drainage structure that has a clear span or internal diameter of greater than 900 mm or any headwall greater than 1.5 m retained height, will be classified as a highway structure and be subject to the specific requirements that apply to highway structures.

D. Catchpits specifications
Unless otherwise specified, you must use catchpits and not manholes on adoptable highway drainage systems. Soakaway structures (typically over-sized chambers and cover slabs which are greater than 1050 mm in diameter). You must provide a catchpit (an access chamber, with sump, on a drainage system) where there is any discharge into an existing ditch or watercourse. On all drainage runs we are to adopt where the pipe diameter is 900 mm or less, you must provide a catchpit at:

− every change of alignment or gradient;

− the head of all main pipelines;

− every junction of pipelines except for single-gulley connections;

− every change in pipe diameter; and

− a maximum spacing of 90 metres.

E. Catchpit and manhole positions
Catchpits or manholes should be located within the verge, and not the carriageway, on classified roads and other roads with a higher status than a residential street or industrial access road. The outside of catchpits and manholes should be at least 500 mm from the kerb line or the edge of the carriageway. Any catchpits or manholes within a carriageway must be located so that they can be accessed while providing the necessary safety zones and without preventing traffic from passing. This will generally mean that you should not site them at or near the centre of the carriageway or within a width restriction. You should also take care when locating catchpits or manholes within junctions or roundabouts, based on the same criteria.

F. Positioning and alignment of highway drains and storm and foul sewers
Highway drains must be laid:

− in straight lengths;

− to straight grades between catchpits; and

− within the carriageway or verge.

You must not lay drains and sewers and their associated catchpits or manholes in footways as this space is required for other utility apparatus.