Salient Drainage Design Aspects for Roads & Highways


Dr. S. S. Seehra, Chief Consultant, Roads & Highways SNC–LAVALIN SPAN Consultants (Member of the SNC–LAVALIN Group Inc.) & Former Director–Grade Scientist & Area Advisor, Central Road Research Institute, New Delhi

Introduction

  • Adequate drainage is a primary requirement for maintaining the structural soundness and functional efficiency of a road.
  • Pavement structure including subgrade must be protected from any ingress of water, otherwise over a period of time, it may weaken the subgrade by saturating it and cause distress in the pavement structure.
  • That is why rapid dispersal of water from pavement and subgrade is a basic consideration in road design.
  • Because of inadequate surface drainage, the structural stability of pavement is undermined by:
  • Weakening of pavement structure and subgrade through infiltration of water from the top, and
  • Erosion of shoulders, verges and embankment slopes caused by water running off the pavement
  • Mechanism of failure on account of inadequate drainage facilities in a pavement system should be understood and suitable remedial measures taken against it to ensure desired performance during the service life of the pavement.

General Drainage Design Aspects

  • Proper geometric design of the road by crowning the carriageway or one side or two side cross fall, giving proper cross slope to the shoulders and verges, providing requisite longitudinal gradient etc.
  • Water from road and the surrounding area shall be successfully intercepted and led away to natural outfalls.
  • The engineer must build adequate cross–drainage structures at river crossings and minor streams.

Survey and Investigations

  • Survey and investigations is a basic necessity for designing a drainage system which may involve the following:
  • Reconrse bearing on road drainage such as Aerial photography
  • Factors bearing on road drainage such as rainfall, topography and natural drainage of the area, cross fall and longitudinal profile, existing drains and internal drainage of pavement layers etc. should be recorded.

Road Geometrics Longitudinal Gradient

  • Flatter slopes both longitudinal and transverse slow down the flow of rain water over the oadway and decrease the drainage capacity.
  • For better internal drainage of pavement layers, especially of granular material, a slight longitudinal gradient is most preferable.
  • A minimum longitudinal drainage gradient is most preferable
  • A minimum longitudinal drainage gradient of 0.3% is adequate.

Pavement Cross Slope/ Camber

  • In geometric design pavement cross-fall or camber could be made to slope either on one side or on both sides with a crown in the middle of the road pavement.
  • On hill roads preference generally is to drain the carriageway water towards the hill side particularly where the road banking is susceptible to erosion so that the drain on the roadway could carry away the discharge safely to proper outfall.
  • Camber should not be less than 1 in 40.

Pavement Cross Slope/ Camber

  • For a given surface type the steeper values may be adopted in the areas having high intensity of rainfall and lower values where the intensity of rainfall is low.
  • For high type bituminous surfacing or cement concrete, the cross fall or camber should be between 1 in 60 to 1 in 50.
  • For thin bituminous surfacing, the camber should be between 1 in 50 to 1 in 40.
  • Earth shoulders 3 to 4% (1 in 25)

Shoulder Drainage

  • The effective method of maintaining the shoulders is to have paved or hard shoulders instead of earth shoulders
  • A common defect in some of the road is occurrence of shoulders at levels higher than pavement surface. In such situations, during rain - the water on road surface does not find a free outlet and accumulates on top of it. Such defect where shoulder blocks the drainage should be rectified.

Shoulder Drainage

  • Hard shoulders are preferable to earth shoulders from overall considerations of improved pavement performance.
  • Earth surfaced median should not be crowned or cross sloped to drain on the road pavement because washed away soil may deposit on road pavement making it slippery and accident prone.

Drainage of High Embankment

  • The problem of erosion of slopes and shoulders is most severe in high embankments (usually more than 8m) having steep slope in longitudinal direction such as approaches in bridges.
  • Where high embankments are on longitudinal slopes, longitudinal and cross drains may be provided.
  • The longitudinal drains may be provided at the edges of the roadway.
  • Once water is channelised in these side drains, it is led down the slopes by means of stepped out falls or lined chutes at about 10 meter intervals ultimately discharging into side channel at the bottom.

Open Drains

  • Open drains are known as side drains, catch water drains, intercepting drains or gutters.
  • Type of road traffic and rainfall intensity are some of the main factors which influence the shape, location and capacity of open drains.
  • The choice of cross-section of open drains is generally limited to 3-types triangular, trapezoidal and rectangular.
  • The following linings are feasible on the drain surface for preventing erosion but it requires proper maintenance so that undesired growth of vegetation may not reduce the flow capacity of drain:
  • Turfing
  • Stone/ Brick MasonryvConcreting
  • Stone Slab Lining
  • Shoulder Pitching
  • Bituminous Treatment
  • Polyethylene Lining

Hydrologic Design

  • Hydrologic analysis is a very important step prior to the hydraulic design of road drainage system
  • Factors which affect run-off are size and shape of drainage area, slope of ground, geology, soil types, surface infiltration and storage.
  • The rational method is an universally accepted empirical formula relating rainfall to run-off:
  • Q = 0.028 PAIc
Where Q = Discharge (Peak run-off) in cu.m/sec.
P = Co-efficient of run-off for the catchment characteristics.
A = Area of catchment in hectares.
c = Critical intensity of rainfall in cm per hour.
The suggested values of ‘P’ for use in formula are given in the Table 2.

Hydraulic Design

For uniform flow in open channels, the basic relationships are expressed by the Manning’s formula:

Q = 1/n AR2/3 S1/2

V = 1/n R2/3 S1/2

WhereQ = discharge in cu.m/sec.
V = Mean Velocity m/sec.
n = Manning’s roughness co-efficient
R = Hydraulic radius in m which is area of flow-cross-section divided by wetted perimeter
S = Energy slope of the channel which is roughly taken as slope of drain bed
A = Area of the flow cross-section in m2

Sub–Surface Drains

  • Two main objectives of subsurface drains are to lower level of water table and to drain out underground water. To be effective, they should not be less than 0.5 m below the subgrade level.
  • The subsurface drains may be provided with perforated pipe or open jointed solid pipe in a trench with backfill around it. The internal diameter of the pipe should not be less than150 mm.
  • The designing of subsurface drain by Darcy’s law is as follows;
Q = Kia
Q = Discharge in m3/sec
A = Cross-sectional area in m2
i = Hydraulic gradient
k = Co-efficient of permeability in m/sec.

NBMCW September 2008

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