Some fundamentals

11.1 The performance of a pavement can be seriously affected if adequate drainage measures to prevent accumulation of moisture in the pavement structure are not taken. Some of the measures to guard against poor drainage conditions are maintenance of transverse section in good shape to reasonable crossfall so as to facilitate quick run-off of surface water and provision of appropriate surface and sub-surface drains where necessary. Drainage measures are especially important when the road is in cutting or built on low permeability soil or situated in heavy rainfall/snow fall area.

11.2 On new roads, the aim should be to construct the pavement as far above the water table as economically practicable. The difference between the bottom of subgrade level and the level of water table/high flood level should, generally, not be less than 1.0 m or 0.6 m in case of existing roads which have no history of being overtopped. In water logged areas, where the subgrade is within the zone of capillary saturation, consideration should be given to the installation of suitable capillary cut-off as per IRC:34 at appropriate level underneath the pavement.

11.3 When the traditional granular construction is provided on a relatively low permeability subgrade, the granular sub-base should be extended over the entire formation width in order to drain the pavement structural section. Care should be exercised to ensure that its exposed ends do not get covered by the embankment soil. The trench type section should not be adopted in any case as it would lead to the entrapment of water in the pavement structure.

11.4 If the granular sub-base is of softer variety which may undergo crushing during rolling leading to denser gradation and low permeability, the top 100 to 150 mm thickness should be substituted by an open graded crushed stone layer of Los Angeles abrasion value not exceeding 40 to ensure proper drainage.

The filter layer must have enough permeability to prevent development of undesirable pore water pressure and it should drain away any free water that enters into it albeit at much lower rate as compared to the drainage layer.

The filter/separation layer should satisfy the following criteria:

D₁₅

To prevent entry of soil particles into the drainage layer And D₅₀

50

of filter layer

D of subgrade ≤ 25 … 11.3

D₈₅ means the size of sieve that allows 85 per cent by weight of the material to pass through it. Similar is the meaning of D50 and D15.

The permeable sub-base when placed on the erodible subgrade soil should be underlain by a layer of filter material to prevent the intrusion of soil fines into the drainage layer (Fig. 11.2).

Non-woven geosynthetic also can be provided to act as a filter/separation layer. Some typical drainage system is illustrated in Figs. 11.1, 11.2 and 11.3.

11.5 When large inflows are to be taken care of, an adequately designed sub-surface drainage system consisting of an open graded drainage layer with collection and outlet pipes should be provided. The system should be designed on a rational basis using seepage principles to estimate the inflow quantities and the outflow conductivity of the drainage system. It should be ensured that the outflow capabilities of the system are at least equal to the total inflow so that no free water accumulates in the pavement structural section. If granular sub-base is not required because of strong subgrade, commercially available geocomposite can be used for drainage and separation. A design example is given in Appendix V.

11.6 Very often, water enters the base, sub-base or the subgrade at the junction of the verges and the bituminous surfacing. To counteract the harmful effects of this water, it is recommended that the shoulders should be well-shaped and if possible, constructed of impermeable material. Major highways should have paved shoulder to keep away water from the subgrade and for other roads also with design traffic of 20 msa or less; the base should be constructed 300-450 mm wider than the required bituminous surfacing so that the run-off water disperses harmlessly well clear off the main carriageway.

11.7 Shoulders should be accorded special attention during subsequent maintenance operation too. They should be dressed periodically so that they always conform to the requisite cross-fall and are not higher than the level of carriageway at any time.

Fig. 11.1 Pavement along a Slope

Fig. 11.2 Pavement with Filter and Drainage Layers

Fig. 11.3 Longitudinal Pipe at the Edge of the Drainage Layer with Outlet Pipe

11.8 Drainage Requirement

Heavy axle loads commonly ply on major roads in India and therefore, it should be ensured that the unbound layers do not undergo unacceptable permanent deformation under repeated loading. The subgrade and the granular layers with entrapped water would be subjected to large pore water pressure under heavy loads causing erosion of the unbound layer. It is necessary to provide a drainage layer to drain away the water entering into the pavement.

The coarse graded granular sub-base (62) would have the necessary permeability of 300 m/day with per cent fines passing 0.075 mm sieve less than 2 per cent. Laboratory test should be conducted for the evaluation of the permeability of the drainage layer. If the surface of the open graded drainage layer is likely to be disturbed by the construction traffic the layer may be treated with 2 per cent cement/2-2.5 per cent of bituminous emulsion without any significant loss of permeability. Field test by Ridgeway in USA indicated that it is the duration of low intensity sustained rainfall rather than high intensity rainfall that is critical for infiltration

of water into the pavement. It was found that the infiltration rate through the joints/cracks was 0.223 m³/day/m and this value can be used for design of drainage layer in the absence of field data. The infiltration rate per unit area qi in m³/day/m² can be expressed as:

q I N

N_c = Number of longitudinal cracks (i.e., number of lanes plus one) W_p = Width of pavement subjected to infiltration

W_c = Length of the transvers cracks or joints (equal to the width of the pavement) C_s = Spacing of transverse cracks (taken as 12 m for bituminous pavement)

K_p = Rate of infiltration through uncracked area (assumed zero for thick bituminous pavements)

An example for the design of drainage layer is given in Appendix V.