Control of sub-surface water and seepage

Sub-surface water (high water table, seepage from higher ground, and capillary and vapour movement) weakens the subgrade and must be kept at a safe depth below the formation level. IRC and NRS 2070 recommend that the subgrade top be at least about 0.6 to 1.0 m above the highest water table (more in silty soils), or that a measure below be provided.

1. Lowering the water table

• Longitudinal (trench) sub-surface drains: a trench below the side drain, filled with graded filter material and a perforated pipe, lowers the water table beneath the pavement.
• Transverse drains: trenches at an angle to the road (herring-bone pattern) in wet cuttings or low-lying lengths.
• Deep side ditches below the formation level in flat country.
• Well points / sand drains: used in soft clay or temporary works.

   Pavement
 ___________________
 ___|_______________|___  Formation level
   \ gravel filter  /
    \  [o o o o]  /     o = perforated
     \_geotextile_/        pipe
      water table lowered ~~~~~~

2. Controlling seepage flow

• Interceptor (cut-off) drains on the uphill side, a trench filled with filter material down to an impervious layer, intercept lateral seepage in hill cuttings.
• Horizontal (bore-hole) drains at the toe of slopes release water from within the slope.
• Drainage blanket (a free-draining sand-gravel layer, or the granular sub-base extended to the slope) drains water under the pavement; a geotextile separates it from the subgrade.
• Counterfort / chimney drains behind retaining walls, with weep holes.

    uphill ground
       \   seepage ->
        \      __ road
   ======\____|__|______
     [filter trench + pipe]
     ======= impervious layer

3. Controlling capillary and vapour rise

• Capillary cut-off: a layer of coarse sand or gravel, 150 to 300 mm thick, between subgrade and sub-base breaks capillary rise (the pores are too large to lift water).
• Impermeable membrane: a bituminous layer, plastic sheet or geomembrane stops both capillary and vapour movement.
• Raising the embankment above the capillary fringe (the height of capillary rise is about $h_c\approx C/(e D_{10})$, large for silts and clays).
• Lime or cement stabilisation of the top subgrade reduces permeability.

 Surfacing
 Base / Sub-base
 ###### coarse sand cut-off ######   <- breaks capillary rise
 Subgrade  (fine soil)
 ~~~~~ water table ~~~~~

The measure depends on the cause: seepage needs an interceptor drain, a high water table needs trench drains or a higher embankment, and capillary rise needs a cut-off layer or membrane.