==>>Sub Surface Highway Drainage<<==

==>>Sub Surface Highway Drainage<<==

• Subgrade may be damaged by sub soil water.
• Sub soil water as free water, when water table is high or it may come up by capillary action to the subgrade when water table is low.
• Subgrade should be of self draining material so that it may pass off the percolation water that comes to it to remain dry and stable.
• But if subgrade is of soft and retentive soil, or there are underground dprings bringing free water to the subgrade fro that reason subsurface drains should be constructed about 1 ½’ to 2’ below the formation level to carry away water from the subgrade and thus keep it dry. ( in easily drainable soil water can be lowered by deep or open side drains, it also takes rain water.
• Cross-drains may be in the form of trapezoidal trenches filled with selected rubble called rubbled drains or trench drains.
• Depth is not much and the discharge is small.
• The pipes are surrounded by filler material and the remaining of the cross trench is filled with graded rubble,  the bigger size rubble being nearer to the pipe. Water of wet subgrade passes through the open joint of pipes and enter the lateral drain which discharge into the longitudinal drain pipe in the two longitudinal side trenches.
• Longitudinal drain carry water to the nearby stream.
• Cross-drains, staggered in herring bone fashion.
• Spacing of lateral drains is less in impermeable soil and more in permeable soil.