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Which statement is true for underground drainage in highways? 1. Ground water surface should be at least 1.5 m below sub grade 2. Transverse drains are also used in low permeability soils 3. Transverse drains are used in all types of soils Select the correct answer from the codes given below

Which statement is true for underground drainage in highways? 1. Ground water surface should be at least 1.5 m below sub grade 2. Transverse drains are also used in low permeability soils 3. Transverse drains are used in all types of soils Select the correct answer from the codes given below Correct Answer only 2

Concept:

Highway Drainage:

  • Highway drainage consists of removing or controlling surface water and subsurface water away from the road surface and the subgrade supporting it.
  • One part of the rainwater flows on the ground or road surface, while the other part percolates into the ground and reaches the groundwater table, raising its level.
  • The subgrade soil above the groundwater table may raise through the soil pores due to the phenomenon of capillarity.

Requirements of a poor Drainage System:

1. Reduction of the strength of the pavement.

2. Spoiling the pavement surface by the formation of pot-holes and ruts.

3. Seeping of surface water through the pavement layers, shoulders, and sides into the subgrade.

4. Reduction in the bearing power of the subgrade through the continued presence of water.

Explanation:

  • Moisture changes in the subgrade occur due to the percolation of rainwater and seepage flow, as also due to the phenomenon of capillary rise.
  • The aim of subsurface drainage is to keep the groundwater table (GWT) sufficiently below the level of the subgrade – at least 1.2 m.
  • If the soil is relatively less permeable, longitudinal as well as transverse drains may be needed to lower the groundwater table

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