Subtopic: Planning, design and layout of surface and sub-surface drainage systems
Question Statement:
In subsurface pipe drainage installations, a granular gravel or synthetic geotextile envelope surround is placed around the perforated pipe primarily to:
Options:
(A) Prevent the ingress of fine soil particles into the drain pipe while reducing entrance hydraulic resistance in the immediate vicinity of pipe perforations
(B) Provide thermal insulation to keep the drainage water warm
(C) Completely seal the perforations to prevent groundwater from entering the pipe
(D) Neutralize acidic soil chemistry through cation exchange
Answer Key:
A
Step-by-Step Solution & Explanation
Drain envelopes (synthetic non-woven geotextiles or graded gravel packs) serve dual hydraulic and mechanical functions:
1. Filtration: Prevent silt and fine sand from washing through the perforations and choking the pipe.
2. Hydraulic conductivity: Provide a high-permeability zone around the pipe wall, minimizing entrance convergence head loss into the discrete perforations. Option A is correct.