--- title: "Control of sub-surface water and seepage" subject: "Transportation Engineering I" chapter: 4 key: "semester_6/transportation_engineering_i:subsurface-drainage-measures" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/subsurface-drainage-measures site: BCT Engineering --- # Control of sub-surface water and seepage Transportation Engineering I · Chapter 4: Highway Drainage - Asked in: 2081 Ashwin, 5 marks; 2078 Chaitra, 5 marks; 2078 Poush, 4 marks; 2077 Chaitra, 4 marks; 2076 Bhadra; 2073 Bhadra; 2072 Ashwin; 2072 Magh; 2071 Magh; 2069 Bhadra; 2068 Magh (old course); 2066 Magh (old course), 8 marks; 2065 Kartik (old course), 4 marks; 2065 Chaitra (old course), 4 marks; 2063 Kartik (old course), 4 marks (asked 15 times) - Video: [BASIC TRANSPORTATION ENGINEERING- SUBSURFACE DRAINAGE AND ITS METHODS](https://www.youtube.com/watch?v=2JioFn_vnho) — Bhavik Joshi - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/subsurface-drainage-measures - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4 ## Question Describe the measures/methods that can be applied for lowering the water table and controlling sub-surface water and seepage flow (including capillary and vapour rise) with suitable neat sketches. ## Short answer Methods include longitudinal trench drains and deep side ditches to lower the water table, interceptor and horizontal drains to control seepage, and capillary cut-off layers or impermeable membranes to stop capillary and vapour rise. The choice depends on the cause. ## Answer 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. ```text 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. ```text 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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How far should the top of the subgrade be above the highest water table? A. About 0.6 to 1.0 m B. About 3 to 5 m C. About 0.1 m D. Exactly at the water table Answer: A. IRC/NRS recommend about 0.6 to 1.0 m (more in silty soils) or else a drainage measure. (Key point: Safe water table depth) 2. What is the function of an interceptor (cut-off) drain? A. Break capillary rise B. Intercept lateral seepage on the uphill side of the road C. Collect rain from the carriageway D. Lower the water table beneath the centre Answer: B. Placed uphill, filled with filter material down to an impervious layer, it stops seepage reaching the road. (Key point: Interceptor drain) 3. How does a coarse sand or gravel layer 150-300 mm thick stop capillary rise? A. It absorbs the water B. Its pores are too large to lift water by capillarity C. It is impermeable D. It increases the water table depth Answer: B. Large pores cannot sustain capillary suction, so rise is broken between subgrade and sub-base. (Key point: Capillary cut-off) 4. Which pairing of cause and measure is correct? A. Seepage - capillary cut-off B. Capillary rise - interceptor drain C. High water table - trench drain or higher embankment D. Vapour rise - well points Answer: C. High water table needs trench drains or a raised embankment; seepage needs interceptor drains; capillary rise needs a cut-off layer or membrane. (Key point: Matching measure to cause) ## Related in Transportation Engineering I - [Surface versus sub-surface drainage](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asurface-vs-subsurface) — Transportation Engineering I, chapter 4, asked 2× - [Surface water drainage in hill roads](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-surface-drainage) — Transportation Engineering I, chapter 5, asked 2× - [Collection and disposal of surface water](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asurface-drainage-collection) — Transportation Engineering I, chapter 4, asked 2× - [Design steps of surface drainage](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asurface-drainage-design-steps) — Transportation Engineering I, chapter 4, asked 3× ## Related in other subjects - [Uniform sand, water table 2 m, capillary](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auniform-sand-effective-stress-capillary) — Soil Mechanics, chapter 6 - [Seepage control measures in embankment dam](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aembankment-seepage-control-measures) — Hydropower Engineering, chapter 4 - [Water pollutants, harmful effects, control measures](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollutants-effects-measures) — Engineering Chemistry, chapter 3 - [Capillary rise exceeding tube height](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Acapillary-rise-exceeds-tube-height) — Fluid Mechanics, chapter 1