--- title: "Causes of moisture variation in subgrade" subject: "Transportation Engineering I" chapter: 4 key: "semester_6/transportation_engineering_i:moisture-causes" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/moisture-causes site: BCT Engineering --- # Causes of moisture variation in subgrade Transportation Engineering I · Chapter 4: Highway Drainage - Asked in: 2079 Chaitra, 6 marks; 2079 Jestha, 2 marks; 2076 Baisakh; 2076 Bhadra; 2073 Bhadra; 2072 Ashwin; 2071 Magh; 2070 Bhadra; 2069 Bhadra; 2068 Magh (old course); 2065 Chaitra (old course), 2 marks; 2064 Shrawan (old course); 2063 Kartik (old course), 3 marks (asked 13 times) - Video: [Causes Of Moisture Variation In SubGrade Soil | Highway Drainage| Surface & SubSurface Drainage |](https://www.youtube.com/watch?v=xSA68v1xg7Y) — DIPAK MANDAL - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/moisture-causes - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4 ## Question What are the causes of moisture variation in sub-grade soil? ## Short answer Moisture varies due to rain infiltration, ground-water seepage, water table fluctuation, capillary rise, vapour condensation under the pavement, ponding, edge drying, and moisture trapped during construction. Design uses soaked CBR. ## Answer The moisture content of subgrade soil changes with time, and a wet subgrade has a lower CBR and bearing strength. The main causes of moisture variation are: 1. **Rain-water infiltration** through the pavement surface, cracks, joints, permeable shoulders and unlined side drains. 2. **Seepage of ground water** laterally from higher ground, springs and leaking canals or ponds, especially in cuttings and hill roads. 3. **Fluctuation of the ground-water table**, seasonal rise during the monsoon, and floods. 4. **Capillary rise** of water from the water table into fine soils such as silt and clay, which can reach 1 m or more above the water table. 5. **Vapour movement and condensation.** Temperature differences between day and night, and between the pavement and soil, drive water vapour upward or downward. It condenses beneath the impermeable pavement and builds up moisture. 6. **Ponding and flooding** of the road side because of poor surface drainage, blocked culverts or low embankments. 7. **Evaporation and drying** at the shoulders, which makes the edge zone drier than the centre. This differential moisture causes cracking and edge failure. 8. **Moisture trapped during construction** (compaction on the wet side of optimum) and the effect of tree roots and vegetation. The moisture content under the centre of a covered pavement slowly reaches an equilibrium value, while the edges vary seasonally. The design therefore uses the **soaked CBR** (worst moisture state) of the subgrade (IRC 37, NRS 2070). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does an increase in subgrade moisture do? A. Raises CBR B. Lowers CBR and bearing strength C. Has no effect on strength D. Raises bearing capacity Answer: B. A wet subgrade has a lower CBR and bearing strength. (Key point: Effect of wet subgrade) 2. Which soils show greatest capillary rise from the water table? A. Coarse gravel B. Clean coarse sand C. Fine soils such as silt and clay D. Rock Answer: C. Fine pores in silt and clay lift water 1 m or more above the water table. (Key point: Capillary rise) 3. How does vapour contribute to moisture build-up under a pavement? A. It evaporates through the pavement freely B. Temperature differences drive vapour which condenses beneath the impermeable pavement C. It is absorbed by the surfacing D. It only occurs during construction Answer: B. Day-night and pavement-soil temperature differences move vapour, which condenses under the covering. (Key point: Vapour movement) 4. Which CBR does design use because of moisture variation? A. Unsoaked CBR B. Soaked CBR (worst moisture state) C. CBR of the surface layer D. Average of dry and soaked Answer: B. Design uses the soaked CBR to cover the worst moisture state (IRC 37, NRS 2070). (Key point: Design moisture condition) ## Related in Transportation Engineering I - [Desirable properties of subgrade soil](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asubgrade-properties) — Transportation Engineering I, chapter 6, asked 11× - [CBR test on subgrade soil](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Acbr-test) — Transportation Engineering I, chapter 6 - [Soil erosion causes and preventive measures](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aerosion-causes-prevention) — Transportation Engineering I, chapter 4 - [Reasons for grade compensation](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Agrade-comp-reasons) — Transportation Engineering I, chapter 3, asked 3× ## Related in other subjects - [Boring type for road sub-grade](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aboring-type-subgrade) — Foundation Engineering, chapter 2 - [Soil-moisture-irrigation relationship](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-irrigation-relationship) — Irrigation and Drainage Engineering, chapter 2, asked 2× - [Soil moisture tension and related terms](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-tension-terms) — Irrigation and Drainage Engineering, chapter 2 - [Forms of soil moisture with diagram](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-forms-diagram) — Irrigation and Drainage Engineering, chapter 2