Civil Engineering: Soil, Water, Roads and Concrete in Sri Lankan Practice
Civil Engineering in the Sri Lanka Engineering Service examination tests the applied design and construction knowledge a public-sector engineer needs on Road Development Authority, National Water Supply and Drainage Board, Urban Development Authority, Irrigation Department and local-authority works: soil and foundation behaviour, hydraulics and hydrology, water supply and sanitation, highway materials, concrete technology, surveying and quantity estimation.
Soil and foundations
- Atterberg limits classify fines; the plasticity index (PI = LL − PL) drives subgrade acceptability.
- Compaction is judged by field dry density against the Standard Proctor maximum, usually by the sand-replacement method and expressed as a percentage of MDD at OMC.
- CBR is the strength index for subgrade, subbase and base, measured at 2.5 mm penetration, using the 5.0 mm value only if it is higher.
Hydrology and hydraulics
- Rational method: Q = CiA/360 with A in hectares and i in mm/h, giving Q in m³/s.
- Manning: V = (1/n)R^(2/3)S^(1/2); use hydraulic radius, not diameter, for circular pipes.
Worked example
Catchment 12 ha, C = 0.50, 10-year intensity 60 mm/h. Q = (0.50 × 60 × 12)/360 = 1.0 m³/s. The constant 360 already converts mm/h × ha to m³/s; converting the intensity to m/s first gives an answer 3 600 times too small.
Traps
- Levelling checks: ΣBS − ΣFS must equal last RL − first RL; a mis-booked inverted staff reading breaks it silently.
- Mixing characteristic and design strengths in concrete; cement to SLS 107, water/cement ratio and curing govern site acceptance.
- Forgetting that the rational method assumes uniform rainfall over a catchment smaller than about 50 ha.
- Reading a nomograph or table without checking the units on its axes.