- 2075 Asoj · 5 marks
Drilling type and field tests, Kathmandu core
Short answer
Rotary wash boring with casing and bentonite is recommended for the soft lake clays and sands. Suitable field tests: SPT every 1.5 m, electric cone penetration with pore pressure (CPTu) for continuous profiling, field vane shear in soft clay, and shear-wave velocity measurement, complemented by laboratory consolidation and triaxial tests.
In a core of Kathmandu valley a geotechnical investigation is to be carried out. As an engineer, recommend the type of drilling and the suitable field tests such that the test data can be used as much as possible.
Site nature: Kathmandu valley is a lacustrine basin filled with soft to stiff lake clays and silts, fine micaceous sands, thin peat/black clay and gravelly layers, with high groundwater table, low strength at shallow depth and high liquefaction/amplification risk.
Recommended drilling
- Rotary (wash) boring with casing and bentonite mud to the required depth (15-30 m or more), because soft clay and sand below water table would collapse in simple augering; percussion is not recommended for disturbance.
- Use a thin-walled piston (Shelby) sampler in clay/silt for undisturbed samples and split-spoon sampling in sands.
Suitable field tests
| Test | Where | Data obtained |
|---|---|---|
| SPT (every 1.5 m) | sand, silt, stiff clay | , , density, liquefaction, bearing capacity |
| Electric cone penetration test with pore pressure (CPTu) | whole profile, good for soft clay and silt | continuous profile, , , , undrained strength, layering |
| Field vane shear | soft clay | undrained strength and sensitivity |
| Groundwater observation / pore pressure | all holes | seasonal water level |
| Shear-wave velocity (downhole/MASW) | seismic design | site class, amplification |
Laboratory tests (consolidation, triaxial/UCS, Atterberg limits) on the undisturbed samples complement these. CPTu and SPT give the greatest amount of directly usable design data for bearing capacity, settlement of soft clay, pile capacity and liquefaction.
✓ Answer checked by Claude Sonnet 5.5
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