Soil exploration and improving bearing capacity

Soil exploration

Soil exploration (site investigation) is the process of studying the soil and rock at a site by borings, sampling, field tests and laboratory tests, to find the type and thickness of strata, groundwater level and the engineering properties (bearing capacity, settlement, strength) for safe foundation design. Methods are test pits, auger boring, wash boring, percussion boring, rotary drilling, SPT, cone penetration, plate load test, geophysical methods.

Methods of improving bearing capacity

1. Compaction of loose soil by rolling, ramming and vibration.
2. Drainage of ground water (sumps, well points, sub-soil drains).
3. Replacement of weak soil with sand, gravel or murrum, compacted in layers (sand cushion).
4. Increasing size and depth of foundation.
5. Sand piles, stone columns, to densify soft soils.
6. Grouting and chemical injection (cement, lime, silicate).
7. Soil stabilisation with cement, lime, bitumen or fly ash.
8. Pre-loading and use of geosynthetics.
9. Confining the soil with sheet piles or sheeting.
10. Piles to reach firm strata.

Method commonly used in Nepal

The most common method is replacement with a compacted sand or gravel cushion, along with compaction and increasing the footing size, for ordinary buildings, because it is cheap, uses local materials and manual labour, and needs no special equipment. For larger works in soft soil, sub-soil drainage, stone columns and pile foundations are used. For many Kathmandu valley sites with soft clay and a high water table, these methods plus raft foundations are adopted.