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Foundation selection factors, excavation in wet areas

Short answer

Foundation selection depends on soil bearing capacity, loads, firm stratum depth, water table, settlement, structure type, seismic zone, expansive soil, adjacent structures, cost, and codes. Wet area excavation methods include sump and ditch, well-point, deep wells, cofferdams, caissons, freezing, and electro-osmosis. Sump and ditch is most common in Nepal.

What are the factors affecting selection criteria of foundation? List various methods used for excavation of foundation in wet areas and explain any three methods. Which method is commonly used in Nepal?

Factors affecting selection of foundation

  1. Bearing capacity and type of soil.
  2. Magnitude and nature of the loads.
  3. Depth of the firm stratum and the water table.
  4. Allowable settlement, particularly differential.
  5. Type and use of structure (flexible or rigid).
  6. Seismic zone and liquefaction potential.
  7. Presence of expansive or filled soils; aggressive chemicals.
  8. Adjacent structures; site topography, access and equipment.
  9. Cost, time and materials.
  10. Codes and bye-laws.

Methods of excavation in wet areas

  1. Sump and ditch (open pumping).
  2. Well-point system.
  3. Deep wells.
  4. Cofferdams or sheet piling.
  5. Caissons.
  6. Freezing and chemical grouting.
  7. Electro-osmosis.

Three methods explained

Sump and ditch method: A drain is cut around the bottom of the excavation, leading to a sump pit deeper than the foundation level. Water collected in the sump is pumped out with a centrifugal pump. Suited to shallow excavation and low inflow, in coarse soils.

Well-point system: Many small perforated pipes are jetted into the ground at 1 to 1.5 m spacing around the excavation and connected by a header to a vacuum pump. This lowers the water table below the excavation floor, and keeps the soil dry and stable. Good for sand and silt, depth up to about 5 m for a single stage.

Sheet piling / cofferdam: Interlocking steel or timber sheet piles are driven around the area to cut off the water and hold back the soil; the water inside is pumped out. Used for deep excavations and near rivers.

 Sump:  \_______/ <- foundation level   Well point: |||  |||
         [sump]--pump                     header + pump

Method commonly used in Nepal

The sump and ditch method (open pumping) is the most common in Nepal because it is simple and cheap, and needs little equipment. The well-point method is used for larger works.

✓ Answer checked by Claude Opus 5.5 (high effort)

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