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Excavating foundation trenches in water-logged ground

Short answer

In Nepal, sumps and ditches collect seepage in a trench leading to a sump pit which is pumped out; simple and cheap for shallow depths. The well-point system uses perforated pipes around the trench connected to a suction pump to lower the water table, suitable for sandy soil and deeper works.

“Excavation in foundation trenches in ground having high water table or in water logged area creates greater problem”. How will you deal in such situation? Critically describe two methods that are commonly used in Nepal.

Excavation in water-logged ground or with a high water table is difficult because the sides collapse, the bottom becomes soft, the soil strength falls, and the work cannot be done in dry condition. Therefore the water must be removed or kept out of the excavation, and the sides supported.

Methods commonly used in Nepal

1. Sumps and ditches (open pumping)

  • A trench (drain) around the excavation at its bottom collects seepage and leads it to a sump (a pit deeper than the foundation level).
  • Water is pumped out from the sump by centrifugal or submersible pumps, so the excavation stays dry.
  • Simple and cheap, used for shallow depth and low inflow. Not suited for fine sand (boiling and piping).
 GL
  \         /
   \_______/   <- foundation level
   ditch  [sump]-- pump

2. Well-point system (dewatering)

  • Many small perforated pipes (well points) about 1 to 1.5 m apart are jetted into the ground around the trench and joined to a header pipe and a suction pump.
  • The water table is lowered below the excavation level, so the soil is dry and stable.
  • Good for sandy and silty soil with deeper trenches. It needs skilled labour and equipment.

3. Other methods (not in common use) Cofferdams with sheet piles, deep wells, and the freezing or chemical method.

In Nepal, the sump and ditch method is most common because it is cheap, and the well-point method is used for large works. Timber or steel shoring is added to support the sides, and the work is done in the dry season where possible.

✓ Answer checked by Claude Opus 5.5 (high effort)

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