---
title: "Excavating foundation trenches in water-logged ground"
subject: "Building Technology"
chapter: 2
key: "semester_6/building_technology:excavation-wet-methods"
source: https://www.bctengineer.com/q/semester_6/building_technology/excavation-wet-methods
site: BCT Engineering
---

# Excavating foundation trenches in water-logged ground

Building Technology · Chapter 2: Foundations

- Asked in: 2079 Jestha, 4 marks; 2078 Poush, 5 marks (asked 2 times)
- Video: [FIGHTING groundwater under the foundation](https://www.youtube.com/watch?v=a9RJgUTOlI4) — NS Builders
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/excavation-wet-methods
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-2

## Question

“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.

## 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.

## Answer

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).

```text
 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is a sump in open pumping?
   A. A pit deeper than foundation level where seepage collects for pumping
   B. A pipe jetted into ground
   C. A steel sheet pile
   D. A chemical grout
   Answer: A. Ditches lead seepage to a sump from which water is pumped. (Key point: Sump)

2. Why is sump pumping unsuited to fine sand?
   A. It is too costly
   B. Sand absorbs pumps
   C. Sand stops water
   D. Boiling and piping occur
   Answer: D. Fine sand can boil and pipe with open pumping. (Key point: Limitation)

3. In the well-point system, what is done?
   A. A single large well is dug
   B. Water is frozen
   C. Trench is filled with concrete
   D. Small perforated pipes join a header and suction pump to lower the water table
   Answer: D. It lowers the water table below excavation level. (Key point: Well point)

4. Which method is most common in Nepal?
   A. Freezing
   B. Cofferdam with sheet piles
   C. Deep wells
   D. Sump and ditch
   Answer: D. Sump and ditch is cheap; well point is for large works. (Key point: Nepal use)

## Related in Building Technology

- [Factors in excavating trenches for pipes](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexcavation-pipes-factors) — Building Technology, chapter 2
- [Foundation selection factors, excavation in wet areas](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexcavation-selection-wet-080c) — Building Technology, chapter 2
- [Formwork for excavation, timbering, RCC formwork](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aformwork-timbering-062p) — Building Technology, chapter 8
- [Formwork and timbering for trench excavation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Atrench-formwork) — Building Technology, chapter 8, asked 3×
- [Deep foundation types and use of shallow foundations](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adeep-shallow-types-079a) — Building Technology, chapter 2
- [Foundation on hard rock](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afoundation-hard-rock) — Building Technology, chapter 2

## Related in other subjects

- [Septic effluent disposal, high water table/rocky](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-effluent-high-water-table-rocky) — Sanitary Engineering, chapter 9
- [Well foundation: excavation, sinking, brick, PCC curb](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Awell-foundation-d-four-items) — Estimating and Costing, chapter 6, asked 2×
- [Rectangular footing 3 m × 4 m, water table cases](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Arect-footing-3x4-backfilled) — Foundation Engineering, chapter 6
- [Circular footing on stiff clay, water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Acircular-footing-stiff-clay) — Foundation Engineering, chapter 6