---
title: "Foundation selection factors, excavation in wet areas"
subject: "Building Technology"
chapter: 2
key: "semester_6/building_technology:excavation-selection-wet-080c"
source: https://www.bctengineer.com/q/semester_6/building_technology/excavation-selection-wet-080c
site: BCT Engineering
---

# Foundation selection factors, excavation in wet areas

Building Technology · Chapter 2: Foundations

- Asked in: 2080 Chaitra, 2+4+2 marks
- Video: [Selecting Type of Foundation from Type of Soil?](https://www.youtube.com/watch?v=ddkroD9aIAQ) — Engineering Motive
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/excavation-selection-wet-080c
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-2

## Question

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?

## 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.

## Answer

#### 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.

```text
 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.

## Check yourself

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

1. Which is a factor in selecting a foundation?
   A. Bearing capacity and soil type
   B. Colour of bricks
   C. Name of contractor
   D. Wall paint
   Answer: A. Soil bearing capacity, loads, water table and settlement govern selection. (Key point: Selection factor)

2. In the sump and ditch method:
   A. A drain leads water to a sump deeper than foundation level, where it is pumped
   B. A vacuum pump lowers the water table
   C. Chemicals are injected
   D. Sheet piles are frozen
   Answer: A. Water collects in the sump and is pumped out with a centrifugal pump. (Key point: Sump and ditch)

3. The well-point system works by:
   A. Freezing soil
   B. Driving sheet piles only
   C. Digging large open ponds
   D. Jetting perforated pipes around the excavation connected to a vacuum pump to lower the water table
   Answer: D. It keeps soil dry and stable; good for sand and silt. (Key point: Well-point)

4. Which method is most common in Nepal?
   A. Electro-osmosis
   B. Sump and ditch
   C. Freezing
   D. Caissons
   Answer: B. It is simple, cheap and needs little equipment. (Key point: Method in Nepal)

## Related in Building Technology

- [Factors affecting foundation design, improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afactors-design-improve-bc) — Building Technology, chapter 2
- [Earthwork excavation in soft soil, hard rock, wet soil](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexcavation-soft-hard-wet) — Building Technology, chapter 2
- [Criteria for site selection](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asite-selection-criteria) — Building Technology, chapter 1
- [Excavating foundation trenches in water-logged ground](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexcavation-wet-methods) — Building Technology, chapter 2, asked 2×
- [Factors in construction stage of trench excavation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Atrench-excavation-factors) — Building Technology, chapter 2
- [Factors affecting thermal performance](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Athermal-perf-factors) — Building Technology, chapter 1

## Related in other subjects

- [Factors affecting selection of pile type](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-type-selection-factors) — Foundation Engineering, chapter 8
- [Common RoR intakes in Nepal; site selection](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-common-intakes-nepal-location) — Hydropower Engineering, chapter 5, asked 2×
- [Areas of utilization of electrical energy in Nepal](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Autilization-areas-nepal-improve-consumption) — Utilization of Electrical Energy, chapter 1
- [Reservoir intake and site selection factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Areservoir-intake-site-factors) — Water Supply Engineering, chapter 5, asked 2×