---
title: "Earthwork excavation in soft soil, hard rock, wet soil"
subject: "Building Technology"
chapter: 2
key: "semester_6/building_technology:excavation-soft-hard-wet"
source: https://www.bctengineer.com/q/semester_6/building_technology/excavation-soft-hard-wet
site: BCT Engineering
---

# Earthwork excavation in soft soil, hard rock, wet soil

Building Technology · Chapter 2: Foundations

- Asked in: 2076 Baisakh, 8 marks
- Video: [FOUNDATION IN WATERLOGGED & FILLED UP LOOSE SOIL-STEP BY STEP CONSTRUCTION-A2Z Construction](https://www.youtube.com/watch?v=VeRsML2hWB8) — A2Z Construction
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/excavation-soft-hard-wet
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-2

## Question

Explain various methods of earthwork excavation in soft soil, hard rock and wet soil.

## Short answer

Soft soil: manual or machine excavation, with sloped sides or timber shoring above 1.5 m. Hard rock: wedging, pneumatic breakers, blasting, rippers, or non-explosive agents. Wet soil: dewater by sump and pump, well-points, deep wells, or sheet piling, and work in dry conditions.

## Answer

#### Excavation in soft soil
- **Manual**: Using spade, pick-axe, and shovel for shallow excavation of earth, clay, sand and murrum; labour is cheap in Nepal.
- **Mechanical**: Backhoe excavators, bulldozers and loaders for large areas.
- Sides are sloped or supported by timber **shoring** (planking and struts) for depth over 1.5 m, to avoid collapse.
- Spoil is placed at least 1 m away from the edge; the bottom is levelled and compacted.

#### Excavation in hard rock
1. **Manual**: Crowbars, wedges, chisel, pick axes, sledge hammers for small works. Wedging with feathers and plugs splits blocks.
2. **Pneumatic breakers and drilling** with compressed air, jackhammers.
3. **Blasting**: Holes are drilled, charged with explosive (gelatin, ANFO), stemmed and fired with detonators and fuse. Needs licensed personnel, warning, and safety distance. Controlled blasting limits damage.
4. **Rippers and hydraulic hammers** on excavators.
5. **Non-explosive demolition agents** (expansive cement) where blasting is not allowed near buildings.
The rock bottom is cleaned and made level or benched.

#### Excavation in wet soil
Needs removal of water so that work is in dry conditions.
1. **Sump and ditch method**: Drains lead water to a sump from which it is pumped.
2. **Well-point system**: Pipes jetted around the trench with a vacuum pump lowering the water table.
3. **Deep wells and sheet piling / cofferdam** for deep works near water.
4. **Shoring** with close timbering for loose, saturated soils.
5. Scheduling in the dry season.

```text
  Soft: sloped sides      Wet:  sump + pump
   \       /               \____/ 
    \_____/                  [sump]
```

## Check yourself

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

1. In soft soil, sides are sloped or shored for depth over:
   A. 1.5 m
   B. 5 m
   C. 10 m
   D. 0.3 m
   Answer: A. Beyond about 1.5 m, timber shoring avoids collapse. (Key point: Shoring depth)

2. In blasting rock, the sequence is:
   A. Fire, then drill
   B. Use only crowbars
   C. Just pour water
   D. Drill holes, charge with explosive, stem and fire with detonators
   Answer: D. Holes are drilled, charged, stemmed and fired; licensed personnel needed. (Key point: Rock blasting)

3. Where is expansive cement used?
   A. To fill pits
   B. Where blasting is not allowed near buildings
   C. In soft soil
   D. For mortar
   Answer: B. Non-explosive demolition agents break rock without blasting. (Key point: Non-explosive agents)

4. The purpose of dewatering wet soil is to:
   A. Work in dry conditions
   B. Increase water pressure
   C. Raise the water table
   D. Soften the soil
   Answer: A. Water must be removed so work proceeds dry. (Key point: Wet excavation)

## Related in Building Technology

- [Earthwork excavation in soft soil and hard soil](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexcavation-soft-hard-compare) — 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
- [Foundation on hard rock](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afoundation-hard-rock) — Building Technology, chapter 2

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