---
title: "Classify shallow and deep foundations"
subject: "Foundation Engineering"
chapter: 1
key: "semester_5/foundation_engineering:classify-foundations-soil"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/classify-foundations-soil
site: BCT Engineering
---

# Classify shallow and deep foundations

Foundation Engineering · Chapter 1: Introduction

- Asked in: 2076 Chaitra, 2 marks; 2074 Chaitra, 2 marks; 2074 Asoj, 2 marks; 2075 Asoj, 2 marks (asked 4 times)
- Video: [Shallow and Deep Foundations | Different Types of Foundations | Geotechnical Engineering](https://www.youtube.com/watch?v=6tKefSUsyuY) — Geotechnical Engineering Consultancy
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/classify-foundations-soil
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-1

## Question

Classify the shallow and deep foundations according to their soil conditions (how would you select the suitable type of foundation according to soil conditions?).

## Short answer

Shallow foundations are used where firm soil is near the surface: isolated footings for light loads, raft when footings cover more than half the plan area. Deep foundations (piles, wells) are used when weak upper soil overlies a deep strong layer or for large scour. Try the cheapest shallow type first.

## Answer

Foundations are chosen by comparing the bearing capacity and compressibility of the soil layers with the building load. They fall in two groups.

#### Shallow foundations ($D_f \le B$)
Used where a firm layer is near the surface.

| Soil condition | Suitable foundation |
|---|---|
| Good soil at shallow depth, light loads | Isolated spread footing / wall (strip) footing |
| Columns close together, or weak soil needing more area | Combined or strap footing |
| Weak soil, heavy load, columns cover more than ~50% of the plan area, differential settlement problem | Raft (mat) foundation |

#### Deep foundations ($D_f > B$)
Used where the upper soil is weak, compressible, or scour-prone and a good layer lies deep.

| Soil condition | Suitable foundation |
|---|---|
| Soft soil over hard stratum or rock at moderate depth | End-bearing pile / pier |
| Deep soft soil with no hard layer | Friction pile |
| Large river bed, scour, bridge piers | Well (caisson) foundation |
| Very heavy loads, uplift or lateral loads | Bored/driven pile group, drilled shaft |

**Selection rule:** try the cheapest shallow type first; if the footings would cover more than about half the plan area or settlement is excessive, go to a raft; if still unsafe, use piles or wells.

## Check yourself

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

1. A foundation is classed as shallow when
   A. Df > B
   B. Df/B > 5
   C. Df <= B
   D. it rests only on rock
   Answer: C. Shallow foundations have Df <= B; deep ones have Df > B. (Key point: Shallow foundation limit)

2. When is a raft (mat) foundation selected?
   A. Good soil near the surface with light loads
   B. Weak soil, heavy load, and columns covering more than about 50% of plan area
   C. Soft soil over rock at moderate depth
   D. A large river bed with scour
   Answer: B. Footings would cover over half the plan area, so one slab under the whole structure is used. (Key point: Raft selection)

3. Which foundation suits deep soft soil with no hard layer within reach?
   A. End-bearing pile
   B. Isolated spread footing
   C. Friction pile
   D. Strip footing
   Answer: C. With no hard stratum to rest on, load is carried by skin friction along the pile. (Key point: Friction vs end-bearing pile)

4. What is the recommended order in selecting a foundation?
   A. Start with piles and move to footings if too costly
   B. Start with a raft in every soil
   C. Choose wells first for all sites
   D. Try the cheapest shallow type first, then raft, then piles or wells if still unsafe
   Answer: D. The selection rule moves from cheap shallow footings to raft to deep foundations only if needed. (Key point: Selection rule)

## Related in Foundation Engineering

- [Shallow versus deep foundation](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Ashallow-vs-deep-foundation) — Foundation Engineering, chapter 1
- [When piles are preferred to shallow foundations](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-vs-shallow-conditions) — Foundation Engineering, chapter 8, asked 3×
- [Define and classify pile foundations](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-classification-define) — Foundation Engineering, chapter 8
- [Why foundations; types of foundation](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awhy-foundations-types) — Foundation Engineering, chapter 1
- [Classification of pile foundations](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-classification-material-load) — Foundation Engineering, chapter 8
- [Purposes of foundation soil improvement](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Asoil-improvement-purposes) — Foundation Engineering, chapter 10

## Related in other subjects

- [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
- [Shallow and deep wells](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ashallow-deep-wells) — Water Supply Engineering, chapter 2, asked 4×
- [Foundations on black cotton soil](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ablack-cotton-foundation) — Building Technology, chapter 2
- [Types of shallow foundation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ashallow-foundation-types) — Building Technology, chapter 2