---
title: "When pile and mat foundations are chosen"
subject: "Building Technology"
chapter: 2
key: "semester_6/building_technology:pile-mat-scenarios"
source: https://www.bctengineer.com/q/semester_6/building_technology/pile-mat-scenarios
site: BCT Engineering
---

# When pile and mat foundations are chosen

Building Technology · Chapter 2: Foundations

- Asked in: 2081 Chaitra, 4 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/pile-mat-scenarios
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-2

## Question

Explain the scenarios in which pile foundation and mat foundation are commonly chosen.

## Short answer

Pile foundations are used where shallow soils are weak, loads are heavy, water table is high, uplift or lateral forces exist, or ground is unstable. Mat foundations suit low bearing capacity soils with closely spaced columns, needing reduced differential settlement, basements below groundwater, or seismic regions.

## Answer

#### Pile foundation is chosen when
- The top soil is weak, soft or loose to a large depth, and a hard layer lies deeper (end-bearing piles); or the soil is uniformly soft and friction piles are used.
- Building loads are heavy (high-rise buildings, bridges, towers) and settlement is critical.
- The site has a high water table, is submerged, or is near a river where scour is expected.
- The foundation must resist uplift or lateral load (towers, retaining structures, jetties).
- Expansive, liquefiable or filled ground is present.
- Shallow excavation near existing structures is not possible.

#### Mat (raft) foundation is chosen when
- The soil has low bearing capacity, so that individual footings would cover more than 50 % of the plan area.
- Loads are heavy and columns are closely spaced, so footings would overlap.
- Differential settlement must be reduced, such as on compressible or non-uniform soils.
- The basement is below the water table, so the raft acts as a waterproof floor and resists uplift.
- The structure is a silo, chimney, tank, or multi-storey building with a basement.
- The site is in an earthquake area, where a single rigid slab gives better behaviour.

Piles transfer the load to depth, while a mat spreads it over a large area at shallow depth; a mat is therefore cheaper when the soil near the surface is moderately firm.

## Check yourself

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

1. A mat foundation is chosen when individual footings would cover
   A. More than 50 % of plan area
   B. Less than 10 % of plan area
   C. Exactly 20 % of plan area
   D. No area
   Answer: A. If footings cover over 50 % of the area, a raft is better. (Key point: Mat when)

2. Why is a raft good for a basement below the water table?
   A. It acts as a waterproof floor and resists uplift
   B. It lowers the water table
   C. It needs no concrete
   D. It increases settlement
   Answer: A. A raft resists uplift and provides a watertight floor. (Key point: Basement)

3. Which suits a pile foundation?
   A. Firm surface soil with light loads
   B. Soft soil to large depth with heavy loads
   C. Dry rocky site
   D. Small single-storey house on hard soil
   Answer: B. Piles transfer load deep when surface soil is weak or loads heavy. (Key point: Pile reasons)

## Related in Building Technology

- [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
- [Situations demanding pile foundation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Apile-situations) — Building Technology, chapter 2, asked 2×
- [Foundations on black cotton soil](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ablack-cotton-foundation) — Building Technology, chapter 2
- [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×
- [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
- [Common problems of existing foundations](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aproblems-existing-foundation) — Building Technology, chapter 2, asked 2×

## Related in other subjects

- [Compensated foundation and types of mat](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Acompensated-foundation-mat-types) — Foundation Engineering, chapter 7, asked 7×
- [Classification of pile foundations](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-classification-material-load) — 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
- [Mat foundation design procedure](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Amat-foundation-design-procedure) — Design of RCC Structure, chapter 11, asked 3×