---
title: "Compensated foundation and types of mat"
subject: "Foundation Engineering"
chapter: 7
key: "semester_5/foundation_engineering:compensated-foundation-mat-types"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/compensated-foundation-mat-types
site: BCT Engineering
---

# Compensated foundation and types of mat

Foundation Engineering · Chapter 7: Mat Foundations

- Asked in: 2078 Bhadra, 6 marks; 2076 Asoj, 2+4 marks; 2074 Chaitra, 1+5 marks; 2072 Chaitra, 2 marks; 2080 Baisakh, 4 marks; 2080 Bhadra, 2 marks; 2074 Asoj, 2 marks (asked 7 times)
- Video: [Unit 3/ Types of Mat foundation](https://www.youtube.com/watch?v=Ip1o7ZNgT7o) — ADLIN
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/compensated-foundation-mat-types
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-7

## Question

Explain the concept of compensated (floating) foundation. Describe with sketches the various types of mat foundations and their suitability.

## Short answer

Compensated foundation: raft placed so weight of excavated soil equals structure weight, giving zero net pressure and minimal settlement. Mat types: flat plate (light loads), thickened under columns (punching resistance), beam-and-slab (heavy unequal loads), slab with basement walls, cellular raft (high-rise, compressible soil), piled raft (very weak soil).

## Answer

#### Compensated (floating) foundation
A **compensated foundation** is a raft or mat placed at a depth such that the weight of soil excavated is equal to (fully compensated) or a large part of (partially compensated) the weight of the structure. The net pressure on the soil is then zero or small, so settlement is very small, like a ship floating in water.

Net pressure: $q_{net} = \dfrac{Q}{A} - \gamma D_f$. When $q_{net} = 0$, $D_f = \dfrac{Q}{\gamma A}$. It is used on soft, compressible clays and for buildings with basements.

#### Types of mat foundation
```text
 (a) Flat plate              (b) Thickened under columns
     |     |     |               |     |     |
 ===================         ====[=====[=====[====   <- slab
                                  \___/ \___/        <- thickened pads

 (c) Beam and slab           (d) Cellular (box) raft
     |     |     |               |     |     |
 ===================         ====================   <- top slab
  |||   |||   |||            |  [  ] | [  ] | [  ] |  <- walls, hollow cells
 -------------------         ====================   <- bottom slab
   (beams below slab)
```
1. **Flat plate (uniform thickness) mat:** a plain slab of uniform thickness. Suitable for light loads and closely spaced columns.
2. **Flat plate thickened under columns:** pedestals or drop panels increase the punching shear resistance. For heavier column loads with uniform spacing.
3. **Beam and slab mat:** beams in both directions with a slab below or above. Stiff and economical for heavy and unequal column loads and large spans.
4. **Slab with basement walls (rigid frame):** the basement walls act as stiffeners. Used where a basement is needed.
5. **Cellular (box) raft:** a bottom slab, a top slab and walls forming hollow cells. Very stiff, reduces differential settlement, and the hollow space reduces net pressure. For high-rise buildings on compressible soils.
6. **Piled raft:** the mat is supported on piles, used when settlement is large or the soil is very weak.

#### Suitability
Mats are used where the allowable pressure is low (spread footings would cover more than about 50% of the plan area), where the soil is erratic or compressible, for uplift due to a high water table, and for tall or heavy buildings needing uniform settlement.

## Related in Foundation Engineering

- [Why foundations; types of foundation](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awhy-foundations-types) — Foundation Engineering, chapter 1
- [Conventional design of mat foundation](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Amat-conventional-design) — Foundation Engineering, chapter 7, asked 3×
- [Why design a mat foundation](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awhy-design-mat) — Foundation Engineering, chapter 7
- [Fully compensated raft and its depth](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Afully-compensated-raft-depth) — Foundation Engineering, chapter 7
- [Classify shallow and deep foundations](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aclassify-foundations-soil) — Foundation Engineering, chapter 1, asked 4×
- [Classification of pile foundations](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-classification-material-load) — Foundation Engineering, chapter 8

## Related in other subjects

- [Piles for Foundations: suitability for apartments](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Apiles-for-foundations-apartments) — Communication English, chapter 2
- [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
- [When pile and mat foundations are chosen](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Apile-mat-scenarios) — Building Technology, chapter 2
- [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×