---
title: "What is an isobar diagram"
subject: "Soil Mechanics"
chapter: 8
key: "semester_4/soil_mechanics:isobar-diagram-define"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/isobar-diagram-define
site: BCT Engineering
---

# What is an isobar diagram

Soil Mechanics · Chapter 8: Vertical Stresses Below Applied Loads

- Asked in: 2075 Baisakh, 1 marks
- Video: [Isobar diagram | Pressure bulb Diagram | for 0.1Q | Soil Mechanics | Chapter 8](https://www.youtube.com/watch?v=h9yvHBhICdw) — Sudip Dhakal
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/isobar-diagram-define
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-8

## Question

What is an isobar diagram?

## Short answer

An isobar diagram (pressure bulb) is a contour plot joining points below a loaded area that experience the same vertical stress increase, typically expressed as a fraction of the contact pressure q (e.g., 0.2q). The bulb-shaped curves show how stress spreads and diminishes with depth.

## Answer

An **isobar diagram** (pressure bulb) is a plot of contours (isobars) joining points below a loaded area that have the same vertical stress increase, usually expressed as a fraction of the contact pressure $q$ (e.g. 0.8q, 0.5q, 0.2q, 0.1q). The contours form bulb-shaped curves under the foundation and show how stress spreads and dies out with depth.

## Check yourself

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

1. An isobar is a contour joining points below a loaded area that have the same
   A. settlement
   B. vertical stress increase
   C. pore pressure only
   D. horizontal distance
   Answer: B. Isobars join points of equal vertical stress increase, usually as a fraction of q. (Key point: Meaning of isobar)

2. What shape do the isobars form under a foundation?
   A. Straight horizontal lines
   B. Concentric squares
   C. Bulb-shaped closed curves
   D. Parallel vertical lines
   Answer: C. The contours form bulb-shaped curves, hence the name pressure bulb. (Key point: Shape)

3. What does an isobar diagram show?
   A. How stress spreads and dies out with depth
   B. The water table position
   C. The shear strength profile
   D. The grain size distribution
   Answer: A. It shows how stress spreads out and decreases with depth below the load. (Key point: Purpose)

## Related in Soil Mechanics

- [Draw isobar diagram of 0.1Q](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aisobar-diagram-point-one-q) — Soil Mechanics, chapter 8