Shallow Foundations MCQ with answer and solution

Concept

Expansive soils undergo volumetric changes with changes in moisture content. Vegetation, especially large trees, exacerbates moisture variations through transpiration, making it critical to maintain a safe distance between trees and building foundations.

Step-by-step solution

1. Analyze Statement 1: Big trees near buildings on expansive soils cause severe moisture depletion and differential settlement due to shrinkage; hence, it is NOT good. 2. Analyze Statement 2: IS:1904 specifies that large trees should not be planted within a minimum distance of 6 m from the foundations of buildings on expansive soils to prevent moisture-related distress. 3. Analyze Statement 3: Having large trees within 3 to 4 m is dangerous and violates safety recommendations. 4. Conclude that only Statement 2 is correct.

Explanation

According to IS 1904 (Code of practice for design and construction of foundations in soils: General requirements), trees extract large amounts of water from the soil, leading to severe shrinkage and cracking in expansive soils (black cotton soils). Therefore, having big trees near buildings on expansive soils is detrimental, and IS 1904 specifically recommends that no tree which grows to a large size should be planted within 6 m of the foundations of buildings in expansive soils.

Why the other options are wrong

A: Statement 1 is incorrect because big trees near expansive soils cause harmful moisture changes and differential settlement. B: Statement 1 is incorrect. D: Statement 3 is incorrect because 3 to 4 m is too close and violates the 6 m safe distance requirement.

Shortcut

Remember the thumb rule for expansive soils: 'No large trees within 6 meters of foundations'.

Common mistake

Believing that trees help stabilize moisture by absorbing water; while they absorb water, in expansive soils this causes extreme drying, shrinkage, and severe structural cracking.