Question

As per the BIS:1904 (1986), the maximum permissible settlement for raft foundations on sand is

• A. 25 mm
• B. 40 mm
• C. 40 mm to 65 mm
• D. 65 mm to 100 mm

Hint:

In geotechnical engineering, permissible settlement limits vary based on the foundation type and soil. Raft foundations on sand are designed to handle larger settlements (e.g., 40 mm to 65 mm per BIS:1904) compared to isolated footings, due to their ability to distribute loads more evenly and reduce differential settlement.

Answer 1

The Correct Option is C

Step 1: Identify the given data.
The question asks for the maximum permissible settlement for raft foundations on sand, as specified by BIS:1904 (1986). The options provided are:
(1) 25 mm
(2) 40 mm
(3) 40 mm to 65 mm
(4) 65 mm to 100 mm
We need to determine the correct value based on the standard.
Step 2: Recall the BIS:1904 (1986) standard for raft foundations.
BIS:1904 (1986) is the Indian Standard Code of Practice for Design and Construction of Foundations in Soils. It provides guidelines for permissible settlements to ensure the stability and functionality of structures. For raft foundations on sand, the standard specifies limits to control differential settlement and ensure the structure’s performance.
Step 3: Determine the permissible settlement for raft foundations on sand.
According to BIS:1904 (1986), the maximum permissible settlement for raft foundations on sand is typically in the range of 40 mm to 65 mm. This range accounts for the fact that:
Raft foundations distribute loads over a large area, reducing settlement compared to isolated footings.
Sand is a granular soil, and its settlement behavior depends on its density and the applied load.
The standard sets this range to ensure that the structure remains serviceable without excessive differential settlement.
Step 4: Compare with the given options.
25 mm (Option 1) is too low and more typical for isolated footings on stiff soils, not rafts on sand.
40 mm (Option 2) is at the lower end but does not cover the full range allowed.
40 mm to 65 mm (Option 3) matches the standard range for raft foundations on sand.
65 mm to 100 mm (Option 4) exceeds the typical permissible limit for sand, as higher values are more applicable to softer soils like clay.
Thus, the correct range is 40 mm to 65 mm.
Step 5: Select the correct option.
Based on the BIS:1904 (1986) standard, the maximum permissible settlement for raft foundations on sand is 40 mm to 65 mm.
The correct option is (3). \[ \boxed{40 \, \text{mm to} \, 65 \, \text{mm}} \]