Maximum settlement in sand and hard clay for reinforced concrete structures according to IS code

In the design of a raft foundation for reinforced concrete structures, it is important to consider the maximum allowable settlement to ensure the stability and durability of the structure. The Indian Standard code for design of concrete structures, IS 456:2000, provides guidelines for the maximum settlement in different types of soils, including sand and hard clay.

Maximum settlement in sand

According to IS code, the maximum settlement allowed in sand for reinforced concrete structures is 75 mm. Sand is a granular material with loose particles that can easily compress and settle under load. Excessive settlement in sand can lead to differential settlements, which can cause structural damage and uneven load distribution. Therefore, the maximum settlement limit of 75 mm is specified to ensure the structural integrity of the foundation.

Maximum settlement in hard clay

Hard clay is a cohesive soil with high strength and low compressibility. It exhibits less settlement compared to loose sand. According to IS code, the maximum settlement allowed in hard clay for reinforced concrete structures is also 75 mm. Although hard clay has better load-bearing capacity than sand, it can still experience settlement under the weight of the structure. Therefore, the same settlement limit as sand is specified to prevent excessive settlements and minimize the risk of structural failure.

Importance of maximum settlement limit

The maximum settlement limit specified in the IS code is based on the desired performance and safety requirements of the structure. Excessive settlements can lead to several issues, including:

1. Differential settlements: Differential settlements occur when different parts of the foundation settle at different rates. This can cause cracks in the structure and uneven load distribution, leading to structural instability.

2. Structural damage: Excessive settlements can cause structural damage such as cracks in walls, floors, and ceilings. This can compromise the integrity of the structure and reduce its service life.

3. Uneven settlement of utilities: Settlements can also affect the alignment and performance of utilities such as water supply pipes, drainage systems, and electrical conduits. Uneven settlements can lead to leaks, blockages, and disruptions in utility services.

Therefore, it is crucial to design the raft foundation with a maximum settlement limit to ensure the safety, stability, and durability of the reinforced concrete structure. The specified limit of 75 mm for both sand and hard clay in the IS code helps to achieve these objectives.