BALANCING RAFT OR FLOATING RAFT

Balancing rafts or floating rafts are used when the bearing capacity of the soil is very low. This foundation protects against soil settlements and works on the principle that the total weight of the soil and water excavated for the foundation equals to the total load of the proposed structure. They are called zero settlement structures and are used for high rise towers with multiple basements. This type of foundation is preferred only when the soil is very compressive and soft and even friction piles are not able to support the loads.

ALSO READ : TYPES OF RAFT FOUNDATIONS

Also Read : PILE FOUNDATIONS-TYPES AND ADVANTAGES

HOW FLOATING RAFT WORKS

How floating raft works
How floating raft works

The basic concept of a floating raft is countering the weight of excavated soil and water with the total load of the structure resting over it. ie. The same weight of excavated materials is replaced by a structure with the same weight. Because of these balancing floating rafts are also known as balancing raft foundations or compensated foundation. The vertical pressure at the foundation depth remains same before and after excavation.

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CONCERNS OF FLOATING RAFT

Execution of a floating foundation demands massive earthwork excavation in high water table areas. Open dewatering may not be sufficient to control a large amount of water flowing in the excavated areas. Wellpoint dewatering systems have to be run simultaneously with the excavation to lower the water table.

Excavation of floating foundations can trigger soil collapse and soil scoring from the bottom of existing foundations. Sheet piles have to be erected to protect the building against any possible damage during excavation.

Materials required for temporary retaining walls like sandbags, PCC blocks, etc has to be organised and mobilised before starting excavation. Temporary retaining walls have to be constructed in tandem with excavation works.

Heave formation
Heave formation

Heave formation can always happen while doing deep excavations because of the weight of soil on both sides of the excavation pit. Heaves formations cannot be avoided in any case but have to be controlled or else it may initiate foundation settlements. Heave formation can be minimized by further lowering the water table to a depth below the founding level of the structure. Installation of friction piles or micro piles on partially completed excavation surfaces can also control heave formation to an extent. Concrete bands are provided at the bottom level to control heave formation.

Balancing raft foundations are adopted for high rise buildings with multiple basements. Execution of these foundations requires highly skilled professionals and micro-level supervision supported by an experienced and skilled workforce. All safety precautions have to be done to protect the nearby structures from any damage.

VIDEO : RAFT FOUNDATION – TYPES AND ADVANTAGES

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