SPECIAL FOUNDATIONS


(a) Stepped Foundations : In this case, the foundation is provided in the form of steps of concrete. Excavation, also, is carried out in the forrn of steps. The height of each step is kept uniform. The top surface of each step is cleaned an concrete is laid over it.

These are used when the ground is sloping as in hilly areas in order to avoid slping foundation bed or excessive depth of foundation.

(b) Grillage Foundations : These are used to transmit heavy loads from steel columns to foundation soils having low bearing power. It avoids deep excavations and provides necessary area at the base to reduce the intensity of pressure to be transmitted into the sub-soil.

The foundation in this case consists of one or two tiers of wooden or rolled steel sections. The space in between the sections is filled up with concrete. It is called grillage foundation as the bed constructed is called grillage. It helps in minimizing the intensity of pressure so that the same does not exceed the safe bearing capacity of the soil.

Construction Detail :

In this case, first of all a trench is excavated. The surface is well compacted so that a uniform level is maintained. After this a layer of lean cement concrete (1 : 8 : 16) is laid for a thickness of about 30 cm. It is uniformly compacted and over it another layer of rich cement (1 : 2 : 4) is laid for about 15 cm in thickness and compacted. It forms a concrete bed. On this bed, the bottom tier consisting of number of steel I-beams are placed. These are separated by spacer bars. The in-between space is filled with cement concrete. Then over this, another layer of I-beams is placed. The space in-between is again filled with cement concrete. This bed is called grillage bed. Over it, the structure in the form of steel stanchion.
Such foundations are useful for structures like stanchion, piers and columns which are to support heavy concentrated loads. Therefore, for supporting heavy loads of the buildings like cinema halls, factories etc., grillage foundations are generally provided.

(c) Inverted Arch Foundations : These are not adopted these days as the end piers have to be specially strengthened by buttresses to prevent arch thrust rupturing the pier function. The advantage is that in soft soils, the depth of foundation is greatly reduced.

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