Grillage foundation consists of one or more tiers of beams (steel or timber) superimposed at right angles to each other on a concrete layer to disperse load over an extensive area.
This type of foundation is generally used for heavy structural columns, piers, and scaffolds where the foundation is expected to disperse heavy loads over large areas.
With the use of a grillage foundation, deep excavations can be avoided, and the depth of the foundation can be limited from 1 m to 1.5 m.
Types of Grillage Foundations
Steel grillage foundation
Timber grillage foundation
1. Steel Grillage Foundation
The steel grillage foundation consists of steel beams placed in single or double tiers and enclosed concrete.
The grillage beams of each tier are held in position by 25 mm-diameter pipe separators and 20 mm spacer bars. The beams are spaced appropriately to allow the placement and compacting of concrete between them.
It is recommended that a minimum clearance of 8 cm be maintained between the two beams. In any event, the distance between the beams’ flanges should not exceed 1.5 to 2 times the flange width, with a maximum of 30 cm.
A minimum cover of 10 cm is maintained on the outside edges of the external beams and above the upper flange of the top tier to protect the beams from corrosion. The concrete cover under the lower beam should not be less than 15 cm.
2. Timber Grillage Foundation
Timber grillage foundation is used where the soil is soft and is permanently water-logged. A suitably designed timber grillage foundation can economically support the building walls.
By restricting the soil weight to 5.5 tonne/sq.mt, a wood platform replaces the concrete block that is generally given below the wall footing in this construction style.
The wood platform comprises boards that are typically 8 to 10 cm thick and are stacked in two layers, one longitudinal and the other across the wall, reaching 45 to 60 cm beyond the footing foundation on each side.
The boards are 5 cm to 10 cm thick at the bottom levels, depending on the loads and site conditions. Rectangular portions of timber spaced at no more than 38 cm center to center, with a depth of 0.75 times the width, divide the two layers of planks.
Construction of Grillage Foundation
The steps followed for the construction of the grillage foundation are listed below:
The framework must hold the grillage in place for a solid monolithic grillage foundation.
The formwork is prepared from edging boards in the form of rectangular troughs.
The height of the formwork ranges from 1-1.5 ft, and the width shall be equal to the minimum wall thickness of the building
Concrete is poured into the formwork to form a concrete pad of 450 mm to 600 mm.
The grillage sections (timber or steel) are placed above the concrete pad with sufficient cover on all the sides.
Concrete is poured into the foundation up to the desired depth.
A minimum cover of 10 mm shall be maintained at the top grillage to avoid corrosion of steel beams.
Placing of Grillage Foundation
Advantages of Grillage Foundation
The time required for the installation and casting of grillage foundation is comparatively less than that of conventional foundation.
Grillage foundation is capable of successfully transforming heavy loads over a larger area.
These foundations are used to construct significant structures such as column piers and scaffolds.
Grillage foundation avoids deep excavations in soft soils, and the depth of the foundation can be limited to 1-1.5 m.