Grillage Foundation

is used when heavy structural loads from columns, piers or

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stanchions are required to be transferred to a soil of low bearing capacity. is often found to be lighter and more economical. This avoids deep excavation and provides necessary area at the base to reduce the intensity  of pressure within safe bearing capacity of soil. soil . Depending upon the material used in construction of grillage foundation it can be broadly  divided in the following two categories :

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consists of steel beams also known as grillage

beams which are provided in single or double tiers. In case of  double tier grillage foundation, the top tier is laid at right angles to the bottom one. The grillage beams of each tier are held in position by 20 mm spacer bars with 25 mm diameter pipe separators. The beams are suitably spaced so as to provide facility for the placing and compacting of concrete between them. A minimum clearance of 8 cm is considered most suitable. In any case, the distance between the flanges of the beams should not be more than one and

half to two times the flange width with a maximum of 30 cm. If  the beams are spaced more distance apart, there is a danger of the concrete filling not acting monolithically with the beams, and as such, may result in the failure of the foundation. In order to protect the beams against corrosion, a minimum cover of 10 cm is kept on the outer sides of the external beams as well as above the upper flange of the top tier, Cover of concrete under the lower beam should not be less than 15 cm.

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Excavations are carried to the designed depth and the bed is well leveled. This foundation bed is covered with a layer of rich

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mixture of concrete not less than 15 cm in thickness. This is well compacted so as to make the layer of concrete an impervious bed. Grillage beams of the designed dimensions are then placed on this bed of concrete at specified distance apart using separators. The upper surface of the grillage beam flanges is brought in a horizontal plane and rich cement grout is poured all round the lower flanges of the beams so as to secure the beams to the concrete bed. The concrete is then placed between and around the beams. If another tier is required to be provided, it is kept at right angles to the already laid tier and the entire space is filled with concrete.

Where the soil encountered is soft and is permanently water-logged building walls can be economically supported by suitably designed timber grillage foundation. Timber grillage foundation can be safely used for light buildings by limiting the loading on the soil to 5.5 tonne/sq.mt. In this type of construction, the concrete block usually provided below the wall footing is replaced by timber platform. The timber platform consists of planks usually 8 cm to 10 cm thick, arranged in two layers, one longitudinal and the other across the wall extending beyond the footing base by about 45 cm to 6O cm on either side. In the lowermost layers, the planks are 5 cm to 10cm thick depending upon the loading and site conditions. The two layers of planks are separated by rectangular sections of  timber spaced at not more than 38cm center to center, the depth of the sections being 0.75 times the width.

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Where reinforcement is usually created by surrounding the structure with a concrete base, here, a steel grillage (which is a rigid construction that's placed at sufficient depth below ground that the weight of soil removed to input the grillage equals it in weight) can be used in replacement. Increasingly, for structures such as road signs or railway gantries, grillages have been designed to consist of one or more steel beams (an interface) mounted on underlying helical piles to give extra support. The fast installation and the fact that the grillage is fabricated off-site make this a very  suitable yet versatile foundation solution.

Steel grillages have many benefits, some of these include 

Speed of installation - using a grillage often permits valuable time savings.

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Convenience - avoidance of poured concrete saves time and simplifies installation.

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Reduced costs - through minimal disruption of transport infrastructure.

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Versatility - the technology suits many different diff erent applications.