Foundation Practical

8/3/2019 Foundation Practical

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Designing a Foundation

Foundation

Foundation is the part of structure below plinth level up to the soil. It is in direct contactof soil and transmits load of super structure to soil. Generally it is below the groundlevel. If some part of foundation is above ground level, it is also covered with earthfilling. This portion of structure is not in contact of air, light etc, or to say that it is thehidden part of the structure.

Depth of Foundation

Depth of foundation depends on following factors.1. Availability of adequate bearing capacity2. Depth of shrinkage and swelling in case of clayey soils, due to seasonal changes whichmay cause appreciable movements.3. Depth of frost penetration in case of fine sand and silt.

4. Possibility of excavation close by5. Depth of ground water table6. Practical minimum depth of foundation should not be less than 50 cm. to allowremoval of top soil and variations in ground level.7. Hence the best recommended depth of foundation is from 1.00 meter to 1.5 meterfrom original ground level.

Footing

Footing is a structure constructed in brick work, masonry or concrete under the base of awall or column for distributing the load over a l arge area.

Width of Foundation/Footings

The width of footings should be laid according to structural design. For light loadedbuildings such as houses, flats, school buildings etc having not more than two storeys,the width of foundation is given below.1. The width of footing should not be l ess than 75 cm for one brick thick wall.2. The width of footing should not be less than 1 meter for one and half brick wall.

Different Processes in Foundation Work

The processes executed in the foundation works are given below.1. Excavation of earth work in trenches for foundation.2. Laying out cement concrete.3. Laying the footing in case of raft or column construction.4. Laying Anti termite treatment.5. Laying Brick work up to plinth level.6. Laying Damp proof course on the walls.7. Refilling of earth around the walls8. Refilling of earth in the building portion up to the required height according to plinthlevel.







Precautions while designing 'Foundation'

1. A foundation should be designed to transmit combined dead load, imposed load

and wind load to the ground.2. Net loading intensity of pressure coming on the soil should not exceed the safebearing capacity.

3. Foundation should be designed in such a way that settlement to the ground islimited and uniform under whole of the building to avoid damage to thestructure.

4. Whole design of the foundation, super structure and characteristics of the groundshould be studied to obtain economy in construction work.

R atio of Cement Concrete and Mortar for Foundation

1. The cement concrete 1:8:16 is generally used in the foundation of walls inconstruction work.

2. In case of column raft cement concrete 1:4:8 is the best recommended ratio forit in the foundation.3. For brick masonry cement mortar 1:4 to 1:6 is used as loading condition.

In case of column and raft footings up to plinth level cement concrete 1:2:4 or 1:1.5:3are used.Soil having Safe Bearing Capacity

Dry coarse and well graded dense sand have maximum shear resistance and maximumbearing capacity. In general submerged soil and clay have less bearing capacity.



Precautions during Excavation of Foundation Work

The depth and width of foundation should be according to structural design.

1. The depth of the foundation should not be less than 1 meter in case the design isnot available.

2. The length, width and depth of excavation should be checked with the help of center line and level marked on the burjis.3. The excavated material/ earth should be dumped at a distance of 1 meter from

the edges.4. Work should be done on dry soil.5. Arrangement of water pump should be made for pumping out rain water.6. The bottom layer of the foundation should be compacted.7. T here should be no soft places in foundation due to roots etc .8. A ny soft/ defective spots should be dug out and be filled with concrete/

hard material

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Foundation Practical