Detail of foundation and method of SBC (soldering capacity)

FOUNDATIONS

The lowest load bearing part of the building, usually located below the ground level, which transfers

the load to the soil is known as foundation.

Functions of foundations are to distribute load on soil, minimize unequal settlement, providing

stability, level surface and safety against undermining and prevention of soil movement.

Foundations are classified as shallow and deep. The minimum depth of shallow foundation should

be

where p = SBC of soil w = unit weight of soil and q = angle of repose of soil. In any case it shall not

be less than the width of wall plus 300 mm. For walls inverted arch footing may be used wherever

SBC of soil is less.

For providing footing for walls and masonry columns, trenches of required sizes are made, the soil

is rammed and a led concrete of 1 : 4 : 8 is laid to a thickness of 150 to 200 mm. Then foundation is

built with stone masonry in courses 150–200 mm depth and projecting at 50 to 75 mm is each course.

For R.C.C. columns isolated column footing, combined footing, cantilever footing, continuous

footings or raft footings are provided. For steel columns, grillage footing are ideal.

Pile foundations are the examples of deep foundations used for buildings. They may be classified as

frictional piles and bearing piles. According to the materials used piles may be classified as timber

piles, concrete piles (cast in site or precast), steel piles, composite piles and sand piles. To facilitate

pile penetration during driving it they are provided with cast iron or steel shoes. Pile driving is

usually by dropping a hammer freely over the pile cap. A pile is supposed to have reached a hard

stratum when it does not settle more than 10 mm with 10 blows of a 30 kN hammer falling freely from

a height of 800 mm.

From geological and agricultural maps and also from local enquiries preliminary investigation may

be carried out to find out type of soil, nature of soil strata, groundwater condition, etc. The detailed

investigations are carried out by collecting and testing soil samples from open trial pit, probing,

boring, sanding, seismic method and by electric resistivity method. SBC is determined by laboratory

tests. For small works presumptive values may be picked up from tables.

The methods of improving SBC of soil are by

1. Increase depth of foundation

2. Compaction

3. Draining the soil

4. Confining

5. Grouting

6. Chemical treatment

7. Using geo-synthesis.

The steps followed for the layout of foundations are:

1. Establish an important line

2. Drive two other pegs to mask width of foundation

3. Trace foundation plane on ground and check from other points

4. Build masonry pillars at a distance of at least 1 m from the foundation trench. Top of the pillars

are plastered and kept at plinth level. The centre line is marked and a nail is fixed.

The width of excavation should be a little more than the width of the footing. However, it should be

wide enough to ensure safe and efficient working. The depth of the foundation is checked by holding a

string along the grooves on the opposite pillars and holding a boning rod. Sheetings, struts, bracing

and waling are used to timber the trenching. Such timbering is known as shoring. Depending upon the

nature of soil and depth of trench different methods of shoring are adopted. Stay bracing, box

sheeting, vertical sheeting, runners and sheet piling are different methods of shoring.

Foundations in B.C. soil: If depth of B.C. soil is 1 to 1.5 m, it is economical to replace B.C. soil.

The sides of foundation trench should be filled with sand and gravel. Below the bed level of

foundation about 600 mm sand filling should be provided. If black cotton soil is more than 1.5 m deep

short columns may be built at regular intervals and connected by concrete beams or masonry arches.

If depth of B.C. soil is more than 3 m, pile foundations should be provided.

If a foundation is to be laid for a new building close to the existing building the minimum horizontal

distance between the two foundations should be equal to the width of the wider one. The line joining

the nearest points of the two foundations should not be more than 30° in soft soil and 45° in average

soil. However, using geosynthesis it is possible to manage closer spacings.

If reclamation is by filling with good soil on layers of 300–400 mm with good consolidation, the

foundation should be started on reclaimed soil only after two years.

For laying foundations in waterlogged areas drains and sumps are made to pump out water. Water

may be drained by well point system, multi-stage well point system, deep well system or by vacuum

method.

To prevent leakage of water in foundation trenches cement grouting, chemical grouting, freezing

process or electro osmosis process are used.

A cofferdam is a temporary structure built around a construction site to make the site reasonably dry.

Sand bag dikes, earthern cofferdams, rock fill cofferdams, rock fill crib cofferdam, single wall

cofferdam, double walled coffer dam or cellular cofferdams are used.

A caisson is a water tight chamber in which underwater constructions can be carried out. Types of

caissons used are box caisson, open caisson, pneumatic caisson.

A foundation may fail due to unequal settlement, sub-soil moisture movement or by movement of

sub-soil.