Type of Soils and their Bearing Capacity

The soils on which a structure rests may be classified into three categories:

(a) Hard soils. These soils are generally rocky in nature, incompressible and can bear fairly good loads. Solid rock, moorum and stony soils are examples.

(b) Soft soils. These are alluvial soils and are compressible when loaded. They can not take much load. Ordinary clay, loam, and common soils are examples of this.

(c) Spreading soils. These soils are compressible when they are confined and prevented from spreading. These soils when load­ed spread out laterally. Sand and gravel are examples of this type of soil.

Bearing capacity of soils. Bearing power or Bearing capa­city of a soil is defined as the maximum load that a soil can bear per unit area (usually Tonnes per sq. m) without yielding or causing cracks, displacement or rupture. This is the ultimate capacity of a soil. But as far as designing is concerned, we are less concerned with the ultimate load or the ultimate capacity of soil.

Methods of determining the Bearing Capacity of Soils

There are many methods of finding out the bearing capacity of soils. A very simple methods of determining the bearing capacity is as follows :

Dig a pit of size 2 mx2 m and of the required depth. The bottom of the pit is levelled by simply spreading the soil by hand. It should never be compacted. At the centre of this pit put a steel plate of 60 cm X 60 cm X 5 cm. Drive to pegs equidistant from the steel plate. Over the steel plate erect a wall 40 cm X40 cm either with bricks or stone or concrete blocks. Nearly 50 cm above the adjoining ground level. The difference: of levels between the top of the pegs and the wall is noted by a dumpy level. Now gently place the loads on the top of the wall by constructing a wooden platform. The load may consist of sand bags, girders or R.S.J. The loads are increased by a suitable amount. usually 0.5 tonne at an interval of 20 to 30 minutes. Before each in­crement of load the difference of levels between the pegs and the,top of the wall is noted. Note that the difference of level will remain constant till the soil yields. The moment the difference is increased, the increment of load must be stopped.

The bearing capacity of the soil will be the total load divided by the area of steel plate, that is

Bearing capacity = wt. of steel plate + wt. of wall + wt. of sand bags etc.

Area of plate (60 x 60 cm)

Safe Bearing Capacity. The safe load to be taken on a soil for the purpose of design is called Safe Bearing Capacity of the soil. The Safe Bearing Capacity of a soil may be defined as the bearing capacity of a soil divided by a number usually by constant and called factors of safety, i.e.

Safe Bearing Capacity = Ultimate Bearing capacity

Factor of Safety

The factor of safety depends on the type of building and the nature of the soil. Generally a factor of safety of 2 to 4 is taken for different purposes. Thus if the ultimate load of a soil is 6 tonnes/m²

and its factors of safety is 3 the working or design loads to be given to that soil will be 6/3 = 2 tonnes/m². This is the safe bearing capa­city of the soil.

Safe Bearing Capacity of Different Soils

Soft clay 2-3.75 tonnes/m²

Black cotton soil 5-7.5 tonnes/m²

Alluvial learn 7-5.16 tonnes/m²

Alluvial soil 5-7.5 tonnes/m²

Moist clay 11-18 tonnes/m²

Made-up ground 5 tonnes/m²

Ordinary clay 22 tonnes/m²

Clay mixed with and 22 tonnes/m²

Compact clay (dry) 33-55 tonnes/m²

Loose sand 22 tonnes/m²

Compact sand 22-32 tonnes/m²

Compact confind sand 44 tonnes/m²

Kankar or sandy gravel 22-32 tonnes/m²

Compact gravel 44-65 tonnes/m²

Moorum 22-44 tonnes/m²

Soft rock 25-85 tonnes/m²

Ordinary rock 85-110 tonnes/m²

Hard rock above 250 tonnes/m²

^{Sources from other website}