Foundation soils

FOUNDATION SOILS

Soil is the natural, aggregated or un-cemented deposits of mineral and organic grains covering major portion of the Earth's crust. Soils are formed by the weathering action of rocks.

Foundation Soil or Sub-Soil

Soil on which the foundation of a building structure rests is called Foundation Soil or Sub-Soil.

 

1. CLASSIFICATION OF SOILS

The classification of soil is done to find its suitability for foundations. Soils are classified as:

1. Indian Standard (IS) Classification [IS : 1498 – 1970]

It is based on the grain size of the soil as well as its plastic properties. The classifications are:

(a) Coarse Grained Soils: Coarse Grained Soils are further divided into two types as:

(i) Gravel (G): Gravel is denoted by the symbol G. It is a very good foundation soil. Gravels are the coarse soil particles of more than 50% by weight, having sizes larger than 4.75 mm IS Sieve size [i.e., larger than 75 microns] up to 80 mm.

(ii) Sand (S): Sand is a good foundation soil. Sand particles are unaltered mineral grains. Sand contains cohesionless particles of more than 50% by weight, having sizes smaller than 4.75 mm IS Sieve size. [i.e., smaller than 75 microns.]

(b) Fine Grained Soils: Fine Grained Soils are further divided into two types as:

(i) Silts: Silts consist of fine particles of weathered rocks. Silts have slight plasticity or no plasticity with minimum or no strength. So, silts are not very much suitable for foundation.

(ii) Clay: It is not a good foundation soil. It becomes plastic in the presence of water.

2. Classification based on Particle Sizes

In this system, soils are arranged according to the grain size of the particles.

Boulder – Grain sizes more than 300 mm;   Cobble - Grain sizes 80 mm to 300 mm

Gravel - Grain sizes 4.75 mm to 80 mm; Sand - Grain sizes 0.075 mm to 4.75 mm

Silt - Grain sizes 0.002 mm to 0.075 mm; Clay - Grain size less than 0.002 mm

 

2. BEARING CAPACITY OF SOIL

It is the Earth that provides the ultimate support for all types of structures including buildings, bridges, dams, etc. The supporting ground is invariably the soil. Soil is weaker than any construction material like masonry, concrete, steel, etc. Hence, a large area or mass of soil is necessarily involved in carrying the load on it.

Thus, the design of foundation is based on the concept of Bearing Capacity of the Soil.

DEFINITIONS

1. Bearing Capacity of Soil

It is defined as the maximum load per unit area that the soil can support without causing undesirable settlements. In other words, it is the inherent capacity of the soil to bear the load imposed on it.

2. Ultimate Bearing Capacity of Soil

It is determined by conducting a Plate Load Test. In this test, a square rigid steel plate is loaded gradually and the settlement of soil for each load is measured. The graph showing the load-settlement relationship is used to determine the maximum load carried, when the soil just fails in shear.

Ultimate Bearing Capacity of Soil = Ultimate Load / Area of the Plate

3. Safe Bearing Capacity of Soil

A load much lower than the ultimate bearing capacity only should be applied on the soil so that perfect safety of the foundation is ensured. This safe load is called the Safe Bearing Capacity of the Soil.

Safe Bearing Capacity of Soil is defined as the maximum load per unit area that the soil can carry safely without the risk of failure (collapse).

Safe Bearing Capacity of Soil = Ultimate Bearing Capacity of Soil / Factor of  Safety

Factor of safety depends on the nature of the soil, depth of the soil and moisture contents of the soil. It is taken as 2 for normal applications and between 1.5 and 3 for special structures.

4. Allowable Soil Pressure

The bearing pressure of soil is the pressure developed under the foundation due to the loads. Allowable Soil Pressure is the safe soil pressure which is safe against failure (collapse). At the same time, it does not cause settlement more than the permissible settlement.

 

3. FACTORS AFFECTING BEARING CAPACITY OF SOIL

1. Physical features of the foundation, namely, Type of foundation, Size and shape of foundation, Depth of foundation below the ground level and Rigidity of the structure.

2. Type of soil and its physical properties such as density, shear strength, etc.

3. Position of ground water-table.

4. Allowable settlement of the soil.

 

4. METHODS OF IMPROVING BEARING CAPACITY OF SOIL

In early days, areas having low bearing capacity soils, were avoided for construction. But with scarcity of land in urban areas, it is not possible to do so. It is possible to improve the bearing capacity of soil at the construction site by adopting any one of the following artificial methods:

1. By increasing the Depth of Foundation

This method can be used only when the increase in the cost of construction by increasing the depth of foundation is not much. This method cannot be adopted where the ground water level is high.

2. By Compacting the Soil :

Compaction (ramming) of the soil reduces the open (air) spaces between the individual particles in the soil mass. Therefore, they are less liable to displacement; And bearing capacity of the soil is increased.

By packing large sized boulders, gravel, etc., to the soil or by driving Piles, compaction can be attained in sandy soils of loose nature.

3. By Draining the Sub-soil Water

Reduction in water content increases the bearing capacity of the soil. The water present in the soil can be drained by laying open-jointed drains in trenches.

4. By Grouting the Soil Mass with Cement Grout

Cracks and fissures in the soil reduce the bearing capacity of soil. Cement grout can be injected under pressure into the foundation soil to fill any cracks or fissures.

5. By Restricting the Movement of Soil

Sheet Piles are driven to form an enclosure. This increases the bearing capacity of the soil. Thus, the movement of the soil is restricted.

6. By Injecting Chemicals

Chemical solutions like silicates are injected under pressure into the soil mass. Hence, the soil mass becomes a compact unit. This method is very costly and is adopted in exceptional cases.