Subsoils and Foundations Loadings in buildings consist of the combined dead and imposed loads which exert a downward pressure upon the soil on which the structure is founded and this in turn promotes a reactive force in the form of an upward pressure from the soil. The structure is in effect sandwiched between these opposite pressure and the design of the building must be able to resist the resultant stresses set up within the structural members and the general building fabric. The supporting subsoil must be able to develop sufficient reactive force to give stability to the structure to prevent failure due to unequal settlement and to prevent failure of the subsoil due to shear. To enable a designer to select, design and detail a suitable foundation he must have adequate data regarding the nature of soil on which the structure will be founded and this is normally obtained from a planned soil investigation programme . Soil investigation. Soil investigation is specific in its requirements whereas site investigation is all embracing, taking into account such factors as topography, location of existing services, means of access and any local restrictions. Soil investigation is a means of obtaining data regarding the properties and characteristics of subsoils by providing samples for testing or capital which can be reasonably expended on any soil investigation programme will depend upon the type of structure proposed and how much previous knowledge the designer has of a particular region or site.The main methods of soil investigation can be enumerated as follows:1. Trail pits small contracts where foundation depths are not likely to exceed 3m.2. Boreholes medium to large contracts with foundations up to 30m deep.Classification of soils. Soils may be classified by any of the following methods:1. Physical properties; 2.Geological origin; 3.Chemical composition; 4.Particle size.It has been established that the physical properties of soils can be closely associated with their particle size both of which are of importance to the foundation engineer, architect or designer. All soils can be defined as being coarse-grained or fine-grained each resulting in different properties.Coarse-grained soils: these would include sands and gravels having a low proportion of voids, negligible cohesion when dry, high permeability and slight compressibility, which takes place almost immediately upon the application of load.Fine-grained soils: these include the cohesive silts and clays having a high proportion of voids, high cohesion, very low permeability and high compressibility which takes place slowly over a long period of time.There are of course soils which can be classified in between the two extremes described above. BS 1377 deals with methods of testing soils and divides particle sizes as follows:Clay particles less than 0.002mmSilts particles between 0.002 and 0.06mmSand particles between 0.06 and 2mmGravel particles between 2 and 60mmCobbles between 60 and 200mmThe silt, sand and gravel particles are also further subdivided into fine, medium and coarse with particle sizes lying between the extremes quoted above.Shear strength of soils. The resistance which can be offered by a soil to the sliding of one portion over another or its shear strength is of importance to the designer since it can be used to calculate the bearing capacity of a soil and the pressure it can exert on such members as timbering in excavations. Resistance to shear in a soil under load depends mainly upon its particle composition. If a soil is granular in form, the frictional resistance between the particles increases with the load applied and consequently its shear strength also increase wish the magnitude of the applied load. Conversely clay particles being small develop no frictional resistance and therefore its shear strength will remain constant whatever the magnitude of the applied load. Intermediate soils such as sandy clays normally give only a slight increase in shear strength as the load is applied.Compressibility. Another important property of soils which must be ascertained before a final choice of foundation type and design can be made is compressibility, and two factors must be taken into account:1. Rate at which compression takes place.2. Total amount of compression when full load is applied.When dealing with non-cohesive soils such as sands and gravels the rate of compression will keep pace with the construction of the building and therefore when the structure is complete there should be no further settlement if the soil remains in the same state. A soil is compressed when loaded by the expulsion of air and/or water from the voids and by the nature rearrangement of the particles. In cohesive soils the voids are very often completely saturated with water which in itself is nearly incompressible and ther