CBD-12. House Foundations Con't.
C. B. Crawford


SOIL IDENTIFICATION

Because of Canada's glacial history it is a very common soil deposit.  Fill is a man-made deposit which must always be identified if it occurs at a building site.  It is characterized by the presence of pieces of masonry, glass, twigs, grass or rubbish.  Usually an organic soil profile will be found at its base.  Organic soil is easily recognized by its dark colour and characteristic odour.  Natural soils may contain a very small amount of organic matter or they may be almost entirely composed of organic matter.  Even small amounts of organic matter should be noted in the soil description.  Completely organic soil, called peat, requires special
investigation.

SOIL PROPERTIES

Natural soil is made up of a skeleton of solid particles the voids of which are filled with water and air. 
Saturated soil is a special case in which the voids are completely filled with water.  Until the interaction
of the three components is understood the performance of soil as a construction material is something of
a mystery.

The water contained in coarse-grained soils does not have a great influence on their properties but the flow of water through them, when they are not confined, may eliminate the frictional resistance between grains and cause a complete loss of strength.  This results in the condition known as "quicksand" which can be
corrected only by reversing the flow of water or by loading the affected area.

The water content of a fine-grained soil has a much greater influence on the properties of the soil.  As water is removed from a fine grained soil it shrinks and the strength increases. Correspondingly some clay soils will take up water when it is available and will swell and decrease in strength.  Fine-grained soils are quite "impervious".  Therefore they are difficult soils to drain.  Further they are often quite compressible and subject to volume change by seasonal drying and wetting.  They do not compact easily. They may "heave" when they freeze and so cause distortion to light structures founded on them.  They do, however, have some
desirable qualities.  They retain moisture necessary for growth during dry periods.  Their impermeability can be turned to advantage to prevent seepage at undesirable locations. They resist erosion and so make good slope material.

FOUNDATION SELECTION

The selection of the type of foundation for a house is usually based on local convention. The load transmitted to the soil is normally so small that it can be neglected.  Concentrated loads such as those imposed by large fireplaces may need special attention. Foundations located on organic soil and fill always need special
attention.  Often, as an economic measure, houses are founded on surface concrete mats.  Where a site
is underlain by solid rock this type of construction has obvious economic advantages.  Where the site is on soil the cost of providing service space above ground must be compared with the cost of a basement.  If a mat is used it is good practice to disturb the natural ground as little as possible and cast the slab on a base
of compacted stone or gravel at least one-foot thick. Insulation must be provided to prevent frost penetration beneath the slab.

Other types of foundations are used in certain regions because of local soil conditions but most house are founded on full basements. Usually it can be shown that the basement provides the most economical space for heating equipment, laundry and other activities which do not require first-class accommodation.
Further, basement footings are less susceptible to various influences such as frost heaving and swelling
or shrinking clays.  Dampness is the main problem with house basements and this can usually be attributed to faulty construction. The most effective way to reduce water infiltration is to collect and control it before
it reaches the basement wall. Surface drainage away from the wall is necessary. Subsurface water is
intercepted by an under-drain placed outside the wall footings in a pervious collecting layer and is
discharged from the site. Even with these measures the outside of a basement wall will usually be damp and there will be a tendency for water to move through the wall. The use of properly placed good materials will result in a practically impervious wall and eliminate this problem.