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

SITE INVESTIGATION (con't)


If the groundwater table is very deep, basement dampness is not likely to be a serious problem.  Special
attention must be paid to seasonal influences on groundwater table.  A low water table during a dry season will not necessarily be the case during other times of the year.  Personal experience will necessarily play a
vital part in this evaluation.  The best source of preliminary subsurface information is the local municipal engineering office.

Further information can be obtained by hand augering or by digging test pits.  The extent to which
subsurface site investigation is carried out will naturally depend on the total investment in the building
or buildings and on the general site characteristics.  With the sprawl of modern urban communities, more and more questionable sites are being developed for housing.  Obviously, if a site is poor from a foundation point of view it will require more detailed subsurface investigation and possibly a special foundation design.  Expert advice on these matters is now available in all parts of Canada.

SOIL IDENTIFICATION

The subsurface examination of a building site requires the identification and description of the soils
encountered.  If it is to be of general use, this information must be recorded in familiar and standard
terminology. ^* Soils are divided into two main groups, "coarse-grained soils" and "fine-grained soils".  Coarse-grained soils are the familiar gravels and sands and include all particles that are clearly visible
to the naked eye.  Fine-grained soils are usually more trouble-some in building operations than are the coarse-grained soils.  Their properties result more from their mineralogical and chemical characteristics than from their grain size.  Accordingly they are distinguished mainly on the basis of their plasticity and structure.

Two classes of soil particles make up fine-grained soils.  The coarser fraction, called "silt" has properties grading from those of very fine sand to those of the finest fraction, called "clay".  The properties of clays
can be attributed largely to the presence of "clay minerals".  These complex minerals are formed by the natural weathering of basic minerals.

It is the interaction between clay minerals and water that gives them their unusual properties.  Because of their wide variation in performance it is essential to distinguish between silt and clay at a building site.  The two materials look so much alike that they have often been identified improperly to the detriment of later construction operations.  They can easily be distinguished in the field by three simple tests.

The first is called the shaking test.  If a pat of the wet soil is shaken vigorously in the hand, the surface will become glossy and will shed the free water.  If the pat of soil is then squeezed in the fingers the free water may disappear, in which case the soil is a silt.  If the free water does not disappear then the soil is probably
a clay.

The second test is called the shine test.  if a moist lump of soil is stroked with considerable pressure either with the flat of a knife blade or with the finger nail the soil can be recognized as a silt if a dull surface is
produced, and as a clay if the surface is shiny.

The third simple test is the dry strength test.  If a small piece of soil is broken after it has been dried, its breaking strength is some indication of its character. If it is very strong it is almost certainly a clay, but if it powders easily, then it is probably a silt.

In addition to these tests, clay sticks to the fingers when wet and it does not wash off easily.  Silt, on the other hand, will wash away easily or will brush off when dry.  When a small quantity of soil is placed between the teeth, the presence of grit will indicate silt or sand but if no grit can be detected with the teeth, then the soil is certainly a clay. 
While all natural soils can be classified as either coarse-grained or fine-grained there are three types of
deposit that warrant special mention.  These are "glacial till", "man-made fill" and "organic soils".  Glacial till, as the name implies, is the unsorted soil deposit which results from glacial action.  It is usually very dense and hard and may contain a complete range of particle sizes from large boulders down to the finest clays. Occasionally it is found to be rather soft but it is always characterized by lack of stratification and an unusual range of particle sizes.