Soil Identifi cation in the Field
In the fi eld, the predominant soil types based on texture are identifi ed by inspection. Gravels and sands are gritty and the individual particles are visible. Silts easily crumble, and water migrates to the surface on application of pressure. Clays fail this water migration test since water fl ows very slowly through clays.
Clays feel smooth, greasy, and sticky to the touch when wet but are very hard and strong when dry. Common descriptive terms and methods of identifi cation are as follows.
Color: Color is not directly related to engineering properties of soils, but is related to soil mineralogy and texture.
Gray and bluish: unoxidized soils
White and cream: calcareous soils
Red and yellow: oxidized soils
Black and dark brown: soils containing organic matter
Moisture: Appearance due to water is described as wet, dry, or moist.
Structure: Homogeneous: Color and texture feel the same throughout.
Nonhomogeneous: Color and texture vary.
Shape: Angular, subangular, subrounded, rounded, fl aky.
Weathering: Fresh, decomposed, weathered.
Carbonate: Effervesces with acid. Add a small amount of hydrochloric acid and check if soil effervesces. If it does, it contains carbonate.
Smell: Organic soils give off a strong odor that intensifi es with heat. Nonorganic soils have a subtle odor with the addition of water.
Feel: Use feel to distinguish between sand, silts, and clays.Sand has a gritty feel. Silt has a rough feel similar to fi ne sandpaper. Clay feels smooth and greasy. It sticks to fi ngers and is powdery when dry.
Consistency: Very stiff: Finger pressure barely dents soil, but it cracks under signifi cant pressure.
Stiff: Finger pressure dents soil.
Firm: Soil can be molded using strong fi nger pressure.
Soft: Easily molded by fi nger.
Very soft: Soil fl ows between fi ngers when fi st is closed.
Dilatancy: Place a small amount of the soil in your palm and shake horizontally. Then strike it with the other hand. If the surface is slurry and water appears, the soil probably has a large amount of silt.
Packing: Coarse-grained soils are described as:
Very loose: collapses with slight disturbance; open structure
Loose: collapses upon disturbance; open structure
Medium dense: indents when pushed fi rmly
Dense: barely deforms when pushed by feet or by stomping
Very dense: impossible to depress with stomping.
Number and Depths of Boreholes:
It is practically impossible and economically infeasible to completely explore the whole project site. You have to make judgments on the number, location, and depths of borings to provide suffi cient information for design and construction. The number and depths of borings should cover the zone of soil that would be affected by the structural loads.
There is no fi xed rule to follow. In most cases, the number and depths of borings are governed by experience based on the geological character of the ground, the importance of the structure, the structural loads, and the availability of equipment. Building codes and regulatory bodies provide guidelines on the minimum number and depths of borings.
The number of boreholes should be adequate to detect variations of the soils at the site. If the locations of the loads on the footprint of the structure are known (this is often not the case), you should consider drilling at least one borehole at the location of the heaviest load. As a guide, a minimum of three boreholes should bedrilled for a building area of about 250 m2 (2500 ft2) and about fi ve for a building area of about 1000 m2 (10,000 ft2). Some guidelines on the minimum number of boreholes for buildings and for due diligence in subdivisions