The purpose of an investigation is to obtain information relating to foundation condition and to natural construction materials commensurate with the type of structure involved and with the stage of the project.
In evaluating soil, the transition is gradual between those properties that serve only as broad guides to the character of material and quantitative properties which define specific performance characteristics. For example, moisture content and dry density are at times used as index properties and at other times used as engineering properties.
Engineering computation using soil strength deal primarily with shear strength, the resistance to sliding of one mass of soil against another, and rarely with compressive or tensile strength. In 1776, a French engineer and scientist, observed that the shear, strength of a soil consisted of two parts
- One part dependent on the stress acting normal to the shear plane,
- The other part independent of that stress.
The parts were called internal friction and cohesion, respectively. The factors relating the normal component of stress to shear strength was designated by tan and the unit of cohesion by c.
