Both boreholes and test pits are commonly used means of subsurface geotechnical investigation and are used to determine soil density, stability and install ground-water monitoring wells.
Boreholes are the most typical sampling and use a steel tube inserted at set depths to take a cross-section of the soil. If continuous depth testing is required, this method can become expensive.
For when you need a continuous profile, test pits are preferable and the most economical. As the name implies, a test pit is dug at the site and allows the engineer to see the layers and test for slope stability. However, this method cannot go into bedrock.