Kelly bars can be divided into friction and interlocking Kelly bars by methods of applying pressure and structural features. Friction Kelly bars(also called as friction bars) are generally suitable for soft soils conditions, such as soils, sands, etc., while interlocking Kelly bars (also called as interlocking bars) are generally suitable for hard soils conditions, such as cobbles, rocks, etc. We are now capable of manufacturing 3-element, 4-element and 5-element interlocking Kelly bars, as well as 4-element, 5-element and 6-element friction Kelly bars.