Drilled shafts are often used to solve deep-seated slope stability problems. When slope failure planes run deep the driving forces are high which require a reinforcing element with a high section modulus placed at a relatively close spacing. Drilled shafts used for slope stability applications typically consist of 24 to 42 inch diameter reinforced concrete shafts. They are placed through the existing failure plane (existing landslides) or a potential critical failure planes (new construction) to increase the factor of safety for slope stability. Typically placed in a single row, at a spacing of 1 to 3 diameters apart, they are drilled to a depth that enables the passive resistance of the soil below the critical failure plane and the stiffness of the concrete shaft to resist the driving force of the landslide or mass of soil above the critical failure surface. These are most cost effective in applications which have:

■ Deep Seated Failure surfaces

■ High Driving forces

■ Confined Areas

■ When controlling lateral movement is important – critical structures

■ When combined with a post and panel or tieback wall for grade separation

36 inch diameter Drilled shafts placed at the face of an MSE wall to improve global stability in highly plastic and fissured “Marine Clay” soils in Alexandria, Virginia.