What would you do to prevent slope failure in a road-cut where you have a marine shale/clay in an area of high groundwater?

• A. Install horizontal hydroaugers
• B. Drill vertical drainage wells
• C. Reinforce the slope with steel tiebacks
• D. Install surface drainage ditches

Answer: (A)

Relieving pore-water pressure inside the slope is the key to stability in clay-rich, groundwater-saturated conditions. In marine shale/clay, shear strength drops as pore pressure rises, so lowering that pressure increases effective stress and the tendency for the slope to fail is reduced. Horizontal drains drilled parallel to the slope provide drainage paths within the mass, allowing water to escape laterally to a collector or toe drain and thus reduce pore pressure along the potential slip surface. This targeted internal drainage is much more effective for weakening the hydraulic conditions that drive failure than simply removing surface water or installing structural supports. Vertical drainage wells mainly pull water down but don’t relieve pressures along the critical horizontal slip zone, surface ditches address only surface runoff, and tying the slope back with steel anchors strengthens the face but doesn’t alter the internal groundwater pressures. Hence, installing horizontal hydroaugers best addresses the driving mechanism of failure in this scenario.