Earth Pressures

Behind a smooth wall with level ground, the tilt of the stress can only be vertical or horizontal, so it is one number, C, and K = σ_h′/σ_v′ = (1 − C)/(1 + C). Friction stops C at ± sin φ′: the active and passive states. Drag along the disc, or move the wall.

What you are seeing

• The tilt on the disc. The same disc as the Mohr circle tool: every stress state is a point (C, S), and friction keeps it inside the blue circle of radius sin φ′. With a smooth wall and level ground there is no shear on horizontal planes, so S = 0 and the state can only move along the horizontal axis: this is the dial. The scale under the axis reads K = (1 − C)/(1 + C). The dots mark passive (purple), water, K = 1 (blue), at rest (green) and active (orange); the arrow from the centre is the tilt now, and the arrow below it the trip from rest. Drag along the axis to move C.
• The dial. The same line opened out: K against C, on a log scale. The curve is solid blue between friction's two stops and dotted beyond them, where the shaded regions are out of reach. Drag along it to move C.
• The wall. The grains push on the wall with Kγz: the pressure diagram grows with depth, with the at-rest diagram dashed in green for comparison. At a stop, the sliding wedge and its slip plane are drawn, at 45° + φ′/2 (active) or 45° − φ′/2 (passive). The arrow at the top shows which way the wall has moved from rest.
• The Mohr circle. The stress at the base of the wall, between σ_v′ and σ_h′, with the friction lines in blue. At a stop the circle touches them.
• The status bar. Whether the soil is between the stops or at one.

The model

The two stops are Rankine's active and passive states. At rest the soil sits wherever deposition left it, C₀ = (1 − K₀)/(1 + K₀). Water has no tilt, and sits at C = 0, K = 1. The soil is dry.