Under a strip footing the tilt of the stress points down; beside it, under the surcharge, it lies
flat. Between them it turns through a right angle, a quarter turn, and turning a tilt that is at the
cap multiplies the squeeze by e2 tan φ′ Δβ. Slide the turn along the fan to follow
it.
What you are seeing
The mechanism. Prandtl's mechanism, to scale: the active wedge under the
footing (orange), the fan (blue) and the passive wedge under the surcharge (purple). Ticks show
the direction of the tilt in each zone. The thick blue ray is the point you have reached round
the fan, and the black tick on it is the tilt there.
The squeeze along the way. The squeeze p′ on a log scale, read from
the footing out to the surcharge, through the active zone, the fan and the passive zone. The
ratio across each zone is written along its bottom. The black dot and dashed line mark where
the slider is.
Assumptions
The soil's own weight is left out, as in Prandtl's solution: it adds the
½γBNγ term, which has no closed form.