ELI5: λ = 1

In geology, λ = 1 means that water trapped deep underground is pushing upward with the exact same amount of force as the heavy weight of the rocks pressing down on it.

Imagine a giant pile of heavy blankets stacked on top of you. Normally, the weight of those blankets pushes down directly onto your body. In the Earth, this is what heavy rock layers do to the ground underneath them, which geologists call lithostatic pressure.

Now, imagine there is a powerful water balloon trapped under that stack of blankets:

• If the balloon is flat, your body carries all the weight of the blankets.
• If the balloon is pumped full of water, the trapped water starts pushing upward to help hold up the blankets.
• If the water pressure inside the balloon becomes strong enough to lift the entire weight of the blankets all by itself, that special balance is called λ = 1 (or a pore fluid pressure ratio of one).