As the diameters of the openings increase the permeability increases. Therefore gravel is more permeable than sand. Of course, no matter how large the spaces in a material are, there must be connections between them if water is to pass through. If the open spaces are not interconnected the rock is impermeable. Compression squeezes the cylinder and makes it shorter and wider.
Extension makes the cylinder longer and narrower.
The strongest force is shown by the red arrows. Both porosity and permeability can be increased when joints and cracks form in the rock. This can happen when stress is applied to the rock. A force applied to material that tends to change its dimensions is called stress. This is commonly unit stress, defined as the total force divided by the area over which it is applied.