There are many different properties and principals that are applied when dealing with geostatics and rock mechanics. So far for calculating geostratic stresses, the layers of rock/soil have only been in a horizontal orientation. So what happens when the layers get tilted? Well the answer just shifts from basic geostatic stress and strain to slope stability analysis. When dealing with inclined layers there is a friction component that corresponds to the individual units. This becomes much more complicated.

Soils and rocks have many underlying properties that contribute to several aspects of geostatics. These properties include:

• Cohesion
• Internal friction angle
• Pore water pressure
• Saturation
  
• Water tables

Pore water pressure is the most important of the above listed. When water is added to a soil rock system the overall pressure is carried by the the material and the water within the pores of the material. Water will also affect the cohesion of the material and the internal friction angle. Now when calculating the stress at a given point below the ground surface it becomes effective stress because of the water component. To get the effective stress of a given point the load that the water is carrying must be taken out to get the rock/soils stress.

Using the previous example, let's add a water table at 4m below the ground surface.

The effective stress at P is now given by 18.1*5 + 17.8*5 + 17.4*5 + 17.6*(5-2) -9.81*(1+5+5+3)  [(kN/m^3)(m)] = 182.6kPa.

The water in the pores of the material, due to the water table, is carrying 137.3kPa of the total stress. Therefore, the strength of the solid material is much less due to the pore water.