Just like any other engineering problem it is wise to start with a free body diagram. This will help you get a feel for the loads and what we need to do to resist them. Then next thing is to have an understanding of what assumptions are being made in the analysis. For reference I will use earth retention system and retaining wall interchangeably for most of following articles.
There are many types of earth retention systems but here are some of the basics:
Soldier Pile wall which may also use soil anchors, deadman or whalers.
Sheet Pile wall which may also use soil anchors, deadman or whalers.
Concrete cantilever retaining walls which may be used in some foundations as well.
Segmental Retaining Walls which may be made up of large concrete units or reinforced with geogrid.
These are just some of the basics but the principles of almost all systems are very similar. No matter what system is chosen the retaining wall must be designed to resist soil pressure, hydro-static pressure, surcharges and any other externally applied loads (typical hand rails, guardrails or fences). Typically there are there areas of stability that should be checked – external, internal and global (or slope stability). External stability typically refers to sliding, overturning and bearing failures of the wall acting as a rigid body. Internal stability is a check of the components which make up the wall to ensure the wall is acting as a rigid body. Overall or global stability is an evaluation of the entire excavation or slope of soil which the wall is bearing on and retaining. This analysis is most commonly preformed by the geotchnical engineer but should be clearly stated on the plans as to whether or not this failure state has been evaluated.
We will now move on to address the lateral earth pressure do to soil and hydrostatic load.