Question
I have a slight hill around 2-3 feet high at its peak, at the foot of the hill is a sidewalk that leads right to my pool. The hill keeps washing dirt into the pool at heavy rainstorms. I was thinking of building a wall out of railroad ties (landscape timbers) with drainage rocks behind it to prevent the mud slides. How would I go about doing this the right way, and how much drainage rock would I need?

Answer
There are a number of excellent "how to" sites on the web that can answer the "nuts and bolts" of how you assemble a pressure-treated wood retaining wall.  Here are a few:

http://www.ehow.com/how_10151_build-simple-reinforced.html

http://www.familyhandyman.com/DIY-Projects/Outdoor-Projects/Backyard-Structures/...

http://www.creativehomeowner.com/index.php?pane=project%26projectid=chwlk173

I would suggest wood larger than the "peeler posts", and as your question implies, you'll be using timbers - good!  The last link above may be the most helpful to you, then.  

I typically specify 12" of drainrock with a drainage pipe wrapped in filter fabric (so it won't clog) behind every wall I construct.  So if the wall is 2 feet high, each lineal foot of wall will take 2 cubic feet of rock.  When ordering bulk materials, remember that there are 27 cubic feet of rock in a cubic yard.  By example, a 20 foot x 2 ft. tall wall will take 40 cubic feet of rock 2 cf of rock per lineal foot of wall), or 1.5 cubic yards of rock (40/27 = 1.48 yards).  Don't forget the pipe %26 fabric, too.

If you have excessive water runoff from this slope, and you have the room, I'd intercept it before it gets to the slope with a rock-lines swale, and bring the water to a point where it can be better discharged.  Here is an example:
What is a swale:

http://acehomeinspec.com/images/Swales2.jpg
How to construct one with rocks:

http://www.handymanhowto.com/2008/08/30/building-a-rock-drainage-swale/

www.wikihow.com/Dig-Swales  ( a bit too mechanical for my tastes, but it works)

Pretty pictures of some after installation, with some text on installation tips:

http://www.thisoldhouse.com/toh/photos/0,,20180459,00.html

Also, any all over 3 feet will require a permit in most jurisdictions.  Hope it helps.  ~M