This build gives you the ability to tank, and support, and do a lot of damage with your spells.
You will be tanky enough to walk out of team fights when you have used your full combo and helped your teammates.
It gives you enough health and armor to tank turrets and give you dodge too with the masteries and runes.
You will support your teammates with clairvoyoance. and will of the aegis, and riyals crystal scepter. Will of the aegis will give you a lot of the armor and magic resist.
but its your choice but i really think its the best build for Amumu.

I don´t copy my builds from anyone, i play the character, and learn how to play it, and then find the best suited build.
the only thing i look on Mobafire is if my build is all ready in there,

All my builds is something there can work in ranked, and normal games, all the builds is something that work good 100% guarantee.

this build will give huge damage on your ult be course it scales 100% on your ability power, all your other spells will do heavy damage too, thats why i noted (Ap carry) on the title.

The dominion build, is much the same as the summoners rift build, the different with the two is that this build, you don´t go support you just ap/tank and you have ghost for cap at the beginning, and so you can cap other things really quick. Garrison is for engage and def too.
So the main thing with this build is, that you can tank the capture points while you deal huge damage to the enemy defending it.