My arborvitae's look green on the outside but if you separate the foliage and look inside there is some brown. Is this normal or is it in distress?

How do you prune them and also when?

They are planted in a raised garden surrounded by 4x4 lumber. There are a lot of leaves that have accumulated underneath,should they be removed? Some have brown needles at the base also.

We have 2 dogs and they do urinate on them from time to time,so that's not good.

I planted them about 8 years ago at about 2-3 feet tall now they are about 8 feet.