Making outside and inside tunnel arch wall

So i’ve been trying to make an arch turn for a tunnel, however the results always result in something like this happening:

https://imgur.com/LfjyV04

(Alternate image here: https://imgur.com/LfjyV04 )

(The problem being that’s there are always gaps between the tunnel arch and the road itself)

The dimensions are 896x896 on the outside and 512x512 on the inside, I’ve searched many tutorials but i still haven’t been get the idea of what i’m doing wrong

source in a nutshell, only way i’ve found is to divide it up into two triangles for every box

Are you using the 1 inch grid? don’t use the 1 inch grid.

also this http://hl2.gamebanana.com/tuts/8444

Unless you’re creating it initially off grid, arches work 99% of the time. The 1% when they don’t work properly is because of maths issues when making a non-square arch, so always make arches square (512512, 10241024).