Graveled area

Hi everyone
I have bought a home which has a very large Japanese Garden area. The previous owners had graveled some of the area which was overgrown by plantation and weeds for a number of years. But i have cleared the area out now.
The area -
As far as a i can see they have lined it with an undersheet
There is a generous amount gravel remaining
The issues -
There may be a number weed roots remaining that i cant get at.
The area is covered in debris and i wont be able to rake it all up without destroying the present gravel / lining.
Could i simply put weed killer down and place more gravel down?
here are some pictures to try help me explain my situation


0
Posts
It is very likely that the undersheet has been damaged by any weed roots growing through it, those grasses have really strong roots
I would definitely use weed killer , it will help to kill the roots , I known lots out there don' t like using the stuff , I am not a fan but needs must
the put some more gravel down , I have some small area of gravel and it works well
best of luck