Hello

I have created a new flower bed area running the side of my garden (20 meters on each side of the garden). The area was once grass and I had someone remove soil and then grass. I still see bits of grass around and it seems it been a bad job by the labourer. I will be adding 50mm of new top soil but today I had pulled hidden grass roots. I am not sure if grass will grow through the new soil.

I questions are;
1. Will grass grow through the new 50mm soil?
2. Is there anything I can do to the existing soil (bits of grass) before adding new soil?
3. Are there any chemical based products that I can use to kill grass and when should it be applied (before or after new top soil)?

Thanks
Rasik