What fillers to use for retaining wall?

[cathaygirl]

I have installed a retaining wall alongside of an existing wall near to a boundary fence. The tradie suggest to fill them with drainage gravel and it’s good to go. I wanted to ask the Workshop community here if there is a cheaper option to fill the gap  Or any other ideas that I could use? 

It is 1meter high and 12 m long with a 12cm gap to fill. I intend to lay turf over it.

Here is a glimpse of it... Apologies for the sad state of my lawn. 

Another option is,  if leave it as it is and put a drainage channel on the top and put weed at and turf of top? Will that be enough? 

Thanks in advance! 

[Jason]

Welcome to Workshop @cathaygirl. I trust our helpful community members will have some suggestions for you.

Feel free to post anytime you need a hand with a project around the house and garden.

Jason

[cadsman]

Blend: a sand and gravel for concrete mix. Put a 50mm socked Ag drain at the base and blend fill. You will have to refill or top up when the fill settles. Any cheaper option will let you down eventually. 

[MitchellM]

@cathaygirl just to clarify, you want to fill the gap between the new retaining wall and the existing wall. You're not going to fill inbetween the retaining wall and the fence are you?

I think you'll find a bulk delivery of bluemetal would be reasonable inexpensive. 1 square metre will just about fill it, then you can lay some topsoil/sand to fill the remainder and turf over that.

[cathaygirl]

Thanks @MitchellM and @cadsman. I’ll certainly look into both of your suggestions. To your question, yes I’m only filling the smaller gap between the new and old retaining wall. 
And thanks @Jason for welcoming me to the community!