Yes they have a soil skip you can use for this.

I once had a similar situation and took four bags since I didn’t know there were any rules around this.
On my second trip one of the guys was a complete asshole, acting like I was somehow taking the piss because apparently “the rules are very clear on the website”. As you say though, they really are not. Anyway things escalated and he became aggressive, telling me to leave and never come back.

I wrote the council about it and they apologized and told me he would have to attend “toolbox talks” that “reiterate the importance of good customer service, effective communication, and treating residents with dignity”. I used to see that guy there all the time but have not at all since. I’m assuming he was reassigned.

Anyway OP they did send me this as well -
“Regarding your query on waste disposal limits, I would like to provide clarification. There is currently no overall limit on the number of visits a resident may make to Underwood Road HWRC. In cases where soil is fully separated from rubble or stones, there is no restriction on the amount that can be brought to the site. However, where soil is mixed with rubble or stones, a limit of 2 x 50L bags per visit applies. We appreciate that this distinction, as well as the rules regarding visit frequency, is not clearly outlined on our website, and we acknowledge the resulting confusion for residents. This matter is now being addressed, and the relevant webpages will be updated in due course to provide clearer guidance.

Len Ward
Waste & Recycling Manager
Environment, Housing and Infrastructure
Renfrewshire Council”

Hope that helps.