When we delete any site collection from a SharePoint site; it stays there for a while. If you have deleted multiple site collection from your web application, it may happen that Content DB gets full. In that case it won’t allow you to create a new site collection even though you see there are not many site collections present in a Web application.
To remove all such site collection in single go you can do something like this;
Get-SPDeletedSite -webapplication http://sharepoint | Remove-SPDeletedSite
In above example http://Sharepoint is the web application address; change the same as per your requirement.
Here it will prompt you to delete them one by one, if you want it to delete all of them without further prompts just enter the letter “A”.