run the following code on a sharepoint server powershell admin console:

Write-Host $siteUrl

#Get the site collection to delete

$Site = Get-SPSite https://ucshare.canterbury.ac.nz/sitename

 

#Get the content database of the site collection

$SiteContentDB = $site.ContentDatabase

 

#Force delete site collection

$SiteContentDB.ForceDeleteSite($Site.Id, $false, $false)

　

If that does not work, you can use this one - you need to get the site id for this to work - see tip 268 (https://ucsharehub.canterbury.ac.nz/management/thehub/Lists/HowToDoThings/DispForm.aspx?ID=268)
Add-PSSnapin Microsoft.SharePoint.Powershell -ErrorAction SilentlyContinue

stsadm -o deletesite -force -siteid "71d8e2dd-a967-4234-83b9-62c0149fb449 " -databaseserver UCSP15SQLDB -databasename SP15content_UCShare_Prod_1203

#Read more: http://www.sharepointdiary.com/2014/09/force-delete-corrupted-site-collections-in-sharepoint.html#ixzz5uYIB4jDu