Remove-SPSite -Identity "<URL>" -GradualDelete

Get-SPSite http://technet.microsoft.com/en-us/library/ff607950.aspx
Set-SPSite
New-SPSite
Backup-SPSite
Remove-SPSite http://technet.microsoft.com/en-us/library/ff607948.aspx
Move-SPSite
Restore-SPSite

http://technet.microsoft.com/en-us/library/cc262392.aspx

1. Verify that you have the following administrative credentials:\\To delete a site collection, the user account that is performing this procedure must be a member of the Farm Administrators SharePoint group.

1. Start SharePoint 2013 Central Administration.\\For Windows Server 2008 R2:

Click Start, click Microsoft SharePoint 2013 Products, and then click SharePoint 2013 Central Administration.

For Windows Server 2012:

On the Start screen, click SharePoint 2013 Central Administration.

If SharePoint 2013 Central Administration is not on the Start screen:

Right-click Computer, click All apps, and then click SharePoint 2013 Central Administration.

For more information about how to interact with Windows Server 2012, see Common Management Tasks and Navigation in Windows Server 2012.

1. On the Central Administration website, on the Quick Launch, click Application Management.

1. On the Application Management page, in the Site Collections section, click Delete a site collection.

1. On the Delete Site Collection page, in the Site Collection list, click Change Site Collection.

The Select Site Collection dialog box appears.

1. In the Web Application list, click Change Web Application.

The Select Web Application dialog box appears.

1. Click the name of the web application that contains the site collection that you want to delete. Relative URLs of sites in the site collections of the web application that you have selected appear on the Select Site Collection dialog box.

1. Click the relative URL of the site collection that you want to delete, and then click OK.

1. Read the Warning section and verify that the site collection information is correct.

1. On the Delete Site Collection page, click Delete.