Revit Site - Cut and Fill analysis
Site Topography in Revit provides the user with simple tools to model the site terrain, however these tools are by no means broad enough in there functionality to model sites with Sloping Car parks and Roads easily and they often take a reasonable amount of time to model so they look authentic. It is all too easy to get pulled into a ‘black hole’ where you spend more time modelling due to an absence of effective tools. For this reason it is wise to balance what you need to provide in information with the time it will take.
Modelling of a Site without the need to provide cut and fill quantities is all very fine in Revit, only problem is insufficient tools available to make this a quick exercise; however, should you required Cut and Fill quantities then there are a few alarms that need to rung and errors that need pointing out. The following article is a high level overview and does not set out to delve into these areas too technically, but wishes to point out the ‘traps and pitfalls’.
Be aware of:
Normal 0 false false false EN-GB X-NONE X-NONE Don’t utilise Pads in the existing phase of a Site Topo, if you are going to amend the proposed Site in that area. Pads in Existing Site Topo’s when demolished in the Proposed phase will not calculate any Cut and Fill. When using the ‘Graded Region’ on a section of Existing Topo containing Pad(s), these Pad(s) are removed from the Existing Phase, therefore not use Pads to model Existing Basements.
- Volumes of Cut and Fill are not all they seem. Volumes Revit specifies can be incorrect – See examples outlined later on in this article that prove these errors, unfortunately we could not determine a logic behind these. We therefore wish to point out that whilst these are not completely accurate they provide approximate values of Cut and Fill. If volumes are required to be as accurate as they can be, then consider exporting to a Civil engineering package such as ‘Civil 3D’. This is not the total BIM Solution that many companies would prefer. Obvious downsides to exporting from Revit are:
Dedicated Staff skilled in extracting the necessary information.
Information split across several packages, therefore amendments to site may be required in both packages and coordination errors may creep in.
Potential bottleneck of projects coming from many seats of Revit.
Cost in software and training.
- Graded Regions performed on Sites sometimes do not produce Cut and Fill quantities (where the site Topo has been split), whereas Copying and Pasting the Topo and changing its phased created parameter will often rectify this. Therefore do not work on a graded Topo too long before checking if Cut and Fill is working, if not, abandon and choose the manual approach of copying/pasting and changing parameters for each Topo in both phases and then check if Cut and Fill is being performed.
- The Way you split a Topo and perform a Graded Region can result in whether or not Cut and Fill is calculated. You should always perform a quick check to see if Cut and Fill is working before continuing too far and then realising that no calculations have/will be performed.
Above is a series of images of the results of splitting Topo surfaces (highlighted by the dashed line) and performing graded regions on the sections highlighted with a dot. These are meant to serve as a guide so that you can evaluate quickly if cut and fill are likely to work or not. However we have been unable to work out why it works sometimes and not in others, therefore perform a quick check before modelling your site too much as this could result in aborted time.
Volumetric errors in Cut and Fill - examples.
To demonstrate these errors we have taken an existing site and graded a region, placing a number of pads of known sizes that we can easily calculate what the volumes should be. As the volume is calculated to the underside of any Pads we have made the thickness of the Pad structure 1m for ease of calculations.
In the example above, we placed all pads with a 9m offset.
1m x 1m Pad Calculated correctly at 10cubic meters
4m x 4m Pad produced a -3cubic meter error
5m x 5m Pad produced a -12.2cubic meter error
8m x 8m Pad produced a -12cubic meter error
20m x 20m Pad produced a +46.9cubic meter error
It is therefore apparent that roughly speaking the larger the Pad the more the error. Furthermore it can be seen that the errors do not appear to follow a pattern.
Written By: Ian Howard