Computing Rock Excavation For Building Pads

In the video below we show how to use the Create Side Slope command to compute the excavation of rock around a building pad.

The Pad cuts through the Rock Surface so that there are some parts of the pad in Rock Cut and others that are in soil. The user wants to determine the amount of rock cut if they offset the pad 5’ around its perimeter when in Rock Cut, and then tie up to the Rock interface vertically, but when not in rock they will do no offset at all.

In this case you need a conditional statement that says

“If” in Rock (i.e. if the pad elevation is below the rock surface at this point along the pad line, and you compute that at intervals along the pad line)

Then Offset the line by 5’ at 0 Elevation Difference
Then do a Tie Slope up to the Rock Surface at e.g. 1:100

If you also want to compute the Dirt Qtys then you may want to add a second tie slope that ties up to Original ground 1:100 or maybe you want a 1.5:1 or 2:1 slope in soils for those quantities.

Then there is the “Else” condition that is what happens if the pad line is not below the rock surface - in this case you can do nothing if all you care about is the rock qty or you can do a surface tie up to Original Ground at 1:100 with no 5’ offset.

Then there is the “End” condition to tell the computations to stop and do no more.

Video shows you how to do this for one or more building pads

If you need to remove the jogs from your pads - this video shows how to do it with the three point rectangle tool

Following On from the above discussion - the following video shows how to structure this when you have Finished Grade for a Pad at e.g. 105’ and then you apply a 5’ subgrade Adjustment and that is where you want your rock overex to be computed from.

You have to be aware of the issues relating to Takeoff here

While Takeoff will allow you to apply an Overex to a Pad, and it will allow you to create a template for your overex that allows for a 5’ offset and a 1:100 tie up, the template does not allow for any conditional If in Rock type scenario - so it will not work in this case. If you were OK to excavate out 5’ all around the pad and then want a vertical tie up then the overex method would work. Since you only want the 5’ Overex in Rock then you have to use either the Corridor Modeler or the Create Sideslope Modeler to model the conditions that you want i.e.

If in Rock Cut then offset 5’ at 0 elevation difference followed by a Sideslope up to Original Ground at 1:100 slope.

Else in Soil i.e. not in rock then just to a 1:100 Sideslope to Original Ground.

End If

Then you have your sideslope lines that you can add to your subgrade adjusted Pad line. i.e. you have your original Building Pad design lines (with the jogs) on Layer FG - Building Pads. You create the rectangles for the pads using 3 Point Rectangle with adjustment and those should go on layer FG - Building Pads (Rectangles) for example. Then you offset those Rectangles at 0 offset and -5’ down to create your subgrade adjusted rectangles - those should go on e.g. FG - Building Pads - Subgrade Adj layer. When you create the Side Slope Lines - you probably want to put the Sideslope Object / template on a layer called FG - Sideslope Templates and the lines that they generate on FG - Sideslope Lines.

Because these are going to be used to model the Rock Cut only and are not really going to be a part of your Takeoff (standard TBC Takeoff Calcs) then I would categorize as follows

FG - Building Pads (Design Category) (these will be used in your standard Takeoff Calcs) (Site Improvements will be applied to these using Identify Site Regions that will give you your Eartworks Qtys down to the bottom of Subgrade - in this case Pads with 5’ excavation below the Finished Grade.

FG - Building Pads (Rectangles) (Categorize as Unused as they will not be used in Takeoff)

FG - Building Pads - Subgrade Adj (Categorize as Unused - same reason)

FG - Sideslope Templates (Categorize as Unused - same reason)

FG - Sideslope Lines (Categorize as Unused - same reason).

Once you have built all of the lines then your rock overex surface can be created using the Create Surface command using the lines on the following layers

FG - Building Pads - Subgrade Adj
FG - Sideslope Lines

Note when you do the volume calc using the Earthwork Report between e.g. Rock and Rock Overex it will give you the total qty of rock excavation in all of the pads - this will include the rock qty in your standard Takeoff Report. The difference between what is in your Takeoff report and in this qty is the 5’ offset material where you are in Rock.

Takeoff Qtys

Earthwork Volume Qtys (separate calc using Surface to Surface)

If you then do a Volume between your Original ground and the Rock Overex surface That will give you the total volume of Excavation (Soil and Rock) for the pad, but remember that the Takeoff Report will have already accounted for most of the material in this scenario - the Rock Overex additional quantity below the Red Line is the only additional material being excavated.

Video hopefully explains this in better detail