Earthworks and Topsoil Stripping Question

I had a great discussion with a relatively new Takeoff User today that I thought would be good to share some of the discussion.

The call started out as a question about the Balance Line function of TBC and led to numerous things that I thought were great concepts that would help many takeoff users.

The customer has a job on an industrial park development. The site has to be stripped and the soils report for the project says that the site has 4 to 6" topsoil to strip. If you take the worst case scenario of 6" the topsoil strip generated creates a lot of Topsoil and they only have a small set of areas in which it can be placed, and they only need a fraction of what is being generated for respread. This created an excess of ~8000 cy of topsoil that has to be wasted because it cannot be placed in the structural fill areas, however they do have embankment areas where it can be placed mixed in with the other cut materials from the project.

From an Earthworks perspective the site is not balanced, there is less cut than is needed for the job, and that can be partially balanced if they include the stripping quantity in the earthworks calcs and then balance the earthworks. So how do you go about this.

Lets say that the site has 10000 CY of Topsoil at 6" depth but they only need 2000 cy for respread so they want to include 8000 CY in the earthworks calculations.

If 6" depth = 10000 CY then 2000 CY = 1.2" thickness (1/5 of the total)

So the first step here is to change the Topsoil Strip depth to 1.2". That will mean that it will now generate 2000 CY of strip that balances the 2000 CY of respread required.

By reducing the Topsoil Strip to 1.2", this raises the Earthworks surface by 4.8" which will increase the amount of Cut Volume and Reduce the amount of Fill Volume by a reciprocal amount. This will then improve the balance of the Earthworks but may not fix it completely.

You have to be clear that of the total Cut and Fill Volume, there will be some areas where you will be placing fill that you will still need to cut off the strip and use it elsewhere in the fill before you place fill in that location.

If you now run the Balance Line calculation which only uses the Earthworks Cut and Fill volumes, it will now tell you how much you need to raise or lower the design in order to achieve a balanced site.

If you need to split out the topsoil element into Topsoil for Respread and Topsoil for embankment fill and soils for general fill, you can use the boring log and strata function to create a single boring log in the middle of the site that has strata

Topsoil Strip - For Respread from 0 to 1.2" deep
Topsoil Strip for Non Structural Fill from 1.2 to 6" deep
Soils for Structural Fill from 6" deep downwards

When you do this - when you define your stripping you can say use top most strata and that will use the first 1.2" of material. The other 2 strata will be broken out in the Earthworks Cut quantity so it is clear in the reports that you have some Topsoil for non structural fill areas and then you have soil for structural fill areas etc.

This will provide a solution to the balance problem.

Remember that the Takeoff calculations has two sides of the computations

Original Ground side of the computations

You have an Original Ground Surface (OG), You have Demolition Areas surface (DEMO) and you have Topsoil strip areas surface (TSSTRIP). The Original Ground with Demolition removed and Topsoil Stripped surface (OG-EWOL) is used for the Earthworks calculation and balance line calculation.

OG - DEMO = Demo Quantities
OG - TSSTRIP = Topsoil Strip Qty

Finished Grade side of the computations

You have a Finished Grade Surface (FG), you have a Subgrade Adjusted surface (SUBG) and you have the Topsoil Respread Surface (TSRESP) on landscaping areas. The Finished Grade with Subgrades adjusted and topsoil respread removed is the surface that is used in the Earthworks calculations (FG-EWOL)

FG - SUBG = Subgrade Quantities
FG - TSRESP = Topsoil Respread Quantity

(OG - EWOL) - (FG - EWOL) = Earthworks Quantity
Note that if you apply strata to OG then the Earthworks Quantity will be split out into the different strata quantities (Topsoil, Soil, Rock etc.)