Overexcavation of the Existing Grade to 1.0' Depth in Fill Areas

This request came up this week and I thought it was a good one to show how to go about it. There are 2 approaches that work (at least) to solve this request - the first is a simple subgrade adjustment work process and the second a Takeoff Overexcavation process.

Simple Subgrade Adjustment Process

The process for this method involves the following steps

  • Import the 2 surface files - Existing and Design (or adjusted versions as necessary
  • Compute the isopach surface between the two surfaces (Use Earthwork Report)
  • Create the 0 contour for the Isopach surface using Contour at Elevation (you could use the Surface Intersection linestring command also for this step of the process - either works)
  • Explode the contours with the Explode command to turn the contour objects into usable lines
  • Create the boundary around the Isopach surface to find the edge line of the surface - this can be used with the 0 contour to find the areas of Cut and Fill - use the Extract Surface Features command to create this boundary (the TBC Create Surface Edge Breakline command will not create an edge breakline for a n Isopach surface.
  • Use the Track Region command to find the boundary lines for each of the Fill areas that you decide you have to over excavate - note that there may be a number of very small fill areas - only you can decide if those need over excavating or not (get your teaspoons out …)
  • Use the Change elevation command to drape the fill boundary lines onto the existing surface.
  • Check the properties of the lines to make sure that the Surface Sharpness Property of the line is set to “Sharp & Texture Boundary” this is needed for the Subgrade adjustment process.
  • Use add / remove surface members to add those lines to the existing surface model
  • Now use the Materials and Site Improvement Manager to define a material for the Over excavation - you can just create a category in Earthen Materials, and then create a material called Over excavation Backfill and give it a color e.g. Orange.
  • Now use the Materials and Site Improvement Manager to create a Site Improvement called Over excavation Backfill and set the layer to be material Over excavation Backfill and make it 12" thick
  • Now you can use the Site Improvement Command in the Surface menu to apply Site Improvements of the Over excavation Backfill to each of the polygonal areas of fill.
  • Now use the Create Subgrade command in the surface menu to create the surface from Existing model to the bottom of Engineered Materials - this will create you the Over excavation Adjusted Existing Model surface.
  • Compute the volume between Existing and the Over excavation Adjusted Existing to determine the volume of dirt to be over excavated and the additional volume of Fill required to back fill the Over excavation areas.

Takeoff Method

The process for this method includes the following steps

  • Create the Original Ground Takeoff Surfaces - use the categorize Layers function to put the Original Ground linework layers in the Original Category of Layers and Build Surfaces to create the Original Ground model.
  • Use the Define Strata command to define the Strata for the project. Use the Original ground model as the top surface, and then define a strata layer called 1 FT Over excavation and set the material below OG to be the Over excavation Material and the Material below the 1 FT Over excavation layer to be Soil. We are going to use this Strata in the Over excavation calculations.
  • In the same way as above - you can use the 0 contour function or the Surface Intersection Linestring command to find the intersection between the Original ground and Finished Design surfaces. Use the Isopach boundary in combination with the 0 contours to create the Fill Boundary areas.
  • Create a Single Boring Log in the middle of the project and enter the strata for the Over excavation layer at 1.0 depth or thickness below Original ground.
  • Run the Takeoff Over excavation command to excavate 1.0’ of material inside all of the Fill Boundaries that you have defined.
  • Run the Build Surfaces command to build the Over excavation adjusted surface.
  • You can now run the Takeoff Report or the Surface to Surface Earthwork Report to determine the volumes of Over excavation.

Video shows you how