Computing Variable Depth Subgrades in Cut or Fill Situations

This question comes up quite a bit.

The typical sections for in this case a riverside walk project state that there is a standard subgrade of e.g. 10" thick which has to be made thicker using an extra 4" of gravel when the path is in a fill condition.

In addition the path has to be excavated 12’ wide to create a 10’ wide path, and this extra thickness of material has to be that additional 1’ wide left and right of the finished grade width.

So the key issue here is to find out where the bottom of the subgrade (10") is above the existing grade and to use an extra 4" of material in those areas.

The approach to use depends a little on the nature of the project - in this case it is a linear project that can be modeled with a corridor - so we can use a conditional instruction to see if the subgrade at Centerline is in Cut or in Fill and if in Fill then to add the 4" subgrade thickness and if in cut to not add the additional 4" thickness of material.

The alternative is to use Takeoff methods to determine the areas in which the base of subgrade is in a fill and then to apply a thicker site improvement in those areas to create the correct subgrade adjustment for the fill conditions. While not an ideal solution I created a mock up of this scenario and used the following commands to determine the necessary areas

  1. Use the Strata definition to create the OG and then a strata 4" thick that I call OX
  2. Use a single borehole with OG and the Top of Soil / base of OX material interface 4" below OG
  3. Use Build surfaces to create the Top of Soil / Base of OX surface
  4. Use the Overexcavation command to determine an Overex surface of OG to the base of the OX material strata layer to be removed in fill
  5. Use the Surface intersection linestring command to find the line(s) along which the overexcavation surface intersects with the OX surface model - these are the locations inside which you need to apply the thicker site improvements. Add those lines to a layer categorized as Design and having potential boundary lines - inside which you can place the thicker site improvement.

Video shows you both methods