For roads and platforms, subgrade strength, often stated as CBR, decides how much aggregate and which geosynthetic are needed. For liners, the subgrade must be smooth, compacted and free of stones, roots and sharp edges that could puncture the geomembrane. Soft, wet subgrades drive many geosynthetic choices, from separation fabric to stabilization grid.
Subgrade CBR is under 1 in the low spot, so it's grid plus woven and a thicker first lift.
Walk the subgrade before deployment and pick out every stone and root; liner installers usually have to accept the subgrade in writing before they start.
Also heard as
- foundation soil
- native soil
- formation
Related terms
Separation
Keeping two different soil layers, such as an aggregate base and a soft subgrade, from mixing under load.
Stabilization
Improving how soft ground or an aggregate base carries traffic by adding geogrids or geotextiles.
Pumping
Fines from a wet subgrade being forced up into the aggregate base by repeated traffic loads.
Undercut
Digging out soft or unsuitable subgrade below grade and replacing it with better fill.