This Stormtech Subgrade Performance technical note explains loading scenarios, subgrade performance considerations and a general foundation overview.

StormTech chambers, as buried arch structures, concentrate overburden loads at the chamber feet and spaces between rows of chambers. A foundation layer of crushed stone under the feet of the chambers is used to partially disperse these concentrated loads to an appropriate bearing pressure on the subgrade.

A loading scenario is offered featuring a typical StormTech system figure. Figure 2 is a conceptual loading diagram of the StormTech subgrade.

Subgrade Performance Considerations determine allowable bearing pressure which is routine geotechnical practice which involves consideration of ultimate bearing capacity (resistance to shear failure), settlement conditions, local expertise, and other site- or project-specific factors. The native subgrade below a StormTech system must be stable and unyielding for proper function of the system and for protection of surface developments. Bearing capacity failure is not tolerable. Local shear (punch shear) is an important design consideration, given the narrow row spacing and relatively thin over-loaded zone of the subgrade. The chamber system is tolerant to minor settlement. Settlement tolerances are intended to provide the basis for foundation design and ensure structural performance of the chamber-soil system. If the system design allows for infiltration (i.e. no impermeable liner is specified) then water content of the subgrade soil is expected to increase above natural levels immediately after a storm event. The degree of saturation will depend on the hydraulic conductivity of the subgrade and the storm intensity among other factors. If an impermeable liner is to be used, then it can be assumed the StormTech system will not increase the saturation of the subgrade.

Minimum foundation depth tables with formulas and general overview of shallow foundation design by ASD & LRFD are provided. Table 1 shows the StormTech Chamber specific dimensions and references.