Retaining Wall Soil and Backfill: What Actually Matters

Two soils control your wall: the backfill behind it (how hard it pushes) and the foundation under it (how much it can carry).

Backfill: lower pressure is better

IBC Table 1610.1 sets the design lateral load as an equivalent fluid pressure:

• Clean sand / gravel (GW, GP, SW, SP): 30 pcf, the goal.
• Silty soils (GM, SM): 40-45 pcf.
• Clayey / low-plasticity (SC, ML, CL): 60 pcf.
• High-plasticity / expansive clay (CH): avoid entirely, it swells and traps water.

That's why engineers so often replace native clay with clean granular fill behind the wall: it halves the push and drains freely.

Foundation: higher bearing is better

IBC Table 1806.2 sets the presumptive allowable bearing:

• Gravel (GW, GP): 3,000 psf
• Sand / silty / clayey sand: 2,000 psf
• Silt / clay (CL, ML): 1,500 psf
• Rock: 4,000 psf (sedimentary) to 12,000 psf (crystalline)

Weak foundation soil means a wider base or a deeper footing. The calculator lets you pick both soils and shows how each changes the result.