Strong soil underlain by weak soil

In practise, uniform and homogenous soils can be only seen on occasion and layered soils are more commonly encountered. A method proposed for estimating the bearing capacity of layered soils under a rectangular foundation has been outlined below.

Metric and Imperial Units

The above formulas may be used with both imperial and metric units. As with all calculations care must be taken to keep consistent units throughout with examples of units which should be adopted listed below:

Notation

• B = width of foundation, ft or m
• L = length of foundation, ft or m
• c' = effective cohesion of soil, lb/ft² or kN/m²
• D_f = depth of foundation measured from ground surface, ft or m
• H = depth of weaker soil beneath foundation, ft or m
• K_p = passive pressure coefficient
• N_c, N_q, N_γ = bearing capacity factors (refer The general bearing capacity equation)
• F_cs, F_qs, F_γs = shape factors with respect to soil layer under consideration (refer The general bearing capacity equation)
• q = effective unit weight of soil (refer modification for a water table), lb/ft³ or kN/m³
• q_u = ultimate bearing capacity, lb/ft² or kN/m² (kPa)
• γ = unit weight of soil, lb/ft³ or kN/m³
• ϕ' = effective angle of internal friction, degrees