Foundation Design
Foundation design bridges structural and geotechnical engineering, focusing on transferring structural loads safely into the ground. Design is governed by two strict, independent criteria: 1) Bearing Capacity (Ultimate Limit State) to ensure the soil does not fail in shear, and 2) Settlement (Serviceability Limit State) to ensure displacements remain within tolerances for the superstructure. Shallow foundations (isolated footings, strip footings, mat/raft foundations) are utilized when competent soil is near the surface. Terzaghi's bearing capacity theory models soil failure as a combination of three resisting forces: soil cohesion, surcharge (the weight of the soil above the foundation base), and the internal friction of the soil wedge beneath the footing. Deep foundations (driven piles, drilled shafts) bypass weak surface layers to transfer loads via skin friction along the shaft and end-bearing resistance at the tip. Structurally, concrete footings must be designed to resist bending moments (dictating flexural reinforcement), one-way (beam) shear evaluated at a distance 'd' from the column face, and two-way (punching) shear evaluated at 'd/2'.
Knowledge Check
1.A square footing supports a concentric vertical load of 1200 kN. Geotechnical reports dictate an allowable bearing pressure of 150 kPa. What is the minimum required footing width to prevent bearing failure?
2.According to ACI 318, at what distance from the face of a column is the critical section for two-way (punching) shear evaluated in a concrete spread footing?
3.In Terzaghi's ultimate bearing capacity equation (q_u = c'N_c + qN_q + 0.5γBN_γ), what physical contribution does the 'qN_q' term account for?
4.For a spread footing subjected to both a vertical axial load (P) and a bending moment (M), the load eccentricity is e = M/P. To maintain compressive stress over the entire soil footprint (preventing uplift), the eccentricity 'e' must not exceed what value for a footing of width B?
5.Why is a Factor of Safety (FoS) of 3.0 commonly applied when calculating the allowable bearing capacity for shallow foundations, which is much higher than typical structural factors of safety?
6.A driven friction pile relies entirely on shaft resistance in a deep clay layer. If the pile is 10 m long, has a diameter of 0.5 m, and the average ultimate skin friction (f_s) of the clay is 50 kPa, what is the ultimate axial capacity of the pile?
7.When is a mat (raft) foundation typically selected by engineers over individual isolated spread footings?
8.What is the ultimate bearing capacity (q_u) of a surface strip footing resting on a purely cohesive clay soil (φ = 0, N_c = 5.14, N_q = 1, N_γ = 0) with an undrained shear strength (s_u or c) of 100 kPa?
9.In geotechnical foundation design, what is the exact distinction between 'gross' bearing pressure and 'net' bearing pressure?
10.A structural concrete footing is to be cast directly against the earth without the use of formwork or a lean concrete mudmat. According to ACI 318, what is the minimum concrete cover required for the steel reinforcement?