Terzaghi’s Principle states that when a rock is subjected to a stress, it is opposed by the fluid pressure of pores in the rock. reading that total stress is equal to the sum of effective stress and pore water pressure.
(1) The soil is semi-infinite, homogeneous and
(2) the problem is two-dimensional,(strip footing)
(3) the base of the footing is rough,
(4) the failure is by general shear,
(5) the load is vertical and symmetrical,
(6) the ground surface is horizontal,
(7) the overburden pressure at foundation level is equivalent to a surcharge load (shear resistance is ignored)
(8) the principle of superposition is valid,
(9) Coulomb’s law is strictly valid,
Zone I of elastic equilibrium (cohesive resistance)⇒
the soil located immediately beneath the base remains permanently in a state of elastic equilibrium, and the soil located within this central Zone I behaves as if it were a part of the footing and sinks with the footing under the superimposed load. The depth of this wedge-shaped body of soil ABC remains practically unchanged, yet the footing sinks. The sinking of Zone I creates two zones of plastic equilibrium, II and III, on either side of the footing. It can be considered as Rankine Active zone. (settlement is necessary for mobilization of shear strength).
Zones II of radial shear state⇒
Frictional resistance resulting from the surcharge from the q0 at the footing level. ∅ (curve shape is logarithmic spiral) Zone II is the radial shear zone whose remote boundaries bd and af meet the horizontal surface at angles (45° – Ø/2), whereas Zone III is a passive Rankine zone. The boundaries de and fg of these zones are straight lines and they meet the surface at angles of (45° – Ø/2).
Zones III of Rankine passive state⟹
Frictional resistance resulting from the weight of soil within the failure zone. it touches the foundation level at an angel of (this zone is in plastic stage) Ultimate Bearing Capacity for shallow strip footing q= is overburden pressure. Zone III is a passive Rankine zone. The boundaries de and fg of these zones are straight lines and they meet the surface at angles of (45° – Ø/2).