DESIGN OF THE COUNTERFORTS (RETAINING WALLS)

COUNTER FORT RETAINING WALLS:-- When the height of the Retaining wall exceed 6 m Counterfort retaining wall generally proves economical .The spacing of Counterforts depends on the height of the wall , allowable unit pressure on soil , relative cost of steel ,concrete and cost of formwork. By keeping Counterforts closer thickness of vertical wall and heel slab is reduced and amount of concrete and steel is reduced .However this is offset by Concrete and steel required in large number of Counterforts, which are required , and also formwork required for Counterforts. Most economical spacing of Counterforts is one in which total cost is minimum.The economical spacing of the Counterforts is governed by the space required to accommodate steel .Vertical slab is supported on the base and by Counterforts at intervals of 2.5 to 4 m.Vertical slab is supported on Counterforts and designed for a continuous slab .The lowest portion of the stem is subjected to maximum earth pressure. The toe is designed as Cantilever subjected to earth pressure acting upward and self load acting downward ,when no front Counterforts are provided ,when the front Counterforts are provided the toe is designed as a continuous slab supported on Counterforts. The heel is designed as a continuous slab supported on Counterforts. It is subjected to downward loads due to earth and self load and upward force due to earth pressure. The Counterfort is designed as a Cantilever beam taking load from the stem portion between two Counterforts.The effective depth of the Counterfort is taken as distance between the junction with stem and base .The main reinforcement is provided on the inclined side .The section is checked for shear and bond .Two legged stirrups are provided horizontally and vertically to connect the Counterfort with stem and the base respectively.These stirrups are in tension. The front Counterfort is designed as a Cantilever beam subjected to self load acting downward and net earth pressure on the toe portion between two Counterforts acting upward .The section is checked for shear and bond.