DHARMCAISSONS AND WELL FOUNDATIONS 763
The perimeter shear stress may be calculated as follows:
The force causing perimeter shear = Ai(H – t)γw
Area resisting this force along the Perimeter of the concrete seal = Pi.t
∴ Perimeter shear stress, σp isσp =AH t
Ptiw
i()
−γ
...(Eq. 19.7)Here Ai is the inside area of the caisson and Pi its inside perimeter.
The value of the perimeter shear stress as calculated from Eq. 19.7 should be less than
the allowable value for concrete. The overall stability of the caisson against buoyancy should
also be ensured.
Total downward force = Weight of Caisson + Weight of Seal + Skin Friction Force. This
should be equal to or greater than the force of buoyancy caused by the pressure γw. H. If
necessary, the thickness of the seal may be increased to ensure this.
19.2.6Cutting Edge
Except for box or floating Caissons which are sunk through water only, the lower ends of the
Caisson wells are made with an inside bevel, reducing the wall thickness to about 100 mm to
450 mm at the bottom. The inside bevel is usually made two vertical to one horizontal. This
bevelled portion of the wall is called the ‘Cutting Edge’, which is meant to facilitate the pen-
etration of the Caisson.
The Cutting Edge also protects the walls of the Caisson against impact and obstacles
encountered during penetration.
A cutting edge is usually made of angles and plates of structural steel or reinforced
concrete and steel. Since sharp edges are easily damaged, blunt edges are more commonly
used. To avoid tearing off the cutting edge, the shell concrete must be anchored to the cutting
edge (Fig. 19.2). The lower portion of the cutting edge is provided with a 12 mm thick steel
plate anchored to the concrete by means of steel straps.
Strap
Anchors(a) Sharp edge (b) Blunt edge
Fig. 19.2 Cutting edge of a caisson19.3 Open Caissons
As mentioned earlier, this type of Caisson is open both at the top and the bottom during con-
struction. It is provided with a cutting edge at the bottom to facilitate sinking. When the