Why shear keys are provided in retailing walls?

 

While designing retaining walls, to determine their stability, we check failure criteria like sliding, overturning criteria etc. In sliding criteria, the sliding resistance is found out by finding base friction between the base of the retaining wall and the soil. To increase this sliding resistance, shear keys are provided at the base of the retaining walls.

The basic principle is that shear key provide extra passive earth pressure which increases stability in sliding. But at the same time, active earth pressure is also increasing on another side. So, the shear key will serve its intended purpose only if the increase in passive earth pressure exceeds the increase in the active earth pressure due to the shear key.

Reasons for providing Raft foundations

 
1.   Low Soil bearing capacity -----------i) Good soil but γ_d is low
                                                             ii) High groundwater table/ seepage pressure
                                                            iii) Filled up soil / organic soil like peat, muck  

2.  Very heavy loads such that overlapping of footing area

3.  Sum of area of individual shallow footings is more than 50 % of land area ( Thumb rule)

4.  Presence of loose compressible pockets. If these pockets are within range of pressure bulb of individual shallow footing, it can create a large settlement. But if we provide raft footing, this effect is minimized due to large pressure bulb area

5. To resist large hydrostatic uplift pressure

6.  Some structures by nature require raft foundations. Ex. Elevated water tank, chimney, silos etc.

7.   When structure and equipment to be supported are very sensitive to differential settlement

Why maximum permissible settlement is more for clayey soil compared to sandy soil?

 

The maximum permissible settlement depends upon the type of the soil, the type of the foundation and structural framing system. 

As per IS 1904-1986, for RC structures;

Type of foundation	Maximum Permissible settlement for Sandy soil	Maximum permissible settlement for Clayey soil
Isolated Foundation	50 mm	75 mm
Raft Foundation	75 mm	100 mm

The progressive settlement on clays permits better strain adjustments in the structural members. And for a unit increase in a total settlement of clay, there is less increase in differential settlement compared to sand. That is why the maximum permissible settlement is more for clayey soil compared to sandy soil.

What is the science behind Quick Sand?

Some may think that "Quick sand" is the type of sand. But it is not. It is the hydraulic condition of sand. 

When the flow of water takes place in an upward direction, the seepage pressure also acts in the upward direction and the effective pressure is reduced. If seepage pressure becomes equal to the total pressure due to the submerged weight of soil, the effective pressure reduced to zero. In such a case, a cohesionless soil loses all its shear strength and the soil particles have a tendency to move up in the direction of flow. This phenomenon of the lifting of soil particles is called quick condition or boiling condition or quick sand.

-Quick condition mainly occurs in fine sands. For clays, the shear strength of soil does not reduce to zero even when effective stress becomes zero, due to its cohesion. Hence, quick sand condition does not occur in clays. 

-During quick condition, the hydraulic gradient becomes critical, i.e. i_c = 1.

Where it can occur? 

-Downstream of Dam site due to high artesian pressure.
-An excavation into the sand below groundwater table where water is pumped out to work in dry condition etc.

How to prevent it?     

-Lowering water table in the area

-keeping the certain depth of water in excavation trenches

Why pore water pressure is called Neutral Pressure?

When soil mass contains water in pores, pressure transmitted through the pore water is called pore water pressure or simply pore pressure. But why it is also called " Neutral Pressure"?

The pore pressure act equally in all directions and in all sides of the soil particles. So it does not cause the particles to press against each other and hence it is not effective in decreasing void ratio or in increasing shear strength of soil mass and remain neutral without changing soil mechanical properties. That is why it is called Neutral pressure.