Utilizing the Cohesive Strength of Soil for Resistance to Sliding and Overturning

For isolated footings designed to the Indian code, the resistance offered by the cohesive strength of soil to lateral forces and overturning moments is another option available (in addition to passive resistance) to augment the resistance. This is an optional item, meaning, the engineer is given a choice as to whether you want to include this resistance.

The general principles involved in this feature are very similar to those described for passive resistance.

The cohesive strength is mobilized on the same face on which passive resistance is calculated. For example, in the figure below, against the force FX, the resistance from both effects are mobilized on the face 4-3-7-8 for the footing pad, and on the face D-C-G-H for the pedestal. Similarly, the resistance against the force FZ is mobilized on the face 2-3-7-6 for the footing pad, and on the face B-C-G-F for the pedestal.

If this is included, the resisting force and moment contributed by this property of soil will be added to those generated from friction and passive resistance.

Procedure Used in Calculating Resistance from Cohesive Strength

The resistance due to cohesive strength is calculated for each of the 2 horizontal directions (X and Z). The steps involved in this calculation are shown below. The text book Soil Mechanics and Foundation Engineering: Geotechnical Engineering series by V.N.S.Murthy is used as the basis for these equations.

Resisting Force along X $= 2. C . \sqrt{K p} \times (h \times b + D \times B)$

Resisting Force along Z $= 2. C . \sqrt{K p} \times (h \times l + D \times L)$

Resisting Moment about global X $= 2. C . \sqrt{K p} \times [h \times b (0.5 h + D) + D \times B (0.5 D)]$

Resisting Moment about global Z $= 2. C . \sqrt{K p} \times [h \times l (0.5 h + D) + D \times L (0.5 D)]$

where

L	=	Footing dimension along global X
B	=	Footing dimension along global Z
l	=	Pedestal dimension along global X
b	=	Pedestal dimension along global Z
D	=	Thickness of footing pad
h	=	height of pedestal

These forces and moments are added to the sliding resisting force and moment as described earlier.

Input for the Program

There are two terms required to instruct the program to calculate the passive pressure resistance.

In the Sliding & Overturning page, select Yes for Consider Cohesion Effect.

In the Cover & Soil page, ensure that the Soil Type selected is Cohesive and provide the value for Cohesion strength.

Output for the Program

In the calculation sheet produced by the program, the output for resistance due to cohesive strength will be reported through the terms shown in the next figure.