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If the stratigraphy is differentiated, foundation soil different from that of elevation, if possible, avoid making the base stratigraphy share perfectly coincide with the foundation dimension.
The figure shows the correct scheme to follow.
Attention: this is only a trick because by making them coincide MDC assumes the geotechnical characteristics of the base layer for limit load checks.

VESIC  Project limit load in short term
In order that a foundation may safely sustain the projected load in regard to general rupture for all combinations of load relative to the ultimate limit state, the following must be satisfied:
where:
Vd is the project load at ultimate limit state normal to the footing, including the weight of the foundation itself;
Rd is the projected foundation ultimate limit load for normal loads, also taking into account eccentric and inclined loads. When estimating Rd for fine grained soils short and long term situations should be considered.
Limit load undrained conditions is calculated by:
where:
Project effective foundation area. There where eccentric loads are involved, use the reduced area at whose center the load is applied. 

cu 
Undrained cohesion. 
q 
Total lithostatic pressure on footing. 
sc 
Foundation shape factor. 

Rectangular shapes. 

Strip foundations


Correction factor for inclination of load H. 
Depth factor. 
VESIC  Project limit load in drained conditions is calculated as follows:
where:
Form factors
Rectangular shape 

Rectangular shape 


Rectangular, square, or circular shape 
Resultant inclination factors due to an horizontal load, H parallel to B'


Depth factor.


HANSEN  Project limit load in short term
Limit load undrained conditions is calculated by:
where:
Project effective foundation area. There where eccentric loads are involved, use the reduced area at whose center the load is applied. 

cu 
Undrained cohesion. 
q 
Total lithostatic pressure on footing. 

Strip foundations


Correction factor for inclination of load H. 
Depth factor. 
HANSEN  Project limit load in drained conditions is calculated as follows:
where:
Form factors
Rectangular shape 

Rectangular shape 


Rectangular, square, or circular shape 

Strip foundations 
Resultant inclination factors due to an horizontal load, H parallel to B'

Depth factor.


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