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Wednesday, April 18, 2018

Calculation of Settlement of Spread Foundation (EC7 Part 2)

In this post, we are going to talk about an approach described in Annex E of EN 1997-2:2006 (Eurocode 7 Part 2) for determining the settlement of spread foundations. This is based on semi-empirical method and the result of MPM test (Menard Pressuremeter Test). 

The formular for evaluating the settlement of a spread footing is given below;

Bo is a reference width of 0.6 m;
B is the width of the foundation;
λd , λc are shape factors given in Table E.2;
α is a rheological factor given in Table E.3;
Ec is the weighted value of EM immediately below the foundation;
Ed is the harmonic mean of EM in all layers up to 8 × B below the foundation;
σv0 is the total (initial) vertical stress at the level of the foundation base;
q is the design normal pressure applied on the foundation.

The Table E.2 and E.3 of EC7-2 are given below;

Solved Example
2m x 2m square footing is carrying a quasi-permanent SLS combination load of 1040 kN. The Menard Pressuremeter moduli values are as given in the sketch below. Calculate the settlement of the foundation if the unit weight of the first layer of soil is 18.5 kN/m3.

σv0 = 18 kN/m3 × 1.2m = 21.6  kN/m2
q = 1040 kN / (2m × 2m) = 260  kN/m2
B = 2.0m (width of the foundation)
λd = 1.12 (square foundation)
λc = 1.1 (square foundation)
α = 0.67 (normally consolidated clay)
Ec = 16.8 Mpa
Ed = 3/[(1/16.8) + (1/27) + (1/33)] =  23.447 Mpa

Substituting these values into the settlement equation;
Therefore, the settlement of the foundation is 5.358mm.

What do you think about this approach?

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