The Concept of Effective Stress in Unsaturated Soils - Multiscale Geomechanics: From Soil to Engineering Projects - page 173

600

1

2

Quasi-saturated tests :
η max = 1.54

3

6

400

5

4

7

8

9

200

11

10

0

0 200 400

Effective mean stress p' = p+ σ ' cap (kPa)

Figure 6.17. Interpretation of results of large strain triaxial tests using the
micromechanical model for determining effective stress [1: w=3.74%;
2: w=6.25%; 3: w=7.5%; 4: w=8%; 5: w=9.5%; 6: w=10% 7: w=10.4%;
8: w=14.1%; 9: w=16.1%; 10: w= 17.9%; 11: w=18.3%]

6.5.2. Interpretation of small strain modulus measurements

The normalized values of the modulus for the different tests, shown in Figure
6.15, have been plotted in Figure 6.18 as a function of the effective vertical stress:
σ' v = σ v + σ ' cap ( p a = 0). To interpret the results, the values of the capillary pressure
were derived from correlations based on the measurements made at different water
contents, void ratios, and confining stresses. For quasi-saturated specimens,
σ ' cap = p a - p w and for dry specimens, σ ' cap = 0. For unsaturated specimens, the
values of σ ' cap were derived from the micromechanical model, using the same
“characteristic dimension” R = 1.7 µm as for the other triaxial tests and the function
shown in Figure 6.16. Expressed against the effective stress, all the results (from
w = 0% to w = 18.3%) are correctly located near the regression line of the dry and
quasi-saturated specimens, with equation

E v0 = 200 (σ' v / p atm ) 0.68

[6.25]

Considering that the results were obtained on samples of different sizes, under
very different conditions, with different experimental devices, from values of pore-
water pressure deduced from correlations and for deformations below 10 -5 , the
amount of scatter in the data is rather small.