Dear colleague

You may also consider the maximum grain size of the soil to the minimum dimension of your model.

The ASTM suggests that the ratio between mold diameter / max. particle diameter is >= 5 otherwise the surroundings will interfere your results.

Good luck

Dear Haider,

In laboratory experiments, there is a kind of experiences where we can take a reduced model from a real project, and we apply experiences to this reduced model, for example, we went to show the behavior of a foundation resting over soft soil, we take a reduced model in laboratory, we apply loading at surface of the reduced model of the foundation, and we measure vertical displacements, but, how we can select dimensions of this reduced model? Are there any correlation’s to choose those dimensions?

Thank you sir.

Hi Tabchouche

My idea is to use a centrifuge apparatus, because the result of this test is very reliable.

Hi Tabchouche,

It depends on what kind of experiment you perform. In case of the centrifuge testing, the scaling factor is N (for lengths) which is same as the factor used to increase the gravitational acceleration (Ng) in centrifuge.

Dear colleague

You may also consider the maximum grain size of the soil to the minimum dimension of your model.

The ASTM suggests that the ratio between mold diameter / max. particle diameter is >= 5 otherwise the surroundings will interfere your results.

Good luck

Dear Laith,

Can you please give the reference of the ASTM paper .

For rockfills, see the paper "The effect of size on the strength of coarse rock aggregates and large rockfill samples through experimental data", Ovalle et al. 2014, Acta Mechanica

The minimum ratio for specimen diameter to height for a oedometer consolidation test is 2.5. ASTM D2435, see below :

6.2.2 Minimum Specimen Height—The minimum initial

specimen height shall be 12 mm [0.5 in.], but shall be not less

than ten times the maximum particle diameter.

6.2.3 Minimum Specimen Diameter-to-Height Ratio—The

minimum specimen diameter-to-height ratio shall be 2.5.

NOTE 5—The use of greater diameter-to-height ratios is recommended.

To minimize the effects of friction between the periphery of the specimen

and the inside of the ring, a diameter-to-height ratio greater than four is

preferable.

for piled raft or piles group or raft foundation: you can choose any dimensions depending on the size of soil tank but you should take into account the stiffness of raft and piles. moreover, for vertical loading the ratio between the diameter of the tank and the raft side should be greater than 3. for horizontal loading the soil tank should be extended to 12x pile dia. The vertical distance between the piles tip and the base of soil tank should be kept at least L, which is the pile length. Depending on these dimensions of the tank, you can choose raft and piles with any material and size, just check their stiffness to get the desired stiffness ( flexible or rigid )

Catalogue of Scaling Laws and Similitude Questions in Geotechnical Centrifuge Modelling

Please refer to:

Authors: J. Garnier; C. Gaudin; S. M. Springman; P. J. Culligan; D. J. Goodings; D. Konig; B. L. Kutter; R Phillips; M. F. Randolph; L. Thorel

in IJPMG

ASTM D3080/D3080M-11 (2012) suggests: the minimum specimen diameter for circular specimens, or width for square specimens, shall be 50 mm, or not less than ten times the maximum particle size; the minimum initial specimen thickness shall be 13 mm, but not less than six times the maximum particle size; the minimum specimen diameter to thickness or width to thickness ratio shall be 2:1.

Ihsan Al-abboodi

Dear sir i read your above comment about small scale piled raft. i am also working on small scale. here is my question that if i want to consider my piled raft of small scale model to show behavior of prototype. to which ratio should i reduce the dimensions so that to get settlement of both system to be same according to scale. i will be waiting for you helpful reply

Wali Ullah

Using dimensional analysis of small-scale data to predict in-situ results directly is not accepted in geotechnical analysis. The ability of small-size tests outputs to be directly extrapolated to a prototype results may be prevented due to the fact that some soil properties (such as density, strength, water content) and foundation characteristics are not easy to simulate accurately in small-scale tests.

Only centrifuge test results can be used for this purpose by using some scaling relationships as in the paper mentioned by Thanh Liem Vo

Dear Professor

Scaling relationships for spiral piles