6.9 Shallow foundations

Three types of loading technique are mainly adopted in bearing capacity tests of shallow foundation; displacement-controlled, stress-controlled and gravity-controlled loadings. Strip, circular or rectangular footing are used in the tests.

Fig. 1 shows a setup of displacement-controlled test. The loading shaft is driven vertically by the motor-jack.

Tatsuoka et al. (1994) conducted a series of model tests on bearing capacities of strip footing on Toyoura sand. They investigated the scale effect which is mainly caused by pressure level and particle size using a modelling of prototype technique.

Fig. 2 shows typical normalized load intensity - settlement curves obtained from 1 G and centrifuge models, and FFMS simulation. Bearing behaviour of centrifuge model is different from the 1 G prototype model (B=50 cm) especially at the portion of large settlement, giving the larger bearing capacity than that of the 1 G model.

Ng (=2q/gnB0) values obtained from the tests are plotted against nB0 in Fig. 3. So-called scale effect that is, Ng decreases with increasing nB0, can be seen in the figure. This difference of Ng between 1 G and centrifuge tests with identical nB0 is due to the effect of sand particle size relative to B0 (the particle size effect). The pressure level effect is typically observed in this series of centrifuge tests changing n with the same B0.Accordingly, the scale effect consists of the particle size effect and the pressure level effect. Note that the particle size effect could be observed also in tests using a same footing but using different types of sand having different particle size but having the same pre-peak deformation properties and peak strength.

 

References

Terashi, M., Endoh, T. and Kitazume, M. (1989) : Planning of model tests and reliability of test results - Bearing capacity of a circular foundation of sand -, Report of the Port and Harbour Research Institute, Vol. 28, No. 3, pp. 59-79 (in Japanese).

Tatsuoka, F., Siddiquee, M.S.A. and Tanaka, T. (1994) : Link among design, model tests, theories and sand properties in bearing capacity of footing on sand, Panel Discussion for Plenary Session B. Foundations, Proc. of 13th ICSMFE, Vol. 5, pp. 87-88.

Tatsuoka, F., Goto, S., Tanaka, T., Tani, K. and Kimura, Y. (1997) : Particle size effect on bearing capacity of footing on granular material, Deformation and Progressive Failure in Geotechnics, IS-NAGOYA, pp. 133-138.

 

Fig. 1 Model setup for a displacement-controlled test (after Terashi et al., 1989)

 

Fig. 2 Comparison of load-settlement relation for an equivalent footing width B = nB0 = 50 cm between 1 G and centrifuge model tests on sand under plane strain conditions and their FEM simulations (Tatsuoka et al., 1994).

 

Fig. 3 Summary of plane strain model bearing capacity tests and their FEM simulations (Tatsuoka et al., 1997)