6585844981

189

Israel

Fig. 6 - Correlation between D_R aild

0 for Israeli Dune Sands

expected blow count for the unloaded sand lies in the increased strength stored in the sand due to prestressing and the resi-dual horizontal stresses after removing the surcharge. Laboratory confined com-pression tests were carried out on similar sands in which lateral pressures were raeasured, while the vertical pressure was reduced from 40 ton/sq.m. to 10 ton/sq.m. (The same as in the field) and showed a corresponding drop in lateral pressure from 16 ton/sq.m. to 8 ton/sq.ra., thus indicating a residual Ko value of 0.8. A residual lateral pressure has an effect on the blow counts similar to an increased overburden pressure. But it should be remembered that in shallow depths the re-bound movements may be enough to cancel this effect.

For proper design of foundations in sand layers it is necessary to evaluate the stress-strain properties which will allow prediction of settlement. In atteropting to develop correlations between the SPT and the stress-strain characteristics of a sand layer, it roust be born in mind that the situation is complicated by the variation in stress levels and the stress distribution as influenced by the type and size of the foundation used. The SPT results in themselves are definitely not a direct measure of the stress-strain characteistics. Therefore indirect correlations are applied to allow SPT values to be used for estimating relative den-sities, load-settlement relations, or a modulus of elasticity. Such a correlation was given by Komornik et al (1970), showing SPT results versus E_o from borings

in sand strata as determined by the Menard pressuremeter (Fig. 7).

Fig. 7 - Pressuremeter E₀ versus Blow

8

Count Ń (Sands)

It has been found that the values re-

s

sulting from first loadings in the pressuremeter tests are about 1/5 to 1/8 of the values found in rebound and reloa-ding determinations (Komornik et al, 1970 a). Current experience has shown that the higher E values agree better with

field observations. Hence, it may be re-commended that approximate evaluation of the E_g may be obtained by the formula:

E = 4N, MN/m² s

In many cases a modulus of subgrade reac-tion, k, is desired for the evaluation of settlements in sands. The most commonly employed raethod involves the use of the SPT values, cotrected for the effect of overburden, to estimate the relative den-sity. The value of k is then selected, based on the charts proposed by Terzaghi and Peck (1967), which furnish the allow-able soil pressures for 1 inch setllements. A chart which conforms to these relations, including the effect of piąte size as based on Kogler*s tests has been prepared by Alpan and is shown in Fig. 8. Details of amethod of analysis based on this approach is included in a publication by Alpan (1964).

SPECIAL CASES

Several cases are presented below where the SPT raethod was useful, but reguired special interpretation because of unusual conditions.

Geological conditions have produced layers of slightly cemented sandstones along the Israeli coast (Kurkar)• These layers may appear in various forms, such as layers of several meters thickness, thin plates with sand formations in between, and as particles of gravelly materiał in sand formations. It has become important, especially for heavily loaded structures, to identify these layers and to utilize them for founding either by spread footings, or deep piles or caiss-ons. While investigating sites along the coastline, the ratę of penetration of the SPT spoon in some borings was some-