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des: point resistance, lateral friction and total resistance, in order to have a continuous chec-king by summing up and see if anything wrong was going on. From this point of view, the use of automatic recorders would be very useful to prevent any change in the field data.

Finally we should mention the misleading rcsults which may be obtained when coarse particles are present in which case dynamie tests are prefera-ble.

3. 2. Intepretation of test results. - The way in which test results are interpreted is difficult to summarize. The International literaturę is today available to everybody and therefore each Engi-neer chooses the methods best suited to his know-legde and particular situation. We will therefore only try to give a rough outline of which are the generałtrends.

a)    The SPT is usually employed for design purpo-ses only in the case of foundations in granular soils. In cohesive soils its use is limited to a qualitative assessment of the layering of the ground, but perhaps for unimportant foundations under well known local conditions.

In the case of granular soils, the interpretation is often based on the Terzaghi-Peck criteria as well as on Meyerhoff work and other morę refi-ned methods and corrections introduced by seve -rai authors.

b)    The cone test is usually intepreted only througi the point resistance R_p. The lateral friction de-termined directly by the friction mantle is consi-dered by most engineers as unreliable.

In the case of deep foundations the methods deve-loped by the main foreign research workers on the subject, as collected in Sanglerat's book, are used. This same book, which has been widely po-pularized among Spanish Engineers, describes several methods for the case of shallow foundations. Besides, other methods are used by some Consultants, like the one developed by Schmert -mann for calculating settlcments of shallow foundations in sand.

c)    The dynamie continuous penetration test is usually employed in a qualitative way. Nevertheless for foundations in sand of little importance where no better information is available, some empiri-cal correlations are used, at least for prelimina-ry design. The Borro's company for instance fur-nishes some criteria for shallow foundations and piles that we are not going to reproduce here.

Also by using some dynamie penetration formulas, simple correlations are deduced for calculating the allowable sur face load and probable values of the cone point resistance R_p, method sometimes used as a first approach by the D.G. C.

The value of N is also approximately correlated with that of n20» obtained with the continuous dynamie penetration test, through simple expres-sions like (only for sands): where k varies between 0, 7 and 1,0 according to the different Departments of the D.G.C.

4. NEEDS OF FUTURĘ DEVELOPMENT.

Penetration tests are not the panacea to solve our geotechnical problems as some people pre-tend, but properly combined with other methods are an extremelly useful tool.

In the case of cohesionless soils, where undis -turbed samples are very difficult to obtain, they become an essential part of the soil survey, sińce they are practically the only available source of quantitative information. Therefore any fur-ther developmcnt of these methods may have a significant practical value.

The SPT has demonstrated to be a useful method as a complement of a boring where samples are to be taken. Nevertheless when used in submer-ged sands it may give very misleading results, either because an improper driving of the casing may compact the soil in the zonę where the test is to be run or because a loosening is originated due to piping at the botton. We can imagine seve-ral ways to try to avoid such effects, but the truth is that we will never be surę that the results obtained have not been influenced by the boring operations. Therefore we think it is unwi-sc designing foundations with only the data of the SPT. But on the other hand it would not either be wise ignoring the results of this test which may be very useful, sińce it combines "rough" aampling with "rough" resistance values. Then the best thing to do is standardizing it as far as possible. And we say "as far as possible" because elaborating too much the procedurę would render too expensive a test that, as we said be-fore, we think will never be completely reliable.

The main things to consider in such a SPT standardization would be:

a)    Detailed way to proceed for perforating the boring, especially in submerged sands in the proximity of the level to be tested, indu-ding advance sequence of the casing and cleaning operations, rangę of diameters for the casing, etc.

b)    Methods to avoid piping or loosening at the bottom, like by keeping the hole filled with water or slurry. Use of smali diameter bailers slowly moved, etc.

c)    Exact definition of the way to apply the drop