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82

Netherlands

An example of this calculation method i8 givcn in figurę 6.

For the calculation of the minimum value of the average cone resistance below the pile point the size of the area must be varied gradualy between the measures 0.75 and 3.75 x the pile toe diame-ter. The lowest value of the various results has to be used in the bearing capacity calculation of the pile toe.

A morę detailed description of this method is given by Sanglerat in his book "The penetrometer and soil exploration" (Sanglerat, 1972),

It must be kept in mind that the application of this calculation method is restricted to driven piles only.

For bored piles reduction factors, determined from pile load tests, have to be introduced in the method.

Also the shape of the cone and the sounding procedures have to be taken into account. In sand the penetration resistance of the cylindric-al electrical cone is somewhat higher in com-parison with penetration resistances of the electrical cone with a narrowed shaft (Reijnen, 1973).

The pile shaft friction is calculated on the basis of the data obtained from sounding tests with the mechanical or electrical adhesion jacket cone.

Various parameters are involved in this calculation procedurę, such as type of piles, type of soil, loading characteristics (tension or com-pression) and sounding method (mechanical or electrical) (Begemann, 1969).

Por sand the local friction measured with the mechanical adhesion jacket cone is approximately 2% of the cone resistance. For the calculation of the friction of a pile shaft this value has to be multiplied by a factor depending on the type of pile. For prefabricated driven concrete piles this factor is approximately 0.33; for driven cast in place piles it may amount to ap-proximately 1.

Other factors have to be applied for different type of soils. For piles under alternating load-ingconditions the shaft friction may decreasc considerably. Therefore an extra reduction factor has to be applied in the bearing capacity calcul-ations for this type of loading.

For soundings in sand layers with the electrical cone of the Delft Soil Hechanics Laboratory the frictional resistance to cone resistance ratio amounts to approximately 1%.

Consequently the above pile type factor has to be adapted to the type of sounding.

The ratio local friction to cone resistance also gives reliable Information of the type of soils in a soil profile (Begemann, 1969). For pure cohesive soils this ratio amounts to about 0.05; for pure cohesionless soils the ratio depends on the type of cone.

The determination of important soil parameters from the results of static penetration tests is a very complicated matter.

For pure cohesive soils rather accurate datę for the cohesion c may be derived from the sounding results.

From several comparison tests between soundings and vane-tests or laboratory triaxial tests on undisturbed samples the following approximate relation has been derived:

c_{r :} 15 . c    (3)

in which: c_r = cone resistance c = cohesion

Such a simple relation does not exist for cohesionless soils.

From empirical tests on different types of sand some relation has been established between the slope of the cone-resistance curve with respect to depth and the density or relative density. This relation depends strongly on the type of sand and must therefore be determined for each sand empirically (Plantema, 1957).

Nevertheless soundings are used frequently for the purpose of controlling the compaction of road embankments and other similar construc-tions. The test is reliable and much morę convenient than other in situ density measure-ments.

From only a few comparison tests between soundings and the Mćnard pressure celi it was found that for sand layers a rather good agreement exists between the cone-resistance value c_r and the modulus of horizontal soil reaction k.

For a pile-diameter D = 1 m the relation may be expre8sed by the next relation:

D . k = 4 . c_r    (4)

From the modulus of horizontal soil reaction Young's modulus of the soil may be derived.

Very seldom the cone resistance values are used for the prediction of settlements of compressi-ble soil layers.

As to be expected the employed relation is not very reliable.

5. FUTURĘ DEVELOPMEMTS

One of the main items for futurę development is the standardization of the dutch cone test. For this purpose a committeo has been forraed by the Soil Hechanics and Foundation Engineering Div-ision of the Netherlands Royal Institute for Engineers.

Cooperation of this committee with international groups may be very useful in this matter.

Further research work of the Delft Soil Mechan-ics Laboratory will be directed to refinements in the method in order to derive relevant soil parameters from sounding results.

A promising starting point may be the so-called rotating penetrometer with different types of cone-shaped points (Smits, 1972).

The effort put into this research will certainly be worthwhile.

The dutch cone test has proved its usefulness in numerous investigations sinco it was de-veloped in the Netherlands in 1930.