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paper by N. Janbu and K. Senneset.)

Rot* of ponotroflon 2 ca/tok

Cono foUihmc* MN/a*

FIG.2.10. Cone resistance and porę

pressure observations in sand.

Norway

lakę, Mjosa, and it has been used for some research works done by The Norwegian Institute of Technology, and by The Norwegian Geotechnical Institute itself.

The intention of the cone penetrometer test research done here at the University, was to find relations between penetration resistance and the soil parameters. The theoretical investigations reąuire know-ledge of the porę pressure conditions around the tip. This necessary "porę pressure"-sounding was done by using an electric piezometer with the same outer dimensions as the cone penetrometer. The piezometer is of NGI-type, originally constructed for pore-pressure measure-ments ir. sea bottom. The porous filter is placed ca. 1 cm above the cone point. Behind the filter is a water-saturated chamber. The outer porę pressure is measured by a vibrating-wire device, placed in contact with the chamber.

This ^,łpore-pressure^u-sound was pushed down with the same ratę of penetration as the compared penetration test. In this way we got three comparable informa-tions for a soil stratum as a function of depth:    the cone resistance, the fric-

tion resistance and the porę pressure around the penetrometer tip. A new cone penetrometer will be built, where the porę pressure gauge is placed in the cone tip. This three informations will then be recorded at the same time.

Port prttMTt kN/

0    J00    400

Roto of    i 2cm/*k.

Cm rmiifono MN/ mf

FIG.2.9. Cone resistance and porę pressure observations in moraine clay.

In Figs. 2.3 and 2.10 the cone resistance and the recorded porę pressure is shown for two soils, the first one is a morene clay, the second one is a sand deposit. The ratę of penetration for the sounding shown is 2 cm/sek. (Interpretation of test results will be shown in a special

2.7 Corrosion sounding

For structures where Steel piles, sheet piling, Steel pipes or other buried Steel components may be used, it is im-portant to know the degree of corrosivity of the subsurface soil layers.

The Norwegian Geotechnical Institute has designed a corrosion sound which permits a rapid in situ determination, in satur-ated soils, of the two most important factors concerning corrosion ratę, i.e. the cathodic depolarization and the specific soil resistance.

Fig. 2.11 shows a photo of point and instrument. The corrosion sound consist of a Steel pipę eąuipped with a magne-sium tip isolated from the Steel pipę.

The Steel pipę and the magnesium tip are connected to the measuring instrument by isolated conductors. The cable have to be pulled through hollow rods, diam. 32 mm. The corrosion sound itself has a length of 0.5 m and a 31 mm diameter.

The instrument readings give, as function of depth, the cathodic depolarization and the earth resistance. There is also a reading for the corrosion in ym/year (1 pm/year corresponds to 1 mm per 1000 years.)

This sounding method has been in use for many years. It has been possible to control the predicted ratę of corrosion of Steel piles, but the corrosion speed