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meter is manually driven at a standard ratc of pcnctration of lcm/sec and for every lOcm depth, thc penetration resistance is rccorded. In the single rod type of( a), the tip resistance cannot be determined correctly on account of skin fric-tion acting on the rod and thcrefore, if nccessary, thc double-tube type of( b) has to be employed}^)

Calculation of the penetration resistance 9_r is madę according to the above-mentioncd deep sounding.

5.2 Appraisal and usc of measured data

Figurę 15 shows an example of the data which comparc the penetration resistance q_c with the unconfined compression strength 9_U of diluvial and alluvial clays using a simple cone with the tip angle of 3C/ and the base area of 6.45 cinS Though the valucs depend on thc type of cohesive soils, the following relation gcnerally holds: 12)

9_f » 5 9_U “ lOr(where0 ? 0)

The penetrometer illustrated in Figurę 14(a) has proven useful for route reconnaissance of extremely weak soil layers and also for construc-tion of fili consisting of cohesive soils of volcanic origin for which thc value of 9_C is taken asa measure of degree of compaction.

The one illustrated in Figurę 14(b) is mtend-ed for obtaining detailed strength data on an ex-tremely weak soil laycr whose presence has been revcaled by reconnaissance. With no skin fric-tion involved during measurement , the cohcsion c can be estimated from the test rcsult and accord-ingly this penetrometer is often utilized for pre-diction of the base failure of fili constructcd on a weak soil layer.

6. FUTURĘ PROBLEMS AND FUTURĘ PROS-PECTS OF PENETRATION TESTS

The most widely-uscd penetration test in Japan is the Standard Penetration Test introduced from the U. S. and now the "JV-value" is consider-cd indispensable for generał evaluation of subsurface conditions. With such popularity In prac-tice, for the first time in 1961 the penetration test was specified and included in theJapanese Indus-trial Standard (JIS). This standard, howcver, suffers from its incompleteness in several points and its revision is presently underconsideratipn^ The major points to be revised are as follows:

1)    Specification of the procedurę to guarantee the free fali of the drop hammer.

2)    Standardization of details of tools used.

3)    Necessity for recording of penetration for each hammer blow.

4)    The maximum depth fortf-yalue measurement and correction of jV-value for depth.

Of the above four points, the most important is l ); at present some experiments are under way to find how much difference there is between thc iV-values measured by the cone-pulley method and those measured by the "Tombi" (triggering) method. In this connection it is felt necessary to establish an international standard for this procedurę including the possible use of a fcw automatic drop-hammer devices which havc been developed. In this regard, exchange of informa-tion is desirable with the countries which prac-tice the standard penetration test. It is also desirable to discuss points 2) and 3) intcr-nationally.

Regarding 4), considerably wide variancc is observed dcpending on thc soil conditions. This problem directlyconcerns the reiiabilityof subsurface investigations in view of the latest tendency in which the foundations are becoming larger and deeper, it is about the time when a certain guide be established.

ln 1963 a standard was proposed for Swed-ish Sounding by the Japanese Society of Soil Mcchanics and Foundation Engineering, but being as old as lOyears. it is now being reviewed for possible revision}l)

The Japanese Society of Soil Mechanics and Foundation Engineering Standard for the Dutch Double-tube Type Cone Penetration Test was proposed in 1968, but some ąuestions have already been raised. As stated earlicr, in Japan wherc such soundings are freąuentlyconducted in weak soils, it is a problem to interpret the data obtain-ed when thc inner rod drops by its own weight.

In cxtremely weak soils, thcrefore, it is necessary to use a portable light weight penetrometer with scrcwKTonnectcd inner rods or else anlsky-meter.

With rapid progress expected in the con-struction of the nationwide SH1NKANSEN (Super-express) network and thc expressway network projects, efficient subsurface investigations over wide areas are callcd for. In order to meet such demand, a few construction firms have developed vehicle-mounted sounding devices such as those illustrated in Figures 16 and 17. In employing such devices uniąue methods have been developed for continuous penetration and have been success-ful to speed up sounding procedurę 13)» 14) The futurę problem would be a morę efficient investi-gation of extremely weak soils and exchange of information is highly desirable with the countries-which are confronted with similar problems.

Fig. - 16 In-situ Sounding Car of Continuous Recording Type (Kajima Const. Co. )