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177

Ule oone is drlven by allowlng the 65 Kg. harnmer to drop freely through a height of 75 cm on the drlvlng head. A drum type winch fixed to the central leg of the trlpod is used £br lifting the drop weight such that the frec fali of the hammer is not affected. The driving of the oone and the pumping in of the slurryis started simultaneously.

Driying is not done morę than 30 cm. at a time aft er which it should be stopped for a minutę or two. Punping i s, nowever, contlnued. Thi s helps in keeping the hole lined and al & avoids the choking of the hol es pro-vided in the cone. The driving rods are giv® a few turns (about 4 or 5 turns) every no w and then so that the hole above the oone is maintained. Bffident circulation of slurry is necessaiy for eliminating friction on rcds. The number of blows is recorded for every 10 cm of penetration.

2.3.1 FACTORS INFLUENCING RESULTS

for a ąuiok exploration Job, it has always De® felt to find out a sdmple test like Dynamie cone which could be oorrelated to SPT. The 5i mm (2") oone has been used to ascertain depth to harder strata but the śtudies carried out (CBRI, 1963 & 70) indicatod that due to friction or. rods, it was not desirabie to draw any relationship with soil properties and Standard Penetration Tests (K values). Oon-seąUently, a detailed study was taken for standardising the shape and ze of oone which could provide co rapa rabie data to N values. Schultze and Knausenberger (1957) have suggested the use of large dLaoeter oone to minimise the &in friction on drill rods. However, a systematic study on thi s was called fbr.

Different shapes of cones were tak® and it was notlced that tne 60° oone provlded the least number of blows fbr a certaln strata. Tf the angle is less, the nim ber of blows go up.

Tf the angle is reduced, again the number of blows go up. The differ®ce upto 30° angle, however, was not considerable in loose sil ty clays but the effect i s morę pronounced in den ser strata. Thus, a 60® cone was finally selected for the detailed study. The slope on the reverse side should also be about the same as an abrupt change towards flattening is apt to provide ir.oonsistent resu.lts due tc marked fluctuations in number of blows for even scali variation in soil stratna.

The second important factor was slze of cone. Varlous sizes rangine from almost the size of ¹ A* rods to 76 mm were tak®. The tests were carried out to shallow depths in the beglnning avoidlng friction on rods. The studles revealed that for the 63 mm cone, the number of blows were morę oonslst®t and comparable to 11 values (Fig. 8). for larger depths, beyond 6 ra, however, the effect of friction was pronounced and use of 5 perc®t bentonite slurry in water was used for retalnlng the sldes of borę hol es (Sengupta <& Agfarwal 1966). Thi s eliminated friction oń rods. In tne process forwarded, the slurry is pushed through the drill rods while drlving. The slurry cemes up through. hol es provlded on the upper slope of the cone and retains the borę. In certaln strata liiere chances of sedlraentation are morę, a vane follow-ing the cone is used. The vanes are welded to a dzlll rod and there are holes proviied through it also. The drill rods while drlving is often rotated. The vanes alongwlth slurry help in keeping any sedlmentatlon to remaln In suspenslon. Thi s ellmlnates friction on rods almcst oompletely even for larger depths.

FIG. 8. AVERA6E RATIO OF H^j/Ng WTTH CONE SIZES FOR OIFFERENT DEPTH

A few factors as in case of Standard P®etratlon tests are applicable more or less in thi s test al ® e.g. weight of rods, shape of hammer and frequ®cy of fali. But the morę pertinent ones e.g. sand boiling, sedlmentatlon, effect of sleeve, effect of size of bores are not there. The slurry used retains the borę and do es not in any way effect the strata being penetrated. The effect of bentonite pressure is also negligible. The test proyides a con-tinuous record of p®etration wlthout any friction on rods. In ssali er size cones. with dry pro bing, the effect of friction is indeterminate.

2.3.2 IMTEI^RETaTTON OF TEST RESULTS

It was first suggested (Sengupta &

Jatn, 1969) that for dry pro bing

L’cd = 1*5 M for depths upto 3 m,

and K_c(j = 1.75 M for depth from 3 to 6 o.