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India 168

66, 68, 71),

2. TYP ES OP PENETROMETERS

Th© technląue of sampllng noncohesive soils has not yet been perfected enough for obtalning rellable results at reascnabLe cost. Thus most of the presait methods of analysl s have been based on lndlrect test like penetro-meter. Three diff©rent types of the penetrometers are used in practlces

1) Oonycntlonal Standard Split Spoon Test descrlbed by Terzaghi and Peck (1957) and adopted by Indian Standard ISi 2131-63.

2a) Uncased dynamie 50.8 ram Gon© tests ISi 4968 (Part-I) - 1968.

b) Dynamie cone test with bentonit© cireulation ISx4968 (Part-II) - 1968.

2.1 STANDARD P EWETRATION TEST

Tal s is broacCLy based on ą>lit Barrel Sampllng of solls as per Canadian Standards Association and Anerlcan Soclety for Testing and Materials* The method ls covered by the Indian Standard ISs 2131-1963, Method for Standard Penetratlon test fbr a>ils.

The split spoon sampler ls of 50*8 mm O.D., 35 mm I.D. and mi nim un 675 mm long. The driving rods have stlffness atleast equlvalent to • A¹ rods. Fbr depths exceeding lOm, ą>ac9rs are pro-vided. A borę 55 to 160 ram in dia-meter is requlred to be madę. Caslng or Drllllng mud ean be used to ayoid caving. Drllllng mud alone ls pre-fered. When a caslng is used, it is not drlyen below the level at vhich the test ls madę. The caslng should be fbrwarded by turalng (to and fro motion) rather than drlving as the latter may dlsturb the density of stratum below. Use of side discharge bits fbr ranoying sedimentatlon are prefered befbre testing. Tb ayoid sand boiling, the borę ls kept fuli of water or drllllng mud*

The split-spoon sampler is lowered into the borę and madę to rest at the bottom. It is than drlven wlth the blows of a 65 Kg* harmer falling freely through 75 cm* The sampler is first drlyen 15 on. called seatlng drlve. The total blows required fbr the seoond and thlrd 15 cm* of penetratlon are temed the penetratlon reslstance N. If the total penetratlon ls less than 45 cm, then the penetratlon resl stance shall be that fbr the last 30 on. of pene-tratlon. Refusal is consldered to have been reached when the ratę of advance is less than 2.5 <sn fbr 50 blows. This ls now being reduced to 20 blows.

2.1.1 FACIO RS INFLU&i&NG RESULTS

In solls \here sand boiling ls likely deep bores can be successfully madę wlth use of drllllng mud (Devendra Shanna, 1960) • A further study also revealed that rellable sampllng of sands was posslble by use of a suitable drllllng muds (Sengupta & others, 1964)* Standard Penetratlon Tests to oonslder-able depths (73 m) have been reported by this technique (Banbrjee, 1967). He used an addltlonal tubę ooupled wlth •A" rods fbr Jetting bentonit© śluriy just above the sampler to ranoy© sedi-mentatlon. The technlque of uslng bentonit© slurry has been reported to be in wlde use In this country. (Dlnesh Mohan & others, 1967). A suggested proceduro was subsecuently laid down (Sengupta & Jain, 1969).

A study carrled out on evaluation of N yalue from the seoond fbot of penetratlon (nisnber of blows in last 3D cm of penetratlon after allowlng first 3D cm. as seatlng penetratlon) showed that there was a break around 30 cm.

In curves drawn between number of blows and depth of penetratlon (S^igup ta & Aggarwal, 1965). Thi s indicated that strata to a depth of about 30 cm. was dlsturbed. These tests proyide & morę conslstent data but have not becorae popular so far.

A study analyslng available literatur© on factors influendng the results of a Standard Penetratlon test (Desai,

1970) indicated negliglble effect of length and weight of rods, slze of borę hole and materiał of the hamraer* As the length inereases, the weight alsa inereases and this oounters each others effect. A study carrled out on effect of borę hole slse on the result of oone penetratlon test9 (Narahari and others, 1966) has shown that the effect of borę hole slze varies wlth the type of test. The effect ls falt morę ln a dynamie test and is analogous to the pit effect ln piąte load tests. Korę work on this is called fbr.

A study on &ape and slze of shoe showed that the resl stance of hollow samplers was almost independent of internal diaoeter (SLkka₉ 1968).

Howeyer, ln actual practice, a sampler wlth a blunt edge snows a oonslderable lncrease ln number of blows in a dense strata, and the ^iape needs standardi-sation. Slmilarly use of shoes of softer materiał like mlld steel tend to lncrease the blows and need harden-ing. In samplers wlth aleeyes, if a gap is left on the sldes due to shorter sleeyes than the slot, ©yen a 2 cm gap is found to lncrease the number of blows conslderably in mediua compact to compact sands. This is due to lncrease