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of struć tur es, Eowever, thi s technique has not been wldely adopted in practlce and a morę detalied study ls called for. The method for finding out relatlve denslty of dlfferent types of solls al so needs standardl satlon. In loose sands and soli s wlth organie conteit, spedal care oay be needed ln acccunt-ing for Terzaghi¹ s slze effect.

Another correction for N_s ln vogue for tests ln saturated silty flne sands is

fig = 15 + ILzlL

2

ytoere N_s ls the corrected N value.

A lower average of the N_s values to a depth equal to wldth of footing ls tafeffi for oomputation of ałlowable lo ads.

The relationehip between angle of lntemal frlction (0)bearing capacity factors and value of N (Peck, Han son and Thanburn, 1953) are also ln vague. Curves diowlng soli pressure oorres-pondlng to 40 mm settlement in non cohesive soils for dlfferent wldths of footing (Fig. 4) are no w prevalent. Barii er, the peimisslble settlemait was 25 mm only*

FIG. 4. SOIL PRESSURE CORRESPONDING TO 25 MM AND 40 MM OF SETTLEMENT OF FOOTING ON SAND

The value of ałlowable pressure q-> is taken 50 percent if the water tacie is

at the base of the footing. When the water table ls deeper wlthin a depth eqUal to the wldth of footing, a prorata reduction is madę. Hóvever, (Desai, 1965) suggested that no reduction need be applied in cases where N values were determined below water tabl e.

N values are qulte in vague for finding out the bearing capacity of plles. Indian Standard Oode of Practice on Design and oonstruction of Pile founda-tlon Part-III, Ihder-roamed pile foundations piovides a safe load table for dlfferent slzes of piles and the safe loads are guided by the N values. Fbr dense sandy (N^-30) and stiff clayey (N<8) soils, the safe loads are inereased by 25 percait. However, the values of lateral thrust are not inereased unless the stability of the top soli (strata to a depth of about 3 times the stem diameter) is ascert-ained. A 25 percent reduction ls madę in case of loose sandy (N^ 10) and soft clayey (^4) soils. for deteiroining the average N value, a weighted average is taken and correction for fineness under water table is applied vhere appli cable.

2.2 DYNAMIC OONE PENETRAHON TESTS -(50 mm oo ne wlthout b en to ni te slurry)

Thi s test i s generally based on (HvorsLev, 1949) and (Terzaghi & Peck, 1957). Ihe method helps in reoonaiss-ance survey of wlde areas ln a shorter time whlch enables selective in-situ testing or sampling for typical pro-flles. It provides useful data for load conditions where reliable oorre-lations have been established. It has been oommonly used as a sounding test to assess depth to hard stratum.

The cone is of 50 mm dla. wlth 60° apex angle. The rear portion can be fiat or wlth 45 to 60° • Drlving pip es are •A* rods (4i.27 mm OD and 28.57 mml.D.) The hapmer used is 25 on high of suit-able diemeter to welgh 65 Kg. A through hole ls provided for the guide pipę wlth a clearance of about 5 mm.

The cone is dilven into the ground by allowlng a 65 Kg. hammer to fali freely through a height of 75 cms. each time. The nunber of blows are oounted for every 30 cm. of penetration till the blow oount is 100 for the last 30 cm. The nunber of blows (N_cd), as a oontinuous reoord for every 30 on, of penetration, ls sho*n in a tabular stateoent or a graph betwetfi N_C(| and depth.

2.2.1 FACTO RS INFLUENCTNG RESDLTS

In India, Central Water and Power Ooirraission started using the test sińce 1958 to miniraise drilling and sampling