6585844974

India

180

50

capacity of pilcs. The safe load on a pile may be worked out (Dlnesh Mohan & Jain. 1963) from the relationi Qc    Ag    ^

Q_s = —    X —— + ąc.Ap x

for aoils having cone penetration values betweor 10 and 100 Kc/sq.cm. (60° cone with a 10 sq.cn. base) the average skin frlction is about 2 per-ccnt of the average eon© penetration resi stance (Fig.iO).

<łc

A_s

^Fi

and Fg

AVERAGE CONE PENETRATION RESISTANCE (N_a)Kę/»q.cm

Where q_c(av.) = average penetration

resl stance over the length of the pile*

= average penetration resi stance around the pile toe (3.75 B above and D below the pile toe) •

= Surface area of the pile shaft.

= Sectlonal area of the pile toe.

= Fhctor of safety for the frlction resi stance

= foctor of safety for the point resi stance.

The factors of safety and Fg may be worked out assuming that the work-lng load should not exceed 60# of the ultimata bearing capacity of the pile arrlved at by the load test. Oon-siderlng th© upper limit of 60# of the correlaiion facto r between skin fric-tlon and cone poretratlon resi stance and an average value of 50#, a factor of safety F, = 60/50x100/60=2 may be adopted. Ihe factor of safety (Pj>) for predicting the point bearing should be 1.5x100/60=2.5 because the cone penetration resi stance ls found to be a maxiiDum of about 1.5 times the actual ultlmate point bearing as obtained from pile load tests (Fig. 9).

CONE PENETRATION RESISTANCE (N_CT1KgAq.cm

FIG. 9 . STATIC CONE PENETRATION RESISTANCE Ł POINT RESSTANCE OF PILES

FIC. 10. STATIC CONE PENETRATION RESISTANCE

A FRICTIONAL RESISTANCE OF PILES

An interesting application of the static cone sounding has been madę in Calcutta near Prlncep Ghat to locate the bed of the river Hooghly that exi sted on the eastern slde of the present bed. Oone penetration tests and advancenent of borę hol es were first done side by side at two loca-tions, one at the present bank and the other at a spoć about a mile east-ward. At the present bank there is a top silty clay layer, 3 met er deep, giving a cone resi stance of 15-aO kg/ cm². It is underlain with a loose fi. ne sand depo sit extending up to 15 metres. Cone resi stance ln this layer varled between 50 to 70 Kg/on². The next layer consisted of ooarse sand with a point resistence of mor© than 250 on². At the second spot ihidh was far from the rlver bed, the top 3 meter layer was silty clay show-ing a cone resi stance of 15-20 Kg/cm². The underlying stratum extending up to 14 metres consisted of silty clay and recorded a resi stance of 3-5 Kg/an².

The next layer was a stiffer one (silty-day) śhowing a point resi stance of 17-40 Kg/cm². &>unding test was carried out at reguł ar intervals start!ng from the presort bank. It was interesting to notę that at a di stance of about 600 metres from the present bank the sounding record took an abrupt change. It was therefore evident that the rlver bed approached upto this di stance once up a time.