Geotechnical Engineering

DHARM

802 GEOTECHNICAL ENGINEERING

Lazard conducted lots of tests and compared the results with the values obtained from

equation 19.61; he claims that the correlation is satisfactory for all types of soils except for

boulder deposits.

Lazard proposed an alternate empirical expression for evaluating the limiting over-

turning moment:

D¢

D

i

Q

H

2 R

(or L or B)

Fig. 19.21 Key figure for Lazard’s hypothesis

(QH)limit =

pR HD

HDD

()*

.( , )

2

10 6

3

4 1 10 000

2

× +− 1

F

H

G

G

G

I

K

J

J

J

...(Eq. 19.62)

(*2R or L or B)

where p = maximum pressure acting at depth D′′ from the ground surface

D 1 = D – D′′

D′′ =

D′

+

2

015.

2 R, L, B, D, D 1 , D′, D′′ are all expressed in metres. Then (QH)limit will be got in the

appropriate units such as kN.m.

The correlation claimed by Lazard was based on tests of short duration; it was shown

that the values from those of longer duration were 75 to 95% of the values from short term

tests. He found that lateral friction in negligible. It could be inferred from Lazard’s data that

the axis of rotation of the well lies well above the base at incipient failure.

19.11 CONSTRUCTION ASPECTS OF WELL FOUNDATIONS

Well Foundations may be constructed on a dry bed or after making a sand island. If the depth

of water is large and the velocity is also high, wells can be fabricated on the banks of the river

and floated to the final position and grounded. Alternatively, it may be constructed in lifts,

each lift being carefully sunk to reach the final position. Once the well has touched the desired

stratum, sand bags are deposited around it to prevent scour. A well may sink into the soil 0.3

to 0.6 m due to its own weight. Excavation through dredge holes, in addition to adding kentledge,

may be carried out to aid the sinking process. Water jets may be used on the exterior surface of

the well to reduce skin friction and help in the sinking operation.