951
Technical Committee 104 /
Comité technique 104
(a)
(b)
Figure 3. (a) Typical impregnation mapping, (b) Response of
impregnation versus D
50
×e (After Sarma, 2000)
As a general trend it was observed that both average and
maximum impregnation depths increase with increase in slurry
pressure. Individual plots between impregnation and other
parameters like, over consolidation ratio, liquid limit, plasticity
index, activity, coefficient of permeability, void ratio, revealed
no definite trends. Therefore, plots were tried with composite
parameters. For plots with maximum impregnation versus
D
50
×e (where D
50
is the size of mesh through which 50% of soil
passes and ‘e’ is the
), no mean line could be
drawn as points were
ay be due to the fact that
t it being a single function of void ratio and
ore size. However, points of average impregnation versus
1 Kg/cm
2
and 2 Kg/cm
2
(2)
itigating the effects of shrinkage.
-soil becoming part of
sive collapse with
irtual
o shrinkage and its effect on the shaft
y been completed. Formation of smear
in-situ void ratio
scattered. This m
maximum impregnation depth depends on the depth of local
fissures and as the depth of fissures vary considerably from
sample to sample, the depth of maximum impregnation too
would vary withou
p
D
50
×e for slurry pressure of
respectively were seen in a trend more or less around a straight
line drawn through the mean points for each pressure (Fig. 3 b).
Regression analysis was done for average impregnation versus
D
50
×e and following equations of straight lines were developed
for slurry pressure of 1 Kg/cm
2
and 2 kg/cm
2
respectively:
05.1 10 23.0
3
V
I
av
12.1
10 54.0
3
V
I
av
(3)
where, ‘V’ is D
50
×e and ‘Iav’ is average impregnation in mm.
The above equations show that average impregnation is a direct
function of ‘D
50
×e’, which is a measure of both pore size and
overall void ratio.
14 EFFECT OF SHRINKAGE
Shrinkage of concrete, which is a phenomenon associated with
curing, is an important factor as it introduces an element of
uncertainty in the location of the potential rupture surface that
results in uncertainty in the prediction of shaft resistance
(Sarma, 1992). It is therefore of considerable importance in
mobilisation of shaft resistance.
With the setting action of concrete, the process of shrinkage
also continues. The magnitude of shrinkage, however, depends
on various factors among which effects of size of aggregate,
elastic properties of aggregate, concrete used, contamination of
concrete by clay particles are important.
Among the influencing factors as stated above, the most
important influence is exerted by aggregates. The size and grade
of aggregate do not influence the magnitude of shrinkage
directly but large aggregate permits use of linear mix and hence
results in lower shrinkage (Nevile, 1981).
The elastic properties of the aggregate determine the degree
of restraint offered. Presence of clay particles in concrete lowers
its restraining effect increasing shrinkage. Even if, aggregates
used in concrete are free from clay particles, during the process
of tremie concreting it may carry clay particles from borehole
wall and prone to higher shrinkage.
Based on the shrinkage strain of 3×10-4, recommended by
Indian Standard (I.S. 456-1978), reduction of diameter is
expected varying from 0.09 to 0.36 mm for diameter of pile 300
to 1200 mm. This reduction of diameter shall give rise to virtual
gap around the shaft leading to virtual loss of contact with
borehole wall.
There may be mixed opinion whether such gap has any
practical significance or not. Generally it is expected that the
soil of the borehole trends to fill-up such gap by collapsing
nder active pressure m
u
However, from the evidence of cemented
such structures, it is possible that progres
gradual shrinkage occurs outside the zone of impregnation as
concrete brings-in cement-impregnated surrounding soil during
its shrinkage. Eventually, shrinkage may have effect outside
impregnation zone and strain softening the potential failure
surface further. Therefore, separate effect of shrinkage has not
been considered while presenting the alternative concept of
shaft resistance mobilisation based on impregnation.
Nevertheless, effect of shrinkage may be prominent in case of
bored cast in-situ deep foundation installed by DMC technique
where, impregnation depth and soil becoming part of the pile
may not be significant.
15 CONTINUING DEVELOPMENTS
The model of average impregnation is presented for the range of
maximum concrete pressure for pile of 400 mm diameter. As
the information on critical depth of maximum concrete pressure
is available, the model can be extended for higher diameter
shaft of deep foundation. With these findings determination of
effective diameter of bored cast in-situ deep foundation is
possible for variety types of soil more importantly information
on the location of potential rupture surface for mobilisation of
shaft resistance. Among other two indeterminate factors,
nam
age and smear zone, development of a v
ely, shrink
collapse model due t
resistance had alread
zone is a problem primarily associated with types of equipments
used and installation workmanship. Developments over the
conventional construction equipments and method of
installation had already been done, new equipments fabricated,
and prototype piles constructed for performance evaluation. The
objectives of these new equipments were to keep the depth of
smear minimum and within the impregnation depth. With these
new patented equipments (viz., valve auger, scraper unit, and