949
Technical Committee 104 /
Comité technique 104
situ deep foundation installed by conventional / hydraulic
assisted augers or bailers where no drilling mud is used. Such
installations where Direct Mud Circulation (DMC) technique is
used, a thick soil becoming a part of the foundation is not
generally observed during excavation for caps. This has been
confirmed during bored pile foundation for the hostel building
of Indian Institute of Technology, Guwahati, and in several
other cases. This is due to the fact that the impregnation of
bentonite that takes places during the process of boring, fills the
voids adjacent to the bored surface. Moreover, the thixotropic
property of the bentonite particles left no room for cement
slurry to impregnate further, thus negating the chances for a
thick skin of soil becoming a part of the structure. This confirms
the possibility of mobilisation of shaft resistance closer to the
surface of the structure or at the surface itself depending upon
the nature of soil. In such a case the adhesion that occurs
between two different materials may govern the process of
mobilisation of shaft resistance rather than cohesion between
adhered to surrounding soil.
LOPMENT OF DEVICE FOR IMPREGNATION
a
hol
ic
pum used to compress air inside the first compartment.
side the rectangular
com
ted
as 1
without any
isturbance to the impregnated and smeared face.
aw without causing any undue damage to the required
lane.
RVATION FOR SMEAR ZONE
se two possibilities might
occur separately or simultaneously.
7 DEVE
STUDY
Notwithstanding the effect of impregnation and soil becoming
part of bored cast in-situ deep foundation, available literatures
do not give much information on the magnitude of impregnation
and its dependent factors. Furthermore, no technique for
measuring such impregnation could be known from present
literatures. Therefore, a new method was developed in which
the pressure exhibited by cementitious slurry during placement
of fresh concrete in borehole was simulated in laboratory for
allowing impregnation through the soil sample collected from
borehole. Such laboratory simulation involved development of
concept, fabrication of device, and performing trial tests. Final
version of the impregnation test equipment, incorporating minor
modification upon trial tests, was used for impregnation study.
The equipment comprised of a closed rectangular concreting
compartment of size 150x200x200 mm fabricated from thick
steel plates with detachable top lid fixed by high tensile nuts
and bolts. At the top lid a non-return-air valve and a pressure
gauge were fixed to pump compressed air in and to monitor air
pressure inside the compartment respectively. In one of the side
plates of the compartment a hole of 75 mm diameter was made
and a threaded socket was welded along the circumference of
the hole so that a sampler could be threaded into the socket.
Cylindrical hollow samplers of 75 mm diameter, 150 mm long,
and 2 mm thick having threads at both the ends were used for
sampling and for fixing with the socket for impregnation test.
At the other end of the sampler, a threaded cap was fixed with
e in it, plugged by jute wick, to allow water to come out.
All the joints have been made airtight. The non-detachable
joints are sealed by resinous epoxy, threaded joints that require
frequent removals are sealed by jute fibre soaked in zinc
solution and non-threaded joints are sealed by rubber gasket
kept in highly a compressed state. A remote control pneumat
p was
8 IMPREGNATION TEST PROCEDURE
Samples for impregnation tests were collected from shallow
depth, generally, two numbers at the same depth from each
auger-borehole by horizontal sampling. Shallow sampling depth
was preferred in order to collect samples experiencing
maximum disturbance from repeated insertions and withdrawals
of boring tool. Collected samples were kept for twenty-four
hours inside the sampler to regain its natural state to the
possible extent. A pumice stone wrapped by filter paper was
placed by trimming soil in the driven end of the sampler. After
covering the end by cap, the sampler was inserted into the
socket with smeared face of the sample towards the rectangular
compartment. Nominal mix of concrete of ratio 1:2:4 (1 part
cement: 2 parts sand: 4 parts 20 mm nominal size aggregate)
with 10% extra cement, having slump of 120 mm was poured in
the rectangular compartment and properly placed. Without any
delay the top lead was fixed and air was pumped by the
pneumatic pump till air pressure in
partment reaches the required limit.
Bowles (1982) indicated that the critical maximum pressure
of concrete would occur at a depth within 10 to 20 times the
diameter of pile. So for 400 mm diameter pile, the extent of
height of fresh concrete was worked out within 4 and 8m and
corresponding extents of impregnation pressures were adop
and 2 Kg/cm
2
for fresh concrete of specific gravity 2.5.
The pressure had to be maintained at required limit as
sometime it was found necessary to compensate little pressure
drops as a result of impregnation. Although initial setting time
of cement is 30 minutes, pressure in the compartment was
maintained for 90 minutes to simulate field condition to the
possible extent. The pressure in the compartment was released
and the sampler was then removed from the compartment.
Sampler cap, pumice stone and pieces of filter paper were also
removed for extrusion of the sample from sampler. For
extrusion, pressure was applied in the face at the outer side
where pumice stone and G.I. cap was fixed such that no
disturbance occurs to the impregnated and smeared face of the
sample. The samples were stored in dry place
d
9 PREPARATION OF BLOCK SAMPLES FOR
OBSERVATIONS
For visual observation it is necessary to prepare block samples
with at least one plane face. Of the whole sample, since the face
through which impregnation occurs was important, it was
necessary to cut along the direction of impregnation. Prior to
cutting, the samples were saturated with toluene-epoxy solution
under vacuum desiccators. With the process of de-airing, the
sample absorbed toluene-epoxy solution and upon curing gets
strengthened. After 15 days of strengthening the samples were
found to be suitable for cutting. The cutting was done by a thin
metal s
p
10 MICROSCOPIC OBSE
AND IMPREGNATION
Microscopic investigation was carried out under a high
resolution polarised microscope. The prepared sample for
investigation was placed under microscope and observed
through lens ‘X100’ and subsequently with ‘X10’. In case of
study of samples under high magnifying lens, the particle re-
orientation or particle crushing due to smear effect was found
very difficult to identify from other randomly oriented soil
particles that naturally exist in the soil. Furthermore, it was
found to be very difficult to distinguish between the particles of
impregnated cement from that of other similar whitish materials
scattered in the soil matrix. Under low magnifying lens (X10),
however, some irregular cylindrical veins of deep brown tinted
materials and whitish materials were seen at random. Later
those deep brown tinted material was identified as the epoxy
resin used for stabilisation of the study samples and whitish
material was identified as impregnated cement particles
respectively. The identification was confirmed upon comparing
the samples of hardened epoxy and hardened cement under the
same microscope. While in case of epoxy resin, cracks formed
due to desiccation of surface during preparation of samples was
identified as the chief reason for impregnation, two possibilities
were identified for impregnation of cementitious materials in
the form of cylindrical veins. The
