50th INDIAN GEOTECHNICAL CONFERENCE thigs/ldh/files/igc 2015 pune... · 5 0 th 50th INDIAN...
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50
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50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
ATTENUATION CHARACTERISTICS OF PILE BORING VIBRATIONS
M. R. Khan1, S. J. Shah
2
ABSTRACT
Ground vibrations originating from construction activities are one of the most alarming issues faced by
structural builders, working on the construction of large scale projects down to the buildings erected for
residential purposes. Much more than the structural damages caused on to the adjacent structures, they
create distress to the crowd of population in close proximity to the work sites. The sense of disturbance to
humans is evoked as a very low threshold than is generally estimated during environmental impact
assessments. This is due to the combination of physically experienced vibrations with the noise being
created during the course of action. Construction of pile foundations have become an inevitable part of
the ever-growing civil engineering industry, since present day constructions are progressing with the
motive of satisfying the dreams of humans to touch the skies. These mega structures impose a very large
load onto the ground and demand the load to be transferred to very great depths, necessitating pile
foundations. Bored cast in-situ pile foundations are generally adopted nowadays, since the transportation
of long precast piles is almost impractical through the crowded spaces available these days. Moreover, the
drivability of slender piles into soils is very limited and tedious due to requirement of very high driving
loads. Such high impacts may also result in the failure of foundation, if not installed with utmost care.
Bored cast in-situ piling demands very less complications in the transportation of materials and provides
complete control over the specification and erection of piles at the site. Direct mud circulation technique
is a common method of pile boring used in soft soils. This method requires use of a very heavy cutting
chisel, which is raised and dropped from a predetermined height for cutting into the ground and taking out
the soil contained within. During this drop, the heavy chisel hits the ground with a very high impact,
imparting all the contained kinetic energy of the chisel on to the incident ground mass. This results in
vibration of the ground, with the waves travelling out in the radial direction due to the impact.
Propagation of these vibrations occurs in three mutually perpendicular directions (vertical, radial and
transverse). These vibrations have a general trend to attenuate with increasing distances away from the
point of impact, due to geometric damping of expanding wave front and material damping occurring
within the soil mass. The distances travelled by these vibrations depend upon a large number of factors,
including the weight of hammer, height of drop and the most complex variable, the characteristics of
ground itself. Based on studies that have already been conducted in this regard, standard codes specify the
1Mr. M. Roshan Khan, Department of Civil Engineering, Indian Institute of Technology Bombay, Mumbai, India,
[email protected] 2Dr. Syed Jalaludeen Shah, Department of Civil Engineering, Universal Engineering College, Thrissur, India,
M. R. Khan & S. J. Shah
maximum intensities of vibrations that the receiving structures or humans may safely be subjected to.
Beyond these limits, vibrations have a destructive attribute and the same must be avoided. Studies are
conducted to find out general trends in attenuation characteristics of vibrations in triple axis directions, in
different soil profiles prevailing at various piling sites. Accelerometers have been used for making
measurements in all the three axes simultaneously and the data is logged onto a computer for analysis.
The readings are measured in terms of peak particle velocities measured in mm/s, which is computed as
the root of sum of squares of vertical, radial and transverse directional vibrations. The variations in peak
ground accelerations are also considered. The effect of depth of boring on the propagation of vibration
has been an important aspect of the study. These results can be used by the piling contractors and
structural builders in practicing sustainable constructions with slightest impacts on the surrounding
environment.
Keywords: Ground vibrations, Direct mud circulation piling, Peak particle velocity
50
th
IG
C
50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
ATTENUATION CHARACTERISTICS OF PILE BORING VIBRATIONS
M. R. Khan, Ph.D. Research Scholar, Indian Institute of Technology Bombay, [email protected]
S. J. Shah, Professor, Universal Engineering College, Thrissur, Kerala, [email protected]
ABSTRACT: Vibrations due to pile boring activities impose serious concern in safe and sustainable construction
practices. Propagation and attenuation characteristics of these vibrations in various soil profiles are studied in detail
using accelerometers that quantify the vibrations in terms of peak particle velocities and accelerations. The analysis
is done considering vibration components in triple axes directions and the general trends are studied both in
component levels and as resultant particle velocities and accelerations. Based on the standardized safe permissible
limits, piling contractors need to ensure environment friendly pile boring sequences, inflicting meagre deteriorating
effects on the proximate structures and least discomfort to the population in vicinity.
INTRODUCTION
Direct Mud Circulation (DMC) is a common
technique used for pile boring in marine soils and
soft soils. The method employs a cutting chisel for
boring, which emanates vibrations during impact
on the ground. These pose serious effects on to the
surroundings and need to be observed critically. A
study was conducted for analyzing the general
trends in propagation of vibration characteristics in
the triple axes (vertical, radial and transverse) and
the results were quantified in terms of Peak Particle
Velocities (PPV) and Peak Ground Accelerations
(PGA).
DIRECT MUD CIRCULATION TECHNIQUE
Boring of piles by DMC involves the use of
various machineries and equipments including
power winch, tripod, drilling chisel, DMC rods,
lifting head, flushing head, concrete mixer,
concrete funnel, tremie pipes, etc for boring the
soil and concreting of piles. Drilling chisel is the
cutting tool used for penetrating the layers of soil
for their subsequent removal. The drilling chisels
are available in a variety of sizes and weights,
which are selected in projects based on the
diameter of piles and the characteristics of soil,
respectively. The DMC rods are connected through
threads for making the drilling chisel reach the
desired depth of boring and also connect onto the
water lines provided inside the chisel. During
flushing procedures, their function is to carry the
suspension of bentonite to the bottom of hole.
Fig. 1 Drilling chisel with casing
Figure 1 shows the photograph of a casing with the
drilling chisel, which is initially used at shallower
depths for preventing caving-in of bore-holes. The
walls of bore-hole at greater depths are stabilized
by a vertical pump system, as shown in Fig. 2. In
the pump system, a suspension of bentonite is
pumped down into the bottom of the bore hole
through drill rods and it overflows at the top of
casing. The system in general should have the
capacity to maintain a velocity of 410-760 mm/s to
float the cuttings.
An additional weight rod was connected to the
chisel when boring was supposed to be past soils
with higher penetration resistance, like lateritic
soils. There was no requirement of a weight rod
attachment when boring was done in clayey soils.
M. R. Khan & S. J. Shah
Fig. 2 Vertical pump system for mud circulation
Concrete funnel is used for pouring in concrete
during the concreting of pile and tremie pipes are
connected at the bottom of concrete funnel to carry
the concrete mix down to the location of
concreting. These are also connected through
threads for varying the height of operation.
Course of piling
After insertion of casing, pile boring is done using
pile chisel as the cutting tool. Chisel is dropped
down straight and then rotated at place to scrape
out the soil to be removed out.
The dropping down of chisel from specified height
onto the ground is the main source of ground
vibration in DMC piling. In this study, vibration
characteristics of the adjacent soil during pile
chisel drops have been the matter of interest.
FIELD MONITORING OF VIBRATION
The information on absolute levels of ground
vibration associated with chisel boring in DMC
piling was collected at various sites in Kerala with
different soil profiles. Piling vibrations were
measured using four accelerometer modules, which
were placed at ground level during the complete
pile boring sequence until the depth of hard stratum
to rest the pile was encountered. Accelerometer
modules had been placed at measured distances of
2.5, 5, 7.5 and 10 m from the point of pile boring
horizontally at ground level, as given in Fig. 3.
Fig. 3 Monitoring Scheme Adopted for Field Study
The accelerations in triple-axes directions were
collected for every point of measurement and
corresponding velocity components were
calculated using a processing microcontroller. The
PPV is then calculated from the three components
of velocity obtained and similarly, triple-axes
accelerations give the resultant PGA.
Monitoring at Piling Location 1
The information on vibration levels was collected
at the site of construction of a commercial complex
in Cochin, Kerala by the method of DMC.
Piles to be erected were of 0.8 m diameter and the
boring equipment being a pile chisel of 0.8 m
diameter, weighing 0.8 tonnes. No additional
weight rod was connected to the chisel since
sufficient advance of boring was achieved with the
weight of chisel itself in the clayey soils. The drop
height was maintained between 0.5-0.75 m. The
profile of layers of soil was obtained by the rotary
drilling of a bore-hole at the piling location, as
given in Table 1.
The horizontal variation of triple-axes ground
velocities and accelerations for various depths of
boring at the site are as given in Figures 4-15.
50
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IG
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50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
Table 1 Soil Profile at Piling Location 1 Depth below Ground (m)
Visual description of
soil
Thickness of layers (m)
1.6 Silty clay with sand
1.6
5.7 Silty clay 4.1 8.8 Sandy clayey silt 3.1 13.6 Silty clay 4.8 14.8 Clayey sand 1.2 27.7 Silty clay 12.9 33.7 Lateritic clay 6.0 37.6 Silty clay with
decayed organic matter
3.9
39.9 Clayey sand 2.3 45.8 Clay with sand 5.9 46.7 Sand 0.9 47.6 Silty (weathered
rock) 0.9
49.6 Hard rock -
Figures 4 and 5 show variations of vertical particle
velocities.
Fig. 4 Variation of Vertical Particle Velocities with
Horizontal Distance at Location 1 (GL-25 m)
The variations of radial particle velocities with
horizontal distances are as shown in Figures 6
and 7.
Fig. 5 Variation of Vertical Particle Velocities with
Horizontal Distance at Location 1 (30-50 m)
Fig. 6 Variation of Radial Particle Velocities with
Horizontal Distance at Location 1 (GL-25 m)
M. R. Khan & S. J. Shah
Fig. 7 Variation of Radial Particle Velocities with
Horizontal Distance at Location 1 (30-50 m)
Figures 8 and 9 show the variations of transverse
particle velocities with horizontal distances for
different depths of boring.
Fig. 8 Variation of Transverse Particle Velocities
with Horizontal Distance at Location 1 (GL-25 m)
Fig. 9 Variation of Transverse Particle Velocities
with Horizontal Distance at Location 1 (30-50 m)
The variations in triple-axes ground accelerations
are given in Figures 10-15. Figures 10 and 11 show
the variation of vertical ground accelerations with
depths.
Fig. 10 Variation of Vertical Ground Accelerations
with Horizontal Distance at Location 1 (GL-25 m)
50
th
IG
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50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
Fig. 11 Variation of Vertical Ground Accelerations
with Horizontal Distance at Location 1 (30-50 m)
Figures 12 and 13 represent the variation of radial
ground accelerations with horizontal distances at
various depths.
Fig. 12 Variation of Radial Ground Accelerations
with Horizontal Distance at Location 1 (GL-25 m)
Fig. 13 Variation of Radial Ground Accelerations
with Horizontal Distance at Location 1 (30-50 m)
Figures 14 and 15 show the variation of transverse
ground accelerations with horizontal distances for
different depths of boring.
Fig. 14 Variation of Transverse Accelerations with
Horizontal Distance at Location 1 (GL-25 m)
M. R. Khan & S. J. Shah
Fig. 15 Variation of Transverse Accelerations with
Horizontal Distance at Location 1 (30-50 m)
From the measured values of triple-axes vibration
velocities and accelerations, resultant PPV and
PGA were calculated. The variation of these values
with boring depths, at different points of
measurements are presented in Figures 16 and 17.
Fig. 16 Variation of PPV with boring depths at
different monitoring points in Location 1
Fig. 17 Variation of PGA with boring depths at
different monitoring points in Location 1
Monitoring at Piling Location 2
Vibration monitoring was next done at the site of
pile boring for a residence apartment in Cochin.
Piles to be erected were of 0.5 m diameter and the
boring equipment consisted of a pile chisel of
diameter 0.5 m connected with a weight rod on top,
weighing a total of 1.2 tonnes together. The drop
height varied between 0.5-0.75 m. Table 2 gives
soil profile obtained by rotary drilling at location of
pile boring.
Table 2 Soil Profile at Piling Location 2 Depth below Ground (m)
Visual description of
soil
Thickness of layers (m)
1.5 Top soil 1.5 10.1 Hard laterite 8.6 10.4 Soft rock 0.3 11.4 Hard rock -
Vibration measurements were done with the same
monitoring scheme adopted for site 1. Similar to
the first site, velocities and accelerations were
measured in vertical, radial and transverse
directions and the resultant PPV and PGA values
were computed. Figures 18 and 19 show the
variation of PPV and PGA with boring depths, at
different points of data measurements considered.
50
th
IG
C
50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
Fig. 18 Variation of PPV with boring depths at
different monitoring points in Location 2
Fig. 19 Variation of PGA with boring depths at
different monitoring points in Location 2
Monitoring at Piling Location 3
Vibration characteristics associated with pile
boring were then measured at the site of
construction of another residence apartment
located near the marine coast of Cochin.
Piles to be erected were of 1 m diameter. Boring
was done using a pile chisel of 1 m diameter
weighing 1 tonne and no weight rod was connected
since the advance was optimally achieved in the
clayey soil. The drop height was maintained in the
regular range of 0.5-0.75 m. Table 3 gives the soil
profile obtained by rotary drilling at piling
location.
Table 3 Soil Profile at Piling Location 3 Depth below Ground (m)
Visual description of
soil
Thickness of layers (m)
1.1 Filling laterite 1.1 3.6 Loose clay 2.5 6.0 Silty clay with
shell dust 2.4
9.5 Silty clay 3.5 15.0 Clay 5.5 16.5 Clayey sand 1.5 22.0 Lateritic clay
with sand 5.5
35.8 Clay with sand 13.8 45.3 Decayed wood 9.5 47.8 Stiff clay 2.5 50.2 Fine little sand
with clay 2.4
56.4 Decayed wood 6.2 63.0 Stiff clay 6.6 65.0 Stiff clay with
silt -
Similar to piling locations 1 and 2, vibration
monitoring was done, with the same monitoring
scheme. The vertical, radial and transverse
components of velocities and accelerations were
measured and the resultant PPV and PGA values
were computed.
The variation of PPV and PGA with boring depths,
at different points of data measurements are shown
in Figures 20 and 21.
M. R. Khan & S. J. Shah
Fig. 20 Variation of PPV with boring depths at
different monitoring points in Location 3
Fig. 21 Variation of PGA with boring depths at
different monitoring points in Location 3
Peak Particle Velocities (PPV)
The measured PPV values were found to have a
general trend to attenuate with horizontal distance
away from impact point of boring, for all the
depths of boring. It was found that the values of
particle velocities were higher at lower depths from
ground and then start reducing with increased
depths from ground surface up to certain depths.
The values were found to again increase when
boring approached soil layers with higher
resistance to penetration of chisel and then rising
substantially high when the depth of boring was
approaching stronger and hard rock strata.
Peak Ground Accelerations (PGA)
The values of PGA were found to be generally
attenuating with horizontal distance away from
impact point of boring, for all the depths of boring
considered. It was found that the values of particle
accelerations were higher at lower depths from
ground and then start reducing with increased
depths from surface of ground up to certain depths.
The values were found to again increase highly
when the boring was approaching stronger rock
strata. Rate of decrease of PPV and PGA was
found to be highest in rock strata, where the
initially high peaks were found to be decaying
significantly to lower values with distances away
from the point of piling.
Triple-axes Vibration Characteristics
Considering the components of the vibration
characteristics in triple-axes separately, the vertical
particle velocities and accelerations were found to
be greater than radial or transverse components,
with the radial characteristics generally being
greater than transverse in most cases. For higher
distances away from piling point, radial
characteristics were approximating to transverse
values.
Correlation between Vibration Characteristics
and SPT-N Value
A general trend in correlation between ground
vibration characteristics and SPT-N value has been
observed during field monitoring of vibration. The
characteristics appear to be directly related to the N
value, meaning that the vibration characteristics
50
th
IG
C
50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
have an analogous increase when the penetration
resistance of soil is higher (higher SPT-N value).
With a lower resistance to chisel penetration in
some layers of soil (lower SPT-N value), the
characteristic values are also found to be lower.
A sample set of velocity and acceleration readings
changing with the depth of boring is considered for
the piling site 1 at an accelerometer position of 7.5
m. The values are as given in Table 4, with the
SPT-N values of respective layers of soil also
mentioned.
Table 4 Variation of vibration characteristics with SPT-N value at various depths in one sample point Depth from Ground (m)
SPT-N value
PPV (mm/s)
PGA (m/s
2)
G.L. 4 0.322125 0.305008 05 3 0.253782 0.146521 10 6 0.293043 0.179451 15 11 0.493264 0.322125 20 11 0.784494 0.343623 25 18 1.122268 0.561134 30 30 1.315976 0.514566 35 19 0.971912 0.485956 40 44 1.254734 0.920872 45 38 1.460321 1.104593 50 50+ 1.854329 1.429365
However, an absolute equated correlation between
the ground acceleration/velocity and SPT-N value
is beyond the scope of this study, since the drop
heights were considerably varied at site when
changes in the soil strata were encountered. When
boring was done in soils having lower penetration
resistance, the drop height was lowered since
sufficient driving rate was attained even with lower
energy of impact. However, when tougher soil
layers were encountered, the drop height needed to
be increased, imparting more energy for driving to
suffice rate of penetration. Due to this, there is no
benchmark impact energy corresponding to chisel
drops, with which one can compare the readings.
CONCLUSIONS
Accelerometers were used for the measurement of
vibration characteristics at various sites and data
measurements were done in terms of PPV and
PGA for studying the variation of the vibration
characteristics propagating away from the impact
point of piling at different depths. PPV was found
to have a general trend to attenuate with horizontal
distance away from impact point of chiseling for
all the depths of boring. The velocities were higher
at the ground level and start to reduce with increase
in depths from ground surface. The same rises
again considerably when boring approached soil
strata having higher resistance to chisel
penetration. PGA was also found to be generally
attenuating with horizontal distances away from
impact point of boring for various depths of boring.
The values of PGA were higher at lower depths
from ground and then start reducing with increased
depths from surface of ground up to certain depths.
The values were found to again increase highly
when boring was approaching soil with greater
penetration resistance or the stronger rock strata.
Maximum measured values of PPV and PGA were
associated with chiseling in hard rock strata. The
rate of attenuation of PPV and PGA was found to
be highest in the rock strata, where the initially
high peaks decayed significantly with increase in
distance away from the chiseling point. Vertical
vibration components were found to be greater
than the radial or transverse components. The
radial vibration characteristics were found to be
generally higher than transverse components in
most of the measurements. At higher distances
away from the point of boring, radial values
approximated to transverse readings. Vibration
characteristics appeared to be directly related to the
SPT-N value, meaning the vibration characteristics
had an analogous increase when penetration
resistance of soil is higher. When boring was done
at sites with soils having higher resistance to
penetration, weight rod attachments were done to
the chisel to attain a higher weight of drop. This in
turn transferred higher energies to the ground,
resulting in greater values of velocities and
accelerations. Monitoring of ground vibrations is
indispensably required for piling monitoring and
controlling to minimize damages to adjacent
structures.
M. R. Khan & S. J. Shah
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