Controlling
foundation length by the parallel seismic method
NORME : NFP 94 - 160 - 3
The
CS 97 equipment enables to apply two methods to control
foundations :
- The sonic coring method (transparency method).
- The parallel seismic method.
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The
parallel seismic method allows one to know the
length of a concrete pile, of a barrette, of
a moulded wall, of a metallic piling, or of
any other buried structure.
The
major interest of this method lies in that it
can be applied even though the foundation is
capped of such a structure as a building or
a bridge. Besides, it is not necessary to access
the area above the foundation to test : all
one needs to access is an area close to the
foundation and that is in rigid connection to
it. |
Only
few methods allow to measure the length of a buried
pile in such conditions.
Principle
of the method :
A
reception probe is taken down a tube step by step.
The tube is set in a drill located parallel to the
foundation to scan. The vertical step can be chosen
by the user on the CS 97 material (1 metre, or 20
or 50 centimetres). A 0.5 m step is advised.
| For
every position of the reception probe, a sound
pulse is broadcast by a stroke a hammer gives
to the foundation or to the structure carried
by the foundation.
The
hammer is fitted with a precise triggering device
which causes the measurement to start as soon
as it strikes the foundation.
Then,
the sound wave propagates through the foundation.
The
CS 97 unit records the signal received by the
reception probe in the tube. The duration between
the hammer stroke and the first arrival of the
sound wave on the reception probe is measured.
The
probe is taken 0.5 m lower down again, a new
hammer stroke is given, and so on...
The
measurement terminates when the probe has reached
the bottom of the tube.
If
two operators make the measurement, it takes
no more than about 25 minutes to wholly test
a 20 metre deep drill. |
|
 |
It
is worth noticing that one can take advantage of the
drill to perform pressiometric measurements with the
suitable material.
This is naturally to be performed before the tube
is set and the concrete is injected.
Knowing the bearing capacity of the ground can be
of a great help to evaluate the accuracy of the foundation
length.
Setting
the tube :
A
drill has to be made parallel to the pile to test.
This drill has to be located as close as possible
to the foundation, that is, closer than 1.5 m apart
from it..
| The
depth of the drill should be at least equal
to that presumed of the foundation (5 metres).
A
plastic tube with an internal diameter of 40
millimetres is placed into the drill ; it is
then sealed to the ground over its whole length.
This injection is to be performed with a cement
slurry by means of a plunge tube (between the
plastic tube and the ground).
The
tube has to be stopped at its base by a strong
stopper and it has to be filled with water.
The
water and the cement injection are meant to
allow a good sonic bond between the reception
probe and the ground.
The
injection has to be achieved with painstacking
care : the fact is, a bad tube to ground bond
does not allow the sound wave to reach the reception
probe. |
|
 |
Determining
the length of a foundation :
Each
measurement of a wave time of flight (or propagation
duration) between the hammer stroke and the arrival
at the reception probe is reported onto a graph.
| The
programme plots the time of flight versus the
measurement depth.
This
plot is called a dromochronic.
It
usually appears as a couple of straight lines
making an angle.
The
slope of the superior part (shallow depths)
corresponds to the probe when located in front
of the foundation, and that of the inferior
part corresponds to the probe when located below
the bottom of the foundation.The change of the
slope takes place at the position if the bottom
of the foundation, which can therefore be determined
after a slight correction.
The
top of the tube is the reference for zero depth.
The length of the foundation is given between
this tube top and the bottom of the foundation..
Two
seismic velocity are calculated by the software
: the seismic velocity in the foundation and
the seismic speed in the ground under the foundation).
They are meant to verify that the two slopes
determined from the graph do represent the part
in front of the foundation and that below its
bottom. |
|
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The
Parallel Seismic software of the CS97 :
The
software built in the CS 97 unit presents a very easy
use, and allows to process data very quickly.
The
mean straight lines, the length of the foundation
and the seismic velocities are computed automatically.
Whenever
wanted, the operator can re-locate the probe to a
given depth and perform one more acquisition, if he
thinks that the signal previously logged was of a
bad quality.
Four
types of documents can be printed straight on the
site of the measurements.
- The signals received, plotted versus depth and time.
- The mean straight lines plotted versus depth and
time.
- The mean straight lines plotted versus depth and
deviations.
- The schematic drawing of the foundation, of the
tube and of its position, onto which the results of
the measurements have been automatically reported.
The
CS 97 equipment for the parallel seismic period :
The
parallel seismic method for with the CS 97 of CEBTP
is particularly recommended in the case of work reception
control in the case of a law suit, of an expertise,
or in the case of a restoration, of a modification
or of the rising of a work when the drawings of the
foundations are lost.
Only
few accessories are to add to the sonic coring CS
97 equipment in order to apply the parallel seismic
method :
 |
the
parallel seismic software of CEBTP allowing
to perform acquisistions, savings, data treatments
and result printing. |
 |
an
instrumented hammer that emits the pulse wave
through the foundation and triggers the measurement. |
 |
a
cable counter allowing to precisely position the
reception probe in the tube. |
Composition of the equipment
 |
- a
CS 97 unit.
- a
laser A4 format printer.
- a
printer linking cable.
- an
amplified reception probe.
- a
cable winch (100 m) for the reception probe.
- a
cable counter.
- an
instrumented hammer.
- a
cable winch (50 m) for the hammer.
- a
tripod for the cable counter.
- two
linking cables to the CS 97 unit.
- a
linking interlink cable.
- an
user's manual in both English and French language.
|
Characteristics
of the CS 97 equipment for the parallel seismic method
:
Power
supply :
- Option1 : AC 110 volt up to 320 volt automatical.
- Option2 : AC 110 volt up to 320 volt automatical,
or 12 volt.
Power
consumption : 40 Watt.
operation
temperature : from 0 to 50°C.
Probe
diameter : 25 mm.
Screen :
LCD black and white 6 in screen.
Keyboard :
Alphanumeric, waterproof IP 65, QUERTY.
Printer :
Laser A4 format printer.
Dimensions
of the CS 97 unit : 43 x 20 x 51 cm.
Weight
of the CS 97 unit : 13.6 Kg (for the mains power supply version).
Sampling
frequencies : From 125 kHz up to 2 MHz.
Vertical
resolution : can be selected between 0.2
m, 0.5 m or 1 m.
Time
base : 5 ms, 10 ms, 20 ms ou, 40 ms.
Resolution
of the time base : 0.1 ms.
Maximum
control depth : 30 meters.
Maximum
space between the foundation and the tube :
3 meters (recommended maximum : 1.5 m)
Maximum
internal storage capacity : 80 000 signals.
Language
of the software : English.
Language
of the prints : English, French, Spanish,
Portuguese language (other alphabetical languages
upon request).
The
software can be used from the keyboard or from the
mouse.
Exploitation
system : Windows 95.
Total
weight of the equipment (CS 97 unit, cables,
probes, printer, etc...) : 52 kg
This material meets with the requirements of the NF P 94 160-3 norm.
Further
informations :
<Historical overview
of the foundation control by the parallel seismic method>
<Sonic
coring with the CS 97>
<The different means
of controlling a foundation>
