The different means of controlling a foundation
There
are four methods producing the length of a foundation
and its integrity, principally.
The sonic coring method with the CS 97 :
The
metallic tubes have to be fixed to the steel reinforcement
of the foundation before the concrete is cast.
The tubes, which have the same length than the foundation,
are filled with water. They allow path for a reception
and an emission probe.
Two tubes are necessary for small diameter piles ;
three or four tubes are required for large (i.e. above
600 mm) piles.
This method also applies to molded walls and barrettes.
This method allows to record the variations of the
time of flight of a sound wave between the tubes.
The document obtained is called a diagraph.
Advantages
of the method :
- The
foundation length is not limited.
- The
depth of a defect can be located precisely.
- The
importance of it can be evaluated precisely.
Drawbacks
of the method :
- it
is a prerequisite that the foundations were fitted
with tubes when fabricated.
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The reflection method with the MIMP 16 :
The
operator places a reception sensor (called a geophone)
at the top of the pile to test, and he creates a sound
signal by hitting the top of a foundation with help
of a simple hammer.
The informations are gathered and stored by the MIMP
16.
This method allows the length of the foundation to
be determined after an analysis is performed.
This analysis can be performed quickly with the MIMP
16, or in a more in-depth way on a PC with the IMPRO2
software.
Advantages
of the method :
- the
foundation does to need to be have previously
been fitted with tubes.
- the
cost of the method is lowered by reference to
the impedance method
(no instrumented hammer)..
Drawbacks
of the method :
- The
length of the foundation to test is limited. This
limit depends on the length and the diameter of
the foundation and on the ground characteristics
(Aspect ratio=length/diameter).
- The
characteristic impedance and the stifness of the
pile cannot be measured.
- The
shape of the foundation cannot be known.
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The
impedance method with the MIMP 16 :
The
process is the same as for the reflection method,
the excitation to the foundation is brought to it
with a hammer fitted with a force sensor. Thus the
force is measured.
The IMPRO2 software allows to analyze the foundation
with three methods : the frequency analysis, the time
analysis, and the impedance profile.
Advantages
of the method :
- The
foundation does to need to be have previously
been fitted with tubes.
- The
characteristic admittance and the stifness of
the pile can be measured.
- Permet
la mesure de la raideur basse fréquence.
- The
impedance profile can be plotted.
Drawbacks
of the method :
- The
length of the foundation to test is limited.
This limit depends on the length and the diameter
of the foundation and on the ground characteristics
(Aspect ratio=Length/diameter).
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The
parallel seismic method with the CS 97 :
This
method is bound to be used in expert analyses, in
case the impedance method would be impossible because
the top of the foundation cannot be reached.
A plastic tube is placed in a coring as close as possible
to the foundation to test.
A hammer stoke is then applied to the structure born
by the foundation, and the propagation time is measured
by a probe for different depths in the tube.
The document produced allows one to know the depth
of the foundation as well as the seismic speeds in
the ground and in the foundation.
This method can be applied to piles, to barrettes,
to claddings, etc...
Advantages
of the method :
- It
allows to know the length the length of the foundations
of a building or of a bridge without having to
access the foundation.
Drawbacks
of the method :
- A
coring, placing a tube and a injection are required.
- Defects
cannot be detected, except for clear ruptures
in the foundation.
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Below are the limits of the réflexion and impedance
methods, as stated by norm NF P 94.160-2 for réflexion
and by norm NF P 94.160-4 for impedance.
The aspect ratio (more commonly called " slenderness
") of a pile has to be included in the 10 to 30 interval
(i.e. 10 £ L/D £ 30).
The aspect ratio of a pile is defined as the ratio
of its length (L) to its diameter (D).
Aspect ratio = length/diameter
If a pile is 1 m in diameter, you can theoretically
test a pile less than 30 m long (aspect ratio = 30).
If a pile is 0,6 m in diameter, the maximum length
for it to be tested is 18 m (aspect ratio = 30).
When the pile happens to cross a thick layer of soft
ground or of water, the maximum aspect ratio can be
increased.
As you cas realize, the réflection or impedance method
applies to a large majority of piles built for building
engineering
