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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 :
Sommaire
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.
Sommaire
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,
-
the impedance profile can be plotted.
Drawbacks of the method :
Sommaire
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 :
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.
Sommaire
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.
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