THE CS 97 EQUIPMENT


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.

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.

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.

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

You can obtain a complete set of documents about these new equipments quickly by simply placing a request at :

M. Frédéric BOURDET

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