EP0126010B1 - Devices for determining the variations of the size of a deformable and inflatable cell thrusted into the ground and subjected to gradients of internal pressure - Google Patents

Description

The invention relates to improvements in apparatus for determining the volume variations of an expandable deformable cell embedded in a ground and subjected to internal gas pressure gradients.

The principle of these devices is set out in French patent 1,117,983 of Mr. Louis MENARD in which the devices are called “Pressure meters” (registered trademark). Essentially, they include an expandable cell to be driven into the ground of which certain mechanical characteristics are to be determined, means for introducing an inflation liquid into the cell, means for creating a gas pressure on this liquid, and means to measure changes in cell volume and gas pressure. In addition, these devices usually include one or more guard cells which accompany the expandable cell and which are subjected to a gas pressure which is in relation to the gas pressure on the liquid of the expandable cell, this to ensure a field of cylindrical stresses during the measurements and to facilitate deflation of the cell when the test is completed.

• ESSAI PRESSIOMETRIQUE NORMAL (Mode opératoire du Laboratoire Central des Ponts et Chaussées). Ed. DUNOD, Paris, 1971)
• SONDAGE PRESSIOMETRIQUE NORMAL (D.T.U 11 1 de la Fédération du Bâtiment)
• SUGGESTED PRACTICE FOR PRESSUREMETER TESTING lN SOILS de Ernest WINTER (1982 - ASTM).

Various devices operating according to this principle have been proposed since 1955 and a recent development has been proposed in French patent application 8 111 630 (FR-A-2 512 860). In addition, this technique for in-situ determination of the characteristics of a site has been subject to certain standardization:

• NORMAL PRESSIOMETRIC TEST (Procedure of the Central Laboratory of Bridges and Roads). Ed. DUNOD, Paris, 1971)
• NORMAL PRESSIOMETRIC SURVEY (DTU 11 1 of the Fédération du Bâtiment)
• SUGGESTED PRACTICE FOR PRESSUREMETER TESTING lN SOILS by Ernest WINTER (1982 - ASTM).

We can also consult the book THE PRESSUREMETER AND FOUNDATION ENGINEERING by F. Baguelin, J.F. Jézéquel and D.H. Shields (Trans Tech Publications, 1978).

The “pressuremeter” test consists in imposing pressure levels on the liquid which fills the expandable cell and in measuring the volumes of the cell corresponding to these levels, these volumes depending on the resistance of the ground around the cell.

The operator, after having determined a priori threshold values for the gas pressure on the liquid of the expandable cell, acts manually on the gas inlet by monitoring the pressure gauge which indicates the gas pressure to maintain this pressure at the desired level value for a certain time and measure the cell liquid volume by repeating this operation for each successive level.

This technique requires great dexterity not to exceed the desired level and to maintain the value of the level throughout the duration of the level. In practice, the pressure drifts over the duration of the bearing and it is difficult, if not impossible, to determine with exactitude the volume corresponding to the desired bearing pressure, because this pressure is not achieved or because this pressure is not not maintained for a sufficient period of time.

The invention aims to avoid these difficulties.

• - une chambre de prédétente à relier à la source de gaz;
• - une canalisation de gaz reliant la chambre de prédétente à une chambre pilote où la pression du gaz s'exerce au-dessus d'un liquide aboutissant à la cellule dilatable;
• - une canalisation de gaz reliant la chambre de prédétente à une chambre tempon reliée à la ou aux cellules de garde;
• - un capteur de pression de gaz apte à fournir un signal Si représentatif de la valeur de la pression du gaz P₁ admis dans la chambre de prédétente;
• - une électrovanne apte à commander l'admission de gaz dans la chambre de prédétente;
• - un capteur de pression de gaz apte à fournir un signal S₂ représentatif de la valeur de la pression de gaz P₂ dans la chambre pilote;
• - une électrovanne apte à commander l'admission da gaz dans la chambre pilote;
• - des moyens de détection aptes à fournir un signal S₃ représentatif de la pression différentielle entre ladite pression P₂ et la pression de gaz P₃ dans la ou les cellules de garde;
• - une électrovanne apte à commander l'admission de gaz dans la cellule tampon, et
• - un bloc électronique de commande qui reçoit les signaux S₁ , S₂, S₃ et qui commande les fonctionnements des électrovannes en fonction du programme prédéterminé des valeurs de P₃ et des relations désirées entre P₃, Pz et P₁ , de façon à réaliser ce programme et ces relations.

According to the invention, this is achieved by means of an apparatus of the type set out in the preamble of claim 1 which comprises:

• - a pre-expansion chamber to be connected to the gas source;
• - a gas pipeline connecting the pre-expansion chamber to a pilot chamber where the gas pressure is exerted over a liquid terminating in the expandable cell;
• - a gas pipeline connecting the pre-expansion chamber to a tempon chamber connected to the guard cell (s);
• a gas pressure sensor capable of supplying a signal Si representative of the value of the pressure of the gas P ₁ admitted into the pre-expansion chamber;
• - a solenoid valve capable of controlling the admission of gas into the pre-expansion chamber;
• a gas pressure sensor able to supply a signal S ₂ representative of the value of the gas pressure P ₂ in the pilot chamber;
• - a solenoid valve capable of controlling the admission of gas into the pilot chamber;
• - Detection means able to supply a signal S ₃ representative of the differential pressure between said pressure P ₂ and the gas pressure P ₃ in the guard cell or cells;
• a solenoid valve capable of controlling the admission of gas into the buffer cell, and
• - an electronic control unit which receives the signals S ₁ , S ₂ , S ₃ and which controls the operations of the solenoid valves as a function of the predetermined program of the values of P ₃ and of the desired relationships between P ₃ , Pz and P ₁ , so to realize this program and these relationships.

The predetermined program of the values of the pressure P ₃ and the desired relationships between P ₃ , P ₂ and P ₁ are chosen by the operator, for example by following the indications of the standards cited above.

The control electronics can be carried out without difficulty by an electronics engineer who is knowledgeable of the rules of the art and informed of this program and of these relationships.

• la figure 1 est une vue de principe du dispositif;
• la figure 2 est une vue du dispositif de l'invention selon un mode de réalisation, et
• la figure 3 est une vue de différentes courbes de montée en pression obtenues à partir d'une commande manuelle ou selon la présente invention.

An exemplary embodiment will be described below, with reference to the figures in the attached drawing in which:

• Figure 1 is a principle view of the device;
• FIG. 2 is a view of the device of the invention according to one embodiment, and
• Figure 3 is a view of different pressure build-up curves obtained from a manual control or according to the present invention.

In the diagram of FIG. 1, the measurement probe 1 inserted into a borehole 2 of a site 3 comprises an expandable cell 4 and two communicating guard cells 5. This is only an exemplary embodiment and the invention is not limited to this embodiment.

The expandable cell is filled with an incompressible fluid, generally water, and is connected to the surface by a pipe 6 which serves to introduce (or to drain) this liquid. The guard cells are connected to the surface by a pipe 7 which makes it possible to introduce a gas, for example air or nitrogen, into these cells.

A source of compressed air 8 is used, after expansion to a pressure P ₁ , to provide a pressure P ₂ which will be exerted on the fluid of the expandable cell and a pressure P ₃ which will be exerted in the guard cells.

• a) si P_lim est la pression limite (estimée à priori) et N le nombre de paliers, le PAS du programme de mon-P_lim tée arithmétique de P₂ est égal à PAS = . N
  • La durée T de chaque palier est fixée habituellement à 60 secondes mais cette durée, en fait, peut être choisie différente (par exemple 30 secondes).
  • Le nombre de pas d'un essai est généralement égal à 10 mais ce nombre peut être choisi différent.
  • Le programme de montée de la pression P₂ est donc choisi à priori à partir d'une valeur estimée de P_lim, d'un choix de N et d'un choix de T.
• b) La pression P₃ est généralement liée à la pression P₂ de façon que la pression différentielle P₃-P₂ soit égale à:
  
  où H est la profondeur de la cellule de mesure exprimée en mètre et où les pressions P sont exprimées en bars.
• c) la pression P, est généralement réglée de façon que
  
  où P_lim est la pression limite (estimée à priori) exprimée en bars. La constante K est choisie de façon que même si la pression limite a été sous estimée, P_i soit cependant supérieure à P_lim réelle. Une marge de sécurité généralement suffisante est K = 20.

During a "presiometric" test, the pressure P ₂ must rise from a zero value to a final value, by an arithmetic series of plateau values, each plateau being maintained for a constant duration chosen a priori.

• a) if P _lim is the limiting pressure (estimated a priori) and N is the number of steps, the PAS of the program of my-P _l arithmetic im p ed of P ₂ is equal to PAS =. NOT
  • The duration T of each level is usually fixed at 60 seconds but this duration, in fact, can be chosen different (for example 30 seconds).
  • The number of steps in a test is generally equal to 10, but this number can be chosen different.
  • The pressure rise program P ₂ is therefore chosen a priori from an estimated value of P _lim , a choice of N and a choice of T.
• b) The pressure P ₃ is generally linked to the pressure P ₂ so that the differential pressure P ₃ -P ₂ is equal to:
  
  where H is the depth of the measuring cell expressed in meters and where the pressures P are expressed in bars.
• c) the pressure P, is generally adjusted so that
  
  where P _lim is the limit pressure (estimated a priori) expressed in bars. The constant K is chosen so that even if the limit pressure has been underestimated, P _i is however greater than actual P _lim . A generally sufficient safety margin is K = 20.

The device of the invention for ensuring an automatic rise in P ₂ according to the chosen program comprises a pre-expansion chamber 9 connected to the gas source 8 via a solenoid valve 10, a sensor 11 for detecting the gas pressure P, in chamber 9 (this sensor can detect this pressure in the chamber or in the amount or downstream of the chamber), a pilot chamber 13 connected to the pre-expansion chamber 9 by a pipe 12 under the control of a solenoid valve 14 , this pilot chamber containing a liquid 15 on which the pressure P ₂ of the gas is exerted in the pilot chamber and this liquid 15 being in communication with the expandable cell 4; a sensor 16 for detecting the pressure P ₂ (this sensor detects the pressure in the pilot chamber or on the path of the liquid 15 towards the expandable cell), a buffer cell 17 connected to the pre-expansion chamber 9 by a pipe 18 under control a solenoid valve 19 and connected to the pipe 7 which leads to the guard cells a differential sensor 20 which detects the pressure difference between the gas pressure P ₂ and the gas pressure P ₃ of the buffer cell, and a block of electronic control 21 which receives the output signals S ₁ , 5 ₂ , S ₃ from the sensors 11, 16, 20 and which controls the solenoid valves 10, 14, 19 so as to carry out the program defined in a) and the relationships defined in b ) and c). In short, this electronic unit, from the data provided by the operator (estimated P _lim , depth of the measuring cell, number and duration of the stages) carries out the pressure rise program P ₂ .

The values of the volume of the expandable cell which can be determined for example by following the variations in the volume V of liquid 15 in the pilot cell 13 are, on the other hand, captured by any suitable means 22 and are read for each pressure level via block 21. A screen 23 displays the pairs P ₂ , V for each level value.

In practice, depending on the type of Pressuremeter used, the device can be more complicated.

Figure 2 is an example of a device designed for a commercial pressuremeter.

We see in this figure that the device is completed by solenoid valves 24, 25, 26 which control, as necessary, the exhaust of the pre-expansion chamber 9, the pilot chamber 13 and the buffer cell 17. These solenoid valves are also controlled by the electronic unit 21 during the pressure lowering phase P ₂ during deflation of the expandable cell 4.

Each pair of inlet solenoid valve and exhaust solenoid valve can be constituted by a single solenoid valve in a manner known per se.

• - terminer prématurément l'essai si le volume de la cellule dilatable approche la capacité limite de cette cellule (par exemple si ce volume dépasse 550 cm³ pour une capacité limité fixée à 600 cm³).
• - terminer prématurement l'essai si la pression P₂ approche une pression de sécurité, par exemple une pression de 70 bars.
• - calculer à partir des premières mesures le module «pressiométrique»
  
  (où V_REF qui dépend de la sonde est par exemple égal à 585),
• - en déduire la valeur limite correspondante de la pression
  
• - Comparer cette valeur à la Pl_im estimée à priori, corriger la valeur du PAS si le rapport P'_lim/ P_lim est extérieur à une gamme choisie A-B (où A est par exemple un nombre compris dans l'intervalle 0,4-1,5 et B est par exemple un nombre compris dans l'intervalle 1,5-3) et
• - provoquer un nouvel essai avec la nouvelle valeur du PAS.

In an improved variant embodiment, the electronic unit is designed to provide a certain number of security features, in particular the following operations:

• - prematurely end the test if the volume of the expandable cell approaches the limit capacity of this cell (for example if this volume exceeds 550 cm ³ for a limited capacity set at 600 cm ³ ).
• - prematurely end the test if the pressure P ₂ approaches a safety pressure, for example a pressure of 70 bars.
• - calculate from the first measurements the “pressuremeter” module
  
  (where V _REF which depends on the probe is for example equal to 585),
• - deduce the corresponding limit value of the pressure
  
• - Compare this value with the Pl _im estimated a priori, correct the value of the PAS if the ratio P ' _lim / P _lim is outside a chosen range AB (where A is for example a number included in the interval 0.4-1.5 and B is for example a number in the range 1.5-3) and
• - trigger a new test with the new PAS value.

Table I is a flowchart which illustrates the operation of the electronic unit. In this table, the index i means that the value indicated corresponds to the level (i) of the arithmetic series of the levels of pressure P ₂ . In this table, RND (N / 2.5) means rounded value of N / 2.5.

Table I is a typical example of the indications useful to those skilled in the art of electronics to make a control unit from microprocessors, transistors, and the like.

FIG. 3 shows for the example of the pressure rise curves obtained from a more or less experienced manual control (curves C _l , C ₂ ) or from an automatic control (curve C ₃ ).

Claims (5)

1. Apparatus for determining the variations of volume of a dilatable cell (4), this cell being filled with an incompressible fluid subjected to the pressure of a gas originating from a source of gas (8) and this cell being accompanied by one or more guard cells (5) which are likewise subjected to the pressure of a gas, the assembly of the dilatable cell and the guard cell or guard cells constituting a probe (1) to be submerged in a soil (3) of which it is desired to determine the mechanical characteristics, the apparatus being characterized in that it comprises:

a pressure reduction chamber (9) to be connected to the source of gas (8);

a gas duct (12) connecting the pressure reduction chamber to a pilot chamber (13) where the pressure of the gas is exerted above a liquid (15) passing to the dilatable cell (4);

a gas duct (18) connecting the pressure reduction chamber (9) to a buffer chamber (17) connected to the guard cell or guard cells (5);

a gas pressure sensor (11) capable of supplying a signal S, representing the value of the gas pressure P, admitted into the pressure reduction chamber;

a solenoid valve (10) capable of controlling the admission of gas into the pressure reduction chamber;

a gas pressure sensor (16) capable of supplying a signal 8₂ representing the value of the gas pressure P₂ in the pilot chamber;

a solenoid valve (14) capable of controlling the admission of gas into the pilot chamber;

detecting means (20) capable of supplying a signal S₃ representing the pressure differente between the said pressure S₂ and the gas pressure P₃ in the guard cell or guard cells;

a solenoid valve (19) capable of controlling the admission of gas into the buffer cell (17), and

an electronic control unit (21) which receives the signals Si, S₂ and S₃ and which controls the operations of the solenoid valves (10, 14, 19) in accordance with the predetermined programme of the values of P₃ and the desired relationships between P₃, P₂ and P₁, in such a manner as to execute this programme and these relationships.

2. Apparatus according to Claim 1, characterized in that it includes solenoid valves (24, 25, 26) which control the connection to exhaust of the pressure reduction chamber (9), of the pilot chamber (13) and of the buffer cell (17) respectively, these solenoid valves being controlled by the said electronic unit (21) during the phase of decrease of the pressure P₂ during the deflation of the dilatable cell (4).

3. Apparatus according to Claim 1 or 2, characterized in thatthe said electronic unit (21 ) is designed to terminate the test prematurely if the volume of the dilatable cell (4) approaches the limiting capacity of this cell.

4. Apparatus according to one of Claims 1 to 3, characterized in that the said electronic unit (21) is designed to terminate the test prematurely if the said gas pressure P₂ of the pilot chamber (13) approaches a safety pressure.

5. Apparatus according to one of Claims 1 to 4, characterized in that the said electronic unit (21) is designed to calculate the limiting value corresponding to the «pressiometric» module, to compare this value with the limiting pressure estimated a priori, to correct the value of the STEP if the ratio of the two values is outside a selected range, and to initiate a new test with the new value of the STEP.

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