EP3669029B1 - Static penetrometer for evaluating the liquefiable character of a soil and associated method - Google Patents

Description

FIELD OF THE INVENTION

The present invention relates to the field of geotechnics and geology. It relates to a device for measuring the resistance to soil penetration, commonly called a penetrometer, and an associated measurement method. It relates in particular to a static penetrometer for carrying out tests for evaluating the liquefiable character of a soil.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

The new earthquake-resistant standards lead to systematizing, during geotechnical studies in seismic zones, the characterization of soils with regard to the risk of liquefaction. The liquefaction of a soil during a seismic movement refers to the decrease in the rigidity of said soil and / or the decrease in its shear resistance, due to the increase in the pressure of the pore water. These modifications of the characteristics of the soil are likely to produce significant permanent deformations (settlements, slips), or even a virtual cancellation of the effective stresses (phenomenon of large deformation, spreading).

Identifying this risk of liquefaction therefore makes it possible to anticipate solutions for constructing foundations to ensure the stability of structures during earthquakes.

Remember that the compactness of soils is usually measured, either in static mode or in dynamic mode, by a measuring device called a penetrometer. The documents FR2584186A1 and WO00 / 17622A1 present examples of devices for measuring mechanical characteristics of soils.

A penetrometer conventionally comprises rods connected end to end to form a string of rods at the end of which is fixed a measuring tip, intended to sink into the ground to depths which may reach several tens of meters.

In static mode, the drill string is pushed by jacks, causing the progressive insertion of the measuring tip; the latter measures the point resistance and possibly the lateral friction on a cylindrical sleeve located above the point. These measurements are recorded continuously or discontinuously at regular intervals. The static measurement of resistance to soil penetration is undoubtedly the most precise because it is carried out directly on the measuring tip, at the bottom of the borehole.

Currently, the potentially liquefiable character of a soil is evaluated by experimental correlations between in situ measurements and the critical cyclic shear stresses (which we know have caused liquefaction during past earthquakes). In situ measurements are usually carried out from standard penetration tests (SPT) or cone penetration tests (CPT), in particular piezocone soundings (CPTU). The raw measurements resulting from these tests are then used to determine standardized variables allowing the evaluation of the standardized resistance of the soil; this normalized resistance is then compared to the normalized stress of the site. Soil samples can also be taken with a view to determining grain size curves in the laboratory.

The challenge is to propose methods for identifying liquefiable soils, on the one hand relatively direct, avoiding empirical correlations, and on the other hand relatively simple, in particular avoiding the complexity of implementing piezocone tests or cyclic loading in the laboratory.

OBJECT OF THE INVENTION

An object of the present invention is to provide an alternative solution to the solutions of the state of the art, in particular a static penetrometer, simple to use and allowing a more direct evaluation of the liquefiable character of a soil.

BRIEF DESCRIPTION OF THE INVENTION

• Au moins une tige centrale terminée à une première extrémité par une pointe de mesure ;
• Au moins un tube creux entourant la tige centrale, cette dernière étant apte à coulisser à l'intérieur du tube creux ;

The present invention relates to a penetrometer for evaluating the liquefiable character of a soil, comprising:

• At least one central rod terminated at a first end by a measuring tip;
• At least one hollow tube surrounding the central rod, the latter being able to slide inside the hollow tube;

• configuré pour transmettre un déplacement à une deuxième extrémité de la tige centrale menant à un enfoncement contrôlé de la pointe de mesure dans le sol, et pour mesurer une force appliquée pour effectuer ledit déplacement,
• et configuré à appliquer à la deuxième extrémité de la tige centrale une vibration, à une fréquence déterminée.

The penetrometer further comprises an electric jack comprising an external body integral with the hollow tube and a movable body, said movable body being:

• configured to transmit a displacement to a second end of the central rod leading to a controlled depression of the measuring tip into the ground, and to measure a force applied to effect said displacement,
• and configured to apply to the second end of the central rod a vibration, at a determined frequency.

• la fréquence déterminée est comprise entre 1 et 5 Hertz ;
• le vérin électrique est asservi électroniquement de sorte que l'actionnement et la vitesse du corps mobile pour effectuer un déplacement, la force appliquée, et l'application ou l'arrêt de la vibration peuvent être effectués selon des séquences programmées ou asservis les uns aux autres ;
• le pénétromètre comprend une cellule solidaire du tube creux, destinée à transmettre une force d'appui, appliquée par des moyens d'appui, de manière à provoquer un enfoncement dans le sol du tube creux et de la tige centrale jusqu'à une profondeur donnée.

According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination:

• the determined frequency is between 1 and 5 Hertz;
• the electric actuator is electronically controlled so that the actuation and speed of the movable body to effect displacement, the force applied, and the application or stopping of the vibration can be carried out according to programmed sequences or slaved to each other. others ;
• the penetrometer comprises a cell integral with the hollow tube, intended to transmit a bearing force, applied by bearing means, so as to cause the hollow tube and the central rod to sink into the ground to a given depth .

• a) enfoncer dans le sol le couple formé par le tube creux et la tige centrale pour amener la pointe de mesure à une profondeur donnée, ladite pointe de mesure étant en butée contre le tube creux ;
• c) actionner le corps mobile pour qu'il effectue un premier déplacement, induisant un premier enfoncement contrôlé de la pointe de mesure dans le sol, et mesurer la force appliquée pour effectuer ledit premier déplacement ;
• d) appliquer une vibration à une fréquence déterminée, à la deuxième extrémité de la tige centrale par l'intermédiaire du corps mobile et simultanément actionner le corps mobile pour qu'il effectue un deuxième déplacement, induisant un deuxième enfoncement contrôlé de la pointe de mesure dans le sol ; et mesurer la force appliquée pour effectuer ledit deuxième déplacement ;
• e) arrêter la vibration ;
• f) actionner le corps mobile pour qu'il effectue un troisième déplacement, induisant un troisième enfoncement contrôlé de la pointe de mesure dans le sol et mesurer la force appliquée pour effectuer ledit troisième déplacement.

The invention also relates to a method for evaluating the liquefiable character of a soil, which can use the penetrometer as above. Said method comprises the following steps:

• a) driving the couple formed by the hollow tube and the central rod into the ground to bring the measuring tip to a given depth, said measuring tip being in abutment against the hollow tube;
• c) actuating the movable body so that it performs a first displacement, inducing a first controlled depression of the measuring tip in the ground, and measuring the force applied to effect said first displacement;
• d) apply a vibration at a determined frequency, to the second end of the central rod through of the movable body and simultaneously actuate the movable body so that it performs a second movement, inducing a second controlled depression of the measuring tip into the ground; and measuring the force applied to effect said second displacement;
• e) stop the vibration;
• f) actuating the movable body so that it performs a third displacement, inducing a third controlled depression of the measuring tip in the ground and measuring the force applied to effect said third displacement.

• le procédé comprend avant l'étape c), l'étape b) suivante : b) actionner le corps mobile pour le placer en contact avec la deuxième extrémité de la tige centrale ;
• l'étape c) comprend également la mesure de la force de réaction transmise par la deuxième extrémité au corps mobile, à l'arrêt, après le premier enfoncement ;
• l'étape f) comprend également la mesure de la force de réaction transmise par la deuxième extrémité au corps mobile, à l'arrêt, après le troisième enfoncement ;
• à l'étape d), la vitesse du deuxième déplacement est ajustée de manière à maintenir la force appliquée mesurée sensiblement constante ;
• à l'étape d), la vitesse du deuxième déplacement est ajustée de manière à ce que la force appliquée mesurée soit maintenue sensiblement égale ou approche au plus près la force appliquée mesurée à l'étape c) ;
• la vitesse de déplacement du corps mobile peut atteindre 16cm/s ;
• le déplacement maximal entre une position effacée et une position déployée du corps mobile est de 75mm ;
• le premier déplacement du corps mobile est de 10mm, le deuxième déplacement est de 30mm et le troisième déplacement est de 10mm.

According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination:

• the method comprises, before step c), the following step b): b) actuating the movable body to place it in contact with the second end of the central rod;
• step c) also comprises measuring the reaction force transmitted by the second end to the movable body, when stationary, after the first depression;
• step f) also comprises measuring the reaction force transmitted by the second end to the movable body, when stationary, after the third depression;
• in step d), the speed of the second displacement is adjusted so as to keep the applied force measured substantially constant;
• in step d), the speed of the second displacement is adjusted so that the measured applied force is kept substantially equal to or closely approaches the applied force measured in step c);
• the moving speed of the moving body can reach 16cm / s;
• the maximum displacement between a retracted position and a deployed position of the mobile body is 75mm;
• the first displacement of the moving body is 10mm, the second displacement is 30mm, and the third displacement is 10mm.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 présente une courbe d'essai avec double mesure de la résistance à la pénétration d'un sol ;
• la figure 2 présente un pénétromètre conforme à l'invention ;
• la figure 3 présente un procédé conforme à l'invention ;
• la figure 4 présente l'évolution de la force appliquée pour effectuer des enfoncements contrôlés, en fonction du temps.

Other characteristics and advantages of the invention will emerge from the detailed description which follows with reference to the appended figures in which:

• the figure 1 presents a test curve with double measurement of the resistance to soil penetration;
• the figure 2 presents a penetrometer in accordance with the invention;
• the figure 3 presents a method according to the invention;
• the figure 4 shows the evolution of the force applied to perform controlled sinks, as a function of time.

DETAILED DESCRIPTION OF THE INVENTION

- mode statique) et une mesure de la résistance de pointe à l'arrêt (notée $Q_c^{arr\mathrm{ê}t}$

).The applicant has developed a method for the pre-identification of liquefiable soils (cf. H. Hosseini-Sadrabadi et al, "Identification of liquefiable soils by static penetrometer: principle and digital modeling", National Days of Geotechnics and Engineering Geology, Nancy 2016 ) using a static penetrometer. This method is based on static penetration tests with double measurement: a measurement of the tip resistance at a constant driving speed of 2cm / s (noted $Q_{\mathrm{vs}}^{2\mathit{cm}/s}$

- static mode) and a measurement of the peak resistance at standstill (noted $Q_{\mathrm{vs}}^{\mathrm{To}rr\mathrm{ê}t}$

).

These double measurements, in static mode and at standstill, at a given depth, can make it possible to identify potentially liquefiable soils in that the difference (noted Δ Q _c ) between the resistance of the soil in static mode and the resistance of the ground at standstill seems characteristic of the evolution of pore pressures in the ground: $$\mathrm{\Delta }{Q}_{\mathrm{vs}}=\frac{Q_{\mathrm{vs}}^{2\mathit{cm}/s}-Q_{\mathrm{vs}}^{\mathrm{To}rr\mathrm{ê}t}}{Q_{\mathrm{vs}}^{2\mathit{cm}/s}}$$

The higher this difference Δ Q _c , the greater the risk of the soil liquefying in the event of an earthquake. In particular, the applicant affects a very high probability of liquefaction when this difference exceeds 40% of the resistance of the ground measured in static mode.

As shown on the figure 1 , it is therefore possible, during a test with double measurement (in static mode and at standstill) of the resistance to penetration of a soil, to detect layers of soil (zone A), at one (or more) ) given depth (s), for which the deviation Δ Q _c is greater than a critical value, in particular 40%. This type of test can constitute an interesting preliminary diagnosis of the liquefiable character of a soil.

The present invention relates to a penetrometer 100 for the evaluation by direct measurement of the liquefiable character of a soil. On the basis of a pre-diagnosis mentioned above, the penetrometer 100 according to the invention will make it possible to carry out tests, directly at the given depth of zone A (to return to the example illustrated in figure 1 ), in order to more directly assess the liquefiable character of the soil layers concerned and assess the associated risks.

The penetrometer 100 ( figure 2 ) comprises at least one central rod 1 terminated at a first end by a measuring tip 11. It also comprises at least one hollow tube 2 surrounding the central rod 1. The respective diameters of the hollow tube 2 and of the central rod 1 are suitable so that the latter can slide freely inside the hollow tube 2.

The couple formed by the central rod 1 and the hollow tube 2 is intended to sink into the ground, the tip 11 at the head. As is well known, to reach a given depth, additional rods 1 and tubes 2 can be connected end to end, to form a train of rod / tube pairs, which can be driven into the ground for several tens of meters.

The penetrometer 100 further comprises an electric jack 6 comprising an external body 61 integral with the hollow tube 2 and a movable body 62 able to come into contact with the central rod 1.

Advantageously, the movable body 62 is capable of achieving a maximum displacement of 75mm between a retracted position (movable body 62 retracted) and a deployed position (movable body 62 extended to the maximum).

The movable body 62 is configured to transmit a displacement to a second end 12 of the central rod 1, which displacement will lead to a controlled depression of the measuring tip 11 into the ground. As will be described in more detail in the method according to the invention, controlled sinks will be carried out successively from the given depth P _A , in the suspect zone A ( figure 1 ). For this, the power of the electric actuator motor is preferably chosen so that the movable body 62 is capable of applying a pressure between 10 and 40 bars: this range of pressures makes it possible to address the characteristic stresses (10-30 bars) measured at peak in soil layers at risk of liquefaction. The motor of the electric jack 6 is also chosen so that the speed of movement of the movable body 62 can vary between a few mm / s and about 16 cm / s, so that the detection of the effective loss of lift of the ground (case of a liquefiable sol) is not affected by the technical limits of the device (as will be detailed later in the process).

The movable body 62 is also configured to measure the force applied to effect said displacement. The applied force is representative of the resistance to soil penetration at the level of the measuring tip 11. The electric jack 6 therefore comprises a force or stress sensor (not shown) for measuring this force.

Finally, the movable body 62 is able to apply a vibration to the second end 12 of the central rod 1, at a determined frequency.

The determined frequency is advantageously between 1 and 5 Hertz. This frequency range is characteristic of earthquakes, according to current seismological knowledge; we will see later in the description of the method according to the invention, that the vibration in this frequency range makes it possible to modify the properties of the ground and to evaluate the evolution of its lift in the event of an earthquake.

Advantageously, the electric jack 6 is electronically controlled: the actuation and the speed of the movable body 62 to effect a movement, the force applied during the depression, the measurement of the reaction force of the rod 1 and the vibration of the movable body 62 can thus be carried out according to programmed sequences or be slaved to one another.

The penetrometer 100 according to the invention advantageously comprises a cell in contact (or integral with) the hollow tube 2. The cell (not shown) may for example be secured to the hollow tube 2 by means of a clamping jaw 3. The cell is intended to transmit a support force, applied by support means, so as to cause a depression in static mode in the ground of the couple formed by the hollow tube 2 and the central rod 1 up to ( or the given depth (s) to be investigated.

The support means may in particular consist of a hydraulic cylinder. The movable part of the hydraulic cylinder fixed to the cell applies to it the support force necessary for the continuous sinking of the tube / rod pair. The fixed part of the hydraulic cylinder must be fixed directly or indirectly to a reaction block. The support means may comprise a self-propelled hydraulic unit, to actuate the hydraulic cylinder.

Advantageously, the support means are held by a frame. The frame is provided with at least one mechanical connecting element intended to be connected to a reaction block. This mechanical connecting element could for example consist of a hydraulic or mechanical clamp, or else a vice of the same type. The fact that the frame is equipped with such a mechanical connecting element makes it connectable to any kind of reaction block.

The present invention also relates to a method for evaluating the liquefiable character of a soil. It will be described in the case of the use of the penetrometer 100 described above. However, it should be noted that a penetrometer of different design, but capable of performing the characteristic functions of the penetrometer 100 according to the invention could very well be used to carry out the steps of the method below.

The process for the evaluation of the liquefiable character of a soil comprises different steps ( figure 3 ).

Step a) consists in driving the couple formed by the hollow tube 2 and the central rod 1 into the ground to bring the measuring tip 11 to a given depth of investigation (for example P _A with reference to the figure 1 ). During this step, the measuring tip 11 is preferably in abutment against the hollow tube 2 and the movable body 62 of the electric cylinder is in the retracted position ( figure 3 - a )). Advantageously, in this position, the movable body 62 is not in contact with the second end 12 of the central rod 1. The support means, via the cell, make it possible to achieve this depression to the given depth. .

After the measuring tip 11 has reached the given depth P _A , step b) is carried out; it consists in actuating the movable body 62 to place it in contact with the second end 12 of the central rod 1. This step b) is optional and would in particular not be useful in the event that the movable body 62, in the retracted position, would be already in contact with the second end 12.

Then, step c) consists in actuating the movable body 62 so that it performs a first displacement D1, inducing a first controlled depression of the measuring tip 11 into the ground. The first displacement D1 is preferably carried out at constant speed. According to an advantageous embodiment, the first displacement D1 (corresponding to the amplitude of the first depression) is 10mm, carried out for example at a speed of 2cm / s.

mesurée à la profondeur donnée P_A est sensiblement identique à celle mesurée lors du pré-diagnostic à cette même profondeur. Préférentiellement, après le premier enfoncement, la mesure de la force de réaction transmise par la deuxième extrémité 12 au corps mobile 62, à l'arrêt (représentative de la résistance de pointe à l'arrêt $Q_c^{arr\mathrm{ê}t}$

) est également prévue à l'étape c).Step c) also provides for the measurement of the force applied to effect said first displacement D1. This measurement of the applied force, representative of the resistance of the ground, makes it possible to verify that the peak resistance $Q_{\mathrm{vs}}^{2\mathit{cm}/s}$

measured at the given depth P _A is substantially identical to that measured during the pre-diagnosis at this same depth. Preferably, after the first depression, the measurement of the reaction force transmitted by the second end 12 to the movable body 62, at standstill (representative of the peak resistance at standstill $Q_{\mathrm{vs}}^{\mathrm{To}rr\mathrm{ê}t}$

) is also provided in step c).

et $Q_c^{arr\mathrm{ê}t}$

extraites des mesures à l'étape c) sont attendues sensiblement identiques à celles mesurées lors du pré-diagnostic : elles constituent le point de départ de l'essai d'évaluation du caractère liquéfiable de la couche de sol investiguée (zone A).Values $Q_{\mathrm{vs}}^{2\mathit{cm}/s}$

and $Q_{\mathrm{vs}}^{\mathrm{To}rr\mathrm{ê}t}$

extracted from the measurements in step c) are expected to be substantially identical to those measured during the pre-diagnosis: they constitute the starting point of the test for evaluating the liquefiable character of the soil layer investigated (zone A).

The next step d) consists in applying a vibration at a determined frequency to the second end 12 of the central rod 1 by means of the movable body 62 and simultaneously actuating the movable body 62 so that it performs a second displacement D2 , inducing a second controlled insertion of the measuring tip 11 into the ground ( figure 3 - d )).

As indicated previously, the determined frequency is between 1 and 5 Hertz, characteristic frequencies of earthquakes. The objective here is to apply locally constraints likely to modify the properties of the soil layer, as could an earthquake. In parallel with this stress, the movable body 62 performs a second displacement D2 which causes a second depression of the measuring tip 11 in the layer of soil placed under vibratory stress.

According to an advantageous embodiment, the second displacement D2 (corresponding to the amplitude of the second depression) is 30mm.

Step d) also provides for the measurement of the force applied to effect the second displacement D2: the measured values of the force applied reflect the change in the peak resistance Q _c during this second controlled depression.

mesurée à l'étape c). La vitesse du deuxième déplacement D₂ est donc automatiquement augmentée ou diminuée en fonction de la force mesurée au cours de l'étape d), dans l'objectif de maintenir celle-ci sensiblement constante.According to an advantageous embodiment, the speed of the second displacement D ₂ is adjusted so as to keep the applied force substantially constant. The second displacement D2 is therefore advantageously carried out with a constant load (applied force). In particular, an applied force is aimed at substantially equal to the value of the applied force representative of the peak resistance. $Q_{\mathrm{vs}}^{2\mathit{cm}/s}$

measured in step c). The speed of the second displacement D ₂ is therefore automatically increased or decreased as a function of the force measured during step d), with the aim of keeping the latter substantially constant.

The speed at which the second displacement D ₂ is carried out, reflecting the time necessary to achieve the second depression, at constant force, gives an important indication of the sudden nature of the loss of resistance of the layer of soil under vibratory stress.

Several scenarios may arise, depending on the properties of the soil layer analyzed. The resistance properties of the soil layer can change very slowly under the vibratory stress: the second displacement D2 is then carried out at a low speed, the latter being adjusted so as to keep the force applied to effect said second displacement substantially constant. D2. In such a case, it appears that the soil layer does not change suddenly with a vibratory stress.

According to another scenario, the layer of soil can very quickly lose its resistance under the vibratory stress: the second displacement D2 is then carried out at a high speed, the latter being adjusted so as to keep the force constant as much as possible. applied to perform said second displacement D2. As indicated above, the motor of the electric jack 6 is chosen so that the speed of movement of the movable body 62 can reach approximately 16 cm / s, in order to be able to follow a sudden loss of lift of the layer of soil studied.

The risks in the event of an earthquake in both cases mentioned are clearly different: the precautions and specifications on the foundations of constructed structures can be adapted to each case.

When the second controlled depression is completed (second displacement D2 performed), the next step e) consists in stopping the vibration.

Step f) then provides for actuating the movable body 62 so that it performs a third displacement D3, inducing a third controlled depression of the measuring tip 11 in the ground ( figure 3 - f )). Step f) also provides for measuring the force applied to effect this third displacement D3. The third displacement D3 is preferably carried out at constant speed.

According to an advantageous embodiment, the third displacement D3 (corresponding to the amplitude of the third depression) is 10mm, carried out for example at a speed of 2cm / s.

During step f), in the absence of vibratory stress, the soil layer can see its resistance evolve in different ways, depending on the characteristics of said layer.

: c'est un comportement de sol liquéfiable (par exemple courbe (a) sur la figure 4).According to a first behavior, a layer having quickly lost its resistance under vibratory stress may, in the absence of the latter, regain its initial resistance. $Q_{\mathrm{vs}}^{2\mathit{cm}/s}$

: it is a behavior of liquefiable soil (for example curve (a) on the figure 4 ).

: cela peut traduire un phénomène de type grande déformation (par exemple, courbe (b) sur la figure 4), les propriétés de résistance de la couche de sol ayant été irréversiblement modifiées par la vibration.According to a second behavior, a layer having quickly lost its resistance under vibratory stress may, in the absence of the latter, return during the third controlled depression to a resistance Q _c lower than its initial resistance. $Q_{\mathrm{vs}}^{2\mathit{cm}/s}$

: this can translate a phenomenon of the large deformation type (for example, curve (b) on the figure 4 ), the resistance properties of the soil layer having been irreversibly modified by the vibration.

: cela peut traduire un phénomène de densification (par exemple, courbe (c) sur la figure 4), la résistance de la couche de sol ayant été renforcée par la vibration.Finally, according to a third behavior, a layer having rapidly lost its resistance under vibratory stress may, in the absence of the latter, return during the third controlled depression to a resistance Q _c greater than its initial resistance. $Q_{\mathrm{vs}}^{2\mathit{cm}/s}$

: this may reflect a phenomenon of densification (for example, curve (c) on the figure 4 ), the resistance of the soil layer having been reinforced by vibration.

Advantageously, after the third depression, the measurement of the reaction force transmitted by the second end 12 to the movable body 62, when stationary, after the third depression, is also provided in step f).

Steps a) to f) may be repeated for other given investigation depths, so as to analyze successive soil layers included in a suspect zone A of greater or lesser thickness.

The method and the penetrometer 100 according to the invention thus make it possible to evaluate the liquefiable character of the soil layer analyzed, from the speed of realization of the second displacement D ₂ . The more or less sudden reduction in the resistance of the soil under vibratory stress is a key criterion of the liquefiable character.

Furthermore, the invention also provides important information on the bearing properties of the soil layer analyzed, following a vibratory stress, making it possible to anticipate potential irreversible changes in the resistance of the soil.

From the foregoing, knowledge of the effective behavior of the soil after vibration makes it possible to guide builders in their design of structures, and companies in their construction methodologies. For example, soil densification will make it possible to consider alternative methods of soil improvement prior to the construction of structures.

Conversely, a loss of lift could lead to the creation of secure foundations with objective knowledge of the risks.

Claims (13)

1. Penetrometer (100) for evaluating the liquefiable character of a soil, comprising:

   • at least one central rod (1), a first end of which finishes in a measuring tip (11);

   • at least one hollow tube (2) surrounding the central rod (1), the latter being able to slide inside the hollow tube (2);

   the penetrometer (100) being characterized in that it comprises an electric cylinder (6) comprising an external body (61) that is rigidly connected to the hollow tube (2) and a movable body (62), said movable body (62) being designed:

   • to transmit a movement to a second end (12) of the central rod (1), leading to a controlled depression of the measuring tip (11) into the soil, and to measure a force applied to perform said movement,

   • and to apply a vibration to the second end (12) of the central rod (1) at a determined frequency.
2. Penetrometer (100) according to the preceding claim, wherein the determined frequency is between 1 and 5 Hertz.
3. Penetrometer (100) according to either of the preceding claims, wherein the electric cylinder (6) is controlled electronically such that the actuation and speed of the movable body (62) to perform a movement, the force applied, and the application or stopping of the vibration can be performed in programmed sequences or slaved to one another.
4. Penetrometer (100) according to any of the preceding claims, comprising a cell that is rigidly connected to the hollow tube (2) for transmitting a support force, applied by support means, so as to cause a depression in the soil of the hollow tube (2) and the central rod (1) to a given depth.
5. Method for evaluating the liquefiable character of a soil using the penetrometer (100) according to any of the preceding claims, and comprising the following steps:

   a) pushing the pair formed by the hollow tube (2) and the central rod (1) into the soil to bring the measuring tip (11) to a given depth, said measuring tip (11) abutting the hollow tube (2);

   c) actuating the movable body (62) such that it performs a first movement, inducing a first controlled depression of the measuring tip (11) into the soil,

   the method being characterized in that step c) provides for measuring the force applied to perform said first movement, and in that it further comprises the following steps:

   d) applying a vibration at a determined frequency to the second end (12) of the central rod (1) by means of the movable body (62) and simultaneously actuating the movable body (62) such that it performs a second movement, inducing a second controlled depression of the measuring tip (11) into the soil; and measuring the force applied to perform said second movement;

   e) stopping the vibration;

   f) actuating the movable body (62) such that it performs a third movement, inducing a third controlled depression of the measuring tip into the soil and measuring the force applied to perform said third movement.
6. Method for evaluating the liquefiable character of a soil according to the preceding claim, comprising, before step c), the following step b):
   b) actuating the movable body (62) to bring it in contact with the second end (12) of the central rod (1).
7. Method for evaluating the liquefiable character of a soil according to either of the two preceding claims, wherein step c) also comprises measuring the reaction force transmitted by the second end (12) to the movable body (62), when stationary, after the first depression.
8. Method for evaluating the liquefiable character of a soil according to any of the three preceding claims, wherein step f) also comprises measuring the reaction force transmitted by the second end (12) to the movable body (62), when stationary, after the third depression.
9. Method for evaluating the liquefiable character of a soil according to any of the four preceding claims, wherein, in step d), the speed of the second movement is adjusted so as to keep the measured applied force substantially constant.
10. Method for evaluating the liquefiable character of a soil according to any of the five preceding claims, wherein, in step d), the speed of the second movement is adjusted so that the measured applied force is kept substantially equal or as close as possible to the applied force measured in step c).
11. Method for evaluating the liquefiable character of a soil according to any of the six preceding claims, wherein the movement speed of the movable body (62) can reach 16 cm/s.
12. Method for evaluating the liquefiable character of a soil according to any of the seven preceding claims, wherein the maximum movement between a retracted position and a deployed position of the movable body (62) is 75 mm.
13. Method for evaluating the liquefiable character of a soil according to any of the eight preceding claims, wherein the first movement of the movable body (62) is 10 mm, the second movement is 30 mm, and the third movement is 10 mm.

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