FR3070170B1 - STATIC PENETROMETER FOR THE EVALUATION OF THE LIQUEFIABLE CHARACTER OF A SOIL AND ASSOCIATED METHOD - Google Patents

Abstract

The invention relates to a penetrometer (100) for assessing the liquefiable nature of a soil, comprising: • at least one central rod (1) terminated at a first end by a measuring tip (11); • At least one hollow tube (2) surrounding the central rod (1), the latter being slidable inside the hollow tube (2); The penetrometer (100) also comprises an electric jack (6) comprising an outer body (61) integral with the hollow tube (2) and a movable body (1), said movable body (1) being: • configured to transmit a displacement to a second end (12) of the central rod (1) leading to a controlled depression of the measuring tip (11) in the ground, and for measuring a force applied to effect said displacement, and able to apply to the second end (12) of the central rod (1) a vibration at a determined frequency.

Description

STATIC PENETROMETER FOR THE EVALUATION OF THE LIQUEFIABLE CHARACTER OF A SOIL AND ASSOCIATED METHOD

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 penetration of a soil, commonly called a penetrometer, and an associated measuring method. In particular, it relates to a static penetrometer for performing tests for evaluating the liquefiable nature of a soil.

BACKGROUND OF THE INVENTION

The new seismic standards lead to systematization in geotechnical studies in seismic zones, soil characterization vis-à-vis the risk of liquefaction. The liquefaction of a soil during a seismic movement designates the decrease of the rigidity of said soil and / or the decrease of its shear strength, due to the increase of the pore water pressure. These changes in soil characteristics are likely to produce significant permanent deformations (settlements, slips), or even a virtual cancellation of effective stresses (phenomenon of large deformation, spreading). The identification of this risk of liquefaction thus makes it possible to anticipate the solutions of realization of the foundations to ensure the stability of the works during the earthquakes.

Recall that the compactness of soils is usually measured, either in static mode or in dynamic mode, by a measuring device called a penetrometer. A penetrometer conventionally comprises rods connected end to end to form a drill string at the end of which is fixed a measuring tip, intended to sink into the ground at depths of up to several tens of meters.

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

Currently, the potentially liquefiable nature of a soil is assessed by experimental correlations between in situ measurements and critical cyclic shear stresses (which are known to have caused liquefaction in past earthquakes). In situ measurements are usually made from standard penetration tests (SPT) or cone penetration tests (CPT), including piezocone soundings (CPTU). The raw measurements from these tests are then used to determine standardized variables for the evaluation of standardized soil resistance; this standardized resistance is then compared to the normalized solicitation of the site. Soil samples can also be taken for the determination of particle 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, especially 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 propose an alternative solution to the solutions of the state of the art, in particular a static penetrometer, simple to implement and allowing a more direct evaluation of the liquefiable nature of a soil.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a penetrometer for assessing the liquefiable nature 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 slidable within the hollow tube;

The penetrometer further comprises an electric ram having an outer body integral with the hollow tube and a movable body, said movable body being: • configured to transmit a displacement at a second end of the central rod leading to a controlled depression of the measuring tip in the ground, and for measuring a force applied to effect said displacement, and capable of applying a vibration at a second frequency to the second end of the central rod.

According to other advantageous and nonlimiting features of the invention, taken alone or in any technically feasible combination: • the determined frequency is between 1 and 5 Hertz; The electric cylinder is electronically controlled so that the actuation and the speed of the moving body to effect a displacement, the applied force, and the application or the stopping of the vibration can be carried out according to programmed sequences or slaved to one another; to 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 a depression in the ground of the hollow tube and of the central rod to a depth given. The invention also relates to a method for evaluating the liquefiable nature of a soil, which can use the penetrometer as above. Said method comprises the following steps: a) driving the torque formed by the hollow tube and the central rod into the ground in order to bring the measurement tip to a given depth, said measuring tip being in abutment against the hollow tube; c) operating the movable body to make a first displacement, inducing a first controlled depression of the measuring tip into the ground, and measuring the force applied to effect said first displacement; d) applying a vibration at a determined frequency, at the second end of the central rod via the movable body and simultaneously actuating the movable body to make a second displacement, inducing a second controlled depression of the measuring tip in the ground ; and measuring the force applied to effect said second displacement; e) stop the vibration; f) operating the movable body to make a third displacement, inducing a third controlled depression of the measuring tip into the ground and measuring the force applied to effect said third displacement.

According to other advantageous and nonlimiting features 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 moving body, when stopped, after the first depression; Step f) also comprises measuring the reaction force transmitted by the second end to the moving body, when stopped, after the third depression; In step d), the speed of the second displacement is adjusted so as to maintain the measured applied force substantially constant; In step d), the speed of the second displacement is adjusted so that the measured applied force is maintained substantially equal to or closer to the applied force measured in step c); • the moving speed of the moving body can reach 16cm / s; • the maximum displacement between an erased position and an extended position of the movable 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 Other features and advantages of the invention will emerge from the following detailed description with reference to the appended figures in which: FIG. 1 presents a test curve with double measurement of the penetration resistance of a soil; Figure 2 shows a penetrometer according to the invention; Figure 3 shows a method according to the invention; Figure 4 shows the evolution of the force applied to effect controlled depressions, as a function of time.

DETAILED DESCRIPTION OF THE INVENTION

The applicant has developed a pre-identification method for liquefiable soils (see H. Hosseini-Sadrabadi et al, "Identification of Liquefiable Soils by Static Penetrometer: Principle and Numerical Modeling", National Engineering and Geological Engineering Days, Nancy 2016) using a static penetrometer. This method is based on static penetration tests with double measurement: a measure of the peak resistance at a constant driving speed of 2cm / s (denoted Q? Crn / S - static mode) and a measurement of the resistance of point at a standstill (denoted Q "stop").

These double measurements, in static and stationary mode, at a given depth, can make it possible to identify potentially liquefiable soils in that the difference (noted AQC) between the ground resistance in static mode and the resistance of the ground at shutdown seems characteristic of the evolution of interstitial pressures in the soil:

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

As illustrated in Figure 1, it is therefore possible in a test with double measurement (in static mode and at rest) of resistance to penetration of a soil, to detect soil layers (zone A) , at a given depth (or depths), for which the AQC difference is greater than a critical value, in particular 40%. This type of test can constitute an interesting pre-diagnosis of the liquefiable nature of a soil.

The present invention relates to a penetrometer 100 for evaluation by direct measurement of the liquefiable nature 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 the zone A (to resume the example illustrated in FIG. 1), with a view to evaluate more directly the liquefiable nature of the soil layers concerned and assess the associated risks.

The penetrometer 100 (FIG. 2) comprises at least one central rod 1 terminated at a first end by a measurement 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 adapted so that the latter can slide freely inside the hollow tube 2.

The pair 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 pair 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 outer body 61 integral with the hollow tube 2 and a movable body 62 adapted to come into contact with the central rod 1.

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

The movable body 62 is configured to transmit a displacement at a second end 12 of the central rod 1, displacement which will lead to a controlled depression of the measuring tip 11 in the ground. As will be described in more detail in the method according to the invention, controlled depressions will be made successively from the given depth PA, in the suspect zone A (FIG. 1). For this, the power of the engine of the electric jack is preferably chosen so that the movable body 62 is capable of applying a pressure of between 10 and 40 bar: this pressure range can address the characteristic stresses (10-30 bar ) measured in points in soil layers at risk of liquefaction. The motor of the electric jack 6 is also chosen so that the moving speed of the movable body 62 can vary between a few mm / s and about 16 cm / s, so that the detection of the actual loss of lift of the ground (case of a liquefiable soil) is not affected by the technical limitations 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 measuring tip 11. The electric ram 6 thus 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 process according to the invention, that the vibration in this frequency range makes it possible to modify the properties of the soil 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 displacement, the force applied during the depression, the measurement of the reaction force of the rod 1 and the vibration of the moving body 62 can thus be performed according to programmed sequences or be slaved to each other.

The penetrometer 100 according to the invention advantageously comprises a cell in contact (or solid) 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 bearing force, applied by support means, so as to cause a static depression in the ground of the pair formed by the hollow tube 2 and the central rod 1 to the ( or the given depth (s) to be investigated.

The support means may in particular consist of a hydraulic cylinder. The moving part of the hydraulic cylinder attached to the cell applies the support force necessary for the continuous depression of the tube / rod torque. The fixed part of the hydraulic cylinder must be attached directly or indirectly to a reaction mass. The support means may comprise a self-propelled hydraulic unit for actuating 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 mass. This mechanical connecting element may for example consist of a hydraulic or mechanical clamp, or a vise of the same type. The fact that the chassis is equipped with such a mechanical connecting element makes it connectable to any kind of massive reaction.

The present invention also relates to a method for evaluating the liquefiable nature 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 quite well be used to carry out the steps of the method below.

The method for assessing the liquefiable nature of a soil comprises different steps (Figure 3). Step a) consists in driving the torque formed by the hollow tube 2 and the central rod 1 into the ground in order to bring the measuring tip 11 to a given depth of investigation (for example PA with reference to FIG. 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 jack is in the erased 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, through the cell, allow to achieve this depression at the given depth .

After the measuring tip 11 has reached the given depth PA / step b) is operated; 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 not be particularly useful in the case where the movable body 62, in the erased position, would be already in contact with the second end 12.

Then, step c) consists in actuating the movable body 62 to make a first displacement D1, inducing a first controlled depression of the measuring tip 11 in the ground. The first displacement D1 is preferably carried out at a constant speed. According to an advantageous embodiment, the first displacement D1 (corresponding to the amplitude of the first depression) is 10 mm, made for example at a speed of 2 cm / s. Step c) also provides the measurement of the force applied to perform said first DI displacement. This measurement of the applied force, representative of the resistance of the ground, makes it possible to verify that the peak resistance Q? Crn / S measured at the given depth PA 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 the stop (representative of the peak resistance at the stop Ç "rret) is also provided for in FIG. step c).

The values obtained in steps c) are expected to be substantially identical to those measured during the pre-diagnosis: they constitute the starting point of the evaluation test of the liquefiable nature of the investigated soil layer (zone A). The following step d) consists of applying a vibration at a determined frequency to the second end 12 of the central rod 1 via the movable body 62 and simultaneously to actuate the movable body 62 so that it performs a second displacement D2 , inducing a second controlled depression of the measuring tip 11 in the ground (Figure 3 - d)).

As indicated above, the determined frequency is between 1 and 5 Hertz, frequencies characteristic of earthquakes. The objective here is to apply locally constraints that can modify the properties of the soil layer, as could an earthquake. In parallel with this bias, the movable body 62 makes a second displacement D2 which causes a second depression of the measuring tip 11 in the ground layer 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 the measurement of the force applied to perform the second displacement D2: the measured values of the force applied reflect the evolution of the peak resistance Qc during this second controlled depression.

According to an advantageous embodiment, the speed of the second displacement D2 is adjusted so as to maintain the applied force substantially constant. The second displacement D2 is therefore advantageously carried out at constant load (applied force). In particular, it is intended a force applied substantially equal to the applied force value representative of the peak resistance q? 0171/5 measured in step c). The speed of the second displacement D2 is thus automatically increased or decreased according to the force measured during step d), with the objective of keeping it substantially constant.

The speed at which the second displacement D2 is performed, reflecting the time required to achieve the second depression, at constant force, gives an important indication of the brutal nature of the loss of resistance of the soil layer under vibration.

Several situations can occur, depending on the properties of the soil layer analyzed. The resistance properties of the soil layer can evolve 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 maintain substantially constant the force applied to perform said second displacement D2. In such a case, it appears that the soil layer does not change abruptly with a vibratory stress.

In another case, the soil layer 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 maintain as much as possible the force applied to effect said second displacement D2. As indicated above, the motor of the electric jack 6 is chosen so that the moving speed of the movable body 62 can reach about 16 cm / s, to be able to follow a sudden loss of lift of the soil layer studied.

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

When the second controlled depression is completed (second displacement D2 performed), the following step e) is to stop the vibration. Step f) then provides to actuate the movable body 62 to make 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 a constant speed.

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

According to a first behavior, a layer having rapidly lost its resistance under vibratory stress can, in the absence of the latter, recover its initial resistance ç cm / s: it is a behavior of liquefiable soil (for example curve (a) in Figure 4).

According to a second behavior, a layer having rapidly lost its resistance under vibratory stress may, in the absence of the latter, return during the third controlled penetration to a resistance Qc lower than its initial resistance ç cm / s: this may reflect a large deformation type phenomenon (for example, curve (b) in Figure 4), the resistance properties of the soil layer having been irreversibly modified by the 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 Qc greater than its initial resistance Q? Cm / S: this can translate a densification phenomenon (for example, curve (c) in FIG. 4), the resistance of the soil layer having been reinforced by the vibration.

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

Steps a) to f) may be repeated for other investigative depths, so as to analyze successive layers of soil included in a suspected area A of greater or lesser thickness.

The method and the penetrometer 100 according to the invention thus make it possible to evaluate the liquefiable nature of the soil layer analyzed, based on the speed of realization of the second displacement D2. The more or less abrupt decrease in soil resistance under vibratory stress is a key criterion of liquefiable nature.

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

From the foregoing, the knowledge of the effective behavior of the ground after vibration makes it possible to guide the builders in their design of works, and the companies in their methodologies of realization. For example, a densification of the soil will make it possible to envisage alternative soil improvement methods prior to the construction of structures. Conversely, a loss of lift may lead to the creation of safe foundations with objective knowledge of the risks.

Of course, the invention is not limited to the embodiments described and variants can be made without departing from the scope of the invention as defined by the claims.

Claims (13)

A penetrometer (100) for assessing the liquefiable nature of a soil, comprising: at least one central rod (1) terminated at a first end by a measuring tip (11); • At least one hollow tube (2) surrounding the central rod (1), the latter being slidable inside the hollow tube (2); The penetrometer (100) being characterized in that it comprises an electric jack (6) comprising an outer body (61) integral with the hollow tube (2) and a movable body (62), said movable body (62) being: configured to transmit a displacement at a second end (12) of the central rod (1) leading to a controlled depression of the measuring tip (11) in the ground, and for measuring a force applied to effect said displacement, • and fit to apply a vibration at 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 one of the preceding claims, wherein the electric cylinder (6) is electronically controlled so that the actuation and the speed of the movable body (62) to perform a displacement, the applied force, and the application or the stop of the vibration can be carried out according to programmed sequences or enslaved to each other. 4. Penetrometer (100) according to one of the preceding claims, comprising a cell secured to the hollow tube (2) for transmitting a bearing force, applied by bearing means, so as to cause a depression in the floor of the hollow tube (2) and the central rod (1) to a given depth. 5. A method for the evaluation of the liquefiable nature of a soil, which can use the penetrometer (100) according to one of the preceding claims, and comprising the following steps: a) driving in the ground the pair formed by the hollow tube ( 2) and the central rod (1) to bring the measuring tip (11) to a given depth, said measuring tip (11) abutting against the hollow tube (2); c) operating the movable body (62) to make a first displacement, inducing a first controlled depression of the measuring tip (11) into the ground, and measuring the force applied to effect said first displacement; d) applying a vibration at a determined frequency, at the second end (12) of the central rod (1) through the movable body (62) and simultaneously actuating the movable body (62) to perform a second displacement, inducing a second controlled depression of the measuring tip (11) in the ground; and measuring the force applied to effect said second displacement; e) stop the vibration; f) operating the movable body (62) to make a third displacement, inducing a third controlled depression of the measuring tip into the ground and measuring the force applied to effect said third displacement. 6. A method for evaluating the liquefiable nature of a soil according to the preceding claim comprising before step c), the following step b): b) actuating the movable body (62) to place it in contact with the second end (12) of the central rod (1). 7. A method for evaluating the liquefiable nature of a soil according to one of the two preceding claims, wherein step c) also comprises measuring the reaction force transmitted by the second end (12) to the moving body. (62), stopped, after the first depression. 8. A method for evaluating the liquefiable nature of a soil according to one of the three preceding claims, wherein step f) also comprises measuring the reaction force transmitted by the second end (12) to the moving body. (62), stopped, after the third sink. 9. A method for evaluating the liquefiable nature of a soil according to one of the preceding claims, wherein in step d), the speed of the second displacement is adjusted so as to maintain the applied force measured substantially constant. . A method for evaluating the liquefiable nature of a soil according to one of the five preceding claims, wherein in step d) the speed of the second displacement is adjusted so that the applied force measured is maintained substantially equal to or closer to the applied force measured in step c). 11. A method for evaluating the liquefiable nature of a soil according to one of the six preceding claims, wherein the moving speed of the movable body (62) can reach 16cm / s. 12. A method for evaluating the liquefiable nature of a floor according to one of the preceding claims, wherein the maximum displacement between an erased position and an extended position of the movable body (62) is 75mm. 13. A method for the evaluation of the liquefiable nature of a soil according to one of the eight preceding claims, wherein the first displacement of the movable body (62) is 10mm, the second displacement is 30mm and the third displacement is 10mm.