EP3874251A1

EP3874251A1 - Method for measuring the elasto-plastic properties of soil using a static penetrometer

Info

Publication number: EP3874251A1
Application number: EP19813097.3A
Authority: EP
Inventors: Pierre RIEGEL
Original assignee: EquatechR&d
Current assignee: EquatechR&d
Priority date: 2018-10-31
Filing date: 2019-10-23
Publication date: 2021-09-08
Also published as: FR3087800A1; WO2020089547A1; FR3087800B1

Abstract

The invention concerns a method for measuring the elasto-plastic properties of soil, using a static penetrometer provided with a hollow tube surrounding a central rod capable of sliding inside the hollow tube and terminated at a first end by a measuring tip, the method comprising the following steps: a) Driving the pair formed by the hollow tube and the central rod into the soil, in order to bring the measuring tip to a desired depth for measuring the elasto-plastic properties of the soil, the measuring tip being in abutment against the hollow tube; b) Applying a first constant force to a second end of the rod, for a determined period; c) Measuring a first final movement, associated with the measuring tip being driven into the soil, at the end of the determined period; d) Repeating steps b) and c) with successive increasing forces, in order to form a curve representing the final movement as a function of the force, until an nth force is reached leading to an nth final movement, the nth final movement being the maximum and reflecting the tensile strength of the soil; e) Extracting a modulus of deformation of the soil from the slope of the curve, in a first elastic domain located before an inflection point.

Description

METHOD FOR MEASURING ELASTO-PLASTIC PROPERTIES OF A SOIL THANKS TO A STATIC PENETROMETER

FIELD OF THE INVENTION

The present invention relates to the field of geotechnics and geology. It relates to a process for measuring the elastoplastic properties of a soil using a static penetrometer. It also relates to a static penetrometer for the implementation of said method.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

The construction of complex structures requires knowledge of the bearing capacity of the soil vis-à-vis future foundations, but also knowledge of their deformation under load, that is to say the intensity of induced settlements.

To date, the determination of said settlements is done either from laboratory tests (oedometric tests, triaxial tests ...), or from pressuremeter tests in situ.

In the first approach, it is necessary to be able to ensure a continuous qualitative chain comprising at least: core sampling, packaging, transport, sample preparation and testing. To this, it should be noted that each sample measures only a few cm3 and that the representativeness of the analysis therefore requires a large number of samples and measurements. The major drawbacks of this first approach are the cost, the reliability of the test and the time taken to obtain a result. In the second approach using a pressuremeter, the in situ expansion tests consist in dilating in the soil, in a previously made borehole, a probe with deformable walls and in determining the relationship between the pressure applied to the soil and the displacement of the wall of the probe. Currently, this is the only in situ test providing both a failure parameter and a complete stress / strain relationship giving access to the elasto-plastic properties of the soil. Indeed, the in situ tests usually implemented such as static penetration tests (CPT for "Cone Penetration Test", from a static penetrometer) or beaten core barrel tests (SPT for "Standard Penetration Test", from a dynamic penetrometer) make it possible to measure the tensile strength of the ground and gives only an estimate of the deformations by experimental correlations (cf. P. Riegel and H. Hosseini Sadrabadi, “The use of the static penetrometer in the 'approach to settlement under construction', National Geotechnical and Engineering Geology Days JNGG 2014).

The pressuremeter approach nevertheless suffers from several shortcomings, in particular the impact of the quality of the drilling on the representativeness of the test, because the soil is precisely measured at the level of the walls of the drilling. Another drawback lies in the complexity of the material used and the frequent bursting of the probe as soon as the test pressure and the volume of the probe increase.

The challenge is therefore to propose a method for measuring the elastoplastic properties of a soil, allowing relatively direct measurements, avoiding empirical correlations, and relatively simple, in particular avoiding the complexity of implementing laboratory or pressuremeter tests. . OBJECT OF THE INVENTION

An object of the present invention is to propose a solution overcoming the drawbacks of the state of the art, in particular a method for in situ measurement of the elastoplastic properties of a soil using a static penetrometer.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a method for measuring the elastoplastic properties of a soil, using a static penetrometer provided with a hollow tube surrounding a central rod capable of sliding inside the hollow tube and terminated at a first end by a measuring tip.

The process includes the following steps:

a) Drive the torque formed by the hollow tube and the central rod into the ground to bring the measuring tip to a desired depth for measuring the elastoplastic properties of the soil, said measuring tip being in abutment against the hollow tube;

b) Apply a first constant force to a second end of the rod, for a determined period, said second end being above the ground;

c) Measure a first final displacement, associated with the insertion of the measuring point into the ground, at the end of the determined period;

d) Repeat steps b) and c) with successive increasing forces, to form a curve representing the final displacement as a function of the force, until reaching an umpteenth force leading to an umpteenth final displacement, said umpteenth displacement being maximum and reflecting the stress at break in the ground; e) Extract a soil deformation module from the slope of said curve, in a first elastic range located before an inflection point.

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

• the force corresponding to the point of inflection of the curve brought back to the section of the measuring point corresponds to the creep stress, elastic limit of the ground;

• an admissible stress value is defined at half the creep stress;

• the determined duration is 60 seconds;

• the method comprises a step c ') for measuring a first intermediate displacement, associated with the insertion of the measuring tip into the ground, at the end of an intermediate duration less than the determined duration;

• step d) includes the reiteration of step c ') in parallel with the reiteration of steps b) and c);

• for the force corresponding to the inflection point of the curve, the difference between the intermediate displacement measured after 15 seconds and the final displacement measured after the determined time is representative of the liquefaction potential of the soil;

• the torque formed by the hollow tube and the central rod of the static penetrometer is introduced into a hole previously drilled, in step a), to bring the measuring tip to the desired depth for the measurement of the elastoplastic properties of the soil . The present invention also relates to a static penetrometer for implementing the method described above, comprising:

• At least one central rod terminated at a first end by a measuring tip, said measuring tip being intended to sink into the ground;

• At least one hollow tube surrounding the central rod, the latter being able to slide inside the hollow tube;

• A jack comprising an external body integral with the hollow tube and a movable body, said movable body being configured to apply a force to a second end of the rod, leading to a depression of the measuring tip in the ground, and to measure a displacement associated with this depression, said second end being intended to remain above the ground,

• An electronic controller to slave the cylinder, so as to apply a given force, for a fixed period, and to record the movement of the moving body as a function of time during the fixed period.

Advantageously, the jack is an electric jack.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 shows a method according to the invention; FIG. 2 shows a curve plotting the final displacement of the measuring tip as a function of the applied force, established according to a process in accordance with the invention, for extracting elastoplastic properties from the soil;

FIG. 3 shows a curve plotting the final displacement of the measuring tip as a function of the applied force and a curve plotting an intermediate displacement as a function of the force applied, the curves being established according to a method according to the invention, for extracting elastoplastic properties of the soil;

Figure 4 shows a static penetrometer for the implementation of a method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a method for measuring the elastoplastic properties of a soil, using a static penetrometer 100.

A penetrometer conventionally comprises rods connected end to end to form a string of rods at the end of which is fixed a measuring point, intended to sink into the ground to depths of up to several tens of meters. In static mode, the drill string is pushed by cylinders, causing the progressive insertion of the measuring tip; this latter measure in particular the peak resistance, representative of the breaking stress Qc of the soil. The measurements are recorded continuously or discontinuously at a regular step.

The penetrometer 100 used in the context of the invention is provided with a hollow tube 2 surrounding a central rod 1; said central rod 1 is capable of sliding inside the hollow tube 2 and is terminated at a first end by a point of sour 11 intended to be driven into the ground. By way of example, a well-known “Gouda” type tip may be used, having a surface at its plane section of 10 cm ² .

The measurement method according to the invention, illustrated in FIG. 1, firstly comprises a step a) consisting in driving the torque formed by the hollow tube 2 and the central rod 1 into the ground to bring the measuring tip 11 at a desired depth P for measuring the elastoplastic properties of the soil. Arriving at said depth P, the measuring tip 11 is erased, that is to say placed in abutment against the hollow tube 2.

The measurement method then comprises a step b) during which a first constant force Fi is applied to a second end of the rod 1, for a determined period t. Recall that the second end of the rod 1 is out of the ground. The constant force Fi applied is transmitted by the rod 1 to the measuring tip 11, which will more or less sink into the ground, depending on the mechanical characteristics thereof.

The first applied force Fi may be between 100N and 500N; considering a 10 cm2 section of the measuring tip 11, this corresponds to an applied stress between 1 and 5 bars.

In a subsequent step c) of the method, a first final displacement D [is measured, associated with the insertion of the measuring tip 11 into the ground, at the end of the determined duration t. In the present description, the term “final” is used to describe a displacement corresponding to the displacement recorded at the end of the determined duration t. Step d) of the method according to the invention consists in repeating steps b) and c), with increasing applied forces, to form a curve representing the final displacement as a function of the force applied.

Thus, as illustrated in FIG. 1, a second constant force F ₂ , greater than Fi, is applied to the second end of the rod 1, for the determined duration t. A measurement is made of the second final displacement D |, associated with the insertion of the measuring tip 11 into the ground, at the end of the determined duration t. Note that this second final displacement D | corresponds to the cumulation of the first final displacement D [and of the additional displacement caused by the application of the second force F ₂ .

A third constant force F ₃ , greater than F ₂ , is then applied, still for the determined duration t, then a measurement of the third final displacement Df is carried out at the end of said duration t, and so on.

Advantageously, the increment between two successive applied forces is between 100 and 500N.

The determined duration t may vary between 15 and 600 seconds; it is preferentially defined at t = 60s because this duration joins the methodological approach of the pressuremeter test.

The sequence of applying a force and measuring the final displacement is repeated n times (application of yet another force F _n constant, greater than F _ni , and measurement of yet another final displacement D) until reaching the stress at break in soil Qc. The stress at rupture of the ground will correspond to a force F _n generating a maximum final displacement D due to a significant depression of the measuring tip 11. Recall that the force F _n is related to the constraint by the relation: Qc = F _n / S, S being the surface of the planar section of the measuring tip 11 (for example 10cm ² ).

In practice, the maximum displacement representative of the rupture of the ground is defined at approximately 5 cm (output amplitude of the tip 11 relative to its position erased against the hollow tube 2).

At the end of the preceding steps, a curve C _t can be drawn representing the final displacement as a function of the force applied (FIG. 2). The curve C _t is formed of a first domain E corresponding to elastic deformations of the soil. It is formed, after an inflection point I, of a second domain PI corresponding to plastic deformations of the soil, until reaching the breaking point (Qc).

The following step e) comprises the extraction of the modulus of deformation M of the soil from the slope of the curve C _t final displacement / force, in the first domain E of elastic deformation situated before a point of inflection I.

The slope p of the curve C _t in the first elastic domain E is expressed:

P (note that Df and F3 are given here as

example, the principle being to use a final displacement / force couple in the elastic domain E, making it possible to precisely calculate the slope of said domain)

The deformation modulus M, in MPa / m, is calculated from the expression: with p the slope of the first

elastic range E and S the surface of the flat section of the measuring tip 11. The module thus extracted allows modeling of soil compaction under load.

The method according to the invention also makes it possible to determine the creep stress (Qf), the elastic limit of the soil. Indeed, starting from the curve C _t final displacement / force, the force F _f corresponding to the inflection point I, translates said creep stress Qf.

The creep stress is expressed:

F _f

Qf = with F _f the force corresponding to the inflection point I and S the surface of the planar section of the measuring tip 11.

From the creep stress Qf, we could safely estimate an admissible stress value Qa at ½ Qf, which would place the admissible stress Qa in the middle part of the elastic domain E. Let us recall that the admissible stress Qa is used to the dimensioning of the foundations of the structure.

According to another approach, as a function of the admissible settlement of the construction project, one could attribute to the admissible stress Qa, a value, always lower than the creep stress Qf, located in the first elastic domain E, and corresponding on the curve C _t at the displacement equal to the known admissible settlement.

Recall that traditionally, the admissible stress Qa is evaluated on the basis of a fraction of the stress at break Qc: Qa = Qc / 10; the coefficient 10 being established semi-empirically and in an extremely safe manner. The method according to the invention gives a value of the permissible stress Qa based on the management of the permissible settlement with respect to the structure, which makes it much more relevant.

According to an advantageous variant, the method comprises a step c ') during which the intermediate displacements are measured as a function of time, said intermediate displacements being associated with the gradual sinking of the measuring tip 11 during the determined duration t . As illustrated in FIG. 3, for example for the first force Fi applied, an intermediate displacement Df ¹ can be measured at tl = 15s. Intermediate displacements D [ ² , D [ ³ , ... could also be measured at t2 = 30s, t3 = 45s ... for example.

We can then draw one (or more) other curve (s) C _ti (C _t 2, ...) representing the intermediate displacement as a function of the applied force (Figure 3). The curve C _ti presents, like the curve C _t , a first domain of elastic deformation E of the soil and, after an inflection point, a second domain of plastic deformation PI of the soil, until reaching the breaking point (Qc ).

For the force F _f corresponding to the inflection point I of the curve C _t , the difference AD between the intermediate displacement Dé measured at the end of tl (for example 15 seconds) and the final displacement D measured at the end of the determined duration (eg 60s) is representative of the liquefaction potential of the soil. A deviation AD greater than 20% could reflect a potentially liquefiable soil.

For reference, 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 numerical modeling ", National Geotechnical and Engineering Geology Days, Nancy 2016) based on static penetration tests with double measurement: a resistance measurement of n2cm / s

pornte Q _c at a constant driving speed of 2cm / s and a measure of the peak resistance when stopped Q _c ^rret . The gap

_ Q ^{2Cm / S} -Q ^stop

^ Qc 2cm / s seems characteristic of the evolution of pore pressures in the soil. The higher this difference Q _C , the greater the risk of soil liquefaction in the event of an earthquake. In particular, there is a very high probability of liquefaction when this difference exceeds 40% of the resistance of the soil measured in static mode.

The method according to the invention, by giving access to the deviation Dϋ, could reinforce the representativeness of the deviation AQ _C vis-à-vis the pore pressures and the liquefaction potential of the soil considered.

Note that according to a first variant, in step a) of the method, the couple hollow tube 2 / central rod 1 of the static penetrometer 100 may be driven into the ground according to a conventional static mode, allowing a measurement of the breaking stress. from the ground as a function of the depth, up to the desired depth P for the elastoplastic measurement.

According to a second variant, the couple hollow tube 2 / central rod 1 can be introduced into a hole previously drilled in the ground up to the depth P desired for the elastoplastic measurement. In both variants, the soil to be analyzed, under the tip, was not modified by the step of bringing the tube / rod couple, which ensures good representativeness of the measurements.

The measurement method according to the invention, using a static penetrometer 100, provides an in situ measurement means for directly reconstructing the action / reaction relationship of a foundation / soil assembly by giving access to the effective behavior under soil load: lift, settlements, creep over time.

Unlike the pressuremeter approach, the measurement process is not conditioned in its result by the quality of the execution of the drilling, nor by the heterogeneity of the soil, nor even by the sensitivity or the complexity of the measurement equipment used. .

The present invention also relates to a static penetrometer 100 for measuring the elastoplastic properties of a soil.

The penetrometer 100 (FIG. 4) comprises at least one central rod 1 terminated at one 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 adapted 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 at the end, to form a train of rod / tube pairs, which can be driven into the ground over several tens of meters. It being understood that a second end of the rod 1 and of the hollow tube (or second end of the drill string and of the train of hollow tubes) is intended to remain above ground, on the surface.

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

Advantageously, the movable body 62 is capable of achieving a maximum displacement of approximately 7 cm between a retracted position (movable body 62 retracted and measuring tip 11 abutting against the hollow tube 2) and a deployed position (movable body 62 extended to the maximum , peak output 11 maximum).

The movable body 62 is configured to apply a force to a second end 12 of the central rod 1, which will lead to a more or less significant depression of the measuring tip 11, depending on the characteristics of the soil. The displacement of the movable body 62, associated with this depression is measured.

The power of the actuator motor is preferably chosen so that the movable body 62 is capable of applying forces typically between 100N and 20kN.

Advantageously, the jack 6 is an electric jack.

The penetrometer 100 also includes an electronic controller 7 for controlling the actuator 6, so as to apply a given force, for a determined duration t, and to record the movement of the mobile body 62 as a function of time, and this for the determined duration t .

The penetrometer according to the present invention offers a practical and industrial solution allowing the efficient and economical implementation of the aforementioned method for measuring the elasto-plastic properties of a soil. The penetrometer 100 according to the invention advantageously comprises support means integral with the hollow tube 2 by means of a clamping jaw 3. The support means allow a depression in static mode in the ground of the couple formed by the hollow tube 2 and the central rod 1 up to the given depth (s) to be investigated.

The support means may in particular consist of a hydraulic cylinder. The mobile part of the hydraulic cylinder fixed to the hollow tube 2 applies to it the pressing force necessary for the continuous sinking of the tube / rod couple. The fixed part of the hydraulic cylinder must be secured to a reaction block. The support means may include a self-propelled hydraulic group, for actuating the hydraulic cylinder.

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

Of course, the invention is not limited to the embodiments described and it is possible to make variant embodiments without departing from the scope of the invention as defined by the claims.

Claims

1. Method for measuring the elastoplastic properties of a soil, using a static penetrometer (100) provided with a hollow tube (2) surrounding a central rod (1) capable of sliding inside the hollow tube (2 ) and terminated at a first end by a measuring tip (11), the method comprising the following steps:

a) Push the couple formed by the hollow tube into the ground

(2) and the central rod (1) for bringing the measuring tip (11) to a depth (P) desired for measuring the elasto-plastic properties of the soil, said measuring tip (11) being in abutment against the tube hollow (2);

b) Apply a first constant force to a second end of the rod (1) above the ground, for a determined period (t);

c) Measure a first final displacement, associated with the insertion of the measuring tip (11) into the ground, at the end of the determined duration (t);

d) Repeat steps b) and c) with successive increasing forces, to form a curve (C _t ) representing the final displacement as a function of the force, until reaching an umpteenth force leading to an umpteenth final displacement reflecting the stress when the ground breaks;

e) Extract a soil deformation module from the slope of said curve (C _t ), in a first elastic domain (E) located before an inflection point (I).

2. Method for measuring the elastoplastic properties of a soil according to the preceding claim, in which the force corresponding to the point of inflection of the curve reduced to the section of the measuring tip (11) corresponds to the creep stress (Qf), elastic limit of the soil.

3. Method for measuring the elastoplastic properties of a soil according to the preceding claim, in which an admissible stress value (Qa) is defined at half of the creep stress (Qf).

4. Method for measuring the elastoplastic properties of a soil according to one of the preceding claims, in which the determined duration is 60 seconds.

5. Method for measuring the elastoplastic properties of a soil according to one of the preceding claims, comprising a step c ') to measure a first intermediate displacement, associated with the insertion of the measuring tip (11) into the soil, after an intermediate duration (tl) less than the determined duration (t).

6. Method for measuring the elastoplastic properties of a soil according to the preceding claim, in which step d) comprises the repetition of step c ').

7. Method for measuring the elastoplastic properties of a soil according to one of the two preceding claims, in which, for the force corresponding to the point of inflection of the curve (C _t ), the difference between the intermediate displacement measured after 15 seconds and the final displacement measured after the determined time is representative of the liquefaction potential of the soil.

8. Method for measuring the elastoplastic properties of a soil according to one of the preceding claims, in which the torque formed by the hollow tube (2) and the central rod (1) of the static penetrometer (100) is introduced into a previously drilled hole, in step a), to bring the measuring tip (11) to the depth (P) desired for the measurement of the elastoplastic properties of the soil .

9. Static penetrometer (100) for implementing the process for measuring the elastoplastic properties of a soil according to the preceding claims, comprising:

• At least one central rod (1) terminated at a first end by a measuring tip (11), said measuring tip being intended to sink into the ground;

• At least one hollow tube (2) surrounding the central rod

(1), the latter being able to slide inside the hollow tube (2);

• A jack (6) comprising an external body (61) integral with the hollow tube (2) and a movable body (62), said movable body (62) being configured to apply a force to a second end (12) of the rod central (1), leading to a depression of the measuring tip (11) in the ground, and to measure a displacement associated with this depression, the second end being intended to remain above the ground,

• An electronic controller to slave the cylinder (6), so as to apply a given force, for a determined period, and to record the movement of the movable body (62) as a function of time during the determined period.

10. Static penetrometer (100) according to the preceding claim, wherein the actuator is an electric actuator.