Classifications

• • E—FIXED CONSTRUCTIONS
  • E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
  • E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
  • E02D1/00—Investigation of foundation soil in situ
  • E02D1/02—Investigation of foundation soil in situ before construction work
  • E02D1/022—Investigation of foundation soil in situ before construction work by investigating mechanical properties of the soil

Definitions

• 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.
• 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).
• 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.
• the drill string 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.
• In situ measurements are usually made from standard penetration tests (SPT) or cone penetration tests (CPT), including piezocone soundings (CPTU).
• SPT standard penetration tests
• CPT cone penetration tests
• CPTU piezocone soundings
• 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.
• 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.
• the present invention relates to a penetrometer for evaluating the liquefiable nature of a soil, comprising:
• At least one central rod terminated at a first end by a measuring tip
• the penetrometer further comprises an electric jack comprising an outer body integral with the hollow tube and a movable body, said movable body being:
• the determined frequency is between 1 and 5 Hertz
• the electric jack 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.
• the method comprises the following steps:
• 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;
• step d the speed of the second displacement is adjusted so as to maintain the measured applied force substantially constant
• 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
• the third displacement is 10mm
• Figure 1 shows a test curve with double measurement of resistance to penetration of a soil
• 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.
• the present invention relates to a penetrometer 100 for evaluation by direct measurement of the liquefiable nature of a soil.
• the penetrometer 100 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.
• 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.
• 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.
• controlled depressions will be made successively from the given depth P A , in the suspect zone A (FIG. 1).
• the power of the motor 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 range of pressures makes it possible to address the Characteristic stresses (10-30 bars) measured in peaks 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.
• 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.
• the electric jack 6 is electronically controlled: 1 '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 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.
• 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 pair formed by the hollow tube 2 and the central rod 1 into the ground so as to bring the measuring tip 11 to a given depth of investigation (for example P A with reference to FIG. 1). .
• 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)).
• 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 .
• step b) is performed; 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.
• 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.
• 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 effect said first displacement D1. This measurement of the applied force, representative of the soil resistance, makes it possible to verify that the peak resistance measured at the given depth P A is substantially identical to that measured during the pre-diagnosis at this same depth.
• the measurement of the reaction force transmitted by the second end 12 to the movable body 62, at a standstill (representative of the peak resistance at the stop Q ⁇ rr t ) is also provided for in FIG. step c).
• step c) The measurements taken in step 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 character of the investigated soil layer (zone A). .
• step d) consists in 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)).
• 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.
• 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.
• 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 Q C during this second controlled depression.
• the speed of the second displacement D 2 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).
• the aim is an applied force substantially equal to the applied force value representative of the peak resistance. measured in step c).
• the speed of the second displacement D 2 is automatically increased or decreased depending on the force measured during step d), with the objective of maintaining it substantially constant.
• 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.
• 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.
• 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.
• step e) is to stop the vibration.
• step f) then plans to actuate the moving body
• 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.
• 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.
• 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.
• a layer having rapidly lost its resistance under vibratory stress can, in the absence of the latter, recover its initial resistance. . it is a liquefiable soil behavior (for example curve (a) in FIG. 4).
• 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 Q C lower than its initial resistance this may reflect a large deformation type phenomenon (for example, curve (b) in FIG. 4), the resistance properties of the soil layer having been irreversibly modified by the vibration.
• a layer having rapidly lost its resistance under vibration stress may, in the absence of the latter, return during the third controlled depression to a resistance Q C
• this may reflect a densification phenomenon (for example, curve (c) in FIG. 4), the resistance of the soil layer having been reinforced by the vibration.
• 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 depths of investigation data, 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 nature of the soil layer analyzed, based on the speed of realization of the second displacement D 2 .
• the more or less abrupt decrease in soil resistance under vibratory stress is a key criterion of liquefiable nature.
• 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.

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• 2017
  • 2017-08-18 FR FR1757732A patent/FR3070170B1/fr active Active
• 2018
  • 2018-08-14 WO PCT/FR2018/052062 patent/WO2019034822A1/fr unknown
  • 2018-08-14 ES ES18765956T patent/ES2902928T3/es active Active
  • 2018-08-14 JP JP2020530733A patent/JP7138173B2/ja active Active
  • 2018-08-14 EP EP18765956.0A patent/EP3669029B1/fr active Active
  • 2018-08-14 PT PT187659560T patent/PT3669029T/pt unknown

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