Abstract:
A measuring head, intended to be fitted to a dynamic penetrometer, and attached to a drill string provided with a tip, includes a driving head intended to receive an impact to be transmitted, via the rest of the measuring head, to the drill string; and a central rod for transmitting the impact from the driving head to the drill string, the central rod having a first end turned towards the driving head, and a second end opposite the first end, and suitable for engaging with the drill string, the central rod being provided with at least one deformation sensor. It includes at least one absorption member interposed between an impact receiving end portion of the driving head and the second end of the central rod and which is suitable for filtering the wave transmitted to the drill string when the end portion of the driving head receives an impact.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a measuring head intended to be fitted to a dynamic penetrometer as well as a method of measurement using such a measuring head. 
     DESCRIPTION OF THE RELATED ART 
     A dynamic penetrometer is a device that makes it possible to measure, in situ, the mechanical characteristics of the ground without it being necessary to take a sample for study in a laboratory. 
     A penetrometer comprises a string of metal rods. The free end of an end rod is provided with a tip which provides for the penetrating into the ground of a portion of the rod string when an impact is exerted on the free end of the other end rod of the rod string. This impact is transmitted to the rod string by the intermediary of a driving head. 
     Such a dynamic penetrometer is described in FR-A-2 817 344 and marketed by the company SOL SOLUTION. This penetrometer makes it possible to measure the compactness of the ground and comprises a driving head, a central rod, a tapered probe and a rod string connecting the driving head to the tapered probe. The operation of the penetrometer is based on the principle consisting in providing energy to the driving head, in particular via a hammer strike. This energy is then transmitted by the central rod to the rod stringlinked to the tapered probe. The latter is then driven into the ground to a depth depending on the density of the ground. Knowing the energy provided to the penetrometer, the value of the displacement of the probe and the section of the probe, it is then possible to determine the level of compactness of the ground. The last two characteristics can be measured easily and the energy transmitted to the penetrometer is measured by the intermediary of a sensor of the piezoelectric type of which the deformation generates an electric signal that is proportional to the intensity of the impact. In order to measure the energy supplied to the penetrometer, strain gauges mounted as a Wheatstone bridge can also be used. The strain gauges are then placed under tension and their electrical resistance varies according to the deformation of the gauge. The electric signal, applied to the strain gauges, therefore varies in proportion to the intensity of the impact and makes it possible to deduce the value of the energy transmitted to the driving head. In this document of prior art, concern is not given to measuring the wave transmitted during the impact and even less to filtering such a transmitted wave. 
     The utilisation of such a penetrometer is limited. Indeed, the impact is, generally, carried out manually using a hammer. Because of this, measuring the characteristics of the ground is carried out at a limited depth, typically between 0 and 7 m. 
     In addition, measuring the energy supplied to the penetrometer lacks in precision and using a piezoelectric material such as quartz increases the cost of the penetrometer. 
     SUMMARY OF THE INVENTION 
     The invention intends to overcome more particularly these disadvantages by proposing a measuring head intended to be fitted on a dynamic penetrometer allowing for the determination, reliably, of the characteristics of the ground by optimising the collection and the processing of the signals provided by a penetrometer, and this for thicknesses of ground ranging up to 15 m. 
     The presence of an absorption member able to filter the shock wave allows for an optimised collection of the signals while still limiting the processing of the latter, with the signals as such being subjected to a first filtration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The invention shall be better understood and other advantages of the latter shall appear more clearly when reading the following description of a measuring head carried out in accordance with the invention, provided solely by way of example and made in reference to the annexed drawings wherein: 
         FIG. 1  is a side view of a measuring head in accordance with the invention, 
         FIG. 2  is a longitudinal cross-section, on the same scale, according to the plane II of the measuring head of  FIG. 1 , 
         FIG. 3  is a simplified side view on another scale of the measuring head of  FIG. 1  connected to a rod string of a penetrometer, with this rod string being partially shown, 
         FIG. 4  is a simplified side view of a first type of penetrometer provided with the measuring head of  FIG. 1 , with a hammer shown in front position before an impact on the driving head, with the ground being shown diagrammatically, 
         FIG. 5  is a simplified perspective view on a smaller scale of another type of penetrometer provided with the measuring head of  FIG. 1 , 
         FIG. 6  is a diagram that shows the method of processing the signals collected by the measuring head and 
         FIG. 7  is a simplified curve of the signals collected by such measuring head representing, once processed, the behaviour of the ground. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As is shown in  FIGS. 1 and 2 , the measuring head  1  comprises a cylindrical main body  2  with a circular base. This hollow body  2  is made of steel. Alternatively, it is made of another rigid material. 
     At least one, advantageously two deformation sensors  3 ,  4 , diagrammatically shown in  FIG. 2 , are mounted on a central rod  5  inserted longitudinally into the hollow body  2  of the measuring head  1 . The central rod  5  is solid and cylindrical and has a circular base. Its main longitudinal axis X is confounded with the main longitudinal axis of the body  2  when it is inserted into the latter. 
     The two deformation sensors  3 ,  4 , also referred to as strain gauges, are aligned according to the axis X and arranged in relation to one another at a distance between 10% and 50% of the useful length L 2  of the main body  2 . 
     Advantageously, as shown in  FIG. 2 , an accelerometer  6  is also arranged on the rod  5  between the two strain gauges  3 ,  4 . The reference portion  60  of the accelerometer  6  is fastened onto the inner wall  20  of the hollow body  2 . 
     The rod  5  has a length L 5  such that it exceeds the main body  2  by these two ends  50 ,  51 . The ends  50 ,  51  extend beyond the body  2  by passing through seals  21 ,  22  that close the ends  23 ,  24  of the body  2 . The seals  21 ,  22  provide, on the one hand, the seal in relation to dust and water for the interior of the body  2  and, on the other hand, make it possible to limit the electromagnetic disturbances on the measuring head  1 , by insulating the electrical and electronic elements, for example the sensors  3 ,  4  and the accelerometer  6 , inserted in the main body  2 . The body  2  also participates, via the nature of the material it is comprised of, in the electromagnetic protection of the elements that it contains. 
     Outputs, not shown, allowing for the connection of the various sensors to a calculation module, are also provided on the main body  2  of the measuring head  1 . 
     The end  51 , referred to as bottom when viewing  FIG. 2 , of the rod  5  forms the end of the measuring head. It comprises a blind hole  7 . The hole  7  is made longitudinally in the end  51  and it is centred on the axis X of the rod  5 . The hole  7  allows for the fastening of the measuring head  1 , via nesting of the male/female type, known per se, onto an end  8  of a string  9  of rods  10 , as shown in  FIG. 3 . The string  9  of rods  10  also extends according to the axis X of the central rod  5  and the end  51  is, in configuration for use of the penetrometer, directed towards the ground. This fastening can be definitive. In this case, the first rod  10  of the string  9  is, for example, welded to the measuring head, with the hole  7  being adapted to such a fastening. Alternatively, the fastening is carried out in a detachable manner, for example by screwing, with the hole  7  then being threaded. A detachable fastening makes it possible to mount such a measuring head  1  “as retrofitting” on a penetrometer provided with a string  9  of rods  10 . More generally, the end  51  of the rod  5  is suitable for engaging mechanically with the string  9  of rods  10 . 
     The upper end  50  of the rod  5 , when viewing  FIG. 2 , is, similarly to the lower end  51 , located outside of the main body  2  beyond the seal  21 . The end  50  is inserted into a blind hole  11  arranged in the main body  200  of a driving head  12 . 
     More generally, the end  50  of the rod  5  is suitable for engaging mechanically with the driving head  12 . The hole  11  is carried out in the central position, according to a direction parallel to a longitudinal axis of the driving head  12 . The body  200  of the driving head  12  has the shape of an elongated cylinder with a circular base. Its dimensions are adapted so that, when the end  50  is in the hole  11 , the longitudinal axis of the driving head  12  is aligned with the axis X of the body  2 . Consequently, the driving head  12 , the central rod  5  and the string  9  of rods  10  are all aligned according to the axis X. 
     From the top to the bottom in  FIG. 1 , the driving head  12 , then the central rod  5  and finally the string  9  of rods  10  can then be found. 
     The hole  11  is arranged in an end  121  of the body  200  received in a counterbore  230  made in the seal  21 . The free end  120  opposite the end  121 , is suitable for receiving an absorption member  13  comprising a material adapted in order to provide for a filtering of the waves transmitted to the string  9  of rods  10 . This material is, advantageously, a high-impact polymer or high-density polyethylene. Alternatively, this is another material, for example rubber. This member  13  is configured as a solid disc covering the end  120 . The member  13  comprises, in central position, two reliefs  130 ,  131  arranged facing one another on two faces  132 ,  133  opposite the member  13  and perpendicular to these faces. Alternatively, the member  13  is configured as a ring, as a double ring, as a solid disc without relief or as any other shape suited to the configuration of the measuring head and/or of the filtering desired. 
     Moreover, the material comprising the member  13  and/or geometrical configuration participate in the electromagnetic protection of the measuring head  1 . 
     The relief  130  is inserted into a housing  122  arranged in the end  120 . The relief  131  is inserted into a housing  140  arranged in an end portion  14  of the driving head  12 , solid and configured substantially as a half-moon. This end portion  14  forms the portion of the driving head  12  intended to receive the impact. The latter is transmitted, via the member  13  and the rod  5 , to all of the string  9  of rods  10  of the penetrometer. The absorption member  13  is therefore interposed, according to the axis X, between the end portion  14  of the driving head  12  and the end  51  of the central rod  5 . As the sensors  3  and  4  are arranged on the central rod  5 , they are therefore positioned between the absorption member  13  and the end  51  of the rod  5 . The height H of the absorption member  13  is advantageously between 10 and 100 mm, with this height being taken between the reliefs  130  and  131  included. The density of the member  13 , de facto the density of the material comprising the member  13  is between ¼ and ¾ of the density of steel. Advantageously, the density of the member  13  is between ⅓ and ½ of the density of steel and, preferably, in the neighbourhood of ⅓ of the density of steel. 
     The end  50  of the rod  5  is inserted into the blind hole  11  of the body  200  of the driving head  12  with clearance. In other terms, the end  50  is not in contact with the bottom of the blind hole  11 . This also participates in the absorption during an impact exerted on the portion  14 . 
     The configuration of the head  12 , in particular of the portion  14  and of the member  13 , makes it possible to carry out an impact plastic and to generate a wave, sinusoidal or not, that is wider than that usually generated with a penetrometer of prior art. In other terms, the member  13  absorbs certain frequencies of the wave referred to as descending, i.e. of the wave generated by the impact and directed towards the string  9  of rods  10 . 
     The presence of the member  13  as such makes it possible to generate an impact and a shock wave of a given type. It also participates in protecting the measuring head  1  in terms of electromagnetic disturbances. 
       FIG. 3  shows a string  9  of rods  10  provided with the measuring head  1 . The bottom end  7  of the measuring head  1  is connected to an end rod  10  made of steel, definitively or in a detachable manner, of a string  9  of rods  10 . The string  9  has a length defined by the number of rods  10  and/or the length of each rod  10 , for the type of tests to be carried out. In general with a so-called hand penetrometer, i.e. a variable energy dynamic penetrometer of which the driving head is intended to receive a variable impact exerted manually, this length is between 0.25 m and 1 m. 
     The end rod  10 A, located in bottom position when viewing  FIG. 3 , is provided with a tip  15 . The geometry of this tip  15 , also made of steel, is adapted so that the latter is tapered with an angle α at the top of at least 60° and, preferably, in the neighbourhood of 90°. The active portion of a variable energy dynamic penetrometer therefore comprises a string  9  of rods  10  of which an end rod  10 A comprises a tip  15  and another end rod  10 B a measuring head  1 . 
     Such a so-called hand penetrometer is shown in  FIG. 4 . This type of penetrometer is easy to transport. It is shown in active configuration, installed vertically and perpendicularly to the ground by the intermediary of a guide  16 , configured as a pierced plate, allowing for the precise positioning of the tip  15  before it is driven into the ground. 
     In  FIGS. 3 and 4 , a sheath  100  surrounds the string  9  of rods  10 . This sheath  100  prevents the direct contact between the ground and the rods  10 . As such, only, the tip  15  is in contact with the ground, with the latter as such exerting no friction on the rods  10  that can distort the measurements. In another embodiment, according to the nature of the ground, the penetrometer is devoid of a sheath  100 . 
     The top end  8  of the top rod  10 B of the string  9  of rods is provided with the measuring head  1  which is connected to the guide  16  by a belt  17  which makes it possible to measure the distance travelled by the tip  15  when it is driven in when an impact is applied on the rounded portion  14  of the driving head  12 . At the time of this impact, carried out using a hammer  18  of which certain characteristics are known, an impact plastic is carried out on the portion  14 . At the time of this impact, the portion  14  transmits to the rod  5  and to the tip  15 , via the string  9  of rods, an energy E from top to bottom, according to an arrow F, allowing the tip  15  of the penetrometer to be driven in the ground. A portion Ed of this energy E is dissipated in the ground, according to the nature and the characteristics of the latter. Another portion Er of the energy E is reflected and transmitted by the tip  15  in the opposite direction, i.e. from bottom to top according to the arrow F 1 , to the measuring head  1  by the string  9  of rods  10 . 
     The reflecting of the energy Er is carried out in a manner similar to the transmission of the energy E, due to the homogeneous properties of the rods  10  all along the string  9  and of the measuring head  1 , with these elements being made from the same material, steel. 
     The presence of the member  13  makes it possible to generate descending waves that are wider than those generated in the absence of the member  13 , and this while still filtering certain frequencies corresponding to so-called unwanted waves. 
     The descending waves are, at least partially, reflected in the direction of the head  1  from the tip  15 . These waves are representative of the behaviour of the penetrometer between the tip  15  and the ground and between the portion of the end rod  10 A driven into the ground and the ground, over the portion of rod  10 A inserted into the ground. In other terms, these waves are the bearers of much information relating to the mechanical behaviour of the ground. 
     It is therefore interesting to process such waves in order to determine characteristics of the ground that until now could not be determined or at least were not able to be determined reliably and repetitively with a penetrometer of prior art. 
       FIG. 5  shows another embodiment of the invention with a so-called heavy penetrometer. This is a motorised, self-propelled dynamic constant-energy penetrometer  19 , able to carry out tests over substantial depths of ground, typically beyond 5 m, with these tests able to be carried out to depths in the neighbourhood of 15 m. 
     This type of penetrometer  19  comprises a frame  190  provided with members that provide for the displacement of the penetrometer, in the form of crawler tracks  191 . This frame  190 , in addition to a module  192  for controlling the penetrometer, also comprises a module  193  for collecting and processing the signals collected by the measuring head  1 . 
     The latter, as hereinabove, is arranged at the end of a string  9  of rods  10 , guided along a mast  194  maintained in vertical position during the tests. 
     The lower end  195  of the mast  194  bears against a guide plate  196  that provides the guiding of the tip  15  in the ground. 
     The measuring head  1  is arranged offset in relation to the longitudinal axis X of the string  9  of rods  10 . It is connected to the string  9  by a plate  197  fixed radially on the outer wall of a sleeve  198 . This sleeve  198  is mounted slidingly on the mast  194 , from the end  199  of the latter opposite the end  195 . A mass  180  is inserted in a sheath  181  for guiding arranged parallel to the mast  194 . The mass  180  is set into action via a cylinder  182  inserted into the end  199  of the mast  194 . The masse  180  is dropped from a determined height, in the sheath  181 , before striking the measuring head  1 . 
     As such, with such a heavy penetrometer  19 , it is possible to easily qualify the energy transmitted during the impact since the mass and the speed of falling of the mass  180  are known and defined before each series of measurements. As hereinabove, the measuring head  1  can be provided definitively or in a detachable manner on such a penetrometer  19 . 
     Note that, in the preferred embodiment, two strain gauges  3 ,  4  are used rather than one which makes it possible, using two deformation measurements and the determining of the driving distance to determine, via calculation, the acceleration without requiring the presence of an accelerometer  6 . The latter can however, as shown in  FIG. 2 , be provided in the measuring head  1 , in order to form a backup measurement device in the event of failure of one of the gauges  3  or  4  or in order to carry out a redundant measurement of acceleration. 
     The measuring head  1 , whether it is fastened to a penetrometer of the type of  FIG. 4  or of  FIG. 5 , allows for the collecting of various signals. A first group of signals comprises data concerning the deformation of the string  9  of rods  10  during the transmission of the shockwave from the driving head  12  to the tip  15 . These signals represent the resistive force Fp(t) of the tip. The measuring, either direct by the accelerometer  6  or via calculation, of the acceleration a(t) makes it possible to determine the driving speed Vp(t) of the tip  15  as well as the value for the driving Sp(t) of the tip  15  in the ground. It is as such possible to determine the resistance of the ground on the tip  15 , by the difference between the energy EFdp(t) transmitted to the ground by the tip  15  and the energy reflected by the tip  15 . 
     The energy EFdp(t), in the form of a compression wave, is transmitted from the driving head  12  to the tip  15  then partially reflected towards the source, i.e. towards the measuring head  1  while a portion is absorbed by the tip and the ground when the penetrometer is driven in. When this reflected wave arrives on the measuring head  1 , it is again reflected, this time in the direction of the tip  15 . This yoyo phenomenon of the wave continues while decreasing in intensity at each reflection by the tip  15  or the measuring head  1 , until it is no longer perceptible. 
     All of these signals are collected in real time, at each passing of the wave on the measuring head  1 . As measurements are taken at about every two microseconds, these signals show, over the period of one measurement, a substantial mass of information. 
     This information represents the mechanical behaviour of the ground. In particular, the plasticity, elasticity and the shock absorbing of the ground. 
     In order to carry out the determination of these characteristics using all of the signals collected by a measuring head  1 , whether it is mounted on a variable- or constant-energy dynamic penetrometer such as shown in  FIGS. 4 and 5 , a discrimination needs to be made between the signals that represent characteristics of the ground and the unwanted signals. This is possible, in part, thanks to the absorption member  13  and to the geometric configuration of the portion  14  of the driving head  12 . In other terms, a mechanical filtering of certain frequencies of the shockwave is carried out by the measuring head  1 . 
     This mechanical filtering is supplemented by a method of measurement of data collected by the measuring head  1  shown in  FIG. 6 . Here, the expression “method of measurement” must be understood as designating the collecting of data by the measuring head  1  as well as the processing and the interpreting of this data, with the understanding that the term signal is considered to be a synonym of data. 
     The method of measurement makes it possible, among other things, to process signals in order to improve the readability of the latter, in addition to the action of the member  13  then, using these processed signals, to interpret them and to determine the characteristics of the ground for example, via comparison for some of them. 
     The first step  30  consists, using the measuring head  1 , in acquiring and in conditioning the signals provided by the sensors  3 ,  4 . Signals F(t) and a(t) are obtained. These signals then undergo a series of processing carried out by a calculation module  193  and different according to the F(t) or a(t) signal. 
     The resistive force signal F(t) is subjected to, during a step  31 , a filtration at 50 Hz in order to remove the electrical noise and a smoothing in order to raise the baseline. In step  32 , a corrected signal F(t) is obtained. If after the step  31  the signal is not corrected or cannot be corrected because it is too weak, a new signal F(t) is to be acquired, as indicated by the arrow  33 , by repeating the acquisition of new signals during another step  30 . The signal of acceleration a(t) is subjected to, during a step  34  a smoothing and a filtering at 50 Hz for the background noise, followed by a correction, at least of the first degree, of the baseline. 
     To the a(t) signal is then applied a temporal integration processing  35  and a frequency integration processing  36 , via fast Fourier transformation, followed by filtering steps  37 . 
     If the values  38 ,  39  of the signal a(t) obtained respectively at the end of the steps  35  and  37  are identical, in the step  32  a corrected signal a(t) is obtained. 
     If the values  38 ,  39  are different, favour is given to the value  39  obtained in the step  37  and this value is considered as being the corrected signal a(t) in the step  32 . 
     If the values of a(t) are visibly in error or too low, a series of measurements is taken again as indicated by the arrow  40 . 
     On the corrected signals F(t) and a(t) obtained in the step  32 , a decoupling of the ascending and descending waves is carried out during a step  41 . This step is followed by a step  42  of determining the speed of propagation of the compression wave in the ground. This involves determining the impact polar and then the swiftness. This value is characteristic of the behaviour of the ground under the effect of a compression wave. 
     These corrections make it possible, during a step  43 , to reconstruct the peak resistance signals qd(t) as MPa, the peak force R(t) as kN and of the driving of the tip Sp(t) as mm and to show these graphically. An example of such a curve is provided in  FIG. 7 . 
     These signals, corrected and able to be used, are either displayed and compared, during a step  44 , with the signals from known ground listed in a database, or stored for later processing during a step  45 . Note that the storage  45  also intervenes after the step  44 . 
     At each measurement campaign, the signals collected in the step  43 , through incrementing, enrich the database. 
     In parallel to the storage and/or the comparison with the known values, another series of measurements is taken, as shown diagrammatically by the arrow  46 . 
       FIG. 7  shows the characteristics determined in the step  43  after the method of measurement of collected signals. Here, this is an illustration of the behaviour of a ground where, according to the driving of the tip in mm, the resistance at the tip qd(t) and also the peak force in R(t) are represented. 
     A first portion P 1  of the curve is said to be dynamic. It corresponds to the impact on the tip  15 , i.e. to the driving of the penetrometer into the ground. This portion P 1  characterises the elasticity of the ground. 
     Then a second portion P 2  corresponds to the absorbing of the impact i.e. to the slowing down of the penetration of the tip in the ground. The portion P 2  lasts substantially throughout the entire duration of the driving in. This is a characterisation of the plasticity. Analysing this portion P 2  also makes it possible to obtain information on the granulometry of the ground, for example via an analysis of the signal. 
     The last portion P 3  is a loop corresponding to a loading and reloading cycle. This is a characterisation of the “pure” elasticity, i.e. the elasticity due solely of the vibrations of the tip  15  in the ground, without interaction of the viscosity of the ground. 
     Note that  FIG. 7  is a simplified representation, in order to facilitate reading the curve. 
     By comparing the characteristics obtained with the characteristics that are known and stored in a database, it is possible to associate the characteristics of this ground with its nature and/or it composition and/or its granulometry. 
     The measuring head  1  makes it possible to determine together, i.e. during a series of measurements, these characteristics and this at the time of each impact. By processing the signal and modelling, the signals collected as such make it possible to obtain a substantial amount of information on the mechanical properties of the ground. The static resistance or ultimate resistance Rs of the ground can be determined, for example. Other characteristics are able to be determined by such a measuring head and such a method.