EP1411177B1 - Process and apparatus for assessment of the bearing capacity of an object inserted into the ground through vibration - Google Patents

Abstract

Method for determination of the force acting on an object (1) forced into the ground using a vibrator (2). Accordingly once the object has been forced into the ground to a given level, the insertion forcing process is stopped. An increasing axial force is then applied to the object until the object just moves. The threshold force applied to move the object corresponds to the static force acting on the object. The invention also relates to a corresponding device.

Description

The present invention relates to a method and a device for determination of the load-bearing capacity of an object (eg a pile or sheet pile) driven into the ground by vibrating.

In general, we know that the interest of vibrating an object using of a vibrator, lies in the reduction of the friction forces between this object and the ground due to the vibration of the object. The vibration of the object replaces the static friction with a dynamic friction about ten times less.

Moreover, the depression of the object results both from the weight of the object and from its accessories, the force that may be applied to it as well as the centrifugal force eccentric weights of the vibrator used to generate vibrations: The advantage of this solution stems from the increase in resulting driving force and decreasing strength exerted by the ground on the object.

It turns out that for many reasons, it is desirable to know before to perform important work on a soil to know the bearing strength of an object embedded in this ground and this, in depth in different layers of this soil.

Indeed, this parameter not only interests the company in charge to push objects into the ground (piles, sheet piles, drains ...) to choose the size of the vibrator to use, the frequency and / or the amplitude of the vibrations ..., but also the public works companies that are responsible for carrying out for the determination of such works, the type of infrastructure to be use, etc.

DE-A-19532931 discloses a method for determining the bearing force of a pile driven into the ground with the help of a vibrator.

In order to determine this bearing capacity, the method according to the invention as defined by claim 1 consists of to push the object (or equivalent object) into the ground and, at least one level determined impulse, to stop the process of depression and then to exercise, on all or part of the object, a growing effort of sinking, to detecting the effort threshold from which a displacement of said object is obtained said part and to deduce from this effort threshold a value corresponding to the static bearing force of the object.

According to a first variant embodiment of the invention, the depression of the object in the soil is made using a vibrator which can be varied centrifugal force.

In this case, for at least one level of depression for which we observe no refusal to sink the object under normal conditions of operation of the vibrator, the method according to the invention will consist in vary the centrifugal force applied to the object, to determine a force threshold centrifuge marking the passage from a refusal to a beginning of depression and find a parameter representative of this centrifugal force (amplitude, frequency of the weights, ...) and to calculate a static bearing load in multiplying the parameter raised by a force proportionality coefficient static / dynamic force.

In the case of a denial of depression, it will be sufficient to raise the centrifugal force applied to the object at the end of depression and to deduce the force static load bearing of the centrifugal force previously raised, being that the static bearing load is proportional to the centrifugal force exerted at the time of refusal.

Of course, the variation of the centrifugal force can be obtained by a variation of the vibration frequency. Nevertheless, to the extent that has a variable moment vibrator, it will be interesting to vary the force centrifuge by varying the moment of eccentricity and maintaining the constant frequency especially to avoid parasitic vibrations.

Advantageously, the measurement of the vibration frequency as well as the determination of vibratory moment and / or amplitude of vibrations (which is a direct measure of centrifugal force) will be performed by a measuring device involving an accelerometer and a system of reading.

According to an embodiment of the method previously described, as defined by claim 12, the aforesaid object to be driven into the ground may include two elements, namely: first rigid element directly engaged with the vibrator and a second guided element along the first element, means being provided for ensure a controllable axial blocking of the second element on the first and to exert a variable force tending to move the second element by relation to the first in a sense opposite to the meaning of depression.

• la solidarisation des deux éléments grâce au moyen de blocage commandable,
• le fonçage des deux éléments ainsi solidarisés au moyen du vibrateur jusqu'à une profondeur déterminée,
• la désolidarisation des deux éléments et le relèvement du premier élément d'une hauteur prédéterminée, sous l'action des vibrations exercées par le vibrateur,
• l'application de la susdite force entre les deux éléments en l'accroissant jusqu'à ce que l'on obtienne un déplacement du deuxième élément par rapport au premier jusqu'à ce qu'il retourne à sa position initiale sur le premier élément,
• la mesure de la force qui a provoqué le déplacement et qui correspond à la force de frottement exercée par le sol sur la surface du second élément et le calcul de la portance du deuxième élément,
• la répétition éventuelle de ce processus jusqu'à ce que l'on ait atteint la profondeur désirée,
• le calcul de la somme des portances calculées à chacune des profondeurs de manière à obtenir la portance totale.

In this case, the method according to the invention may comprise the following steps:

• the joining of the two elements thanks to the controllable locking means,
• the sinking of the two elements thus secured by means of the vibrator to a determined depth,
• separating the two elements and raising the first element by a predetermined height, under the action of the vibrations exerted by the vibrator,
• applying the aforesaid force between the two elements by increasing it until a displacement of the second element with respect to the first element is obtained until it returns to its initial position on the first element,
• the measurement of the force that caused the displacement and which corresponds to the friction force exerted by the ground on the surface of the second element and the calculation of the lift of the second element,
• the eventual repetition of this process until the desired depth has been reached,
• calculating the sum of the lift calculated at each depth so as to obtain the total lift.

Les figures 1 et 2 représentent en vue de face (figure 1) et en vue de côté (figure 2) un dispositif pour la mise en oeuvre du procédé selon l'invention utilisant un système à vérin pour exercer la susdite force ;

Les figures 3 à 5 sont des vues de côté du dispositif représenté figure 1, illustrant le principe du procédé selon l'invention ;

La figure 6 est une courbe de pression en fonction du temps, du fluide hydraulique des vérins faisant apparaítre l'instant de déclenchement du glissement ;

La figure 7 est une représentation schématique montrant plusieurs cycles de mesure successifs, effectués depuis une profondeur de trois mètres jusqu'à une profondeur de dix mètres ;

Les figures 8 et 9 sont des vues de face (figure 8) et de côté (figure 9) d'une variante du dispositif selon l'invention utilisant un système à treuils et à câbles au lieu des vérins.

Embodiments of the invention will be described below, by way of non-limiting examples, with reference to the accompanying drawings in which:

Figures 1 and 2 show in front view (Figure 1) and in side view (Figure 2) a device for carrying out the method according to the invention using a jack system to exert the aforesaid force;

Figures 3 to 5 are side views of the device shown in Figure 1, illustrating the principle of the method according to the invention;

FIG. 6 is a curve of pressure as a function of time, of the hydraulic fluid of the cylinders showing the instant of initiation of the slip;

Figure 7 is a schematic representation showing several successive measurement cycles, carried out from a depth of three meters to a depth of ten meters;

Figures 8 and 9 are front views (Figure 8) and side (Figure 9) of a variant of the device according to the invention using a winch system and cables instead of cylinders.

• un train de masselottes excentrées 3 montées rotatives à l'intérieur d'un boítier 4 et entraínées par une motorisation portée par le boítier ;
• un dispositif de suspension 5 du boítier 4 par exemple sur le crochet d'une grue ;
• une pince hydraulique 6 solidaire du boítier 4 qui sert à assurer la fixation du vibrateur sur l'extrémité supérieure du pieu 1 à enfoncer.

In the example illustrated in Figures 1 to 4, the method according to the invention is applicable to the determination of the bearing capacity of a tubular pile 1 of circular section driven into the ground by means of a conventional vibrator 2 comprising :

• a train of eccentric flyweights 3 rotatably mounted inside a housing 4 and driven by a motor carried by the housing;
• a suspension device 5 of the housing 4 for example on the hook of a crane;
• a hydraulic clamp 6 secured to the housing 4 which serves to secure the vibrator on the upper end of the pile 1 to drive.

In the lower end of this tubular pile 1 engages telescopically and is fixed a guide tube 7 which extends the pile 1 over a length predetermined.

On this guide tube 7 engages telescopically and is slidably mounted shuttle tube 8 of diameter substantially equal to the diameter of the pile 1.

The relative displacements of the shuttle tube 8 relative to the tubular pile 1 are provided by two hydraulic cylinders V ₁ , V ₂ each bearing on the pile 1 and on the shuttle tube 8. These two cylinders V ₁ , V ₂ , which are oriented parallel to the longitudinal axis of the pile 1, are diametrically opposed with respect to said axis.

Each jack is housed in a protective casing 10 integral with the shuttle tube 8 and slidably mounted along the pile 1.

• une position fermée dans laquelle l'extrémité supérieure du tube navette 8 vient en butée contre l'extrémité inférieure du pieu 1 (position représentée figure 3) de manière à assurer une transmission des vibrations entre le pieu 1 et le tube navette 8 ; cette position est utilisée pour effectuer le fonçage de l'ensemble pieu 1/tube navette 8 ;
• une position ouverte dans laquelle le tube navette 8 se trouve écarté du pieu 1 d'une distance égale à la course des vérins V₁, V₂, cette distance restant inférieure à la longueur du tube guide dépassant du pieu (figure 4).

These cylinders V ₁ , V ₂ , double-acting type, can take two positions, namely:

• a closed position in which the upper end of the shuttle tube 8 abuts against the lower end of the pile 1 (position shown in Figure 3) so as to ensure a transmission of vibrations between the pile 1 and the shuttle tube 8; this position is used to effect the driving of the pile 1 / shuttle tube 8 assembly;
• an open position in which the shuttle tube 8 is spaced from the pile 1 by a distance equal to the stroke of the cylinders V ₁ , V ₂ , this distance remaining less than the length of the guide tube protruding from the pile (Figure 4).

As an indication, the pile 1 may have a length of the order of 10 m and an inner section of 300 mm, the tube guide 7 may have a length of the order of 1.2 m and an outer section substantially equal to 300 mm. The shuttle tube 8 may have a length of 1 m and an inner section substantially equal to 300 mm. The stroke of the cylinders V ₁ , V ₂ may be equal to 0.5 m.

As illustrated in these figures, the operating mode is as follows:

In a first step, a first phase of driving of the pile 1 / shuttle tube 8 assembly is carried out using the vibrator. This first phase aims to reach an appropriate depth (for example 3 m) so that the frictional forces exerted by the earth on the pile 1 are sufficient for the pile 1 to act as a reaction tube. Above this depth, the action of the cylinders V ₁ , V ₂ on the shuttle tube, may drive the pile 1 instead of lifting the shuttle tube 8.

Of course, this depth will vary depending on the data for example, the continuity of the layers.

During this first driving phase, the cylinders V ₁ , V ₂ are in the closed position, the shuttle tube 8 being abutted on the pile 1 so as to obtain a normal vibrofonçage.

During a second phase (FIG. 4), the jacks V ₁ , V _{2 are released} and a pull is made on the pile 1 to raise it by about 50 cm. The cylinders being free, the shuttle tube 8 does not vibrate and it remains in place.

During a third phase, the measurement is triggered by gradually putting the cylinders V ₁ , V ₂ in pressure in the direction of the rise of the shuttle tube 8 to the pile 1. It then proceeds to the reading of the value of the pressure which triggers the sliding of the shuttle tube 8 on the guide tube 7. This reading makes it possible to deduce the force exerted by the two jacks V ₁ , V ₂ , which corresponds to the friction force exerted by the ground on the surface of the tube shuttle 8.

The progressive pressurization of the cylinders can be carried out by means of a Auxiliary power supply with very low constant flow. In this case representation of the pressure curve as a function of time (FIG. to easily visualize the triggering pressure of the slip. Indeed, in the absence of slip, the pressure rises steadily and then stabilizes as soon as the slip occurs.

Once this measurement is done, we start a fourth step (figure 6) of closing the cylinders again to bring the shuttle tube 8 to contact with pile 1 (and keep them under pressure) and then carry out a new step of sinking by the vibrator to bring the whole pile 1 / tube shuttle 8 at a second level of depth, for example 4 m.

The cylinders V ₁ , V _{2 are} then released and then the pile is raised by vibrating at an intermediate level, for example 50 cm, leaving the shuttle tube in place.

We then perform a measurement similar to the previous one by increasing gradually the pressure of the cylinders and by raising the pressure that causes a beginning of sliding of the shuttle tube.

Once this measurement is done, the cylinders are closed. The device is then ready to perform a new measurement cycle.

This process is repeated in successive contiguous increments until we have reaches the desired maximum depth.

Figure 7 shows three measuring cycles C ₁ , C ₂ , C ₃ at 3 m, 4 m and 10 m, respectively.

The sum of the experimental liftings recorded at each depth allows then to obtain the total lift of the pile.

The last measurement cycle at the desired maximum depth (the cycle in which the initially dark pile at this maximum depth is raised to an intermediate level (here 9.5 m)) can be supplemented by an additional step of driving back the pile 1 to the maximum depth (here 10 m) and to measure the value of the thrust of the cylinders V ₁ , V ₂ downwards, necessary to cause a beginning of movement down the shuttle tube 8.

This additional step C ₄ makes it possible to measure both the contribution to the lift by lateral friction and by peak resistance.

More precisely, the sum of the doubles of each measured force gives the all of the lateral friction on the pile 1, friction to which is added the peak reaction to obtain the actual bearing strength.

Once the actual bearing strength of pile 1 used for measurement needs, it is possible to determine the load-bearing capacity of piles neighboring dimensions, it being understood that the bearing force of a pile is proportional to its surface in contact with the ground and therefore, at equal lengths and similar situations, to its diameter.

In the examples previously described, the measurement of the depression of the pile in the soil is obtained by means of a graduation G carried by the pile 1 (may for example consist of marks inscribed on the pile every meters or 50 cm).

Of course, the invention is not limited to this provision. So, this measure can be provided by means of an electronic measuring device, in order to obtain automatic operation of the system.

Thus, it will be possible to use a cable mounted on an instrumented winder of which the free end is attached to the lower end of the pile. The winder can be then connected to a processor designed to determine the depth (possibly depending on the time) from the information provided by the reel.

It will also be possible to use a laser rangefinder attached to the housing shackle of the vibrator and point a reflective target attached to the ground.

In the example shown in FIGS. 8 and 9, the actuation of the shuttle tube is no longer ensured by cylinders but by two cables CA ₁ , CA ₂ coming to wind on two respective winches T ₁ , T ₂ carried by the hydraulic clamp 6 on both sides of the upper end of the pile 1.

These two cables CA ₁ , CA ₂ pass in two protection tubes P ₁ , P ₂ arranged along the pile 1 before being fixed by their free ends at two diametrically opposite points of the shuttle tube 8.

Of course, the winches can be instrumented to provide a information on the depth of insertion of the pile.

Claims (19)

1. Method for determining the bearing capacity of an object (1) driven into the ground by means of a vibrator (2), characterized in that its consists:

   firstly, when driving the object (1) into the ground and at various successive depths, of performing operating sequences each comprising:

   halting the driving process,

   applying an increasing force, in the driving axis, to all or part of the object (1), or to an equivalent object,

   detecting a threshold force on and after which initiated displacement of said object or said part (8) is obtained, and

   secondly, of determining a value corresponding to the static bearing capacity of the object using the threshold values detected throughout said sequences.
2. Method as in claim 1, characterized in that the driving of the object (1) into the ground is performed using a vibrator (2) whose centrifugal force can be caused to vary, and in that, at least for a driving depth at which no refused downward displacement of the object (1) is encountered under normal vibrator operating conditions, the centrifugal force applied to the object (1) is caused to vary, a threshold centrifugal force is determined marking the changeover from refused downward displacement to initiated displacement, a parameter representing this centrifugal force is identified and the static bearing capacity is calculated by multiplying the identified parameter by a proportionality coefficient of static force/dynamic force.
3. Method as in claim 2, characterized in that in the event of refused downward displacement, the centrifugal force applied to the object (1) at the end of the driving operation is identified, and the static bearing capacity is deduced from the previously identified centrifugal force, the static bearing capacity being proportional to the centrifugal force applied at the time of refusal.
4. Method as in any of the preceding claims, characterized in that the variation in centrifugal force is obtained with a variation in vibration frequency.
5. Method as in any of claims 1 to 3, characterized in that a variable moment vibrator (2) is used, and in that the variation in centrifugal force is obtained by a variation in the eccentric moment.
6. Method as in any of the preceding claims, characterized in that the measurement of vibration frequency and/or determination of the vibration moment and/or of vibration amplitude are performed by a measurement device using an accelerometer.
7. Method as in any of the preceding claims, characterized in that the object (1) to be driven into the ground comprises a first rigid element engaged with the vibrator and a second element (8) guided along the lower end of the first element, actuation means (V₁, V₂) being provided to ensure controllable axial locking of the second element (8) on the first and to exert a variable force tending to displace the second element (8) with respect to the first in a direction opposite to the driving direction.
8. Method as in claim 7, characterized in that it comprises the following steps:

   joining together the two elements (1, 8) by means of the controllable locking means,

   driving downwards the two joined elements (1, 8) by means of a vibrator (2) to a determined depth,

   separating the two elements (1, 8) and raising the first element (1) over a predetermined height, under the action of the vibrations exerted by the vibrator,

   applying the above-mentioned force between the two elements (1, 8) increasing this force until displacement of the second element (8) is obtained with respect to the first (1) until it returns to its initial relative position locked on the first element (1),

   measuring the force which caused the displacement and which corresponds to the friction force exerted by the ground on the surface of the second element (8) and calculating the bearing capacity of the second element (8).

   optional repetition of this process until the desired depth is reached,

   calculating the sum of the bearing capacities calculated at each depth so as to obtain total bearing capacity.
9. Method as in either of claims 7 and 8, characterized in that it comprises determining the bearing capacity for a succession of driving depths, and in that it comprises the sum of the bearing capacities identified at each of these depths so as to obtain the total bearing capacity of the object.
10. Method as in any of claims 7 to 9, characterized in that it comprises an additional step consisting, when the object is at maximum depth, of measuring the value of the downward thrust exerted by said above actuation means (V₁, V₂) that is required to cause initiation of the downward displacement of the said second element (8) so as to be able to measure the contribution of both side friction and tip resistance towards bearing capacity.
11. Method as in any of the preceding claims, characterized in that the bearing capacity of said object is used to deduce the bearing capacity of other objects of similar size in relation to the ratio of ground contact surface of said object and said other objects.
12. Device for implementing the method as in any of the preceding claims, in which the object to be driven into the ground comprises a first rigid element (1) engaged on a vibrator and a second mobile element (8) guided along the lower end of the first element (1) and actuation means (V₁, V₂) provided to ensure the controllable axial locking of the second element (8) on the first (1) and to exert a variable force tending to displace the second element (8) with respect to the first (1) in a direction opposite to the driving direction, means being provided to detect the initiation of displacement of the second element and to measure the force exerted to obtain this initiated downward displacement.
13. Device as in claim 12, characterized in that said above actuation means comprise at least one actuating drive (V₁, V₂).
14. Device as in claim 12, characterized in that said above actuation means comprise at least one winch (T₁,T₂) integral with the first element (1) and a cable (CA₁, CA₂) which winds around this winch (T₁, T₂) and one of whose ends is fixed onto the second element (8).
15. Device as in any of claims 12 to 14, characterized in that said first element (1) is of tubular shape, and in that said second element of tubular shape (8) is slidably mounted on a guide tube (7) engaging telescopically into the lower end of the first element and secured thereto.
16. Device as in any of claims 12 to 15, characterized in that it comprises means for measuring the downward driving of said object (1).
17. Device as in claim 16, characterized in that the measurement means consist of a graduation carried by the first element (1).
18. Device as in claim 16, characterized in that said measurement means comprise a cable mounted on an instrumented reel.
19. Device as in claim 16, characterized in that said above measurement means comprise a laser telemeter integral with the first element (1) and directed towards a reflective target integral with the ground.

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