DE3434565A1 - Method for measuring the deformation of sunken pile-driving bodies and drill bodies - Google Patents

Abstract

This method can be used in the case of sheet pilings, columns and ground anchors as well as drilling linkages both while they are being fed forward as well as subsequently, and is based on the analysis of the body-sound frequency spectrum of the object to be investigated, which spectrum is governed by the operation or is externally excited. In this case, the magnitude and distribution of the deformation-typical internal stress are measured indirectly by means of the resonant-frequency shift caused by the latter and are continuously compared with case-specific reference values which are theoretically calculated or are determined in model experiments. The magnitude of the deviation from the required values gives the machine operator continuous information on the quality of drilling progress and thus initiates corrective measures in good time, when necessary. In addition, typical damage responses can be diagnosed from the measurement curves, such as yoke deviations in the case of divergent sheet pilings, and in addition crack formation and the curvature of the longitudinal axis. The latter value can advantageously be used to identify directional deviations of the actual drilling hole axis from that planned, and furthermore, it is possible continuously to check the drilling linkage for excessive material stress.

Description

Method of measuring deformation

of submerged piling and drilling bodies Description The invention relates to offers a non-destructive method for measuring and monitoring deformations submerged metal ramming or drilling bodies, which are both propulsion-accompanying as can also be used on the stationary object without the latter in its position or shape changes nor affects its surrounding medium in any way will.

Such a process is of great economic importance in hydraulic engineering and foundation engineering as well as of practical interest in drilling technology.

The use of sheet piling and steel pillars as permanent or Provisional construction work has become an indispensable part of today's civil engineering technology, be it for underground railway constructions, road or railway underpasses, excavation of building sites and shaft construction in general; Line laying as well as bridge, river, canal or dam construction.

In all of these cases the sheet pile construction is mechanical Stability and sufficient tightness against groundwater ingress are required which is only given if all planks are hooked on both sides over their entire length interlocking longitudinal profiles (so-called locks) are connected to one another. That however, it is only in the originally deformed or uniformly slightly deformed state of the planks possible.

The costs of a construction project can be due to inadequate Sheet piling can be increased in several ways: 1. The intended capacity of the pumping equipment for lowering the groundwater level and subsequent dewatering in the The excavation pit has to be increased with a loss of time.

2. The resulting delay in construction progress usually contractual penalties for the construction companies carrying out the work and downtime losses of workers and machines.

3. If the stability of the sheet pile wall is endangered are time-consuming and difficult-to-perform rework required.

4. The deformed sheet piles can usually no longer be pulled out are and are thus lost.

This opens up in drilling technology and especially in deep drilling technology Process the possibility of deviations of the borehole axis from the straight line or the determine intentional compliance with the same during the drilling process or to monitor. In addition, tension in the drill pipe, which under Circumstances lead to the breakage and thus to the abandonment of the borehole, recognized at an early stage and corrected, thus reducing the considerable losses incurred when releasing especially avoid deep drilling.

The current state of the art does not know any process that leads to the integral, Not locally stepwise deformation measurement on almost completely covered and thus inaccessible, large-dimensional steel workpieces would be suitable. All the usual Material testing methods such as strain gauges, ultrasound and X-ray technology require the accessibility of the workpiece surface and are application-related sometimes too complicated to withstand tough construction site requirements.

At the moment, the relatively new test method is jckallemissionsanalyse under construction, but active signals from materials (e.g. crackling) analyzed, on the one hand sporadically, i.e. not directly from the test set-up triggered and, on the other hand, independent of frequency the spatial dimensions of the test object.

Furthermore, the test object is only supplied with energy in the form in which it is also supplied to him in the operational state (e.g. process control during the welding process), only the amount of energy supplied can be increased (e.g. pressure test on pressure vessels).

In contrast to this, the method according to the invention always only the same form of excitation energy, namely vibration energy, to the test object supplied in the same form again, possibly changed in terms of frequency or intensity, get back. In the acoustic emission analysis, on the other hand, is the output measurement energy always a vibration energy, regardless of the type of energy supplied.

With the exception of the strain gauge technology, all of the above relate to Methods also intentionally on the material of the test object and not as in the invention Process aimed at the geometric shape of the same.

Accordingly, the following types of deformation become in this connection differentiated: 1. Large radial curvature of the longitudinal axis (compression) and / or torsion (Fig. 1) 2. Small radial curvature (blade-shaped bending of the cutting edge or Front area, Fig. 2) 3. Bending of the longitudinal profile (Fig. 3) 4. Tearing along the longitudinal axis.

Common to all of these cases is the transient (temporarily occurring) or permanent (permanent) internal tension, the latter type only occurring as long as the object is not removed from its envelope and thus no internal tension can occur.

The invention is therefore based on the object of creating a method which, despite the inaccessibility of submerged objects, is able to maintain their outer Form during and after driving in for deviations from the original condition to investigate.

The control and thus the measurement of these deformation-accompanying Internal tension is therefore the decisive instrument for avoiding this Deformations.

This object is achieved in that the inner Stress measured indirectly via the natural frequency, which is dependent on the latter and is continuously compared with reference values, these values either theoretically calculated or determined in model tests. From the size of the target deviation conclusions can then be drawn about the type of deformation occurring, whereby the case of the large radial curvature is of further importance is the deviation of the actual direction of advance over the radius of curvature can be determined from the planned.

The direction control is special in the case of sheet pile wall drilling Significance too, because the stress-free driving in of each individual plank is decisive (based on the principle of the weakest link in a chain) on the compressive strength the whole wall.

It is therefore important that each plank is strictly parallel to the one that has already been set otherwise there will be a tension between the two in the divergence pallet Builds planks, which leads to the partial or complete bending of the locks. If it is ensured that no internal stresses * occur when driving in, this also ensures that the screed is correctly set.

The internal stress as the ratio of constraining force to cross-sectional area of the body is because the cross-section during permanent nature of Driving in remains unchanged, a function of the constraining force and this in turn is the counterforce against the external force (impact or pressure force of the ramming or Vibrator). The coercive force is therefore that provoked by the external force Resistance of the body to deformation, which is within certain limits is always proportional to the external force exerted and in the case of pile drivers decreases from top to bottom as a result of increasing frictional losses.

The constraining force and thus the internal tension is, however, as an integral sum over the entire length of the body always equal in magnitude to the external force regardless of the amount of their distribution along the body.

The dependence of the natural frequency position of such a body as an elongated, A structure capable of vibrating from the internal tension can be broken down into a more physical way Derive considerations on the assumption that the material properties remain unchanged and only insignificant changes in length occur. The relation is positive, i.e. with increasing compression or compression, the basic frequency of one increases Free oscillation generated by impact (structure-borne noise) in both longitudinal and also with transverse waves.

The nature of the piling and drilling bodies, the material and Cross-sectional profile designed for high flexural rigidity is suitable especially for this method, because even with small radii of curvature a high internal stress is generated which increases the sensitivity of the procedure.

The determination of uniform or inconsistent distribution the internal tension along the body is solely due to the natural frequency It is possible, in fact, that if the distribution is uneven, according to the occurring different amounts of force a wide range of different Frequencies is generated whereas with uniform distribution (pure compression without propulsion) a very narrow frequency band.

This stress distribution is therefore the essential diagnostic point Instrument for detecting propulsion resistance in the form of natural obstacles (Boulders) or lock shearings in sheet pile piles and thus serves to differentiate between frictional resistance and propulsion resistance. To identify this stress distribution it is sufficient to indicate one of the two sizes; in the following we will refer to the propulsion resistance Reference is made, which is referred to as relative compression.

Besides the qualitative evaluation of the internal tension just explained the quantitative evaluation is important, whereby a distinction must be made here between transient and permanent type. The permanent voltage values are based on absolute values relatively low as they result exclusively from curvatures of the longitudinal axis, the transient values, on the other hand, reach the maximum amounts of the ramming or

Compressive force.

The quantitative evaluation of the internal tension is based on the object-specific limit values for force-wise stress in general and in particular at both the maximum value and the range within which Reliable conclusions can still be drawn about the degree of curvature that occurs.

First of all, the material properties of the respective material are required decisive, essentially the relationship between internal tension and the resulting relative elongation. Knowing this can be based on the amount of the relative Compression determine whether the body has stresses in the elastic, over-elastic or exposed in the fracture area.

The curvatures of the great radial type are generally below the elastic limit (or proportional limit), those of the small radial type between elasticity and the yield point, cracking beyond the breaking point. Thus, especially after model tests have been carried out a qualitative and, within certain tolerances, also a quantitative statement Make the type and degree of expected deformations, provided the course and distribution the transient as well as the course of the permanent internal tension are in the propulsion or constantly registered during breaks.

An application of the invention for sheet pile wall testing during the advance is presented below and explained using various drawings: Fig. 4 shows a sheet pile wall with two neighboring piles in cross section, FIG. 5 the typical one Curve course of a lock disengagement, Fig. 6 the basic structure of the test device, Fig. 7, 8, and 9 the frequency / intensity distribution of a resonance spectrum without and with an additional maximum value and a pure compression.

The decisive criterion for the quality of sheet pile wall drilling is the integrity of the locks, in the optimal case according to FIG. 4, the represents the cross-section of the planks. The one on the outside, in a spiral shape Edged profiles are referred to as locks, which are usually on the Interlock in a hook shape over the entire length of the plank. The end edges run in a wedge shape to achieve a better sealing effect.

The optimal propulsion method is the so-called shaking, whereby the whole Bohle vibrates and at the same time an axial pressure is exerted on the upper end. If the ground is too hard, the ramming method is used, with ramming strokes to be run on the head end.

In both cases it is essential that the attacking propulsive force is directed parallel to the neighboring pile, otherwise with increasing penetration depth the amount the relative compression increases until the locks are bent up and the planks diverge.

This process can be based on the characteristic course of the Diagnosing curve of FIG. 5: The relative compression (Sr)) increases as a result of the increasing friction with increasing depth of penetration (-H) steadily up to the Point at which the flexural rigidity of the locks noticeably inhibits propulsion (1) after which up to the complete bending of the locks (2) a sharp increase in the relative Compression takes place in accordance with the force required for deformation. This is followed by a short-term relief, i.e. propulsion resistance and thus relative compression decrease, however, as a result, due to the for the now continuous curvature of the screed required additional effort, to rise more than in the case of a straight screed.

During this whole process there is also a steady increase in permanent internal tension, expressed here in the increase in natural frequency (Fe) to register.

A similar curve progression also occurs in isolation when smashed Observing boulders, but the changes in incline are more abrupt, brings through the brittleness of the rock and besides, there is no permanent interior Voltage present.

The latter is also missing in the case of cracks that are at the maximum value the relative compression as a function of the distribution of the internal stress reveal.

The reliable diagnosis of the above and other cases is a more detailed matter Investigations on ongoing construction projects and model tests, the ones listed here Examples are only intended to determine the suitability of the relative compression as a parameter for Detect deformations.

For the early detection of this damage progression it is necessary that the Already harvest object-specific limit values must be adhered to and the current Comparison of the current ellen measured values (transient) with case-specific Reference values necessary.

The determination of these reference values is based in all cases on the soil mechanical data of the respective subsoil, which are usually derived from Any geological exploration boreholes that have already been sunk can be derived.

The prerequisite for this, however, is that for each thus identified Layer (sand, clay, rubble, etc.) the coefficient of friction against the rusty steel surface of the sheet pile profile used has been determined beforehand in model tests is taking into account the respective water content. The immensely different Compositions of relatively narrowly defined sediment layers as well as all forms flowing transitions and alternating positions allow a theoretical determination the course of the coefficient of friction with increasing depth either only in exceptional cases or if there is extensive documentation of comparable cases.

Therefore, the so-called pilot procedure is recommended in practice: The route, along which the sheet pile wall is to be set, is first of all with the help of regular Distances of set exploration core drillings (possibly also partially with ram soundings) geologically explored as well as possible. Now the first sheet pile is used as a pilot pile exactly in the borehole of the first on final borehole Niedergbra and the perpendicular position with an inclined core drilling (which the lower edge of the plank in the corresponding depth must be found).

During the advance of the pilot screed, all relevant data, i.e. quantitative and qualitative transient internal tension, propulsion speed and exerted pushing or ramming force recorded and used as reference values for the subsequent Plank drives used. ün: a falsification of the pilot values by the cavity To prevent exploration drilling, the drilling diameter should be as small as possible or the borehole is filled again accordingly the. Any changes to the soil quality along the sheet pile must of course as local adjustments of the reference values of the pilot screed what is easily possible as long as no new types of sediment appear. Isn't this the one? If so, the first screed that cuts the new formation becomes the pilot screed used and in further progress any fluctuations in thickness as a reference value Taken into account.

This process can be repeated any number of times, but must be repeated in each Each individual case should be carried out very carefully as it is not an absolute one it is a relative control procedure.

The measurement method is for ramming and vibrating processes with regard to the Vibration excitation is different, but it applies to both that the structure-borne sound with increasing propulsive force and increasing permanent internal tension, more high-frequency the permanent internal tension has a frequency-shifting effect comes to.

With the pile-driving method, the pile-driving blows themselves can in principle be used for frequency evaluation can be used, other methods must be used for the vibration process: Either you let hammer blows from a suitable apparatus at regular intervals perform or you use a vibration exciter and recognize when driving through of the corresponding bandwidth, the natural frequency of the screed at the relative intensity.

These excitation and measurement operations can all be carried out by the neighboring pile go out, since this is mechanically coupled with the screed to be monitored, on this there are therefore no measuring sensors, thus avoiding an obstruction during the advance and the eating apparatus itself is not overstrained by strong vibrations is exposed.

Fig. 6 shows the basic structure of such a device, the as a compact unit at the head end of the Nact, barboard clamped will.

The vibration exciter (1) consists of an impact weight that is about a lever in the device on the neighboring screed is movably mounted and like a hammer beats in time on the broad side of the screed to be driven. The integrated in the device Structure-borne sound microphone (2) converts the sound events into electrical signals, those in the sound frequency analyzer (3) in natural frequency and maximum frequency signals disassembled and on separate channels the natural frequency comparator (4) and the maximum frequency comparator (5) are forwarded. The penetration depth measuring device (6) supplies both comparators with the current depth information, so that these are the incoming natural frequency and Maximum frequency values with the corresponding reference values on the data carriers (7) and (8) can be compared. When the specified difference values are exceeded an optical and acoustic warning device (9) is set in motion, which also built into the device and thus in the machine operator's field of vision. A central one Registration unit (10) stores all measured and converted data and records simultaneously on an XY multi-channel recorder.

This arrangement is designed for a constant pressure or ramming force, However, if this is changed during the advance, the connection is also made to attach a load cell (11) or another between the ram head and the screed to create a suitable measuring device which continuously measures the force exerted. The frequency shift caused by the change in this force becomes the most favorable compensated by a proportional change in the limit values in the comparators.

In the case of particularly critical planks, it may be appropriate to monitor the data elsewhere by means of remote data transmission from one in the diagnosis of Let specially trained persons carry out damage processes.

The structure-borne sound signal generated by striking the screed from the side according to its evaluation, should be something like in Fig. 7 look like where the different frequency values up to the natural frequency Fo increasing intensities or signal level heights I. The external excitation is always the undisturbed resonance case, the natural frequency can therefore be clearly determined and also the only usable measurand.

With the operational vibration excitation of the ramming, however is uniform distribution of the internal stress and V is present at t t low propulsion resistance assume a curve as in FIG. 8.

As a result of the relative compression, in addition to the natural frequency maximum the separate maximum belonging to the compression can be seen (1) and the curve should be because of the uniform distribution of the internal tension a simple, clear one Have shape. If the distribution is inconsistent, the accumulation results in different Expect frequencies with several maxima.

The uniformity of the internal stress distribution increases with increasing Homogeneity of the subsoil; So the screed crosses very different layers Coefficient of friction, an inconsistent distribution is to be expected. Different Friction coefficients usually result in different damping properties of the layers, which exert a very complex influence on vibrations by constantly changing intensity act and in extreme cases also change the frequency.

The simple case of a complete compression is illustrated in Fig. 9, the characteristic is the low frequency bandwidth, with medium frequency and intensity depend essentially on the impact force.

From the representation of the evaluation options 1es externally excited and the operationally excited Körershallsignal shows that the sheet pile pile driving method it is actually better to check than the Rüttelverfarn.

However, this is not a disadvantage insofar as the jogging method is used in the Relatively problem-free within the scope of its application (only in light subsoil) and the difficult problems only arise when ramming.

The question about the evaluation options with the vibration method is also not to be answered from the start, since it behaves in such a way that the oscillation excitation takes place on an object that is under compressive stress and is being propelled and not on a dormant one, so it is to be expected with some certainty that a modification of the resonance spectrum will take place.

This procedure is in addition to checking the setting up of sheet piling It is also possible to check large walls that have already been set.

Two methods are suitable for this, either the frequency-critical or the critical one.

With the frequency-critical method, each individual screed is equipped with a hit a heavy hammer and determine the frequency of the structure-borne noise. Will If larger frequency differences are registered in a continuous series, then the probability is given by deformations.

Since this procedure is relative, it would be contingency in some cases first of all a calibration frequency if the piles of the same row have been set correctly with certainty to determine with which the following measurement results are to be compared.

In this case, the technical equipment would look like this: A hammer to strike, a structure-borne sound microphone connected to a portable one Frequency analyzer and a comparator with a pre-programmable calibration frequency, its Difference values together with the entered screed number are recorded in a memory unit will.

The possibilities of this method are limited to the degree the internal tension, lock disengagement and thus leakage cannot deal with Certainty and 'crack formations cannot be ascertained at all.

In the case of the intensity-critical method, the distribution of vibration nodal lines examines how it works on vibrating plates in the form of Chladny sound figures appear.

In the middle of the top edge of a sheet pile wall (Fig. 10) generates a vibration exciter Vibrations in the natural frequency range of the wall or in another suitable frequency to form standing waves, which by the localization of their nodal lines (intensity minima) are determinable.

Now draw the distribution of the If the sheet pile wall is in perfect condition, one obtains a regular arrangement as in Fig. 10. However, if the registered arrangement is irregular, In any case, there are internal tensions and possibly also lock shearing. These could possibly be identified insofar as the vibrations the at the edges of the open locks experience additional flexion Use of special investigation techniques filtered out of the frequency spectrum could be.

In the simple case, a vibration exciter would be required in terms of equipment variable frequency, a structure-borne sound microphone and a signal level meter with data storage and data identification input.

Both methods are less suitable for damage diagnosis purposes than for a blanket coverage of zones with different levels of probability of damage inside a sheet pile wall.

The application of the sheet pile wall test methods presented here to drill rods, Pillars, ground anchors and other objects is just a matter of adaptation the measuring arrangement to the special requirements of the test object.

It is also not necessary for the application of the procedure, as Measured variable for the vibrations to be examined necessarily the variable frequency use, other suitable parameters such as the size acceleration can for example are used if appropriate analysis devices are available.

The types of analyzers that can be used range from simple, portable or measuring devices installed in motor vehicles up to multi-channel computer-aided ones Analyzers that carry out modal analyzes based on the principle of Fast Fourier analysis.

The interpretation of the diverse data regarding such an analysis the deformation of the object, taking into account the influence of the composition of the substrate is ultimately a matter of in-depth practical and theoretical investigations, carried out both on ongoing construction projects and in the context of model tests Need to become.

The effort required for this appears, however, in view of the Follow-up costs of damage to sheet pile walls mentioned at the beginning are entirely justified.

When using the direction control method for drilling In the subsurface as well as in material processing, the friction between drill rods falls or the drill bit and surrounding material largely gone thanks to the flushing the number of variables to be measured is reduced to the permanent internal tension. This can be done in the simplest way with the frequency-critical method for finished Sheet pile walls are measured and the bending stiffness is mathematically taken into account can be expressed as the radius of curvature from which the drilling direction can be extrapolated emerges.

By suitable adaptation of the arrangement shown in Fig. Ó can also continuous monitoring can be achieved.

which responds to a critical: material load and thus helping to prevent expensive downtimes or leakage from boreholes.

Claims (20)

Method for measuring the deformation of submerged piling and drilling bodies Claims 1. A method for measuring the deformation of submerged ramming or Drill bodies, characterized in that after they close individually or in association Vibrations were excited, these vibrations on natural frequency position, frequency bandwidth and maximum frequency analyzed and depending on the depth of penetration of the body with theoretically calculated or determined in model tests, in each case of deformation-related changes in the resonance frequency spectrum free reference values is compared, according to which resulting target deviations as a measure of the degree of Deformation can be displayed or used as a controlled variable. 2. The method according to claim 1, characterized in that any further suitable resonance oscillation characteristics measured and with corresponding Reference values are compared. 3. The method according to any one of claims 1 to 2, characterized indicates that suitable for the excitation of the oscillation, through the specific propulsion technique conditional vibrations can be used and evaluated. 4, method according to one of claims 1 to 2, characterized in that that in the case of non-usable propulsion-related vibration excitation a suitable one Excitation either by a mechanical impulse in the form of a dampened, decaying one Vibration or by a vibration exciter in a continuously frequency-changing mode of operation or at another suitable frequency. 5. The method according to any one of claims 1 to A, characterized in that that the resonance vibration analysis with frequency analyzer or acoustic emission analyzer takes place and the determination of the natural frequency in the case of the variable frequency Excitation based on the signal level maximum. 6. The method according to any one of claims 1 to 5, characterized in that that the analysis equipment used for the computer-aided modal analysis of vibration processes are suitable. 7. The method according to any one of claims 1 to 6, characterized in that that based on the evaluation of the sound signals according to frequency bandwidth the distribution the internal stress and the coefficient of friction and damping properties assigned to the layers traversed by the object. 8. The method according to any one of claims 1 to 7, characterized in that that the frequency and signal level of the shortest-wave oscillation contained in the spectrum measured and used as a criterion for small radial curvatures or tears will. 9. The method according to any one of claims 1 to 8, characterized in that that the recording of the body vibrations either with a structure-borne sound microphone, force, Acceleration or vibration sensors are carried out and the evaluation is carried out accordingly Analysis devices also in parameters other than the size frequency is possible. 10. The method according to any one of claims 1 to 9, characterized in, that the vibration recording devices of any type either on the vibrating body itself or on a mechanically vibrationally coupled to the body to be measured second body are attached or an already existing one or to this Use the transmission medium introduced for the purpose. 11. The method according to any one of claims 1 to 10, characterized in, that the measurement of the depth of penetration of the body is continuous and relative to one absolute or relative fixed point, the latter in the case of sheet piling tests can also be the last set screed. 12. The method according to any one of claims 1 to 11, characterized in that that during the sheet pile wall test, the pile being driven is on top of the neighboring pile mounted friction wheel pressing against its side lock edge set in rotation and a pulse frequency proportional to the speed by means of a suitable measuring arrangement is used to determine the penetration depth. 13. The method according to any one of claims 1 to 12, characterized in that that when used for sheet pile wall testing, the geological connection boreholes determined soil mechanical characteristics depending on the depth traveled can be used to calculate or determine the reference data. 14. The method according to any one of claims 1 to 13, characterized in, that the device unit required for the sheet pile wall test, consisting of the penetration depth sensor, Vibration transducers, ramming or compression force transducers as well as frequency analyzers with data storage and processing device, using a clamping device on the last driven screed is mounted and via acoustic and optical signaling devices to address the machine operator. 15. The method according to any one of claims 1 to 14, characterized in that that the device unit has a data memory for recording all measurement and evaluation data and has an XY multi-channel recorder or to a remote data transmission system can be connected. 16. The method according to any one of claims 1 to 15, characterized in that that in the so-called pilot process the first pile of a sheet pile wall in the center of an exploration borehole is brought down and all relevant registered during the drilling process Jacking and vibration data used as reference values for the subsequent bores will. 17. The method according to any one of claims 1 to 16, characterized in that that the pilot process as often as desired when placing a continuous sheet pile wall can be repeated as soon as a change in the soil mechanical data occurs. 18. The method according to any one of claims 1 to 17, characterized in, that for frequency-critical testing of already installed sheet pile walls every pile individually set in suitable vibrations, after which the frequency position of the resonance vibrations determined by means of a transducer and analyzer and saved in the device at flawless sheet pile walls or sheet pile wall parts compared to the recorded reference values will. 19. The method according to any one of claims 4 to 9, characterized in, that for the intensity-critical examination of already installed sheet pile walls, a vibration exciter, Mounted on the upper edge of the sheet pile wall, standing waves in the natural frequency range generated whose vibration maxima and minima by means of the along the upper edge Acoustic emission analyzer can be determined, the narrow frequency bandwidth of the resonance vibrations and their regular arrangement along the top of the sheet pile wall serve as criteria for the wall's absence of deformation. 20. The method according to any one of claims 1 to 19, characterized in that that when testing drill rods or other propulsion bodies and drill bits in the natural or artificial subsurface or in materials based on the resonance oscillation the deviation of the actual drilling direction from the intended one is calculated and impending material breaks can be detected.