METHOD FOR LOAD MONITORING AND DETERMINATION OF THE LIFESPAN OF GEOCLASTIC-REINFORCED EARTH BODIES

Description

[0001] The invention relates to a method for load monitoring and for determining the service life of geosynthetic-reinforced earth bodies with the features of the preamble of claim 1.

[0002] Geosynthetic reinforcements, such as geogrids in particular, serve, among other things, to significantly improve the stress-deformation behavior of a reinforced soil composite body, so that it is consequently able to withstand significantly increased external stresses and internal forces without damage and with minimal deformation compared to an unreinforced soil body.

[0003] Geosynthetic reinforcement plays a crucial role in many applications, ensuring the stability and serviceability of the overall structure. Failure of the reinforcement can therefore lead to the immediate failure of the entire structure. Examples of such applications include sinkhole bridging, dam foundations on vertical load-bearing elements, and reinforcement layers in surface sealing systems. Besides their comparatively low cost, geosynthetic reinforcement offers numerous advantages in construction practice. Furthermore, its use enables a significant reduction in the so-called CO₂ footprint and the conservation of mineral resources such as gravel, crushed stone, or sand.

[0004] However, successful use requires that the time-dependent material behavior is adequately considered. The strength properties of geosynthetic reinforcements are essentially influenced by the following parameters: chemical and physical environmental conditions, ambient temperature, load duration, load degree

[0005] A mutual influence of these parameters cannot be ruled out in principle.

[0006] In particular, the time- and load-dependent tensile force and strain behavior – relaxation and creep – is therefore a crucial design parameter. An important characteristic here is the nonlinearity of these processes; that is, reducing the load level, for example, does not merely double the time until creep failure occurs, but increases it many times over, and vice versa. Knowledge of the load level thus has a fundamental influence on the expected service life of geosynthetic reinforcement.

[0007] When designing geosynthetic-reinforced earth structures, this specific material behavior is taken into account by separately investigating failure and serviceability states. In the design for the failure state, partial safety factors are applied, which mathematically increase the loads and mathematically reduce the resistances. The combination of these factors yields the desired safety level. In the serviceability test, characteristic properties are used to realistically model the actual deformation behavior and assess the true deformation states.

[0008] The assessment of the degree of stress is therefore essential in order to make a statement about the service life of a building, which in turn has a significant influence on safety in public spaces and the resulting economic decisions.

[0009] In their installed state, geogrids can currently only be measured by recording deformations and using these to assess the degree of stress. This involves measuring the state of deformation over time and correlating it with tensile force-strain curves. These curves are generated under laboratory conditions, which means that external influences can only be selectively considered. For example, humidity and temperature in laboratory measurements are controlled (constantly) according to standard climate conditions, but therefore do not typically reflect the actual behavior of these parameters. Furthermore, the accuracy of the deformation measurement depends on the chosen measurement system and is thus subject to measurement errors. The same applies to determining the properties in the laboratory. Therefore, deriving the degree of stress from deformation measurements under real-world conditions and laboratory measurements is associated with considerable uncertainties.

[0010] In order to accurately determine the actual load state of the geosynthetic reinforcement in the structure, a deformation measurement must also be carried out from the beginning of the loading, i.e., directly after installation and before the cover, because subsequent strain measurements only allow statements about differential changes in the load state during the observation period.

[0011] Determining the degree of stress from deformation measurements is therefore not always unambiguous: Over the service life, the effects on the reinforcement can change, for example, due to redistribution processes or altered loads. It is therefore impossible to determine whether a measured deformation is caused by creep deformation due to constant loading or by a potentially temporary increase in the degree of stress. Assigning a measured deformation to a degree of stress is only possible if the other influencing parameters are known and it is assumed that the load is constant.

[0012] Electrical sensors, such as inductive displacement transducers, are often used to measure deformation in the field, providing information about the deformation state of the geogrid at specific points for the respective sensor location.

[0013] Alternatively, deformation can be measured, for example, by incorporating a polymer optical fiber into the lattice structure. This is described, for instance, in CN 203320507 U. Deformations can be detected and, if necessary, localized using this method. However, force measurement is not possible with this technique.

[0014] As an alternative to polymer optical fibers, a suitable optical fiber can also be used for distributed measurement of deformations. Optical fibers allow for higher measurement accuracy; however, their application range is limited to small strains, approximately 1%.

[0015] WO 2010 / 085243 A1 specifies a measurement method for geosynthetic structures based on the addition of electrically conductive fillers to the polymer used to produce the structure. However, this makes the structure expensive and alters the mechanical properties of the material.

[0016] An early warning system for the early detection of sinkholes and similar phenomena is described in KR 101244304 B1. However, this system uses pressure sensors that measure the ground pressure perpendicular to the plane of the geogrid. The weight of the overlying soil layers does not directly correspond to the tensile stresses on the geogrid. Therefore, while a change in the ground can be detected, it cannot be determined whether the geogrid, installed as a safety barrier, is only moderately stressed by this change, thus requiring no immediate repairs, or so severely stressed that immediate intervention is necessary.

[0017] Also known are so-called fiber Bragg gratings. These are fiber optic sensors embedded in the base structure of geosynthetic reinforcements. Fiber Bragg gratings enable high-frequency and comparatively high-resolution measurements. As with distributed fiber optic measurement technology, the operating range of fiber Bragg gratings is limited to approximately 1% strain. Furthermore, fiber Bragg gratings, like polymer optical fibers, are extremely temperature-sensitive, as the fiber undergoes a change in length under the influence of temperature. The refractive index also changes. Without temperature compensation, strain would be measured in such cases that was not caused by mechanical stress, but solely by temperature fluctuations. Compensating for the temperature influence requires sophisticated equipment and computational effort.

[0018] CN 203 320 507 U refers to a stress measurement in a geogrid, but makes no statements about how the specific sensor arrangement in the geogrid should be carried out.

[0019] US 4,429,580 A refers to a textile fabric, not a geogrid. However, the described mechanisms of action in the load strands of a grid differ from those of a multiaxially loaded, solid-surface textile.

[0020] US 2016 / 341614 A1 concerns the monitoring of a geogrid for the detection of sinkholes, whereby all stresses are to be measured from the edge. This is to be achieved through a circular and multi-layered cross-sectional structure of the grid components. The detection method is not applicable to grids with ribbon-shaped load strands.

[0021] Publication DE 20 2004 020 950 U1 relates to a device for determining the tensile force in tensioned traction elements, in particular in cables in elevators. An insert sensor that is inserted into a cut strap is not disclosed. A surface-mounted sensor designed for testing wire ropes for elevators is described. However, metallic wire ropes exhibit a different relationship between tension and strain than plastic geogrids.

[0022] The publication XP055623152, H. Zanzinger, Gartung, E.: 7th International Conference on Geosynthetics, 2002-09-01, Nice, France, URL: https: / / www.researchgate.net / profile / Helmut Zanzinger / publication / 305723319 Performance of a geogrid reinforced railway embankment on piles / links / 579c784f08ae5d5e1e14b819 / Performance-of-a-geogrid-reinforced-railway-embankment-on-piles.pdf describes load monitoring of a geogrid. Stress and strain are measured directly at the geogrid; however, the resulting values ​​are correlated with the results of simultaneous measurements of vertical ground displacements near the measuring point, thus questioning their validity.

[0023] The object of the present invention is therefore to determine the current load state of a geogrid before installation and, in particular, also afterwards, i.e., when the installed geogrid is already covered with at least one cover layer, and, if necessary, to be able to make a prediction of the service life of the geogrid by considering this over time.

[0024] This problem is solved by a method for load monitoring and for determining the service life of geosynthetic-reinforced earth bodies with the features of claim 1.

[0025] The invention is therefore based on one or more point-based force measurements on the geosynthetic using externally mounted force transducers. The advantage of using such a measurement method for the present application lies particularly in the fact that a force measurement is unambiguous. The force is measured directly and not – as in the prior art – indirectly calculated via strain measurement and material properties that are subject to the influence of time and the environment.

[0026] The use of a geogrid as geosynthetic reinforcement in a structure is particularly advantageous because, in addition to its primary reinforcement function, it also forms a measurement grid. This allows for the simple placement of a sensor at a specific location or multiple sensors in a two-dimensional measurement setup. Placement can be determined simply by counting the meshes and requires no other surveying work. The geogrid thus serves as both reinforcement and sensor carrier.

[0027] For the practical implementation of the method according to the invention, a further advantage arises from the fact that neither a special selection of a particular geogrid type nor a special dimensioning is necessary.

[0028] Furthermore, a grid structure offers the advantage that a force sensor can easily be attached to the individual strands in the longitudinal and / or transverse direction. The force sensor, positioned at least at a single point and attached to the outside of the geosynthetic reinforcement, allows for the subsequent non-destructive determination of the load level at any time, provided the sensor has been calibrated beforehand on the geogrid being measured or using reference samples. Therefore, an existing structure can be measured at any time.

[0029] The geogrids used in the inventive method are generally woven or raschel-patterned geogrids, i.e., those forms in which more pronounced load strands are provided, running in the direction of the load, as well as less pronounced cross-webs that do not enable any significant load transfer but merely define the geometric position of the parallel load strands, thus being necessary for the grid structure. Therefore, each load strand can be considered computationally on its own, whereby influences from neighboring strands are negligible.

[0030] The force measurement method according to the invention can be easily transferred to geogrids manufactured in other ways, in particular to all forms of so-called laid geogrids made of welded and bonded rods and strips or stretched and drawn geogrids. Furthermore, its use is also possible on geogrids that have the same strength properties in the unrolling direction as well as perpendicular to it.

[0031] According to the invention, a further possible measure is to interrupt at least one of the load strands in the geogrid and close it again by installing a force-measuring sensor unit that forms a bridge. The force-measuring sensor unit comprises at least one strain gauge applied to a substrate, preferably a metallic one. Thus, the acting forces can be calculated from the deformation of the sensor base element on which the electronic sensor element is mounted, in a manner known per se. The sensor element has at least one strain gauge. The relationship between the strain of the sensor base element and the force is defined by Hooke's law. This makes it possible to measure the force in a load strand of the geogrid at a measurement position that represents a partial area of ​​the geogrid.For this purpose, at least one load string is disconnected and then reconnected, whereby any slack is removed from the disconnected load string either directly during or after the installation of the force-measuring sensor unit. The voltage in the load string bridged by the force-measuring sensor unit is therefore preferably just slightly above zero. In some cases, a higher preload can be applied, provided that a preload is also present in the adjacent load strings.

[0032] This sensor arrangement is very easy to mount, and only a few sensors are needed per area, whereas according to the state of the art, all strands of the grid must be specially equipped in order to be able to record measurement data.

[0033] Surprisingly, according to the aforementioned embodiment of the invention, a self-leveling measuring system can be created simply by inserting the force measuring sensor unit into the previously interrupted load train:Due to the large number of adjacent load strands, the applied tensile force is distributed essentially evenly across the parallel load strands in the geogrid. If the prestress is too high when installing the force-measuring sensor unit in the interrupted load strand, this load strand will initially be subjected to a higher load than the neighboring strands. However, the higher partial force in the load strand equipped with the force-measuring sensor unit also leads to faster creep behavior; that is, the load strand made of a polymer material is stretched more than the others. As a result, after a relatively short time, the interrupted load strand and its neighboring strands are stretched uniformly and subjected to approximately the same forces. Conversely, if the prestress in the interrupted load strand is too low, the neighboring load strands will be subjected to a higher load than the interrupted load strand.In this case, too, creep behavior occurs, so that after a certain time delay, the situation arises where all adjacent strands are stretched evenly and transmit correspondingly equal partial forces.

[0034] To create a self-leveling measurement system, it is only necessary to secure the force-measuring sensor unit in the interrupted load string in such a way that the string ends cannot pull out of the force-measuring sensor unit, and to ensure that the fastening is tight enough to allow the self-leveling effect to occur. If the connection is significantly too loose, the deformation path in the adjacent strings would not be long enough under a typical load situation to induce tension in the interrupted load string. In other words, when installing the force-measuring sensor unit, care must be taken to ensure that the preload in the load string is greater than or equal to zero. As soon as a load is applied to the geogrid, a tensile stress is generated, which also leads to strain in the load string containing the force-measuring sensor unit.

[0035] This self-leveling effect allows for a purely computational, yet reliable, calculation of the load on several parallel adjacent strands using only a few integrated sensor elements. Crucially, the geogrid used must be designed in such a way that no force is transmitted between the parallel load strands via the crossbeams.

[0036] Temperature compensation can be easily achieved by using a known design of the force measuring sensor unit with at least two strain gauges offset by 90° to each other.

[0037] According to the invention, at least one sensor element is connected to a data acquisition and monitoring unit. Forces can be measured continuously or at predetermined intervals, and tensile stresses can be calculated from these measurements.

[0038] Should shear stresses occur in the structure that act parallel to the plane of the geogrid, strains are induced in the geogrid that can be detected using the method according to the invention. In the event of impending damage to the structure, a warning can be issued or even preventive action can be taken in time.

[0039] Damage can also occur due to displacements or erosion in the plane where the geogrid is installed, causing the geogrid to lose its edge restraint. As a result of such processes, tensile stresses in the load strands can be completely relieved, allowing such a failure to be detected early by monitoring the data provided by the force-measuring sensor unit in the geogrid.

[0040] When voids form in the substructure of the geogrid, the load acting from above remains, but the support for the loads acting on the geogrid from above has failed. Due to the stress exerted by forces perpendicular to the laying plane of the geogrid, high tension develops in the geogrid in the area of ​​the defect. According to the invention, this stress can be detected early, allowing countermeasures to be taken before a sinkhole extends to the top of the structure.

[0041] Based on the load levels measured in the field, the service life of the geosynthetic can then be determined. Various damage accumulation models are available for this purpose: from linear to nonlinear models, which take into account the influence of the geogrid's load history on its service life in a complex manner. All existing long-term strength criteria consider the fact that micro-damage accumulates in the material long before the geogrid fails. To illustrate this, the concept of total damage (sg) is introduced. If the total damage exceeds 1, immediate failure is to be expected when the critical load is reached.

[0042] As an example, the total damage in a geosynthetic made of polymeric material can be determined as a function of force or stress according to MOSKVITIN using the following formula: η = m + 1 ∫ 0 t t − τ m dτ t 1 + m σ τ With: t Time for the break m Constant that takes into account the non-linearity of damage accumulation; is determined experimentally. t [ σ ( τ )] experimentally determined stress function

[0043] The time to failure at constant stress can be approximated for most geogrids by the power law: t = Bσ − b With: B, b - empirically determined quantities

[0044] Taking into account the power law dependency, the total damage to the geosynthetic takes the following form: η = 1 + m B 1 + m ∫ 0 t t − τ m σ 1 + m b τ dτ

[0045] Using the measured values ​​from the field, the load collectives σ can now be recorded as a function of time and approximated by a "best fit function".

[0046] The determination of the total damage and the estimation of any remaining lifespan can then be carried out using appropriate calculation algorithms, e.g. with the help of software.

[0047] For the practical implementation of the method according to the invention, a geogrid measuring arrangement is used which, in addition to the geogrid, comprises at least one force sensor which is either attached to at least one strand of the geogrid from the outside or which is inserted into an interrupted strand and closes it again.

[0048] According to the invention, a further advantage arises from the fact that neither a specific selection or design of a particular geogrid type nor a specific dimensioning of the geogrid measuring arrangement is required. This also makes it possible to retrofit existing geogrid reinforcements by attaching one or more sensors and thereby creating a geogrid measuring arrangement.

[0049] The location and number of sensors, as well as the positioning relative to each other in the case of multiple sensors, are determined depending on the structure to be tested or monitored.

[0050] The selection of measurement points is governed by the general principles of experimental stress analysis. Therefore, when designing a mechanically stressed structure for optimal function, it is necessary to gain knowledge about the nature and local distribution of stresses through theoretical considerations and appropriate analytical calculations according to recognized technical standards or, for example, using the finite element method.

[0051] The decisive factors for earthworks are the maximum forces occurring in the geogrid, which ultimately determine the nominal strength of the geogrid. These forces must be determined in advance as accurately as possible, considering their magnitude, location, and the intended service life of the structure.

[0052] Experimental force analysis offers the possibility of verifying the assumed force distributions within the structure and thus serves as a link between theoretical calculations and the conditions in the actual structure. Ideally, the number and location of the required sensors should therefore be determined in advance as part of a sensitivity analysis.

[0053] The force sensors are positioned accordingly at critically stressed points or at points of particular interest, depending on the geometry of the structure and its load condition.

[0054] If there is no foreseeable critical zone, then it makes sense to equip the geogrid to be surveyed with a correspondingly generous number of sensors.

[0055] A significant advantage of the force measurement according to the invention is that a force sensor can be retrofitted to the geogrid if it is reasonably accessible, and in some cases, measurements can be taken at any number of points with just one sensor, albeit with a time delay. Furthermore, no special installation method for the geogrid is required.

[0056] Attaching the force sensors to the geogrid to create a geogrid measurement setup is remarkably simple. After calibrating the sensor for the geosynthetic material being surveyed, the sensor simply needs to be attached to the geosynthetic on-site using a simple tool such as a wrench.

[0057] The invention distinguishes between a geogrid measuring arrangement with an attachment sensor or with an insertion sensor: With an external sensor, the sensor device is attached to a continuous strand of the unaltered geogrid from the outside. With an internal sensor, a strand of the geogrid is cut, and the sensor housing then closes the interrupted strand.

[0058] The calibration of the sensor is performed individually for each geogrid type, as the sensor sensitivity depends on the cross-sectional geometry of the tension member. For this purpose, before installation, the grid element—that is, the respective grid strand with the installed sensor—is clamped in a tensile testing device and subjected to a defined tensile force. The sensor signal is then determined as a function of the tensile force in the grid element and stored as a reference measurement. These reference values ​​can be accessed during subsequent installation in the geogrid measurement setup. Pre-tensioning is not required for the sensor during or after installation. This is because the spaced abutments on which the strand rests are designed as sliding bearings, preferably with lateral guidance. Tightening occurs automatically when the geogrid is loaded.In slope structures, this stress typically occurs as soon as the first layers of cover are installed. In horizontal installations, e.g., for sinkhole stabilization, the tightening only occurs in the event of damage and triggers a corresponding signal.

[0059] Another advantageous embodiment of the method according to the invention involves combining it with deformation measurement, e.g., a distributed sensor system for detecting the most heavily loaded areas of the structure along the entire reinforcement length. This allows for further improved statements regarding the degree of load.

[0060] In addition to the aforementioned considerations for identifying the correct measuring point, a combination of distributed measurement of the deformation, e.g. with POF sensors (polymer optical fibers), is useful.

[0061] The invention is explained in more detail below with reference to an exemplary embodiment of the method and the drawings. The figures show in detail: Fig. 1: A section of a geogrid with a broken load string and a force-measuring sensor unit in a top view; Fig. 2: A section of a geogrid with a force-measuring sensor unit inserted into a load string in a top view; Figs. 3 and 4: Sections of a geogrid in a schematic top view with an incorrectly installed force-measuring sensor unit in a top view; Fig. 5: A diagram showing the force profile measured in a load string over time; Fig. 6: A landfill with surface sealing in cross-section; Fig. 7: A force-measuring sensor unit according to a first embodiment in a perspective view; Fig. 8: A force-measuring sensor unit according to a second embodiment in a perspective view; Fig. 9: The force-measuring sensor unit according to Fig. 7on a geogrid, in a perspective view from above; Fig. 10 the force measuring sensor unit on the geogrid, in a perspective view from below; Fig. 11 the force measuring sensor unit with a cover, in a perspective view from above; and Fig. 12 the force measuring sensor unit on the geogrid, in a side view.

[0062] Figure 1 Figure 1 shows a schematic view of a section of a geogrid 10 with several parallel load strands 11, 13, which are connected to each other by textile crossbars 12. At a measuring position 1, the load strand 13 is cut to allow the placement of an inset sensor.

[0063] Figure 2Figure 1 shows the completed geogrid measuring arrangement 100 with the laid geogrid 10 before the installation of the cover layer. The free strand ends 13.1, 13.2 of the severed load strand 13 have been reconnected via an inserted force-measuring sensor unit 20. For this purpose, the force-measuring sensor unit 20 has a terminal element 21, 22 on each side, into which the strand ends 13.1, 13.2 are enclosed. At least one of the terminal elements 21, 22 is adjustable axially, i.e., in its position in the longitudinal direction of the load strand 13, and relative to a base element 23.

[0064] Figure 3Figure 100 shows the geogrid measuring arrangement again, deliberately illustrating that the length of the section cut from the load strand 13 at measuring position 1 was not correctly dimensioned and / or the adjustment options for length matching between terminals 21 and 22 on the force-measuring sensor unit 20 were not used correctly. The load strand 13 bridged by the sensor element 20 is therefore shorter than the adjacent load strands 11. Consequently, all forces in this section of the geogrid are transmitted exclusively via load strand 3 and the force-measuring sensor unit 20. The measured forces there are therefore higher than expected for the geogrid as a whole, but still insufficient to tighten the neighboring load strands 11 and thus achieve a uniform distribution of forces across the parallel individual strands 11 and 13.

[0065] In Figure 4The reverse case is shown. Here, the distance between the terminal elements 21, 22, which clamp the strand ends 13.1, 13.2 and connect them to the force-measuring sensor unit 20, is too large after assembly, resulting in loop formation in the load strand 13. Due to the strains occurring in the adjacent load strands 11 during normal operation, the loops are no longer tightened, and consequently, no forces can be measured at the force-measuring sensor unit 20.

[0066] Therefore, for the use of a geogrid measuring arrangement 100 with an insert sensor for force measurement, it is essential that the strand ends 13.1, 13.2 together with the force measuring sensor unit 20 are matched in length such that neither the separated load strand 13 nor the adjacent load strands 11 lie untaut when the geogrid is tensioned from both side edges. Ideally, all load strands must lie stretched side by side without pretension or be tensioned with the same pretension.

[0067] In Figure 7 The force-measuring sensor unit 20 is shown in perspective as an insert sensor according to one possible embodiment. It comprises a base element 23, which has a threaded bore on each of its two end faces. A terminal element 21, 22, each with an external thread, is screwed into each of these bores. The strand ends 13.1, 13.2 (see Figure 2The strands are guided through a bore in the terminal elements 21, 22 and clamped, glued, or thickened and positively locked on the side facing the base element 23 by a splice or knot. If the locking is achieved by a thickening of the strand end accommodated in the terminal element, the bore in the terminal element 21, 22 should terminate in a conical or convex opening to avoid stress concentrations at this point.

[0068] The base element 23 has a measuring surface 24 located approximately at the height of the central axis of the force-measuring sensor unit 20. A strain sensor element 24 is bonded to the measuring surface 24. The base element 23 is made of steel, aluminum, or another homogeneous material with a known modulus of elasticity, so that the forces proportional to the strain measured electronically at the base element 23 can be calculated in the load branches.

[0069] The self-leveling effect in a measuring arrangement 100 constructed and operated according to the invention is described with reference to Figure 5 This is explained. A diagram showing the force curve over time is included. This diagram is based on the following experiment: A section of a geogrid with three parallel load strands is clamped on both sides in a measuring frame with two parallel yokes. Two threaded spindles, each with a load cell, are installed between the yokes. According to the invention, the middle of the three load strands of the geogrid section is separated, with the gap being closed by an inserted force-measuring sensor unit. The terminals are adjusted so that the strand ends are taut but free of preload. A specific force is applied to the three jointly clamped load strands via the measuring frame, specifically three times the calculated nominal force that is intended to apply to each load strand individually. The recorded graph shows the force measured in the middle load strand by the force-measuring sensor unit.

[0070] The study considers a mid-term period of the test between points 2 and 3, during which the target force per strand was set to 400 N. Accordingly, the tested section of the geogrid, clamped between the yokes, was subjected to three times this force, totaling 1200 N.

[0071] The graph shows that the force initially applied to the central load strand was higher than the specified target force. The shortening of the interrupted load strand was slightly too great compared to the lengths of the neighboring strands, leading to an overload of the central load strand when the force was applied. A very short-term, visible peak in the force drop occurs, caused by immediate settling in the connection between the strand ends and the terminals, as well as in the threads of the terminals to the base element. This is followed by a very continuous creep process within the load strand itself, ending at point 3. At this point, the force measured by the force-measuring sensor unit in the load strand has reached the specified target level; that is, the initial differences between the adjacent load strands have been equalized due to creep.From then on, all adjacent load strands will be equally loaded.

[0072] After point 3, the target force per strand is increased to 500 N and the described process is repeated, but is no longer as pronounced, since a tightening has already taken place in the preceding steps.

[0073] When evaluating the test, it must be taken into account that the sample section clamped in the measuring apparatus is only about three times the length of the sensor element. The differences in strain behavior between the two strand ends and the central section formed by the sensor element are therefore significantly greater in the test than in practical applications, where the sections to be monitored can regularly be 10 meters or more. In practice, the influence of the metallic sensor element is thus negligible, and accordingly, it can be expected that after a single force application, the creep effect begins and ceases without recurring under subsequent heavier loads. In any case, multiple abrupt force applications, as in the described test setup, do not occur in an installed geogrid.

[0074] Figure 8Figure 1 shows another embodiment of a force-measuring sensor unit 30, which is designed as an attachment sensor. This means it can be attached to an unmodified single load strand or a crossbar of the geogrid, even retroactively if the geogrid is already under tension. The force-measuring sensor unit 30 comprises a substantially cuboid base 31, which has a raised section with a groove at each of its two lateral ends. This forms abutments 32 for supporting a strand of the geogrid. The lateral flanks of the groove guide the strand and prevent the force-measuring sensor unit 30 from slipping. It is important, however, that the strand can slide freely over the abutments 32 under the assumed forces within the strand, because according to the invention, the intention is not to measure strain, but rather force.Therefore, an important feature of the invention is that in the force measuring sensor unit 30 to be used there is no fixed clamping of the string on the spaced-apart abutments.

[0075] An adjustable yoke 33 is arranged centrally between the abutments 32. This yoke is mounted and secured at its base by two adjusting screws 34 in receptacles 37. The yoke 33 forms a crossbeam that is perpendicular to the alignment of the section between the abutments 32 and positioned centrally thereon. It also has a groove-shaped indentation on its underside to guide a clamped strand of geogrid. The yoke is adjustable relative to the bearing plane formed by the abutments 32 and can be adapted to the thickness of the load strand or crossbeam. However, the yoke is typically extended to its maximum extent.

[0076] A key aspect of the invention is that, when using the force-measuring sensor unit 30, which is designed as an attachment sensor, the sensor is essentially "floating." This means that it is only guided laterally by positive locking. Longitudinally, however, the sensor is not connected to the strand. There is therefore no mechanical clamping or positive locking connection. The sensor is held only by the friction on the strand that occurs when forces are introduced into the geogrid strand. This is significant because no pre-tensioning is required during installation of the geogrid to eliminate any slack. As long as no force acts on the geogrid, no measurement is obtained from the sensor. As soon as a force is applied, the strand gradually tightens, without the force-measuring sensor unit acting as an obstruction.The load initially leads to a change in the shape of the geogrid, which, although not measurable as such according to the invention, does not hinder the invention's objective of monitoring the geogrid under load and / or service life, because the geogrid is not yet under load during the shape change phase. Only as the load increases does the strand become taut, and a tension builds up that can be measured as a force in the sensor.

[0077] The cuboid base 31 represents a bending beam, typically made of steel. Strain gauges are attached to a sensor insert 35 and connected to an evaluation unit via a cable 36. Due to the known geometry and material, the force associated with the deformation of the base 31 can be calculated directly.

[0078] Furthermore, the force-measuring sensor unit 30 is preferably calibrated on the geogrid to be installed itself or on a geogrid of the same type in order to correlate the acquired electrical measurement signals with a force in the load string. For this purpose, the geogrid used as a reference is subjected to a test force in a test fixture, and the resulting electrical measurement value at the force-measuring sensor unit can then later be assigned to the magnitude of the test force. It is also possible to mechanically adjust the yoke 33 until the force measurement value readable by the force-measuring sensor unit 30 corresponds to the applied test force.

[0079] Figure 9 Figure 1 shows a section of a geogrid 100 with load strands 11 and crossbeams 12. A force measuring sensor unit 30 according to the second embodiment is attached to one of the load strands 11, which is located in Figure 7As shown, the load string is attached without having to cut the string. The load string 11 is inserted between the abutments 32 and the yoke 33. A preload can be applied and then changed via the adjusting screws 34 to perform a calibration.

[0080] Figure 10 The arrangement includes an additional cover element 38, which covers the force-measuring sensor unit 30. It can rest on the upper edges of the lateral guides at the abutment 32, but leaves the load string 11 guided there unobstructed. The cover element 38 prevents the load string 11 and / or the yoke 33 from being pinched during the subsequent covering of the geogrid 100, or from subjecting the base 31 to additional forces not caused by forces in the load string 11.

[0081] Figure 11Figure 1 shows the same connection between geogrid 100 and force-measuring sensor unit 30 from the other side of the geogrid 100, i.e., viewed from below in the installed position. In this illustration, the triple support of the load string 11 at the two abutments 32 and the yoke 33 is visible. The illustration depicts the beginning of the assembly process. The load string 11 is already resting on the abutments 32. The yoke 33, which is removable for inserting the load string 11, has already been replaced, but not yet moved into its working position, so the load string 11 has not yet been deflected out of the bearing plane of the abutments 32.

[0082] In Figure 12 The force measurement sensor unit 30 is shown from the side. At this stage, the yoke 33 is positioned over the in Figure 11The visible adjusting screws 34 have been adjusted so that it rests on the housing 31. The distinct deflection of the section of the load string 11 located between the abutments 32, which is pressed downwards by the yoke 33, is clearly visible. The bending stresses induced in the housing 31 as a result are detected by the sensor insert 35 and converted into electrical signals.

[0083] How the expected remaining service life of a geogrid can be determined using the previously described force-measuring sensor unit 30 is described below using the example of reinforcement against slope-parallel sliding of a landfill surface seal. Figure 6 The cross-section of a slope 200 at a landfill body 203 is shown in section.

[0084] The following steps are required to prepare for the subsequent determination of the service life: Determination of a representative design cross-section based on terrain models of the planned surface sealing. Selection of suitable geosynthetic components of the sealing system from a functional perspective (sealing). To prevent the sealing element of a state-of-the-art multi-layer structure with a layer package 204, consisting of sealing, protective layer, and drainage, as well as a vegetation soil layer 201, from being subjected to tensile stresses, it is necessary that the sealing element exhibits lower friction on its upper surface, and ideally also on its lower surface, than the overlying boundary layers. If the slope of the embankment 200 with the sealing system is to be greater than the angle of friction at this critical layer boundary, a geogrid 100 must be used, which absorbs the deficit in holding forces in the layer package 204 and at the anchorage on an embankment crest 202.

[0085] The tensile strength of the geogrid 100 and its anchorage are calculated separately for the ultimate limit state and the serviceability limit state based on established recommendations, standards, and approvals, e.g., EBGEO and BAM approval. The analytical and, where applicable, numerical calculations performed in this context allow for a relatively precise determination of the location of the maximum tensile force in the geogrid 100. This is typically the inflection point 205 at the crest of the slope 202, where the anchorage of the geogrid 100 begins and from which the tensile forces successively applied to the slope 200 are dissipated along the downward slope.

[0086] Based on the design, the sealing system is first professionally constructed layer by layer on the landfill body 203, thus creating the layer package 204. After the geogrid 100 is laid on top, it is preferably equipped directly with a force-measuring sensor unit, as the effort required for this is less than for subsequent installation.

[0087] After measuring the points identified from the analytical analysis as the location of maximum tensile force, such as the bending point 205, the force measuring sensor unit 30 for the geogrid 100 must first be calibrated. This can be carried out in advance for the force measuring sensor unit 30 under laboratory conditions, which is advantageous when dealing with a larger number of measuring points.

[0088] To maintain flexibility, the force-measuring sensor units 30 can also be delivered without calibration and then adapted on-site to the specific geogrid installed at the measuring point. This geogrid is prepared for mounting using a mechanism adapted to the base length of the force-measuring sensor unit 30. A suitable option for this is a frame made of two threaded rods to which two crossbeams are attached at the top and bottom. Each crossbeam is designed so that one or more geogrid strands can be attached to the crossbeam using a clamping plate. The frame is also equipped with one or two calibrated load cells. After clamping the geogrid, deformation can be gradually applied using a wrench, which is then registered by the load cells.Once the calibration values ​​have been stored in the measurement data acquisition for the corresponding sensor, the mounting frame with the mounting sensor can be removed and, if necessary, used for mounting another force measurement sensor unit elsewhere.

[0089] After cabling and connection to a data acquisition unit such as a data logger, construction work can continue. All tensile forces occurring at this point in the geogrid 100 are recorded and, depending on the design of the data acquisition unit, can be read out online or at specific measurement times.

[0090] Based on the short-term tensile strength determined in the laboratory, the actual utilization rate of the geosynthetic reinforcement can now be determined at any time, forming the basis for subsequent calculations of the remaining service life of the reinforcement. Whether any change in tensile strength arises from creep or relaxation processes, or from a change in the force distribution within the sealing system, is irrelevant.

[0091] The procedure described above is to be carried out analogously when retrofitting the force measurement sensor unit 30 to the slope 200. Since the geosynthetic reinforcement is already under tension and an additional load during calibration in the field could potentially result in an unacceptably high degree of stress, prior calibration of the sensor to the geogrid to be instrumented in the laboratory is preferable.

Claims

1. Method for load monitoring and for determining the operational life of geosynthetic reinforced soils, having at least the following steps: - installing a geogrid (100), which has a plurality of parallel load lines (11, 13) connected to each other via cross bars (12), in a building (200), - attaching a sensor unit (20, 30) to at least one measurement position (1) arranged on one of the load lines (11, 13) and / or cross bars (12) and / or at a point of intersection between a load line (11, 13) and a cross bar (12), before or after covering the geogrid (10) with at least one covering layer (204); - connecting the sensor unit (20, 30) to an electronic evaluation and / or monitoring unit; - comparing the measurement data provided by the sensor unit (20, 30) with at least one threshold value and / or reference value; wherein the sensor unit used is a force measurement sensor unit (20, 30) in which the force acting on the load line (11, 13) and / or the cross bar (12) at the measurement position (1) is measured by means of an electrical sensor, characterized - in that a force measurement sensor unit (30) designed as an attachable sensor is used, which sensor is designed to be attached to a load line (11, 13) or to a cross bar (12) of the geogrid (100), wherein the load line (11, 13) or cross bar (12) rests on two abutments (32) spaced apart from each other and is deflected out of the support plane of the abutments (32) from the other side by means of a yoke (33) which is coupled to at least one load cell, and wherein the force measurement sensor unit (30) is mounted in a floating manner on the load line (11, 13) or cross bar (12); - or in that a force measurement sensor unit (20) designed as an insertable sensor is used and, in order to attach the force measurement sensor unit (20) designed as an insertable sensor, the following further steps are carried out: ∘ disconnecting at least one load line (13) at at least one measurement position (1) in the geogrid (100); ∘ connecting the line ends (13.1, 13.2) of the interrupted load line (13) by means of the force measurement sensor unit (20) which comprises at least two terminal elements (21, 22) for each line end (13.1, 13.2), and at least one base element (23) which is arranged between the terminal elements (21, 22) and has at least one electrical strain sensor element (24); ∘ eliminating looseness in the load line (13) or applying a preload when inserting the force measurement sensor unit (20) into the interrupted load line (13) or after; ∘ connecting the sensor unit (20) to the electronic monitoring unit; ∘ covering the geogrid (10) with at least one covering layer; ∘ calculating the force in the interrupted load line (13) of the geogrid (10) by means of the strain measured in the force measurement sensor unit (20).

2. Method according to Claim 1, characterized in that the force measurement sensor unit (20, 30) is calibrated prior to being installed on the geogrid (100) that is to be subsequently installed, or on a geogrid of the same type, in order to ascertain a correlation between the electrical output signal and the force existing at the measurement point in the load line (11, 13) and / or cross bar (12), whereby at least one test force is applied to the geogrid (100) and a corresponding electrical measurement value at the force measurement sensor unit (20, 30) is ascertained.

3. Method according to Claim 1 or 2, characterized in that a slope (200) is monitored, wherein a planar sealing system (204) and a geogrid (100) lying thereon are applied to the slope body and wherein the force measurement sensor unit (20, 30) is arranged in the transition region between a slope top (205) and the slope (200).