A method and system for evaluating a dam The invention provides a method and system for evaluating a dam with respect to its stability.

Dams, generally used as barriers impounding water or under ^¬ ground streams, are well-known in the art. Hereinafter, floodgates, levees and/or dikes are generally referred to as dams .

In the maintenance of dams, particular interest is directed toward a prevention of dam failures. Dam failures are poten- tially catastrophic if the structure of the dam is breached or significantly damaged.

Routine deformation monitoring is useful to anticipate any problems and permit remedial action to be taken before struc- tural failure occurs. However, most of the known monitoring methods cannot address hidden dam damages . Hidden dam sub- grade damages, which are e.g. caused by water weeping forms cavity and water encroachment of soil, can lead to general dam weakness .

The known monitoring methods have fundamental flaws in the detection method of hidden damages or changes inside the dam structure. Usually, these methods can detect only abnormal warps in dam body geometry but not hidden internal damages.

Another approach is based on analyzing methods which require in-depth geological exploration. This approach enables experts to gain detailed information about the dam body. However, this approach requires essential human involvement into data collection process and relatively big amount of time to cover the whole dam surface . Hence, there is a need in the art for a practical evaluation in terms of internal changes in the dam structure.

Accordingly it is an object of an embodiment of the invention to provide a method for evaluating a dam, by which changes, occurring inside the dam structure can be found without the need of repeating geological explorations and without the drawbacks of known monitoring methods . According to an embodiment of the invention, a method for evaluating a dam is provided, the method including the steps of:

exerting a seismic excitation to the dam;

acquiring seismic waves by a plurality of sensors located in a vicinity of the seismic excitation;

reading current measurement values from the plurality of sensors ;

reading historic measurement values from a database, the historic measurement values being determined by an exci- tation and a location of sensors which are substantially similar to the current measurement;

conducting a comparison of the current measurement values with the historic measurement values by considering an analytical model of the dam;

- determining a degree of overlap of the comparison and interpreting the dam stability in view of said degree.

Due to the fact that a considerable degree of potentially harmful changes in the dam structure are occurring inside said structure, this embodiment of the invention suggests an active dam body observation using a defined seismic excitation. This inventive measure bears significant advantages over traditional observation methods which are merely relying on natural phenomena like the optical inspection of the dam or natural seismic occurrences.

Exerting a seismic excitation has exceptional advantages in evaluating the inner dam structure, particularly parameters of the subsoil layers including layer thickness, shear wave velocity or dynamic Young's modulus.

The defined seismic excitation advantageously contributes in conducting a comparison of currently measured sensor measure ^¬ ments with preceding or historic · measurement values in order to determine a degree of overlap. The inventive method does not aim to conduct a detailed analysis of a current dam body structure. Instead, the invention aims a comparison with re- suits of other historic measurements in order to conduct a comparison with historic measurements. In other words, the invention aims to interpret the dam stability in view of changes. If there are no changes in current measurement values or it current measurement values are expected or can be explained, than a conclusion is done that there are no anomalies present inside the dam structure. Otherwise further research may be mandatory, the research directed in detecting reasons of anomalies. Although the inventive method does not aim to conduct a detailed analysis of a current dam body for every single evaluation, it is however advantageous to support the comparison of the current measurement values with the historic measurement values by considering an analytical model of the dam ac- cording to the invention. This analytical model is usually maintained over a series of evaluations according to the invention, which advantageously minimizes expenditures of such evaluations. However, there may be an occasional demand to update this analytical model, particularly on the occurrence of force majeure or structural modifications to the dam.

It is a further object of an embodiment of the invention to provide a mobile system for evaluating the dam stability. Accordingly, a system for evaluating a dam stability is provid- ed according to an embodiment of the invention, the system including

a first mobile vehicle for exerting a seismic excitation to the dam; a plurality of sensors located in a vicinity of the seis ^¬ mic excitation for acquiring seismic waves ;

a second mobile vehicle including a evaluation unit, the evaluation united adapted for:

- reading current measurement values from the sensors; reading historic measurement values from a database, the historic measurement values being determined by an excitation and a location of sensors measurement values which are substantially similar to the cur- rent measurement;

conducting a comparison of the current measurement values with the historic measurement values by considering an analytical model of the dam;

determining a degree of overlap of the comparison and interpreting the dam stability in view of said degree .

This embodiment using a mobile system remarkably reduces expenditures of traditionally known fixed installations and ad- vantageously enables a usage of one mobile system for a plurality of dams.

In a possible embodiment of the method according to the present invention the method comprises a plurality of sensors located in pre-defined arrays near a surface of the dam.

In a further possible embodiment of the method according to the present invention, the method provides an acquisition of direct and refracted seismic waves by the plurality of sen- sors .

In a possible embodiment of the method according to the present invention the seismic excitation is conducted by a vibration test and/or using a falling weight.

These and other objects and advantages of the present invention will become more apparent and readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawing of which:

Fig. 1 shows a side view of a mobile dam monitoring com ^¬ plex according to an embodiment of the invention;

Fig. 2 shows a cross- sectional view of a mobile dam moni ^¬ toring complex according to an embodiment of the invention; and;

Fig. 3 shows a schematic view of the method proposed according to an embodiment of the invention.

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawing.

Figure 1 shows a first mobile vehicle VHl for exerting a seismic excitation to a dam DM, a plurality of sensors S located in a vicinity of the seismic excitation for acquiring seismic waves and a second mobile vehicle VH2 including a not shown - evaluation unit. Preferably, direct and refracted seismic waves are acquired by the plurality of sensors S. As shown in figure 1, the seismic excitation is conducted using a falling weight W.

As can be seen form figure 2, the plurality of sensors S is preferably located in pre-defined arrays near a surface of the dam .

The mobile dam monitoring complex according to Figures 1 and 2 can be built on base of several vehicles VHl, VH2. Preferably, one vehicle VHl is used for transportation of a standard weight for excitation of vibrations. Other vehicles VH2 are used for a collection and evaluation of data, which are acquired by the installed sensors S. In such kind of system, sensors are preferably likewise transportable and can be used on different dams D. Vibration tests using a defined falling weight W have excep ^¬ tional advantages concerning the parameters of the subsoil layers. In order to acquire detailed information including layer thickness, shear wave velocity or dynamic Young's modu ^¬ lus, an advantageous use of a defined input-source supports a correct interpretation of the recorded response.

Usage a standard weight W and the same displacement of moni- toring complex on a dam D lead to strict results interpretation and easier soil changes detection. Such tests can help to determine changes within soil parameters. Direct and re ^¬ fracted seismic waves can be recorded by sensors S, e.g.

three dimensional geophones, located in pre-defined arrays near the dam surface.

Wave velocities increase with depth corresponding to the geology and increasing soil consolidation. Low velocity anomaly can indicate very soft sediments, for example mud.

Forced vibration tests are advantageously carried out regularly with respect to dam importance and environmental changes . The aim of every test is not gaining an entire exploration of a current dam body structure, but rather the ability of comparing the sensor measurement values with results of previous sensor measurement values .

Monitoring and decision taking processes using this system are generally conducted according to a schematic view of the proposed method as shown in figure 3.

A current experiment is conducted by exerting a seismic excitation to the dam. Thereby, seismic waves by a plurality of sensors located in a vicinity of the seismic excitation are conducted. The current measurement values E of the current experiment are read from the plurality of sensors and input to a - not shown - evaluation unit. The evaluation unit includes a comparison unit CMP for conducting a comparison of the current measurement values E with historic measurement values taken from a database DB. The historic measurement values have been determined by previous experiment by an ex ^¬ citation and a location of sensors measurement values which are substantially similar to the current experiment. The com ^¬ parison unit CMP additionally includes a consideration of an analytical model AM of the dam.

The results of the comparison are output to a decision unit R. With the aid of the decision unit R a degree of overlap of the comparison is determined. The stability of the dam is interpreted in view of said degree overlap. If the results of the comparison overlap (branch Y or »Yes« in Fig. 3) or if the results are as expected or can be explained, the dam is considered as being in a condition PS which requires no further action.

If the results of the comparison do not overlap (branch N or »No« in Fig. 3) the dam is considered as being in a condition NPS which requires further action. Such required further action may include a detailed geographic analysis of the dam structure or further research aiming to detect reasons of anomalies . An additional goal of this method is gaining a large database

DB of sensor data. Every dataset collected from current experiments is aiding to gain a base for future experiments.

The invention provides an active test approach by causing a seismic response of the dam which allows early monitoring of body damages .

The invention provides a standard test methodic. Defined input excitation helps to get a correct interpretation of the recorded response.

The invention further enables a comparison of test results. Advantageously, it is not necessary for each test to entirely analyze the dam body structure. Instead, comparing a current result with results of previous tests can indicate internal changes of the dam which can possibly harm the dam stability. The invention further provides a mobile system which can be used for monitoring a set of dams in a region.