Director, PARSAN Overseas Pvt. Ltd., New Delhi  

1.  Introduction

   As large dams age, it becomes increasingly important to determine the condition of their material and to track changes in this condition over time. Inspections of concrete, masonry and earthen dams, dikes and spillways often require testing to evaluate for leakage paths, soil settlement/voiding, or weakened deteriorated concrete/ masonry.  

NDT uses geophysical testing methods (ground penetrating radar, 2D/ 3D electrical resistivity imaging, streaming potential) to identify potential leakage paths under earthen dams, dikes and concrete spillways. Sonic/ultrasonic impact echo, cross-face sonic tomography and GPR non-destructive testing methods are used to identify voided areas under spillways. Sonic/ultrasonic pulse velocity, cross-face sonic tomography, MASW and impact-echo measurements are also used to determine the condition and integrity of dam and spillway concrete.

2.         CONCRETE DAMS

The elements that are of the most concern, because they are the most critical structural elements of the dam, include the spillway concrete, spillway sub-grade, dam wall concrete, and interiors of thin arch dams.

Periodic inspection is a dam owners best defence against these threats. However, visual inspections often only reveal problems that have developed into major degradation. In addition, some types of damage such as voids under a spillway slab or cracking in the dam interior are difficult or impossible to uncover using only visual means.

2.1       MASW

MASW first measures seismic surface waves generated from various types of seismic sources such as sledge hammer analyses the propagation velocities of those surface waves, and then finally deduces shear-wave velocity (Vs) variations below the surveyed area that is most responsible for the analyzed propagation velocity pattern of surface waves.  Shear-wave velocity (Vs) is one of the elastic constants and closely related to Young's modulus.  Under most circumstances, Vs is a direct indicator of the ground strength. After a relatively simple procedure, final Vs information is provided in 1-D, 2-D, and 3-D formats. 


      Fig.1- Field setup and results of MASW


The test provides an excellent tool to determine strength of concrete within the dam body.


2.2       Ground penetrating radar


The GPR method involves moving an antenna across a test surface while periodically pulsing the antenna and recording the received echoes. Pulses are sent out from the GPR computer driving the antenna, at a frequency range cantered on the design centre frequency of the antenna. Antenna centre frequencies used in NDE investigations vary widely, depending on the structural geometry and the information desired. To locate rebar or detect voids under thin slab-type spillways, a 900 to 1,500 mega-Hertz (MHz) antenna typically is used. For thicker slabs or other thick concrete elements, antennae with a centre frequency of 400 MHz or lower often are selected. Lower-frequency antennae allow for deeper penetration, but at the sacrifice of resolution.


These scans showed clear evidence of water-filled voids under the spillway. They also showed the presence of rebar in the slab concrete, at nominal 12-inch centres.


Fig.2- Typical GPR results


2.3       Seismic Tomography

Unlike other methods discussed till here, used from the surface of Dam or Ground, Seismic Tomography is conducted between a pair of boreholes or between upstream and downstream face of the Dam, to provide high resolution details of internal structure. The resulting tomogram shown physical property of each unit cell of dam body. In a concrete dam the information can be interpreted in terms of fractures, weathered concrete etc., as shown in Figure hereunder:


Figure 3. Seismic Tomography Across Faces of a Concrete Dam


3.         MASONRY DAMS


The techniques discussed for concrete dams above are equally applicable for masonry dams. Additional techniques of Electrical Resistivity Imaging, Streaming Potential and ReMi (Refraction Micro-tremor) provides invaluable information on seepage paths, water saturation and material strength of masonry. Acoustic/ sonic tomography results, when concerted to density, provide information on material loss due to leaching and grout quantification.


3.1       Electrical Resistivity Imaging


The results of electrical surveys carried out on the crest of a dam are presented as vertical sections showing the electrical properties of the dam materials. Electrical currents travel along preferential pathways in the most conductive materials such as dam core composed on fine grained materials. The method provides picture of internal resistivity distribution of the dam structure, identifying areas of water saturation in the dam body, and thus identifying the zones of water accumulation and wetting.




                                            Figure 4. Electrical Resistivity Section Showing Zone of Saturation (Blue)

3.2       Streaming Potential Survey

Interpretation of SP measurements to infer seepage patterns and concentrated seepage flows ranges from simple qualitative to more advanced quantitative numerical modelling approaches.

Most common application of SP study is to identify the zones in the dam body through which seepage is taking place. The results are correlated with resistivity sections. An example of such correlation is shown below in Fig. 5 wherein zone of saturation in dam body, as noted in electrical resistivity imaging section, shows a profound negative SP development.



Figure 5. SP Results along with Resistivity Section

3.3       Seismic Tomography


Seismic Tomography is an excellent tool to examine anomalous areas in high resolution, providing detailed properties along the section where it is conducted. It should, however, be chosen based on initial results of surface geophysical methods conducted all along the dam length, as it provides information only across the investigated plane. Typical results obtained from seismic tomography across a plane in masonry dam are as shown in Figure hereunder (velocity has been converted to density based on standard empirical relationships):


                                                        Fig.6 Seismic Tomography Across Faces of a Masonry Dam


                  Seismic tomography also allows similar investigations in spillway section, as shown in Fig. 7.





Fig.7 Seismic Tomography across Spillway of a Masonry Dam


Another application of seismic tomography is from dam gallery to dam crest, or between dam galleries (in case of multiple galleries). This provides a high resolution information on material properties along the length of the dam in gallery portion. The example (Fig. 8) shows results of seismic tomography conducted between dam gallery and dam top.





Fig.8 Seismic Tomography between Gallery & Dam Top



Typically, tomography images are analysed to look at the velocity changes within the concrete. Areas with lower velocity correspond to weaker, less dense concrete, while those with higher velocities are considered to be sound concrete. The results also can show areas with cracking damage or other discontinuities.


3.4       Refraction Micro Tremor (Remi)

Innovative technique of ReMi (Refraction Micro-tremor) has distinct edge over MASW and SASW in terms of logistics, execution and results. ReMi can be performed under the same layout as used for seismic refraction, to obtain excellent shear wave velocity profiles of subsurface.

Refraction Micro-tremor (ReMi) provides detailed S wave profiles of subsurface, providing a detailed insight into material strength (independent of water saturation). Fig. 9 is a typical example from a masonry dam, clearly showing weak blocks after chainage 265m (Blue low velocity zones). The rock interface is visible as red interface.





    Fig.9 ReMi Results on a Masonry Dam Showing Weak Zone



4.         EARTHEN DAMS


Techniques of Electrical Resistivity Imaging and Streaming Potential as discussed earlier are primary investigation tools for Earthen Dams. In addition, seismic refraction tomography provides valuable information on bedrock status and foundation flows, if any.


4.1       Seismic Refraction Tomography

The seismic refraction method detects changes in lateral- seismic velocity and/or layer thickness. Seismic techniques are extremely useful since seismic velocity is generally the most sensitive to slight changes in density and saturation in the types of materials commonly used in dams.

typical results obtained from Seismic Refraction Tomography, with blue to red representing increasing seismic velocity, and red line representing the bedrock topography.


Figure 10. Geophysical Methods for Common Dam Problems


Seismic Refraction results can also be used to determine Phreatic line. Fig. 11 below are typical results obtained from seismic refraction.




Fig.11 Seismic Refraction Results







It is desirable to have a 'base line' data of dam soon after completion of dam. For older dams, this time is 'now'. Availability of this base line data makes it possible to compare periodic measurements and detect 'changes' in physical properties, which are much easier to interpret than one time measurement values. Under 'normal' conditions, such measurements should be repeated every couple of years as a routine dam inspection program.



7.         CONCLUSIONS


Dam geophysics is still in its early days. It holds a great potential as health check and monitoring tool for dams of all types, provided the geophysical program is well designed, executed and interpreted. A strong cooperation between the geophysicist and engineer is essential to improve the interpretation and usefulness of the results. The principal objective of a geophysical investigation is usually to measure material properties, and locate anomalies in dams based on contrast in physical properties. Usually it is best to utilize more than one geophysical method to remove ambiguity to the extent possible. The objectives of investigation program must be well defined, and then the geophysical program should be designed involving primary and secondary tools/ techniques.


8.         REFERENCES

1.   CEATI publication- A Guide to Resistivity Investigation and Monitoring of Embankment Dams

2.   CEATI publication- Investigation of Geophysical Methods for Assessing Seepage and Internal Erosion in Embankment Dams: Self-Potential Field Data Acquisition Manual

3.   Ken Y. Lum and Megan R. Sheffer, Dam Safety: Review of Geophysical Methods to Detect Seepage and Internal Erosion in Embankment Dams

4.   Rana Sanjay, Integrated Geophysical Approach for Dam Health Checks & Dam Condition Monitoring

5.   Robbilard Claude, Seismic Tomography of a Concrete Dam in Canada (Project Report)

6.   M/s Solgeo report on investigation of a masonry dam in India

7.   Dennis A. Sack, Larry D. Olson, and Hunter A. Yarbrough- Non-destructive Techniques for InspectingConcrete Dams and Spillways