Hydraulic structures

Unique methodology for carrying out work – Monitoring the condition of the underwater part of concrete structures using GPR

Geophysical methods are non-destructive methods for monitoring the condition of engineering objects, as well as studying and monitoring the environment surrounding these objects. The possibilities of the methods are quite wide. In some cases, one method with the ability to obtain quick results is sufficient. However sometimes, for more complex problems, extended work using several types of research is necessary. Most often, electrical prospecting, seismic prospecting and ground penetrating radar are used in hydraulic structures. The following problems are solved with the help of engineering geophysics:

Analysis of the condition of concrete structures

• localization of waterlogging areas,
• the presence and condition of the reinforcing frame and the depth of the protective layer of concrete,
• determination of the spatial position of reinforcement,
• presence or absence of major defects,
• search and localization of cracks and delaminations,
• condition of the underwater part of concrete structures (a unique development – the underwater GPR).

Study of a soil dams array

• localization of waterlogging zones, filtration zones,
• determination of bedrock roof,
• condition of the underwater part of the structure,
• search for buried objects in the body of the dam.

Hydrological problems

• reservoir bottom profile,
• features of sedimentation in lakes and river beds,
• determination of the thickness of loose and consolidated sediments,
• studying the structural features of floodplains and river terraces,
• monitoring the condition of earth-fill dams,
• localization of places (directions) of greatest filtration through the dam body,
• direction of lake and artificial reservoir runoffs,
• search for local objects, contouring in plan and in depth,
• determination of the position of utilities (pipelines, cables, etc.).

Hydrogeological conditions

• determination of the depth of the groundwater level,
• determination of the depth and thickness of aquitards,
• determination of filtration properties of rocks.

Study of geological structure

• lithological division of surface and bedrock sediments,
• determination of rock depth,
• determination of the thickness and structure of the weathering zone of the rock base,
• identification of zones of tectonic disturbances,
• identification of places of development of karst and suffusion processes.

Seismic monitoring

• Seismometric monitoring of buildings and structures is specialized seismic monitoring, within which continuous observations of buildings (structures, large industrial facilities) are carried out in order to ensure safety and prevent possible negative consequences.
• Monitoring of natural and induced seismicity is seismic monitoring of the territory in order to record seismicity of various natures. Natural seismicity monitoring is necessary in areas with moderate to high levels of seismic activity. The main task of such monitoring is the rapid assessment of macroseismic manifestations, the impact on the population and technical personnel at industrial facilities. Another important observation task is to analyze the development of seismic activity, taking into account the industrial impact on the geological environment.

Study of the condition of concrete structures at hydraulic structures. Concrete scopes CS-1700 and CS-2500

Study of the condition of the underwater part of concrete structures. Underwater OKO-3 GPR with AB-700M3P

A set of geophysical methods for examining the condition of soil hydroelectric dams

An important element of ensuring the safe operation of hydraulic structures is the early and prompt implementation of comprehensive work to assess the technical condition of soil dams in order to reduce the risk of emergency situations. The leading role is given to geophysical methods, which allow, in an almost continuous mode, unlike other methods, to assess the nature of the distribution of parameters of technogenic soils composing the body of the dam and quickly change the detail of the work if “problem” areas are identified.

A typical complex of geophysical methods consists of seismic prospecting methods (seismic tomography), electrical prospecting (resistivity methods, inductive methods and natural field) and ground penetrating radar. But, each earth dam has its own structural features, which leads to the need for an individual approach when choosing methods and modifications of geophysical methods. In any case, the best results are shown by a set of methods focused on different physical properties of soils. The use of only one method often leads to ambiguous interpretation of the results.

Below is a set of methods and problems that they solve.

Seismic exploration, when used with two types of waves in the seismic tomography technique, allows evaluating the physical and mechanical properties of the soil, identifying local zones of their decrease, and monitoring the groundwater level.

Seismic surveys made it possible to determine the position of the top of the rock foundation. At depths of 2-4 meters, high-velocity anomalies are distinguished, which can be interpreted as areas of denser soils in the thickness of the embankment. No significant anomalous zones associated with watering or decompaction of soils were noted.

Electrical tomography, which differs from the classical VES and SEP methods in its high detail and focus on complex environments, identifies local zones of reduced electrical resistivity. On the one hand, this can be associated with zones of water saturation of the dam body soils, and, on the other hand, with the soil composition changes. In case of difficult grounding conditions, it is recommended to switch to inductive electrical prospecting methods.

In this case, according to the results of electrical tomography, the top of rocky soils is identified, areas of low values of electrical resistivity in the technogenic soils of the dam are noted, which can be classified as zones of high humidity.

The natural field, the distribution of which is determined by filtration potentials, shows high efficiency in searching for leaks. However the implementation of the EP method is often impossible due to a large amount of man-made interference and difficult grounding conditions.

Using the GPR method, one can study the structure of the dam, identify structural layers and watering zones. Still, the depth of the method depends on the composition of the soil making up the body of the dam. Often, the body of the dam is made of clay soils, which sharply limit the depth of the ground penetrating radar method.

Before the appearance of visible destruction of the fastenings of the dam slopes, hidden negative processes take place inside the bulk body of the dams (formation of voids, zones of decompaction of the subgrade soil, infiltration of groundwater along expansion and inter-slab joints), the early identification of which would allow appropriate measures to be taken in a timely manner. GPR devices make it possible to determine the thickness of slope fastening slabs, the condition of the earth embankment of dams, the boundaries of soil moisture zones, foreign inclusions in the soil, the spatial outlines of layer boundaries, groundwater infiltration paths, and also to identify defects made during construction work.

During the GPR survey of this object, the following interesting features were revealed:

• The horizontally layered type of wave pattern predominates.
• Numerous reflective boundaries associated with the stages of filling and formation of the embankment have been identified.
• At one of the survey sites along the dam crest, an anomalous section of low-frequency recording was identified. Such a low shift of the spectrum to the low-frequency region, as well as the presence of a characteristic “ringing” throughout the entire length of the signal recording, may indicate a zone of watering and decompaction in bulk soils.

The combined use of methods has shown to be highly effective. As a result, the top of rocky soils was identified, the layered structure of the embankment associated with different stages of filling and compaction was determined, and local areas of watering were identified.