| Earth-rock dams are one of the most widely used types of dams in China’s hydraulic engineering,and their safety is an important guarantee for the normal operation of all types of hydraulic engineering.The earth-rock dam containment structure consists of two parts,the panel and the dam body,the panel medium is concrete and the dam body is made of soil and stone in accordance with a certain gradation of rolling and stacking.Most of the existing earth-rock dams in China were built during the middle and end of the last century.Due to historical reasons,a large number of earth-rock dams have a series of problems such as low engineering standards,poor construction quality,serious aging and dilapidation,backward engineering management and incomplete supporting facilities,etc.After decades of operation,many earth and rock dams have hidden problems such as panel cracking and dam leakage,which have a negative impact on the normal operation of hydraulic engineering.Therefore,the timely and accurate detection of dam panel and body damage can provide the basis for comprehensive treatment of potential dam leakage and thus reduce the damage caused by potential leakage to the greatest extent.On the basis of current research on earth-rock dam damage detection,this paper conducts research on the geophysical waves of earth-rock dam damage to address the problems in dam damage detection,establishes a discrete random media earth and rock dam model,realizes the simulation of three-dimensional seismic wave field of earth and rock dams,clarifies the influence of water content changes on the leakage wave field during the formation of earth and rock dam leakage.It also reveals the effective seismic attributes for leakage hazard identification,summarizes the characteristics of radar wave field and transient attributes under different void conditions of concrete panels,thus forming an effective earth-rock dam hazard detection technique.Key research results achieved are as follows:(1)Based on the conventional random media theory,the discrete random media modelling method is proposed to effectively reflect the difference between soil and stone elastic media in earth-stone dam media with a certain soil to stone ratio.Analysing the influence of stochastic parameters such as correlation length,roughness factor and partial window scale on the modelling of discrete random media,the correlation between different stochastic parameters and the particle size of soil and rock media is illustrated.(2)The numerical simulation of the seismic wave field in homogeneous medium,conventional random medium and discrete random medium is realized,the differences of the wave field in the three mediums are compared and analyzed,the influence of scattered waves on the identification of the effective wave field in random medium is analyzed,as well as the correspondence between the random parameters,the change of soil to stone ratio and the strength of scattered waves is summarized.The results show that as the autocorrelation length increases,the intensity of the scattered wave energy gradually increases,the scattering of the wave field gradually decreases as the stone content in the model increases,the scattered wave energy gradually weakens,and the wave field characteristics of the discrete random medium gradually converge to that of a homogeneous medium.(3)According to the characteristics of the external morphology and boundary conditions of the earth-rock dam,the tilted AEA free boundary,M-PML and C-PML absorption boundaries are used to handle the boundaries of different parts of the dam body in different regions to realize the 3D seismic wave field simulation of the earth-rock dam.The simulation results show that in the presence of panels upstream of the dam,the arrival time of the surface waves received at the upstream detection point of the dam becomes shorter,and as the slope ratio of the dam slope increases,the reflected transverse waves at the bedrock interface produce gradually weaker bypass energy at the top of the upstream dam.The scattering generated by the surface wave in the wave field of a discrete random earth and rock dam has a greater effect on the overall wave field.The in-phase axis of the reflected wave before the arrival time of the surface wave is largely unaffected by it,and the scattering effect of the stone particles in the dam medium becomes more prominent as the soil content increases,and the scattered wave energy in the wave field becomes stronger.(4)By comparing and analysing the 3D seismic wavefield records with different excitation points,different planes and different fractions of leakage hazards,the ability of the longitudinal,converted and transverse waves to identify the leakage hazards was compared.After the superimposition of the reflected longitudinal waves at the bedrock interface and the reflected longitudinal waves of leakage,it is difficult to identify the hidden leakage by the longitudinal wave information,while the superimposition of the reflected transverse wave at the bedrock interface and the reflected transverse wave of leakage shows an increase in energy and a decrease in frequency,which means that the transverse wave information is more capable of identifying leakage than the longitudinal wave.(5)A comparative analysis of the seismic wave fields of the leakage models with different water contents in the leakage area of a discrete random earth-rock dam shows that as the water content of the leakage area decreases,the energy of the leakage reflected wave field gradually decreases,and when the leakage is located at the bottom of the dam,only when the water content of the leakage area reaches more than 25% can the leakage anomaly be effectively identified from the seismic record;And when the leakage area is located at a distance of 5m from the bottom of the dam,leakage anomalies can only be effectively identified when the water content of the leakage area reaches more than 10%.(6)When there are leakage hazards inside the earth-rock dam,the amplitude and frequency of the leakage reflection region in the seismic attribute of the dam will change,and the leakage reflection homogeneous axis will show strong energy and low frequency characteristics.As the water content of the leakage area decreases,the energy of the leakage reflection homogeneous axis in the transient amplitude gradually decreases,and the continuity of the leakage reflection low-frequency homogeneous axis in the transient frequency deteriorates.The transient amplitude and transient frequency mainly reflect the strong energy reflection homogeneous axis,and are not sensitive to the weak energy reflection wave field;the transient phase is not affected by the amplitude energy,and can highlight the weak energy reflection homogeneous axis.The wavelet transform is more resistant to interference energy and can suppress the scattered wave field in the seismic record to some extent.When the water content of the leakage area is low and the leakage reflection energy is weak,the instantaneous phase can be used for leakage hazard identification;when the signal-to-noise ratio of the seismic record is not high,the frequency slicing of the wavelet transform can be used to analyse the leakage reflection information in the seismic record.(7)The radar wave field simulations of panel devoid without reinforcement,single-layer and double-layer reinforcement are carried out to illustrate the response characteristics of panel devoid radar wave field in the presence of different types of reinforcement,reveal the “pull-down” phenomenon of the reflected homogeneous axis of devoid,and give the conditions for quantitative identification of panel devoid thickness.When reinforcement is present inside the panel,the continuity of the reflected in-phase axis of the devoid becomes poor.As the number of layers of reinforcement inside the panel increases and the spacing between the reinforcement decreases,the“pull-down” phenomenon of the devoid reflection wave pattern and the reflected in-phase axis gradually disappears.When the signal-to-noise ratio of the radar data is low and the presence of voids cannot be identified from the radar profile,transient attribute can be used to identify the potential for voids.The main innovations of this paper are as following:(1)Based on the theory of random media,a discrete random media modeling method that can effectively reflect the characteristics of earth-rock dam media is proposed,and the influence of random parameters on the size of grain in the model building process is analyzed,which provides a theoretical basis for setting up an earth-rock dam model that is closer to the actual one.(2)According to the characteristics of the external morphology of earth and rock dams,the free boundary or absorption boundary of the top,slope and bottom of the dam are processed in different regions to realize the simulation of the three-dimensional seismic wave field of earth and rock dams,and the influence of the change of water content in the leakage area on the seismic wave field during the formation of the potential leakage of earth and rock dams is analyzed.(3)Based on theoretical simulations of geo-radar and actual measurement data,the factors influencing the identification of de-voiding by internal reinforcement in earth-rock dam panels are analysed,and the conditions that can quantitatively identify the thickness of de-voiding are given to make up for the lack of accuracy in seismic detection resolution. |
