Numerical analysis of the effect of arching Eyvashan dam in the first dewatering period and its comparison with Instrumental results

In earth dams, due to different properties of materials core and shell, heterogeneous settlement occurs between different sections of the dam. This action leads to a phenomenon of arching which creates some transverse cracks in the impermeable section of the dam, especially near the dam connection to the lateral walls, which can be expanded during the first period of dewatering and with the sudden application of reservoir water pressure to core. Ultimately, in more critical situations, it leads to hydraulic failure and causes the scouring phenomenon and jeopardizes the safety of the dam. In this research, the results of instrumental analysis of main stresses and arching coefficients of the Eyvashan earth Dam core during the first dewatering period were compared with the results of Geostudio and Plaxis software. The results show that the highest arching rate after dewatering has been applied at the upper levels, but the highest percentage of arching from the beginning of construction to the end of the dewatering stage was equal to 46% and at 1/3 of the height of the core from the floor. In addition, due to the ratio of arching coefficient in the range of 0.55-0.66, the dam in terms of arching in the first dewatering period is normal and there is no problem in terms of hydraulic deflection.

The Eyvashan earth Dam is located 1.5 km from the upstream of the village of Eyvashan and about 57 km from Khorramabad city on the Horoud River. The dam is rock fill type with a Vertical core, with a height of 62 m, a crest height of 1868 m and a normal elevation of 1864 m above sea level. The strain-strain analysis of the Eyvashan dam was performed based on Mohr-Columb behavioral models in terms of strain condition using Geostudio and Plaxis software. For analysis of stress-strain and consolidation settlement, the modeling has been done in eight layers.

By study and comparing the arching coefficient obtained from the Electrical Pressure Cell installed in the Eyvashan earth dam and Numerical analysis results, it was determined that the arching coefficient in the different levels of the core is in the conventional range and the risk of arching do not notice the dam. Αccording to the graphs from the results of the observational data, the highest percentage of arc in the near-filter elements occurred. In addition, the comparison of instrumental results with the results of the numerical analysis of matching is about 90%.

By comparing the results of measured and predicted total stresses with Geostudio and Plaxis software, it was found that the total stresses obtained from numerical analysis in the Total Pressure Cell installed at one level are approximately the same and are not significantly different, and the function of the software is appropriate. In addition, the difference in the total pressure values obtained from the instrumentation and the numerical analysis can be due to differences in the specific gravity of the materials in the design and the reality or due to errors and inappropriate performance of the tool.