Comparison of SEEP/W Simulations with Field Observations for Seepage Analysis through an Earthen Dam (Case Study: Hub Dam – Pakistan)

Abstract

The present research work is designed to model seepage analysis of an earthen dam by using finite element approach. For this purpose a research study was conducted on Hub dam, which is a small earthen dam located at about 35 km, north-east of Karachi city, Pakistan. In the study the amount of seepage through and under body of the main dam is computed, profile of phreatic line is simulated for different scenarios and compared with the observed data. For the purpose of this study, SEEP/W the sub-program of Geo-Slope software is used. Data pertaining to design parameters and dam geometry are given as input to the software to compute the unknown parameters. Finally results are validated by comparing them with the observed data. The main dam is composed of three different kinds of reaches; therefore in this research only one reach with core wall i.e. Zoned Embankment Section at CH: 48+75 is studied. Computations are carried out for three different scenarios, that is: maximum pool level, normal pool level, and minimum pool level.

Calibration of the material properties is made on the basis of minimization of error while comparing observed hydraulic heads with the simulated ones. The flownet has been drawn comprising of streamlines, equipotential lines, velocity vectors showing dominant flow (seepage) field and phreatic line depicting seepage behavior of the Hub dam. The seepage flux (discharge), exit gradient and maximum seepage velocity for the entire pond level scenarios and for all the selected section are computed. At lowest pond level minimum seepage occurs at highest pond level maximum seepage occurs. It is also ascertained that the exit gradient is within the permissible limits that is that less than unity for all the scenarios; thus it also conforms to safety criteria of the dam. Seepage velocities for the entire pond level scenarios and for the selected section are computed; at lowest pond level minimum seepage velocity is observed and at highest pond level maximum velocity occurs.

Residual head dissipation trend is modeled and predicted for all the sections of interest for different scenarios. At selected section i.e. Zoned Embankment Section at CH: 48+75 for low pond level slightly smoother dissipation rate is followed, however, at higher pond levels a somewhat rapid dissipation of head occurs at sheet pile positions; this of course signifies the effectiveness of sheet pile. Initially dead dissipation follows somewhat smoother trend, however at the position of core wall and sheet pile an abrupt rise in dissipation of head is exhibited, which again signifies the effectiveness of the two seepage control devices.

Validation of any model is made by comparing simulated results against the observed ones; this is done to ensure model applicability. If this comparison shows a good coincidence, then the model developed can be recommended for practice. Table 4 contains the data pertaining to observed piezometeric heads and simulated ones and the relative error. Performance of the model is assessed evaluated on the basis of statistical parameters, i.e. mean error, root mean square error and model efficiency; these results are presented in Table 6.