Assessment of Optimal Location of Cutoff Wall with Drainage System Beneath Concrete Dams in Non-Homogenous Foundation

Seepage flow through the base of hydraulic structures such as concrete dams causes different issues in these types of structures. It may cause under-scouring of foundations that are constructed in previous material like loose sands or non-cohesive soils. Moreover, seepage flow creates uplift pressure that may destroy the whole structure by overturning or sliding. Engineers have tried to reduce upward pressure created by seepage flow using different methods and techniques. Normally, applying drainage systems such as well-graded granular material, geo-materials, and artificial filters could control the moving of tiny soil particles from spaces between large particles. However, selecting optimal location of the mentioned elements to control seepage forces as well to fix bed material under the foundation of a hydraulic structure is the main challenge for engineers and designers. Nevertheless, by using appropriate filters and drainage system, moving of bed material can be controled; this method couldn’t control the harmful impacts of seepage flow singly. Since the last decades, researchers have proposed different methods to control uplift forces by using cut-off walls due to increasing seepage passway. This techniques drops the hydraulic gradient of seepage flow. Employing the relief well is another practical method to break high pour water pressure and reduce acting uplift pressure to the hydraulic structures.Conducted studies showed that the closer the drainage systems to the toe of the dam, the lower pressure (Yaghan, 1998). Ahmadi et al, (2010) confirmed that the best location for relief well and the downstream cut-off wall is 0.28L and 0.72L from the toe, respectively in homogenous materials to reduce uplift pressure and seepage flow (L is the length of the dam foundation). The literature review revealed that almost all the research and studies in the field of seepage problem and uplift issues beneath concrete structure have been done under homogenous material, meanwhile, in nature, the base materials are in non-homogeneous conditions especially in the layered form with different permeability and properties. Therefore, in this research, we tried to study the effects of non-homogeneities of foundation materials in the seepage and uplift problem beneath concrete dams.

To study the effects of drainage and cutoff wall application in concrete or diversion dams and to determine their optimum location simultaneously, the hydraulic behavior of cutoff wall with installed drainage system in different depth at the non-homogenous sandy foundation has been studied experimentally and numerically. At the prepared experimental setup seepage rate and piezometric pressure under 4 different hydraulic head have been recorded using by 18 mounted piezometers. To study the various condition of non-homogeneities of foundation material as the ratio of permeability of the upper layer to the lower seep/w, a module of Geostudio as a finite element base mathematical model has been used. Calibration and validation of mathematical model have been conducted based on observed experimental data by modifying the permeability of each layer to reach the same seepage flow rate in both model and experimental setup. The optimal location of cutoff walls also has been determined numerically. To study the under scouring phenomenon beneath the dam, critical seepage gradient has been checked under different heads of water at upstream and downstream of the dam.

The results showed that employing a drainage system within two cutoff wall led to increase in seepage flow and at the same time a significant reduction in uplift force and exit gradient in compare to the ordinary state (without drainage well). As well, increasing the depth of drainage installation makes the growth of seepage flow and dropping in the exit gradient but it does not have any effect on the uplift pressure. Also based on achieved results, in case of implementation of two cutoff walls with drainage, the optimum location for the upper cutoff wall would be exactly at the heel and the down cutoff should be installed at the distance of 0.73 of lengths of foundation from the heel. In current condition, all three parameters as seepage flow, uplift pressure, and exit gradient take in the minimum magnitude.

Assessment of the achieved results about the effects of non-homogenous material of the foundation of the dam in uplift pressure acting to the dam proved that layered strata didn’t have significant effects on the pressure distribution and its magnitude. However, analyzing of flow-networks showed that the equipotential lines lean horizontally in the case of the ratio of permeability of the upper layer to the lower is smaller than one. In fact, in such cases, seepage streams lines tend to pass through the lower layer.

The authors would like to express special gratitude to the vice-chancellor of research and technology of Urmia University for provhding the opportunity and facilities to conduct this research.