This article presents a procedure for designing rockfill toe protections with a highly permeable downstream shoulder to avoid mass sliding in dams during extremely high through-flow episodes. This accidental through-flow may be caused by reasons such as overtopping or leakage flow due to the loss of sealing at the impervious element of the dam or its foundation. The proposed protection is located at the downstream toe of the dam, and it is composed of highly permeable material, typically compacted rockfill. This material can be different from the rockfill forming the downstream shoulder of the main dam. The work is a result of a combination of numerical and experimental research carried out to analyze the influence of the geometry of the toe protection for given material properties of both dam shoulder and protection, when they can be considered highly isotropic. As a result of these studies, a design methodology is obtained. A series of validation tests are presented to support the reliability of the method.

Introduction and background

During the last decades, the social demand for dam safety standards has significantly increased, especially in most developed countries. This has yielded new and more demanding dam regulations and technical guidelines, some of them imposing the need of protecting existing dams [1,2]. The dam protection techniques involve different adaptations of the designs of the dams to protect them against foreseeable failure mechanisms so as to improve their safety [3,4]. There are different types of dam protections, mainly depending on the type of dam, the associated failure mechanisms, and the specified degree of protection [5]. In the particular case of rockfill or earthfill dams, the development of heavy through-flows within the downstream shoulder due to overtopping or extremely high leakages (Fig. 1) is the main cause of their partial or total failure [6]. Reinforced rockfill, riprap, and gabions, among others, have been the most common rockfill dam protections used in the past [7–۱۱]. In addition to this, countries such as Norway or Sweden are currently using rockfill protections on the downstream slope in rockfill dams where potential risk for loss of life and significant downstream damage is high. This is the case of, for example, Suorva, Seitevare and Trängslet Dams in Sweden, and Svartevann Dam in Norway. So-called “rockfill toe berms” are installed on these three Swedish dams, whereas rip-rap is placed on the downstream slope of the Norwegian dam. These particular types of protection have been recommended by their respective National Dam Safety Guidelines [2,12] to improve the stability of dams against accidental leakage [13–۱۶].