Evaluation of Hydraulic and Stability Behavior of Homogeneous Earth Dam with Combined Drain

Introduction
Different types of drainage systems are used for seepage control through earth dams; including: horizontal toe drain, triangular toe drain, chimney drain and combined toe drain. Earth dams are constructed by soil compaction, so horizontal permeability is more than vertical permeability. As a result, hydraulic performance of chimney drain is better than triangular toe drain and operation of triangular toe drain is more suitable than horizontal toe drain. On the other hand, triangular toe drain can be repaired due to the more accessible position, if necessary, and is easier to implement than chimney drain. In this research, a combined drain, as a replacement for triangular toe drain is introduced, and its hydraulic performance is studied and compared with triangular toe drain. In order to achieve this goal, first, the hydraulic efficiency of the combined drain is investigated through physical modeling. After assuring the positive performance of combined drain and acceptable comparing of numerical and physical modeling results, sensitivity analyses of seepage and stability were performed numerically. For this, the height of triangular toe drain was decreased from 20 to 50%, and a part of the saved material was attached to triangular toe drain as horizontal toe drain. This kind of drain called combined drain.
Methodology
In this study, first, hydraulic performance of 3 physical models with different drainage system (without drain, with triangular toe drain, and with combined drain) is studied. Models were constructed in a box with 11 piezometers and 2 spillways in upstream and downstream which are used to keep the water level fixed in reservoirs. Height, length and width of this box are 1m, 1.1m, and 0.15m, respectively. Height of physical models, considering the dimensional constraints of box considered 49 cm. The slopes angles were kept about 45 degrees and the crest width was dictated 21 cm. The height of triangular toe drain, considering Creager’s recommendation was choosed 17 cm. In models with combined toe drain, height of triangular part, considering 15% reduction, was assumed to be 14.5 cm and length and thickness of the horizontal part of combined drain were held to be 11.5 and 5 cm respectively. Upstream and downstream water level were set as 47 and 4 cm for all models. Piezometric water level as a representative of phereatic surface, and volumetric flux were recorded from physical models. Then numerical models were run using Geo-studio software. Hydraulic performance comparison between these two physical and numerical analyses illustrated acceptable agreement; In the following, additional analyses were performed just numerically in order to assurance the adequacy of combined drain. The size and characteristics of numerical models were assumed based on real earth dams characteristics. Seepage analyses were performed for both steady state and rapid drawdown conditions, then stability analyses were done for downstream slope (end of construction, steady flow condition and quasi-static condition) and upstream slope (drawdown condition) for these mpdels.
Results and discussion
Comparison between models having triangular toe drain and combined toe drain, in whih half of the remaining material from toe triangular drain height reduction were horizontally atached to toe drain, showed an increase in cover length of downstream on phreatic line, and also noticeable rise in amount of discharged water; so in the next step, models having combined toe drain, with the same hydraulic performance as models with triangular toe drain were compared. Also, stability performance of these two models were evaluated. Stability analyses of models, showed ignorable difference in factors of safety, due to little share of drain area in slip surface in which causes slight change in shear strength, and also, phreatic surface dropping down in which causes an increase in dry area of earth dam downstream and induces an increase in unit weight of soil and subsequently, expansion of slope stability.
Conclusion.
The process described in the previous parts, approved proper performance of combined drain when it is used as a replacement for triangular toe drain. It was revealed that when a triangular toe drain substitute with combined drain, it will improve the hydraulic performance of the earth dam and will also result in a noicable reduction in drain material usage in which is more expensive than body material. In addition, this replacement will have a negligible effect on the static and quasi-static stability of the reservoir slopes. Therefore, the proposed drainage system’s adequacy is confirmed as a suitable alternative for triangular toe drain in homogeneous soil dams. Results indicated 11 to 157% increase in cover length on downstream phreatic line and 25 to 50% reduction in drain material compared with triangular toe drain, in models which half of the saved drain material, was used as horizontal toe drain. On the other hand, using combined drain with the same hydraulic performance instead of triangular toe drain results in 17 to 60% decrease in volume of drain material.