What I felt when I went down the inspection gallery of a dam for the first time
About 15 years ago, when I was a student at the university , one of our assistant professors suggested that we have a class trip to a dam in Ardabil Province. I had never seen a dam up close before and I didn’t know anything about them. We were about 30 students and we started moving towards the dam site early in the morning. When we arrived there, the huge trucks, lorries, cars and other heavy machinery were moving outside of the dam area. First, we went toward the dam crest and all appurtenant structures could be seen there except the gallery. I had some information about the tunnel, spillway and the dam body but I didn’t know anything about the gallery.
When I passed the dam engineering course at the Power and Water University of Technology (PWUT), I realized that the function of a dam gallery, which is shaped like a horseshoe, is to provide facilities for inspection of the foundation, dam behavior, and also draining the seep water. The gallery entrance is located in one of the dam abutments, outside of the dam body.
We walked on the dam crest until we arrived at the opposite abutment where the gallery entrance was located. In order to block access to unauthorized individuals, the entrance was fenced and locked, just like prison doors. A guard came and opened the door for us and then we were escorted by a specialist who acted as our guide. As we went down a special access stair-case, I felt darkness and cold slowly creeping up my spine. The only source of light was our guide’s flashlight which wobbled as we went down the hundred-long stairs.
The first question that came to my mind was; why hasn’t the gallery been designed wider or larger? Wasn’t it possible or maybe it hasn’t been designed based on any standards? I found out later that the gallery dimensions should have sufficient height to allow easy movement of personnel and minor equipment. Also, wide openings induce quite a lot of local stress with consequent cracking of the concrete. Therefore, it’s width and height has been determined to be 4.40 and 4.70 m, respectively.
The second question; wasn’t it possible to reduce the number of stairs or design another type? The answer was that the number of stairs were determined according to the dam height and the slope of the valley. The spiral staircase was another type that could be used but it isn’t suitable in large dams.
As I was slowly following the only source of light, I was looking everywhere carefully to avoid the potential snakes or mice. I was cold, my hands were freezing and sometimes I could feel a drop of cold water falling down on my face. Our guide was providing a detailed explanation of the equipment as his calm voice was reflecting off the wet interior walls, but I really couldn’t make much of what he was talking about, since I was too afraid to pay attention.
Inspection gallery of the embankment dam
I carefully looked up at the joints on both sides along the gallery for any potential hazards, but I realized that there was an interesting instrument in every 10 meters. Later, I found out that it was called a jointmeter or crackmeter which is installed for controlling the crack opening rate.
Crack-meter that was installed in the gallery
On the ceiling of the gallery there was a white plastic thing like an instrument. It was an extensometer which is used for measuring horizontal and vertical displacement of the gallery. Also, some cables were passed through the wall like black snakes which were used for connecting the instruments to the control room outside the gallery.
I only thought about the things that cause death; what would happen if the ceiling of the gallery collapsed? Would the gallery fill with water and we sink? Or would the dam body settle over our heads and we suffocate? If there was a fire, what would happen? There was nowhere to escape. We were following the guide in a narrow, cold and dark corridor with only a glimmer of light. I was confused and couldn’t concentrate on our guide talking about the instruments, cables and equipment and when he asked me ‘’didn’t you have any questions?’’ I said to him ‘’No I understood everything well’’.
That was of course a lie; I had a lot of questions about the gallery structure, excavation and construction, but I didn’t want to stay inside any longer and wanted to go out as soon as possible. One of the things that I really wanted to ask him was a question about some holes that were drilled on the walls and the ceiling and the fact that a little amount of water was poured out of some holes. Later, I realized that the holes were drains which were placed to drain the water in the foundation into the gallery.
On both sides of the gallery, a conduit was constructed to convey the run-off water into a low-level downstream outlet. Doing this, the amount of water which was drained from foundation and abutments was measured. A change in the total quantity of flow within the conduit or from individual drains is indicative of potential problems developing with the concrete, waterstops, or other seepage control systems of the dam. Unusual seepage may indicate increasing pressures along concrete lift lines or joints within or along the base of the dam that may lead to eventual instability. A change in seepage quantity is a cause for concern, because it may indicate seepage pressure is approaching levels that exceed the design loads for the structure or that the drainage system is not working properly. It is essential that the internal drains be regularly maintained to avoid plugging due to precipitation.
I was feeling uneasy and I wasn’t getting anything about my gallery inspection while I should have prepared a technical report for that mission. I comforted myself and thought if I could take some photographs I could find some text about the gallery and prepare the report. All in all, I was so angry with myself for choosing this course and told myself it’s not in line with my feelings and abilities. After all, It was only a class trip and I didn’t have any responsibilities about the proper performance of gallery, instruments and facilities. Was I really able to work in this field? Could I come to the gallery one day all on my own without any guides? No, it wasn’t possible, I couldn’t think about that. Damn Me!
At that moment, all I could think about was how I could die in the gallery. The guide told us “be careful, we want to go to the access gallery at the downstream of the dam body”. I was legitimately terrified at that moment. “should we go even lower?” I said to myself. I couldn’t believe it when I saw hundreds of stairs in front of my feet again. I didn’t have the energy anymore. At that time, I only could think about when and how I could get out of this underground nightmare.
I should have gone down the stairs but I couldn’t stand on my feet and all I wanted was to find a place to sit, on the other hand I really wanted to escape from that closed atmosphere. Reluctantly, I started going down the dark staircase and as we got to the bottom, a shimmering beam of light could be seen from far away.
As we were getting closer to the outdoors, the interior of the gallery was getting brighter and warmer. It gave me a fresh feeling and I forgot the fatigue, disappointment and death. I took a deep breath and continued walking and listening. Finally, we got outside of the gallery at the downstream of the dam body, I could breath, I was alive! I could see the blue sky with white scattered clouds. It was shiny and I felt like I was getting warmed up and I could really use a drink.
The downstream of the dam body was a green and flat plain and the river was flowing up. We washed our faces, hands and feet and sat on the rocks near the river and rested for about one hour. I came back to the moments I was in the gallery; what was I thinking? I laughed at my thoughts, fears, worries and the terrible things that came to my mind.
Now, it has been a long time since that visit and I have had several missions to different dams in Iran and the other countries. Sometimes, I have gone as a guide by myself or participated in technical meetings as a dam designer. Now, I can determine the risk of a dam construction or prepare a dam safety program. I know what to do in a critical position, I have no fear of a dam.
All in all, it was a very interesting experience, although that visit was full of fear, horror, disappointment and the thought of death, but I would like to go there again and reminisce.