1 Introduction
2 Geology of the Area
3 Geological and Engineering Problems
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The karsts prevailing at the dam site and in the reservoir area.
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The existence of gypsum/anhydrite rock formations in the dam foundation alternating with soft marl layers and weathered and cavernous limestone beddings.
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The presence of an extensive ground water aquifer called Wadi Malleh aquifer which affects considerably the ground water regime in the right bank.
4 General Status of the Geological Conditions
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The dissolution phenomenon is not recent in Mosul Dam area. The impounding of Mosul Dam reservoir increased the rate of dissolution of the rocks. This had led to the sudden appearance of sinkholes in the area.
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The bedding plains under the foundation of the dam represent weak areas which had eased the water movement along these surfaces. The movement of water along these surfaces increased the dissolution of rocks vertically and horizontally. Furthermore, the supply of fresh unsaturated water from Mosul Dam reservoir tremendously increased this process.
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The dip of the beds on the eastern side of the dam that does not exceed 6 degrees SE direction and this makes the groundwater to move along this direction. It was noticed that when the water level in the reservoir exceeds 318 m. a. s. l. the rate of dissolution increases. This is attributed to the rock types within the area. The beds on the far East part of the dam are horizontal and that condition decreases the rate of dissolution. It should be also mentioned that the high dip of beds on the right side of Wadi Deer Almaleh fold is so high and this reduces the rates of dissolution.
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The concentration of sulfates in the seepage water increased indicating increased dissolution of gypsum.
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Caverns and sinkholes are increasing under the dam. There were signs of increased formation of cavities under the dam. Dissolution of gypsum reached 10,000 m3 during the period August 2014 until beginning of 2016 due to discontinuation of grouting works.
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Increased monolith movement in the grouting gallery and cracks opening were noticed.
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It was a very well known fact to the Engineers working in the dam that there was continuous settlement in the grouting gallery under the dam which is beyond the expected elastic compression values. This indicated that even with the continuous grouting under the foundation for all the past years, dissolution of gypsum continued making room for these high settlements. Records were kept and plotted from 1986 up to summer of 2015. The plot in Fig. 15 shows this settlement in five locations along the grouting gallery in the deep section of the dam.×
5 What Happens if the Dam Fails?
Case | 1 | 2 | 3 | 4 | 5 | 6 |
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Manning “n” | 0.033 | 0.050 | 0.033 | 0.050 | 0.033 | 0.050 |
Width of Breach(m) | 700 | 700 | 700 | 700 | 200 | 200 |
Breaching time (h) | 4 | 4 | 5 | 5 | 2 | 2 |
Time in hours (h) | Q × 1000 m3/s | |||||
0 | 1 | 1 | 1 | 1 | 1 | 1 |
1 | 13 | 13 | 13 | 13 | 50 | 50 |
1.5 | 80 | 80 | 80 | 80 | 385 | 380 |
2.0 | 215 | 210 | 215 | 212 | 425 | 415 |
2.5 | 372 | 356 | 335 | 325 | 405 | 390 |
3.0 | 474 | 452 | 422 | 404 | 385 | 365 |
3.5 | 535 | 499 | 480 | 453 | 375 | 330 |
4.0 | 551 | 510 | 509 | 475 | 360 | 310 |
4.5 | 538 | 469 | 497 | 460 | 345 | 290 |
5.0 | 507 | 469 | 497 | 460 | 330 | 275 |
6.0 | 405 | 382 | 435 | 405 | 280 | 260 |
8.0 | 271 | 266 | 186 | 278 | 205 | 210 |
10.0 | 186 | 192 | 195 | 198 | 180 | 160 |
12.0 | 123 | 136 | 130 | 142 | 150 | 80 |
18.0 | 37 | 47 | 39 | 49 | 50 | 65 |
24.0 | 18 | 2 | 19 | 22 | 20 | 40 |
City | Mosul | Baeji | Tikrit | Sammara | Baghdad | ||||||||||
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RL | T1 | T2 | H | T1 | T2 | H | T1 | T2 | H | T1 | T2 | H | T1 | T2 | H |
330 | 1.7 | 6.2 | 26.3 | 16,8 | 23.5 | 14.3 | 21.08 | 26.8 | 14.5 | 26.12 | 30.08 | 16.1 | 67.0 | 67.0 | 8 |
319 | 2.29 | – | 22.7 | 18.59 | – | 11.8 | 24.07 | – | 11.8 | 30.05 | – | 13.8 | 76.20 | – | 7.6 |
309 | 2.55 | – | 19.2 | 22.54 | – | 9.3 | 29.00 | – | 9.5 | 36.48 | – | 11.1 | 96.40 | – | 7.5 |
307 | 2.54 | – | 18.6 | 23.45 | – | 8.9 | 30.04 | – | 9.1 | 38.12 | – | 10.4 | 103.05 | – | 7.4 |
305 | 3.17 | – | 17.9 | 24.53 | – | 8.3 | 31.19 | – | 8.7 | 40.12 | – | 9.9 | 111.31 | – | 7.3 |
300 | 3.40 | – | 16.0 | 28,32 | – | 7.3 | 35.58 | – | 7,8 | 46.18 | – | 8.3 | 141.33 | – | 7.1 |
Inundation | Population | Area (km2) |
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0.1–0.5 m | 948,000 | 637 |
0.5–2.0 m | 3,144,000 | 2022 |
2–5 m | 1,626,000 | 1150 |
5–10 | 260,000 | 916 |
> 10 m | 270,000 | 916 |
Total | 6,248,000 | 7202 |
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Reservoir water level = 330 which is the maximum design operation water level.
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Reservoir water level = 319 which the maximum operation water level adopted in (2006).
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Reservoir water level = 300 which is the dead storage water level as per design.
RWL | T | Inundation | Mosul | Bayji | Tikrit | Samara | Baghdad |
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330 | 6 | 0.1–0.5 | 21,000 | 0 | 0 | 0 | 746,000 |
0.5–2.0 | 55,000 | 300 | 100 | 3000 | 2,949,000 | ||
2–5 | 41,000 | 400 | 2500 | 100 | 1,134,000 | ||
5–10 | 60,000 | 17,000 | 14,000 | 5500 | 26,000 | ||
> 10 | 183,000 | 2000 | 4000 | 3500 | 0 | ||
319 | 6 | 0.1–0.5 | 27,000 | 0 | 0 | 0 | 803,000 |
0.5–2.0 | 12,000 | 500 | 0 | 0 | 1,756,000 | ||
2–5 | 510,000 | 15,000 | 3500 | 1000 | 750,000 | ||
5–10 | 66,000 | 4500 | 14,000 | 10,000 | 78,000 | ||
> 10 | 125,000 | 0 | 500 | 1000 | 0 | ||
300 | 12 | 0.1–0.5 | 14,000 | 500 | 500 | 1000 | 382,000 |
0.5–2.0 m | 15,000 | 9500 | 7000 | 7000 | 850,000 | ||
2–5 | 86,000 | 3500 | 8000 | 3000 | 229,000 | ||
5–10 | 54,000 | 0 | 500 | 500 | 60,000 | ||
> 10 | 4000 | 0 | 0 | 0 | 0 |
6 Is There a Possible Solution?
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In the context of strengthening the grout curtain in the problem areas where massive grouting had to be repeated widening the curtain was recommended. Additional rows of boreholes ought to be drilled consisting of one row upstream of the present curtain and slightly inclined towards the upstream, another row in the downstream of the present curtain and inclined towards downstream, and finally a central vertical row in between. The central row was to be grouted first followed by the upstream row and then the downstream. Finally, the central row would be re-drilled and fine grouting to be performed using silica gel. The Board did not object to this proposal as machinery and grouting capacity were available.
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The second solution was to construct a tunnel the length of the chalky series from which grouting would be performed. The Board thought that such work was very specialized and would need expert studies to check its feasibility.
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The third solution was to construct a series of tunnels and galleries to replace risky material. This alternative received the same comments as in (b) above.
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The fourth alternative was to construct a diaphragm wall from the upstream berm, with a sloping concrete facing from the top of the diaphragm to the top of the dam (Fig. 21). Or even to remove part of the top of the dam and install the diaphragm through the core in a location upstream of the gallery. This arrangement was attributed to the unavailability of machines that could cut to the desired level. In a later update, the removal of the dam top was thought unnecessary due to new development in diaphragm machines. The Board; however, judged this solution undesirable due to the required lowering of the reservoir level, which could extend 2–3 years in addition to the very high cost.×
7 Conclusion and Recommendations
Mosul Dam is suffering from the seepage problem under the foundation of the dam since the start of its operation in 1986. Various works had been done to overcome the problem, but it seems that the dam is showing more and more signs of weakness. In view of all the published work and meetings it is well understood that the geology of the dam site is very complicated and it suffers from severe problems due to the presence of badly jointed and cavernous soluble gypsum/anhydrite layers, gypsum breccias layers, weathered and jointed limestone and soft marls. The consultants had underestimated these problems where impounding have increased the solubility of the rocks within the vicinity of the dam and enhance the formation of sinkholes. Despite all the intensive maintenance work the dissolution of gypsum continued and the gypsum breccias layers were very much resistant to grouting at the deep grout curtain zones. Furthermore, it is evident from the work done that grouting can only be a temporary solution where it cannot stop permanently the dissolution of gypsum. In addition, it is causing weakening of the rocks by the re-grouting process; it cannot stop the progressive formation of sinkholes and, moreover it is giving a false sense of security.Looking at the report, papers and discussions of researchers and experts, it is believed that the following points are to be considered:
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Keeping the reservoir water level as low as possible for such a period that might be necessary subject to updated risk analysis.
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It is very important to continue grouting operations and to evaluate the existing monitoring program and upgrade the system with new available instrumentation to pin point the most critical locations to be grouted.
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Providing possible early warning system for changing conditions that may indicate where and when a failure mechanism is developing.
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Bathymetric survey should be conducted to determine the points of excessive leakage from the reservoir and to detect any sinkhole, spring and seeps within the upstream area in the reservoir close to the dam.
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Emergency Action plan should be developed and implemented and the public should be aware of such a plan.
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A new panel of experts is to be established to meet every three months to review the status of the grouting program and any new development.
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Since grouting is not the final solution for the problem, other alternatives are to be carefully studied and a decision should be taken to solve this problem. These solutions may require to include the construction of a diaphragm in Mosul Dam, completion of Badush Dam or a hybrid solution of both.