The site of the river-cutoff dam of the Three Gorges Project is located near the late Sandouping Town of Yichang, Hubei Province. The Sandouping Town used to be an ordinary mountainous market town at the med-length of the beautiful Xiling Gorge and by the right bank of the Yangtze River. With the completion of the Gezhouba Hydropower Project, the late Sandouping Town had been submerged then. A new Sandouping Town has been built up at the new location downstream of the site with the repayment funds of the Gezhouba Hydropower Project.
The siting of the dam of the Three Gorges Project had been made with a great number of geological surveys and a great many proofs by the experts by the preliminary selection. The study and proof on the dam-length siting had lasted 24 years by then with the careful study and investigation of the two dam regions, the 15 dam sites in the two dam regions and their dozens of dam axes.
The two dam regions and the 15 dam sites in the two dam regions had been surveyed and studied for the first surveys and studies as follows.
1. Nanjinguan Limestone Dam Region and the Selected Five Dam Sites within the Region
The Nanjinguan Limestone Dam Region stretches 13 km upstream from the Nanjinguan, the outlet of the Three Gorges of the Yangtze River, to the Stone Tablet, where 5 dam sites had been selected by the first surveys and studies for the final choice of the dam site of the Three Gorges Project;
2. Meirentuo Granite Dam Region and the Selected Ten Dam Sites within the Region
The Meirentuo Granite Dam Region stretches 25 km upstream from Liantuo to Meirentuo, where 10 dam sites have been selected by the first surveys and studies for the final choice of the dam site of the Three Gorges Project.
The Nanjinguan Dam Site of the Nanjinguan Limestone Dam Region and the Sandouping Dam Site of the Meringue Granite Dam Region have been both selected out of all the above 15 dam lengths for comparison with the further geological surveys and inv estigations so that the final dam site can be chosen for the Three Gorges Project. Three years since then, the location surveys of the large and small scales on the two dam sites had been made with the about 53,000 m drilling and large quantities of studies and researches of hydrograph geology and engineering geology, and the tests of rock mechanism.
The further geological survey then showed that there were serious problems with the geology of the limestone dam region.
The problems were
1. The river valley was narrow; the overburden layer was very thick;
2. The low-angle-dip fault was relatively developed and its structural rock was weak;
3. The karst was developed;
4. The conditions for the engineering geology and hydrograph of the site were very complex.
On the contrary, the further geological survey showed that there were relatively minor problems with the geology of the granite dam region. The dam region was proved as follows.
5. The valley of the granite dam site was relatively broad with the overburden layer of more than 10 m.
6. The riverbed rock was relatively complete and hard.
7. Although the fracture structure was quite developed, the cementation of the structure rock had been proved with the test of recrystallization.
It was concluded on the comparison among the geographies, geologies, layouts of the structures of the complex project, and the conditions on the construction in 1959 that the conditions of the granite dam region were obviously better than those of the limestone dam region.
The structures and the rock conditions of the 10 dam lengths of the granite dam region were generally the same at that time. The final selection was then based on the valley geography and the superficial weathering of the rocks on the riverbed. The 10 dam lengths were accordingly divided into two types as follows.
1. First Type
The dam sites of the first type were of the medium river width;
2. Second Type
The dam sites of the sec ond type were of the large river width.
The Taipingxi Dam Site was selected out of all the dam sites of the first type, where it was relatively good to lay out the underground power house and the conditions for the protection of the construction were relatively superior.
The Sandouping Dam Site was selected out of all the dam sites of the second type, where it was relatively good to lay out the power houses behind the dams and the site of the construction of the project was relatively broad.
Both the dam sites were proper for the construction of the high concrete dam geologically. By the time of the final selection of the dam site, the drillings at the above two dam sites had been made respectively by 30,000 m and 53,000 m.
Then there were Ha, Hb, Ma, Mb, La and Lb axes for comparison even at the Sandouping Dam Site. With the comprehensive comparison, the Planning Office of the Yangtze River Basin recommended the Ha axis for that of the final dam site. This proposal was then made for the discussion of the experts from all over China. Quite a few discussions had been made nationwide until the Ha axis at the Sandouping Dam Site was selected at the nationwide meeting for the final dam-site selection in 1979 for the dam site of the Three Gorges Project.
Professor Muler from Austria is a founder of the method for the excavation of tunnels and caves, and a famous international authority of rock mechanism and engineering geology. He inspected the Sandouping Dam Site in May of 1986 and praised that the dam site was really idealistic and it was really a gift to the Chinese people from God. All the experts either from China or from all over the world have all thought that the dam site was conditionally good. Such a conclusion is based on the following.
1. Dam Site on the Idealistic Granite Belt
The river channel of the Three Gorges with the beautiful scenery on both the river banks lasts 192 km from the Baidi City upstream to Nanjinguan downstream. But on ly the bed of the 31 km river channel between Mache to Liantuo is geologically proper for the construction of the high concrete dam, whose rock structure is composed of pyrogenetic-amphibolitic granite bodies (called granite in the following ), and where the Sandouping Dam Site is located. The bed of the 141 km river channel upstream from the Baidi City is composed of metamorphic rocks, sandstones and lime rocks; and the bed of the 20 km river channel downstream from Liantuo to Nanjinguan is composed of developed lime rocks. It is very difficult to select a idealistic dam site on either of the above river channels. If we selected such a dam site within an even longer river channel 658 km upstream from Nanjinguan to Chongqing, we would find an idealistic dam site only within the above 31 km river channel with the granite riverbed. This is the very conclusion on the basis of a great many studies and surveys of the dam site. It is noted that the bed of the river channel between Baidi City and Chongqing is mostly of sandstones and mudstones.
2. Complete Rock Body for the Construction of the Dam
The river bed at the dam site of the Three Gorges Project is mostly of granite. The granite bodies there are relatively complete with high mechanical strength. Their maximum compress is up to 100 MPa equal to the pressure of 10,000 m water column. In addition, the two fractures in the dam site, which are geologically called fracture structures, are not large mostly with the tilt angles of above 60o and the good cementation. Such a geological condition is rare and superior either in China or in the world because there have been one or more large fractures at most of the high-dam sites and most of the fractures have a crushed zone/ crushed zones and even a mud zone/ mud zones in between.
3. Rock Bodies with the Excellent Waterproof
The rock bodies at the dam site of the Three Gorges Project are of extremely weak hydraulic conductivity. In a word, the unit hydraul ic conductivity is generally less than 0.01 l/ decimeter, that is, the hydraulic conductivity is generally less than 0.01 liter at a 1 m water head within 1 m hole length within one minute. This is rare for all the high-dam sites all over China. The dam sites are mostly of weak hydraulic conductivity, but partially of relatively strong hydraulic conductivity which particularly needs making waterproof against the seepage.
4. Good Geological Conditions for the Dam Safety
The dam site is located on the central area of the crystalline base formed during the frequent pre-sinian magma flows/ eruptions. Based on the analysis on the regional geological background and the structure formation, the crystalline base has neither active fractures nor earthquake structures where a great earthquake might occur. It is concluded that the massif on which the dam site is located is relatively more reliable and rigid. It has been proved from the historical records of nearly 2000 years that the area with the radius of 320 km around the dam site is an area with few earthquakes. Even if an earthquake occurred, that earthquake would be a very weak quake with low frequency. That area is a typical weak-earthquake area geologically. The seismic intensity of the area where the dam site is located is 6o in accordance with the appraisals by the related state earthquake administrations, the reexamination by the state earthquake administrations in 1987 and the approval by the Seismic-Intensity Commission of the State Seismic Bureau. All this shows that the dam site is really qualified and superior for the construction of the dam of the Three Gorges Project. The above superior conditions of the Sandouping Dam Site have been rarely found either in China and in the world to prove very qualified for the construction of the high dam of the Three Gorges Project. The Chinese and foreign experts of engineering geology have all thought it extremely proper to select the Sandouping Dam Site for the construction of the high dam of the Three Gorges Project. All this proves it correct to finally select the Sandouping Dam Site for the project.
But the thick weathered layer over the granite base of the dam site had formed before the construction of the dam because the granite there had long exposed to the air in the moist subtropical environments and the environments have relatively rarely varied. For example, the weathered layer on the first terrace of the granite base was about 20 m thick before the construction of the dam and the weathered layers on the mountain ranges on both the banks were then up to 30 m. There had been really a great deal of clearance of the weathered soil and rocks until the fresh granite base was exposed.
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