Chapter 3: Hydraulic Engineering Earthquake Damage to Hydraulic Structures

243 CHAPTER 3: HYDRAULIC ENGINEERING EARTHQUAKE DAMAGE TO HYDRAULIC STRUCTURES IN THE TANGSHAN PREFECTURE1 Yuan Guiji2 1. Introduction Before the earthquake of 1976 the Tangshan Prefecture consisted of the cities of Tangshan and Qinhuangdao, the counties of Fengrun, Fengnan, Luanxian, Luannan, Leting, Changli, Funing, Lulong, Qianan, Qianxi, Zunhua, and Yutian as well as the Baigezhuang reclamation area. The total area of cultivated land amounted to 11.65 million mu (one mu equals 660 square meters). According to the topography of the Tangshan Prefecture, the land was classified into four areas with high elevation in the northeast and a low elevation in the southwest part as shown in Figure 1. The Yanshan Hill area was in the north part having an elevation of about 100 m for the hill area and an elevation of 300 to 500 m for the mountainous area. There were 3 large B size, 4 medium size, 48 small A size and 357 small B size reservoirs as well as 1,141 water ponds, and 13 pump stations built in this Prefecture after 1949. Terraced cultivated land amounted to 0.4884 million mu. (The reservoirs having a water impounding capacity greater than a billion, one hundred million, ten million, and a million and fifty thousand cubic meters classified as the large A, large B, medium, small A and small B sizes respectively). The plain area was in the south and could be classified into the following three areas: (1) Luanji alluvial fan, a very fertile and level land where irrigation canals, small sluices, conduits, as well as pumping wells were the main hydraulic structures; (2) Jidong coastal area with an elevation of less than 10 m; and (3) Jidong swampy area with an elevation of less than 3 m. Most of the large and medium sluices and drainage pumping stations were built in the two low and flat areas in (2) and (3), and also 3 reservoirs and a 120 km long coastal dike. 1 Thanks are expressed to Messrs. Wen Zhili, Zheng Chunyuan, Zheng Guoqing, Liu Ruixiang, Li Linhua, Zhang Dingwen, Li Chengzhou, Dong Xiuzhong, Shi Chongyi, Li Shouzhen, Xiao Diancai, Liu Qi, Pang Jun, Wang Zhengqi, Wang Jingyao, Liang Baohua, Tong Zuoxiang, Yang Wentian, Sun Jizhen, Yang Lixian, Di Liang, Zhao Qingxian, Zhang Hongmin, Ge Jiguang, Wang Jianyong and Yang Jingsi for their assistance in providing and checking information. Thanks are also expressed to Messrs. Ma Pingjun and Lu Xingui for their preparation of the draft of Section 2 which deals with damage to reservoirs. 2 Bureau of Water Resources of the Tangshan Prefecture 244 There are about 10 large and medium rivers in this Prefecture which includes the Jiyuanhe, Huanxianghe, Luanhe, Douhe, Shahe, Yinmahe, Yanghe, Daihe, Tanghe and Shihe rivers in addition to tens of small rivers. Some rocks outcropped in the Yanshan Hill area but in the valley the bedrock was covered by soils with a maximum depth of 10 to 20 m and an average depth of 3 to 6 m. Soils originated from rock of the Cenozoic era, which are found south of the Luanji alluvial fan. The soils were sand, gravel, clayey sand, and organic clay, their thickness increased from 20 m in the north to 800 m in the south. The thickness of soil deposits at the Jidong coast and swampy area ranged from 800 to 1,700 m, and even up to more than 2,000 m. The main damages to the Yanshan Hill area were land cracks, sandboils, cracks and settlements of embankment dams, leaks through dam foundations, cracks of conduits, collapse of parapet walls, disappearance of individual springs, and leaks at individual reservoir areas; 330 or 80% of hydraulic structures were damaged. The collapsed terraced land amounted to 0.114 million mu, and some water wells became dry. Since the plain area south of the Luanji alluvial fan consisted mainly of saturated sand, sandboils and subsidence occurred and salt-water springs appeared from place to place resulting in blockage of irrigation canals, siltation of sand and alkalization of cultivated land of 1.594 million mu. The discharge capacity of some rivers decreased due to blockage, sliding of riverbanks and cracks. Rivers, dikes, irrigation canals, sluices, pumping stations, and wells and also small conduits and bridges within the cultivated land were damaged to various degrees. Table 1 shows the degree of damage to important reservoirs, sluices and pumping stations under different earthquake intensities. 2. Damage to Reservoirs (1) Introduction There were 412 reservoirs in the Tangshan Prefecture before the 1976 earthquake including 3 large B size, 4 medium size, 48 small A size and 357 small B size reservoirs. At these reservoirs 386 out of 412 dams were earth structures among which 152 were homogeneous dams, 145 were sloping core dams and 89 were central core dams. The construction material used for the homogeneous dams was mainly silty clay, and mostly clay for the core dams. All were rolled compacted embankment dams. Other types were earth and rock fill dams and stone masonry dams. Figure 1 shows the location of the reservoirs. The large and medium sized dams were on the main stream of rivers. River deposits on the bedrock were Quaternary in nature having a wide range of compositions. The thickness of river deposits for small sized dams was only 4 to 6 m, only 2% of reservoirs had a river deposit thickness exceeding 10 m. No consideration for earthquakes was taken into account in the design of the dams except for the Douhe, Qiuzhuang, Yanghe, Banruoyuan, Shihe and Gaojiadian reservoirs which were being designed to resist an earthquake having an intensity of VII. Most small sized dams had a dam crest width of 4 to 5 m, an upstream slope of 1 on 2.75 to 3.50, and a downstream slope of 1 on 2.50 to 245 3.00. There was a berm on both the up and downstream slopes. A cut off wall having a base width of 3 to 4 m was mainly in direct contact with the bedrock. Outlet conduits were usually founded on bedrock and concrete or masonry structures were built under the dam. Before the earthquake it rained continuously in the north mountainous region though the rain was light. However, it rained heavily within 3 to 5 days before the earthquake, the precipitation amounted to 80 to 200 mm. As a result, most of the reservoir water levels reached the normal or high water level and the dams were subjected to a full hydrostatic load. Inspection of the dams after the earthquake revealed that 330 or 83% of the 399 usable dams were damaged. All the dams were classified into four categories in accordance to the seriousness of damage. They were serious, less serious, slight and no damage. 1) Serious damage. Piping at downstream of the dam; large and long longitudinal cracks with a width of more than 20 cm; transverse cracks whose width exceeds 3 cm; visible settlement; cracked conduit; tilting of parapet wall. 2) Less serious damage than (1). Similar damage but to a lesser degree. 3) Slight damage. Slightly damaged, needing only very simple repair work. 4) No damage. No visible damage. Table 2 shows the severity of damage to dams subjected to different earthquake intensities. (2) Investigation of earthquake damage There were 55 reservoirs of large, medium and small A sizes in this Prefecture among which 50 were earth dams, and 5 masonry dams. Table 3 briefly shows their damages. In Table 3 the No. 1 to 3 reservoirs were large B size; No. 4 to 7 were medium size, No. 8 to 55 were small A size and No. 56 to 68 were small B size. The locations of these reservoirs are shown in Figure 1 in reference to their label number in Table 3. There were 15 masonry dams within the earthquake region among which 12 dams were investigated including 5 stone masonry gravity dams, 3 stone masonry and rock fill dams, 2 stone masonry framework gravity dams and 2 stone masonry arch dams. Their damages are shown in Table 4. (3) Typical examples of damage 1) Qiuzhuang Reservoir A. Introduction The reservoir was located on the middle up-reach of Whuanxianghe River, its drainage area was 525 square kilometers and its main purpose was flood control and irrigation. It was a large B size reservoir. The dam was a homogeneous earth structure with a height of 24.8 m and a length of 926 m. The widths of the crest and base were 5 m and 161.55 m respectively. The elevation of the dam crest was 73.8 m and there was one berm at an elevation 69 m on both the up and downstream slopes. Figure 2 shows its cross-section. An open spillway along the bank consisted of three 246 openings controlled by three steel tainter gates with a width of 13.5 m and a height of 10.9 m. Its maximum discharge was 3,387 cubic meters per second. A concrete conduit built in a trench was capable of discharging 333 cubic meters per second through a horseshoe cross-section. B. Foundation and its treatment The thickness of river deposits on siliceous limestone varied from 17.75 m near the left abutment to 33.60 m at the main riverbed and decreased to 17.13 m near the right abutment. The deposits from top to bottom were successively sandy clay, fine sand, gravelly sand, gravel and cobble. The sandy clay layer on the surface had a thickness of 3.7 to 5.8 m and was dark yellow in color and hard plastic in state.

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