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Published on December 10, 2007

Author: Carlton

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Underground Dam:  Underground Dam - Illustrated guide of underground dam - Part of data were taken from that provided by Okinawa General Bureau, Land Improvement Section Slide2:  Underground dam is... A facility that dams up groundwater flow, stores in the pores of the stratum and uses groundwater in a sustainable way. Furthermore, a facility for preventing saltwater intrusion is also included to definition of the underground dam. In a wide sense, it is called as underground dam including reservoir area. Underground dam have no huge “Tank” under the ground, generally have a lot of porosity in the aquifer (underground). In other words, underground dam reserves the groundwater in “hard” porous sponges. What is underground dam... Slide3:  Various types of underground dam Underground dam is classified into various types by the purpose, the construction method and the reservoir form. Underground dam is mainly classified into two types by its purpose. <1> Dam up type (Storage type, Run-off control type) This dam type is planned to store groundwater. The reservoir, which dams up groundwater and regulates its discharge, accordingly increases the groundwater level and allows stable intake of groundwater. <2> Saltwater intrusion prevention type This dam type is planned to prevent intrusion of saltwater into the groundwater and to protect available water resources. The reservoir unconditionally allows groundwater pumping and the resultant adjustment of the groundwater level. Classification by purpose Classification by purpose of underground dam Slide5:  Dam up type (storage type) Saltwater intrusion prevention type Slide6:  Classification by construction method Construction methods of underground dam are mainly classified into the following 3 categories. <1> Ground improvement method (grouting method) In general, is applied by foundation improvement of surface dam, using injection of cement milk under the ground and hardening the milk, and so constructing the impermeable barrier (in other words, grout curtain). This method is applied to underground dam in small scale. <2> Impermeable body driving method This method is to construct a dam body by driving steel sheet pile (or concrete sheet pile). This method is used for shallow unconsolidated layer. <3> Diaphragm wall method Diaphragm wall method is applied to underground dam in large scale. Among of the several types of diaphragm method used for underground dam construction, SMW method (Soil Mixing Wall method) is the most popular method in Japan. Slide7:  Classification by construction method of underground dam Remarks; As to the diaphragm wall method, the following methods were also performed as testing construction of the underground dam projects in Japan. - Bucket excavation method - Horizontal multi-axis excavation method - Large-diameter drilling method - Thin cut-off wall by chainsaw type method - TRD (trench cutting re-mixing deep wall) method etc. Slide8:  <Example of diaphragm wall method (actually used method for test construction (in Japan)> Bucket excavation method Horizontal multi-axis excavation method Slide9:  <Example of diaphragm wall method (actually used method for test construction (in Japan)> Large-diameter drilling method Thin cut-off wall by chainsaw type method Slide10:  <Example of diaphragm wall method (actually used method for test construction (in Japan)> Slide11:  Items of ground improvement method and diaphragm wall method Slide12:  SMW method is... - Many large scale underground dams in Japan are constructed by SMW method, taking geological condition, working efficiency and cost performance into consideration. - SMW is an acronym for Soil Mixing Wall, which is one of the diaphragm wall methods. - Drilling of SMW method is performed by process of casing drilling, pilot drilling and multi-shaft auger (tri-axial auger) drilling, and the wall elements by soil-cement are formed to be mixed the soil in-place with cement liquid etc.. These elements are overlapped to form a continuous wall.        - The general construction process of SMW method is shown in the following figures. Slide13:  Image for construction process of SMW method Main machine of SMW method Casing drilling Pilot drilling Triaxial drilling Construction process Crest cleaning Slide14:  Classification by reservoir form Reservoir forms of underground dam are mainly classified into the following 3 categories. <1> Fully subsurface storage type This dam type is ordinary case of underground dam and reservoir is not visible directly. <2> Partially surface storage type This dam type has functions not only to reserve groundwater but also to store surface water on the ground in the reservoir area. <3> Surface dam hybrid type At the surface dam, reservoir water is stored in the ground in addition to on the ground by the effectiveness of the watertightness barrier, which is created by the foundation treatment such as grouting works. Slide15:  Classification by reservoir form Classification by reservoir form of underground dam Slide16:  Example of fully surface storage type underground dam Slide17:  Example of partially surface storage type underground dam (Kanjin underground dam) Surface reservoir of Kanjin underground dam Slide18:  Merits and demerits of underground dam Underground dam has some merits and demerits because of its characteristics (in comparison with surface water intake facilities such as surface dam and weir etc.). Slide19:  Technical terms of underground dam Underground dam Dam body (cut-off wall)・・・The construction of underground in order to shut out the groundwater flow. Drainage facility・・・The facility to drain the surplus water from the underground dam. Intake facility・・・・The facility to take the reserved water from the underground dam. Operation and Maintenance facilities・・・The facility to operate and maintain the underground dam. Recharge facility・・・The facility in order to increase the storage volume of underground dam by infiltration. Slide20:  Technical terms of underground dam Dam crest : The crest of the cut-off wall. Dam length : Length of the dam body. Dam height : Height of the dam body (including penetration part). Penetration part : Part penetrated into the basement layer. Slide21:  Technical terms of underground dam Cut-off area : The sectional area constructed by cut-off wall (including Penetration part area. Uncut-off area : The sectional area between working floor and dam crest. Penetration part area : The sectional area constructed in the basement by cut-off wall. Working floor : Flat plain for construction of cut-off wall. Slide22:  Critical water level : Permitted maximum water level in the reservoir. Full water level:Water level when an overflow starts from the dam crest. Low water level (Dead water level):Minimum water table that is reached as a result of groundwater use in the reservoir area. Effective water capacity:Storage capacity obtained by subtracting the dead water capacity from the gross water capacity. Dead water capacity:Water volume below low water level. Gross reservoir capacity:The sum of effective water capacity and dead water capacity. Technical terms of underground dam Slide23:  Requested natural condition for underground dam The following three natural conditions are of particular importance for planned underground dam. In general, the specialty of underground dam development is attributed to the fact that regions satisfying all three requirements below are rare. 1. Existence of excellent storage aquifer An aquifer with large effective porosity and hydraulic conductivity must be available in the planned area. The former influences the storage capacity, and the latter influences the groundwater intake. 2. Existence of impermeable basement stratum such as underground valley The basement stratum forming the reservoir floor and side boundaries must be relatively watertight so as to form efficient groundwater reservoir. The underground dam is created by cut-off wall reaching the basement stratum securely. In this case, suitable length and depth of the cut-off wall must be considered for economic reason. The maximum depth for the construction of the cut-off wall in Japan is about 65m. 3. Sufficient recharge to reservoir area Sufficient and appropriate groundwater recharge must be available in the reservoir area of underground dam. Normally, high precipitation and infiltration will be required corresponding to the planned amount of storing water. In addition, it may be necessary to confirm the absence of stratum which would be subjected to volume change by the storage and intake in the reservoir area. Because the volume changes in a stratum due to abstraction could leads to subsidence. Furthermore, other deleterious environments such as soil and groundwater salinization and risk of pollution and contamination should be carefully considered in the site selection. Slide24:  Examples of suitable condition for underground dam (1) Limestone plateau (2) Coastal plain (4) Alluvial fan (3) The footplace of volcano Slide25:  Location map of existing underground dam There are many completed underground dam in the world, but those are small scale excluding those in Japan. Slide26:  List of underground dam in Japan Furthermore, the following underground dam is being planned at the Okinawa area.   Nakahara, Bora, Tarama, Yomitan, Tsuken, Yonaguni and Motobu underground dam Slide27:  How the idea of underground dam in large scale create at the Okinawa areas. 1. In Okinawa, annual rainfall exceeds 2,000mm, however, they concentrate rainy season (tsuyu season) and tyhoon season, the other season is short of water. 2. Many islands in Okinawa are covered with limestone with high permeability and high effective porosity. 3. A major part of rainfall water is easily infiltrated into the ground and flow out into the sea, and groundwater level is to low in order to pump up the necessary amount. 4. By the topographical and geological conditions, the construction of surface dam is not suitable. However, at the underground, there are under valley structures which consist of impermeable layer. 5. These matters have led to the idea of underground dam in large scale, which dams up groundwater flow by closing of aquifer limestone. Slide28:  Sketch of underground structure in Miyako Island 沖縄の位置:Location of OKINAWA:  沖縄の位置:Location of OKINAWA 沖縄県は,日本の南西部に位置し,九州と台湾の間に弓なりに連なる琉球弧に属し,東西1,000Km,南北400Kmの広大な海域に点在する大小160の島嶼(うち有人島49)からなっている。 Okinawa Prefecture is located in the area between lat. 24° and 28° , long.131° and 123°E, stretching over the distance of 400km from North to South and 900km from East to West. Okinawa Prefecture consists of 112 uninhabited islands and 49 inhabited islands. These island are scattered along Ryukyu-Arc. The distance from Okinawa to Kagosima(Kyuusyu island) is 600km and to Tokyo is 1,560km Slide30:  Year            Events Ryukyu is first mentioned in the annuals of Chinese history Sho Hashi unifies the three principalities into one centralized Kingdom Satsuma (Kagoshima) invades the Ryukyu Kingdom U.S. Commondore Matthew C. Perry visits Naha on his way to JAPN The Ryukyu Kingdom is abolished. OKINAWA Prefecture is established. 1945 OKINAWA is invaded by U.S. forces on April 1 and fighting ends with Japanese surrender of the Ryukyu Islands on June 23. The Government of the Ryukyu is established. In a joint statement, President Nixon and Prime Minister Sato announce agreement for the return of OKINAWA to Japan in 1972 OKINAWA returned to Japan after 27 years of U.S. control. OKINAWA prefectural Government is established. 2000 G8 Leaders’ Conference (Kyusyu-KINAWA Summit) held in July. 沖縄の歴史年表(主要事項) 推古13|605AD| [琉球」はじめて中国史にあらわれる。 永亨元|1429| 三山統一(群雄割拠時代終わる) 慶長14|1609| 薩摩軍琉球を攻略 嘉永6|1853| ペルリ提督来航 明治5|1872| 「琉球藩」となる。 明治12|1879| 「沖縄県」となる。(琉球処分) 昭和20|1945| 米軍上陸(4.1),沖縄戦終結(6.23) 昭和27|1952| 琉球政府創立(4.1) 昭和44|1969| 1972年の沖縄返還きまる 昭和47|1972|沖縄の本土復帰が実現,沖縄県復活 平成12|2000|九州沖縄サミット開催 沖縄の歴史:History of OKINAWA Slide31:  人口,土地面積などの沖縄県の主要指標は,全国シェアの概ね1%を占めている。 主要指標比較(全国~沖縄) Major Index Comparison (National ~ OKINAWA) 単位 全  国 沖  縄 シェア(%) unite National OKINAWA share 土地面積 Km2 377,829 2,266 0.6 Land area 人口 千人(th) 124,944 1,287 1.0 Population 世帯数 千戸(th) 44,072 404 0.9 Number of families 就業人口 千人(th) 64,141 542 0.8 Working population 沖縄の主要社会指標:Major Social Index of OKINAWA OKINAWA has 1% share of Major social index in JAPAN. Slide32:  農業の主要指標と農業の特徴:Agricultural Major Index and its Feature 我が国唯一の亜熱帯気候を活かし,サトウキビが基幹として栽培されている。また、収益性の高い花卉、熱帯果樹等の栽培が展開されている。 Sugar cane is cultivated as a main crop in Okinawa by good use of its location in the subtropical climate area, and furthermore high profitability crops such as flowers, tropical fruit, etc... are newly being promoted. Mangos Orchid Slide33:  沖縄の農業の課題としての 水資源の確保 Securing Water Resources as a problem to be solved for promoting Agricultural Condition in OKINAWA. 沖縄の年間降水量は,平年で2,037mm(那覇)と全国と比較して多いものの, 降雨が梅雨期と台風期に集中することもあって,季節毎、年毎のばらつきが大きい。 OKINAWA has an average annual rainfall of some 2,000 mm, higher than the National average, however, that the greater part of the total rainfall comes from “TSUYU” or typhoons , and for this reason the quantity of water varies greatly from season to season and from year to year. 一方,沖縄の河川の延長は,一般的に短く,かつ,河川勾配も急であり利水がしにくい。 Most of rivers in OKINAWA tend to be short and precipitous, thus difficult to be used. Drought Disaster (Sugarcane) Annual rainfall and temperature in average in Okinawa Slide34:  琉球石灰岩が分布する沖縄本島の一部,宮古島やその他の中小離島では,降雨の多くが地下に浸透し,河川が発達しにくいなどの理由により,本土復帰以前には大規模な農業用水開発が行われず不安定な農業経営を強いられてきている。 The Ryukyu Island are marked be a wide spread of porous coral reef limestone called Ryukyu Limestone. The Ryukyu Limestone is so porous that a major part of the rainfall permeates to subsurface, and river in such area is difficult to be develop. Therefore, agricultural water resource development project, such as construction dam, head works …etc, could be conducted before reversion to Japan, and agricultural products was unstable. Water Circulation Image in Miyako Island Simajiri layer (Mud stone) Ryukyu Limestone Evapotranspiration 40% Surface runoff 10% Runoff through subsurface Rainfall 100% Ryukyu Limestone 40% Slide35:  < Outcrop of Ryukyu limestone (Quaternary, Pleistocene limestone), at the Ie underground dam> Slide36:  The Prospect of the underground dam technology in future The 1st generation underground dam The underground dam technology is already established and is used as practical facilities in Japan. These underground dams have been constructed to the area where the geological condition and groundwater condition are suitable for the construction of that. These underground dams can be stated as the 1st generation underground dam. The establishment of new technology for the next generation underground dam At the southeast island area in Japan, there are many areas accompanying with lack of water. In these areas, there are some cases that the site conditions for underground dam are not always suitable sites, and it hardly seems possible that underground dam constructs using of only the 1st generation underground dam technology. For example, - Dam site has not clear underground valley condition. - Impermeable basement is located on the deep depth (or is not recognized within dam construction limit). The underground dam projects accompanying with the above new technology development is called as the next generation (the 2nd, the 3rd generation) underground dam. Slide37:  The 2nd generation underground dam technology The underground dams (Ie, Yokatsu and Izena underground dam) by the use of the 2nd generation technology are under construction at present. Examples of site condition of the 2nd generation underground dam are as follows. The Prospect of the underground dam technology in future Slide38:  The Prospect of the underground dam technology in future The 3rd generation underground dam technology Examples of site condition of the 3rd generation underground dam are as follows. The dam type of this generation is called as the floating type underground dam Slide39:  As to the 3rd generation underground dam, the test construction of cut-off wall was already performed for the confirmation of recharge and storage effect at the Tsuken island (in the last year). The results of test construction revealed that project plan has potentially high effectiveness. The Prospect of the underground dam technology in future Slide40:  The Prospect of the underground dam technology in future THE END Thank for your enjoy presentation:  THE END Thank for your enjoy presentation

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