buttress Dam

Introduction• Roman engineers, often stability erroneously, judged the of a dam wall to be inefficient, they backed it up by irregularly spaced buttresses. • A third of the Roman dams in the Iberian peninsula were buttress dams. • The most remarkable of the one Roman buttress dams is the near the village of Esparragalejo. This is the first multiple arch dam Evolution of the Modern Buttress Dam  Today buttress dams as derivations from the massive gravity type with the introduction of intermediate spaces.  These spaces allows the discharge of water seeping through the dam and its foundation, thus greatly reducing uplift pressures.  Given the absence of uplift, more substantial savings were possible by inclining the upstream face, thereby mobilizing the vertical water load on the Classifications OF BUTTRESS DAMS  Massive head buttress dams  Deck slab buttress dam  Multiple arch buttress dam Massive head buttress dams • Massive round head • Massive flat head Deck slab buttress dam • Simple slab deck • Continuous slab deck Multiple arch buttress dam The first multiple arch dam of reinforced concrete had been completed in 1908.  It impounded the Hume Lake fluming reservoir on the Ten Mile creek in the California Sierra Nevada Mountains. The dam was designed and supervised during constuction by John S. Eastwood (1857-1924). DESIGN CONSIDERATION 1. Geologic investigation for the foundation 3. Assessment of the area to be inundated by the upstream lake (also called a reservoir) and its associated environmental and ecological impacts 5. Selection of materials and construction techniques 7. Designation of methods for diverting stream flow during construction of the dam 9. Evaluation of the potential for sediments to accumulate on the reservoir bottom and subsequently reduce storage capacity Forces on the buttress dams R0 WATER P R1 R1 R2 F2 F1 w Prestessing is used to minimize the quantity of concrete and counteract tensions. Prestressing can be applied in at least three manners to a buttress dam. » To 'pull down' the upstream face » To 'jack up' the downstream face » To compact the buttress on to the foundation CONSTRUCTION PROCESS Stream must be diverted or blocked from flowing through the site. CONSTRUCTION PROCESS • A coffer-dam (a temporary structure to impound the water) must be built. • Another small coffer dam is built to block the leakages. CONSTRUCTION PROCESS Contd… • Pumps are used to remove the water from site. • Compact the site. CONSTRUCTION PROCESS Contd… • Foundation area - must be cleaned before the first concrete is placed. • Form work-Modern steel formwork is of cantilever design. • Forms made of wood or steel are constructed. CONSTRUCTION PROCESS Contd… • May be necessary to install extensive systems of rock bolts or anchor bolts. • Instruments has to be installed: » monitor groundwater levels » joint movement » potential seepage » slope movements » seismic activity FOUNDATION PREPERATION  Excavation in bedrock.  Construction on un consolidated deposits.  Grouting.  Pour the concrete. CONCRETE HANDLING  Pre-cast reinforced concrete planks & reinforced concrete piles are used. CONCRETE HANDLING Contd…  Steel reinforcing rods are cast into the body.  Buttresses also poured with reinforcing rods. Functional Importance of Elements of the Buttress Dam Structure of a Buttress Dam Upstream MWL Max. level NWL Normal water level Crest Down stream Spillway (inside dam) Buttress Free board Gallery Sluice way Heel Toe Buttress Upstream MWL Max. level NWL Normal water level Down stream Buttress Buttress A thin, erect, tabular concrete supporting member used in construction of slab and buttress dams. Also a projecting structure providing lateral support to a rock face or a portion of a dam. Heel Upstream MWL Max. level NWL Normal water level Down stream Heel Heel The upstream contact of a dam with its foundation. Toe Upstream MWL Max. level NWL Normal water level Down stream Toe Toe The downstream contact of a dam with its foundation. Crest Upstream MWL Max. level NWL Normal water level Crest Down stream Crest The top of a Dam Pavements  The top part of the dam can be designed as a road.  Same time it can act as a bridge. Spillway Upstream MWL Max. level NWL Normal water level Down stream Spillway (inside dam) Spillway The structure on or at the side of a dam that contains and guides the flow of the excess water supplied to a reservoir. Spillways inside the reservoir are called glory holes and consist of a vertical shaft a tunnel which exits below the dam. Spillway Controlled Spillway Water flow control by using gates Uncontrolled Spillway The elevation of spillway crest use to control the water flow. Spillway Types  Auxiliary Spillways/Emergency Spillways  Primary Spillways  Side Channel Spillways  Siphon Spillways  Shaft Spillways/Morning Glory Spillways Sluiceway Upstream MWL Max. level NWL Normal water level Down stream Sluice way Sluice way An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side. Gallery Upstream MWL Max. level NWL Normal water level Down stream Gallery Gallery A long, narrow passage inside of a dam used for inspection, grouting, or spillway. Free board Upstream MWL Max. level NWL Normal water level Down stream Free board Freeboar d That portion of a dam maximum water level in a reservoir. Penstock A conduit, commonly steel pipe, leading from the reservoir to a power generating plant downstream from the reservoir. Usage of Penstock Turbine Fish Ladders A structure built at the side or up the face of a dam to enable migration of fish upstream and downstream. Materials Depend On  Construction Method  Size of Construction  Type of Project  Soil Condition  Nature  Durability Types of Materials Reinforcement Aggregate Admixtures Water proofing Pozzolan Cement Cement  For massive hydraulic structures, Ordinary Portland cement has been used  Proper chemical composition of cement is important Pozzolan  This is a type of a cement replacement  The heat producing and cost of cement is lead to use pozzolan to reduce the cement content of mass concrete structures.  Natural pozzolan materials - Clays  May be used to improve the Aggregates Coarse Aggregate  Materials within the range of 5 to 150mm in size for hydraulic  Should be made of clean, hard, durable, uncoated rock fragments Fine Aggregate  Can be natural or crushed  Grading of fine aggregate has much greater effect on workability  Sea sand, suitably graded may be used Reinforcement  Used according to the structural requirements  Different diameter of tor steel and mild steel bars used Water  Should be free from materials that affect the hydration of Portland cement  Where choice is available, the cleanest and the purest source of water has to be selected  Accelerators Admixtures CaCl2 (2% by weight of cement) for cold weather  Air entraining agents Improve workability  Water reducing and set controlling agents Native gypsum from 2.5%-5% weight of cement Waterproofing  The water leakage should be prevent perfectly Water Stops:  Provided in transfers joints for stopping the flow of water into the joints eg:- Copper (20mm gauge) - An alloy of Nickel & Copper - Stainless Steel PLANT & EQUIPMENT EXCAVATORS • Dragline • Backhoe • Bulldozers •Scraper • Dumpers CRANES CONCRETE EQUIPMENT ADVANTAGES • Less material required. • Strong rock foundation not required (as for arch dams) • Strength of the dam is high. • uplift forces acting on the dam are minimal. DISADVANTAGES • Height limitations. • Construction difficulty. • Time consuming construction. Examples Bartlett Dam Verde River, AZ Examples Bartlett Dam Verde River, AZ Cost analysis • • • Materials Formwork Plant Purchase 19% Plant Operation 19% Placing and consolidation of concrete 4% Precooling concrete Concrete Treatment 3% 3% 25% 20% • • • •