dam..types

CONCRETE AND MASNRY DAMS

Department of Earth Sciences IIT Roorkee

CONCRETE AND MASONRY DAMS (Engineering Geology Assignment)

Submitted by: Submitted to: Roopak Malik Prof. Anbalagan (11410024) Dept. of Earth Sciences IIT Roorkee

CONTENT

GRAVITY DAM

CONCRETE DAM

ARCH DAM

BUTTRESS DAM

CUPOLA DAM

RCC DAM

MASONRY DAM

A dam is a barrier built across a stream, river or estuary to hold and control the flow of water for such uses as drinking water supplies, irrigation, flood control and hydropower generation etc.On the basis of STRUCTURAL BEHAVIOUR dams can be classified as:- GRAVITY DAM EMBANKMENT DAMGRAVITY DAMA Gravity dam has been defined as a structure which is designed in such a way that its own weight resist the external forces. A gravity dam is a massive sized dam fabricated from concrete or stone masonry. They are designed to hold back large volumes of water. By using concrete, the weight of the dam is actually able to resist the horizontal thrust of water pushing against it. This is why it is called a gravity dam. Gravity essentially holds the dam down to the ground, stopping water from toppling it over.The foundation of a Gravity Dam must be capable of resisting the applied forces without overstressing of the dam or its foundation. The major forces acting on dam are:-Horizontal Forces:- The horizontal forces on the dam tend to make it slide in the downstream direction, which results in horizontal stresses at the base of the dam. These in turn may try to induce shear failure in the concrete at the base or along the concrete-rock contact or within the rock foundation. Uplift forces:- Uplift forces , in combination with other loads, tend to overturn/topple the dam, which in turn may cause the crushing of rock at the toe of dam.Gravity dams are well suited for blocking rivers in wide valleys or narrow gorge ways. Since gravity dams must rely on their own weight to hold back water, it is necessary that they are built on a solid foundation of bedrock.One of the greater advantage of Gravity dams is that these dams are very durable and require very less maintenance.

Now based on the construction material used while making a Gravity dam, these dams are further classified as:-

Concrete Dam Masonry dam

Concrete Dam

A concrete dam is a structure designed and built for the purpose of holding back water that is usually placed across the path of a river.Concrete Gravity Dams rely on the weight of the concrete of which they are built to resist the forces (gravity, water pressure, earthquake) to which they are subjected. A concrete dam is the strongest type of dam built in modern times and may take several forms. Concrete itself is a building material made from water, cement, sand and gravel, or aggregate.

In modern times, nearly every dam is made either partially or entirely from concrete. Concrete is an excellent material for constructing dams because it is very strong when under compression i.e. when pressed down or pushed together. Many designs for concrete dams take advantage of this property to produce extremely large dams, capable of holding back many cubic miles (1 cu. mile = 4.2 cu. kms) of water.These dams are heavy and massive wall-like structures of concrete in which the whole weight acts vertically downwards. As the entire load is transmitted on the small area of foundation, such dams are constructed where rocks are competent and stable.

Dams constructed from concrete have these basic designs:- Arch DamsAn arch dam is curved in plan, with its convexity towards the upstream side. They transfers the water pressure and other forces mainly to the abutments by arch action.An arch dam is quite suitable for narrow canyons with strong flanks which are capable of resisting the thrust produced by the arch action. The section of an arch dam is approximately triangular like a gravity dam but the section is comparatively thinner. The arch dam may have a single curvature or double curvature in the vertical plane. Generally, the arch dams of double curvature are more economical and are used in practice.

The arch dam is one of the most elegant of civil engineering structures. In cross section, the dam is narrow in width, but, when viewed from above, it is curved so the arch faces the water and the bowl of the curve looks downstream. This design uses the properties of concrete as its strength. Concrete is not strong in tension (when it is pulled or stretched), but it is very strong in compression (when it is pushed or weighed down). The arch dam uses the weight of the water behind it to push against the concrete and close any joints; the force of the water is part of the design of the dam. Arch dams are built across narrow, deep river gorges, but now in recent years they have been considered even for little wider valleys. C-H ratio greater than equal to 3 is preferred. Buttress DamsThese type of dams consists of thin upstream sloping deck, supported by buttresses. The name "buttress" dam comes from the structure of the dam itself. The dam is supported at intervals by several buttresses, concrete slabs reinforced with steel, which form a watertight seal against the river.Buttress dams are of three types: (i) Deck type, (ii) Multiple-arch type, and (iii) Massive-head type. A deck type buttress dam consists of a sloping deck supported by buttresses. Buttresses are triangular concrete walls which transmit the water pressure from the deck slab to the foundation. Buttresses are compression members. Buttresses are typically spaced across the dam site every 6 to 30 meter, depending upon the size and design of the dam. Buttress dams are sometimes called hollow dams because the buttresses do not form a solid wall stretching across a river valley. The deck is usually a reinforced concrete slab supported between the buttresses, which are usually equally spaced. In a multiple-arch type buttress dam the deck slab is replaced by horizontal arches supported by buttresses. The arches are usually of small span and made of concrete. Multiple-arch buttress dams were the first type of buttress dam ever built.In a massive-head type buttress dam, there is no deck slab. Instead of the deck, the upstream edges of the buttresses are flared to form massive heads which span the distance between the buttresses. The buttress dams require less concrete than gravity dams. But they are not necessarily cheaper than the gravity dams because of extra cost of form work, reinforcement and more skilled labor. The foundation requirements of a buttress are usually less stringent than those in a gravity dam.The deck slope influences stability parameters and the stresses in the buttress. A relatively flat slope, while providing good stability, may result in high tensile stresses parallel to the upstream face. The optimum slope is found by trial, and for economy should be as steep as possible while still satisfying stability and stress requirements. The shape of the buttress also influences stresses.The buttresses may be of a single- or double-wall (hollow) construction and must satisfy the same stress and stability requirements as a gravity dam. In addition, buttresses must be designed to prevent buckling in accordance with reinforced concrete design practices. For this reason, bracing or struts may be placed between adjacent buttresses; stiffeners or counterforts may be placed on the sides of the buttresses, or, in the case of double-wall buttresses, diaphragms or curtain walls may be used between the walls. Buttress spacing is based upon economy and is usually established by trial studies.Placing a roadway, spillway, power intake, or outlet in a buttress dam involves no special problems. The spillway crest and downstream apron can be supported by the buttresses and are designed in the same manner as deck slabs. Piers could be constructed on the weir and the crest gated on buttress dams if desired. Buttress dams also can accommodate outlets. Outlet openings are provided either in the deck between buttresses or, for thick buttresses, in the buttress itself. Cupola damIn wider valleys with gentle sloping sides, the stresses near the center of the arch may become excessive and, to withstand this, the wall is designed with curvature in both plan view as well as in sectional view too. They are called as Double Curvature Dams also. Due to its characteristic design the transfer of load is uniform throughout the foundation. However, the design and construction of this type of dams is very challenging.As the dam section is quite thin so there is every possibility that we get more seepage of water below the dam. As this will increase the erosion of foundation rocks so we are required with foundation rocks which are quite resistant to erosion.Due to arching action, the maximum load is shifted to abutments so we require strong abutment rocks with very less geological discontinuities. Tight narrow gorges with C-H ratio 3 or less are preferred for the construction of Cupola dams. Roller-Compacted Concrete (RCC) DamsDams constructed of RCC are concrete gravity dams, and their stability requirements and methods of analysis are similar to those of gravity dams. In addition to construction methods, they differ from gravity dams principally in mix design and details of appurtenances. The major advantage of RCC dam construction is the speed at which the dam can be constructed, with the associated resulting economies. Special facing techniques usually are employed for forming the faces of RCC dams. These include precast concrete face panels, conventional concrete facing mix placed in conventional formwork, and extruded concrete curbs. The interlocking precast concrete panels are held in place by steel anchors embedded in the RCC as it is placed. Temporary panel supports must be utilized until sufficient RCC strength is developed at the anchor embedment. Conventional forms have been used on a number of projects to support conventional concrete facing mixes. The use of concrete curbs involves the placing of the facing elements by a slip form paving machine. Appurtenant features are designed to minimize their interference with the continuity of RCC construction. For example, diversion conduits and outlet works, if possible, should be embedded by conventional concrete in abutment trenches or below the main dam section.Advantages of RCC Dams:-(A) Dams are a vital, but aging, part of our public works infrastructure. The challenge is to find cost-effective repair and replacement methods without sacrificing safety and reliability. RCC has three key properties that make it uniquely suited for dams: economy, performance, and high-speed construction. It has the required strength and durability. (B) It protects dams from over-topping failure, earthquakes, and erosion. It can be placed quickly and easily with large-volume earth-moving equipment. Its generally transported by dump trucks, spread by bulldozers, and compacted by vibratory rollers.(C) Sections are built lift-by-lift in successive horizontal layers so the downstream slope resembles a concrete staircase. Once a layer is placed, it can immediately support the earth-moving equipment to place the next layer. After RCC is deposited on the lift surface, small dozers typically spread it in thick layers. Workers also place it with motor graders, spreader boxes, and paving machines. Older concrete and masonry dams can be buttressed with RCC to increase resistance to earthquake loading and to improve stability to prevent overturning and sliding.(E) RCC is used as backfill to support conventional concrete spillways. Due to its high resistance to abrasion, RCC is also used to construct stilling basins, build liners for outlet channels, and form grad the first successful application of RCC technology was demonstrated in 1974. The repair of the collapsed intake tunnel of Tarbela Dam proved that the material had more than adequate strength and durability. The maximum placement of 18,000 m3 of RCC in one day, which is still the worlds record, was a clear evidence of the potential of this new construction method (Refer Figure 4).

Masonry damAny dam constructed mainly of stone, brick or concrete blocks jointed with mortar. A dam having only a masonry facing should not be referred to as a masonry dam. Masonry dams differ from rock-fill dams in that the stone is hand-placed with mortar resulting in the entire dam being impermeable.Structure wise it is similar to that of a Gravity Dam except that the material used for the construction of such type of dams are stone masonry blocks. Stone masonry blocks can be carved out of hard massive igneous rocks like granite, syenite etc. Fine to medium grained rocks are preferred because smaller grains have a large surface area which comes in contact with other grain thus less concentration of stresses thus preventing the partial melting of the grains, which might otherwise be the case if coarse grained rocks are used. Fairly wide to wide valleys are preferred to accommodate the main dam and appurtenant structures. For this purpose C-H ratio >4 is required. Hard massive rocks are preferred as foundation rock however we can also manage with moderately hard rocks also. As maximum load is concentrated on river bed foundation so it should be taken care of that there should be no faults or thrusts below the dam i.e. there should be less number of Geological discontinuities in the foundation rock. Height in general preferable less than 50m sometimes up to 80m.There should be availability of sufficient reserves of masonry blocks near the dam site so as to minimize the cost of transportation. To cut out the masonry blocks of a given dimension we require skilled man power also.Ex.. 63m high and 510m long Gandhisagar dam across Chambal river in M. P

Geological Investigation for Concrete and Masonry Dams1. Rock should be sound and resistant to static and dynamic forces.2. Valley slopes should be stable.3. Dam foundation should be safe from sliding.4. The foundation rocks should be of same geological properties.5. Reservoir should be free from seepage/leakage.6. Rocks should be resistant to corrosion, erosion, solution, drying and wetting.7. Reservoir drainage and overburdens should be resistant to erosion and sliding.8. In case of arch dams the abutments should be extremely strong.9. The site should have a permissible:- (a) Spillway and (b) Diversion tunnel and/or outlet works, if any.10. Construction material should be economically available.11. Overall economy and safety.