A buttress dam is a modification or improvisation of gravity dam. Buttresses are nothing but solid walls which are constructed parallel to the water flow with some specified thickness ate regular intervals. On these buttresses, an arch slab or inclined deck slab is provided to support upstream water.
Generally, gravity dam requires more concrete. To minimize the cost, concrete quantity is minimized by clearing it from the unstressed area of dam. Finally, an important innovation takes place which is nothing but buttress dam.
Types of Buttress Dams
There are five types of buttresses dams, out of which deck slab type and multiple arch type are most commonly used. Those five are as follows.
Deck slab buttress dam
Multiple arch buttress dam
Massive head buttress dam
Multiple dome buttress dam
Columnar buttress dam
Deck Slab Buttress Dam
In this type of dam, a deck slab is provided which is supported by the corbels of buttresses. This type of dam is constructed to smaller heights generally from 20 to 50 meters.
The slab provided is inclined to the horizontal by about 40 to 55 degrees. This inclination is needed to stabilize the dam and support the dead load of stored reservoir water and also it prevents sliding due to self-weight of dam.
In this case each deck slab unit on two adjacent buttresses acts as one independent single unit. Hence, if one unit gets effected or damaged then no need to worry about other units.
Design consideration of deck slab buttress dam are same as gravity dam considerations. Again, in this case we have three types and they are
Fixed deck slab
Free deck slab
Fixed Deck Slab Buttress Dams
In case of fixed deck slab type dam, deck slab and buttresses are casted monolithically.
Free Deck Slab Buttress Dams
This is also called as simple deck slab buttress dam. This is constructed when the foundation soil is very weak. In this case reinforcement is provided at the downstream face of the deck slab.
Cantilever Type Buttress Dams
In this case, deck slab is cantilevered at both ends and upstream face is provided with reinforcement.
Multiple Arch Buttress Dam
Multiple arch dam contains series of arches and in this case arch slabs are provided at the upstream face of the dam. These slabs are supported by buttresses. The buttress wall is constructed as single stiffened wall or double hollow wall.
This type of dam can be preferred for larger heights about more than 50 meters. As compared to deck slab-type buttress dams, multiple arch buttress dams are more flexible and stable.
The main disadvantage in this case is the adjacent buttress units depends on each other. So, if one unit gets affected then it causes severe problems for the whole series of arches. To overcome this good foundation for each of buttresses wall should be provided.
The thickness of arch provided varies in case of larger spans and constant in the case of shorter spans. The central angle of arch should be in between 180 degrees to 150 degrees.
The space between buttresses should be in between 15 to 21 meters. However, larger spacing can also be provided by taking some consideration.
Massive Head Buttress Dam
In this type of buttress dam, no slab or arch is provided at the upstream face, instead of that buttress head is enlarged and joined with adjacent buttress head. Like this all buttresses heads makes strong water supporting surface.
The enlargement of buttress heads can be done in different shapes like rounded, diamond shape etc. So, these shapes can resist the water pressure very well. The joining of heads is strengthened by providing copper strips.
Construction of massive head buttress dam is easier compared to other types and no reinforcement is required only mass concrete is laid whole over the dam body. Because of such a huge concrete body, it is heavier and resists sliding.
Multiple Dome Buttress Dam
It is almost similar to multiple arch type buttress dam but in this case domes are constructed in the place of arches. Apart from that almost all the features are same.
Provision of such domes influence the setting of larger spacing between buttresses. Hence, longer spans can be provided and also number of buttress also reduced.
Above picture represents the upstream side of multiple dome buttress dam (Coolidge dam, Arizona)