A buttress dam is made up of a water-retentive membrane or deck that slopes upward, it is supported by few buttresses that are positioned at right angles to the dam’s axis.
The reinforced concrete slab, a series of arches, or a thickened buttress head can serve as the upstream water retaining slope surface.
A cut-off is provided at the upstream end to stop or lessen water seepage.
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What is Buttress Dam?
A hollow dam is alternative name for a buttress dam.
It is a dam with an upstream side that is solid, watertight and a downstream side that is supported at regular intervals by several supports.
A gravity dam is modified or improvised into a buttress dam.
Buttresses are nothing more than solid walls that are built perpendicular to the water’s flow and spaced at regular intervals.
Purpose of buttress dams:
- It is originally constructed to hold onto water for irrigation or mining in regions with expensive or scarce resources but cheap labour.
- By removing it from the dam’s unstressed area, the quantity of concrete is reduced in order to reduce costs.
- They are employed to raise the amount of water available for hydroelectric power generation and to lessen the ultimate discharge of floodwater brought on by powerful storms.
Types of Buttress Dams:
Classification based on Sloping Membrane:
1. Dam Deck Slab Buttress:
The deck slab is provided, and it is supported by the buttress corbels.
This kind of dam is built to lower heights, typically between 20 and 50 meters.
The provided slab has an angle of between 40 and 55 degrees with respect to the horizontal.
This inclination is required to maintain the dam’s stability, support the dead load of the water that has been stored in the reservoir, and stop the dam from sliding under its own weight.
In this instance, each deck slab unit on two nearby buttresses functions as a separate, independent unit.
As a result, if one unit is impacted or damaged, other units need not be a source of anxiety.
The deck slab can be further classified as follows:
Simple Slab-Style Buttress Dams for Decks:
Instead of being firmly fastened to the buttresses, the deck slab in this type of buttress dam is simply supported and keeps in contact with it.
Bituminous mastic, asphaltic putty, or another flexible joint compound is used to fill the joint between the slab and buttress tongue.
Continuous or Fixed Deck Slab Buttress Dams:
In this buttress dam, the deck slab is cast monolithically with the buttress intended to be one continuous slab.
Due to reversing bending moments and the potential of upstream reinforcement becoming corroded.
Hence reinforcement is provided on both the upstream and downstream faces of the slab in this instance.
The deck slab in this type of dam is thin.
Buttress dams with a Cantilever Deck Slab Design:
In this buttress dam, the deck slab is cast homogeneously with the buttress so that it cantilevers across each buttress from either side.
Columnar type buttress dams:
These dams are supported by a number of slanted columns rather than buttresses and consist of a continuous flat slab of reinforced concrete.
A two-directional system of struts stiffens and reinforces the columns.
Truss-type buttress dams:
Rather than using buttresses, these buttress dams support the reinforced concrete deck slab on a number of trusses made of reinforced concrete.
2. Multiple-arch Buttress Dam:
In this buttress dam, the sloping surface or deck is made of numerous reinforced concrete arches that are supported by multiple buttresses.
The dam’s upstream face is typically 45 degrees incline, while buttresses and the arches are casts in one piece.
As compared to other buttress dams like a deck slab buttress dam, multiple arch buttress dams are stronger and more adaptable.
The dam can be built as either a single hollow wall or a double stiffened wall, the buttresses on the dam depend on one another is its biggest drawback.
It implies that if one buttress experiences issues, the entire dam will become ineffective.
This dam can support higher heights, ideally those over 50 meters.
3. Multiple Dome Buttress Dam:
This buttress dam has many reinforced concrete domes that make up the sloping surface or deck, which is supported by many buttresses.
The majority of the traits are the same as those of a multiple arch dam, but domes are used in place of the arches.
The number of buttresses needed to stabilize the dam can be decreased by using multiple domes.
The domes can be placed at greater distances than arches, it aids in saving money and materials when designing the dam.
4. Massive or Bulky Head-Type Buttress Dams:
The primary characteristic is the lack of separate water retaining members, which is instead created solely by enlarging the buttress’s upstream end side.
As a result, the dam is constructed using numerous buttresses and enormous heads that are set side by side.
Based on the design of the buttress head, the three types of massive head buttress dams listed below can be further divided:
- Dams with round head buttresses.
- Dams with a diamond-shaped buttress.
- Dams with a tee head buttress.
Classification based on Correlation between Sloping Membrane and Buttresses:
The intersection between the buttresses and sloping membrane can be used to classify buttress dams into the following three groups:
6. Rigid Buttress Dams:
In this instance, the buttresses are not incorporated into the sloping membrane or deck in a single piece.
As a result, for these dams, foundation settlement, temperature variation, and concrete shrinkage all have a significant impact and it must be considered in the design.
7. Articulated Buttress Dams:
In this case, the buttresses are not built monolithically into the sloping membrane or deck.
Due to their flexibility, these dams are not significantly affected by foundation settlement, temperature changes, or concrete shrinkage.
8. Intermediate or Semi-Rigid Buttress Dams:
Intermediate or semi-rigid buttress dams are a type of buttress dam between rigid and articulated buttress dams.
The unpleasant rigidity is thus eliminated by these dams, but they also lack a high degree of flexibility or articulation.
Advantages of Buttress Dam:
- It is possible to build the Buttress dam on a flimsy foundation.
- Buttress dams can be built in soil that settles differently because they can be designed to accommodate modest foundation movement without suffering serious damages.
- A buttress dam needs between 1/3 and 1/2 the amount of concrete that a gravity dam of equal height needs.
- Uplift and drainage of the foundation are not issues.
- A buttress dam experiences significantly less uplift pressure, which saves concrete and increases the dam’s overall stability.
- The location of the powerhouse, switchyard between the buttresses can reduce some construction costs.
Disadvantages of Buttress Dam:
- More effort is needed to operate a buttress dam than a solid concrete dam.
- Since the concrete surface upstream is thinner, it is more likely to deteriorate.
- It needs ongoing maintenance and oversight.
- The dam has a shorter lifespan than other dams.
- A buttress dam requires more skilled labour and it has a higher shuttering to concrete ratio than a gravity dam resulting in concrete costing more per unit.
- When compared to other types of dams, buttress dams typically require more water seals to be installed and maintained.
Also read: Earthen dam | Types of Bridges | Arch Dam
Buttress dams are beneficial for sites with wide or narrow valleys, it must be built on a solid foundation of bedrock.
The stability of a buttress dam depends on the bracing action of the buttress where most of the load is concentrated.