Arch dam and buttress dams come under the category of medium dams, although their use is not very common. Earth dams and gravity dams are the most commonly used dams. Steel dams and timber dams are also sometimes adopted but only as a temporary measure of storing water.
1. Medium Dams:
i. Arch Dams:
Arch dam is curved dam in plan. It transmits major part of the water pressure to the abutments by arch action. The remaining part of the horizontal pressure is transferred to the foundation by cantilever action. Since most of the horizontal thrust is transmitted to abutments through arch action, it is very essential for the abutments to be very strong.
The weight of the arch itself is not considered as resisting any horizontal water pressure. For this reason, the uplift on the base is not an important design factor. Early arch dams were used to be built of rubble ashlar masonry. But now almost all die arch dams are constructed in cement concrete.
The arch dams may be of the following three types:
1. Constant Radius Arch Dam:
This dam is shown in Fig. 11.1. The face of the dam coming in contact with water or upstream face is maintained vertical. In plan dam is circular. Thickness of arch is minimum at the top of the dam but goes on increasing as we proceed downwards. Taper is always given towards the inside of the arch curvature. In this arch dam, the radius of the arch with respect to outer face is kept constant.
The centre point for all the curves remains same as shown in Fig. 11.1. This type of arch dam is adopted to U-shaped valleys. This arch dam is less economical than constant angle arch dam. However form work for constant radius arch dams is much simplex.
2. Variable Radius Arch Dam:
Figure 11.2 shows this type of arch dam. This dam is adopted for narrow V-shaped valleys. In this dam radii of the intrudes curves and extrudes curves vary at various elevations which is maximum at the top and minimum at the bottom.
3. Constant Angle Arch Dam:
It is a special variable radius arch dam in which central angle of the horizontal arch rings is of the same value at all the elevations. It has been found that the volume of concrete is minimum when the central angle is 133° 34’.
If constant radius arch dam requires say 100 m3 of concrete, the variable radius arch dam for the same height will require 58 m3 of concrete. Hence considering the amount of concrete, constant angle arch dam is most economical.
Forces Acting on Arch Dams:
Self-weight, water pressure, silt pressure, uplift pressure, ice pressure, earthquake pressure etc. all the forces may be acting on the dam. But all these forces do not carry equal important. Self-weight, up-lift pressure, are negligibly small. The most important forces to be considered in arch dams are the internal stresses caused by ice pressure, temperature changes and yielding of abutments. Ice pressure causes a continuous concentrated load along the arch at the level of ice i.e. water level.
During summer, temperature change causes shifting of dam towards upstream direction while in winter towards downstream direction. Winter conditions are considered more appropriate for stress analysis since they act with reservoir loads. The slight yielding of abutment may also develop high internal stresses in the arch. Stress analysis has not been given here.
ii. Buttress Dams:
This dam consists of a number of piers that divide the total length of the dam into a number of spans. All the spans are then covered either with inclined concrete slab or arches on the upstream side of the piers. If spans are covered with flat inclined slab it is known as deck type buttress dam.
If spans are covered by arches the resulting dam is known as multiple arches buttress dam. The elements of this dam are the sloping slab, buttress, mat foundation, and lateral braces. Braces are provided to reinforce the buttress. Cut-off walls may also be provided to prevent seepage.
Types of Buttress Dams:
Buttress dams may broadly be categorised under three heads:
(i) Rigid type.
(ii) Articulated type.
(iii) Semi-rigid or intermediate type.
In the case of rigid type buttress dam sloping deck slab is cast monolithically with the buttresses. No allowance for any settlement of foundation is made. Multiple arch dam and multiple dome dams are the examples of this type of dam.
Articulated buttress dams are quite flexible. The slabs are not cast monopolithically with the buttresses. Flat slab buttress dam is the example of this type.
Round head buttress dam and the diamond-head buttress dam are the examples of semi-rigid type dams. No undesirable rigidity is considered in the design of such dams.
Following are the types of buttress dams which fall under the above three categories:
1. Flat Slab Buttress Dam:
It is also known as Ambursen type of buttress dam named after its inventor.
2. Multiple Arch Type Buttress Dam:
It consists of series of arches transmitting water pressure to buttresses.
3. Multiple Dome Type Dam:
It consists of dome shaped deck instead of arch or flat slab.
4. Cylinder or Massive Head Type Dam:
In this water supporting member is formed by enlarging the upstream end of the buttresses until they meet adjacent members.
5. Columnar Buttress Dam:
In this case deck slab is supported on inclined Columns rather than buttresses.
6. Truss Buttress Dam:
In this, deck slab is supported on reinforced concrete trussed buttresses.
2. Minor Dams:
i. Steel Dams:
These dams are not in common use. No dam in Indian has yet been constructed of steel. In USA some dams had been constructed in steel in early part of this century. These dams consist of a deck slab made of steel plates.
These plates are supported by inclined struts. These dams may also be of cantilever type. In this, a frame work of steel sections is installed in the rigid foundation and deck plate is fixed on them. In this case tensile forces one induced in deck plate and girders.
ii. Timber Dams:
They are also a sort of temporary dams. They consist of a frame work of wooden bullies and planks.
Timber dams are usually of three types as follows:
1. Framed Timber Dam:
In this type of dam a triangular frame work of timber bullies is formed and face which has to come in contact with water is covered by fixing studs and planks. Such dams are usually of very short heights, seldom exceeding 10 m.
2. Rock Filled Crib Timber:
In such dams, crib piers of wooden members are erected at centre to centre distances of 2 to 2.5 m. In order to impart stability to the piers, rock boulders are filled in the open spaces left in the wooden members. After having erected all the crib piers, along the centre line of the dam, timber planks are fixed to prepare the face for retaining water.
If rock foundation is available at the site, the bottom of the cribs is anchored with the rock. In case of earth foundation, sheet piles are driven both at the U/ S as well as D/S side of the cribs. System of sheet piles or that of anchoring with rock foundation is essential to maintain the cribs in position.
3. Beaver Type Timber Dam:
This dam does not involve even driving of sheet piles or anchoring with foundation rocks. This dam is suitable for small heights and also where plentry of timber is easily available. An inclined stack of timber logs is developed. The stock is finally covered on water face by wooden planks. To avoid scouring and also to add to the stability of the dam, the lower U/S end should be covered with boulders and sand.