ADVANTAGES AND DISADVANTAGES OF DIFFERENT TYPES OF DAMS
(A) Gravity Dams
- Maintenance cost is negligible.
- They are specially suitable for deep steep valley conditions where no other dam is possible.
- If suitable foundation is available, such dams can be constructed for very large heights.
- Because they can be constructed in very large heights, they can store more amount of water.
- If suitable separate place is not available for installation of spillways, they can be installed in the dam section itself.
- This dam gives prior indication of instability. \If remedial measures are taken in time, unsafe dams may even be rendered safe.
- Even if they cannot be made sale they give sufficient time for the people to move out the area likely to be submerged due to failure of the dam.
- Silting rate of the reservoir can be reduced considerably by installing undersluices in the dam near the bed of the reservoir.
- Sluices can be operated from time to time and silt may be scoured out of the reservoir.
- They are not affected by very heavy rainfall. Earth dams cannot sustain very heavy rainfall because of heavy erosions.
- They are very costly in initial construction.
- They take lot of time of construct.
- They require skilled labour for construction.
- Such dams can be constructed only on good foundation.
- If height of the dam is to be raised, it cannot be done unless provision for it had been made in the construction of the lower part of the dam.
(B) Earthfill and Rockfill Dam.
- They can be constructed on any type of foundation
- They can be constructed in comparatively less time.
- They do notrequire skilled labour.
- Initial cost of construction is low as locally available soils, and rock boulders are normally used.
- Their height can be increased without any difficulty.
- They are specially suitable for conditions where slopes of river banks are very flat.
- Gravity dams under such conditions are not found suitable.
- They fail all of the sudden without giving any pre-warning.
- Flood waters affect the dam safety.
- Spillway have to be located independent of the dam.
- They cannot be constructed as overflow dams.
- They require continuous maintenance.
- They cannot be constructed in narrow steep valleys.
- They cannot withstand heavy rains unless properly protected.
- They cannot be constructed in large heights.
The usual height is 30 m for which most of the earthen dams are constructed.
(C) 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.
- 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.
- This type of arch dam is adopted to U-shaped valleys.
- This arch dam is less economical than constant angle arch dam, which has been discussed ahead.
- However form work for constant radius arch dams is much simplex.
- Variable radius arch dam.
2. Variable Radius Arch Dam.
- 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.
- 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 down stream 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.
- They are particularly suitable for narrow deep gorges.
- In such conditions arch dams prove even more suitable than gravity dams.
- They require comparatively very small amount of construction material.
- The section or thickness of the dam is very small in comparison to the gravity dam.
- Uplift pressure is not very large as base width of thedam is quite small.
- They can be constructed on moderate foundations.
- They require very complicated form work which is very costly.
- They require very skilled labour. 3. Design of the dam is also difficult.
- Speed of construction is very slow.
- They require very strong solid rocky abutments to resist the thrust of the arch.
(D) 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 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.
- They can be constructed on weak foundation because they are less massive than gravity dams.
- The load of water lying on the inclined deck slab adds to the vertical component. of the dam and as such helps in increasing the stability of the dam.
- The factor of safety in case of buttress dams is far greater than for gravity dams.
- Since ice tends to slide over the inclined U/S surface, ice pressure does not carry any significance.
- Height of buttress dams can be increased by extending both buttresses and deck slab
- Power houses or other plants can be located in the empty space between buttresses.
- They require only half to one-third amount of cement concrete in relation to the concrete required for gravity dam for the same height.
- The cost is not reduced by the same proportions as increased cost is required for form work and reinforcement.
- Because of space between buttresses, it is possible to reach the back space of deck slab.
- By reaching here periodic inspection of foundations etc. can be carried out.
- Since exposed surface of concrete is more but volume of concrete is less than the gravity dam, heat dissipation during construction is achieved better.
- Also speed of construction can be increased as thinner sections do not cause any problem for cooling.
- More skilled labour is required.
- Shuttering cost is more.
- The slab being very thin, its face in contact with water is likely to deteriorate and cause damage to the dam.
- It is more susceptible to delibrated damage.
- It is so because thickness of slab is very small and also there is approach to reach back of the slab through the space between buttresses.
- Steel dams and timber dams have been discussed in Chapter 14 where arch dams have also been discussed in brief.
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 is 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:
- Flat slab buttress dam.
- It is also known as Ambursen type of buttress dam named after its inventor.
- Multiple arch type buttress dam.
- It consists of series of arches transmitting water pressure to buttresses.
- Multiple dome type dam. It consists of dome shaped deck instead of arch or flat slab.
- 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.
- Columnar buttress dam.
- In this case deck slab is supported on inclined Columns rather than buttresses.
- Truss buttress dam.
- In this, deck slab is supported on reinforced concrete trussed buttresses.