Dam structures are subjected to horizontal loading from the water head behind, the pressure from the water towards the dam materials, the neighboring geology as well as from the size of the reservoir. These features are unique in the case of dam structures.
Common Causes of Failures of Dam Structures
The most common reason behind the failure of dam structures from the review taken from past studies is overtopping. The overtopping is caused either due to overtopping, which is something concerned with the design of the spillway.
Overtopping is a major failure that is created due to heavy floods. There are two main factors that cause the overtopping failure. One is the continuous flow that is created due to surface elevation that will exceed the complete structural elevation profile.
The second main reason is the over wash from the waves, where the surface of the water stays below the structure elevation profile. Most of the embankment dams does not have much withstanding power towards the overtopping effect. But the concrete dams will withstand this to certain level as they have strong rock foundation.
Fig.2. Failure of Dam due to Overtopping
Many other causes of dam failures are mentioned below:
- Water Breaching
- The foundation of the dam is inadequate
- Piping causes erosion of the foundation
- Subsidence and the movement of the foundation
- Uplift from the ground and sliding of the structure
- Variation in temperature unequally throughout the dam structure
- Water breaching issues
- Dynamite blasting in the nearby areas causing vibration to the dam structure
- Seismic load action
- Wave action on the structure and weak energy absorption characteristics of the structure
- Higher amount of silting causing greater loads that expected during the design of the dam.
- The concrete and nearby rock or the soil surface loosing shear capacity.
Causes of Failures of Earth Dam Structures
In the case of earth dam, the unique feature is that the material used for the structure is made from earth materials or the rocks that is nearby the site. This makes the structure to be heterogeneous in nature that it shows different properties for different conditions of moisture as well as internal pore pressures. This material will bring variations in the physical properties like density, shear strength and permeability.
Other primary phenomena that will create failures for the earthen dam structures are:
- Inadequate spillways constructed causing overtopping issues
- Piping erosion
- Failure of the structure along the outlet pipes
- Animals creating undermining issues on the foundation
- The water will cause the clay blanket to swell. The water pressure will force the front wall of the structure
- Puddle core settlement
- Drainage and saturation of the drainage
- Shrinkage in clay caused from droughts
- Compaction not up to the desired level
- Breach formed at the intake device
- Slip of the upstream slope and use of material that is unsuitable
- Upstream clay face subjected to sloughing
- The clay blanket will be subjected to swelling due to the water. This is then forced on to the front wall.
- Slope instability creating rapid draw down and slide failures
Causes of Failures of Gravity Dam Structures
Another type of dams are the gravity dams which are constructed from concrete and masonry. The major causes of failures of gravity dam structures are:
- Erosion of soil near to the foundation causing settlement.
- Construction joints undergoing failure
- The quality of the material used is very poor
- The degradation of the material due to excessive exposures.
- Rock foundation subjected to horizontal shear.
- The footing constructed found to be too shallow in nature. This creates scour or blowout of the foundations.
Causes of Failures in Arch Dam Structures
Another type of dam is the arch dam. This dam differs from other dams in terms of in plane stresses existing and the thrust. These factors are employed for the design of the structure in large extend. These factors are completely dependent on the soil and the conditions of the rock.
Other major reasons causing failure in arch dams are:
- The structure lacks the ability to adjust with the existence of the slip plane. This instability of the structure may not be identified in the soil investigation.
- Penstock vibration resulting in cracks on the structure.
- Inadequate grouting action of the contraction joints causing cracks in the structure.
Causes of Failures in Buttress Dam Structures
Another type of dam is the buttress dam. This dam is unique in the case that the horizontal forces are transferred to the rock foundation. This is carried out by means of a vertical component of the water pressure. This is the force that is analyzed for the sliding and overturning failure of the dam structure. The main reasons behind the damage of these structures are:
- Alternate freeze and thaw cycles undergone by the abutments will cause frost damage.
- Masonry bases that are constructed poorly for multiple set of concrete dams causing failure of the concrete masonry joints
- Abutments subjected to cracking
- Permeability in concrete or formation of porous concrete