Ground Motion During Earthquake

Earthquake are the sudden, rapid release of energy stored in rocks and becomes perceptible by the constant motion of the earth's surface. The earth's rock layer is broken into large pieces. These pieces are in slow but constant motion. They may slide by each other smoothly and almost imperceptibly. From time to time, the pieces may lock together and energy that accumulates between the pieces may suddenly released. The energy that is released travels through the earth in the form of waves. People on the surface of the earth than experience an earthquake.


Ground Motion

The energy released at each material point within the three dimensional volume that ruptures at the fault, results seismic waves which produce motion to earth’s surface. The motion at any site on ground is random in nature with its amplitude and direction varying randomly with time as these waves arrives at various instant of time having different amplitudes and carry different levels of energy. Knowing how, fast, for how long, and how much the ground moves during an earthquake is important for estimating how ground motion will affect the built environment. Seismologist use several concepts to express these measurements:

1) Acceleration
2) Duration
3) Velocity
4) Displacement

Acceleration

A series of vibrations are created when seismic waves move through the ground. These movements are transmitted into dynamic loads or inertial forces that cause the ground and any thing attached to it (i.e., the built environment) to vibrate in a complex manner. These inertial forces cause damage to buildings and other structures. Inertial forces are created when an outside force tries to make an object move or change its rate of travel.
Acceleration is the rate of change of motion. The variation of ground acceleration with time recorded at a point on ground during an earthquake is called an accelerogram. The nature of accelerogram may vary depending on

- Energy released at source
- Type of slip at fault surface
- Geology along the travel path from fault rupture to the earth surface
- Local soil

They carry distinct information regarding ground shaking; peak amplitude, duration of strong shaking, frequency content ( amplitude of shaking associated with each frequency) and energy content (i.e., energy carried by ground shaking at each frequency) are often used to distinguish them.

Normally acceleration is not associated with buildings since building is not expected to move. During an earthquake, however, inertial forces may cause the upper part of the building to sway while the foundation remain stationary, or they may cause whole building to “move”. Structures built in seismically active areas must be built to withstand predicted acceleration levels.


Duration


The duration of ground motion is very important because the destructive effects increase greatly with increase in length of duration. The damage will occur the whole time the ground is moving. So more damage is likely to occur the longer an earthquake lasts. In predicting the amount of potential damage that could occur in a specific should include the duration of ground motion.


Velocity


Velocity is mathematically related to acceleration. Velocity is the speed of an object at an instant in time. Velocity is quickly becoming as important as acceleration in determining the building damage. Consider an example: if your car decelerate suddenly, the inertial force may cause your head to hit windshield. The velocity at which your body id traveling at the instant your head hits the windshield determines whether you get a little bump or a fractured skull. For a building, this could mean the difference between superficial damage and building collapse.


Displacement

Displacement is the distance an object is moved from a resting position, such a how far a building is moved or displaced from its foundation. Seismologist use measurements of displacement to judge the impact of an earthquake on a community.


Conclusion

None of the scales (The Richter Magnitude Scale and The Mercalli Intensity Scale ) used to measure earthquake, are not enough to develop a seismic resistant design method or further development of scientific study. The Richter scale does not give ground motion information that is important for designers. The Mercalli scale is subjective and does not cover many new kinds of construction used today. Together with theses information, scientists can define an earthquake having following information:
- Where seismic events takes place
- How large it was
- What its impact was on the built environment.

Failure of Dam

Dam

A dam can be defined as an obstruction or a barrier built across a stream or a river. At the back of this barrier, water gets collected, forming a pool of water. The side on which water gets collected is called the upstream side, and the other side of barrier is called the downstream side. The lake of water which is formed upstream is often called reservoir.

Function of Dam

The function of dams is to block the flow of a river creating an natural storage tank for the now trapped water. It provides water for irrigation and water supply for domestic use. It can improve navigation and create a reservoir of water for to supply industrial uses, generates hydro-electric power, create recreation areas. It is also a habitat for fish and wild life. The most important function of dam is to control flood. It can control effluent from industrial sites such as mines or factories.

Dam Failure

A functional dam needs to be strong enough to hold back its reservoir and it needs to be able to pass flood water through the structure during heavy rains and periods of increased run-off. Dams are classified by the material and design used in construction and the failure is also depends on such classification. The Federal Emergency Management Agency (FEMA) defines dam failure as a catastrophic type of failure, resulting a sudden, rapid and uncontrolled releases of confined water.

History of Failure

The failure of the dam may be caused either due to bad workmanship or due to faulty design or due the occurrence of unanticipated floods. Luckily, these disasters have been comparatively rare in this century. Dams used to give away in olden times, but due to engineering advancement in modern times their failure has been considerably reduced taking care of design, keeping in view the various forces which are going to face, proper and rational design, good supervision and constant vigil and watch during maintenance periods ensures their safety. The Boulder Dam on Colorado River in U.S.A can not fail in one attempt, how furiously these river may try to move their foundations. But sometimes we have to pay for this confidence with tragedies.

Malpasset Dam

The "Reyran" river flowing well few hundred meters below the dam.

In 1954 the Malpasset Dam, a 200 feet high arch dam on the Reyran river, was completed. This dam failed in December 1956 causing 421 persons to die in floods. This was due to failure of foundation.

Vega de Tera Dam

A very confident dam called Vega de Tera Dam in Spain failed in January 1959. The town Rivaldelago was fattened. Telephone poles were snapped like matchsticks. With in moments, 123 villages were drowned. Several hundred luckier ones were saved, but were rendered homeless. This time this dam was not built strong enough to bear the full weight of its intended reservoir. Heavy rains wrecked it.

Vega de Tera Dam

Sometimes excessive and unanticipated earthquakes may result failure to a dam. In 1968 earthquake, the Koyna Dam in india was at the verge of failure. The engineers saved that dam by toiling hard day and neight.

Shakidor Dam Brust

The other failures are South Fork Dam due to spillway design error, Shakidor Dam due to extreme rainfall and Dale Dike reservoir, Taum Sauk pumped storage plant due to human, computer or design error.

Leading Cause of Dam Failure

a) Hydraulic failures

More than one-third of all dam failures have been attributes to these causes. These type of failure may occur due to the following reasons:

1) Overtopping

When water levels rise rapidly and without adequate warning due to flash floods, heavy rains, a landslide in the reservoir that creates a tsunami, or if a dam upstream collapses, overtopping occurs and rise in level of level of a reservoir exceeding the capacity or height of the dam. If the spillways become blocked with debris, like silt, mud or trees, or the spillway gates are not operated properly and water can not be released, there is a danger that the water level in the reservoir will rise higher than the crest of the dam and spill over resulting overtopping.

2) Erosion of Upstream Face

Wind produced waves near the top water surface and try to notch out the soil from the upstream face and may even, sometime cause the slip of upstream slope.

3) Cracking Due to Frost Action

The upper portion of dam may subject to heaving and cracking of soil due to frost action with dangerous seepage and consequent failure.

4) Erosion of Downstream Face by Gully Formation

The erosive action on downstream face of moving water from heavy rainfall, may lead to formation of gullies on the downstream face, ultimately leading to the dam failure.

b) Foundation Defects

This failure is responsible for one-third of all dam failure of whole world. The main cause of concrete dam failure is a problem with foundation. Dams built on slopes must be properly engineered to avoid issues with instability or landslides. Only the weight of a dam structures has an impact on the ground underneath. If this weight is not properly taken into account in the engineering of the dam, the ground underneath can settle unequally and compromise the foundation. Any event causing the movement of a foundation, such as earthquake, can also result failure of the dam’s foundation. High uplift pressure and uncontrolled foundation seepage can also lead the dam foundation to failure.

c) Seepage Failure

The semi-permeable dam like embankment type can be failed when too much seeps or leaks through the structure. Controlled seepage or limited uniform seepage is inevitable in all earth dams and ordinarily it does not produce any harm. Uncontrolled or concentrated seepage through the dam body or through its foundation results piping. Piping is the progressive erosion and subsequent removal of the soil grains from with in the body of the dam or the foundation of the dam. More than one-fifth of all failure of dam occur because of these reasons.

1) Piping Through Foundation

Water, through the foundation of dam, may start seeping at a huge rate when highly permeable cavities or fissures or strata of coarse sand or gravel are available in the foundation of the dam. This concentrated flow at a high gradient, may erode the soil. This leads to increase flow of water and soil, ultimately resulting in a rush of water and soil, thereby creating hollows below the foundation. The dam may sink down into hollow so formed, causing its failure.

2) Piping Through the Dam Body

When the concentrated flow channels get developed in the body of the dam, soil may be removed in the same manner as was explained in the foundation piping, leading to the formation of hollows in the dam body, and subsequent subsidence of the dam. These flow channels may develop due to faulty construction, insufficient compaction, cracks developed in embankment due to formation of settlement, shrinkage cracks, animal burrows etc.

4) Armed Conflict

The dams creates dangers in wars, especially in modern atomic age. The resultant failure of such a dam will create catastrophes, but also it will get contaminated by radioactivity from which there could be no escape.

This is an important point which results stress on the host of the dam. In World War II, British Royal Air Force Dambusters operate raid on Germany, in which three German dams were selected to breached in order to have an impact on German infrastructure and manufacturing and power capabilities deriving from Ruhr and Eder River.

To protect possible destruction on the civilian population and the environment, the rules of International Humanitarian Law ( IHL) are made and dams are defined as object not to be attacked. During armed conflict, dam is to be considered as an “ installation containing dangerous forces”. A protective sign consisting of three bright orange circles placed on the same axis is provided to facilitate the identification.

5) Other Reasons

Dams which are improperly maintained or built with inadequate materials or unsound design can result in structural weakness that lead to catastrophic dam failure.

REINFORCEMENT REQUIREMENT OF PILE

The amount of reinforcement and its arrangement vary with the loading condition and installation and driving condition. The amount and arrangement of reinforcement for different types of pile are discussed below briefly.
Precast Pile

Longitudinal Reinforcement

The minimum amount of longitudinal reinforcement should be 1.5% of concrete section. At least 4 bars have to provide in a symmetrical pattern.

Lateral Tie Spacing

At each end of pile lateral tie reinforcement consisting of 6mm dia or larger have to place at a spacing not more than 75 mm center to center, or an equivalent spiral shall have to provide through a length equal to at least three times or the pitch of spiral may be increased to 300 mm.

Clear Cover

The cover of concrete over all the reinforcements including ties, should not be less than 70 mm anywhere through out the length of piles.

Cover should be measured clear from the main or longitudinal reinforcement.

Cast-in-situ Piles

- Minimum vertical reinforcement in bored cast-in-situ piles shall be four 13 mm dia bars and embedded at least half of the pile.

- Reinforcement should be assembled and and tied together and should be placed in the piles as a unit before the reinforced portion of the pile is filled with concrete.

- When hollow stem auger is used to install piles, the longitudinal steel reinforcement should be placed through ducts in the auger prior to filling the piles with concrete.

- All pile reinforcements should have a concrete cover of not less than 65 mm.

Under-reamed Bored Cast-in-Situ Piles

Longitudinal Reinforcement

- Minimum longitudinal reinforcement in stem shall be 0.4 %.

- Reinforcement is to be provided in the full length.

- Minimum 3 bars o 10 mm of diameter mild steel or 3 bars of 8 mm diameter high strength steel be used.

Transverse Reinforcement

Transverse reinforcement should be provided with bars not less than 6 mm in diameter and at a spacing not less than the stem diameter or 300 mm, whichever is less.

In case of cohesion less soil, under-reaming should not be done in both above and below ground water table.

The minimum depth of under-reaming bulb shall be either 2.75 m or below the level of stabilized moisture content, whichever is deeper.

Under-reaming Compaction Piles

Vertical reinforcement

Minimum four 12 mm dia bars should be provided, but for piles of length exceeding 5 m and diameter exceeding 375 mm, a minimum of six 12 mm dia bars shall be provided. For piles exceeding 400mm in dia, a minimum of six 12 mm dia bars shall be provided.

Transverse Reinforcement

The circular stirrups of such piles shall be provided with a minimum of 8 mm dia bars.