Foundation, Concrete and Earthquake Engineering

Earthquake Impact on Water Systems

To determine earthquake hazards, information about earthquake sources and mean values of their attenuation, displacement rates, uncertainty of these parameters and local design standards are essential and professional personnel having expertise in particular techniques involved in analysis of seismic risk along with technical persons of water supply authority are required. They should have knowledge about system components and their importance in water supply system.

In some region earthquake may sometimes associated with volcanic activity. A violent volcanic activity may generate eruptions which may even block water courses or produce diversions.

The eruption materials like lava flows, ash and gases may severely damage exposed structures. Avalanches and landslides may even produce total collapse of them. Metal structures like valves & tanks and treatment plants may become damaged while acid rain and ashfall.

Severity of earthquake:

Depending on energy released (i.e. magnitude), an earthquake can produce:

• Settlement of ground surface
• Fault propagation in rocks
• Landslides
• Caves-ins
• Mudslides

Vibration in saturated cohesionless soil can produce liquefaction which in combination with seismic waves can result severe damage and even total destruction of water supply system components.

Severity, in other word degree of damage depends usually on:

• Magnitude of earthquake, depth is also important as it defines extent of tremor.
• Behavior of soil as site amplification may enhance earthquake damage. Say a landslide near water system components or failure of dam destructing or reducing reservoir capacity; contamination of existing or treated water may also occur.

2) Elevated tanks.

Elevated tanks of average or large size are usually constructed from steel or reinforced concrete.

a) Tanks supported by steel frames with adequate diagonal bracing perform well in earthquakes. Their most vulnerable point is where pipes (which form the supporting structure) penetrate the ground. However, different kinds of design, construction, and maintenance of steel tanks, combined with diverse earthquake magnitude and response of the supporting soil, can produce:

• Light damage, such as shear of the diagonal supports, which can be repaired or replaced quickly;

• Damage in the supporting structure and/or in the storage tank can vary from minor to very serious. The most severe damage will likely occur in the connection between the supporting structure and the pipes;

• Collapse of the structure.

b) Concrete tanks can be affected by earthquakes in the following ways:

• Loss of exterior stucco. This is easily repaired although scaffolding may be required;

• Damage to pipes entering or leaving the tank or to superimposed elements such as access ladders. These elements do not compromise the structure and their repairs can range from slightly to moderately difficult;

• Cracks in the supporting structure or storage tank which can occur in the areas of overlap of an excessive number of steel reinforcements, at points where the pipes cross the concrete walls, in the connection between the storage tank and support structure, or in the foundation of the support structure;

• Toppling or leaning of the structure, or foundation failure. This is usually of serious significance;

• Collapse of the structure.

According to the UNDRO study (1982), the survival index of elevated reinforced concrete tanks is less than that of steel tanks, and the precautions for their construction are less clearly defined. Reinforced concrete structures can hide more damage than steel structures, so any damage that exceeds superficial loss of stucco should be examined by a specialist. What appear to be simple cracks can cause major problems when a subsequent earthquake occurs.

c) Small elevated tanks.

Small water storage tanks used for individual dwellings, small groups of houses, schools, small industry, etc., are built of a large variety of materials. The support structure may be built of wood, structural steel, reinforced concrete, etc. The tank may be of corrugated or smooth iron, asbestos cement, fiberglass, reinforced concrete, etc.

• Corrugated iron tanks collapse frequently during earthquakes, but experience shows that this is more often due to poor maintenance than to instability.

• Damage in the support structure and/or in the tank may require simple repairs, or if the structure collapses, require tank replacement. It may be possible to salvage part of the material from wooden and metal structures (except where there is corrosion).

iii) Dams and Reservoirs:

Only dams and reservoirs for drinking water supplies are addressed here. Seismic activity in reservoirs can cause large waves that will overtop the dam. Cave-ins or landslides falling into the reservoir can generate damaging “internal” tidal waves. Floods resulting from the rupture of a dam can have very serious and unpredictable consequences for populations located downstream from the dam.

a) Rock-fill dams are more flexible than those of concrete and more resistant than earth dams. However, the clay or concrete used to make these dams water-tight can crack in an earthquake, resulting in leaks. Possible damage would include:

• Small, medium, or large cracks or leaks;

• Collapse of reservoir embankments;

• Total collapse of the dam.

b) In earth dams, earthquakes cause failure of foundations, cracks in the core, landslides in the dams, waves in the reservoir causing landslides in the dykes, and overtopping or collapse of the core wall. Other damages include:

• Small leaks which should be immediately repaired to avoid the increase of erosion;

• Accumulation of soil because of landslides, which may need to be dredged;

• Collapse of the dam.

c) Concrete dams can crack or the foundations can fail. As in all dams, there is the danger that waves will overtop the dam. Possible damage could include:

• Cracks or small leaks that should be repaired immediately;

• Cracks that would require the reservoir to be emptied for repair (implying loss of stored water);

• Accumulation of soil due to slides;

• Collapse of the dam.

B) Earthquake Damage to Underground or Buried Works:

Underground works include:

a) piping and conduits of drinking water, sewage, and storm water; chambers, valves and domestic installations;

b) Underground water intakes such as wells, drains, and galleries.

These works differ significantly from surface works since, for the most part, damage will not be visible, making actual damage assessment much slower and more labor intensive. For example, within 15 days of the Mexico City e a r t h q u a k e, the major damage to the drinking water mains had been repaired, but months were required to complete smaller repairs, and it was much more complex and time-consuming to repair the sewage and storm drain networks.

The earthquake exerts inertial force on above-ground structures, but buried structures such as pipes and rigid connections can be damaged as the earth undergoes deformation. Less damage can be expected in relatively more flexible pipelines (PVC or steel, for example) compared with rigid pipes such as compressed mortar, concrete, cast iron, and asbestos cement, especially if they have rigid joints.

i) Influence of Soil Type on Damage.

I n embankments built of infill, or in soft soils, earthquakes can break buried pipes. Failures also occur in pipelines located in areas where there is a change of soil type, as in changes in density of natural fill. The liquefaction of soil is one of the most damaging effects of the earthquakes since it reduces foundation support. A large part of damage to pipes in alluvial terrain or water saturated sand occurs because of liquefaction. For example, in Japan, in an area of saturated sands, earthquake vibrations practically converted the soil into a liquid in which the pipes and chambers “floated”, causing major damage to the installations. Large diameter pipes placed at a shallow level suffer more damage than those of smaller diameter, since they have less resistance to “Rayleigh waves” which are dispersed over the earth’s surface in a similar, though less obvious, way as waves of water. Another area of potential damage is in the proximity of pipes to buildings that collapse. The rupture of pipes that enter or leave buildings can wash out public network pipelines to which they are connected.

ii) Seismic Risk Maps Showing Ground Quality.

Given the difficulty of locating damage in existing pipelines, a review of seismic risk maps of the areas affected will show the most vulnerable areas, for example:

• Areas with deep layers of soft soils, sands and sedimentary gravel, swamps and infilled areas (i.e., subsoils that do not absorb seismic vibrations as do hard rock);

• Areas with layers of loose sand that are saturated with water and other non-cohesive soil strata in which the soil can soften;

• Faults in the rock strata (pipelines that cross these faults can suffer damage).

iii) Locating Damage in Pipes:

a) Damage to drinking water pipelines. Damage commonly produces water seepage in areas close to the breaks in the pipes or connections. To determine the magnitude and extent of damage and to make urgent repairs, it is necessary to excavate the lines to find the broken pipe. However, where there are highly permeable soils or low water pressure, it is possible that breaks will be detected only after service is restored. Some indications of this kind of damage are as follows:

1) New leaks evidenced by increased pressure in the network after the breaks initially discovered are repaired;

2) Areas of a city or town that continue without water service or have lower pressure after repairs have been made. This might be due to damage in pipes feeding these zones, which should be identified and repaired.

3) Detecting leaks can be very time-consuming, especially if the necessary equipment and expertise are not locally available. It can be difficult to determine which leaks were caused by the earthquake and which existed before the event.

4) Flow meters installed at appropriate points in the mains of the network can detect the existence of leaks.

b) Damage to Sewage Pipes.

Surface seepage of waste water can be indicative of an area of damage. However, since these are usually open channel flow pipelines, without pressure, there may be fewer visible leaks than in drinking water pipes where pressure can facilitate detection of damage. Manholes can facilitate the visual assessment in successive chambers to locate sections with leaks (by comparing the levels of waste water in neighboring chambers). Breaks in the pipes, if they did not exist before, can be a product of the earthquake. Where the drinking water supply is interrupted as a result of the disaster, there will be no return waste water. Normalization of the drinking water supply must occur before final inspection of the waste water system can take place.

c) Storm-water Drainage System

If a disaster occurs during the rainy season, the review of this system would be similar to that discussed for the sewerage system. However, if it occurs in the dry season, a visual inspection of damage could be carried out by following waste water channels and major sewer mains, accessible sewer mains, if they exist, and by inspection of neighboring reaches from adjacent manholes.

iv) Risk of Contamination of the Drinking Water System.

If pipes from the drinking water and waste water systems break simultaneously, the waste water will penetrate the drinking water system (especially if there is a considerable volume of waste water spread on the ground). This occurs because pipes for drinking and waste water are usually built parallel to each other, along the same streets. In certain cases there is ground water that covers the drinking water and sewerage networks. Ground water contaminated by breaks in the sewage system can infiltrate the drinking water system through broken joints. This is likely to occur if there is negative pressure as a result of breaks in the system or because of rationed drinking water.

C) Effects of Earthquakes on Ground Water Collecting Works.

In areas where water is taken from deep wells or filter galleries, the earthquake can cause the ground water to flow into newly opened fissures resulting in a decrease, and even the exhaustion of the flow obtained from these intakes. Contamination is also a hazard when cracks or faults connect surface water or water from latrines with ground water, rendering intakes useless.

1) Damage to Medium, Deep, or Large Diameter Wells:

Given the variety of wells that exist, various types of damage can occur, including:

• Settling of soil around the well, resulting in slight to severe damage;

• Collapse and total loss of the well (for example, as a result of a fault that traverses the well and causes its collapse, or because of cave-ins that cover it);

• Slight to severe damage in the pumping mechanism.

2) Damage to Filter Galleries or Drains:

In underground galleries or drains, the earthquake can cause various types of damage, including:

• Cracks in the walls, pipes or beams that form the drain or filter gallery. Cracks may be relatively easy to repair (if the filter gallery is accessible) or require interior reinforcements or replacement of the facing of the drain.

• Partial cave-in of part of the filter galley, drain, or manholes;

• Total collapse of the filter gallery or drain;

• Damage to pumping equipment (if it exists).

D) Contamination of Drinking Water Sources.

The risk of ground water contamination was mentioned in the previous section, but a more common hazard is the contamination of surface sources of drinking water. This may occur because of the presence of animal carcasses, or the discharge of petroleum, industrial or toxic wastes into bodies of water, posing one of the greatest large-scale hazards to health in the event of an earthquake. In such cases, it will be necessary to immediately identify alternative sources, and construct (or rehabilitate) intakes and distribution systems for drinking water.

To estimate damage as a result of seismic action, special attention should be given to the stability of foundation soils, including the points described above. The typing of components should consider the interaction with other components that could modify their dynamic response during ground shaking.
Table shown below provide a synthesis of the expected performance of pipes during intense earthquakes.

The expected effects of earthquakes on drinking water and sewerage systems can be summarized as follows:

• Total or partial destruction of intakes, conveyance structures, treatment facilities, storage, and distribution;

• Breaks in delivery and distribution pipes and damage in connections between pipes or with tanks, resulting in a loss of water;

• Interruption of electric power, communications, and access routes;

• Change in water quality because of landslides;

• Variation (decrease) in the flow of underground or surface collector works;

• Change in the site of water outlets in springs;

• Damage from interior coastal flooding caused by tsunamis.

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