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How can thrust blocks resist unbalanced force in horizontal bends in watermain?

Thrust block resists the unbalanced force in two common approaches. In the first approach, thrust block serves as gravity block which makes use of its own dead weight to resist the thrust forces. An example of this application is vertical down bends.’

The second approach of thrust block to resist unbalanced forces in watermain involves providing a larger bearing area so that the resulting pressure against the soils does not exceed the bearing capacity of soils. Therefore, the function of thrust block in this case is to make use of stiffness of concrete to spread the thrust force into larger area. An example of this application is horizontal bends in watermain.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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Why are hydrodynamic forces not considered in the design of thrust blocks?

Liquids in motion produce forces whenever the velocity or flow direction changes. The forces produced by changes in direction of fluid is called hydrodynamic forces and is equal to (density of fluid x discharge x change in flow velocity).

In underground pressurized pipelines, the configuration of pipelines causes unbalanced forces of hydrostatic and hydrodynamic and joint separation shall result if these forces are not properly balanced. In general, the unbalanced hydrostatic and hydrodynamic forces are called thrust forces. In normal applications of pressurized pipelines in wastewater works and waterworks, it is observed that the range of fluid velocity and discharge is quite limited. As such, the resulting unbalanced hydrodynamic forces induced are insignificant when compared with unbalanced hydrostatic forces and they are often neglected in the design of thrust blocks.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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What is the difference between air chamber and surge tank in pressurized pipelines?

Air chambers and surge tanks are normally installed in watermain to ease the stress on the system when valves or pumps suddenly start up and shut down. A surge tank is a chamber containing fluid which is in direct contact with the atmosphere. For positive surge, the tank can store excess water, thus preventing the water pipes from expansion and the water from compression. In case of downsurge, the surge tank can supply fluid to prevent the formation of vapour column separation. However, if the relief of surge pressure is significant, the height of surge tank has to be large and sometimes it is not cost-effective to build such a large tank. On the contrary, an air chamber can be adopted in this case because air chamber is an enclosed chamber with pressurized gases inside. The pressure head of the gas inside the air chamber can combat the hydraulic transient. The volume of liquid inside the air chamber should be adequate to avoid the pressure in the pipelines falling to vapour pressure. The air volume should be sufficient to produce cushioning effect to positive surge pressures. In essence, air chambers can usually be designed to be more compact than surge tanks. Air chamber has the demerits that regular maintenance has to be carried out to check the volume of air and proper design of pressure level of gas has to be conducted.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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In road opening, it is sometimes noted that asbestos cement pipes are broken up into pieces. Why?

Asbestos cement is mainly a mixture of cement and asbestos fibres with density greater than 1000kg/m3. It contains about 10% asbestos fibres and it is a light grey hard material. The fibres are tightly held in cement mixture and they shall be discharged if asbestos cement undergoes significant disturbance such as drilling and sawing.

However, one of the important characteristics of asbestos cement is the relative brittleness. As such, asbestos cement pipes can be broken easily when falling at height or driven over by heavy vehicles. Hence, it is not uncommon that asbestos cement pipes are observed to break up into pieces in inspection pits.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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What are the controversial health issues of using asbestos cement pipes for watermain?

There are suspected health hazard for using asbestos cement pipes for watermain. In drinking water the gastrointestinal tract cancer risk depends on the amount of asbestos swallowed. When asbestos cement pipes are in good condition, there should be little safety problem. However, when the pipes become aged so that some may break down, it then become a great hazard. When someone takes a little asbestos which are distributed to other parts of the body so that no single parts of the body have excessive amount of asbestos, the risk should be theoretically on the low side. The harmful effect of asbestos is its ability to accumulate in human body. The microscopic fibers lodged in tissues can act like time bombs and cause cancer years later. Since asbestos exposure is cumulative, adults have three or four decades to develop cancer after exposure while youngsters have six or seven.

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However, according to the findings of WHO, asbestos fibres are too large to be absorbed during the digestion. Therefore, the chance of significant transmission of asbestos fibres would seem to be low. Some evidence suggests that high density asbestos-cement products pose no detectable isk to the public because asbestos fibres are carcinogenic only when inhaled but not ingested.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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