Hydraulics

The original intention of adding a thin asphaltic seal-coat on freshly placed cement-mortar lining is to reduce water loss during hydration so as to achieve better curing of the linings. In fact, it also helps hinder the leaching of cement by corrosive water. Otherwise, leachates from cement linings may cause a rise in pH in water.

However, asphaltic seal-coat is considered undesirable from environmental point of view. The seal-coat material is solvent-based which contains volatile organic compounds, which is an air pollutant.

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.

Cement mortar lining provides a high pH in the inner surface of ductile iron pipes and it serves as a physical barrier to water to guard against corrosion of iron from acidic water. Moreover, owing to smooth nature of cement lining, it results in the provision of high flow.

For cement-mortar lined ductile iron pipes to convey water, water infiltrates the pores of lining and releases some calcium hydrate. The freed calcium hydrate form calcium carbonate with calcium bicarbonate in water so that it serves to clog the pores of linings and avoid further permeation of water. Moreover, iron also reacts with lime to precipitate iron hydroxide which also seals the pores of linings. As such, the lining provides both chemical and physical barrier to aggressive water.

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.

For watermain pipe size less than 600mm, ductile iron is normally used because internal welding for steel pipes below 600mm is difficult to be carried out. Moreover, it requires only simple jointing details which allow for faster rate of construction. For watermain pipe size above 600mm, steel pipes are recommended because steel pipes are lighter than ductile iron pipes for the same material strength and therefore the cost of steel pipes is less than that of ductile iron pipes. In addition, in areas of difficult access the lighter mild steel pipes pose an advantage over ductile iron pipes for easy handling.

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.

Both steel pipes and ductile iron pipes use hoop stress equation to model internal pressure design. The difference in pipe thickness arises as a result of more conservative approach in DI pipes.

For ductile iron pipes, surge pressure is considered as part of design pressure and they are added together before applying a safety factor of 2 as follows:

t=[F(P+S)D]/2Y
where t = Pipe thickness
F = Factor of Safety of 2
P = Working pressure
S = Surge pressure
Y = Yield strength of ductile iron

For steel pipes the design of working pressure is based on 50% of steel yield strength (i.e. a factor of safety of 2). The presence of surges could be allowed to increase the stress in pipe to 75% of yield strength. The design is based on the following steps:

(A) If surge pressure is less than or equal to one-half of working pressure, the pipe shall be designed using working pressure only with 50% yield strength as allowable stress.

t= PD/2Y where Y = 50% of yield strength

(B) If surge pressure is more than or equal to one-half of working pressure, the pipe shall be designed using working pressure and surge pressure only with 75% yield strength as allowable stress.

t= [(P+S)D]/2Y where Y = 75% of yield strength

For case A, the use of 50% yield strength is essentially the same of adopting a safety factor of 2 in DI pipe design. However, as surge pressure is not considered, the thickness calculated is smaller than that in DI pipe design.

For case B, the use of 75% yield strength is essentially the same of adopting a safety factor of 1.33 in DI pipe design. As such, the thickness calculated is smaller than that in DI pipe design.

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.

Ductile iron pipes normally possess thicker pipe walls and are generally stiffer than steel pipes. As such, it replies less on side fill soils to support external loads. Hence, it is not necessary to achieve highly-compacted soils for ductile iron pipes for sustaining external loads.

For steel pipes, owing to less stiffness associated with thinner pipe walls, it relies heavily on the stiffness of backfill soils in resisting external loads. Hence, to enhance the external load-carrying capacity of steel pipes, the most convenient methods are to improve the quality of backfill materials and to increase the level of soil compaction.

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.

For watermain pipe size less than 600mm, ductile iron is normally used because internal welding for steel pipes below 600mm is difficult to be carried out. Moreover, it requires only simple jointing details which allows for a faster rate of construction. For watermain pipe size above 600mm, steel pipes are recommended because steel pipes are lighter than ductile iron pipes for the same material strength and therefore the cost of steel pipes is normally less than that of ductile iron pipes. In addition, in areas of difficult access the use of lighter mild steel pipes has an advantage over ductile iron pipes for easy handling.

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.

Any soils that can be excavated with handtools is suitable provided that the void or pore openings in the soil mass are small enough to prevent the calibrated sand used in the test from getting into the natural voids.

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Cement will mix with more water than is required to eventually combine during hydration of cement paste. As such, some voids will be left behind after the hydration process which affects the strength and durability of concrete. With the presence of air voids in concrete, it is vulnerable to penetration and attack by aggressive chemicals.

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For gravity pipes, air tests or water tests are carried out after completion of laying and jointing of the pipes. These tests are conducted to check the watertightness of joints and to ensure the pipelines are free from damage where leakage may occur.

Air test has the advantage that the test itself is simple and faster to be carried out. It does not require the disposal of significant quantities of water used in the test which is a mandatory requirement for water test. However, in case leakage exists in the constructed segment of gravity pipelines, the position of leakage can hardly be located in air test. Moreover, the rate of water leakage cannot be determined from air tests. In addition, air test is readily affected by atmospheric condition because air has a relatively high coefficient of thermal expansion. The test is also influenced by the moisture condition of the test pipelines because it affects the passage of air through the pipelines.

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Froude number is used when gravitation forces is predominant in the channel flow. Reynolds number is adopted when viscous forces are predominant in the channel flow. It is almost impossible to make Froude number and Reynolds number identical in model and prototype.

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