Hydraulics

The crack width formation is dependent on the early tensile strength of concrete. The principle of critical steel ratio also applies in this situation. The amount of reinforcement required to control early thermal and shrinkage movement is determined by the capability of reinforcement to induce cracks on concrete structures. If an upper limit is set on the early tensile strength of immature concrete, then a range of tiny cracks would be formed by failing in concrete tension [4]. However, if the trength of reinforcement is lower than immature concrete, then the subsequent yielding of reinforcement will produce isolated and wide cracks which are undesirable for water-retaining structures. Therefore, in order to control the formation of such wide crack widths, the concrete mix is specified to have a tensile strength (normally measured by Brazilian test) at 7 days not exceeding a certain value (e.g. 2.8N/mm² for potable 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 crack width less than 0.2mm, it is assumed that the mechanism of autogenous healing will take place in which the crack will automatically seal up and this would not cause the problem of leakage and reinforcement corrosion in water retaining structure.

When the cracks are in inactive state where no movement takes places, autogenous healing occurs in the presence of water. However, when there is a continuous flow of water through these cracks, autogenous healing would not take place because the flow removes the lime. One of the mechanisms of autogenous healing is that calcium hydroxide (generated from the hydration of tricalcium silicate and dicalcium silicate) in concrete cement reacts with carbon dioxide in the atmosphere, resulting in the formation of calcium carbonate crystals. Gradually these crystals accumulate and grow in these tiny cracks and form bonding so that the cracks are sealed. Since the first documented discovery of autogenous healing by the French Academy of Science in 1836, there have been numerous previous proofs that cracks are sealed up naturally by autogenous healing. Because of its self-sealing property, designers normally limit crack width to 0.2mm for water retaining structures.

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.

Service reservoirs, other than normal reservoirs, are provided because of the following reasons:

(i) In case of the breakdown of pumping stations and water treatment plants, it provides a temporary storage of water in emergency situation like fire fighting.

(ii) Since the demand of water supply from customers varies with time, the provision of service reservoirs aims to balance the fluctuation rate of water demand.

(iii) It provides a constant head of water to the distribution system under the design pressure.

(iv) In the event of the occurrence of water hammer or surge during the rapid closure and opening of pumping stations, the reservoir acts to attenuate the surge and performs like a surge tank.

(v) It leads to a reduction of the size of pumps and trunk mains connecting to the distribution system as the pumps are not required to directly cope with the peak rates of water demand by the introduction of service reservoirs. As such, there is substantial cost savings arising from the use of smaller pumping pipelines and smaller pumps.

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 unreinforced concrete thrust blocks in bends and tees for pressurized pipelines, it is recommended that the contact surface between the pipelines and concrete thrust blocks should not exceed 45^o from either side of the pipe in the direction of thrust force through the center of pipelines. The reason is to prevent the occurrence of potential cracking arising from the deformation of pipelines under loading condition. If it is necessary to embed the whole section of pipelines into concrete, it is suggested to coat the pipe with a flexible material

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.

In normal situation, reinforcement is not required for thrust blocks in pressurized pipelines. However, certain amount of reinforcement has to be added in thrust blocks in the following situations:

(i) The structure integrity of huge thrust block could be enhanced by the introduction of reinforcement.

(ii) At the anchorages for straps in thrust blocks, some reinforcement has to be designed to avoid the development of tensile stresses.

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.

The unbalanced thrust forces in pressurized pipelines cause the line to move and joints to separate unless the unbalanced force is counterbalanced by some means such as thrust blocks.

Restrained joints can be adopted to resist the thrust forces. The mechanism of restrain joint involves gripping and locking the pipe joints together to avoid axial movement and joint separation. For the unbalanced thrust forces, they are distributed to the surrounding soils in such as way that the bearing area is assumed to decrease linearly from the location of thrust forces to the end of restrained pipes. The soil bearing against the pipelines and soil friction provide resistance to movement of pipelines.

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.

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.

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.

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.

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.

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.

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.

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.

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.