Professional Pre Engineering Interview Questions & Answers:
1. Tell me the mechanism of cavitations in pipes and drains?
The formation of air bubbles in a fluid due to low pressure conditions lower than the saturation pressure is known as cavitations. This is considered to be a high potential damage condition where the strength and durability of the pipes can be greatly reduced. Cavitation works on the principle of Bernoulli's Equation. When fluids are at high velocities the pressure head of fluids reduce accordingly. But since the fluid pressure is lower than the saturation pressure the dissolved gases get released from the flowing fluid. These air bubbles suddenly collapse on entering a region of high pressure. This leads to the damage of the pipelines as a high level of dynamic pressure is created.
2. Explain me why are pull-out tests performed for soil nails?
Pull out tests are performed for primarily the following reasons:
☛ In order to detect and the verification of the bond strength among the soil and the grout adopted during the design of soil nails. This is considered to be as the primary objective of performing pull out tests for soil nails.
☛ For the detection of any slippage or occurrence of creeps.
☛ To detect the elastic and deformations (plastic) of any of the test nails employed. Observations are made during the loading and unloading cycles of the soil nails repetitively.
☛ To achieve the perfect balance the test nails should always be loaded so that the ultimate soil/grout mixture with an upper limit of 80%.
3. Explain me what reinforcements are used in the process of prestressing?
The major types of reinforcements used in prestressing are:
☛ Spalling Reinforcement: The spalling stresses leads to stress behind the loaded area of the anchor blocks. This results in the breaking off of the surface concrete. The most likely causes of such types of stresses are Poisson`s effects strain interoperability or by the stress trajectory shapes.
☛ Equilibrium reinforcements: This type of reinforcements are required where several anchorages exist where the prestressing loads are applied in a sequential manner.
☛ Bursting Reinforcements: These kinds of stresses occur in cases where the stress trajectories are concave towards the line of action of load. In order to reduce such stresses reinforcements in the form of bursting is required.
4. Tell me for a washout valve why are two gate valves required in normal practice?
The washout valves are primarily used for normal maintenance works such as that of water main. This can be like to allow water to flow out during the cleaning of the water main. The junctions at which a pipe branches out to a washout point usually a gate valve is installed so that the two pipelines are separated. The gate valve installed above usually remains open during normal operation. Another gate valve is installed further downstream and this remains closed during normal operation of the washout valve. In case this valve is not installed then the pipe section of the branched out pipe would remain dry during normal operation and high chances of damage and leakage can take place. When the downstream valve is installed the branched out water main contains water under normal operation. With two gate valves installed a leakage can be detected immediately.
5. Explain me why are steel plates inserted inside bearings in elastomeric bearings?
In order to make a elastomeric bearing act/ function as a soft spring it should be made to allow it to bulge laterally and also the stiffness compression can be increased by simply increasing the limiting amount of the lateral bulging. In many cases in order to increase the compression stiffness of the bearing the usage of metal plates is made. Once steel plates are included in the bearings the freedom of the bulge is restricted dramatically, also the deflection of the bearing is reduced as compared to a bearing without the presence of steel plates. The tensile stresses of the bearings are induced into the steel plates. But the presence of the metal plates does not affect the shear stiffness of the bearings.
6. Tell me why should curing not be done by ponding and polythene sheets?
The primary purpose of curing is to reduce the heat loss of concrete that is freshly placed to the atmosphere and in order to reduce the temperature gradient across the cross-section of the concrete. Ponding is not preferred for curing as this method of thermal curing is greatly affected by cold winds. In addition to that in ponding large amounts of water is used and has to be disposed off from the construction sites. Polythene sheets are used on the basis that it creates an airtight environment around the concrete surface henceforth reducing the chances of evaporation over fresh concrete surfaces. But the usage of polythene can be a drawback as it can be easily blown away by winds and also the water lost by self-desiccation cannot be replenished.
7. What are the different approaches in designing the floors of the service reservoirs?
In general there are two main approaches of designing the reservoir floors to prevent leakage of water due to seasonal and shrinkage movements:
☛ In this approach the movement joints of the reservoir floor panels are such that the free expansion and contraction of the panels takes place. Every panel is isolated from the other panels and two panels have a sliding layer between them to help in sliding.
☛ The second method does not provide any room for free movement. With seasonal and shrinkage movements, some cracks are designed to voluntarily occur on the floors of the service reservoirs. These tiny cracks are spread throughout the floor and are simply too minute to cause any leakage or corrosion of the floors. But the difference also in this method is that the amount of reinforcement used is much more than the first approach.
8. Explain me why should pumping be not used in case of concreting works?
During the pumping operation the pump exerted pressure must overcome any friction between the pumping pipes and the concrete, also the weight of the concrete and the pressure head when the concrete is placed above the pumps. Since only water is pump able, all the pressure generated is by the water that is present in the concrete. The major problem due to pumping are segregation effects and bleeding. In order to rectify and reduce these effects, generally the proportion of the cement is increased in order to increase the cohesion , which leads to the reduction of segregation and bleeding. Also if a proper selection of the aggregate grading can vastly improve the concrete pump ability.
9. Tell me why is the use of granular sub-base in concrete carriageways not preferred?
Some of the reasons why granular sub-base is not preferred in concrete carriageways:
☛ Sub bases are permeable and hence water can seep through them easily. The soil particles get pumped out through the joints on the application of traffic loads. This results in the creation of voids underneath the pavement structure. This leads to the weakening of the concrete surface and it can crack easily upon intense traffic loads.
☛ Instead if lean concrete is used for carriageways it greatly increases the strength of the roads and the load carrying capacity of the roads is increased.
☛ Sub-bases implementation requires a lot of workmanship which can lead to an un-uniform distribution of the sub-base. This can lead to the cracking of the carriageway when there is severe traffic loading.
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