# Civil Engineering Interview Questions Part – 21

Question No. 01
Should design life be the same as return period for design conditions?
Answer: Design life means the minimum duration a structure is expected to last. The longer is the design life; the higher is the cost of a project. Therefore, in choosing the design life for a structure, engineers should consider the design life which generates a economical project without sacrificing the required function.
In selection of return period of certain design conditions, winds, waves, etc., one should consider the consequences of exceedance. In fact, there are normally no extreme maximum values of these design conditions and its selection is based on the probability of exceedance which is related to return period.
Therefore, design life may not be equal to return period of design conditions because their selections are based on different considerations.

Question No. 02
How are freeway bridges built?
Answer: After calculating the anticipated traffic for the bridge, cement/reinforced-with-rebar stanchions are spaced over the freeway to accommodate the bridge. An ‘off-ramp’ from the freeway to the bridge is constructed, as is an ‘on-ramp’ to the subsequent road. Cement/rebar slabs are built and lifted with cranes to form the platform, and voila! Drive carefully. Although the bridge deck/roadway is almost always a concrete slab, the structure that holds up the bridge deck can be reinforced concrete, structural steel, or a combination of steel and concrete.

Question No. 03
What is the difference between air chamber and surge tank?
Answer: 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 water from compression. In case of down-surge, the surge tank could supply fluid to prevent the formation of vapour column separation. However, if the surge pressure to be relieved is very large, the height of surge tank has to be designed to be excessively large and sometimes it is not cost-effective to build such a chamber. On the contrary, an air chamber can be adopted in this case because air chamber is a enclosed chamber with pressurized gases inside. The pressure head of gas inside the air chamber is the component to combat the hydraulic transient. However, air chamber has the demerits that regular maintenance has to be carried out and proper design of pressure level of gas has to be conducted.

Question No. 04
What is Gravity flow?
Answer: Gravity flow is fluid flowing due to the forces of gravity alone and not to an applied pressure head. In the Bernoulli equation, the pressure term is omitted, and the height and velocity terms are the only ones included.

Question No. 05
How do rock sockets take up loads?
Answer: The load transfer mechanism is summarized as follows: When a socketed foundation is loaded, the resistance is provided by both rock socket wall and the socket base and the load distribution is a function of relative stiffness of foundation concrete and rock mass, socket geometry, socket roughness and strength. At small displacements the rock-socket system behaves in an elastic manner and the load distribution between socket wall and socket end can be obtained from elastic analysis. At displacements beyond 10-15 mm, relative displacement occurs between rock and foundation and the socket bond begins to fail. This result in reduction of loads in rock-socket interface and more loads are transferred to the socket end. At further displacements, the interface strength drops to a residual value with total rupture of bond and more loads are then distributed to the socket end.

Question No. 06
What is made from large rocks, which protects the base of cliffs?

Question No. 07
In designing the lateral resistance of piles, should engineers only use the earth pressure against pile caps only?

Answer: In some design lateral loads are assumed to be resisted by earth pressure exerted against the side of pile caps only. However, it is demonstrated that the soil resistance of pile lengths do contribute a substantial part of lateral resistance. Therefore, in designing lateral resistance of piles, earth pressure exerted on piles should also be taken into consideration.
In analysis of lateral resistance provided by soils, a series of soil springs are adopted with modulus of reaction kept constant or varying with depth. The normal practice of using a constant modulus of reaction for soils is incorrect because it overestimates the maximum reaction force and underestimates the maximum bending moment. To obtain the profile of modulus of sub-grade reaction, pressure-meter tests shall be conducted in boreholes in site investigation. Reference ismade to Bryan Leach (1980).

Question No. 08
What is the density of most of construction materials?
Answer: If it floats it is less dense than water, 62.4 lbs/cubic ft. Wood is about 40 lbs/ cu.ft.it floats. Concrete is 150 lbs. / cu.ft. It does not float.

Question No. 09
What are the differences in function between rock anchors and rock sockets?
Answer: Rock anchors are used primarily for resisting uplift forces. On the contrary, rock sockets serve three main purposes:
(i) Rock socket friction and end bearing to resist vertical load;
(ii) Passive resistance of rock sockets contribute to resistance of lateral load; and

(iii) Socket shaft friction is also used for resisting uplifting forces. But only 70% of this capacity should be used because of the effect of negative Poisson ratio.
Note: Rock anchors, which may consist of a high tensile bar or a stranded cable, are provided for tension piles when there are insufficient soil covers to develop the required uplifting resistance.

Question No. 10
In designing concrete structures, normally maximum aggregate sizes are adopted with ranges from10 mm to 20 mm. Does an increase of maximum aggregate size benefit the structures?
ratio of a cube is 6/b where b is the length of the cube. This implies that the surface area to volume ratio decreases with an increase in volume. Therefore, when the size of maximum aggregate is increased, the surface area to be wetted by water per unit volume is reduced. Consequently, the water requirement of the concrete mixes is reduced accordingly so that the water/cement ratio can be lowered, resulting in a rise in concrete strength.
However, an increase of aggregate size is also accompanied by the effect of reduced contact areas and discontinuities created by these larger sized particles. In general, for maximum aggregate sizes below 40 mm, the effect of lower water requirement can offset the disadvantages brought about by discontinuities as suggested by Longman Scientific and Technical (1987).

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