1) Difference
between seepage, infiltration and percolation.
Seepage - When water enters the
ground surface at the upstream side of a retaining structure like a dam and
comes out at the downstream side.
Infiltration
- When water enters the ground surface but doesn't come out, thus increasing
the moisture content of the soil.
Percolation
- When water enters the ground surface and travels through the soil to finally
join the ground water reserves.
2) Under-Reinforced vs Over-Reinforced sections in beam
design.
Under-Reinforced structures
fail by the failure of steel which is ductile in nature. Hence the failure is
gradual and ductile and presents itself with sufficient warning.
Though Over-Reinforced
structures have a greater load bearing capacity than the former, they fail by
the failure of concrete which is brittle. Hence the failure is usually sudden.
3)What are some
quick tests to differentiate between silt and clay in the field?
Dilatancy test - A wet soil
sample placed in the palm of the hand is shaken rapidly. In case of silt, water
will be reflected on the palm on account of greater permeability.
Dispersion test - A small
quantity of soil is added in a glass of water. If soil settles in a few
minutes(10-15 minutes), it is silt. If a turbid suspension is formed its clay.
4) What
advantage does an arch have over a beam?
An arched structure carries less bending
moment than a beam structure for the same loading and span. A part of the
moment due to vertical loading is offset by the horizontal thrust in case of
arches.
For a parabolic arch subjected
to a uniformly distributed load, bending moment is zero at every section of the
arch.
5)Describe briefly the various methods of concrete
curing.
Curing is the process of maintaining the moisture and temperature
conditions for freshly deployed concrete. This is done for small duration of
time to allow the hardening of concrete. The methods that are involved in
saving the shrinkage of the concrete includes:
(a) Spraying of water: on walls, and columns can be cured by sprinkling
water.
(b) Wet covering of surface: can be cured by using the surface with wet
gunny bags or straw
(c) Ponding: the horizontal surfaces including the slab and floors can be
cured by stagnating the water.
(d) Steam curing: of pre-fabricated concrete units steam can be cured by
passing it over the units that are under closed chambers. It allows faster
curing process and results in faster recovery.
(e) Application of curing compounds: compounds having calcium chloride can
be applied on curing surface. This keeps the surface wet for a very long time.
6)What do you understand by “preset” during the
installation process of bridge bearings?
During the installation of bridge bearings the size of the upper plates is
reduced to save the material costs. This process is known as preset. Generally
the upper bearing plate comprises of the following components:
> Length of bearing
> 2 x irreversible movement.
> 2 x reversible movement.
The bearing initially is placed right in the middle point of the upper
bearing plate. No directional effects of irreversible movement is considered.
But since the irreversible movement usually takes place in one direction only
the displaced direction is placed away from the midpoint. In such cases the
length of the upper plate is equal to the length of the length of the bearing +
irreversible movement + 2 x reversible movement.
7)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.
8)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.
9) In the design of bridge arguments what considerations
should be made to select the orientation of the wing walls?
Some of the most common arrangements of wing walls in cases of bridge
arguments are as follows:
> Wing walls parallel to abutments: This method is considered to take
least amount of time to build and is simple as well. But on the downside this
method is not the most economical. The advantage of this type of design being
that they cause the least amount of disturbance to the slope embankment.
> Wing walls at an angle to abutments: This design method is considered
to be the most economical in terms of material cost.
> Wing walls perpendicular to abutments: The characteristic of this
design is it provides an alignment continuous with the bridge decks lending a
support to the parapets.
10) In case if concrete box girder bridges how is the
number of cells determined?
When the depth of a box girder bridge exceed 1/6th or 1/5th of the bridge
width then the design recommended is that of a single cell box girder bridge.
But in case the depth of the bridge is lower than 1/6th of the bridge width
then a twin-cell or in some cases multiple cell is the preferred choice. One
should also note that even in the cases of wider bridges where there depths are
comparatively low the number of cells should be minimized. This is so as there is
noticeably not much improvement in the transverse load distribution when the
number of cells of the box girder is higher than three or more.
11) Under what circumstances should pot bearings be used
instead of elastomeric bearings?
Pot bearings are preferred over elastomeric bearings in situations where
there are chances of high vertical loads in combinations of very large angle of
rotations. Elastomeric bearings always require a large bearing surface so that
a compression is maintained between the contact surfaces in between the piers
and the bearings. This is not possible to maintained in high load and rotation
environment. Also the usage of elastomeric bearings leads to the uneven
distribution of stress on the piers. This results in some highly induced stresses
to be targeted at the piers henceforth damaging them. Due to the above reasons
pot bearings are preferred over elastomeric bearings in such cases.
12)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.
13) 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
14) What are the different type of slump test indications?
Slump tests are performed to empirically measure the workability of fresh
concrete. It is used to measure the consistency of the concrete. In general
there are three different types of slumps that occur in slump tests. They are
as follows:
> True Slump
> Shear Slump
> Collapse Slump
True Slump: This type of slump is characterized by the general drop of the
concrete mass evenly without visible signs of deterioration or disintegration.
Shear Slump: It indicates that the concrete mix is deficient in cohesion.
This type of slump leads to segregation and bleeding. Henceforth in the long
run effecting the durability of the concrete.
Collapse Slump: This type of slump is indicates that the mix of concrete is
simply too wet. The mix is considered to be harsh and lean.
15) Why is propping required for long structures once the
formwork is removed?
Once the process of concreting is performed the striking of the formworks
should be done as soon as possible as delay in this process can lead to the
discoloration of the concrete structures. In case of long structures
particularly long span structures once the structures have attained enough
strength to support themselves it is essential to provide them with propping as
creep deflection can take place which can greatly reduce the integrity of the
structure. Due to the above mentioned reasons propping should be done after the
removal of formwork. Also the props should not be made to stand long as it can
lead to overstress for the structures.
16) Explain 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.
17)For what purpose bedding is used under storm water
drains, explain?
Beddings are primarily made up of granular or concrete materials. They are
primarily used for the following purposes:
> They are used to provide a more uniform support for the under pipes so
that the bending moment longitudinally can be reduced greatly.
> In order to enable the pipes to get more load-supporting strength.
> They are also used to act as a platform to achieve a more correct
alignment and level pre and post construction.
> In case of pipes which contain spigot and socket joints, it enables
pipes to get supported along pipe lengths in place of sockets. Otherwise it can
lead to uneven stress being induced on the pipes eventually damaging it.
18) 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%.
19 )Describe briefly the advantages and disadvantages of
using plastic and timber fenders?
The advantages and disadvantages of using plastic fenders are as follows:
Plastic fenders are low in strength with a relatively high resistance to
abrasion. Plastic fenders are resistant to chemical and biological attacks.
Plastic fenders have moderate energy absorption capacity. The berthing
reactions are also comparatively moderate and depends on the point of contact.
Also since they are made from recycled material they are environmental
friendly.
The advantages and disadvantages of using timber fenders are:
timber fenders are low in strength and are very susceptible to marine borer
attacks and rotting. The energy absorption capacity is very low. Also generally
the contact pressure between the vessels and the fender are high.
20)Explain why concrete barriers have curved surface
profiles?
The concrete safety fencings are made to contain vehicles in their
carriageway being travelled so as to reduce the chances of rebounding into the
roads leading to more hazards. In the case of normal fencings upon vehicle
crashes the fencings give away so as to absorb as much energy as possible
henceforth reducing the impact on the vehicles. But in the case of concrete
barriers their purpose is not to absorb energy of vehicles crashing into the
barrier but to retain them. They have a curved design so as to allow the
vehicles that hit them to slightly go up on the barrier but not overturn. They
also prevent the vehicle from again getting back on the road by rebounds. This
helps in vastly reducing the chances of other vehicle hazards.
21)Why is the use of granular sub-base in concrete carriageways
not preferred, explain?
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.
22) Why are separation membranes used between concrete
pavement slab and sub-base?
The purpose of separation membrane between the concrete pavement slab and
the sub-base are as follows:
> The separation membrane reduces the frictional forces between the
concrete slabs and the sub-base. The membrane aids the movement of the concrete
slab in reference to the sub-base when changes in the level of the moisture and
temperature occurs.
> It aids in the segregation of sub-base materials from freshly placed
concrete.
> The separation membrane also helps in the reduction of cement and
water loss in the form of immature concrete. Immature concrete greatly affects
the strength of the concrete. It also affects the durability of it.
A good example of a separation membranes is polythene sheeting which is
commonly used.
23)In the roof of a pumping station explain briefly the
components of a waterproofing system.
The components of a typical waterproofing system on the roof of a pumping
station are as follows:
> Right above the structural finish level of the roof ( concrete ) a
uniform thickness screed is applied so as to facilitate the application of the
waterproofing membrane. The surface provide for the membrane should always
possess good cohesion properties and must be thin so as to prevent any
un-uniformity. This thin layer also acts as a layer of thermal insulation.
> Right above this layer the waterproofing membrane is deployed to
secure the water tightness of the roof.
> In order to enhance the thermal insulation of the roof an insulation
board is sometime placed right above the waterproof membrane. The insulation
board helps in the maintenance of a stable temperature in both weathers.
24) During reclamation how can the occurrence of mud
waves can be rectified?
There are several solution to the rectification of the problem of mud
waves:
> Complete removal of all the disturbed mud: This method can be
considered to be one of the fastest methods. As soon as the disturbed mud is
removed some filling material is used to replace the disturbed mud. But
economically this method can be expensive as compared to others.
> Accelerated consolidation of disturbed mud: In this method surcharging
loads are placed on top of the mud waves. Along with this band drains are
installed to accelerate the consolidation process. This method is quite slow
compared to the other methods.
> Partial removal of the disturbed mud: This method is the hybrid of the
above two methods where the top layer is removed whereas the lower level is
treated with the surcharging process.
25) In reclamation works what are the importance of
geotextiles and sand?
The primary purposes of geotextiles and sand in reclamation works are as
follows:
> Geotextiles: They are used to separate the marine mud from the
reclamation fill. Also geotextiles are used as reinforcements in reclamation
processes to increase its stability. It is still debated as to whether the
usage of geotextiles is better or are the old processes followed are better as
the performance has not been comparable to the conventional methods.
> Sand: In reclamation process sand is used to spread the load of any
future public dumps placed on top of it. Sand also acts as a drainage for the excess
pore water pressure of band drain installations.
