465 october-2019 time-three hours part-a 1. define void ...6. different types of shallow foundation...
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465
October-2019
Time-Three hours
(Maximum marks: 75)
Part-A
1. DEFINE VOID RATIO AND POROSITY.
Void ratio:
The void ratio of a mixture is the ratio of the volume of voids to
volume of solids.
Porosity:
The porosity is the ratio of VS is the volume of solids to VT is the total
or bulk volume.
2. DEFINE DARCYS LAW
Darcy's law states that the rate of fluid flow through porous medium is
proportional to the potential energy gradient within that fluid.
The constant of proportionality is the Darcy's permeability of soil.
Darcy's permeability is a property of both porous medium and the
fluid moving through the porous medium.
3. WHAT IS O.M.C
Optimum moisture content (OMC) is the percentage of water present
in soil mass at which a specific compaction force can dry
the soil mass to its maximum dry weight. ... For different types
of soils, OMC and maximum dry density curves are different.
4. WHAT IS EFFECTIVE STRESS
Effective stress is the force at contact particles of soil but divided by
total area. The contact area is very less between the particles here.
It cannot be obtained practically but we can calculate the effective
stress by measuring total stress and pore water pressure.
5. DEFINE SAFE BEARING CAPACITY OF SOIL
Bearing capacity is the capacity of soil to support the loads applied to
the ground.
Ultimate bearing capacity is the theoretical maximum pressure which
can be supported without failure; allowable bearing capacity is
the ultimate bearing capacity divided by a factor of safety is known as
safe bearing capacity of soil.
6. DIFFERENT TYPES OF SHALLOW FOUNDATION
Strip footing
Spread or isolated footing
Combined footing
Strap or cantilever footing Mat or raft Foundation
7. WHAT ARE THE FORCES ACTING ON THE TRANSMISSION
LINE TOWER
Shearing force
Bering force
Uplift pressure
Seapage pressure
8. WHAT IS SOIL EXPLORATION
Soil exploration consists of determining the profile of the
natural soil deposits at the site, taking the soil samples and
determining the engineering properties of soils using laboratory tests
as well as in-situ testing methods.
PART-B
9. Define plasticity index and liquidity index.
Plasticity Index
It is the difference between the liquid limit and plastic limit of the soil.
It shows volume of range of moisture content at which the soil
remains in plastic condition.
PI=LL - PL
Here,
PI - plasticity index,
LL - liquid limit
PL - plastic limit
LiquidityIndex
The relative consistency of a fine grained soil in the original state can
be defined by a proportion called as liquidity index.
LI=(W-PL)/(LL-PL)
Here,
LI - liquidity index,
w -sample water content,
PL -plastic limit
LL- liquid limit.
10. Explain the Mohr-coulomb failure theory
Material fails essentially by shear.
The critical shear stress causing failure depends upon the properties
of the material as well as on normal stress on the failure plane.
The ultimate strength of the material is determined by the stresses on
the potential failure plane (or plane of shear)
When the material is subjected to three dimensional principal stress
(i.e.1, 2, 3) the intermediate principal stress does not have any
influence on the strength of material.
In other words, the failure criterion is independent of the intermediate
principal stress.
The theory can be expressed algebraically by the equation.
f s F()
Where
f =s= shear stress on failure plane,
F () = function of normal stress
11. Brief notes on Quick sand condition.
Quick sand condition or boiling Quick sand condition is a condition of
flow, not a type of soil, in which a vertical upward seepage flow causes
floating condition of a particle in cohesion less soil such as Sand and fine
grave.
Where the effective pressure becomes zero.
12. What are the factors affecting the bearing capacity of the soil?
soil strength
foundation width
foundation depth,
soil weight and surcharge,
spacing between foundations.
These factors are related to the loads exerted on the soil and considerably
affect the bearing capacity
13. Mention any six uses of pile.
If a high groundwater table exists beneath the structure.
If the superstructure load is high and non-uniform.
If highly compressible soil is present at shallow depth.
If the structure is located near the river bed or sea shore etc, pile
foundation is suggested to secure the structure form the possible
scouring.
If a canal or deep drainage systems pass near the structure, pile
foundation is suggested.
If soil condition is very poor and it is not possible to excavate the soil up
to the desired depth.
If it becomes impossible to keep the foundation trenches dry by any
measure due to heavy inflow of seepage
14. Define swelling potentials, swelling pressure and free swell.
swelling potentials
Soil swelling is a term generally applied to the ability of a soil to undergo
large changes in volume due to increased moisture content
swelling pressure
The expansive clays increase in their volume when they come in contact
with water owing to surface properties of these clay types.
The pressure which the expansive soil exerts , if it is not allowed to swell
or the volume change of the soil is arrested , is known as Swelling
Pressure of Soil.
free swell
Free swell is the increase in volume of a soil, without any external
onstraints, on submergence in water
15. Distinguish between free vibration and forced vibration.
Free vibrations involve no transfer of energy between the vibrating object
and its surroundings,
whereas forced vibrations occur when there's an external driving force
and thus transfer of energy between the vibrating object and its
surroundings.
16. Write short notes on Isolated Footing.
Isolated footing is the single or individual footing which transfers load to
the underground soil.
It is provided when a single column is to be provided.
Isolated footing is generally provided for shallow depths.
Shallow foundations have their depths less than the widths.
Footing is provided, either as simple footing, sloped footing or stepped
footing.
In simple isolated footing, base of uniform depth is provided.
In sloped footing, base of uniformly sloping downward pattern is
provided.
In stepped footing, base is constructed in steps to distribute the load
uniformly to the foundation soil.
Part-C
17. (A)DESCRIBE SHRINKAGE LIMIT DETERMINATION WITH
NEAT SKETCH
The shrinkage limit of soil is the water content of the soil when the water is just
sufficient to fill all the pores of the soil and the soil is just saturated.
The volume of the soil does not decrease when the water content is reduced
below the shrinkage limit.
Shrinkage limit can be determined from the relation
Where M1= initial wet mass,
V1= initial volume
Ms = dry mass of soil
V2 = volume after drying.
Take a sample of mass about 100g from a thoroughly mixed soil passing 425
micron sieve.
Take about 30g of soil sample in a large evaporating dish.
Mix it with distilled water to make a creamy paste which can be readily worked
without entrapping the air bubbles.
Take the shrinkage dish.
Clean it and determine its mass.
Fill the mercury in the shrinkage dish.
Remove the excess mercury by pressing the plain glass plate over the top of the
shrinkage dish.
The plate should be flush with the top of the dish. And no air should be
entrapped.
Transfer the mercury of the shrinkage dish to a mercury weighing dish and
determine the mass of the mercury to an accuracy of 0.1g. the volume of the
shrinkage dish is equal to the mass of mercury in grams divided by the specific
gravity of the mercury.
Coat the inside of the shrinkage dish with a thin layer of silicon grease or
Vaseline.
Place the soil specimen in the center of the shrinkage dish equal to about one-
third the volume of the shrinkage dish.
Tap the shrinkage dish on a firm cushioned surface and allow the paste to flow
to the edges.
Add more soil and continue the tapping till the shrinkage dish is completely
filled and excess soil paste projects out about its edge.
Strike out the top surface of the plate with a straight edge. Wipe of all soil
adhering to the outside of the shrinkage dish. Dry the soil in the shrinkage dish
in air until the colour of the pat turns from dark to light.
Then dry the pat in the oven at 105 to 110 0C to constant mass. Cool the dry pat
in a desiccator.
Remove the dry pat from the desiccator after cooling, and weight the shrinkage
dish with the dry pat to determine the dry mass of the soil.Place a glass cup in a
large evaporating dish and fill it with mercury.
Remove the excess mercury by pressing the glass plate with prongs firmly over
the top of the cup.
Wipe off any mercury adhering to the outside of the cup.
Remove the glass cup full of mercury and place it in another evaporating dish
taking care not to spill any mercury from the cup.
Take out the dry pat of the soil from the shrinkage dish and immerse it in the
glass cup full of mercury.
Take care not to entrap air under the pat.
Press the plate with prongs on the top of the cup firmly.
Collect the mercury displaced by the dry pat in the evaporating dish and transfer
it to the mercury weighing dish.
Determine the mass of the mercury to an accuracy of 0.1g.
The volume of the dry pat (V2) is equal to the mass of the mercury divided by
the specific gravity of the mercury.Repeat the test atleast 3 times.
Stages for derivation of Shrinkage Limit
(B) DESCRIBE CONSTANT HEAD PERMEABILITY TEST WITH
NEAT SKETCH
Constant Head Permeability Test
The constant head permeability test is a laboratory experiment conducted to
determine the permeability of soil.
The soils that are suitable for this tests are sand and gravels.
Soils with silt content cannot be tested with this method.
The test can be employed to test granular soils either reconstituted or disturbed.
Objective and Scope
The objective of constant head permeability test is to determine the coefficient of
permeability of a soil.
Coefficient of permeability helps in solving issues related to:
Yield of water bearing strata
Stability of earthen dams
Embankments of canal bank
Seepage in earthen dams
Settlement Issues
The coefficient of permeability, k is defined as the rate of flow of water under
laminar flow conditions through a porous medium area of unit cross section under
unit hydraulic gradient.
Procedure
Remove the collar of the mould. Measure the internal dimensions of the mould.
Weigh the mould with dummy plate to the nearest gram.
Apply a little grease on the inside to the mould. Clamp the mould between the base
plate and the extension collar and place the assembly on a solid base.
Take about 2.5kg of the soil sample, from a thoroughly mixed wet soil, in the
mould. Compact the soil at the required dry density using a suitable compacting
device.
Take a small specimen of the soil in a container for the water content
determination.
Remove the collar and base plate. Trim the excess soil level with the top of the
mould.
Clean the outside of the mould and the dummy plate. Find the mass of the soil in
the mould.
The mould with the sample is now placed over the permeameter. This will have
drainage and cap discs properly saturated
Test Procedure
Through the top inlet of the constant head reservoir, the specimen is connected.
The bottom outlet is opened and a steady flow is established
For a particular time interval, the quantity of flow can be collected.
Measure the difference of head (h) in levels between the constant head reservoir
and the outlet in the base.
For the same interval, this is repeated three times.
18. (A)EXPLAIN WITH NEAT SKETCH THE STANDARD PROCTOR
COMPACTION TEST TO DETERMINE THE DENSITY OF SOIL
Compaction is the process of densification of soil by reducing air voids.
The degree of compaction of a given soil is measured in terms of its dry
density.
The dry density is maximum at the optimum water content.
A curve is drawn between the water content and the dry density to obtain the
maximum dry density and the optimum water content.
Dry density of soil:
Where M = total mass of the soil, V= volume of soil, w= water content.
Proctor Soil Compaction Test Procedure:
Take about 20kg of air-dried soil. Sieve it through 20mm and 4.7mm sieve.
Calculate the percentage retained on 20mm sieve and 4.75mm sieve, and
the percentage passing 4.75mm sieve.
If the percentage retained on 4.75mm sieve is greater than 20, use the large
mould of 150mm diameter. If it is less than 20%, the standard mould of
100mm diameter can be used. The following procedure is for the standard
mould.
Mix the soil retained on 4.75mm sieve and that passing 4.75mm sieve in
proportions determined in step (2) to obtain about 16 to 18 kg of soil
specimen.
Clean and dry the mould and the base plate. Grease them lightly.
Weigh the mould with the base plate to the nearest 1 gram.
Take about 16 – 18 kg of soil specimen.
Add water to it to bring the water content to about 4% if the soil is sandy
and to about 8% if the soil is clayey.
Keep the soil in an air-tight container for about 18 to 20 hours for maturing.
Mix the soil thoroughly. Divide the processed soil into 6 to 8 parts.
Attach the collar to the mould. Place the mould on a solid base.
Take about 2.5kg of the processed soil, and hence place it in the mould in 3
equal layers. Take about one-third the quantity first, and compact it by
giving 25 blows of the rammer. The blows should be uniformly distributed
over the surface of each layer.
The top surface of the first layer be scratched with spatula before placing
the second layer. The second layer should also be compacted by 25 blows
of rammer. Likewise, place the third layer and compact it.
The amount of the soil used should be just sufficient to fill the mould ad
leaving about 5 mm above the top of the mould to be struck off when the
collar is removed.
Remove the collar and trim off the excess soil projecting above the mould
using a straight edge.
Clean the base plate and the mould from outside. Weigh it to the nearest
gram.
Remove the soil from the mould. The soil may also be ejected out.
Take the soil samples for the water content determination from the top,
middle and bottom portions. Determine the water content.
Add about 3% of the water to a fresh portion of the processed soil, and
repeat the steps 10 to 14.
(B) WRITE NOTES ON MECHANICAL AND CHEMICAL
STABILIZATION
MechanicalStabilization:
Mechanical stabilization is the methodology, which improves selected
engineering properties of soils without the addition of agents or other
particle binding energy.
The methodologies are as follows, compaction, blasting, dynamic
compaction, preloading, sand drains, etc.
Factors affecting the mechanical stabilization are, aggregate mechanical
strength, mineral composition, gradation, compaction, and properties of
soil.If the mixture is not properly designed or not compacted well, the
mechanical stabilization will get affected.
The composition of the mineral is linked with the mechanical stability of the
mixed soil.
The mineral present in the soil should resist weathering action.To attain a
high density in the mixed soil, the pore space between the coarse aggregate
have to be filled with the fine aggregate.Effective compaction is necessary in
the mixed soil to produce high density and stability mix.Plasticity index in
the soil has to be in control, as it reduces the soil stability.
When the clayey soil is in the saturated condition, plasticity index can affect
the stability.
Chemical stabilisation
soil stabilization is the process of altering properties of soil by changing the
gradation through mixing with other oils or chemicals to improve strength
and durability.
Mechanical stabilization and chemical stabilization are the main two
methods employed in stabilization.
Chemical processes such as mixing with cement, fly ash, lime, lime by
products and blends of any one of these materials can be used to alter soil
properties such as strength, compressibility, hydraulic conductivity, swelling
potential and volume change properties.
The additives are combined with the help of machines.
The method used depends on the location and availability of the machine.
There are many types of additives used for chemical stabilization.
But the selection of additive is based on the type of soil.
A single additive act differently with different type of soils.Cement
stabilization offers better strength and improves soil quality.
Normally Portland cement isused for this purpose.
Stabilization using lime creates long-lasting changes in soil properties.
Lime reacts with medium, moderately fine and fine-grained soils to produce
decreased plasticity, increased workability, reduced swelling, and increased
strength.
To enhance the effectiveness of cement, lime, fly ash stabilization or a
combination of the three are used in required proportion.
Fly ash is the byproduct of combustion of coal and contains Silicon and
Aluminum and is mainly used as a filler product to reduce voids.
The silicate aluminates-amide system is widely used for strength
improvement and water cut-off as this system can be used in acidic soils as
well.
19.(A)SHOW THE FLOWNET DIAGRAM BY SKETCH AND ALSO
EXPLAIN THE VARIOUS TYPES OF FLOWS
Flow net is a graphical representation of flow of water through a soil mass.
It is a curvilinear net formed by the combination of flow lines and
equipotential lines.
Properties and application of flow net are explained in this article.Flow lines
represent the path of flow along which the water will seep through the soil.
Equipotential lines are formed by connecting the points of equal total head.
Properties of flow net are, the angle of intersection between each flow line
and an equipotential line must be 90o which means they should be
orthogonal to each other.
Two flow lines or two equipotential lines can never cross each
other.Equal quantity of seepage occurs in each flow channel.
A flow channel is a space between two flow lines.
Head loss is the same between two adjacent potential lines.
Flow nets are drawn based on the boundary conditions only.
They are independent of the permeability of soil and the head causing flow.
The space formed between two flow lines and two equipotential lines is
called a flow field.
It should be in a square form.Either flow lines or equipotential lines are
smoothly drawn curves.
Applications of FlowNet
Flow net is useful to determine the following parameters in seepage analysis of soil
Rate of Seepage loss
Seepage Pressure
Uplift Pressure
Exit Gradient
(B) DESCRIBE ABOUT TERZHAGHI ANALYSIS WITH SKETCH
Terzaghi analysis theory, column load P is resisted by shear stresses at edges of
three zones under the footing and the overburden pressure, q (=γD) above the
footing.
The first term in the equation is related to cohesion of the soil.
The second term is related to the depth of the footing and overburden pressure.
The third term is related to the width of the footing and the length of shear
stress area.
The bearing capacity factors, Nc, Nq, Nγ, are function of internal friction angle,
φ. Terzaghi's Bearing capacity equations:
Strip footings: Qu = c Nc + γ D Nq + 0.5 γ B Nγ
Square footings: Qu = 1.3 c Nc + γ D Nq + 0.4 γ B Nγ
Circular footings: Qu = 1.3 c Nc + γ D Nq + 0.3 γ B Nγ
20. (A )HOW DO YOU DETERMINE LOAD CARRYING CAPACITY OF
PILES?EXPLAIN
The ultimate bearing capacity of a pile is the maximum load which it can carry
without failure or excessive settlement of the ground.
The bearing capacity of a pile depends primarily on 3 factors as given below,
1. Type of soil through which pile is embedded
2. Method of pile installation
3. Pile dimension (cross section & length of pile)
While calculating pile load capacity for cast in situ concrete piles, using static
analysis, we need to use soil shear strength parameter and dimension of pile.
The pile transfers the load into the soil in two ways.
Firstly, through the tip-in compression, termed as “end-bearing” or “point-
bearing”; secondly, by shear along the surface termed as “skin friction”.
LOAD CARRYING CAPACITY OF CAST IN-SITU PILES IN COHESIVE
SOIL
The ultimate load carrying capacity (Qu) of pile in cohesive soils is given by the
formula given below, where the first term represents the end bearing
resistance (Qb) and the second term gives the skin friction resistance (Qs).
LOAD CARRYING CAPACITY OF CAST IN-SITU PILES IN COHESION
LESS SOIL
The ultimate load carrying capacity of pile, “Qu”, consists of two parts. One
part is due to friction, called skin friction or shaft friction or side shear denoted
as “Qs” and the other is due to end bearing at the base or tip of the pile
toe, “Qb”.
The equation given below is used to calculate the ultimate load carrying
capacity of pile.
(B)EXPLAIN UNDER-REAMED PILE FOUNDATION.
Under reamed piles are bored cast-in-situ concrete piles having one or more
number of bulbs formed by enlarging the pile stem.
These piles are best suited in soils where considerable ground movements occur
due to seasonal variations, filled up grounds or in soft soil strata.
Provision of under reamed bulbs has the advantage of increasing the bearing
and uplift capacities. It also provides better anchorage at greater depths.
These piles are efficiently used in machine foundations, over bridges, electrical
transmission tower foundation sand water tanks.
Indian Standard IS 2911 (Part III) - 1980 covers the design and construction of
under reamed piles having one or more bulbs.
According to the code the diameter of under reamed bulbs may vary from 2 to 3
times the stem diameter depending upon the feasibility of construction and
design requirements.
The code suggests a spacing of 1.25 to 1.5 times the bulb diameter for the
bulbs. An angle of 45 0 with horizontal is recommended for all under reamed
bulbs.
21. (A) Explain Barkan’s method.
Refer the books………
(B) Explain the choice and types of foundation for transmission line tower.
There are seven basic types of tower foundations:
a) Steel grillage
b) Concrete spread footing
c) Concrete auger or caisson
d) Pile foundation
e) Rock foundation
f) Raft foundation
g) Novel foundations.
The foundation is excavated to the desired depth.
Generally, the depth of foundation is shallow, just sufficient to
accommodate the two tiers of grillage beams and the gusset plates etc.
connecting the stanchion to the base.
However this depth should not be less than 90 cm in any case.
After leveling the foundation base, rich concrete is poured and compacted,
so that the formed thickness is not less than 15 cm.
Compaction should be done properly so that the layer of concrete becomes
an impervious bed.
This would protect the steel joists against ground water.
After levelling the concrete bed, first layer of grillage beams of designed
sizes are laid over it, at proper distances, with the help of separaters.
The upper surface of all the beams should lie in one horizontal plane.
Rich cement grout is then poured all around the lower flanges of the beams
so that they are secured to the concrete bed.
Cement concrete is then poured betwecn and around the beams of the first
tier.
The second tier of beams is then placed at right angles to the first tier and
over the top flanges of the beams of the first tier.
They are properly spaced with the help of separators.
Concrete is then poured between and around the steel beams.
The steel stanchion is then connected to the upper tier with the help of a
base plate, side angles and gusset plate.
These connecting elements are also embedded in the concrete so that joint
becomes rigid.
Steel grillage foundation may also be provided for a masonry wall on soils
of low bearing capacity.
The grillage foundation for such a case consists of only on tier, though in
some circumstances when the wall is wider and it carries heavy loads, two
tiers may also be provided.
TYPICAL GRILLAGE FOUNDATION FOR STEEL STANCHION.
STEEL GRILLAGE FOUNDATIONS FOR WALLS.
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