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11.What is coefficient of compressibility? The coefficient of compressibility represents the compression of clay due to a unit increase of the pressure. 12. Explain how volume change occurs in soil masses. . When a soil mass is subjected to a compressive force, its volume decreases. The compression of soils can occur due to the following causes. i. Compression of solid particles and water in the voids. ii. Compression and expulsion of air in the voids. iii. Expulsion of water in the voids. 13. What is swelUng pressure? Swelling pressure is defmed as the pressure required for preventing volume expansion in soil in contact with water. 14. What is Free Swell ? Free Swell is defmed as Vf-Vi _m_- X 100 Sf Vi Sf Free Swell in percent Vf Final volume of poured soil = Vi Initial dry volume of poured soil 15. What are the two major limitations of plate load test? 1. Plate load tests should not be relied on to detennine ultimate bearing capacity of sandy soils as the scale effect gives misleading results since the bearing capacity of footin gs in sands varies with size of the footings. 2. Consolidation settlements in cohesive soils, which may take years, can not be predicted, as the plate load test is essentially short duration test. 16. What is the advantage of method of proportioning? In the method of proportioning, the footing sizes are arrived based on the assumption that the allowable soil pressure under service loads below all the footings is same. This will ensure that all the footings would settle by equal amounts. 17. Differentiate immediate and consolidation settlement. ,

S.No Immediate Settlement .J Consolidation Settlement 1. Immediate settlement are those which takes Consolidation settlement which are time 2.

3.

place as the load is applied immediately (with in a time period of 7 days) It forms the major contribution to the total settlement in case of cohesion less soil. The rate of settlement does not depends upon drainage condition. .

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department and taking months to years to develop. In forms the major contribution to the total settlement in case of cohesive soils. The rate of settlem;:nt depends upon the drainage conditions of the clay layer. J

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are the corrections to be applied for computed settlement values? are two corrections to be applied for computed consolidation settlement. ] Correction for geometry of the footing and history of the clay. COlTectionfor depth.

19. The correction factor for geometry of the footing and history' of the clay depends upon the pore pressure coefficient A. Where as The depth cOlTection = Factor may be defmed as Depth factor

=

Correctedsettlementfor a footing at depth of

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Calculated settlement for a surface footing 20. Is code permissible settlement values. ..,

I Maximum Permissible Settlement 65mm Isolated foundations on clay 40mm Isolated foundations on sand 65 to 100 mm I Raft foundation on clay 40 to 65 mm I Raftfoundation on sand 40mm Maximum differential settlementon clay Maximum differentialsettlement on sand 25mm Angular distortion incase of large framed 1/500 Normally and 1/1000 if all structmes kinds of minor damages-~C!J?~_P!~vente4___Note: For columnsand load bearing wall structural damage is expected when angular distortion exceeds 1/150. I

Type of Foundation

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Architectural damage is expected wlJ.enangular distortion exceeds 1/300. 21. Settlement due to dewatering occurs because of the increase in effective stress.

22. Expansive clays cause settlement characteristics.

problems

because of their swelling and shrinkage -'

23. If the coefficient mv is known, the settlement can be estimated mvl\crH.

using the expression

24. Three components of settlements of footings. on clay are !!nmediate consolidation settlement and secondary compression.

settlement,

~5. Safe' limit of angular distortion ,for building where cracking is not permissible 1/1000, \Ibr

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30. "Vhat is the type of foundation failure relative density is less than 200/0?

in a sand

deposit

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The type of foundation failure in a sand deposit if the relative density is less than 20% is local shear failure. 31. What is the nature of the footing assumed in bearing capacity? . Terzaghi gave a general theory for the making the following assumptions. (a) The footing is strip footing. ' (b) The footing is laid at shallow depth. (c) The base of the footing is rough. (d) TI1e load on the footing is vertical and 32. List on sand?

the

factors

that influence

the

Terzaghi's

theory of

bearing capacity of soils

i.e. Dt< B. is unifonnly distributed.

bearing capacity

of a

According to Terzaghi' s bearing capacity theory, The ultimate bearing capacity of a footing on sand is qu=

yDfNq +0.5ByNy Where Nq, Ny

So (a) (b) (c) (d)

= feet»~

the factors influencing the bearing capacity of sandy soils are Unit weight of the soil. Angle of internal friction. Width of foundation. Depth of foundation.

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footing

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26. What is the factor of safety in the design of the footing

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Factor of safety is defmed as the ratio between ultimate bearing .capacity an<;lsafe bearing capacity. The factor of safety in foundation may range from 2 to 5,depending upon the impol1ance of structure and soil profIle at the sight. '. 27. What is the maximum permissible settlement for a footing on clay as per B.I.S The maximum permissible settlement for a isolated footing on clay is 65 mm. For raft

foundation on clayis 65 to 100mm. . more 28. Why;s difJl!._ut settlement allowed in clay than sand The allowable differential sett'ement higher for clays because progressive settlements (consolidation settlements) on clayey soils permit better strain adjustments in the structural member. In sands the differentialsettlementscan occur as soon as the total.settlementitself has occurred, thus leavingno time for gradualadjustment.

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UNIT IV PILE FOUNDATIONS

1. . Explain the following terms i. ii. iii. iv. v. vi.

End bearing pile Friction pile Tension pile (or) Uplift pile Anchor pile. Compaction piles Fender piles

End bearingpile :

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When the top soil is soft or too weak to support the super structure, piles are employed to transmit the load to the underlying bed rock, such pile. are called end bearing piles (or) point bearing piles. Friction pile : When the bed rock is not existing at a reasonable depth the load is transferred through friction along the pile shaft such piles are called friction pile.

Combined end bearing andfriction piles: These piles transfer loads by a combination of end bearing at the bottom of the pileand frictionalongthe surface of the pile shaft. Compaction piles: Short piles are sometimes used for compacting loose sand deposits \vhich gets densified by the vibrations set up on driving. Such piles are known as compaction piles. Uplift pile (or) tension piles: Transmission towers, off shore plat fonus, and ba.sement mats are subjected to uplift forces and piles are used to resist the uplift forces, which are called uplift piles (or) tension piles. .

Anchor piles: These piles are used to provide anchorage for anchored sheet piles. These piles provide resistance against horizontal pull for a sheet pile wall.

Fender piles: Fender piles are sheet piles which are used to protect water from stmctures from impact of ships and vessels.

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2. What are steel piles? Steel piles are generally in the form of thick pipes or rolled steel H sections. Pipe steel piles are driven in to the ground with their ends open (or) closed. Piles are provided with a driving point (or) shoe at the lower end. They may cany loads upto 100 tonnes (or) more. 3. What is displacement piles? All driven piles are displacementpiles as the soil is displaced laterally when the pile is installed. The installation may cause heaving the surrounding ground. 4. What are non displacement piles? Bored piles are non displacement piles. As the soil is removed when the hole is bored, there is no displacement of the soil during installation. The installation of these piles causes very little change in the strt:.:ssesin the surrounding soil. 5. What are cast in situ piles? The cast in situ piles may be either driven or bored type. Driven cast in situ piles are formed by driving a casing with a closed bottom end into the soil. The casing is later filled with concrete. The casing may (or) may not with drawn. Bored cast in situ piles are fonned by excavating a hole in to the ground and then filling it with concrete.

6. Mention four disadvantages of ca."Itin situ piles. a. Installationof cast in situ piles require great care and are not suitable in undergroundflowingwater. b. Boring methods in case of fonning bored cast in situ piles, may loosen sandy (or) gravellysoils. c. Limitationin lengthbecause of liftingforce required to with draw casing. d. The ,.:astin situ pileswilltakemuch lessload as compared to driven piles. 7. Mention advantages of driven piles. a. The drivenpiles can be pre cast and installedaccordingto specifications. b. Pile lengthsare easilyadjustable. c. Driven' piles compacts the soil around there by increases the bearing capacity. d. Piles can be driven with a closed end so as to exclude the effects of growld water. 8. :Mention four advantages of driven cast in situ piles. a. The pile work can be programmed in advance. b. Pile lengths are easily adjustable. c. An enlarged base can be formed which will increases the bealing capacity. d. Piles can be driven with a closed end so as to exclude the effect of ground water:

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9. Mention four disadvantages of driven cast in situ piles. a. Heave of adjoining ground which could affect the service or structures

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b. Limitationin lengthbecause of liftingforce required to'with draw casing. c. Possible damage to uncased piles by necking and waisting due to lateral

forceswhenthe concreteis green.

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d. Concretecan not be inspectedafter completion. 10. Mention advantages of bore cast in situ piles.

.

a. Very littledisplacementand no risk of ground heave. b. Lengthcan be readilyvaried. c. Piles can be installed with much noise and vibration with limited head room. d. Piles can be installed in very great length and very large diameters and end enlargements up to 2 to 3 shaft diameters are possible in days (or) soft rocks. 11. What are the disadvantages of bored cast in situ piles? a. Boring methods may loosen sandy (or) gravelly soils. b. Concreting under water will pose problems. c. Concrete can not be subsequently inspected. d. Susceptible to waisting or necking in squeezing ground. e. Enlarged bases can not be formed in granular soils. 12. What is drop hammer? Drop hammer is the simplest tram of hammer are solid masses of steel, IOta 50KN in mass, titted with a lifting eye and lugs for sliding in the leaders. Drop hammer is raised by a winch and allowed to drop in the top of the pile under gravity form ce11ainheight. 13. What is single acting hammer? In a single acting hammer, the ram is raised by air (or stream) pressure to the required height. It is then allowed to drop under gravily on the pile cap provided with a hammer cushion.

14. What are the factors governing selection of pile? The factors governingselectionof piles are 1.' Length of the pile in relationto the load and type of soil. 2. Character of structure. 3. Availabilityof materials. 4. Type ofloaaing. 5. Ease of maintenance. 6. Estimated cost of types of piles, taking into account the initial cost, life expectancyand cost of maintenance. 7. Availabilityof funds.

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15.What are the factors that decide the carrying capacity of a single pile? The bearing capacity of a single pile depends upon 1. Type, size and length of pile 2. Type of soil 3. The method of installation 16. What is a test pile? A test pile is a pile which does not cany the lo~ds eoming from the super structure. The maximum load that can be put on such piles may be about 2.5 .times the design load (or) the load imposed must be such as ro give a total settlement not less than 1I1Othof pile diameter. 17. What is working pile? A working pile is a pile driven (or) cast in situ along with the other piles to carry the loads from the superstructure. The maximum test load on such piles should not exceed on and half times the design load. 18. \-Vrite down and explain each term in two dynamic pile driving formulae used for baring capacity of piles. i. The general form of engineering News formula for the allowable load is Qa = Wh/6(S+C)

=

W h s e

= =

=

weight of hammer height of fall in em fmal penetration in cm per blow empirical constant

2.5 for drop hammer

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0.25 for single acting and double acting hammer The general fOlm of Hiley's formula for ultimate load. . 110WH 1+CR2R Qu = { } l+R S + (C/2) '10 W h R CR

=

=

efficiency of hammer weight of haIIU)1er

heightof fall of hammer Wp/W Coefficientof restitution

19. Comment on group efficiency of piles. The efficiency of a pile group is defined as the ratio of group capacity to the sum of ..;apacitiesof number of piles. Qg fig =

-------

nQs Group efficiencydepends upon i. Type of soilin which piles are embedded. ii. Method of installation iii. Spacingof piles .

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. 20. Which type of foundation is used in clay soil?

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Whenis the top soil soft clay and rock available at is reasonable depthbed end bearing piles (Bor~d type) When the top soil is soft clay and bed Friction piles (Bored type) rock is not existing at a reasonable. J depth l\

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21. What are Batter piles? Some of the water and earth retaining stl11ctures are subjected to horizontal and inclined forces. Such piles are referred to as batter.piles. , 2,2.\-Vhat is Feld's rule? According to the Feld's l111e,the value of each pile is reduced by one-sixteenth owing to the effect of the nearest pile in each diagonal or st~aight row of which the pal1icular pile is a member.

23. What is negative skin friction? Negative skin friction is a downward drag acting on the piles due to relative movementbetween the piles and surroundingsoil. 24. \Vhen the load is transferred through the pile tip such a pile is called end bearing pile 25. The factor of safety in Engineering news fOlIDulafor pile capacity is 2. 26. According to Feld's rule the efficiency of four pile group is 82~/O. 27. Piles generally carry load by end bearing and friction. 28. In compressible fills piles carry additional load due to negative skinfriciton. 29. Dynamic fOlIDulais applicable to driven piles in coarse sands. 30. Based load transfer, the piles are classified as end beari1}gpiles _and friction piles. 31. Group efficiency of a pile group is defined as the ratio between group capacity and sum of individual pile capacities.

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32. What are timber piles The timber piles are made of tree trunks with branches trimmed of .It shall be of sound quality and free of defects. TIle length of the pile may go up to 15 mm or more. If greater lengths are required they are to be spliced. The diameter of the piles at the butt end may vary from 30 to 40 em. The diameter at the tip end should be less than 15 em. TIle maximum design load does not exceed 25 tones. 33. When do you use an under reamed pile Under reamed pile are very mush used in expansive soils to take up the uplift loads caused due to swelling of soils. The enlarge base in the under reamed pile is called bulb and the bulb is'placed in the stable zone where there will be no change in moisture content. 34. What ar'e displacement piles All driven piles are displacement piles as the soil is displaced laterally, when the pile is inst~\lled. The soil also gets densified. Precast concrete piles and closed end pile are high displacement piles. Steel H piles are low displacement piles. 35. What are non displacement piles Bored piles are non displacement piles .As the soil is removed wh~n the hole is bored there is no displacement of the soil during installation. The installation f these piles causes very little change in the stresses in the sun-ounding soil. I ,

36. What is feld's pile According to feld's pile ," the value of each pile if reduced by one sixteenth owing to the effect of the nearest pile in each diagonal on straight row of which the particular pile is a member under what conditional are pile driving fonnulas are reliabk. Dynamic fonnulae could be used with more confidence in freely drawing material such as coarse sand .What is the efficiency of a pile group in a square pattern according to feld's pile

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