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./UNIT I

'SUBSURFACE INVESTIGATION

@ Explain the purpose of site exploration? (OR) What is the object of siteexploration" ~

The purposeof site explorationis toa. knowthe geologicalconditionof rock and soilformation.b. Establishground water level.c. Selectthe type and depth of fOWldation.d. Determinethe bearingcapacityof th,esite. ,e. Evaluatethe anticipatedsettlementof the structure.

2. "'bat is site reconnaissance?Site Reconnaissance is the first step in a subsurface exploration program. Itinvolves an inspection of the site and study of topographical features. It helps indeciding future program of site investigations, scope of the work, methods ofexploration to be adopted, types of samples to be taken and the laboratory testingand in situ-testing.

3. What is open excavation in site explorations?Test pits (or) trenches are open type, normally excavated at the site to inspect thestrata. 'Test pits are considered suitable only for small depths - up to 3m. Forgreater depths, especially in pervious soils, lateral supports (or) bracing ofexcavations will be necessary. The necessary soil samples are taken usingsampling teclmiques and used for ascertaining engineering properties.

4. What is Boring?Boring is the process of making (or) drilling bore holes into the ground with aview to obtain soil (or) rock samples from specified (or) known depths. Samplesare taken from the borehole and tested in a laboratory. The borehole may be usedfor conducting in situ tests and for locating water table.

5. What is Auger borings?Soil Auger is a device that is useful for advancing borehole in to the ground. Thesoil auger is advanced by rotatingc it while pressing if in to the soil at the 'sametime. As soon as the auger gets tilled with the soil, it is taken out and the soilsample collected Augers may be hand operated or power driven. (Eg.) Post hole, .

auger, HelicalAuger.

6. What is wash boring?The method consists of driving a casing pipe usually through a heavy drophammer supported on a tripod and pulley. Water is forced under pressure througha hollow drill rod. The lower end of the drill rod, fitted with sharp cutting edge(or) chopping bit cuts the soil. The soil thus cut gets mixed with water and flowsthrough the annular space between.the casing pipe and the drill rod. The slunyflowingout providesindicationof soil type.

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7. What is rock core sampling? (OR) What is core drilling?Rotary drillingis another method of wash boring normallyused to collect the rocksamples. In this method core barrels with diamond bits are attachedat the end ofa string of drillrods which is rotated by power rig. This enables the simultaneouscollectionof rock cores. The method is known as core drilling.

8. What is.chunk sampling .,

Chunk sample are obtained from open excavations like test pits. For chunksampling it is essential that the soil has a trace of cohesion.

. .During excavationa block of soil about 40cm x 40cm in plan is left undisturbed.An undisturbedblock of about 30cm x 30cm x 30cm is usually trimmed with flatknife (or) trowel. An open ended box is then slides over the trimmed block. Thespace between the side of the box and the sample is filled with dry sand. The endof the box is sealedwith paraffinwax.

9. Write short notes on i. Undisturbed sample, Ii. Disturbed sample, iii.Representative sample, iv. Non representative sample.

L Undisturbedsample:These are the samples in which the natural structure of the soil and the water

content are retained. For all practicalpurposes, however, an undisturbed sampleis considered as one n which the materialhas been subjected to smaIldisturbance.Undisturbed samples are used for determining engineeringproperties of the soilsuch as shear strength, compressibilityand permeability. (Eg). Tube samplers,Chunk samplers.

ii. Disturbed sample :Disturbed samples are those where the natural structure of the soil gers

modified (or) destroyed during sampling operation.(Eg) Soil sample collected in split spoon sampler in SPT test.

They can be used for determining the index properties, specific gra"ity andgrain size distribution.

iii. Representativesample: .

Representative sample is a disturbed sample where the natural soil structuregets modified during sampling operation. With suitable precautions, the naturalmoisture content and the proportion of mineral constituents can be preserved.

. (Eg) Soil sample collected in SPT test.

iv. Non representativesample: .Non representative sample is a disturbed sample consists of nrixture of soils

from the other layers(or) som~of the mineralconstituentshave been lost.(Eg) Samplescollectedin wash boring.These are suitable only for providing qualitative information such as major

.changesin subswface strata.

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10. What is open drive sampler?This sampler essenti~lly consists of a seamless open end steel tube with a cutting.edge. The tube is connected through a head to the drill rod. . The sampler h~ad isprovided with vents to pennit water and air to escape during the with drawl ofsampler. The sampling tube may be thick walled (or) thin walled.

11. What is piston sampler?A piston sampler consists of thin walled tube with a piston inside. The piston.

keeps the lower end of the sampling tube closed when the sampler is lowered tothe bottom of hole.

After the sampler has been lowered to the desired depth, the piston isprevented from moving downward by a suitable arrangement. The thin tubesampler is pushed past the piston to obtain soil samples fonn soft and sensitiveclays. .

12. Name the geophysical methods of subsurface exploration.The two commonly used geophysical methods in civil engineering arei. Seismic refraction methodii. Electrical resistivity method

13.What is subsurface sounding?Subsurface sounding nonnally consists of driving a sampling tube (or) a cone(Penetrometers)with a "iew to measure the resistanceto penetration of soil strata,and thereby try to identifythe soil and some of its Engineeringcharacteristics.

14. \Vhat is the use of penetration test?Penetration tests are field test conducted at the site with a \~ew to measure theresistance to penetration of soil strata, and thereby try to identify the soil and someof its Engineering characteristics.SPT TEST - Sampling TubeSPT TEST - Cone

IIis used to penetrate the soil

15. What is dynamic cone penetration test?The dynamic cone penetration test is conducted by driving the cone by 65kghammer falling from a height of 75cm. The number of specified distance (30cm)is a measure of dynamic cone resistance.

16. What is critical depth of boring? (OR) What is significant depth ofexploration?The depth of exploration in any soil investigation should extend through all soilstrata where the str~s~ increment due to the super imposed loads can producesignificant settlement (more than 80% of the settlement) .md shear stresses isknown as significant depth (or) critical depth of boring.

17. What are different types of borings adopted in soil exploration?The common methods of advancing bore holes are:a. Auger boringb. Auger and shell boringc. Wash boringd. Percussion drillinge. Rotary drilling

II more commonly employed for sampling in rock sti-ata.

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18. What are the advantages and disadvantages of Auger boring?A.dvantages :

a. Thismethod uses simple,light,inexpensiveand flexibleequipment.b. It is very efficientin mediwn soft to stiff cohesivesoils.c. It is suitablefor determiningground water level.d. The soil sample collected using auger boring technique are suitable for

identific.ltionand for the determinationof certain physical properties such asAtterberglimitsand specificgravity.

Limitations:

a. However, this method is not well suited in very hard (or) cemented soils, verysoft soils and fully saturated cohesion less soils.

b. Soft soils have tendency to flow in to the hole.

19. What are the advantages and disadvantages of wash boring?Advantages:a. Wash boring uses light and inexpensive equipment and can be used for both

small and large holes in most common soils that do not contain numerousstones or boulders.

b. It does not obstruct ground water observation.

Disadvantages:a. This method should bot be used when undisturbed samples are desired above

ground water level.b. It is not well suited in very soft soils, fme to medium grained cohesion less

soils and in hard and cemented soils.

. 20. What is recovery ratio?Recoveryratio is the ratio betweenlength of the samplewith in the tube and depthof penetrationof the samplingtube. .

21. Minimum depth of soil explorationbelow the foundation level will be 1.5 timeswidthof foundation.

22. If the 'N' value is 30 for a sand deposit, the sand can be considered to be in densestate. .

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23. Distinguish between undisturbed

i UNDISTURBED SAMPLESI Undisturbed samples are types ofI samples in which

(a) All the minerals are present(b) There is no change in structureand water content

These samples may be used forpermeability, consolidation, andstrength tests.

and representative samples. .

REPRESENTATIVE. SANWLES .

Representative samples are types ofsamples in which(a) All the mineraL') in each layerare present.(b) 'The structure and water content.of the sample are seriously altered.These samples are suitable foridentification and classificationpurposes. It is also used for thedetermination of certain physicalproperties such as .Atterberg.limits,and. grain specific gravity. .

24. State the quality of foundation soil ifThe foundation soil should be either

If N= 30.If it is dense sand,

range of 36° - 42°. and theIf it is a stiff clay,

the range of 200- 400 KN/m2.

N= 30;dense sand (or) stiff clay.

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it possesses angle of internal friction in therelative density lies in the range of 65-85%

Th,,: unconfmed compressive strength lies in

25.\\'hat is the correction for water table in a SPT.The correction for watertable in a SPT is correction due to

dilatancy. In saturated fme or silty dense or very dense ,deposits, the NValue observed may be greater than the actual value because of such

materials to dilate during shear under undrained conditions. Terzaghi andPeck recommended that, if the observed N value is greater than 15, itshould be con-ected for dilation effect as

Nc = 15+ Yz (No - 15)No = Observed SPT Value.Nc = Corrected value for dilation effect.

26. Distingljjsh between Area ratio and Recovery ratioAREA RATIO RECOVERY RATIO !

i Area ratio of a sanipling tube is'" Recovery ratio of the sampling I

I Defmed as I I tube is dejined as II 2 2

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I Do -Dj , Rr= L/H! Ar= D/ L = Actual length of the s,ample. I

I \Vhere Di =, internal diameter of the sampler I H= Total length of the sampling I

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= External diameter of I tube .. .. I

I The sampler. --l ---1IIArea ratio is a measure of volume of the Recovery ratio measures I

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I collected sample. disturbance due to II (a) presence large stones,roots, and Ii I other materials. !

(b) friction between the soil and

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27. What are samplers?The sampling.. tubes used to extract soil samples from bore holes are

called samplers.Disturbed samples are obtained by theUn disturbed samples are obtained

drive sampler andlor (b) a piston sampler.

use of split spoon sampler.either by the use of (a) open

28. What is seismic refraction?

Seismic refraction method is based on the 'fact that, seismic waveshave different velocities in different types soils (or) rock.

Further the waves are refracted when they cross boundary betweenthe different types of soil.

The method enables the determination of the general soil typesand the approximate depth of boundaries of strata or bed rock.

29. What is detailed exploration?Detailed investigation follows the preliminary investigation and is

normally canied out to determine the nature, sequence and. thickness ofvarious subsoil layers, their lateral variation, their physical properties, and theposition of water table. Borings and detailed sampling are usually undertakento obtain this infOlmation. Various in situ field tests also form a part ofdetailed investigation.

30. Compute the area ratio of a sampler with inside diameter 70 mmand thickness 2mm. Comment.

Dj = 70mm Do= 70+2>1<2=74mm;

Ar=

2 .,Do :DiwDt

742-702702 = 11.75% >10%

The sampler is used to collect disturbed sample.

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UNITn

BEARING CAPACITY OF SHALLOW FOUNDATIONS

1. Define the followingtermsi. Gross pressureintensityii. Net pressureintensityiii. Ultimatebearingcapacityiv. Net ultimatebearingcapac~tyv. Net safe bearingcapacityvi. Allowablebearingpressure

i. Grosspressure inrensiJy: .

Gross pressure intensity is the total pressure at the base of the footingdue to

i. Weight of the superstructure, ii. Self weight of the footing

ill. Weight of the soil fill over the footing

ii. Net pressure inrensity :The difference between the gross pressure and overburden pressure(yDf) at the base of the footing is called net pressun: intensity.

qnp = qp - yDr

iii. ultimate bearing capacity :It is the maximum gross pressure intensity that the soil can supportbefore it fails in shear.

i,'. Net ultimate bearinl!. capciJv :It is the maximum net pressure intensity that the soil can supportbefore it fails in shear.

qnu= qu- yDr

v. Net safe bearing capacity :It is the maximum net pressure intensity that the soil can support without risk of shear failure. It is the ratio between bet ultimate bearingcapacity and factor of safety.

qns = qnu/ F. S.

vi. Safe bearing capacity (or) Gross safe bearing capacity.It is the maximum gross pressure intensity that thewith out risk of shear failure.

qs =ql\J +yDr

soil can support

vii.Allowahk bearingpressure:It is the maximum net pressure intensity that can be allowed on thesoil with no possibilityof shear failure (or) the possibilityof excessivesettlement.

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2. List the factors affecting bearing capacity of soil.a. Natureof soil and its physicaland Engineeringproperties.b. Nature of foundationand other detailssuch as the size, shape, depth below

the ground surface, and rigidityof the structure.c. Locationof ground water table.d. Mode of loading,whethervertica~(or) inclined,axial(or) eccentric.e. Initialstressesif any.

3. What the effective surcharge at the base level of the foundation?The over burden pressure (stresses due to self eight of the soil) at thefoundation level is equivalent to effective surcharge load (qo)

So qo = yDr\Vhere y = unit weight of soil.

Dr = depth of foundation

4. State Terzaghi's equation for a. Circular footing, b. square footing.Terzaghi's bearingcapacityequationfor a circularfootingrestingon c-<psoil1S

qu = 1.3 CNc + yDrNq + 0.3 ByNyFor square footingqu = 1.3 CNc + yDfNq+ 0.4 ByNy ,

Wl1ere C = cohesion

y = unitweight of soil.Dr =Depth of foundationB = widthof foundation

Nc, Nq, Ny = Bearingcapacityfactors.

5. Stnte Meyerhofrs formula.The fonn of equationproposedby Meyerhoff is

qu= CNc dc Sc ic+ yDfNqSqdq iq + 1,'2ByNy.Sy.Dy.iyWhere C =cohesion,

Nc, Nq, Ny. = Bearingcapacityfactors for stripfoundationdc, dq, dy =depth factorSc, sq, sy = shape factorsie, iq, iy = inclinationfactors'Y =Effective:unit weight ,,[ ~hesoilabove base levelof

foundation.

= Depth of foundation.D

6. WI:<' <.' - al failure?'Ih(~g,,,;w.'la1~li",al'failure mo~e occurs in soils possessing brittle type stresstrain behavior (ex: dense sand) is characterized bya. well defmed failure patternb. tilting of foundation'

.c. bulging of ground surfaced. well defined failure load in load settlement curve

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7. What is local shear failure?The local shear (ailure is usually associated with medium dense (or) mediurll '

stiff days is characterized by ,

a. welldefmedwedge and slipsurfacesonly hl..'ncaththe foundation.b. Significantcumpressionof the soildirectlybelowthe foundation.c. Slightbulgingof ground surfaceadjacent to the foundation.d. There is no indicationof clearfailure l(,)adin load settlementcurve.

8. What is punching shear failure? .

Punching shear failure is usually associated in soils possessing stress strain "

characteristics of very plastic soil (loose sand (or) soft clay) is characterizedby

ii.b.

Poorly de\fined shear planes' .There will be a significant compression beneath the footing accompaniedby shearing in vertical direction.There is no bulging of ground surfaceUltimate load con not be clc..rl~ recognized in load - settlement curve.

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9. St~tr the assumptions in Terzaghi's analysis.Tcrzaghi made the following assumptions for developing an equation fordetermining qufor a c-<psoil.a. The soil is semi infmite, homogenous and isotropic.b. The failure is by general shear.c. The shear strength of the soil above the base of the footing is negligible.d. The overburden pressure at foundation level is equival~r.t to surcharge

load qo= yDf.e. Coulomb's lawis strictlyvalidis S =C + 0' tan <p

10. Explain how the water table affects the allowable soil pressure.The bearingcapacityequationfor c -<psoils is

qu =CNc + yDf Nq + YzByNy

The fIrst term CNc in the above equation is not affected by position of watertable. The third term (112 BYNy) is not affected if the water level is at a depthequal to greater than the width.

Both second and third term get affected when the water table rises to groundsurface. Since submerged unit weight is roughly 50% of saturated unit weightthere will be reduFtion of bearing capacity by nearly 50~'o.

11. What is spread footing? .

Spread footings is basically a pad used to spread out loads from walls (or)columns over sufficiently larger area of foundation soil. Spread footingsrequired to support a wall is known as continuous footing while that requiredto support a column is known as isolated footing.

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12. What is meant by compensating foundation? (OR) What is meant byfloating found~tion?The floating foundation is a special type of foundation construction useful inlocations where deep deposits of compressible cohesive soils exists .md theuse of piles is impractical. The concept of a floating foundation requires thatthe substructure be assembled as a combination of a raft and caisson to createa rigid box. This foundation is installed at a depth that total weight of the soilexcavated for the rigid box equals to the total weight of planned structure.

13. What is strap foundation?A strap footing comprises two or more footings connected by a beam called"strap" . TIns may be required when the footing of an exterior ..olumn can notextend to the adjoining propel1y.

14. What is raft foundation?A raft (or) mat foundation is a large footing usually supporting walls as well asseveral columns in two (or) more rows. This is adopteda. When the allowable soil pressure is low.b. The loading is hea")'.c. Spread footings cover more than 50~o of the plan area.

15. What is combined footing?A combined footing supports two (or) more columns in a row, nonnallyadopted whena. Two columns are closely spaced so that individual footings are not

practicable.b. \\-'hen a wall column is so close to the property line so that it is impossible

to center an individual footing under the column.

16. What is deep foundation?A foundation is deep if its depth is more that 15 times the \vidth of thefoundation is, Dr / B ratio is greater than 15. Deep foundations are nonnallypreferred when,

i. The loading is heavyii. When the allowable soil pressure is low.

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17. Differentiate s~allow and deep foundations.

18. Where do you recommend spread footings?Spread footings are commonly used to supportrequired.

i. When the allowable soil pressure is high.ii. When the loading is low to medium type.

a wall or column, that

19. Terzaglri's fonnula is valid for shallow foundations.

20. Skempton's fonnula for baring capacity is used for cohesive soils.

21. Rise of water table to the ground level reduces the bearing capacity of afooting by approximately flfty percent.

22. Liquid limit gives an idea of the compressible characteristics of the clay.

23. Accordingto bureau of Indian standards the safe bearing capacityfor compactsand may be taken as 450 Knlm2.

24. \Vhen the relative density of a sand deposit is greater than 70%. General shearfailure conditions will prevail.

25. The bearing capacity factor in Skemption's fonnula is a function of shape anddepth of foundatiem.

26. If ex is the eccentricity of the load in the width (B) direction the effectivewidth i" taken as B - 2e1\'

27. The bearing capacity of footings on sand depends on the width of footings.

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I S.No Shallow foundations1.I In case of shallow foundation In case of deep

foundations I.

I Dti'BlDti'B>15

2. Shallow foundations are Deep foundations are nonnallynonnally adopted when the adopted when the loading is heavyloading is lo}v to medium and and the allowable soil pressure is

, the allowable soil pressure is low.high.

I 3. Shear strength of the soil above The shear strength of the soilthe be of the footing is above the base 1S considered

neglected in. case of shallow (Meyerhoff)I- foundations. (Terzaghi) .

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28. What are general requirements of foundation?1. The foundation must be properly locating consideringany future influence

which could adverselyaffect its perfonnance which could advcrscl.va~et~ particularlyfor footingsand mats.

2. The soilsupportingthe foundationmust be safe againstshear failure.3. The foundation must not settle or deflect to a degree that can result in a

damaGe to the structure.4. The foundationshouldbe safe againstslidingand over turning.

29. What are the factors influence the choice of a particular foundation?, 'The type of foundation most appropriate for a given structure depends uponseveralfactors:a. The functionof the structureand loads it must carry.b. The subsurfaceconditions.c. The cost of foundationin comparisonwith the cost of the superstructure.

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F()U~DAliON SETl CEMENTS.

1. What is immediate settlement? 0

hnmediate (or) elastic settlement takes place immediately (or) shod time (less than about 0

7 days)aft-Qrthe constructionof the structure. 0 . 0

2. What is consolidation settlement?

This component of settlement occurs due to grad~alexpulsion of water from the voids of 0

the soil, resulting in a dissipation of excess pore water pressure and an increase in effectivestress.

3. What is secondary compression? (OR) What is secondary consolidation settlement?Secondary compression settlement which occurs at constant effective stress, with volume

change occurring due to rearrangement of soil particles. This settlement occurs aftercompletion of primary consolidation.

4. What is the effect of settlement on the structure?If the structure as a whole settles uniformly in to the ground there will nohbe any deter

mental effect it can have is on the service lines, such as water and sanitary pipe coimections,

telephoneand electriccables etc; ,

But the differentialsettlementin a structure can cause.a. Structureto tilt. and.b. Cracksin the exteriormasonry walls it interior plaster walls of buildings.

5. Explain "Settlement of footing".Foundations of all the structures have to be placed on the soil. The structure may

undergosettlementdependingupon the characteristicssuch as compressibilityof the strata ofthe soil on which it is founded. Thus the tenn settlement indicates the sinking of a structuredue to compressionand defonnation of the underlyingsoil.

6. Differentiate uniform and differential settlement.

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Uniform Settlement

Unifonn settlement is possible onlyif the subsoil is homogeneous andload distribution is unifonn.The unifonn settlement is due toi. Elastic compression of the soil.

0 ii. ConsolidationI iii. Secondarycompression

jl There is no harrn:(ul effectuniform settlement.

Differential Settlement I

Differential settlement occurs if the load 0

\distribution is non uniform and the subsoilis hetero enous. I

The difference of settlement between twoadjacent column foundations. I

2.

3. due to IDifferential settlement causes structural

damage and architecturaldamage.

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7. Define the following terms.i. Differential Settlementii. Tilt.

iii. Angular distortion

i. Differential Settlement:

The differencein total settlementbetween any two points is the differentia1settleme'nt.

ii. Tilt:

It is the ratio of difference in settlement of the two ends of the foundation to the width offoundation. When the columns are monolithic with the foundation slab, tilt of the foundationtakes place such that the angle between the colunm and foundations remain 90°.

iii. Angular Distortion:

It is the ratio of differential settlement between tow colunms to the spacing betweenthem.

8. What is "Seat of settlement" ?

The seat of settlement can be defmed as the stressed zone beneath a foundation with inwhich the stresses induced by the load are large enough to cause significant orders ofsettlement.

The seat of settlement is taken as the zone bounded by the 10% vertical stress contour. Ina square footing this extends up to a depth of 1.5B and upto 3.5B in a strip footing.

9. What is coefficient of volume change?The coefficient of volume change represents the compression of the clay per unit of

originalthicknessdue to a unit increaseof the pressure.

10. Define the following terms.a. . Normally Consolidated Clayb. Over Consolidated Clayc. Under Consolidated Clay

,--1. Normally Consolidated Clay:A normally consolidated clay is one which had been subjected to a pressure in the

p'ast equals to the present existing pressure. The over consolidation ratio for normally/" consolidated clay is equals to one.

2. Qver Consolidated Clay:. A clay deposit that has been fully consolidated under a pressure in the past, larger

. than the present over burden pressure is called an oven consolidated clay. The over.1' consolidation ratio for over consolidated clay is more than 1.

J.. Under Consolidated Clay:Rapid natural deposition or deposits. under recent fillings may not be fully

consolidated under the present overburden pressure, such clay deposit is called under. consolidated clay.

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