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UNIT I 'SUBSURFACE INVESTIGATION

@ Explain

the purpose of site exploration? exploration"

(OR) What is the object of site ~

The purpose of site explorationis to a. 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. It involves an inspection of the site and study of topographical features. It helps in deciding future program of site investigations, scope of the work, methods of exploration to be adopted, types of samples to be taken and the laboratory testing and 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 the strata. 'Test pits are considered suitable only for small depths - up to 3m. For greater depths, especially in pervious soils, lateral supports (or) bracing of excavations will be necessary. The necessary soil samples are taken using sampling 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 a view to obtain soil (or) rock samples from specified (or) known depths. Samples are taken from the borehole and tested in a laboratory. The borehole may be used for 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. The soil auger is advanced by rotatingc it while pressing if in to the soil at the 'same time. As soon as the auger gets tilled with the soil, it is taken out and the soil sample 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 drop hammer supported on a tripod and pulley. Water is forced under pressure through a 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 flows through the annular space between.the casing pipe and the drill rod. The sluny flowingout 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 collectthe rock samples. In this method core barrels with diamond bits are attachedat the end of a string of drillrods which is rotated by power rig. This enables the simultaneous collectionof 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 chunk sampling 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 flat knife (or) trowel. An open ended box is then slides over the trimmed block. The space between the side of the box and the sample is filled with dry sand. The end of the box is sealedwith paraffinwax. 9. Write short notes on i. Undisturbed sample, Ii. Disturbed Representative sample, iv. Non representative sample.

sample,

iii.

L Undisturbedsample: These are the samples in which the natural structure of the soil and the water content are retained. For all practical purposes, however, an undisturbed sample is considered as one n which the materialhas been subjected to smaIldisturbance. Undisturbed samples are used for determining engineeringproperties of the soil such 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 and grain size distribution.

iii. Representativesample:

.

.

Representative sample is a disturbed sample where the natural soil structure gets modified during sampling operation. With suitable precautions, the natural moisture 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 is provided with vents to pennit water and air to escape during the with drawl of sampler. 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 to the bottom of hole. After the sampler has been lowered to the desired depth, the piston is prevented from moving downward by a suitable arrangement. The thin tube sampler is pushed past the piston to obtain soil samples fonn soft and sensitive

clays. . 12. Name the geophysical methods of subsurface exploration. The two commonly used geophysical methods in civil engineering are i. Seismic refraction method ii. 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 the resistance to penetration of soil strata, and thereby try to identify the soil and some of its Engineering characteristics. I SPT TEST - Sampling Tube SPT TEST - Cone Iis used to penetrate the soil 15. What is dynamic cone penetration test? The dynamic cone penetration test is conducted by driving the cone by 65kg hammer 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 of exploration? The depth of exploration in any soil investigation should extend through all soil strata where the str~s~ increment due to the super imposed loads can produce significant settlement (more than 80% of the settlement) .md shear stresses is known 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 boring b. Auger and shell boring c. Wash boring I d. Percussion drilling e. Rotary drilling I 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. b. c. d.

This method uses simple,light,inexpensiveand flexibleequipment. It is very efficientin mediwn soft to stiff cohesivesoils. It is suitablefor determiningground water level. The soil sample collected using auger boring technique are suitable for identific.ltionand for the determination of certain physical properties such as Atterberglimitsand specificgravity.

Limitations: a. However, this method is not well suited in very hard (or) cemented soils, very soft 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 numerous stones 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 between length of the samplewith in the tube and depth of penetrationof the samplingtube.

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21. Minimum depth of soil explorationbelow the foundation level will be 1.5 times width of foundation. 22. If the 'N' value is 30 for a sand deposit, the sand can be considered to be in dense state. .

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23. Distinguish

between

undisturbed

and

UNDISTURBED SAMPLES

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representative

REPRESENTATIVE.

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Undisturbed samples are types of I samples in which (a) All the minerals are present (b) There is no change in structure and water content These samples may be used for permeability, consolidation, and strength tests.

samples.

.

SANWLES

.

Representative samples are types of samples in which (a) All the mineraL') in each layer are present. (b) 'The structure and water content. of the sample are seriously altered. These samples are suitable for identification and classification purposes. It is also used for the determination of certain physical

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properties such as . Atterberg.limits, and. grain specific gravity. . 24. State the quality of foundation soil if N= 30; The foundation soil should be either dense sand (or) stiff clay. If N= 30. If it is dense sand, it possesses angle of internal friction in the range of 36° - 42°. and the relative density lies in the range of 65-85% If it is a stiff clay, Th,,: unconfmed compressive strength lies in the range of 200- 400 KN/m2. 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 N Value observed may be greater than the actual value because of such materials to dilate during shear under undrained conditions. Terzaghi and Peck recommended that, if the observed N value is greater than 15, it should be con-ected for dilation effect as Nc

No = Nc

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as

2

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Do -Dj D/

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15+

Yz (No

Observed

- 15)

SPT

Ar=

Value.

= Corrected value for dilation effect.

26. Distingljjsh between Area ratio and AREA RATIO Area ratio of a sanipling tube is'"

I Defmed I

=

Recovery ratio RECOVERY RATIO ! Recovery ratio of the sampling

,

is dejined

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as

Rr= L/H L = Actual length of the s,ample. I I

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\Vhere Di Do =, internal diameter of the sampler

= External diameter of

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IThe Area sampler. ratio is a measure of volume of the I

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H= Total tube ..

length of

the sampling

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Recovery ratio measures

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disturbance due to I (a) presence large stones,roots, and I I

other materials. (b) friction between the soil and inside tube wall.

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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 the use of split spoon sampler. Un disturbed samples are obtained either by the use of (a) open drive sampler andlor (b) a piston sampler. 28. What is seismic refraction? Seismic refraction method is based on the 'fact that, seismic waves have different velocities in different types soils (or) rock. Further the waves are refracted when they cross boundary between the different types of soil. The method enables the determination of the general soil types and 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 of various subsoil layers, their lateral variation, their physical properties, and the position of water table. Borings and detailed sampling are usually undertaken to obtain this infOlmation. Various in situ field tests also form a part of detailed investigation. 30. Compute the area ratio of a sampler with inside diameter and thickness 2mm. Comment. Dj = 70mm Do= 70+2>1<2=74mm;

Ar=

2 ., Do :Diw Dt 742-702 702

The

sampler is used

= 11.75% >10% to collect

disturbed sample.

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70 mm

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UNITn BEARING CAPACITY OF SHALLOW FOUNDATIONS 1. Define the following terms i. Gross pressureintensity ii. Net pressureintensity iii. Ultimatebearing capacity iv. Net ultimatebearingcapac~ty v. Net safe bearingcapacity vi. Allowablebearing pressure i. Grosspressure inrensiJy:

.

Gross pressure intensity is the total pressure at the base of the footing due 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 support before it fails in shear. i,'. Net ultimate bearinl!. capciJv : It is the maximum net pressure intensity that the soil can support before it fails in shear. qnu= qu- yDr

v. Net safe bearing capacity : It is the maximum net pressure intensity that the soil can support with out risk of shear failure. It is the ratio between bet ultimate bearing capacity 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 the soil can support with out risk of shear failure. qs = ql\J + yDr vii.Allowahk bearingpressure: It is the maximum net pressure intensity that can be allowed on the soil with no possibilityof shear failure (or) the possibilityof excessive settlement.

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2. List the factors affecting bearing capacity of soil. a. Nature of soil and its physicaland Engineeringproperties. b. Nature of foundation and 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 the foundation 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-
Dr = Depth of foundation B = width of foundation Nc, Nq, Ny = Bearing capacityfactors. 5. Stnte Meyerhofrs formula. The fonn of equationproposedby Meyerhoff is qu= CNc dc Sc ic+ yDfNqSqdq iq + 1,'2ByNy.Sy.Dy.iy Where C = cohesion, Nc, Nq, Ny. = Bearing capacityfactors for strip foundation dc, dq, dy = depth factor Sc, sq, sy = shape factors ie, iq, iy = inclinationfactors 'Y = Effective:unit weight ,,[ ~hesoil above base levelof foundation.

D 6. WI: <' <.'

= Depth of foundation. -

al failure?

'Ih(~g,,,;w.'la1 ~li",al'failure mo~e occurs in soils possessing brittle type stress train behavior (ex: dense sand) is characterized by a. well defmed failure pattern b. tilting of foundation' .c. bulging of ground surface d. 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

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a. b. c. d.

well defmedwedge and slip surfacesonly hl..'ncaththe foundation. Significantcumpressionof the soil directlybelow the foundation. Slightbulgingof ground surface adjacent to the foundation. 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 by

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

. ii. Poorly de\fined shear planes' b. There will be a significant compression beneath the footing accompanied by shearing in vertical direction. c. There is no bulging of ground surface d. Ultimate load con not be clc..rl~ recognized in load - settlement curve. 9. St~tr the assumptions in Terzaghi's analysis. Tcrzaghi made the following assumptions for developing an equation for determining qufor a c-
e. Coulomb's law is strictlyvalidis S = C + 0' tan

-

The bearingcapacityequationfor c
The fIrst term CNc in the above equation is not affected by position of water table. The third term (112 BYNy) is not affected if the water level is at a depth equal to greater than the width. Both second and third term get affected when the water table rises to ground surface. Since submerged unit weight is roughly 50% of saturated unit weight there 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 footings required to support a wall is known as continuous footing while that required to support a column is known as isolated footing.

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12. What is meant by compensating foundation? (OR) What is meant by floating found~tion? The floating foundation is a special type of foundation construction useful in locations where deep deposits of compressible cohesive soils exists .md the use of piles is impractical. The concept of a floating foundation requires that the substructure be assembled as a combination of a raft and caisson to create a rigid box. This foundation is installed at a depth that total weight of the soil excavated 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 not extend to the adjoining propel1y. 14. What is raft foundation? A raft (or) mat foundation is a large footing usually supporting walls as well as several columns in two (or) more rows. This is adopted a. 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, nonnally adopted when a. 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 the foundation is, Dr / B ratio is greater than 15. Deep foundations are nonnally preferred when, i. The loading is heavy ii. When the allowable soil pressure is low.

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17. Differentiate s~allow and deep foundations. rI

S.No Shallow foundations 1. I In

Deep foundations In case of deep Dti'B>15

case of shallow foundation

Dti'Bl 2. Shallow foundations are nonnally adopted when the loading is lo}v to medium and , the allowable soil pressure is high.

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foundations I.

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Deep foundations are nonnally adopted when the loading is heavy and the allowable soil pressure is low.

3. Shear strength of the soil above The shear strength of the soil

the be of the footing is above the base 1S considered neglected in. case of shallow (Meyerhoff) foundations. (Terzaghi) I

.

18. Where do you recommend spread footings? Spread footings are commonly used to support a wall or column, required. i. When the allowable soil pressure is high. ii. When the loading is low to medium type.

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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 a footing 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 capacity for compact sand may be taken as 450 Knlm2. 24. \Vhen the relative density of a sand deposit is greater than 70%. General shear failure conditions will prevail. 25. The bearing capacity factor in Skemption's fonnula is a function of shape and depth of foundatiem.

26. If ex is the eccentricity of the load in the width (B) direction the effective width i" taken as B - 2e1\' 27. The bearing capacity of footings on sand depends on the width of footings.

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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 upon ,

severalfactors: a. The function of 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? hnmediate (or) elastic settlement takes place immediately (or) shod time (less than about 7 days) aft-Qrthe constructionof the structure. 0 . 0

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2. What is consolidation settlement?

This component of settlement occurs due to grad~al expulsion of water from the voids of

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the soil, resulting in a dissipation of excess pore water pressure and an increase in effective stress. 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 after completion 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,

telephone and electriccables etc;

,

But the differentialsettlementin a structure can cause. a. Structureto tilt. and. b. Cracks in 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 compressibility of the strata of the soil on which it is founded. Thus the tenn settlement indicates the sinking of a structure due to compressionand defonnation of the underlying soil. 6. Differentiate uniform and differential settlement. S.No Uniform Settlement Differential Settlement -,,-1. Unifonn settlement is possible only Differential settlement occurs if the load if the subsoil is homogeneous and distribution is non uniform and the subsoil load distribution is unifonn. is hetero enous. 2. The unifonn settlement is due to The difference of settlement between two adjacent column foundations. i. Elastic compression of the soil.

ii. Consolidation I iii. Secondarycompression 3. l There is no harrn:(ul effect due to I Differential settlement causes structural j uniform settlement. damage and architectural damage. 0

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7. Define i. ii. iii.

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the following terms. Differential Settlement Tilt. Angular distortion

i. Differential Settlement: The difference in 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 of foundation. When the columns are monolithic with the foundation slab, tilt of the foundation takes 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 between them. 8. What is "Seat of settlement" ? The seat of settlement can be defmed as the stressed zone beneath a foundation with in which the stresses induced by the load are large enough to cause significant orders of settlement. The seat of settlement is taken as the zone bounded by the 10% vertical stress contour. In a 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 originalthickness due to a unit increase of the pressure. 10. Define the following terms. a. . Normally Consolidated Clay b. Over Consolidated Clay c. 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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