preloading or precompression

CE-464 Ground ImprovementCE-464 Ground Improvement

PRELOADING OR PRELOADING OR PRECOMPRESSIONPRECOMPRESSION

PRELOADING IS SURCHARGING THE GROUND WITH A UNIFORMLY DISTRIBUTED SURFACE LOAD PRIOR TO THE CONSTRUCTION OF THE INTENDED STRUCTURE (BUILDINGS, EMBANKMENTS, MOTORWAYS, RUNWAYS TANKS ETC.).

THE PURPOSE IS TO TAKE UP THE SETTLEMENTS UNDER THE CIVIL ENGINEERING STRUCTURES BEFORE THEY ARE BUILT.


IF GREATER LOAD LEVEL (SURCHARGE) IS USED, THE CONSOLIDATION IS FORCED TO BE COMPLETED IN A SHORTER TIME THAN WOULD OCCUR UNDER DESIGN LOAD, THEN THE LOAD IS REMOVED.

SOILS SUITABLE FOR PRELOADING: COMPRESSIBLE SOFT TO MEDIUM SOFT SATURATED CLAYS AND SILTS, ORGANIC CLAYS, PEATS.

TYPES OF PRELOADS: EARTH FILLS (MOST COMMON), WATER IN TANKS OR PONDS, VACUUM APPLICATION UNDER A MEMBRANE, SPECIAL ANCHOR AND JACK SYSTEMS, GROUNDWATER LOWERING, ELECTROOSMOSIS.

THE SURCHARGE RESULTS IN;

•PRIMARY CONSOLIDATION SETTLEMENT•SECONDARY CONSOLIDATION SETTLEMENT•INCREASE IN THE UNDRAINED SHEAR STRENGTH OF THE SOIL.


SURCHARGE LOADS AND VERTICAL DRAINS ARE FREQUENTLY USED TOGETHER WITH THE PRELOADING TECHNIQUE.

SURCHARGE LOADS (LOADS IN EXCESS OF THOSE TO BE APPLIED BY A PERMANENT FILL OR STRUCTURE) CAN BE USED TO ACCELERATE THE PROCESS FOR A PERIOD OF TIME TSR.

TERZAGHI'S ONE-DIMENSIONAL CONSOLIDATION THEORY CAN BE USED.

WHEN THE ANTICIPATED TIME OF COMPRESSION IS EXCESSIVE, VERTICAL DRAINS MAY BE USED TO SHORTEN THE TIME REQUIRED (IF COMPRESSION IS PRIMARY CONSOLIDATION). SURCHARGING AND PRIMARY CONSOLIDATION HAS BEEN DISCUSSED BY ALDRICH (1965) AND JOHNSON (1970A).


USE OF SURCHARGE LOADING (EXTRA PS) AND DURATION OF ITS

APPLICATION WHICH MEANS EXTRA COST IS TO REDUCE THE MAGNITUDE OF SETTLEMENT AFTER CONSTRUCTION. IT IS USUALLY AIMED AT EITHER;

1.TO DETERMINE THE MAGNITUDE OF SURCHARGE (PS) REQUIRED TO ENSURE THAT THE TOTAL SETTLEMENT ANTICIPATED UNDER THE FINAL PRESSURE (PR) WILL BE COMPLETED (OR MOSTLY COMPLETED) IN A GIVEN LENGTH OF TIME.

2.TO DETERMINE THE LENGTH OF TIME REQUIRED TO ACHIEVE A GIVEN AMOUNT OF SETTLEMENT UNDER A GIVEN SURCHARGE LOAD.

“


ONE-DIMENSIONAL CONSOLIDATION SETTLEMENT OF A LAYER 2H IN THICKNESS (NC CLAY);

IF Uf+s DENOTES THE AVERAGE DEGREE OF CONSOLIDATION OF

THE CLAY LAYER UNDER (Pf+Ps) AT THE TIME tSR , THEN FOR NO

FURTHER SETTELEMENT TO OCCUR UNDER Pf ALONE;

IF THE SURCHARGE WERE LEFT IN PLACE UNTIL THE TIME tSR ,

THEN THE LAYER WILL HAVE SETTLED AN AMOUNT EQUAL TO THAT TO BE EXPECTED UNDER THE PERMANENT FILL ALONE; i.e. StR = Sf. AT THIS TIME THE LAYER WILL HAVE REACHED AN

AVERAGE DEGREE OF CONSOLIDATION U(SR).


FROM EQUATIONS (1) AND (2)

THE RELATIONSHIP BETWEEN Uf+s , Pf / 'vo AND Ps / Pf CAN ALSO BE PREPARED IN A GRAPHICAL FORM. ONCE THE AVERAGE PERCENT OF CONSOLIDATION IS SPECIFIED, THEN THE ASSOCIATED TIME FACTOR AND THE REAL TIME CAN BE FOUND.

IMPORTANT: THE ABOVE PROCEDURE IS VALID ONLY IF THE MAXIMUM PAST PRESSURE IS EXCEEDED , i.e (Pf + Ps + 'vo ) > ’vmax.

THE DISTRIBUTION OF EXCESS PORE PRESSURES AND EFFECTIVE STRESSES IN A LAYER OF 2H THICKNESS WITH DOUBLE DRAINAGE BOUNDARIES BEFORE AND AFTER SURCHARGE REMOVAL ARE SHOWN IN THE FOLLOWING FIGURE.


DEGREE OF CONSOLIDATION (OR CONSOLIDATION RATIO) AT ANY POINT IS;

where : uz = EXCESS PORE WATER PRESSURE AT DEPTH Z AT ANY TIME uo = INITIAL EXCESS PORE WATER PRESSURE UNDER

THE APPLIED SURFACE LOAD.FOR NO SETTLEMENT TO OCCUR AFTER THE SURCHARGE (Ps) REMOVAL AT THE TIME tSR , EFFECTIVE STRESS AT THE CENTRE OF THE LAYER MUST BE EQUAL OR GREATER THAN THE PERMANENT LOAD (DESIGN LOAD) Pf. IN OTHER WORDS;


SOME SOILS LIKE ORGANIC SOILS,PEATS SETTLE AFTER PRIMARY CONSOLIDATION IS COMPLETED (SEE FIGURE 3)

where ; Ca = VERTICAL STRAIN PER LOG CYCLE (COEFFICIENT OF SECONDARY COMPRESSION) tp = TIME FOR PRIMARY CONSOLIDATION Hp = THICKNESS AT t=tp

SECONDARY COMPRESSION SECONDARY COMPRESSION EFFECTSEFFECTS


IF NO PRIMARY AND SECONDARY SETTLEMENTS ARE DESIRED UNDER THE DESIGN LOAD Pf ; THE LOAD Pf + Ps MUST BE SUSTAINED FOR A PERIOD tSR SUCH THAT THE TOTAL SETTLEMENT SSR IS THE SUM OF PRIMARY AND SETTLEMENTS UNDER Pf ALONE (FIGURE 4).


THE DESIRED SETTLEMENT SSR IS RELATED TO THE PRIMARY SETTLEMENT UNDER THE LOAD Pf+Ps.

SUBSTITUTING 1b, 10, 7, 8, INTO 9

PRELOADING VERSUS GAIN IN UNDRAINED SHEAR STRENGTH (FIGURE 3);

•IF Pf < s'vc (ab), e = CONSTANT, NO cu CHANGE. •PRIMARY CONSOLIDATION : (bc) cu INCREASES FROM B TO C. •SECONDARY COMPRESSION (CD) RESULTS IN INCREASE OF cu FROM C TO D.•DISTRIBUTION OF DCU INCREASE IN THE LAYER IS VARIABLE AFTER Ps IS REMOVED.


PREDICTION OF cu INCREASE; Ip and cu / 'vp or cu TRIAXIAL TESTS cu≈0.2’v

THAT IS THE REASON FOR STAGE CONSTRUCTION OF EMBANKMENTS ON VERY SOFT CLAYS.

VERTICAL DRAINSVERTICAL DRAINS

PRELOADING TECHNIQUE MAY NOT WORK SOMETIMES PRELOADING TECHNIQUE MAY NOT WORK SOMETIMES

ALONE DUE TO A THICK UNIFORM SOFT CLAY LAYER OR ALONE DUE TO A THICK UNIFORM SOFT CLAY LAYER OR

PERMEABILITY OF THE CLAY IS VERY LOW SO THAT TIME PERMEABILITY OF THE CLAY IS VERY LOW SO THAT TIME

FOR PRECOMPRESSION (TSR) IS VERY LONG AND NOT FOR PRECOMPRESSION (TSR) IS VERY LONG AND NOT

PRACTICAL OR SURCHARGE WILL BE VERY HIGH FOR PRACTICAL OR SURCHARGE WILL BE VERY HIGH FOR

REASONABLE WAITING PERIODS. SOMETIMES RATE OF REASONABLE WAITING PERIODS. SOMETIMES RATE OF

UNDRAINED SHEAR STRENGTH GAIN IS VERY SMALL WITH UNDRAINED SHEAR STRENGTH GAIN IS VERY SMALL WITH

TIME SO THAT RAPID PLACEMENT OF A HIGH FILL WILL TIME SO THAT RAPID PLACEMENT OF A HIGH FILL WILL

CAUSE FAILURE.CAUSE FAILURE.


TO ACCELERATE THE RATES OF SETTLEMENT HENCE TO TO ACCELERATE THE RATES OF SETTLEMENT HENCE TO

DECREASE THEPRELOADING TIMES, VERTICAL DRAINS ARE DECREASE THEPRELOADING TIMES, VERTICAL DRAINS ARE

INSTALLED TO SHORTEN THE DRAINAGE PATHS. IT IS INSTALLED TO SHORTEN THE DRAINAGE PATHS. IT IS

ESPECIALLY EFFECTIVE IN PRIMARY CONSOLIDATION. ESPECIALLY EFFECTIVE IN PRIMARY CONSOLIDATION.

PORE WATER PRESSURES DISSIPATE QUICKLY, T a Hdr2 , IN PORE WATER PRESSURES DISSIPATE QUICKLY, T a Hdr2 , IN

MOST DEPOSITS kh>kv . IT IS NOT EFFECTIVE IN ORGANIC MOST DEPOSITS kh>kv . IT IS NOT EFFECTIVE IN ORGANIC

SOILS AND PEATS IN WHICH COMPRESSIONS ARE SOILS AND PEATS IN WHICH COMPRESSIONS ARE

DOMINATED BY SECONDARY COMPRESSION.DOMINATED BY SECONDARY COMPRESSION.


THEORY OF CONSOLIDATION FOR RADIAL FLOW AND BOTH THEORY OF CONSOLIDATION FOR RADIAL FLOW AND BOTH

RADIAL-VERTICAL CONSOLIDATION (COMBINED) HAVE BEEN RADIAL-VERTICAL CONSOLIDATION (COMBINED) HAVE BEEN

DEVELOPED FOR A LONG TIME (BARREN I948;CARILLO. 1942). DEVELOPED FOR A LONG TIME (BARREN I948;CARILLO. 1942).

CONSOLIDATION TIME IS MAINLY AFFECTED BY THE DRAIN CONSOLIDATION TIME IS MAINLY AFFECTED BY THE DRAIN

SPACING RATHER THAN THE DRAIN DIAMETER.SPACING RATHER THAN THE DRAIN DIAMETER.

EFFICIENCY OF DRAINS: IS THERE A SIGNIFICANT EFFICIENCY OF DRAINS: IS THERE A SIGNIFICANT

DIFFERENCE IN THE PRIMARY CONSOLIDATION RATE WHEN DIFFERENCE IN THE PRIMARY CONSOLIDATION RATE WHEN

DRAINS ARE USED?DRAINS ARE USED?

FOR SUCCESSFUL PROJECTS :FOR SUCCESSFUL PROJECTS :

1. (1. (vovo’+P’+Pff)>)>vovo’ (PRECONSOLIDATION PRESSURE) (FIgure 7)’ (PRECONSOLIDATION PRESSURE) (FIgure 7)

2. Primary settlement/(Primary Settlement+Secondary settlement) 2. Primary settlement/(Primary Settlement+Secondary settlement)

MUST BE LARGE.MUST BE LARGE.

3. THERE SHOULD NOT BE NATURAL DRAINAGE LAYERS.3. THERE SHOULD NOT BE NATURAL DRAINAGE LAYERS.



EFFICIENCY FACTOR OF VERTICAL DRAINS ;

FIELD MEASUREMENTS FROM 5 SITES INDICATE THAT FIELD MEASUREMENTS FROM 5 SITES INDICATE THAT

SATISFACTORY e VALUES RANGE BETWEEN 0.6 AND 0.8.SATISFACTORY e VALUES RANGE BETWEEN 0.6 AND 0.8.

THE NEED AND EFFICIENCY OF DRAINS ARE LARGELY DEPENDENT THE NEED AND EFFICIENCY OF DRAINS ARE LARGELY DEPENDENT

ON SOIL CHARACTERISTICS, SOIL PERMEABILITY AND ON SOIL CHARACTERISTICS, SOIL PERMEABILITY AND

COEFFICIENT OF CONSOLIDATION.COEFFICIENT OF CONSOLIDATION.

RECENT ALLUVIAL DEPOSITS CONTAIN FREQUENT HORIZONTAL RECENT ALLUVIAL DEPOSITS CONTAIN FREQUENT HORIZONTAL

BANDS OF SAND OR GRAVEL ETC.BANDS OF SAND OR GRAVEL ETC.

THESE ARE USUALLY THIN AND VERY PERMEABLE COMPARED TO THESE ARE USUALLY THIN AND VERY PERMEABLE COMPARED TO

CLAYS.CLAYS.

1. HIGHLY PERMEABLE BANDS OR SEAMS GREATLY INCREASE 1. HIGHLY PERMEABLE BANDS OR SEAMS GREATLY INCREASE

EFFICIENCY OF DRAINS SINCE THEY ACT AS HORIZONTAL DRAINS EFFICIENCY OF DRAINS SINCE THEY ACT AS HORIZONTAL DRAINS

CONNECTED TO MAIN ARTERIES.CONNECTED TO MAIN ARTERIES.

2. CONTINUOUS AND FREQUENT SEAMS OR BANDS OF HIGH 2. CONTINUOUS AND FREQUENT SEAMS OR BANDS OF HIGH

PERMEABILITY SOILS OFTEN MAKE VERTICAL DRAINS PERMEABILITY SOILS OFTEN MAKE VERTICAL DRAINS

UNNECESSARY OR GREATLY REDUCE THEIR REAL UNNECESSARY OR GREATLY REDUCE THEIR REAL

EFFECTIVENESS (FIGURE 8).EFFECTIVENESS (FIGURE 8).


3. SOIL INVESTIGATIONS ARE VERY IMPORTANT3. SOIL INVESTIGATIONS ARE VERY IMPORTANT

CONTINUOUS SAMPLINGCONTINUOUS SAMPLING

K FIELD AT LOW HEADSK FIELD AT LOW HEADS

LARGE DIAMETER (25-30 CM) LABORATORY LARGE DIAMETER (25-30 CM) LABORATORY CONSOLIDATION TESTS.CONSOLIDATION TESTS.


FIgure 8. ContInuous And Frequent

Permeable SoIl Bands Often Make DraIns

Unnecessary

A NUMBER OF DRAINS AVAILABLE :A NUMBER OF DRAINS AVAILABLE :

SAND DRAINSSAND DRAINS

CARD BOARD DRAINSCARD BOARD DRAINS

SAND WICKSSAND WICKS

PLASTIC DRAINSPLASTIC DRAINS

SOME FACTORS AFFECTING THE DRAIN PERFORMANCE :SOME FACTORS AFFECTING THE DRAIN PERFORMANCE :

1. SMEAR AND DISTORTION OF DRAIN WALLS WHICH REDUCE 1. SMEAR AND DISTORTION OF DRAIN WALLS WHICH REDUCE

DRAIN PERMEABILITY.DRAIN PERMEABILITY.

2. DISTURBANCE AND LATERAL DEFORMATIONS OF SOFT 2. DISTURBANCE AND LATERAL DEFORMATIONS OF SOFT

GROUND RESULTING FROM DRAIN INSTALLATION. GROUND RESULTING FROM DRAIN INSTALLATION.

PERMEABILITY DECREASES, UNDRAINED SHEAR STRENGTH PERMEABILITY DECREASES, UNDRAINED SHEAR STRENGTH

DECREASES AND PORE WATER PRESSURES INCREASE DECREASES AND PORE WATER PRESSURES INCREASE

(ROWE, 1968)(ROWE, 1968)


SAND DRAINSSAND DRAINSTHEY WERE WIDELY USED BETWEEN 1930 -1980 WITH DIAMETERS THEY WERE WIDELY USED BETWEEN 1930 -1980 WITH DIAMETERS

CHANGING BETWEEN 20 -60 CM AND WITH 1.5 TO 6 M SPACING.CHANGING BETWEEN 20 -60 CM AND WITH 1.5 TO 6 M SPACING.

CLOSED MANDREL METHODCLOSED MANDREL METHOD : APPLIED BY PERCUSSION OR VIBRATION : APPLIED BY PERCUSSION OR VIBRATION

OR JETTING. THE TUBE IS PUSHED DISPLACING THE SOIL. THERE IS A OR JETTING. THE TUBE IS PUSHED DISPLACING THE SOIL. THERE IS A

LOOSE CAP AT THE END WHICH IS DETACHED AFTER PUSHING IS LOOSE CAP AT THE END WHICH IS DETACHED AFTER PUSHING IS

COMPLETE. THEN THE TUBE IS FILLED AND EXTRACTED. IN THIS METHOD COMPLETE. THEN THE TUBE IS FILLED AND EXTRACTED. IN THIS METHOD

THERE IS DISPLACEMENT AND DISTURBANCE WHICH RESULTS IN A THERE IS DISPLACEMENT AND DISTURBANCE WHICH RESULTS IN A

DECREASE OF UNDRAINED SHEAR STRENGTH, PERMEABILITY DETECTED DECREASE OF UNDRAINED SHEAR STRENGTH, PERMEABILITY DETECTED

BY MEASURING PORE WATER PRESSURES, EU AND SURFACE HEAVE.BY MEASURING PORE WATER PRESSURES, EU AND SURFACE HEAVE.

OPEN MANDREL METHODOPEN MANDREL METHOD : IN THIS METHOD THE SOIL IN THE TUBE IS : IN THIS METHOD THE SOIL IN THE TUBE IS

REMOVED BY JETTING OR AUGERING. THE PROBLEM OF SMEAR STILL REMOVED BY JETTING OR AUGERING. THE PROBLEM OF SMEAR STILL

EXISTS. AUGER METHOD USING SOLID STEM OR HOLLOW STEM AUGERS EXISTS. AUGER METHOD USING SOLID STEM OR HOLLOW STEM AUGERS

WHICH IS A NON-DISPLACEMENT METHOD MAY BE CONSIDERED AS THE WHICH IS A NON-DISPLACEMENT METHOD MAY BE CONSIDERED AS THE

BEST AS COMPARED TO THE OTHERS. ROTARY JETTING METHOD MAY BEST AS COMPARED TO THE OTHERS. ROTARY JETTING METHOD MAY

ALSO BE APPLIED.ALSO BE APPLIED.

SAND DRAIN APPLICATIONS ARE COMING TO AN END IN THE WESTERN SAND DRAIN APPLICATIONS ARE COMING TO AN END IN THE WESTERN

COUNTRIES.COUNTRIES.CE-464 Ground ImprovementCE-464 Ground Improvement

CARDBOARD DRAINSCARDBOARD DRAINS

THEY ARE FIRST TRIED IN 1937 AND 1948 BY KJELLMAN. THEY ARE FIRST TRIED IN 1937 AND 1948 BY KJELLMAN.

THERE ARE DYNAMIC AND STATIC METHODS OF THERE ARE DYNAMIC AND STATIC METHODS OF

INSTALLATION. CARDBOARD DRAINS ARE DRIVEN INTO THE INSTALLATION. CARDBOARD DRAINS ARE DRIVEN INTO THE

GROUND BY PURPOSE-MADE MANDREL WHICH IS THEN GROUND BY PURPOSE-MADE MANDREL WHICH IS THEN

REMOVED. THE ADVANTAGES CAN BE LISTED AS FOLLOWS:REMOVED. THE ADVANTAGES CAN BE LISTED AS FOLLOWS:

THEY ARE EASY TO INSTALLTHEY ARE EASY TO INSTALL

THEY CAN BE SPACED CLOSELYTHEY CAN BE SPACED CLOSELY

THEY HAVE LONG LIFETHEY HAVE LONG LIFE

THEY HAVE THE ABILITY TO RESIST LARGE DEFORMATIONS.THEY HAVE THE ABILITY TO RESIST LARGE DEFORMATIONS.

CARDBOARD WICK DRAINS VISUALLY CONSIST OF A CORE CARDBOARD WICK DRAINS VISUALLY CONSIST OF A CORE

PLASTIC AND FILTER SLEEVE OF PAPER, FIBROUS PLASTIC AND FILTER SLEEVE OF PAPER, FIBROUS

MATERIAL OR POROUS PLASTIC.MATERIAL OR POROUS PLASTIC.


PLASTIC DRAINSPLASTIC DRAINS THESE ARE THE NEW GENERATION DRAINS WHICH ARE VERY THESE ARE THE NEW GENERATION DRAINS WHICH ARE VERY SIMILAR TO CARDBOARD DRAINS. THERE ARE SEVERAL SIMILAR TO CARDBOARD DRAINS. THERE ARE SEVERAL COMMERCIAL BRANDS IN THE MARKET AT PRESENT LIKE COMMERCIAL BRANDS IN THE MARKET AT PRESENT LIKE ALIDRAIN, GEPDRAIN, CASTLEBOARD, COLBONT, MEBRADRAIN ALIDRAIN, GEPDRAIN, CASTLEBOARD, COLBONT, MEBRADRAIN AND PVC DRAIN. SEE FIGURE 10 FOR GEODRAIN.AND PVC DRAIN. SEE FIGURE 10 FOR GEODRAIN.


SAND-WICKSSAND-WICKS

THESE ARE READY-MADE SMALL DIAMETER SAND DRAINS THESE ARE READY-MADE SMALL DIAMETER SAND DRAINS

WHICH ARE CONTAINED IN LONG CANVAS BAGS WHICH ARE CONTAINED IN LONG CANVAS BAGS

(APPROXIMATELY 10 CM IN DIAMETER). THEY ARE (APPROXIMATELY 10 CM IN DIAMETER). THEY ARE

USUALLY INSTALLED BY CLOSE MANDREL TECHNIQUE. USUALLY INSTALLED BY CLOSE MANDREL TECHNIQUE.

THEY ARE RELATIVELY CHEAP AND FIRST USED IN INDIA BY THEY ARE RELATIVELY CHEAP AND FIRST USED IN INDIA BY

DASTIDAR ET AL. (1969) AND THEN BY SUBBARAJU ETAL. DASTIDAR ET AL. (1969) AND THEN BY SUBBARAJU ETAL.

(1973).(1973).


DESIGN OF VERTICAL DRAINSDESIGN OF VERTICAL DRAINS

THE MAIN ASSUMPTIONS MADE FOR THE DESIGN OF VERTICAL THE MAIN ASSUMPTIONS MADE FOR THE DESIGN OF VERTICAL DRAINS ARE ;DRAINS ARE ;

EACH DRAIN IS INDEPENDENT AT THE CENTRE OF A EACH DRAIN IS INDEPENDENT AT THE CENTRE OF A CYLINDRICAL SOIL MASS AND IS ONLY AFFECTED BY THE CYLINDRICAL SOIL MASS AND IS ONLY AFFECTED BY THE DRAINAGE OF THE SOIL IN IT.DRAINAGE OF THE SOIL IN IT.

INSTANTANEOUS LOADING OF THE HOMOGENEOUS SOIL INSTANTANEOUS LOADING OF THE HOMOGENEOUS SOIL RESULTS IN SOLELY RADIAL CONSOLIDATION (AND RESULTS IN SOLELY RADIAL CONSOLIDATION (AND THEREFORE RADIAL FLOW) UNDER CONDITIONS OF THEREFORE RADIAL FLOW) UNDER CONDITIONS OF CONSTANT PERMEABILITY (kh ) AND RADIAL CONSOLIDATION CONSTANT PERMEABILITY (kh ) AND RADIAL CONSOLIDATION COEFFICIENT (ch) .COEFFICIENT (ch) .


THE EQUATION WHICH GOVERNS THE THE EQUATION WHICH GOVERNS THE RELATIONSHIP BETWEEN PORE PRESSURE, U, RELATIONSHIP BETWEEN PORE PRESSURE, U, RADIAL DISTANCE FROM THE DRAIN , r, AND TIME, RADIAL DISTANCE FROM THE DRAIN , r, AND TIME, t,(IN FACT kt,(IN FACT khh=f(t) AND c=f(t) AND chh=f(t) ) IS GIVEN BELOW. =f(t) ) IS GIVEN BELOW.

DRAIN EFFECTS, SMEAR DISTURBANCE, WELL DRAIN EFFECTS, SMEAR DISTURBANCE, WELL RESISTANCE, LOADING RATE, CREEP EFFECTS, RESISTANCE, LOADING RATE, CREEP EFFECTS, APPROPRIATE HYDRAULIC FLOW FORMULATION APPROPRIATE HYDRAULIC FLOW FORMULATION CAN BE ALL INCLUDED IN THE ANALYSES.CAN BE ALL INCLUDED IN THE ANALYSES.


t

u

r

u

rr

uch

1

2

2u=u0 at t=0 at all place

u=u0 In the draIn at any tIme

SOLUTIONS MAY BE OBTAINED FOR TWO TYPES OF SOLUTIONS MAY BE OBTAINED FOR TWO TYPES OF BOUNDARY CONDITIONS.BOUNDARY CONDITIONS.

1.1. UNIFORM SURCHARGE ON THE GROUND SURFACE (FREE UNIFORM SURCHARGE ON THE GROUND SURFACE (FREE STRAIN)STRAIN)

2.2. UNIFORM VERTICAL DEFORMATION OF THE SURFACE UNIFORM VERTICAL DEFORMATION OF THE SURFACE (EQUAL STRAIN)(EQUAL STRAIN)BOTH SOLUTIONS ARE SIMILAR BUT UNIFORM VERTICAL BOTH SOLUTIONS ARE SIMILAR BUT UNIFORM VERTICAL STRAIN CONDITION IS SIMPLER.STRAIN CONDITION IS SIMPLER.ASSUMING THAT THE VERTICAL FLOW IS NEGLIGIBLE, ASSUMING THAT THE VERTICAL FLOW IS NEGLIGIBLE, THE EXPRESSION FOR RADIAL (HORIZONTAL) DEGREE OF THE EXPRESSION FOR RADIAL (HORIZONTAL) DEGREE OF CONSOLIDATION IS ;CONSOLIDATION IS ;


)(

8

1 nF

T

h

h

eU

2

.

e

hh

d

tcT where, (horIzontal tIme factor)

HANSBO(1979);F = F(n) + Fs + FrHANSBO(1979);F = F(n) + Fs + FrWHERE; WHERE;

F(n) F(n) : DUE TO SPACING OF DRAINS: DUE TO SPACING OF DRAINSFs Fs : SMEAR EFFECT: SMEAR EFFECTFrFr : WELL-RESISTANCE: WELL-RESISTANCE

TIME FOR CONSOLIDATION IS;TIME FOR CONSOLIDATION IS;

WHERE WHERE Uh, IS THE AVERAGE DEGREE OF RADIAL CONSOLIDATION Uh, IS THE AVERAGE DEGREE OF RADIAL CONSOLIDATION

USUALLY n>12 AND USUALLY n>12 AND

F(n)= F(n)= = ln(n) - 0.75 MAY BE USED. = ln(n) - 0.75 MAY BE USED. n= de/dw, n= de/dw, SPACING RATIOSPACING RATIO


hh

e

Uc

dt

1

1ln

.8

.2

VARIATION OF Uh WITH HORIZONTAL TIME FACTOR FORVARIATION OF Uh WITH HORIZONTAL TIME FACTOR FOR

VARIOUS n VALUES (EQUAL VERTICAL STRAIN) IS SHOWN ISVARIOUS n VALUES (EQUAL VERTICAL STRAIN) IS SHOWN IS

FIGURE 12. THE EQUATIONS OF Cv AND t FOR VERTICAL ANDFIGURE 12. THE EQUATIONS OF Cv AND t FOR VERTICAL AND

RADIAL CONSOLIDATION ARE AS FOLLOWS;RADIAL CONSOLIDATION ARE AS FOLLOWS;


t

TH

a

ekc v

wv

vv

.

.

)1( 20

v

v

c

HTt

2. (VERTICAL CONSOLIDATION)

t

Td

a

ekc he

wv

hh

.

.

)1( 20

or

h

eh

c

dTt

2. (RADIAL CONSOLIDATION)

NOTE THAT mv = av / (1+e0)


FIgure 12. VarIatIon Of Uh WIth TIme Factor For VarIous N Values (Equal StraIn)

CONSOLIDATION OF THE CYLINDRICAL BODY OF SOIL AROUNDCONSOLIDATION OF THE CYLINDRICAL BODY OF SOIL AROUND

A VERTICAL DRAIN IS, IN FACT, THREE DIMENSIONAL AND ISA VERTICAL DRAIN IS, IN FACT, THREE DIMENSIONAL AND IS

GOVERNED BY THE EQUATION ; GOVERNED BY THE EQUATION ;

OVERALL (THREE DIMENSIONAL) DEGREE OF CONSOLIDATION,OVERALL (THREE DIMENSIONAL) DEGREE OF CONSOLIDATION,

U=1-U=(1-UU=1-U=(1-UHH).(1-U).(1-UVV) WHERE U) WHERE UV V IS THE AVERAGE VERTICALIS THE AVERAGE VERTICAL

DEGREE OF CONSOLIDATION.DEGREE OF CONSOLIDATION.

THE ABOVE CONSIDERATIONS ARE FOR ALL TYPES OFTHE ABOVE CONSIDERATIONS ARE FOR ALL TYPES OF

VERTICAL DRAINS. THE EQUIVALENT DIAMETER DVERTICAL DRAINS. THE EQUIVALENT DIAMETER DWW OF A OF A

BAND SHAPED DRAIN OF WIDTH B, AND THICKNESS T, ISBAND SHAPED DRAIN OF WIDTH B, AND THICKNESS T, IS

CALCULATED BY USING THE RELATIONSHIP GIVEN BELOW.CALCULATED BY USING THE RELATIONSHIP GIVEN BELOW.


t

u

z

uc

x

u

xx

uc vh

2

2

2

2

.1

)(2 tb

d w


BOTH WELL RESISTANCE AND WELL DISTURBANCE DURING

INSTALLATION MAY CAUSE THE ACTUAL TIMES FOR CONSOLIDATION

TO BE GREATER THAN PREDICTED BY THE ABOVE EQUATIONS.

DIFFERENT ASPECTS OF THE TOPIC ARE STUDIED BY VARIOUS

RESEARCHERS. SOME OF THEM ARE;

•SOLUTION FOR GRADUALLY APPLIED LOADING BY CHAPUT AND THOMANN (1975)

•SMEAR AND WELL RESISTANCE STUDIED BY, YOSHIKUNI AND NAKANODO (1974) , BARREN(1948),-ABOSHI (1969), RICHART (1959), BERRY AND WILKINSON (1969)

•EFFECT OF STRATIFICATION IS STUDIED BY LEE AND VALIAPPAN (1974)

REVIEW - DESIGN CONSIDERATIONSREVIEW - DESIGN CONSIDERATIONS

THE COEFFICIENT OF CONSOLIDATION FOR VERTICALTHE COEFFICIENT OF CONSOLIDATION FOR VERTICAL

COMPRESSION AND HORIZONTAL FLOW DOMINATES THECOMPRESSION AND HORIZONTAL FLOW DOMINATES THE

DESIGN OF SAND DRAINS.DESIGN OF SAND DRAINS.


wv

hh a

ekc

.)1( 0

h

eh

c

dTt

2.

or

wv

hh m

kc

.

wv

vv m

kc

.

(NOTE THE DIFFERENCE BETWEEN THE ABOVE EQUATIONS)

CChh IS PRINCIPALLY DEPENDENT ON kh BECAUSE m IS PRINCIPALLY DEPENDENT ON kh BECAUSE mvv IS LESS IS LESS

VARIABLE. ONE WAY OF ESTIMATING kVARIABLE. ONE WAY OF ESTIMATING khh, IS TO CONDUCT , IS TO CONDUCT

SPECIAL LABORATORY TESTS FOR RADIAL DRAINAGE IN SPECIAL LABORATORY TESTS FOR RADIAL DRAINAGE IN

UNIFORM SOILS USUALLY kUNIFORM SOILS USUALLY khh>k>kvv (HANSBO(1960), (HANSBO(1960),

ROWE(1964), BERRY AND WILKINSON(1969), PAUTE(1973).ROWE(1964), BERRY AND WILKINSON(1969), PAUTE(1973).

MITCHEL AND GARDNER NOTED THAT THE BEST WAY IS TO MITCHEL AND GARDNER NOTED THAT THE BEST WAY IS TO

MEASURE AV OR MV IN THE LAB AND KH, IN THE MEASURE AV OR MV IN THE LAB AND KH, IN THE

FIELD(USUALLY CH/CV RANGES BETWEEN 2- 10).FIELD(USUALLY CH/CV RANGES BETWEEN 2- 10).


FIELD CONTROL OF THE CONSTRUCTIONFIELD CONTROL OF THE CONSTRUCTION

IT IS VERY IMPORTANT TO CHECK WHAT IS GOING ON IT IS VERY IMPORTANT TO CHECK WHAT IS GOING ON

AFTER THE DESIGN HAS BEEN DONE AND THE LOADING AFTER THE DESIGN HAS BEEN DONE AND THE LOADING

STARTED. MEASUREMENTS ARE INTEGRAL PART OF THE STARTED. MEASUREMENTS ARE INTEGRAL PART OF THE

DESIGN PROCESS AND IT MAY NOT BE POSSIBLE TO DESIGN PROCESS AND IT MAY NOT BE POSSIBLE TO

COMPLETE A PRELOADING-DRAINS PROJECT WITHOUT COMPLETE A PRELOADING-DRAINS PROJECT WITHOUT

FIELD INSTRUMENTATION. FIELD INSTRUMENTATION.

THE SIMPLEST WAY IS TO MEASURE THE SURFACE THE SIMPLEST WAY IS TO MEASURE THE SURFACE

SETTLEMENTS. PIEZOMETERS AT THE CENTRE AND OTHER SETTLEMENTS. PIEZOMETERS AT THE CENTRE AND OTHER

ELEVATIONS OF THE SOFT CLAY AT THE MID-DISTANCE OF ELEVATIONS OF THE SOFT CLAY AT THE MID-DISTANCE OF

THE DRAINS AND THE SETTLEMENT GAUGES AT VARIOUS THE DRAINS AND THE SETTLEMENT GAUGES AT VARIOUS

DEPTHS (BOREHOLE GAUGES) ARE USUALLY USED TO DEPTHS (BOREHOLE GAUGES) ARE USUALLY USED TO

ASSESS AND TO CHECK THE DESIGN ASSUMPTIONS. ASSESS AND TO CHECK THE DESIGN ASSUMPTIONS.

NECESSARY CHANGES ARE MADE IF REQUIRED.NECESSARY CHANGES ARE MADE IF REQUIRED.CE-464 Ground ImprovementCE-464 Ground Improvement

VARIOUS CASE HISTORIES INVOKING VERTICAL DRAINS TOVARIOUS CASE HISTORIES INVOKING VERTICAL DRAINS TO

SPEED UP THE CONSOLIDATION PROCESS WERE DESCRIBEDSPEED UP THE CONSOLIDATION PROCESS WERE DESCRIBED

BY JOHNSON (1970B), BJERRUM (1972) AND PILOT (1981).BY JOHNSON (1970B), BJERRUM (1972) AND PILOT (1981).

VERTICAL DRAINS ARE GENERALLY EFFECTIVE EXCEPT IN VERTICAL DRAINS ARE GENERALLY EFFECTIVE EXCEPT IN ORGANIC CLAYS, HIGHLY STRATIFIED SOILS OR IN SOILS WHERE ORGANIC CLAYS, HIGHLY STRATIFIED SOILS OR IN SOILS WHERE SEVERE STABILITY PROBLEMS EXIST. THESE SOILS ALREADY SEVERE STABILITY PROBLEMS EXIST. THESE SOILS ALREADY SETTLE IN A SHORT TIME.SETTLE IN A SHORT TIME.

THE METHOD OFTEN SAVES LARGE SUMS OF MONEY AND CAN THE METHOD OFTEN SAVES LARGE SUMS OF MONEY AND CAN BE USED IN VERY POOR SUBSOIL CONDITIONS. LARGE POST BE USED IN VERY POOR SUBSOIL CONDITIONS. LARGE POST CONSTRUCTION SETTLEMENTS MAY BE ELIMINATED, COSTS OF CONSTRUCTION SETTLEMENTS MAY BE ELIMINATED, COSTS OF SURCHARGE AND/OR VERTICAL DRAINS AND PRELOADING SURCHARGE AND/OR VERTICAL DRAINS AND PRELOADING PERIODS SHOULD BE COMPARED.PERIODS SHOULD BE COMPARED.


PLASTIC DRAINS ARE INCREASINGLY BEING USED.PLASTIC DRAINS ARE INCREASINGLY BEING USED.

BEST METHOD IN THE INSTALLATION OF SAND DRAINS IS BEST METHOD IN THE INSTALLATION OF SAND DRAINS IS AUGERING. OPEN MANDREL METHOD RESULTS IN MORE AUGERING. OPEN MANDREL METHOD RESULTS IN MORE EFFICIENT DRAINS THAN CLOSE MANDREL METHOD.EFFICIENT DRAINS THAN CLOSE MANDREL METHOD.

TIMES FOR CONSOLIDATION CHANGES FROM MONTHS TO A TIMES FOR CONSOLIDATION CHANGES FROM MONTHS TO A YEAR OR MORE.YEAR OR MORE.

IF PIF Pff < < c' DO NOT USE SAND DRAINS.c' DO NOT USE SAND DRAINS.


FIELD TESTS (ESPECIALLY FIELD PERMEABILITY) IN STRATIFIED FIELD TESTS (ESPECIALLY FIELD PERMEABILITY) IN STRATIFIED SOILS MAY BE REQUIRED TO DECIDE EFFICIENCY OF THIN SOILS MAY BE REQUIRED TO DECIDE EFFICIENCY OF THIN PERVIOUS SOILS AS DRAINAGE LAYERS. IN MANY CASES PERVIOUS SOILS AS DRAINAGE LAYERS. IN MANY CASES CONSOLIDATION RATES ARE MUCH FASTER THAN USUALLY CONSOLIDATION RATES ARE MUCH FASTER THAN USUALLY PREDICTED DUE TO CONVENTIONAL LABORATORY PREDICTED DUE TO CONVENTIONAL LABORATORY APPROACHES AND THERE MAY NOT BE ANY NEED FOR DRAINS APPROACHES AND THERE MAY NOT BE ANY NEED FOR DRAINS IN SOILS HAVING CONTINUOUS PERMEABLE BANDS.IN SOILS HAVING CONTINUOUS PERMEABLE BANDS.

ON THE OTHER HAND PREDICTIONS OF RATES OF ON THE OTHER HAND PREDICTIONS OF RATES OF CONSOLIDATION WHERE DRAINS ARE INSTALLED CAN NOT BE CONSOLIDATION WHERE DRAINS ARE INSTALLED CAN NOT BE MADE RELIABLY BECAUSE OF THE DIFFICULTIES IN MADE RELIABLY BECAUSE OF THE DIFFICULTIES IN DETERMINING A REPRESENTATIVE VALUE OF ch , AND DETERMINING A REPRESENTATIVE VALUE OF ch , AND ACCOUNTING FOR THE EFFECTS OF THE DRAIN INSTALLATION ACCOUNTING FOR THE EFFECTS OF THE DRAIN INSTALLATION (DISTURBANCE AND SMEAR).(DISTURBANCE AND SMEAR).

SITE INVESTIGATION EFFORTS MUST BE OF GOOD QUALITY.SITE INVESTIGATION EFFORTS MUST BE OF GOOD QUALITY.


WELL RESISTANCEWELL RESISTANCE

DISCHARGE CAPACITY OF THE DRAINS = qDISCHARGE CAPACITY OF THE DRAINS = qw w

PERMEABILITY OF THE SOIL = kPERMEABILITY OF THE SOIL = kcc

WHEREWHERE

l = LENGTH OF THE DRAIN WHEN OPEN AT ONE END ONLY l = LENGTH OF THE DRAIN WHEN OPEN AT ONE END ONLY (HALF LENGTH OF THE DRAIN WHEN OPEN AT BOTH ENDS) (HALF LENGTH OF THE DRAIN WHEN OPEN AT BOTH ENDS)

z = DISTANCE FROM OPEN END OF DRAIN (0 < z < 2 l ) z = DISTANCE FROM OPEN END OF DRAIN (0 < z < 2 l )

WHEN qWHEN qww / k/ kcc < 3000 m < 3000 m22 WELL RESISTANCE CAN NOT BE WELL RESISTANCE CAN NOT BE

IGNORED.IGNORED.


w

cr q

kzlzn )2(75.0)ln(

SMEAR DISTURBANCESMEAR DISTURBANCE

WHERE s=dWHERE s=dss / d / dww

ds = DIAMETER OF THE DISTURBED ZONE ds = DIAMETER OF THE DISTURBED ZONE

kc' = PERMEABILITY OF THE DISTURBED ZONEkc' = PERMEABILITY OF THE DISTURBED ZONE


2

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Documents

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magnitude of surcharge

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