PRELOADING OR PRECOMPRESSION

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.

CE-464 Ground Improvement

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 LOWERING, ELECTROOSMOSIS.

SYSTEMS, GROUNDWATER THE SURCHARGE RESULTS IN;

•

PRIMARY CONSOLIDATION SETTLEMENT

•

SECONDARY CONSOLIDATION SETTLEMENT

•

INCREASE IN THE UNDRAINED SHEAR STRENGTH OF THE SOIL.

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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 T S R.

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).

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USE OF SURCHARGE LOADING (EXTRA P

“

S ) AND DURATION OF ITS MAGNITUDE OF SETTLEMENT AFTER CONSTRUCTION. IT IS USUALLY AIMED AT EITHER; 1.TO

DETERMINE REQUIRED TO THE ENSURE MAGNITUDE THAT THE OF SURCHARGE TOTAL (PS) 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.

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ONE-DIMENSIONAL CONSOLIDATION SETTLEMENT OF A LAYER 2H IN THICKNESS (NC CLAY); IF U f+s DENOTES THE AVERAGE DEGREE OF CONSOLIDATION OF THE CLAY LAYER UNDER (P f +P s ) AT THE TIME t SR , THEN FOR NO FURTHER SETTELEMENT TO OCCUR UNDER P f ALONE; IF THE SURCHARGE WERE LEFT IN PLACE UNTIL THE TIME t SR THEN THE LAYER WILL HAVE SETTLED AN AMOUNT EQUAL TO , THAT TO BE EXPECTED UNDER THE PERMANENT FILL ALONE; i.e.

S tR = S f . AT THIS TIME THE LAYER WILL HAVE REACHED AN AVERAGE DEGREE OF CONSOLIDATION U (SR) .

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FROM EQUATIONS (1) AND (2) THE RELATIONSHIP BETWEEN U f+s , P f /

s

' vo AND P s / P f 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 (P

s

’ vmax.

f + P s +

s

' vo ) > 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.

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DEGREE OF CONSOLIDATION (OR CONSOLIDATION RATIO) AT ANY POINT IS; where : u z = EXCESS PORE WATER PRESSURE AT DEPTH Z AT ANY TIME u o = INITIAL EXCESS PORE WATER PRESSURE UNDER THE APPLIED SURFACE LOAD.

FOR NO SETTLEMENT TO OCCUR AFTER THE SURCHARGE (P s ) REMOVAL AT THE TIME t SR , EFFECTIVE STRESS AT THE CENTRE OF THE LAYER MUST BE EQUAL OR GREATER THAN THE PERMANENT LOAD (DESIGN LOAD) P f . IN OTHER WORDS;

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SECONDARY COMPRESSION EFFECTS

SOME SOILS LIKE ORGANIC SOILS,PEATS SETTLE AFTER PRIMARY CONSOLIDATION IS COMPLETED (SEE FIGURE 3) where ; C a = VERTICAL STRAIN PER LOG CYCLE (COEFFICIENT OF SECONDARY COMPRESSION) t p = TIME FOR PRIMARY CONSOLIDATION H p = THICKNESS AT t=t p

CE-464 Ground Improvement

IF NO PRIMARY AND SECONDARY SETTLEMENTS ARE DESIRED UNDER THE DESIGN LOAD P SUSTAINED FOR A PERIOD f ; THE LOAD P f t SR SUCH + P THAT s MUST BE THE TOTAL SETTLEMENT S SR UNDER P f IS THE SUM OF PRIMARY AND SETTLEMENTS ALONE (FIGURE 4).

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THE DESIRED SETTLEMENT S SR IS RELATED TO THE PRIMARY SETTLEMENT UNDER THE LOAD P f +P s .

SUBSTITUTING 1b, 10, 7, 8, INTO 9 PRELOADING VERSUS GAIN IN UNDRAINED SHEAR STRENGTH (FIGURE 3);

•

IF P f < s' vc (ab), e = CONSTANT, NO c u

•

PRIMARY CONSOLIDATION : (bc) c u CHANGE.

INCREASES FROM B TO C.

•

SECONDARY COMPRESSION (CD) RESULTS IN INCREASE OF c u FROM C TO D.

•

DISTRIBUTION OF DC U AFTER P s IS REMOVED.

INCREASE IN THE LAYER IS VARIABLE

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PREDICTION OF TESTS

D

c u ≈0.2

Ds

’ v

D

c u INCREASE; I p and c u /

s

' vp or c u TRIAXIAL THAT IS THE REASON FOR STAGE EMBANKMENTS ON VERY SOFT CLAYS.

CONSTRUCTION OF

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VERTICAL DRAINS

PRELOADING TECHNIQUE MAY NOT WORK SOMETIMES ALONE DUE TO A THICK UNIFORM SOFT CLAY LAYER OR PERMEABILITY OF THE CLAY IS VERY LOW SO THAT TIME FOR PRECOMPRESSION (TSR) IS VERY LONG AND NOT PRACTICAL OR SURCHARGE WILL BE VERY HIGH FOR REASONABLE WAITING PERIODS. SOMETIMES RATE OF UNDRAINED SHEAR STRENGTH GAIN IS VERY SMALL WITH TIME SO THAT RAPID PLACEMENT OF A HIGH FILL WILL CAUSE FAILURE.

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TO ACCELERATE THE RATES OF SETTLEMENT HENCE TO DECREASE THEPRELOADING TIMES, VERTICAL DRAINS ARE INSTALLED TO SHORTEN THE DRAINAGE PATHS. IT IS ESPECIALLY EFFECTIVE IN PRIMARY CONSOLIDATION.

PORE WATER PRESSURES DISSIPATE QUICKLY, T a Hdr2 , IN MOST DEPOSITS kh>kv . IT IS NOT EFFECTIVE IN ORGANIC SOILS AND PEATS IN WHICH COMPRESSIONS DOMINATED BY SECONDARY COMPRESSION.

ARE

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THEORY OF CONSOLIDATION FOR RADIAL FLOW AND BOTH RADIAL-VERTICAL CONSOLIDATION (COMBINED) HAVE BEEN DEVELOPED FOR A LONG TIME (BARREN I948;CARILLO.

1942). CONSOLIDATION TIME IS MAINLY AFFECTED BY THE DRAIN SPACING RATHER THAN THE DRAIN DIAMETER.

EFFICIENCY OF DRAINS: IS THERE A SIGNIFICANT DIFFERENCE IN THE PRIMARY CONSOLIDATION RATE WHEN DRAINS ARE USED?

FOR SUCCESSFUL PROJECTS : 1. (

s

vo ’+P f )>

s

vo ’ (PRECONSOLIDATION PRESSURE) (FIgure 7) 2. Primary settlement/(Primary Settlement+Secondary settlement) MUST BE LARGE.

3. THERE SHOULD NOT BE NATURAL DRAINAGE LAYERS.

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EFFICIENCY FACTOR VERTICAL DRAINS ; OF

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FIELD MEASUREMENTS FROM 5 SITES INDICATE SATISFACTORY e VALUES RANGE BETWEEN 0.6 AND 0.8.

THAT THE NEED AND EFFICIENCY OF DRAINS ARE LARGELY DEPENDENT ON SOIL CHARACTERISTICS, COEFFICIENT OF CONSOLIDATION.

SOIL PERMEABILITY AND RECENT ALLUVIAL DEPOSITS CONTAIN FREQUENT HORIZONTAL BANDS OF SAND OR GRAVEL ETC.

THESE ARE USUALLY THIN AND VERY PERMEABLE COMPARED TO CLAYS.

1. HIGHLY PERMEABLE BANDS OR SEAMS GREATLY INCREASE EFFICIENCY OF DRAINS SINCE THEY ACT AS HORIZONTAL DRAINS CONNECTED TO MAIN ARTERIES.

2.

CONTINUOUS PERMEABILITY UNNECESSARY AND FREQUENT SOILS OFTEN SEAMS MAKE OR BANDS VERTICAL OR GREATLY EFFECTIVENESS (FIGURE 8).

REDUCE THEIR OF HIGH DRAINS REAL

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FIgure 8. ContInuous And Frequent Permeable SoIl Bands Often Make DraIns Unnecessary 3. SOIL INVESTIGATIONS ARE VERY IMPORTANT CONTINUOUS SAMPLING K FIELD AT LOW HEADS LARGE DIAMETER (25-30 CM) LABORATORY CONSOLIDATION TESTS.

CE-464 Ground Improvement

A NUMBER OF DRAINS AVAILABLE : SAND DRAINS CARD BOARD DRAINS SAND WICKS PLASTIC DRAINS SOME FACTORS AFFECTING THE DRAIN PERFORMANCE : 1. SMEAR AND DISTORTION OF DRAIN WALLS WHICH REDUCE DRAIN PERMEABILITY.

2. DISTURBANCE AND LATERAL DEFORMATIONS OF SOFT GROUND RESULTING FROM DRAIN INSTALLATION. PERMEABILITY DECREASES, UNDRAINED SHEAR STRENGTH DECREASES AND PORE WATER PRESSURES INCREASE (ROWE, 1968)

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SAND DRAINS

THEY WERE WIDELY USED BETWEEN 1930 -1980 WITH DIAMETERS CHANGING BETWEEN 20 -60 CM AND WITH 1.5 TO 6 M SPACING.

CLOSED MANDREL METHOD : APPLIED BY PERCUSSION OR VIBRATION OR JETTING. THE TUBE IS PUSHED DISPLACING THE SOIL. THERE IS A LOOSE CAP AT THE END WHICH IS DETACHED AFTER PUSHING IS COMPLETE. THEN THE TUBE IS FILLED AND EXTRACTED. IN THIS METHOD THERE IS DISPLACEMENT AND DISTURBANCE WHICH RESULTS IN A DECREASE OF UNDRAINED SHEAR STRENGTH, PERMEABILITY DETECTED BY MEASURING PORE WATER PRESSURES, EU AND SURFACE HEAVE.

OPEN MANDREL METHOD : IN THIS METHOD THE SOIL IN THE TUBE IS REMOVED BY JETTING OR AUGERING. THE PROBLEM OF SMEAR STILL EXISTS. AUGER METHOD USING SOLID STEM OR HOLLOW STEM AUGERS WHICH IS A NON-DISPLACEMENT METHOD MAY BE CONSIDERED AS THE BEST AS COMPARED TO THE OTHERS. ROTARY JETTING METHOD MAY ALSO BE APPLIED.

SAND DRAIN APPLICATIONS ARE COMING TO AN END IN THE WESTERN

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COUNTRIES.

CARDBOARD DRAINS

THEY ARE FIRST TRIED IN 1937 AND 1948 BY KJELLMAN.

THERE ARE DYNAMIC AND STATIC METHODS OF INSTALLATION. CARDBOARD DRAINS ARE DRIVEN INTO THE GROUND BY PURPOSE-MADE MANDREL WHICH IS THEN REMOVED. THE ADVANTAGES CAN BE LISTED AS FOLLOWS: THEY ARE EASY TO INSTALL THEY CAN BE SPACED CLOSELY THEY HAVE LONG LIFE THEY HAVE THE ABILITY TO RESIST LARGE DEFORMATIONS.

CARDBOARD PLASTIC AND WICK DRAINS VISUALLY CONSIST OF A CORE FILTER SLEEVE OF PAPER, FIBROUS MATERIAL OR POROUS PLASTIC.

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PLASTIC DRAINS

THESE ARE THE NEW GENERATION DRAINS WHICH ARE VERY SIMILAR TO CARDBOARD DRAINS.

THERE ARE SEVERAL PRESENT COMMERCIAL LIKE BRANDS ALIDRAIN, IN THE GEPDRAIN, MARKET AT CASTLEBOARD, COLBONT, MEBRADRAIN AND PVC DRAIN. SEE FIGURE 10 FOR GEODRAIN.

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SAND-WICKS

THESE ARE READY-MADE SMALL DIAMETER SAND DRAINS WHICH ARE CONTAINED IN LONG CANVAS BAGS (APPROXIMATELY 10 CM IN DIAMETER). THEY ARE USUALLY INSTALLED BY CLOSE MANDREL TECHNIQUE. THEY ARE RELATIVELY CHEAP AND FIRST USED IN INDIA BY DASTIDAR ET AL. (1969) AND THEN BY SUBBARAJU ETAL. (1973).

CE-464 Ground Improvement

DESIGN OF VERTICAL DRAINS

THE MAIN ASSUMPTIONS MADE FOR THE DESIGN OF VERTICAL DRAINS ARE ; EACH DRAIN IS INDEPENDENT AT THE CENTRE OF A CYLINDRICAL SOIL MASS AND IS ONLY AFFECTED BY THE DRAINAGE OF THE SOIL IN IT.

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

CE-464 Ground Improvement

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

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

c h

    2

u



r

2  1

r



u



r

    

u



t

u=u 0 u=u 0 at t=0 at all place In the draIn at any tIme CE-464 Ground Improvement

1.

2.

SOLUTIONS MAY BE OBTAINED FOR TWO TYPES OF BOUNDARY CONDITIONS.

UNIFORM SURCHARGE ON THE GROUND SURFACE (FREE STRAIN) UNIFORM VERTICAL DEFORMATION OF THE SURFACE (EQUAL STRAIN) BOTH SOLUTIONS ARE SIMILAR BUT UNIFORM VERTICAL STRAIN CONDITION IS SIMPLER.

ASSUMING THAT THE VERTICAL FLOW IS NEGLIGIBLE, THE EXPRESSION FOR RADIAL (HORIZONTAL) DEGREE OF CONSOLIDATION IS ;

U h

 8

T h

 1 

e F

(

n

) where,

T h



c h

.

t d e

2 (horIzontal tIme factor) CE-464 Ground Improvement

HANSBO(1979);F = F(n) + Fs + Fr WHERE; F(n) Fs Fr : DUE TO SPACING OF DRAINS : SMEAR EFFECT : WELL-RESISTANCE TIME FOR CONSOLIDATION IS;

t



d e

2 .

m 8 .

c h

ln     1  1

U h

   

WHERE Uh, IS THE AVERAGE DEGREE OF RADIAL CONSOLIDATION USUALLY n>12 AND F(n)=

m

= ln(n) - 0.75 MAY BE USED.

n= de/dw, SPACING RATIO

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VARIATION OF Uh WITH HORIZONTAL TIME FACTOR FOR VARIOUS n VALUES (EQUAL VERTICAL STRAIN) IS SHOWN IS FIGURE 12. THE EQUATIONS OF Cv AND t FOR VERTICAL AND RADIAL CONSOLIDATION ARE AS FOLLOWS;

c v



k v

( 1 

a v

.



w e

0 ) 

H

2 .

T v t t



T v

.

H

2

c v

(VERTICAL CONSOLIDATION)

or

c h



k h

( 1 

a v

.



w

NOTE THAT

e

0 ) 

d e

2 .

T h t t



T h

.

d e

2

c h

mv = av / (1+e0)

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(RADIAL CONSOLIDATION)

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

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CONSOLIDATION OF THE CYLINDRICAL BODY OF SOIL AROUND A VERTICAL DRAIN IS, IN FACT, THREE DIMENSIONAL AND IS GOVERNED BY THE EQUATION ;

c h

    2

u



x

2  1

x

.



u



x

   

c v

 2

u



z

2  

u



t

OVERALL (THREE DIMENSIONAL) DEGREE OF CONSOLIDATION, U=1-U=(1-U H ).(1-U V ) WHERE U V IS THE AVERAGE VERTICAL DEGREE OF CONSOLIDATION.

THE ABOVE CONSIDERATIONS ARE FOR ALL TYPES OF VERTICAL DRAINS. THE EQUIVALENT DIAMETER D W OF A BAND SHAPED DRAIN OF WIDTH B, AND THICKNESS T, IS CALCULATED BY USING THE RELATIONSHIP GIVEN BELOW.

CE-464 Ground Improvement

d w

 2 (

b

 

t

)

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)

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REVIEW - DESIGN CONSIDERATIONS

THE COEFFICIENT OF CONSOLIDATION FOR VERTICAL COMPRESSION AND HORIZONTAL FLOW DOMINATES THE DESIGN OF SAND DRAINS.

c h



k h

( 1 

a v

.



w e

0 )

t



T h

.

d e

2

c h

or

c h



m v k

.

h



w c v



m v k

.

v



w

(NOTE THE DIFFERENCE BETWEEN THE ABOVE EQUATIONS)

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C h IS PRINCIPALLY DEPENDENT ON kh BECAUSE m v IS LESS VARIABLE.

ONE WAY OF ESTIMATING k h , IS TO CONDUCT SPECIAL LABORATORY TESTS FOR RADIAL DRAINAGE IN UNIFORM SOILS USUALLY k h >k v (HANSBO(1960), ROWE(1964), BERRY AND WILKINSON(1969), PAUTE(1973).

MITCHEL AND GARDNER NOTED THAT THE BEST WAY IS TO MEASURE AV OR MV IN THE LAB AND KH, IN THE FIELD(USUALLY CH/CV RANGES BETWEEN 2- 10).

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FIELD CONTROL OF THE CONSTRUCTION

IT IS VERY IMPORTANT TO CHECK WHAT IS GOING ON AFTER THE DESIGN HAS BEEN DONE AND THE LOADING STARTED. MEASUREMENTS ARE INTEGRAL PART OF THE DESIGN PROCESS AND IT MAY NOT BE POSSIBLE TO COMPLETE A PRELOADING-DRAINS FIELD INSTRUMENTATION.

PROJECT WITHOUT THE SIMPLEST WAY IS TO MEASURE THE SURFACE SETTLEMENTS. PIEZOMETERS AT THE CENTRE AND OTHER ELEVATIONS OF THE SOFT CLAY AT THE MID-DISTANCE OF THE DRAINS AND THE SETTLEMENT GAUGES AT VARIOUS DEPTHS (BOREHOLE GAUGES) ARE USUALLY USED TO ASSESS AND TO CHECK THE DESIGN ASSUMPTIONS.

NECESSARY CHANGES ARE MADE IF REQUIRED.

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VARIOUS CASE HISTORIES INVOKING VERTICAL DRAINS TO SPEED UP THE CONSOLIDATION PROCESS WERE DESCRIBED BY JOHNSON (1970B), BJERRUM (1972) AND PILOT (1981).

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

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

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PLASTIC DRAINS ARE INCREASINGLY BEING USED.

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

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

IF P f <

s

c' DO NOT USE SAND DRAINS.

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

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

SITE INVESTIGATION EFFORTS MUST BE OF GOOD QUALITY.

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WELL RESISTANCE

DISCHARGE CAPACITY OF THE DRAINS = q w PERMEABILITY OF THE SOIL = k c

m

r

 ln(

n

)  0 .

75  

z

( 2

l



k z

)

q w c

WHERE l = LENGTH OF THE DRAIN WHEN OPEN AT ONE END ONLY (HALF LENGTH OF THE DRAIN WHEN OPEN AT BOTH ENDS) z = DISTANCE FROM OPEN END OF DRAIN (0 < z < 2 l ) WHEN q w / k c < 3000 m 2 WELL RESISTANCE CAN NOT BE IGNORED.

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SMEAR DISTURBANCE

m

s

 ln(

n

)

s



k k c c

' ln(

s

)  0 .

75 

z

( 2

l



z

)

k q w c

    1       

k k c c

'   

k k c c

'        1

n s

   2    

WHERE s=d s / d w ds = DIAMETER OF THE DISTURBED ZONE kc' = PERMEABILITY OF THE DISTURBED ZONE

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