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PRELOADING OR PRECOMPRESSION

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.).

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PRELOADING OR PRECOMPRESSION

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  1. 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

  2. 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. CE-464 GroundImprovement

  3. 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). CE-464 Ground Improvement

  4. “ 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; • 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. • TO DETERMINE THE LENGTH OF TIME REQUIRED TO ACHIEVE A GIVEN AMOUNT OF SETTLEMENT UNDER A GIVEN SURCHARGE LOAD. CE-464 Ground Improvement

  5. ONE-DIMENSIONAL CONSOLIDATION SETTLEMENT OF A LAYER 2H INTHICKNESS (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). CE-464 Ground Improvement

  6. FROM EQUATIONS (1) AND (2) THE RELATIONSHIP BETWEEN Uf+s , Pf / s'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 + s'vo ) > s’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. CE-464 Ground Improvement

  7. 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; CE-464 Ground Improvement

  8. SECONDARY COMPRESSION EFFECTS 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 CE-464 Ground Improvement

  9. IF NO PRIMARY AND SECONDARY SETTLEMENTS ARE DESIRED UNDER THE DESIGN LOAD Pf ; THE LOAD Pf + Ps MUST BESUSTAINED FOR A PERIOD tSR SUCH THAT THE TOTAL SETTLEMENT SSRIS THE SUM OF PRIMARY AND SETTLEMENTS UNDER PfALONE (FIGURE 4). CE-464 Ground Improvement

  10. THE DESIRED SETTLEMENT SSRIS 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. CE-464 Ground Improvement

  11. PREDICTION OF Dcu INCREASE; Ip and cu / s'vp or cu TRIAXIAL TESTS Dcu≈0.2Ds’v THAT IS THE REASON FOR STAGE CONSTRUCTION OF EMBANKMENTS ON VERY SOFT CLAYS. CE-464 Ground Improvement

  12. VERTICAL DRAINS CE-464 Ground Improvement 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.

  13. 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 ARE DOMINATED BY SECONDARY COMPRESSION. CE-464 Ground Improvement

  14. CE-464 Ground Improvement 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. (svo’+Pf)>svo’ (PRECONSOLIDATION PRESSURE) (FIgure 7) 2. Primary settlement/(Primary Settlement+Secondary settlement) MUST BE LARGE. 3. THERE SHOULD NOT BE NATURAL DRAINAGE LAYERS.

  15. EFFICIENCY FACTOR OF VERTICAL DRAINS ; CE-464 Ground Improvement

  16. CE-464 Ground Improvement FIELD MEASUREMENTS FROM 5 SITES INDICATE THAT SATISFACTORY e VALUES RANGE BETWEEN 0.6 AND 0.8. THE NEED AND EFFICIENCY OF DRAINS ARE LARGELY DEPENDENT ON SOIL CHARACTERISTICS, SOIL PERMEABILITY AND COEFFICIENT OF CONSOLIDATION. 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 AND FREQUENT SEAMS OR BANDS OF HIGH PERMEABILITY SOILS OFTEN MAKE VERTICAL DRAINS UNNECESSARY OR GREATLY REDUCE THEIR REAL EFFECTIVENESS (FIGURE 8).

  17. FIgure 8. ContInuous And Frequent Permeable SoIl Bands Often Make DraIns Unnecessary CE-464 Ground Improvement 3. SOIL INVESTIGATIONS ARE VERY IMPORTANT CONTINUOUS SAMPLING K FIELD AT LOW HEADS LARGE DIAMETER (25-30 CM) LABORATORY CONSOLIDATION TESTS.

  18. 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)

  19. SAND DRAINS CE-464 Ground Improvement 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 COUNTRIES.

  20. CARDBOARD DRAINS CE-464 Ground Improvement 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 WICK DRAINS VISUALLY CONSIST OF A CORE PLASTIC AND FILTER SLEEVE OF PAPER, FIBROUS MATERIAL OR POROUS PLASTIC.

  21. CE-464 Ground Improvement

  22. PLASTIC DRAINS THESE ARE THE NEW GENERATION DRAINS WHICH ARE VERY SIMILAR TO CARDBOARD DRAINS. THERE ARE SEVERAL COMMERCIAL BRANDS IN THE MARKET AT PRESENT LIKE ALIDRAIN, GEPDRAIN, CASTLEBOARD, COLBONT, MEBRADRAIN AND PVC DRAIN. SEE FIGURE 10 FOR GEODRAIN. CE-464 Ground Improvement

  23. SAND-WICKS CE-464 Ground Improvement 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).

  24. 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

  25. u=u0 at t=0 at all place u=u0 In the draIn at any tIme 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 kh=f(t) AND ch=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.

  26. where, (horIzontal tIme factor) CE-464 Ground Improvement 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 ;

  27. CE-464 Ground Improvement HANSBO(1979);F = F(n) + Fs + Fr WHERE; F(n) : DUE TO SPACING OF DRAINS Fs : SMEAR EFFECT Fr : WELL-RESISTANCE TIME FOR CONSOLIDATION IS; 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

  28. CE-464 Ground Improvement

  29. (VERTICAL CONSOLIDATION) (RADIAL CONSOLIDATION) or NOTE THAT mv = av / (1+e0) CE-464 Ground Improvement 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;

  30. FIgure 12. VarIatIon Of Uh WIth TIme Factor For VarIous N Values (Equal StraIn) CE-464 Ground Improvement

  31. CE-464 Ground Improvement CONSOLIDATION OF THE CYLINDRICAL BODY OF SOIL AROUND A VERTICAL DRAIN IS, IN FACT, THREE DIMENSIONAL AND IS GOVERNED BY THE EQUATION ; OVERALL (THREE DIMENSIONAL) DEGREE OF CONSOLIDATION, U=1-U=(1-UH).(1-UV) WHERE UV IS THE AVERAGE VERTICAL DEGREE OF CONSOLIDATION. THE ABOVE CONSIDERATIONS ARE FOR ALL TYPES OF VERTICAL DRAINS. THE EQUIVALENT DIAMETER DW OF A BAND SHAPED DRAIN OF WIDTH B, AND THICKNESS T, IS CALCULATED BY USING THE RELATIONSHIP GIVEN BELOW.

  32. 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) CE-464 Ground Improvement

  33. REVIEW - DESIGN CONSIDERATIONS or (NOTE THE DIFFERENCE BETWEEN THE ABOVE EQUATIONS) CE-464 Ground Improvement THE COEFFICIENT OF CONSOLIDATION FOR VERTICAL COMPRESSION AND HORIZONTAL FLOW DOMINATES THE DESIGN OF SAND DRAINS.

  34. CE-464 Ground Improvement Ch IS PRINCIPALLY DEPENDENT ON kh BECAUSE mv IS LESS VARIABLE. ONE WAY OF ESTIMATING kh, IS TO CONDUCT SPECIAL LABORATORY TESTS FOR RADIAL DRAINAGE IN UNIFORM SOILS USUALLY kh>kv (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).

  35. FIELD CONTROL OF THE CONSTRUCTION CE-464 Ground Improvement 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 PROJECT WITHOUT FIELD INSTRUMENTATION. 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.

  36. CE-464 Ground Improvement 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.

  37. CE-464 Ground Improvement 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 Pf < sc' DO NOT USE SAND DRAINS.

  38. CE-464 Ground Improvement 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.

  39. WELL RESISTANCE CE-464 Ground Improvement DISCHARGE CAPACITY OF THE DRAINS = qw PERMEABILITY OF THE SOIL = kc 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 qw/ kc < 3000 m2 WELL RESISTANCE CAN NOT BE IGNORED.

  40. SMEAR DISTURBANCE CE-464 Ground Improvement WHERE s=ds / dw ds = DIAMETER OF THE DISTURBED ZONE kc' = PERMEABILITY OF THE DISTURBED ZONE

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