deep compaction

CHAPTER 2DEEP COMPACTION : 

CE-464 Ground Improvement CHAPTER 2DEEP COMPACTION THE MAIN PURPOSE OF TREATING OR IMPROVING THE SOILS IS TO ELIMINATE THE DANGER OF EXCESSIVE SETTLEMENTS. LOOSE COHESIONLESS SOILS DO NOT POSE SERIOUS PROBLEMS IN TERMS OF STABILITY AND SETTLEMENT. LIQUEFACTION IS THE MAIN CONCERN FOR LOOSE COHESIONLESS SOILS UNDER WATER TABLE. OTHER DYNAMIC LOADS ALSO AFFECT THESE SOILS. METHODS FOR COHESIONLESS SOILS : - VIBROCOMPACTION TECHNIQUES - COMPACTION PILES

1. VIBROFLOATATION : 

CE-464 Ground Improvement 1. VIBROFLOATATION THIS IS NOT A NEW METHOD. (50 YEARS OLD). THE EQUIPMENT HAS THREE ELEMENTS. A) THE VIBRATOR OR VIBROFLOT B) EXTENSION TUBES C) CRANE (-T-WATER JET SUPPLY WATER, PUMP+GENERATOR, CABLES). VIBROFLOT IS A CYLINDRICAL PROBE WHICH PENETRATES INTO THE GROUND WITH THE AID OF VIBRATION (VERTICAL AND/OR HORIZONTAL) AND ITS OWN WEIGHT AND JETTING (WATER MOSTLY, FOR DEEP PENETRATIONS AIR OR AIR AND WATER).

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CE-464 Ground Improvement VIBROFLOT DIAMETERS ARE IN THE RANGE OF 0.35 - 0.50 m, LENGTHS 2-6 m. AN ECCENTRIC MASS IS AT THE LOWER PART OF THE VIBRO PROBE WHICH IS A HOLLOW STEEL TUBE WEIGHTS OF VIBROFLOTS ARE USUALLY IN THE RANGE OF 20 - 40 kN (2-4 ton) AND THEY DEVELOP HORIZONTAL CENTRIFUGAL FORCES OF 80-160 kN. VIBRATION AMPLITUDES ARE UP TO 20-25 mm AND USUAL OPERATING FREQUENCIES ARE 30-50 Hz (1800 - 3000 rpm) (35-100 kW POWER) WATER JETS MAY BE APPLIED AT TOP AND/OR BOTTOM.

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CE-464 Ground Improvement TYPICAL VIBROFLOT PENETRATION RATES: 1-2 m/mIn, VIBROFLOT WITHDRAWAL / COMPACTION RATES : 0.3 m/mIn. WATER JETTING PRESSURES UP TO 800 kN/m2, FLOW RATES UP TO 3000 lt/mIn (50 lt/sec). SAND BACKFILL IS SUPPLIED AT A RATE UP TO 1.5 m3/m RADIUS OF INFLUENCE, R FROM THE VIBRATOR DEPENDS ON THE TYPE OF VIBRATOR AND SOIL TYPE 0.6–0.9 m FOR 20 % OR MORE SMALLER THAN NO. 200 SIEVE AND 2 m FOR CLEAN SANDS.

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CE-464 Ground Improvement MOST EFFECTIVE IN CLEAN SANDS, HOWEVER WEBB & HALL (1968) REPORT GOOD SUCCESS IN SOILS CONTAINING APPRECIABLE PORTIONS OF SILT AND CLAY AS MUCH AS 30%. CAQUAT - KERISEL (1966) RECOMMEND COARSE SAND, GRAVEL OR COBBLES AS FILLING MATERIAL. WHEN DENSIFYING FINE SANDS AND SILTY SAND COARSE PARTICLES ARE MORE EFFECTIVE TO IMPROVE TRANSMISSION OF VIBRATION TO SURROUNDING SOIL BETTER. THERE ARE ALSO CASES ON RECORD WITH VERY FINE SAND, FINE SAND WITH LENSES OF MUD, FINE SAND WITH BALLS OF CLAY AND SILTS AND SANDS WITH CLAY LAYERS UP TO A METER.

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CE-464 Ground Improvement RELATIVE DENSITY, Dr,OF AT LEAST 70 % CAN BE OBTAINED BETWEEN CENTERS. INCREASE IN RELATIVE DENSITY IS BETWEEN 20% - 40%. qall AFTER TREATMENT IS USUALLY AROUND 2 - 3 kg/cm2 (COST ROUGHLY 2- 4 $/m3 USA RATE). THE DESIGN OF A VIBROFLOATATION TREATMENT REQUIRES SPECIFICATION OF THE DESIRED RELATIVE DENSITY AND THE PATTERN AND SPACING OF VIBROFLOT HOLES. A RELATIVE DENSITY OF AT LEAST 75 % HAS BEEN A FREQUENTLY USED REQUIREMENT TO INSURE AGAINST EXCESSIVE SETTLEMENT OR LIQUEFACTION, HOWEVER THE VALUE SPECIFIED IS A FUNCTION OF THE TYPE OF PROJECT AND THE LOCAL CONDITIONS.

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CE-464 Ground Improvement D'APPOLONIA, MILLER AND WARE (1955) SUMMARIZE SOME OF THEIR FINDINGS; TRIANGULAR PATTERN PREFERRED DUE TO GREATEST COMPACTION EFFORT OVERLAP. D, IS NOT INCREASED ABOVE 70% AT POINTS MORE THAN 1 m FROM A SINGLE VIBROFLOT COMPACTION. THE OVERLAPPING EFFECT FOR SPACINGS GREATER THAN 2.4 m IS SMALL. SPACINGS LESS THAN 1.8 m SHOULD GIVE Dr HIGHER THAN 70 % WITHIN THE COMPACTED AREA. THE EFFECT OF ADJACENT COMPACTIONS CAN BE SUPERIMPOSED. A PROCEDURE FOR DETERMINING VIBROFLOT SPACING REQUIRED TO OBTAIN A SPECIFIED Dr (mIn.) IS PRESENTED.

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CE-464 Ground Improvement WEBB & HALL (1968) ALSO DESCRIBE A SIMILAR METHOD FOR SILTY AND CLAYEY SAND. A SPACING OF 2.1 – 2.4 m IS OFTEN CHOSEN. CLEAN COARSE SANDS AND COARSER SOILS MAY REQUIRE 2.4 m OR MORE. FINER MATERIAL AND MATERIAL CONTAINING CLAY INCLUSIONS MAY REQUIRE MUCH CLOSER SPACINGS. RANGE (1.5 -3M) DEPENDS ON; - TYPE OF SOIL AND BACKFILL - PROBE TYPE AND ENERGY - LEVEL OF IMPROVEMENT REQUIRED

2. VIBRATING PROBES : 

CE-464 Ground Improvement 2. VIBRATING PROBES VIBRATORY PIPES OR PROBES MAY BE PUSHED INTO THE SOIL TO DENSIFY THE SOIL LAYERS WITHOUT ADDING MATERIAL AT DEPTH, ONLY SURFACE ADDITION IS DONE. ANDERSON(1974) USES PILE VIBRO-DRIVER HAMMER ON TOP OF A 0.76 m DIAMETER PIPE PILE (3-5 m LONGER THAN THE DEPTH OF TREATMENT). TYPICAL APPLICATION CHARACTERISTICS ARE; FREQUENCY 15 Hz, VERTICAL AMPLITUDE 10-25 mm, APPROXIMATELY 15 Probes /hr, 1-3 m SPACING, UPPER FEW METERS ARE NOT EFFECTIVELY DENSIFIED. SAITO (1977) USES AGAIN VIBRATORY PILE DRIVING HAMMER AND VIBRO-RODS

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3. VIBRO - COMPOZER METHOD : 

CE-464 Ground Improvement 3. VIBRO - COMPOZER METHOD COMPRESSED AIR IS USED INSIDE THE CASING TO HOLD SAND IN PLACE WHILE WITHDRAWING THE CASING MURAYAMA (1958)

4. SOIL VIBRATORY STABILIZING METHOD (SVS) : 

CE-464 Ground Improvement 4. SOIL VIBRATORY STABILIZING METHOD (SVS) IN SVS VERTICAL VIBRATION AND HORIZONTAL VIBRATION ARE APPLIED. SIMILAR TO VIBROFLOT SAND BACKFILL USED, WATER IS NOT USED.

5. COMPACTION PILES : 

CE-464 Ground Improvement 5. COMPACTION PILES DENSIFICATION FROM TWO EFFECTS DISPLACEMENT OF SOIL VIBRATION DURING DRIVING A CASING WITH DETACHABLE (LOST POINT) END PLATE OR WITH A SPECIAL OPENING MECHANISM AT THE TIP IS DRIVEN BACKFILL IS FILLED AS THE CASING IS WITHDRAWN. USUALLY 0.9 - 1.5M SPACING IS USED. BETTER IN FULLY SATURATED OR DRY SANDS.

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CE-464 Ground Improvement FRANKI MACHINES AND TECHNIQUE ARE ALSO VERY SUITABLE TO FORM SAND OR GRAVEL PILES. IF THE PERMEABILITY OF THE SOIL IS NOT HIGH, SAND OR GRAVEL PILES ALSO ACT AS DRAINAGE COLUMNS SAND DRAINS TO ACCELERATE SETTLEMENTS ARE DIFFERENT CONCEPT AS WE HAVE DISCUSSED PREVIOUSLY HORIZONTAL EXTENT OF THE COMPACTED ZONE ALONG THE SHAFT IS ABOUT 5 RADII. FRANKI TECHNIQUE IS MORE EFFECTIVE THAN A DRIVEN PIPE, MEYERHOF (1959).

SUITABILITY OF SOILS AND METHODS : 

CE-464 Ground Improvement SUITABILITY OF SOILS AND METHODS SOIL TYPE, GRADATION, FINES CONTENT ETC. DEGREE OF SATURATION, LEVEL OF G.W.T INITIAL DR s’V0 , s’H0 (OCR, sVMAX) INITIAL SOIL STRUCTURE, CEMENTATION, AGE ETC. SPECIAL CHARACTERISTICS OF THE METHOD USED VIBROCOMPACTION METHODS ARE BEST SUITED TO CLEAN COHESIONLESS SOILS WITH 20-25% MAXIMUM FINE CONTENT DUE TO LOW PERMEABILITY AND COHESION IN LOESS, SILTY SANDS WHERE FINES ARE HIGH IN %, TRY OTHER TECHNIQUES LIKE HEAVY TAMPING (DYNAMIC CONSOLIDATION)

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CE-464 Ground Improvement EVALUATION OF THE TREATED GROUND MAY BE DONE BY ONE OR TWO OF THE FOLLOWING METHODS; SURFACE SETTLEMENT MARKERS VOLUME OF ADDED SOIL (CRATERS OR VIBROCOMPACTION DISP.) STANDARD PENERATION TEST, SPT. CONE PENETRATION TEST, DUTCH CONE, CPT PRESSUREMETER TEST, PMT SEISMIC SHEAR WAVE VELOCITY DETERMINATIONS, VS PILE DRIVING RESISTANCES PLATE LOADING TESTS DOWN - HOLE DENSITY METERS.

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VIBROCOMPACTION GRIDS ARE SQUARE OR TRIANGULAR ASSUMING DENSIFICATION ONLY IN LATERAL DIRECTION (D<80CM)SQUARE TRIANGULAR (CONSIDER AVERAGE De) : 

CE-464 Ground Improvement VIBROCOMPACTION GRIDS ARE SQUARE OR TRIANGULAR ASSUMING DENSIFICATION ONLY IN LATERAL DIRECTION (D<80CM)SQUARE TRIANGULAR (CONSIDER AVERAGE De)

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CE-464 Ground Improvement