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CE-464 Ground Improvement1 4.3 STONE OR SAND COLUMNS IN SOFT CLAYEY MATERIALS :  VIBRO - COMPOZER (JAPAN)  VIBROFLOATATION (VIBROREPLACEMENT) (EUROPEAN)

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Presentation on theme: "CE-464 Ground Improvement1 4.3 STONE OR SAND COLUMNS IN SOFT CLAYEY MATERIALS :  VIBRO - COMPOZER (JAPAN)  VIBROFLOATATION (VIBROREPLACEMENT) (EUROPEAN)"— Presentation transcript:

1 CE-464 Ground Improvement1 4.3 STONE OR SAND COLUMNS IN SOFT CLAYEY MATERIALS :  VIBRO - COMPOZER (JAPAN)  VIBROFLOATATION (VIBROREPLACEMENT) (EUROPEAN) m LAYERS ARE COMPACTED  CASING DRIVING  BORING (VERY SIMILAR TO PROCEDURES IN SANDS)  SOMETIMES INJECTION  MORTAR COLUMNS  m DIAMETER DEPENDING ON THE MATERIAL (up to 20 m)  CRUSHED ROCK mm popular  SQUARE OR TRIANGULAR PATTERN, m spacing

2 CE-464 Ground Improvement2  THEY PROVIDE STRENGTH REINFORCEMENT TO THE SOIL.  SETTLEMENT  STABILITY  THEY ACT LIKE VERTICAL DRAINS.  0.3 m BLANKET: DRAINAGE AND STRUCTURAL STRESS DISTRIBUTING LAYER.  COLUMNS SHOULD EXTEND TO A FIRMER SOIL BELOW.  BECAUSE OF THE RELATIVELY HIGH MODULUS OF THE COLUMNS A LARGE PROPORTION OF THE VERTICAL LOAD APPLIED TO THE GROUND SURFACE TRANSFERRED TO THE COLUMNS. THEY ARE SIMILAR TO PILE FOUNDATIONS (NO CAPS, STRUCTURAL CONNECTIONS!).

3 CE-464 Ground Improvement3  THERE ARE PROBLEMS IN APPLYING THE METHOD IN SENSITIVE SOILS.  BEARING CAPACITY OF THE SOIL WILL BE INCREASED AND SETTLEMENT WILL BE DECREASED.  FACTORS WHICH GOVERN THE SOIL-COLUMN BEHAVIOUR HUGHES ET.AL. (1975).  UNDRAINED SHEAR STRENGTH OF THE SOIL  IN- SITU LATERAL STRESS OF THE SOIL  RADIAL STRESS - STRAIN CHARACTERISTICS OF THE SOIL  INITIAL COLUMN DIMENSIONS   ’ & STRESS - STRAIN CHARACTERISTICS OF THE COLUMN MATERIAL  kN TYPICAL DESIGN VALUES IN SOFT TO MEDIUM CLAYS  ALLOWABLE VERTICAL STRESS  v ON A SINGLE COLUMN

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6 6  SETTLEMENT OF A STONE COLUMN FOUNDATION DEPENDS ON;  COLUMN SPACING  SOIL STRENGTH  IN SINGLE TESTS :  = mm SETTLEMENT UNDER THE DESIGN LOAD.  EXPERIENCE AND ANALYSES FOR FOUNDATIONS : s = 5-10 x (5-10 mm).

7 CE-464 Ground Improvement7

8 8  A CONSERVATIVE ASSUMPTION : TREATING LIKE PILES MORE RATIONAL APPROACH  LOADS ARE DISTRIBUTED BETWEEN SOIL & STONE COLUMNS vertical stress in the soft ground vertical stress in the compacted column

9 CE-464 Ground Improvement9 CONSOLIDATION SETTLEMENT OF THE UNTREATED GROUND: s = m s. . Hm s = COMPRESSIBILITY CONSOLIDATION SETTLEMENT OF THE COMPOSITE FOUNDATION SOIL: s ’ = m s.  c. H = m s.  c. .H AND THE SETTLEMENT REDUCTION RATIO  WILL BE ASSUMING THE CLAY AND THE COLUMNS SETTLE EQUAL AMOUNT

10 CE-464 Ground Improvement10  WHEN STRESS CONCENTRATION RATIO, N AND REPLACEMENT RATIO, AS INCREASE THERE IS GREATER REDUCTION IN SETTLEMENT  ALTERNATIVE ANALYSES BY PRIEBE (1976) AND HUGHES ET.AL.(1975) ARE BASED ON RADIAL EXPANSION AND VERTICAL COMPRESSION OF THE COLUMN  ISOLATED COLUMN CAPACITY for GRAVEL  r  MINIMUM PASSIVE SUPPORT BY THE SOIL  ' r = F.c u +  r – U 0 Bell 2c u +  ros

11 CE-464 Ground Improvement11  STONE COLUMNS ARE SOMETIMES USED ALSO FOR STABILITY INCREASE.

12 CE-464 Ground Improvement12 1.DRILL CAVITY USING AUGERS, INSTALL CASING IF CAVE-INS OCCUR. 2.PLACE CRUSHED STONE AT THE BOTTOM OF CAVITY. 3.RAM BOTTOM STONE WITH BEVELED TAMPER TO PRODUCE BULB. 4.DENSIFY CRUSHED STONE IN LAYERS OF 30 cm WITH TAMPER. 5.PRELOAD TOP OF RAP ELEMENT. Fig. Step Construction procedure of aggregate pier element

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14 CE-464 Ground Improvement14 Table Comparison of aggregate piers versus stone columns (courtesy of Geopier Co., 2003) STONE COLUMNSAGGREGATE PIERS TYPICAL LENGTH5-15m2-8m TYPICAL CENTER TO CENTER SPACING 4d2d THICKNESS OF LIFTS1.5-3 m20-30 cm ALLOWABLE FOUNDATION PRESSURE kPa kPa TYPICAL LENGTH DIAMETER RATIO CONSTRUCTION EQUIPMENT 6 m PROBE MOUNTED CRANE BACKHOE WITH 4 m LONG TAMPER & ACES

15 CE-464 Ground Improvement LIMITATIONS OF AGGREGATE PIERS DISADVANTAGES ASSOCIATED WITH AGGREGATE PIERS CAN BE CATEGORIZED INTO TWO CONSISTING OF ECONOMIC LIMITATIONS AND PERFORMANCE LIMITATIONS. THE REQUIREMENT OF A DRILLED CAVITY, AND THE FACT THAT ALMOST ALL THE SOILS REQUIRING IMPROVEMENT WITH AGGREGATE PIERS, BEING VERY SOFT AND COMPRESSIBLE, CAVITY COLLAPSE IS AN INEVITABLE ISSUE. TO PREVENT THIS, TEMPORARY CASING IS PLACED, AND ADVANCED ONCE THE BACKFILLING STAGE ONSETS. THIS SLOWS DOWN THE APPLICATION RATE AND INCREASES THE COST PER ELEMENT. ADDITIONALLY WHERE TREATMENT ZONE DEPTHS ARE REQUIRED TO BE GREATER THAN SAY 8 m, AGGREGATE PIERS SHALL NOT BE CONSIDERED AS A SOLUTION BECAUSE THEY GIVE BEST PERFORMANCE WHEN USED IN COMPRESSIBLE STRATA AS A FLOATING PILE TO DEPTHS UP TO 8 m.


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