ce-464 ground improvement1 4.3 stone or sand columns in soft clayey materials : vibro - compozer...

CE-464 Ground ImprovementCE-464 Ground Improvement 11

4.3 STONE OR SAND COLUMNS4.3 STONE OR SAND COLUMNS

IN SOFT CLAYEY MATERIALS : IN SOFT CLAYEY MATERIALS : VIBRO - COMPOZER (JAPAN)VIBRO - COMPOZER (JAPAN) VIBROFLOATATION (VIBROREPLACEMENT) (EUROPEAN)VIBROFLOATATION (VIBROREPLACEMENT) (EUROPEAN)

0.4 -1.0 m LAYERS ARE COMPACTED0.4 -1.0 m LAYERS ARE COMPACTED CASING DRIVINGCASING DRIVING BORINGBORING (VERY SIMILAR TO PROCEDURES IN SANDS)(VERY SIMILAR TO PROCEDURES IN SANDS) SOMETIMES INJECTION SOMETIMES INJECTION MORTAR COLUMNS MORTAR COLUMNS

0.6 -1.0 m DIAMETER DEPENDING ON THE MATERIAL (up to 20 0.6 -1.0 m DIAMETER DEPENDING ON THE MATERIAL (up to 20 m)m)

CRUSHED ROCK 20 - 75 mm popularCRUSHED ROCK 20 - 75 mm popular SQUARE OR TRIANGULAR PATTERN, SQUARE OR TRIANGULAR PATTERN, 1.5 -3.5m 1.5 -3.5m spacingspacing


THEY PROVIDE STRENGTH REINFORCEMENT TO THE SOILTHEY PROVIDE STRENGTH REINFORCEMENT TO THE SOIL..

SETTLEMENTSETTLEMENT

STABILITYSTABILITY

THEY ACT LIKE VERTICAL DRAINSTHEY ACT LIKE VERTICAL DRAINS..

0.3 m BLANKET: DRAINAGE AND STRUCTURAL STRESS 0.3 m BLANKET: DRAINAGE AND STRUCTURAL STRESS

DISTRIBUTING LAYERDISTRIBUTING LAYER..

COLUMNS SHOULD EXTEND TO A FIRMER SOIL BELOWCOLUMNS SHOULD EXTEND TO A FIRMER SOIL BELOW..

BECAUSE OF THE RELATIVELY HIGH MODULUS OF THE BECAUSE OF THE RELATIVELY HIGH MODULUS OF THE

COLUMNS A LARGE PROPORTION OF THECOLUMNS A LARGE PROPORTION OF THE VERTICAL VERTICAL LLOAD OAD

APPLIED TO THE GROUND SURFACE TRANSFERRED TO APPLIED TO THE GROUND SURFACE TRANSFERRED TO

THE COLUMNS. THEY ARETHE COLUMNS. THEY ARE SIMILAR TO PILE FOUNDATIONS SIMILAR TO PILE FOUNDATIONS

(NO CAPS, STRUCTURAL CONNECTIONS!)(NO CAPS, STRUCTURAL CONNECTIONS!)..


THERE ARE PROBLEMS IN APPLYING THE METHOD IN SENSITIVE THERE ARE PROBLEMS IN APPLYING THE METHOD IN SENSITIVE

SOILS.SOILS.

BEARING CAPACITY OF THE SOIL WILL BE INCREASED AND BEARING CAPACITY OF THE SOIL WILL BE INCREASED AND

SETTLEMENT WILL BE DECREASED.SETTLEMENT WILL BE DECREASED.

FACTORS WHICH GOVERN THE SOIL-COLUMN BEHAVIOUR FACTORS WHICH GOVERN THE SOIL-COLUMN BEHAVIOUR

HUGHES ET.AL. (1975).HUGHES ET.AL. (1975). UNDRAINED SHEAR STRENGTH OF THE SOILUNDRAINED SHEAR STRENGTH OF THE SOIL

IN- IN- SITU LATERAL STRESS OF THE SOILSITU LATERAL STRESS OF THE SOIL

RADIAL STRESS - STRAIN CHARACTERISTICS OF THE SOILRADIAL STRESS - STRAIN CHARACTERISTICS OF THE SOIL

INITIAL COLUMN DIMENSIONSINITIAL COLUMN DIMENSIONS

’ ’ & STRESS - STRAIN CHARACTERISTICS OF THE COLUMN & STRESS - STRAIN CHARACTERISTICS OF THE COLUMN

MATERIALMATERIAL

200200- 300 kN TYPICAL DESIGN VALUES IN SOFT TO MEDIUM- 300 kN TYPICAL DESIGN VALUES IN SOFT TO MEDIUM

CLAYSCLAYS

ALLOWABLE VERTICAL STRESS ALLOWABLE VERTICAL STRESS v v ON A SINGLE COLUMNON A SINGLE COLUMN


SETTLEMENT OF A STONE COLUMN FOUNDATION SETTLEMENT OF A STONE COLUMN FOUNDATION DEPENDS ONDEPENDS ON;;

COLUMN SPACINGCOLUMN SPACING SOIL STRENGTHSOIL STRENGTH

IN SINGLE TESTS : IN SINGLE TESTS : = 5 - 10 mm SETTLEMENT UNDER THE = 5 - 10 mm SETTLEMENT UNDER THE DESIGN DESIGN LLOADOAD..

EXPERIENCE AND ANALYSES FOR FOUNDATIONS : EXPERIENCE AND ANALYSES FOR FOUNDATIONS :

s = 5-10 x (5-10 mm)s = 5-10 x (5-10 mm)..

..

25

FS

cuv


A CONSERVATIVE ASSUMPTION : TREATING LIKE PILES A CONSERVATIVE ASSUMPTION : TREATING LIKE PILES

MORE RATIONAL APPROACH MORE RATIONAL APPROACH LOADS ARE DISTRIBUTED LOADS ARE DISTRIBUTED

BETWEEN SOIL & STONE COLUMNSBETWEEN SOIL & STONE COLUMNS

c

sn

vertical stress in the soft ground

vertical stress in the compacted column


.)1(1

.

.)1(1

cs

s

cs

c

an

n

an

CONSOLIDATION SETTLEMENT OF THE UNTREATED GROUND:

s = ms. . H ms= COMPRESSIBILITY

CONSOLIDATION SETTLEMENT OF THE COMPOSITE FOUNDATION

SOIL:

s’ = ms. c . H = ms.c..H

AND THE SETTLEMENT REDUCTION RATIO WILL BE

sc an )1(1

1

ASSUMING THE CLAY AND THE COLUMNS

SETTLE EQUAL AMOUNT


WHEN STRESS CONCENTRATION RATIO, N AND REPLACEMENT RATIO, AS INCREASE THERE IS GREATER REDUCTION IN SETTLEMENTWHEN 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 ALTERNATIVE ANALYSES BY PRIEBE (1976) AND HUGHES ET.AL.(1975) ARE BASED ON RADIAL EXPANSION AND VERTICAL

COMPRESSION OF THE COLUMNCOMPRESSION OF THE COLUMN ISOLATED COLUMN CAPACITYISOLATED COLUMN CAPACITY

for GRAVELfor GRAVEL

r r MINIMUM PASSIVE SUPPORT BY THE SOIL MINIMUM PASSIVE SUPPORT BY THE SOIL

''rr = F.c = F.cuu + + rr – – UU00

Bell 2cBell 2cuu + +rosros

)2

'45(tan

'

' 2

r

v


STONE COLUMNS ARE SOMETIMES USED ALSO FOR STONE COLUMNS ARE SOMETIMES USED ALSO FOR

STABILITY INCREASE.STABILITY INCREASE.


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


Table 2.2 - Comparison of aggregate piers versus stone columnsTable 2.2 - Comparison of aggregate piers versus stone columns(courtesy of Geopier Co., 2003)(courtesy of Geopier Co., 2003)

STONE COLUMNS AGGREGATE PIERS

TYPICAL LENGTH 5-15m 2-8m

TYPICAL CENTER TO CENTER SPACING

4d 2d

THICKNESS OF LIFTS 1.5-3 m 20-30 cm

ALLOWABLE FOUNDATION

PRESSURE25-150 kPa 250-300 kPa

TYPICAL LENGTH

DIAMETER RATIO5-30 2-4

CONSTRUCTION EQUIPMENT

6 m PROBE MOUNTED CRANE

BACKHOE WITH 4 m LONG TAMPER &

ACES


2.3.4 LIMITATIONS OF AGGREGATE PIERS2.3.4 LIMITATIONS OF AGGREGATE PIERS

DISADVANTAGES ASSOCIATED WITH AGGREGATE PIERS DISADVANTAGES ASSOCIATED WITH AGGREGATE PIERS CAN BE CATEGORIZED INTO TWO CONSISTING OF CAN BE CATEGORIZED INTO TWO CONSISTING OF ECONOMIC LIMITATIONS AND PERFORMANCE LIMITATIONS.ECONOMIC LIMITATIONS AND PERFORMANCE LIMITATIONS.

THE REQUIREMENT OF A DRILLED CAVITY, AND THE FACT THE REQUIREMENT OF A DRILLED CAVITY, AND THE FACT THAT ALMOST ALL THE SOILS REQUIRING IMPROVEMENT THAT ALMOST ALL THE SOILS REQUIRING IMPROVEMENT WITH AGGREGATE PIERS, BEING VERY SOFT AND WITH AGGREGATE PIERS, BEING VERY SOFT AND COMPRESSIBLE, CAVITY COLLAPSE IS AN INEVITABLE COMPRESSIBLE, CAVITY COLLAPSE IS AN INEVITABLE ISSUE. TO PREVENT THIS, TEMPORARY CASING IS PLACED, ISSUE. TO PREVENT THIS, TEMPORARY CASING IS PLACED, AND ADVANCED ONCE THE BACKFILLING STAGE ONSETS. AND ADVANCED ONCE THE BACKFILLING STAGE ONSETS. THIS SLOWS DOWN THE APPLICATION RATE AND THIS SLOWS DOWN THE APPLICATION RATE AND INCREASES THE COST PER ELEMENT.INCREASES THE COST PER ELEMENT.

ADDITIONALLY WHERE TREATMENT ZONE DEPTHS ARE ADDITIONALLY WHERE TREATMENT ZONE DEPTHS ARE REQUIRED TO BE GREATER THAN SAY 8 m, AGGREGATE REQUIRED TO BE GREATER THAN SAY 8 m, AGGREGATE PIERS SHALL NOT BE CONSIDERED AS A SOLUTION PIERS SHALL NOT BE CONSIDERED AS A SOLUTION BECAUSE THEY GIVE BEST PERFORMANCE WHEN USED IN BECAUSE THEY GIVE BEST PERFORMANCE WHEN USED IN COMPRESSIBLE STRATA AS A FLOATING PILE TO DEPTHS COMPRESSIBLE STRATA AS A FLOATING PILE TO DEPTHS UP TO 8 m.UP TO 8 m.

ce-464 ground improvement1 4.3 stone or sand columns in soft clayey materials : vibro - compozer...

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