Chapter 11 A full-scale study on cement deep mixing in soft Bangkok clay

Chapter 11

A Full-Scale Study on Cement Deep Mixing in Soft Bangkok Clay Dennes T. Bergado and Glen A. Lorenzo

Geotechnical and Geoenvironmental Engineering Program School of Civil Engineering, Asian Institute of Technology, Bangkok, Thailand

ABSTRACT A case history of full-scale deep mixing improved soft clay ground overlain by a 6.0 m high reinforced test embankment is presented. The deep mixing piles were constructed in the ground using the jet mixing technique with cement slurry employing a jet pressure of 20 MPa. Comparison was made to another reinforced test embankment of almost the same height constructed previously on unimproved soft clay foundation. Excess pore pressure buildup during soil-cement pile installation by jet mixing was monitored. The surface and settlements as well as lateral movements were monitored during and after embankment construction. The deep mixing improvement has effectively reduced the settlement and lateral movement of the foundation soil by as much as 70% and 80%, respectively. The local differential settlement between deep mixing pile and its surrounding soil amounted to 8-20% of the average total settlement of the improved ground, and could induce downdrag skin friction on the pile.

1. INTRODUCTION Soft clay deposits which are inherently very low in strength, very high in compressibility and prone to subsidence when there is excessive deep well pumping in nearby areas are widespread in coastal and lowland regions; and several major cities in the world are strategically situated on this type of geological deposits. Due to these inherent undesirable engineering characteristics of soft clay deposits, geotechnical engineers have always been confronted with not only the apparent but also subtle problems in providing the most appropriate shallow and deep ground improvement techniques so as to meet the engineering requirements necessary for the design and construction of associated infrastructure facilities. 305

306

Chapter 11

One of the techniques of improving thick deposit of soft ground is deep mixing method (DMM). This technique has been successfully applied in Thailand for almost a decade as foundations to highway embankments as well as to low to medium rise buildings, as retaining structure for excavation works, etc. (see e.g. Bergado et al., 1999; Petchgate et al., 2003). In DMM, the chemical agents, which are either powder or slurries of lime or cement are mixed into the ground to form soil-cement piles. When cured these improved column of soil-cement piles would stabilize and harden and, then, act as reinforcements of the soil thereby increasing the load-bearing capacity of the soft ground. In Asia, the use of cement in deep mixing practice is more common than the use of lime. Moreover, the methods of mixing generally applied in the installation of DMM piles are either mechanical mixing or jet mixing (Kamon and Bergado, 1991; Porbaha, 1998). In the mechanical mixing, the chemical admixtures are mixed into the soil by mixing blades; while in jet grouting or jet mixing, the mixing is done by jet of water or slurries of admixtures. The deep mixing and jet mixing methods would normally produce high water content cementadmixed clay; besides the soft clay deposit normally has high water content. The consequent stabilization of the soil after mixing the cement admixture is attributed to ion exchange, flocculation, and pozzolanic reactions that happen in the mixtures over a span of time. In this chapter, a case history of full-scale cement deep mixing improved soft Bangkok clay in Thailand is presented. This full-scale improved ground was loaded with a 6.0 m high reinforced test embankment, and was instrumented and monitored within 1 year. The purpose of this full-scale test is to study the characteristics of deep mixing employing jet mixing method as a ground improvement technique for soft Bangkok clay. The behavior of this full-scale test embankment was then compared to that of another reinforced fullscale test embankment of almost the same height but constructed on unimproved foundation, in order to clearly illustrate the effects of cement deep mixing in soft clay ground.

2.

REINFORCED TEST EMBANKMENT ON CEMENT DEEP MIXING (TEDM)

2.1. Project site and subsoil profile The site of this full-scale study was near the Electricity Generating Authority of Thailand (EGAT) Power Station Site at Amphur Wangnoi, Ayuthaya, Thailand. The site was underlain by the well-known soft Bangkok clay deposit. The foundation soils and their properties at the site are shown in Figure 1. The site had a newly placed 1.5 m-thick clay backfill, since it was situated within a reclaimed parcel of land. The soft clay, which is overlain by 1.0-m-thick weathered crust and 1.5-m-thick clay backfill, was encountered from 2.5 to 9.0 m depth. The undrained shear strength obtained from field vane test of the soft clay was less than 15 kPa. Underlying the soft clay layer is medium to stiff clay layer, having strength of more than 50 kPa.

A Full-Scale Study on Cement Deep Mixing in Soft Bangkok Clay

307

Unit weight (kN/m 3) 12

14 16 18 20

0 -1 -2

-g_.

Clay backfill

Weathered clay

i !/

-3

-5

~tm

-6 -7

Soft clay

I1\

t llt

-4 ,,..,. vE

\\\

/

t'

l

-8

,

-9 -10

Po

9 Unit weight Medium stiff clay + Gs

-11

ii it ......

2.6 2.65 2.7 2.75 2.8 20 40 60 80 100120 0

Gs

PL, w N, LL

20 40 60

80 0

P

, max

50 100 150 200 0

CorrectedSu(kPa) P'oand P'max (kPa) from Vane Shear Test

0.2

0.4 CR

0.6 0

0.04

0.08

RR

Figure 1. Soil profile below the reinforcedtest embankmentTEDM.

2.2.

Installation of deep mixing piles by jet grouting

The foundation subsoil was first improved with deep mixing piles which were installed in situ by jet mixing method employing a jet pressure of 20 MPa. The jet mixing equipment used for the deep mixing is shown in Figure 2. The deep mixing piles were installed at 1.5 m spacing in square pattern, except for the perimeter soil-cement piles which were installed at 2.0 m spacing (Figures 3-5). The water-cement ratio (W/C) of the cement slurry and the cement content employed for the construction of deep mixing piles were 1.5 and 150 kg/(m 3 of soil), respectively. Each deep mixing pile has diameter of improvement of 0.5 m and length of 9.0 m, which is up to the bottom of the soft clay layer, as shown in the section views of the embankments (Figures 4 and 5). The deep mixing piles were allowed to cure and the dissipation of excess pore water pressure was monitored until about 80 days before the embankment was constructed. The strength properties of the deep mixing piles with depth are shown in Figure 6.

2.3.

Construction of reinforced embankment

The embankment was made of well-compacted silty-sand backfill reinforced with PVCcoated hexagonal wire mesh. The backfill soil has compacted unit weight of 18.20 kN/m 3, cohesion of 7.70 kPa and angle of internal friction of 22 ~ and it has maximum dry density and optimum water content of 16.1 kN/m 3 and 15%, respectively. During construction, the embankment filling was done at 0.375 m lift thickness and was compacted to at least 98% of the maximum dry density of fill material (Bergado et al., 2002). To support the vertical side of the embankment, concrete facing with dimensions of 1.50 m X 1.50 m • 0.15 m were installed, each being held by two layers of hexagonal wire mesh reinforcements resulting to

Chapter 11

308

Figure 2. Jet mixing machine used in deep mixing.

~..

21

r L Z.5_m, 1

7.5 m. L2m. 1

o

{

~, 2 m . ]

o

m.

,.

6 m.

o

0 0

0

o

O 0

{

7.5 m.

o

0 0

.

t, 2 m . t. 2 m . l "1<~1

o

0

0

.

j,

i, 2 m - t , 2 m. 1 , 2 m : 1 1 1

o

.

]o

TM

m.

t, 1

o/Y

1 rn.

-t

+

'o/4 I

0 0

t, 2 m . . ~ . 2 . 5 I

~ ~I

1.5

m.

1.5

m.

f

0

2

o

o

o~

o~

o

c~olo

o

-.~

m. 12

m.

i

0

0

o

c)

_

o~~ .~~ ~4 ~

""

'"~

".",,'E

o

~

0

lO

p71~Z/rTE3 ~

""

"~'~"

() os ~ ~

t

~ . . . .

:I" :~a:i)~::: ~

i

_

_

.~

........

"'

85Tu~m~t

~....

~

-t

Scale 1 ". 200

Figure 3.

2m.

Plan view of the reinforced test embankment TEoM.

O.5

4 m.

A Full-Scale Study on Cement Deep Mixing in Soft Bangkok Clay l

17m

t

4m 6m J

l

6m

l

6m

l

309

14.5m

1"i 2m ,tl.5ml]1

HEXAGONAL WIRE MESH REINFORCEMENT

1 DUMMYREINFORCEMENTFOR I FIELDPULL-OUTTEST /IIII II II III I It~t]-t111I I I t . . . . IXEXTENSOMETERWIRES--~

Y VERTIC,~LPRE-CAST llFrrt'tlHl~ [ I_.. _ __J.[ CONCRIFTE FACING ~] [I [LI!....!!....!!:"[ .......]~'*~'X~;-~MPACTED----]-~ \ / l-----.l~:H'lt:Zff:::ff:~~qO'~YA-SA.b------T-t A I ,,~ 11III I,I, II II-IFI I BAGKFILXI~ | r \

/ II

ItHH-ktHt-~:t.----t

P'EZOMETERS/ 1t~ . . . . . . .

REFERENCE / I

I

I

~

................ \

~tI~H H n l

f

~ i ~ ~ : : ~ : l l : l * ff~ 1T~'\ ~"

lii ii. ,i

.it t.H..

1

~

E

8ml

E

I

P1-P

II

!

PII'P5 1611 1711 ! 1 1 1

/I

~-~ \

E----~NSOMETER

SETTLEMENT

!

I I

I

NOTE: PLEASESEETHE

41 6 @ 1.5m : 9m PLANVIEWFORDETAILS ' I OFTHELOCATIONOFOTHER I INTRUMENTATIONPOINTS | / J.15m depth

i

REFERENCE

,,\

................... \

Ill I ,T"IolI'IEI'I'

3m|

.........

\

...................... ~ . . . . . . ~ ~.IPPQRT

1 l I'5mcLAYBACKFILL l m WEATHERED CRUST

//

L

~

~ 2m 41m~ ~ MEDIUMSTIFFCLAY " " 9 0.5m DIAMETERSOIL-CEMENT PILESAT 1.5MSPACING IN SQUAREPATTERN

SECTION OF MSE E M B A N K M E N T

Figure 4. Longitudinal cross-section of the reinforced test embankment TEDM.

,1 i Illi

1/11

SUMMYREINFORCEMENT FORFILOPULL-OUTTEST i -,._ ~ 6.0rn

ittl !

.

,

J - 1. ! ll~_l_ll t I:__IZ l I

~ I

1 1

P5/3

I

!

I 5

3 P Pr

I

I

~

[~

r 5

S

5.7 m

1. Figure 5. Transverse cross-section of the reinforced test embankment TEDM.

310

Chapter 11 O-1 -2 -3

~

Clay backfill l

@

Weatheredclay i Typical ideep mixing pile

-4

~-5 -6

Soft clay

-7

-8

-9 -10 -11

Mediumstiff clay '

'

I

'

'

I

'

'

I

.

.

.

.

.

'

I

'

'

'

'

I

'

'

'

'

300 600 900 1200 1500 40000 60000 80000 100000 Unconfined compression strength, qu Modulusof elasticity, Eu50,p (kPa) (kPa) Figure 6. Strengthproperties of the deep mixingpiles. a vertical spacing of the reinforcements of 0.75 m (Figure 4). All reinforcements were 4 m long and were laid horizontally behind the concrete facing. Dummy hexagonal wire mesh reinforcements were also placed at selected heights of the embankment for subsequent fullscale field pullout test. The finished embankment is 6 m high. The embankment construction was completed within 15 days, started on 28 January 2002 and ended on 12 February 2002, as shown in the stages of construction in Figure 7.

2.4. Instrumentations and monitoring The embankment and the improved foundation were instrumented. Piezometers (P), which monitored the dissipation of excess pore water pressures in the foundation soils during and after deep mixing (jet grouting), were installed at various points underground within and outside the embankment zone (Figures 3 and 4). Surface settlement plates (S) were installed both "on pile" and "on clay" at the bottom of embankment. Deep settlement (DS) plates were also installed at 3.0 and 6.0 m depths at few locations as shown in Figures 3 and 4. In addition, vertical and horizontal inclinometers were placed near the vertical side of the embankment to measure the lateral displacement and settlement profile, respectively. Due to the subsequent development in the site made by the landowner, the embankment was demolished and all monitoring was eventually terminated after 12 months.

A Full-Scale Study on Cement Deep Mixing in Soft Bangkok Clay

I

~" 5 v

.E 4 |

t--

311

"

3

S

rr rl=

LT_ 2 1

,

o

,

3

,

,

i

6

,

,

i

9

,

,

!

,

,

,

,

,

,

12 15 18 Time (Days)

,

,

i

21

,

,

i

24

,

,

i

,

27

,

30

Figure 7. Stagesof construction of test embankmentTEDM.

3. FULL-SCALE REINFORCED TEST EMBANKMENT ON SOFT GROUND (TEtj) This full-scale test embankment on soft ground is hereinafter designated as TE u, which means Test Embankment on unimproved soft ground. The Test Embankment TE u, a 5.7 m height embankment reinforced with steel wire grid reinforcement, was constructed within the campus of Asian Institute of Technology, Bangkok, Thailand as part of the US Agency for International Development (USAID) sponsored research project. There was no improvement being made in the foundation subsoil. The three uppermost layers of the subsoil from the existing ground surface at the site of TE u consist of 2.0-m-thick weathered clay, 6.0-mthick soft to medium-stiff clay layer, and stiff clay layer, as shown in Figure 8. The steel wire grid reinforcement used in this test embankment system consisted of W4.5(6.1 mm) • W3.5(5.4 mm) galvanized steel wire mesh with 152 mm • 228 mm grid openings in the longitudinal and transverse directions, respectively. Each reinforcement unit had dimension of 2.44 m wide and 5.72 m long including the facing. The embankment was divided into three sections along its length, and three different backfill materials, namely, clayey sand, lateritic soil, and weathered clay were used, respectively, in each section (Figure 9a). The embankment construction was completed within 1 month, started on April 24, and ended on May 24, 1989. The configurations of entire embankment system are shown in Figures 9a and b. The embankment and its foundation subsoil were extensively instrumented to monitor their construction and post-construction performances (Bergado et al., 1991).

4.

BEHAVIOR OF DEEP MIXING IMPROVED SOFT CLAY UNDER TEST

EMBANKMENT TEl)M

4.1.

Effect of deep mixing pile installation by jet mixing on the surrounding soil Excess pore water pressure was developed in the foundation soil during the installation of deep mixing piles by jet mixing method employing a jet pressure of 20 MPa. The excess pore pressures in the foundation soils at 3 and 6 m depths after installation of deep mixing

Chapter 11

312

SOIL LAYERS

ep 4o 6o ~,.!oo

L

I-

WEIGHT ( kN/ms )

ATTEflBERG UMrrS AND NATURAL m r m C O N m ~ ( % ) 15 |

16 !

IT i

VANE SHEAR STRENGTH kPo

~,

~o

4o,

CONE RESISTANCE kPo 2000 4000 6 0 0 0 800(

BROWN TO REDOISH BROWN WEATHERED CLAY C

2

;

0 : ;0

C:

C

5-

-" ~,0

C 4-

~mJc ~ m r s o ~ Ct.AV, OFTEN WITH DEOOMPO5S[O WOODANO SAM)Y ~dVS E

6-

C . . . .

C

-"

C

-_.~

0

~

89LIBHT ~m~t' STIFF Ct,AY PL Wn

LL

0 TEST NO. I A TEST NO. 2 13 TEST NO. 3

II12IS-

Figure 8. Subsoilat the site of test embankmentTEv. piles are shown in Figures 10 and 11, respectively. The piezometers installed outside but near the improved foundation at 3 m depth were designated as p 1/3, p2/3, p3/3, p4/3, and p5/3 in Figure 10, while those installed within the improved ground were designated as p6/3, p7/3, and p8/3 in Figure 10 (refer to Figures 3 and 4 for the locations of these piezometers). Similarly, the piezometers installed outside but near the improved foundation at 6 m depth were designated as p 1/6, p2/6, p3/6, p4/6, and p5/6 in Figure 11, while those installed within the improved ground were designated as p6/6, p7/6, and p8/6 in Figure 11. The data indicated that there was relatively higher excess pore pressure being developed at 6 m depth than at 3 m depth, and this trend was observed both within and outside the improved foundation. Therefore, the development of excess pore water pressure was affected by the overburden pressure, and it tended to be higher at deeper depths. The average excess pore pressures just after the installation were 9.2 and 27 kPa at 3 and 6 m depth within the improvement zone, respectively. After 70 days of dissipation, these excess pore pressures decreased to 1.6 and 8.3 kPa, respectively. In addition, Figure 11 demonstrated that higher excess pore pressure was developed at points located within the improved foundation than at those points outside the improvement zone at 6 m depth; however, this phenomenon was not obvious at 3 m depth as shown in Figure 10. The latter observation is indicative that the jet of water during jet mixing operation must have traversed a wider distance at shallower depth, thereby causing higher excess pore water pressure even to those points located at the proximity of the improvement zone as demonstrated by the piezometers installed near the perimeter of the improvement zone.

A Full-Scale Study on Cement Deep Mbcing in Soft Bangkok Clay

i

J

i

i

I I

..-_.-.. INS'rlUIEN'T~

t

W4.6 z al' 3.5, 6"xS"

313

i

.J.44m.

,

,

EaCH !111~

|0.15 m z 0.325m) ; " - " " ~- - - ' - ' r

~

~

To.4Sm TIX46m

T S.70 m

t i~_ ~

,

. t

4.88 m-'---q,e--- 4,88 m

-f,

ill

t,,~--.-,-S,'tO m . . . . .

-,--

,. i . |

IECTIOII I w,----.-----CLAyEy

IN4)

l

It -

-

-'~

~

II .II IdL'rl~rl~ ~

~

4.68 m ~

5.70 m

~ III WEAI'I'IEREI~ CLAY -

-

""

A.-.I (a) r,,_;,..

5.03 m . .

.

.

.

.

.

~

0.61m ~;:.

--'

0.45 mI 0.45 m~. 0.45 m~ 5.70m

INS'rRu!dENTATION

W 4.5 = W 3.5, Q":t 9"(0.15m : 0.225

~.

L

t4 I.AYIgtS (TYPICAL.)

NOTE " MAT NOll. I TO ?' ARE INS~rRUIIIEN'rED MAT NOS. 8 "TO14 ARE NOT INSTRUMENTED (b) F i g u r e 9.

(a) L o n g i t u d i n a l s e c t i o n o f test e m b a n k m e n t T E v, (b) T r a n s v e r s e s e c t i o n o f test e m b a n k m e n t T E v.

Therefore, jet mixing operation may not be possible near an existing structure because it can cause excess pore water pressure to develop at the proximity of adjacent existing foundation. The development of excess pore water pressure can eventually reduce the bearing

314

C h a p t e r 11

70

~at

9

0

3m depth

I

/1!:~',~!

50-

r~

---qz>- P1/3 ---o-- P2/3 + P3/3 P4/3

/ / ~

60 --

9

I

9

[

9

9

oPt/3 [

----~--P5'3~ P6'3 ~ P7/3

I

40-

30-20 V

1 0 ~ !~'LY

/,,_,

,_,

_o

o~ =~

I

'

10

I

t

20

I

'

I

'

I

I

'=

30 40 50 Time after installation (days)

I\~

'

60

70

Figure 10. Excess pore pressure at 3 m depth after installation of deep mixing piles by jet mixing method.

70 -

60 "~"

_

4P8/~

~~ pp54/ /66

]P7/~ ~P6/6

e

p6/6

.....

~

p7/6

P1/6 P2/6 P3/6 P4/6 P5/6

e

-

9

40-

~ 82

~,,,,,,,,,

0 -J

0

10

20

pl/6

----o-- p2/6 ~ p3/6

at 6m depth /

50-

~D =

---c>-

~

"D~_

p8/6

_zU/r7 ; :o~~~,

. ~ g

30 40 50 Time after installation (days)

60

70

Figure 11. Excess pore pressure at 6 m depth after installation of deep mixing piles by jet mixing method.

A Full-Scale Study on

Cement

Deep Mixing in Soft Bangkok Clay

315

resistance of the nearby foundation and, hence, can lead to excessive settlement and even collapse of the existing structure. 4.2. Construction and consolidation settlement Figure 12 shows the settlements on top of deep mixing piles and on the surface of surrounding clay during and after construction up to 1 year of full embankment loading. From these actual observed data, the average settlements on deep mixing pile and on clay amounted to about 122 and 162 mm, respectively, after embankment construction. One year after embankment construction, the average settlements on deep mixing pile and on clay amounted to about 285 and 335 mm, respectively. Hence, the corresponding average settlement at the bottom of the reinforced soil is about 310 mm 1 year after embankment construction. Using the method of Asaoka (1978), the average total settlements of deep mixing pile and of the surrounding soil were predicted as shown in Figure 13 using the data recorded from settlement plates S 11 and S 15, which demonstrated the average settlement of pile and surrounding soil, respectively. The average total settlements of deep mixing pile and the surrounding soil amounted to 340 and 440 mm, respectively. Thus, about 40% of the total settlement occurred during construction of embankment. Moreover, if there had been no improvement in the foundation soil, the settlement of embankment 1 year after construction could have been > 1000 mm (Bergado and Lorenzo, 2003). Thus, the embankment load (weight of embankment) has been transferred to the deep mixing piles, thereby not only reducing the intensity of pressure on the surrounding

~,8 E 6

j

azz

9~~D 4 .----2 l,)

,

,

,

i

,

,

,

i

|

,

,

i

i

,

,

,

i

0

60

120

180

240 Time (Days)

300

360

0

60

120

180

240

300

360

0

I

I

I

I

l

,

,

Hollow symbols: on clay[

~ 150

-"

- i~ ~

.= i250 ~,~ 300 350

~

[

|

480

480

.... '

"$9 O S13 ....

.....o*".......S1451o "- Sll z~ S 1 5 - 9 $2 i $6 I--- S1 [] $5

4O0

Figure 12.

,

I

50 :flit E 100 " ] k_.. 200

,

420

I

Solid symbols: on pile

i

420

Comparison of surface settlements at designated settlement plates.

316

Chapter 11

500

I I I I I I I

450

9

on clay, S15

ill

on pile, S 11

400

i

/

45 degree line

f

/

/

/I

I

350

/

J

~vj

/

r"

300 r~" 250 d

200 m

150 |

100 50 0

!11

|

/

/

0

/

/

,r

V

f

/

100

200

300

400

500

Si_l F i g u r e 13.

Prediction of total settlement using Asaoka's observational method.

clay, hence the magnitude of its settlement, but also increasing the bearing capacity of the improved foundation. The deep mixing piles have, therefore, transferred the load down to their bottom ends and, consequently, effected a settlement reduction in the soft clay foundation (Figure 1) of about 70%. The deep mixing piles also promoted faster rate of consolidation of the improved foundation. The degree of consolidation of the improved ground was almost 90% 1 year after construction, as can be interpreted from the predicted total settlement and the settlement after 1 year. For S 11 and S 15, for example, the settlement of pile and clay were 298 and 362, respectively, 1 year after embankment construction; hence, the corresponding degree of consolidation of the improved ground was about 86% on the average, which is almost 90%. Besides, the settlement-time plot in Figure 12 also confirmed this observation. If there had been no improvement in the 6.5-m-thick soft clay (Figure 1), the 90% consolidation settlement could have been attained 9 years after construction (assuming actual coefficient of consolidation of soft clay, C v = 4 m2/year). Moreover, the time-settlement plot obtained from deep settlement plates installed at 3 m and 6 m depth (Figure 13) also confirmed the faster rate of consolidation settlement of the deep mixing improved ground. Figure 14 demonstrated that both settlements at the surface, at 3 m depth and at 6 m depth indicated the same pattern of consolidation behavior, which implied that the rate of consolidation became almost uniform over the entire depth of improvement due to the presence of deep mixing piles.

A Full-Scale Study on Cement Deep Mixing in Soft Bangkok Clay

317

Time (Days) 0

60

0 I ~------'-,-J~-_ , 50 i ~

120

180

240

300

360

420

6mdenth

,00

480 DS2

O DS5

150 ^

200

DS7

-" DS10

i 250

ffi S15

300

Surface

350

--II- S16

40O Figure 14. Comparisonof settlements at designated depths below the embankment.

4.3.

Local differential settlement between deep mixing pile and surrounding clay

The local differential settlements between pile and adjacent clay range from 25 to 60 mm (Figure 12) when the average settlement of deep mixing piles amounted to 285 mm after 1 year of full embankment loading. This implies that the local differential settlement between the deep mixing pile and the surrounding clay under the hexagonal wire reinforced embankment can range from 8% to 20% of the average settlement. However, this amount of local differential settlement was almost eliminated at the surface of embankment due to the combined effect of compaction as well as reinforcement stiffness and arching of overlying reinforced soil. Significantly, Figure 12 also demonstrated that the magnitude of local differential settlements between piles and surrounding clay has been almost fully attained just after 1 month of full embankment loading. This practically implies that for road embankment constructed on deep mixing piles, the final surfacing could be better done at least 1 month after embankment construction.

4.4.

Lateral movement

The lateral movement profiles of the improved foundation soils as well as the wall facing of the reinforced embankment are shown together in Figure 15. The maximum measured lateral movements in the foundation subsoil after embankment construction and 7 months after embankment construction amounted to 5 and 45 mm, respectively; and both of them occurred at the weakest zone of soft clay layer located at about 3.5 m depth below the surface of the clay backfill (Figures 1 and 15). Since the average settlement on clay amounted to 162 and 325 mm after embankment construction and 7 months after embankment construction, respectively, so these magnitudes of lateral movement were only 3% and 14% of the corresponding vertical settlement of embankment.

318

Chapter 11

Time(day7~.~~ ! ~ ~ i

6 5 4

1i

REINFORCED E M B A N K M E N T ~ (SEE DETAIL)

3 2 1

Unit weight (kN/m3)'' 1211 ,16,18 ,20

0 -1

'' I ' ' ' ' I ' ' ' 00101~ 300 200 1 Lateral movement (mm)~

Clay backfill

-2 -4

Weathered

~" v

t--

i i i

clay

Soft clay

-E 121

4

-7

!

i

i

i ! !

I| l| !! I! I1

Figure 15.

-10 -11

Medium

stiff

PL WN LL

clay 9 Unit weight 4.Gs . , . , . ,

.

.

.

.

.

.

,,,,

,,,,,,,,,,,,,,,

2.62.652.72.752.820 40 60 8010012002060 40 Gs

PL, WN, LL

80

Corrected Su(kPa )

fromVaneShearTest

Lateral movement profiles of test embankment TEDM.

In addition, the top of vertical facing experienced a forward movement of only 30 mm after embankment construction; it increased afterwards, amounting to 230 mm after 7 months. The time-dependent behavior of the lateral movement of the wall is attributed to the time-dependent lateral movement of the foundation soil as well as the time-dependent rotation of the embankment body due to the uneven consolidation settlement of the improved foundation soil. From Figure 15, the bottom of the embankment just after construction underwent translational movement of 16 mm; thus, for the 30 mm forward movement at the top of vertical facing just after construction, the remaining 14 mm movement could be attributed to the consequent forward movement of the precast concrete facing panel owing to the mobilization and the subsequent elongation of hexagonal wire mesh reinforcements. After 7 months the lateral (forward) movement at the bottom of embankment amounted to 90 mm; and this translational movement is caused by the horizontal thrust of the sloping side of the embankment. Moreover, after 7 months the embankment underwent rotation, resulting from the uneven consolidation settlement of the foundation soil as shown in Figure 16. The gradient of the settlement profile at the bottom of embankment after 7 months, as can be interpreted from Figure 16, is 0.02; or simply 20 mm vertical per meter horizontal. Assuming the reinforced portion of the embankment after construction behaved rigidly after mobilizing the pullout resistance of the

A Full-Scale Study on Cement Deep M&ing in Soft Bangkok Clay

319

EMBANKMEN

0

m

100

--

n_n--n--n

_

-300 -400 -500 -600700 - 800 -900 1000 -1100 -1200 -1300 -1400 -1500 -1600 -200

_

_

~ D

_

I~1 m

With deep mixing piles (TEDM), after construction With deep mixing piles (TEDM), 1 year after construction

A

No improvement (TEu), after construction

A

No improvement (TEu), 1 year after construction

_

_

_

5

Figure

10 Horizontaldistance (m)

15

20

16. Comparisonof settlement profiles between TEUand TEDM.

reinforcements, this rotation would consequently cause an additional forward movement at the top of the 6 m height embankment of 120 mm. Thus, after 7 months, the lateral movement at the top of the vertical facing is estimated to be comprised of: 14 mm due to mobilization and elongation of reinforcements, plus 90 mm translational movement of the embankment body, plus 120 mm due to the rotation of embankment body, which yielded to a total magnitude of 224 mm. This estimated lateral movement of 224 mm at the top of vertical facing agrees to the measured value of 230 mm. The slight underestimation of the calculated lateral movement could be attributed to the subsequent effect of rotation of the embankment that might have increased slightly the horizontal thrust of the soil in the reinforced zone and, thus, increased the elongation of the reinforcements. Therefore, the time-dependent lateral movement of the vertical facing was greatly affected by the unsymmetrical configuration and loading of the embankment and the consequent uneven consolidation settlement of the foundation soil.

4.5.

Salient effects of deep mixing piles on soft clay improvement

The degree of improvement of the soft clay foundation improved by deep mixing piles using Portland cement can be assessed by comparing the settlements as well as the lateral

320

Chapter 11

movements of the full-scale reinforced test embankment (TEu) constructed previously on unimproved soft clay foundation with the present full-scale reinforced test embankment on deep mixing improved soft clay foundation (TEDM). Both test embankments, TE u and TEDM, were reinforced embankments, and were underlain by similar soil profiles, both having a 6.5 m thick soft clay layer with similar properties. Therefore, even though the two test embankments were constructed at two different locations, their results can still be compared for practical assessment of the performance of deep mixing by jet mixing method for soft Bangkok clay improvement. The installation of deep mixing piles in the soft clay foundation could bring favorable effects on the engineering performance not only on the improved foundation soil itself but also on the reinforced soil wall/embankment. The soil-cement piles installation could decrease the overall compressibility of the improved soft clay foundation. The decrease in compressibility of the improved foundation is evident from the settlements of TE u and TEoM. The comparison of settlements between the test embankment TE u on unimproved foundation soil and the test embankment TEoM on improved foundation soil is presented in Figure 16. One year after construction, the settlement of TE u was 1000 mm, while that of TEDM was only 310 mm. Surely, the settlement of TE u could have been higher than 1000 mm if it had been constructed up to a height of 6.0 m as in the case of TEoM. Thus, the compression of the soft clay foundation was reduced by 70% as a result of deep mixing improvement. The soil-cement piles installation could also increase the bearing capacity of the improved soft clay foundation. The increase in bearing capacity can be assessed based on the lateral movement of the foundation soil. As demonstrated in Figure 17, after construction, TE U, which was constructed on unimproved foundation, had caused maximum lateral movement of 130 mm in the weakest zone of the clay layer of the foundation soils; while TEDM, which was constructed on deep mixing piles improved foundation, had caused lateral movement of only 5 mm, also in the weakest zone of the clay layer. Since the height of TE U was only 5.7 m, lower by 0.3 m than the TEoM, so the maximum lateral movement in the soft clay layer under TE u could have been higher than 130 mm if it had been constructed up to 6.0 m height. After 7 months, the lateral movement of the soft clay foundation under TE v amounted to 220 mm, while that of the improved soft clay foundation under TEoM was only 50 mm. Thus, the lateral movement of the soft clay foundation was reduced by as much as 80% due to deep mixing improvement. The substantial reduction of the lateral movement in the case of deep mixing improved foundation simply indicated that the lateral resistance and, hence, the overall bearing capacity of the foundation soil was significantly increased as a result of deep mixing improvement. Moreover, the increase in lateral resistance and the decrease in compressibility of the improved foundation soil have favorably affected the overall lateral movement of the reinforced wall. After embankment construction, the maximum lateral movement, which occurred at the top of the wall, was 450 mm for TE u, but only 30 mm for TEDM (Figure 17). Though the 450 mm maximum lateral movement of TE u was brought back to 350 mm after

A Full-Scale Study on Cement Deep Mixing in Soft Bangkok Clay

321

l Wall face 6 m

5-4-3-2-1--

200

Lateral movement(mm) 300 400

5OO

I

0 Backfill/ W e a t h e r e d clay "2 -3

~" J=

\

--

\ \

-4--

I I #

-5--

/

-6

Soft clay

t /

"7 =8 "9

m

M e d i u m to Stiff Clay

-10 - -11 a

Iffl With deep mixing piles (TEDM), after construction

-12

II

-13

With deep mixing piles (TEDM), 7 months after construction

Zk No improvement (TEu), after construction

-14 --

&

No improvement (TEu), 7 months after construction

Figure 17. Comparisonof lateral movementbetween TEu and TEDM.

7 months of consolidation of the foundation soils, the lateral movement of TEDM after 7 months was 230 mm and was still lower than that of TE U. Therefore, the installation of deep mixing piles in the soft clay foundation has also reduced the overall lateral movement of the overlying reinforced wall/embankment.

4.6. Negative skin friction-inherent in deep mixing under embankment loading Why the negative skin friction (or downdrag skin friction) in deep mixing pile improved foundation under embankment loading is inherent? Unlike the conventional pile foundation system (e.g., concrete pile foundation), where concrete (rigid) pile caps are provided, the deep mixing pile foundation system, normally, does not have rigid pile cap. There are two ways commonly applied for spreading/transferring the load from the embankment to

322

Chapter 11

the improved ground: either by placing a well-compacted mixture of sand and gravel or by using a reinforced compacted soil/fill on the surface of the improved ground. Neither of these load transfer devices is rigid, so the load from the superstructure is expected to be shared by the deep mixing piles and the surrounding soil. The load sharing is dependent on the stiffness of the load transfer device and of the deep mixing piles. As mentioned earlier, the local differential settlement between the deep mixing pile and the surrounding soil, as observed from the full-scale deep mixing improved foundation under the test embankment TEDM, varies from 25 to 60 mm for pile spacing of 1.5 m in square pattern. Moreover, the case history of deep mixing application in road embankment on soft clay (Bergado et al., 1999) revealed that the clay surrounding the deep mixing pile underwent settlement always higher than that of the deep mixing pile. This local differential settlement occurs in deep mixing pile not only because of the considerable difference in stiffness of deep mixing pile and the surrounding soil but also because of the nature of the load transfer devices normally used for deep mixing piles. Thus, the local differential settlement between the deep mixing pile and the surrounding clay is expected in deep mixing improved foundation. Hence, the negative skin friction should be incorporated in the analysis of bearing capacity and in the calculation of settlements of deep mixing piles. Figures 18a and b show the schematic diagrams of the long-term settlement of an idealized deep mixing pile unit cell with and without friction at the interface of deep mixing pile, respectively. Figure 18a illustrates the actual settlement profile of the soil surrounding the deep mixing pile in a unit cell, and this figure demonstrated the possibility of having nonuniform settlement of the clay surrounding the deep mixing pile, being smallest near the interface of deep mixing and increasing radialy up to a maximum value at the point midway between two adjacent piles. This behavior of settlement profile is caused by the arching effect of the load transfer devices or of the reinforced embankment fill and the friction resistance at the apparent interface of deep mixing pile. Figure 18b, which illustrated the schematic of ideal settlement profile if soil-to-pile interface is frictionless, further explains the effect of the friction at the apparent interface between pile and the surrounding soil. If there would be no friction at the interface of deep mixing pile, then the settlement of the surrounding clay would be uniform all throughout the area of clay. However, the difference in settlement between pile and soil is expected to vary from nearly zero at the apparent interface of deep mixing pile up to a maximum value at the point midway between two deep mixing piles (Figure 18a). Thus, the surrounding soil will be subjected to distortional stresses, which can eventually cause a down-drag force on the apparent interface or skin (negative skin friction) of deep mixing pile. 5. CONCLUSION Excess pore water pressures developed in the foundation soil during the installation of deep mixing piles by jet mixing method employing a jet pressure of 20 MPa. The buildup of excess pore water pressure was affected by the overburden pressure, and it tended to be

A Full-Scale Study on Cement Deep Mbcing in Soft Bangkok Clay

323

Average applied stress, q

~ > . r

V~yings~ea~mo~u,os+ I ant shear modulus,~5

\ ............ ~

J Surrounding clay dp .

4 ct~

[_

j .,

i

Boundary, of unit cell--~.' Harderlayer I De/2 ~.,I

rm

Radial distance, r Schematic diagram of mobilized shear stress in soil at a certain depth 'z'

(a) Soc -['7"?'7") . . . . . ;i7"7"7 ..... ")'7"7";," ........ 77:

i Tint= 0

Surrounding clay

(b)

Harder layer

Figure 18. Schematic of the long-term settlement profile of deep mixing pile unit cell. (a) Typical settlement profile of deep mixing pile unit cell. (b) Settlement profile of surrounding soil if pile-soil interface is frictionless.

higher at deeper depths. In addition, higher excess pore pressure buildup was measured at points located within the improved foundation than those points outside the improved zone at 6 m depth. However, this phenomenon was not obvious at 3 m depth. Thus, the jet of water during jet mixing operation must have traversed a longer distance at shallower depth, thereby, causing higher excess pore water pressure even to those points located at the proximity of the improvement zone. The degree of improvement of the performance of the soft clay foundation improved by deep mixing piles using Portland cement has been assessed by comparing the settlements as well as the lateral movements of the previous full-scale reinforced test embankment (TEu) constructed on unimproved soft clay foundation with the full-scale reinforced

324

Chapter 11

test embankment on deep mixing improved soft clay foundation (TEDM). In this study, both test embankments TEtj and TEDM were reinforced embankments, and were underlain by similar soil profiles with both having a 6.5-m-thick soft clay layer with similar properties. Thus, their performances could be reasonably compared to assess the effects of cement deep mixing ground improvement on the performance of the improved foundation as well as on the overlying reinforced embankment. The maximum surface settlement 1 year after embankment construction was about 1.0 m for unimproved soft clay foundation, but only about 0.325 m for the deep mixing improved foundation. Therefore, the soil-cement piles installation in the soft clay foundation has effectively reduced the settlement of reinforced embankment by at least 70%. The degree of consolidation of the improved ground was already 86%, which was almost 90%, 1 year after the embankment construction. If there had been no deep mixing improvement in the soft clay foundation, the 90% consolidation settlement would have been attained 9 years after the embankment construction. In addition, the local differential settlement between deep mixing pile and the adjacent surrounding clay was observed in the full-scale improved foundation. From this full-scale test, the local differential settlement amounted to 25-60 mm which are about 8-20% of the average settlement. The local differential settlement could induce downdrag skin friction on the deep mixing piles. This local differential settlement, however, was not obvious at the top surface of the embankment due to the combined effect of compaction as well as reinforcement stiffness and arching of the reinforced soil. The maximum lateral movements in the foundation soils after embankment construction were 130 and 5 mm under TEtj and under TEDM, respectively. These maximum lateral movements occurred at 3.5 m depth having the least value of undrained shear strength within the soft clay layer. After 7 months, the lateral movement of the soft clay foundation under TE u amounted to 220 mm, while that of the improved soft clay foundation under TEDM was only 50 mm. Thus, the lateral movement of the soft clay foundation was reduced by as much as 80% due to deep mixing improvement. The substantial reduction of the lateral movement simply indicates that the deep mixing piles have effectively increased the lateral resistance of the soft clay foundation and, hence, the bearing capacity of the improved foundation. In addition, the forward movement at the top of the wall after embankment construction amounted to 450 mm for TEv, but only 30 mm for TEDM. The forward movement of TEDM increased afterwards, amounting to 230 mm after 7 months; even so, the lateral movement of TEDM was still lower than that of TF_v 7 months after embankment construction. The timedependent behavior of the lateral movement of the wall is attributed to the time-dependent lateral movement of the foundation soil as well as the time-dependent rotation of the reinforced embankment due to the uneven consolidation settlement of the improved foundation soil. Significantly, from the results of the analysis, after 7 months, the lateral movement at the top of the vertical facing is estimated to be comprised of 14 mm due to mobilization and elongation of reinforcements, plus 90 mm translational movement of the reinforced embankment

A Full-Scale Study on Cement Deep Mixing in Soft Bangkok Clay

325

caused by the lateral movement of the foundation soil, plus 120 mm due to the rotation of embankment body as result of uneven settlement of the foundation soil. Thus, the estimated total magnitude of the forward movement at the top of vertical facing amounted to 224 mm, which agreed with the measured value of 230 mm. The time-dependent lateral movement of the vertical facing was greatly affected by the unsymmetrical configuration and loading of the embankment and the consequent uneven consolidation settlements of the foundation soil. The effectiveness of cement deep mixing ground improvement employing jet mixing technique in improving thick deposit of soft clay for foundation support of reinforced embankment has been confirmed from field observations of two full-scale reinforced test embankments. The implementation of this ground improvement technique enable the following improvements in the engineering performances of both the improved ground and the overlying reinforced embankment: (1) increase the lateral resistance and the bearing capacity of the soft clay foundation; (2) minimize the lateral movement of the reinforced wall/embankment and improve the integrity of the reinforced soil mass; (3) increase the rate of consolidation of the soft clay foundation; and (4) reduce the compressibility of the improved foundation and the settlement of the reinforced embankment.

REFERENCES

Asaoka, A. (1978) Observational procedure of settlement prediction, Soils Found., 18(4), 53-66. Bergado, D.T. & Lorenzo, G.A. (2003) Behavior of Reinforced Embankment on Soft Ground with and without Jet Grouted Soil-Cement Piles (in TC9 Lecture), Proceedings of the 12th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering, Singapore, August 2003, pp 1311-1316. Bergado, D.T., Lorenzo, G.A. & Chai, X.C. (2002) Construction Behavior of Hexagonal Wire Reinforced Embankment on Deep Jet Grouted Soil-Cement Piles Improved Soft Bangkok Clay,

Proceedings of the Symposium of Ground Improvement and Geosynthetics, KMUTT, Bangkok, Thailand, pp 115-124. Bergado, D.T., Ruenkrairergsa, T., Taesiri, Y. & Balasubramaniam, A.S. (1999) Deep soil mixing to reduce embankment settlement, Ground Improvement J., 3(3), 1-18. Bergado, D.T., Sampaco, C.L., Shivashankar, R., Alfaro, M.C., Anderson, L.R. & Balasubramaniam, A.S. (1991) Performance of a Welded Wire Wall with Poor Quality Backfill Soft, ASCE Geotechnical Engineeering Congress at Boulder, CO, USA, pp 909-922. Kamon, M & Bergado, D.T. (1991). Ground Improvement Techniques, Proceedings of the 9th Asian Regional Conference on Soil Mechanics and Foundation Engineering, Bangkok, Thailand, u 2, pp 526-546. Petchgate, K., Jongpradist, E & Panmanajareonphol, S. (2003) Field Pile Load Test of Soil-Cement Column in Soft Clay, Proceedings of the International Symposium 2003 on Soil/Ground Improvement and Geosynthetics in Waste Containment and Erosion Control Applications, 2-3 December 2003, Asian Institute of Technology, Thailand, pp 175-184. Porbaha, A. (1998) State of the art in deep mixing technology. Part I: Basic concepts and overview, Ground Improvement, 2, 81-92.