lime mixing ground improvement for road construction on soft ground

Chapter 10 Cement/Lime Mixing Ground Improvement for Road Construction on Soft Ground J.-C. Chai 1 and N. M i u r a 2

1Institute of Lowland Technology, Saga University, Japan 2Institute of Soft Ground Engineering, Co. Ltd., Japan

ABSTRACT A road test section of about i km long on soft Ariake clay deposit, Saga, Japan, with 10 different ground improvement methods is described, and the field-monitored results on settlements in a period of more than 1 year and the deflection test results are reported. Both the settlement and deflection test results indicate that for a low embankment (<3.0 m) road, increase in the strength/stiffness of base-course and subgrade and/or increase in the improvement depth of subgrade can reduce the traffic-load-induced settlement significantly. Also, the field data indicate that cement treatment is more effective than lime treatment for soft Ariake clay, and a layer of geogrid in base-course has a positive effect on reducing deflection of the test road. For a low embankment road, the traffic-load-induced settlement consists of a large part of residual settlement and controls the maintenance cost. The methods for predicting the traffic-load-induced settlement are briefly reviewed. Comparing the predictions with the measurements of some subsections of the test road shows that Chai and Miura's (2002). J. Geotech. Geocnnion. Eng., ASCE, 128(11) 907-916, empirical method is useful for predicting the traffic-load-induced settlement. Finally, the concept of total cost (construction cost and maintenance cost) is introduced, and it is demonstrated that for a road with intensive traffic (D-traffic in Japan) and on soft subsoil, using a relatively costly ground improvement scheme is attractive in terms of total cost.

1. INTRODUCTION In lowland areas of soft clay deposits, to reduce embankment load-induced settlement and construction cost, normally, low embankment (<3.0 m height) road is constructed (Fujikawa et al., 1996; Miura et al., 2000; Miura, 2002). However, the low embankment

279

280

Chapter 10

road can be easily affected by traffic load, causing the differential settlement and the cracking of pavement. Then periodic repairing (over-layering) is required in the lifetime of the road. This kind of periodical repairing activity not only influences the traffic, but also the costs. There are some options for reducing the traffic-load-induced settlement, differential settlement and the cracking of pavement, namely, cement/lime mixing improvement of soft clayey subsoil, reinforcing the base-course and/or subgrade of a road, using lightweighted fill material to reduce embankment load and some combinations of the methods. Practically, to select an improvement method, not only the technical availability but also economic feasibility must be considered. To find an economic and effective way of constructing low embankment roads on soft subsoil, a test section about 1 km long with different ground improvement techniques was constructed on soft Ariake clay deposit, Saga, Japan, and monitored for a period of more than 1 year. This chapter first reports the field conditions, ground improvement construction and the field monitoring results. Then, the methods for predicting the traffic-load-induced permanent settlement of a low embankment road are briefly reviewed, and the predicted values by a recommended method are compared with the field data of the test road. Finally, the concept of total cost (construction cost and maintenance cost) is introduced and some possible ways to reduce the total cost are briefly discussed.

2. SUBSOIL CONDITION AND TEST SECTION CONSTRUCTION The test section was located at Kawazoe Machi, Saga, Japan. At the site, nine piezocone penetration tests and three boreholes were made within 800 m length to investigate the subsoil conditions. The measured cone tip resistances (qt) are given in Figure 1 (after Miura, 2002). The estimated soil strata are also indicated in Figure 1. The total thickness of soft deposit is about 20 m and can be subdivided into five alluvial clay layers (Acl -AcS) and two thin alluvial sand layers (Asl and As2). The softest layer is located at about 0 m elevation with a qt value of 0.1-0.2 MPa. On the basis of the piezocone test results the subsoil has been divided into four zones, I-IV, as indicated in Figure 1. Zones I and IV are similar where sand layer As1 is thin. In zone II, there is no Ac3 layer, and the sand layer As1 is relatively thicker. In zone III, the thickness of As~ layer varies from that of zone II and zone IV. Generally, zones II and III are stronger. The three boreholes were located at No. 1, No. 6 and No. 8 locations as shown in Figure 1. The soil properties from laboratory tests with soil samples recovered from the three boreholes are summarized in Figure 2 (after Miura, 2002). The natural water contents of the clay were 80-130% and slightly higher than the corresponding liquid limits. Within about 10 m depth, the compression index (Cc) of the soil was generally 1.0-2.0. The groundwater level was about 1.0 m below the ground surface.

Cement~Lime Mixing Ground Improvement for Road Construction No.1 No.2 qt(MPa) q/(MPa) Elevation 0 0.5 1 0.5 1 (m) ,~'", .... "IP'" .... 'qAcllayer -~_

[~

No.3 qt(MPa) 00.5 1

No.6 No.7 No.8 No.9 qt(MPa) qt(MPa) qt(MPa) qt(MPa) 00.5 1 00.5 1 00.5 1 00.5 1

No.4 No.5 qt(MPa) qt(MPa) 00.51 00.51

" '1....!~l

I~L''l....I' I

I~

k

'\

tAc21ayer_. '

-':..0

9

..o

9

-5 ~

o

~

.~'.-~ . . . . . 9 ,As2 As2 .~. 999

ay:

~~_

" "I.'..'N.':'.:.""::': ":;""" _ _

See.o.9 9

|

ground condition

_~L

::::::::::::::::::::::::::::::::

;ayer

-15 - ~

9- b.?.-~...

"...~

layer

-20 - "

281

@

I

I

I

0

100

200

|

I ~ II~

I

300

| IV

400

500

600

I

I

700

800

m

Figure 1. Estimated soil profile based on piezocone test results. (After Miura, 2002; reproduced with the permission of International Association of Lowland Technology.)

Wp, W n, WL(% ) ])t qu Pc mv Cv Depth e0 (kN/m3) (kPa) (kPa) Cc (m2/kN) (cm2/day) (m) No.1 No6 No.8 o x 9 100 1 2 3 4 5 13 15 0 50 100 100 200 1 2 10-4 10-3 10-2 103 104 0 ~ A 0 Acl Acl

5

Ac2AC2 c2 /*-oX~ / -22i As, Ac3 Ac3

i

c3

10 ~ ~ " 15

~ ....

' "'l""'"'"'""l'"'"' I

o _

[

/.~x/

~

....

T

F" l ~

%

~~

'

~

~

~' T ' ' ~ / ~

'

'

'

I'''

; ....

'""'

'"'""1

'";'"'

'"'""1

,,,,

,,,,,?,

,,,,

/

T /

+ t~

Ac4 Ac4 Ac4 -

/

-

/

Ac5 Ac5

20 Figure 2.

,,,,~,,,,, .... ,,,I,,,,,,,,,,,,,,,I,,,,,,~

,

,

,

,,,,

,P,,,,,,,

Soil properties at test section site. (After Miura, 2002; reproduced with the permission of International Association of Lowland Technology.)

About 1-km-long test section had 10 subsections. Each subsection had a different ground improvement method. The location of each subsection is indicated in Figure 1 and the cross-section of each subsection is illustrated in Figure 3 (after Miura, 2002). The Ascon in the figure means asphalt-concrete. The amounts of admixtures added and the

Chapter 10

282 Ls

Es

Ascon [ Base [I course

I Ascon [ [I course Base [I

Cement Sravel [b 3[ treated ll0

Lime Ireated

Mf CI CII [ Ascon Ascon I I Ascon [I course Base __ [I Base I Base | course course

3[

forced

I [0 3[ cement

[i. I +EpS II "[ Gla/ssIll" I treate Cement grid 1+EPS 1 0 " 6 t r.6e a tCement ed [

I/I ,.o

1111

!! !i II

GI GII I Ascon I I Ascon ] /Base I Gravel

[ Base I

SF Ascon Base

.3 Grave] 0.3 Gravel! 0.3 Gravel

I'll '0 tre ............... rf.i..... entlI,tCemen 06 I I::m te

ii II !!

[~ [treated [l [ oelSO~nnt [ /I I!

c: ::n, ico,~m.

Ss I Ascon Base course

Glass

I treated ] l / treated l i

Fe

Soil

Fe

Cm

I Ascon - 0.05 Base ~_01 cours~ 0.31Grav~

r_~

3.31Cemel treatec

0.6

Lime

treated 2.0 I Column: ~=0.8m

1~ (Unit m) Figure 3.

Subgrade and base-course structures of 10 kinds of test road. (after Miura, 2002; reproduced with the permission of International Association of Lowland Technology.)

resulting unconfined compressive strengths (qu) from laboratory tests using the field samples are summarized in Table 1. In M e subsection, 0.3- m-thick base course was treated by air-bubbled cement (lightweight with a resulting unit weight of 13.1 kN/m 3) and reinforced by a geogrid. The layout of the geogrid is illustrated in Figure 3. The strength of the geogrid was 98 kN/m and failure strain was 5 %. The nominal grid size was 25 mm • 25 mm and the unit weight was 170 g/m 2. Then a 0.6 m thick subgrade was treated with cement and the amount of cement used was 1.10 kN/m 3. For two soil--cement column-slab subsections (CI and Cu), the diameter of the column was 0.8 m and the area replacement ratio of the soil-cement column was 36%. The difference between those two geogrid plus the cement treatment subsections (G I and GIi) results in the layout of the geogrid. In GII, the two ends of the geogrid were connected, but in G I they were not. In subsection E s, expanded polystyrol beads were mixed in a 0.95-m-thick layer, and the resulting unit weight of the treated layer was 14.9-15.1 kN/m 3. The methods for constructing surface cement-lime mixing and soil-cement column are as follows:

Surface mixing method. There are two methods of mixing admixture with surface soil, namely dry mixing and wet mixing. The machine used for dry mixing was modified from a backhoe. The modification was made by replacing the normal bucket of a backhoe with one having several open slits as shown in Figure 4. In the case of wet mixing, the admixture and the soil were mixed in a mixer and poured on the ground surface. The field tests (two test yards with a length of 5 m, width of 3 m and thickness of 0.5 m) showed that the scatter of the strength of the samples from dry mixing yard was larger than that from wet mixing. However, the average strength of the dry mixed sample was higher, and dry mixing was easier to operate in the field (Fujikawa, 1996). Therefore, for the test road, the dry mixing method was adopted.

"6

n~ 9 .in

o

o

n~

c~

o

O

O

o

0 0 o~ O

9 o

0

o

"-- ~----

0 0 0 0 u'~ o o ~ "

~

c5 ~ e~ ~

9~ . ~

,-~

O

0

",-

++~

0

9

o

+.a

c5

0 0 c',l c q

+.a

0 ,--.~ ~l-

Cement~Lime M & i n g G r o u n d I m p r o v e m e n t f o r R o a d C o n s t r u c t i o n

on

o~ "o

o o

o

<~

o

9 r

o

J

~D

0

283

Chapter 10

284

Figure 4.

.

Modified backhoe bucket.

Soil-cement column construction method. The soil-cement column was constructed by Teno-column method. The method of mixing cement slurry with clayey soil by a mixer is as shown in Figure 5. The mixer consists of a mixing wing and a free wing. The function of the free wing is to prevent the co-rotation of the soil with mixing wing and achieve a better mixing.

Average embankment thickness of the test section was about 0.75 m (including pavement). The total width of the test section was 10 m, in which 7.5 m was for traffic and 2.5 m for sidewalk. Figure 6 shows a typical cross-section of the road

3. PERFORMANCE EVALUATIONS

3.1. Surface settlement The settlement was measured after the completion of the road and the settlement during the construction was not monitored. About 100 days after the completion of the construction, the road was opened to traffic. Figures 7(a) and (b) show the measured settlement variation of each subsection. The field data are from the Civil Engineering Department, Saga Prefecture, Japan (1996). Figure 7(a) shows the data of the relative stronger subsoil zones, zone II and III, and Figure 7(b) shows the data of the relative weaker subsoil zones, zone I and IV. It can be seen that the subsoil condition had an obvious effect on settlement, and zones II and III had a smaller settlement than zones I and IV. Another point is that

Cement~Lime Mixing Ground Improvement for Road Construction

285

mixing axle \

}

I

i

m i x i n g wings

/ free w!ng p r e v e n t i n g soil

\ 9

/co-rotating

.

[///////////////////////A ~.. ~ b o r i n g

wing

U

~ outlet hole

column ~__ diame[er Figure 5. Sketch of Teno-column mixer (after Shen, 1998; reproduced with the permission of S.-L. Shen).

300i- 2soo ~00 "~q~00 ' 2 < ~ , 1 -~

Ill

10,000 3000 ,. ' --1

._22,0 ---

--

/ / Lime treated subgrade CBR 6.0% t=30cm / / Subgrade t=30cm /

300 3000 , --i ~ . . . ' 35_~~75~ [ un,t mm

,

~urface (asp-~alt)t=5-g--c-c~m \ \ Upper base course t=10cm \ Lower base course t=30cm

Figure 6. Typicalcross-section of the road. (After Miura; 2002; reproduced with the permission of International Association of Lowland Technology.)

before the road has opened to traffic, all subsections (except subsections S F and Ss), the e m b a n k m e n t (including pavement) load - induced consolidation settlement was almost finished. The reason that the e m b a n k m e n t load-induced settlement for S F and S s was not finished, when the road opened to traffic, is not clear. In later discussion, we consider the settlement after the road opened to traffic was the traffic-load-induced settlement. (1) Comparing lime treatment with cement treatment. Subsections L s, S F and S s were treated with lime and the improvement depth varied from 0.9 to 2.1 m. These three subsections were all in weaker subsoil zones, subsection L s in zone I, subsections S F

Chapter 10

286

9

50

~

|~

"

A

GII

--

GI

'

I

'

I

'

50 " - ' - ~ - E s -----D--M F - - v - - CI ---O-- CII

9

= 100

100

r~

I

SF

•

Ss

~

k k ~ \ ' ~ \ "~ ' " - , , , ~ ~

Subsoil zone II and III Subsoil zone I and IV 150

, 0

(a)

i 100

,

i 200

,

i 300

Elapsed time (days) F i g u r e 7.

Nv

150 400

0

100

(b)

200

300

400

Elapsed time (days)

Settlement versus time curves.

and S S in zone IV. Compared with other subsections in zones I and IV that have the same or less improvement depth (Es, M 0, the settlements of lime-treated subsections are larger (Figure 7(b)). Therefore, generally, the lime treatment was not as effective as cement treatment for soft Ariake clay. However, when comparing CII (in zone IV with cement treatment and improvement depth of 2.7 m) with L~ (in zone I with lime treatment and improvement depth of 1.3 m), they had almost the same amount of settlement (Figure 7(b)). This means that not only the admixtures used, but also the quality of treatment is an important factor. (2) Effect of improvement depth. C~ and CII used the similar improvement method but C I had a deeper improvement depth and smaller settlement (Figure 7(b)). The major difference between C I and C n was the settlement after opened for traffic (i.e. the trafficload-induced settlement). Subsection C H might have a poor improvement quality as discussed above. On comparing C I (4.2 m improvement depth) and C m (0.9 m improvement depth) we can emphasize the effect of improvement depth (Figures 7(a) and (b)). Also, SF and S s had a similar improvement technique, but S s had a deeper improvement depth and a smaller settlement (Figure 7(b)). Normally, traffic load has a limited influence depth (Fujikawa et al., 1996; Chai and Miura, 2002; Miura, 2002). If the improvement depth exceeds this influence depth, the traffic-load-induced settlement can be substantially reduced. This will be discussed later in detail. (3) Effect of strength of base-course/subgrade. The importance of the strength/stiffness of improved layer is already mentioned when comparing C I and CII, and C n and L s. This point can be further demonstrated by comparing the subsections M e and G v These two subsections had the similar subgrade improvement method and improvement depth. G I was in the relatively stronger subsoil zone, zone III, but the improved subgrade had an unconfined compressive strength (qu) of 580 kPa (Table 1). M e is in zone I and improved subgrade had a qu value of 1980 kPa (Table 1). As a result, M e had less settlement than G I (Figures 7(a) and (b)).

Cement~Lime Mixing Ground Improvementfor Road Construction

287

(4) Effect ofgeogrid reinforcement. Cm, G~ and G n w e r e in zone II and III and had the same improvement depth, but G I and G n had a geogrid reinforcement later and C m did not. Also, C m had higher subgrade strength than G I and G n (Table 1). However, G I and GII had a less settlement, which is considered an effect of geogrid. The difference between G I and G n is that for G n the reinforcement was connected at two ends but in the case of G I it was not. G n had a smaller settlement than G I (Figure 7(b)), but the subsoil of subsection GII was stronger than that of subsection G~. So, the effect of different layouts of reinforcement is inconclusive. Miura et al. (1990) suggested that a layer of geogrid can have an equivalent effect of increase in thickness of improved layer by 0.1 m. (5) Effect of light-weighted material. E s and M e w e r e constructed with light-weighted materials. Subsection M e also had a layer of geogrid reinforcement and a higher subgrade strength, and therefore, the effect of light-weighted material cannot be identified. For Es, EPS beads were mixed with a 0.95 -m-thick layer below the base-course. Although the strength of the treated layer was relatively lower (Table 1), the settlement was relatively smaller, and it is considered as an effect of light-weighted material. The settlement curve of E~ shows that after open to traffic, the settlement increment rate is relatively higher, which may indicate that light-weighted material can reduce embankment load-induced settlement but has insignificant effect on the traffic-load-induced settlement.

3.2. Deflection measurement results Benkelman beam tests (BBT) and falling weight deflection (FWD) tests were conducted to evaluate the deflection behavior of each subsection. The tests were conducted twice, immediately after construction and 1 year after construction. The results are depicted in Figures 8 and 9 for BBT and FWD, respectively (after Miura, 2002). In Figure 8, D Ois the deflection directly under the loading point, and D150 the deflection at a point 1.5 m away from the loading point in the direction along the road. D B in Figure 9 is the calculated deflection at the base of subgrade directly under the loading point. The calculation was made under the assumption that the load was transmitted from the loading plate at the surface to the base of subgrade with a spreading angle of 45 ~ (Miura, 2002). For FWD test, with a load spread angle of 45 ~ the deflection at x m from the edge of the loading plate mainly influenced by the properties of the soil layers below x m depth rather than the layers above. Let us denote the thickness of base-course and subgrade as H b. It is further assumed that the value of D B will be the surface deflection at distance H B from the edge of the loading plate (Fujikawa, 1996). Generally, tendency is the same as settlement measurement. The thicker the improved layer and higher the strength/stiffness of the base-course and the subgrade, the smaller the deflection. C~ and Mf subsections had smaller settlements (Figure 7(b)) and smaller deflections (Figures 8 and 9). However, there are some contradictions with regard to settlement measurement. Subsections CII and S s had a relatively larger settlement (Figure 7(b)) but a

Chapter 10

288

0 9

.o

0 month --

12 month

...,, ;',,

1

tD

o

~"O

o

o

'

I

I

I

I

I

I

I

I

I

I

Ls

Es

Mf

CI

Cm

On

OI

GII

SF

SS

Subgrade and base course method Figure 8.

Deflection from Benkelman beam test. (After Miura, 2002; reproduced with the permission of International Association of Lowland Technology.)

I

I

I

I

I

I

I

I

I

0 month 12 month 0.8 -7 0

/

A

~

DO

A D,~o ~

,~,

__

--

i/v,,

DB

0.6

~

0.4

0.2

0 Ls

Figure 9.

Es

Mf CI Cm GII GI Cii Subgrade and base course method

Sf

Ss

Results of falling weight diflectometer (FWD) test. (After Miura, 2002; reproduced with the permission of International Association of Lowland Technology.)

Cement~Lime Mixing Ground Improvement for Road Construction

289

smaller deflection (Figures 8 and 9). The exact reason for this is not yet clear. Both CII and S s had a relatively thicker improved layer, and it might contribute to the smaller deflection measurement. From Figure 9, the factor that subsections C~, CII and Ss had smaller deflections at the base of subgrade (DB) supports this argument. Another point is that subsection C mhad a modest settlement which is consistent with FWD measurement, but different from BBT result. BBT resulted in the largest deflection of subsection C m, and the reason is not clear. On comparing the two measurements, immediately after the construction and 1 year after the construction, there was no significant difference between these two measurements except for subsections GI and GII in case of BBT results. For G I and GII, BBT results indicate that 1 year after the construction, the measurement is about three times of that immediately after the construction. Again, the exact reason is not clear yet, possibly owing to the deterioration of reinforcement effect, such as creep deformation of the geogrid, etc. From the above discussion it can be said that both the improvement depth and strength/stiffness of improved layer have a significant influence on the traffic-load-induced settlement and deflection of a road. The test results show that an improvement depth of more than 1.3 m from the bottom of the base-course is required to substantially reduce the traffic-load-induced settlement and the deflection of a road. If the improvement depth is about 1.0 m, an unconfined compressive strength of more than about 1000 kPa for improved layer is desirable. For low embankment road, the traffic-load-induced settlement is the main part of the post-construction settlement and therefore it controls the maintenance cost. In the following section, the methods for predicting the traffic-load-induced settlement will be discussed and some subsections of the test road reported in this chapter will be analyzed.

4.

P R E D I C T I N G THE T R A F F I C - L O A D - I N D U C E D P E R M A N E N T S E T T L E M E N T

4.1. A brief review on existing methods Several factors affect the traffic-load-induced deformation, namely, (a) the strength and deformation characteristics of soft subsoil, (b) the properties of pavement, base-course and subgrade of a road and (c) the magnitude and number of applications of traffic load. Any useful settlement prediction method should consider the above-mentioned factors directly or indirectly. The existing methods can be divided into three groups: (1) numerical methods; (2) equivalent static loading methods; and (3) empirical equations. (1) Hyodo et al.'s (1996) dynamic analysis method. Theoretically, explicit simulation of the response of subsoil under repeated load is preferable. A method proposed by Hyodo et al. (1996) combines dynamic numerical analysis (two-dimensional (2D)) and dynamic triaxial test results to predict the traffic-load-induced deformation. However, the methods of explicit simulation are difficult to use in engineering practices. The response of subsoil

Chapter 10

290

under traffic load is a three-dimensional (3D) problem and the number of load applications is usually extremely large. (2) Fujikawa et al.'s (1996) equivalent static load method. On the basis of field measurement, Fujikawa et al. (1996) proposed a method to estimate the distribution of the traffic-load-induced consolidation settlement in the subsoil. In Fujikawa et al.'s method, a triangular distribution of traffic-load-induced stress increments is assumed, i.e. maximum at ground surface and linearly decreased to zero at a depth of influence. Fujikawa et al.'s method can only calculate the final settlement and the variation of the settlement with time cannot be predicted. Furthermore, the traffic-load-induced stress is not calculated explicitly; the properties of pavement, base-course and subgrade, and the behavior of subsoil under repeated load are not considered. (3) Li and Selig's (1996) empirical method. A number of empirical equations have been proposed to predict the permanent deformation of cohesive soil under repeated load. Among them, the power equation proposed by Monismith et al. (1975) has been widely used. Li and Selig (1996) proposed a method to determine the constants in the power equation, in which the magnitude of traffic load applications and the strength of subsoil are directly included in the equation, and the physical state of subsoil is considered indirectly. Li and Selig (1998) showed some successful applications of the equation to predict the settlement of cohesive soils under train loading. The values of the constants suggested by Li and Selig seem to be applicable to compacted cohesive soils but may be less suitable for natural clay deposit. Also, the effect of initial static deviator stress in subsoil is not considered. (4) Chai and Miura's (2002) empirical method. Chai and Miura's (2002) method is a modification of Li and Selig's (1996) method. The main modifications were (a) considering the effect of initial deviator stress and (b) linking the constant, a, in Li and Selig's (1996) equation to the compression index (C c) of subsoil. This constant mainly controls the magnitude of the traffic-load-induced settlement. This method was used to analyze some subsections of the test road in Saga, Japan, and a brief description is given below. The equation for calculating the cumulative plastic strain (ep) of soft cohesive soil under repeated loading is ep = a

(/m()n q__s qf

1 + q--Ls N b qf

(1)

where qd is the traffic-load-induced dynamic deviator stress, qf the static failure deviator stress of soil, N the number of repeated load applications, qs the initial static deviator stress and a, b, m and n are the constants. The methods for determining the variables and constants in Eq. (1) are as follows:

Dynamic deviator stress qd. To estimate the value of qa, 3D traffic load transfer mechanism and the characteristics of multilayer foundation must be considered.

Cement~Lime Mixing Ground Improvement for Road Construction

291

Burmister's multilayer elastic solution (Burmister, 1945) is considered suitable for this purpose. Using numerical techniques, the non-linearity of subsoil behavior can be considered. In calculation, equivalent uniform distributed loads over circular areas simulate the tire loads of a truck. A relationship between the magnitude of the tire load and the radius of the equivalent loading area reported by Uchida (1988) is recommended (Figure 10). If there are no measured data, the traffic load amplification factor of 1.0 is suggested (considering the traffic load as a static load). Static failure deviator stress qf. qf = 2Su, and Su is the undrained shear strength of soft subsoil and can be measured by field vane shear test or determined by the following empirical equation (Ladd, 1991):

SU

"--"

(2)

! m ~ S~Yv(OCR)

where cr'v is the effective vertical stress, OCR the over-consolidation ratio and S and ml are constants. Initial deviator stress qs. After a road embankment construction, the static stress distribution in subsoil can be calculated by one of the following two methods: (1) 2D finite element (FE) analysis. (2) Hand calculation. After embankment construction (assume the consolidation is finished), the vertical stress in the subsoil is calculated as !

!

O"v = ~ h

300

'

I

'

I

(3)

i + AO"v

'

I

'

I

Double tires

200 ..o

.~

.-~ingle'/~tire

~

100

0

i

0

I 20

i

I 40

I

I 60

i

I 80

100

Tire load (kN) Figure 10.

Relationship between the magnitude of the tire load and the equivalent radius of loading area. (Data from Uchida, 1988).

292

Chapter 10

where ] ~ h i is the effective stress due to gravitational force of subsoil (below water level, buoyancy unit weight should be used) and Act'v the embankment-load-induced stress increment, which can be estimated by Osterberg's (1957) method. For calculating the horizontal stress, the following equation is suggested to estimate the horizontal earth pressure coefficient, K o (Mayne and Kulhawy, 1982): K o = (1 - sin ~')(OCR) sinr

4.

5.

(4)

where q~' is the the effective stress internal friction angle of subsoil. Constants b and m. At present, the values suggested by Li and Selig (1996) are recommended (Table 2). The parameter b controls the incremental rate of plastic strain with the number of repeated load applications. Li and Selig (1996) showed that b is not sensitive to the magnitude of dynamic deviator stress. It is mainly influenced by soil type. The parameter m influences both the magnitude and distribution of plastic strain with depth. Since qd/qf is < 1.0 (and not a failure problem), the larger the m value, the faster the decrease in ep with depth. Constant a. The parameter a influences the magnitude of plastic strain. The trafficload-induced deformation mainly consists of two parts, namely, dynamic consolidation and shear deformation. The amount of dynamic consolidation deformation is directly related to the compression index (Cc) of soil. Also, C c is one of the parameters affecting the magnitude of shear deformation. Therefore, it is rational to relate a with the compression index (Cc) of subsoil: a = aC c

.

4.2.

(5)

where a is a constant. On the basis of back-calculated results, Chai and Miura (2002) proposed that a = 8.0. Constant n. n is a parameter controlling the degree of the effect of initial deviator stress and Chai and Miura (2002) suggested that n = 1.0.

Comparing predicted settlements with the field measurements

In the analyses, only trucks were considered. This is because in comparison, the weight of passenger cars is 1/10 to 1/30 of that of heavy trucks. The effect of a passenger car on the Table 2. Valuesof constants a, b and m (suggested by Li and Selig, 1996) Soil Type

a

b

M

CH (high plasticity clay) CL (low plasticity clay) MH (elastic silt) ML (silt)

1.2 1.1 0.84 0.64

0.18 0.16 0.13 0.10

2.4 2.0 2.0 1.7

Cement~Lime Mixing Ground Improvementfor Road Construction

293

permanent deformation of a road may be about 1/100 to 1/900 of that of a heavy truck. Another factor is that there are many types of trucks, and the number of axle and the distance between the axles are different. For simplicity, a load distribution pattern as shown in Figure 11 was adopted. According to Japanese standard, the rear axle supports 80% and the front axle supports 20% of the total load (Uchida, 1988). The adopted values of constants in Eq. (1) were a = 8.0, b = 0.18, m = 2.0 and n = 1.0 (Chai and Miura, 2002). Among 10 subsections, 4 of them (C I, C m, Mf and GI) were analyzed to investigate the effect of improvement depth (comparing C I and Cm) and the strength/stiffness of subgrade (comparing Mf and GI). (1) Traffic intensity and the parameters adopted. The total traffic intensity was about 2500 cars/day (two-way) and most of them were passenger cars. The trucks were about 6% of total traffics (150 trucks/day) (Chai and Miura, 2002). Since the road is relatively narrow, the two-way traffic was used for calculating the traffic-load-induced permanent deformation. The trucks that passed this road were mainly light-weighted ones, and heavy trucks were very rare. In the analysis, it was assumed that the average weight of the truck was 100 kN/truck and equivalent radii of tire/road contact areas were 200 mm for back tires (double wheels) and 100 mm for front tires (Figure 10). For analyzing the traffic-loadinduced stress in subsoil, the adopted Young's modulus (E) and thickness (H) of each layer are summarized in Table 3. Poisson's ratios adopted were 0.2 for pavement and basecourse, 0.25 for subgrade and 0.4 for soft subsoil. Modulus for cement treated layers was estimated as 100 times of the unconfined compressive strength (qu) (Kitazume, 1996). Modulus for natural subsoil was assumed to be about 200 times the undrained shear strength (Su) (Fujikawa, 1996). For the four subsections considered, the calculated dynamic deviator stresses were 3 - 5 kPa just below the subgrade, and reduced to about 1.0 kPa at a depth of about 6 m. (2) Traffic-load-induced settlement. The calculated traffic-load-induced settlements are compared with measured data in Figures 12 and 13. The calculation predicted the field

40% of Total load

~.~

7

6.0m

..~

"-\

Figure 11. Loaddistribution pattern of a truck (after Chai and Miura, 2002).

Chapter 10

294 Table 3. Subsection

Cm

CI Mf GI

Thickness and Young's modulus of each subsoil layer of Cm, Ci, Mf and G I subsections Lower subsoil

Pavement base course H (m) E (kPa)

Subgrade base course H (m) E (kPa)

H (m)

E (kPa)

H (m)

E (kPa)

0.45 0.15 0.45 0.45

0.6 0.7 0.6 0.6

3.5 3.5 2.0 2.0

3,000 86,400 3,000 3,000

2.5 2.5 4.0 4.0

2,500 2,500 2,500 2,500

35,000 35,000 175,000 35,000

Upper subsoil

140,000 244,000 198,000 58,000

Notes: (1) H is the layer thickness and E the Young's modulus. (2) Below the lower subsoil is a semi-infinite half-space.

0700

O'

9 I0

'

I

9

'

I

'

i

,

I

,

I

ci

c..)

40

-

'x::l .~

-

o

60 -

C3

_

80

Figure

12.

'

I

,

i

200 400 600 800 Elapsed time after open to traffic (days)

Comparing the traffic-load-induced settlement of subsections C I and C m (modified from Chai and Miura, 2002).

data reasonably well. Note, that for these four subsections, we were able to get the measured data more than 2 years after the road opened to traffic. In Figure 12, subsections C m and C I are compared, which had different depth of improvement. For C I, soil-cement columns (36% replacement ratio by area) improved the upper subsoil. Dynamic deviator stress below the columns was small and the traffic-load-induced settlement was much smaller than C m. In Figure 13, M e and G I are compared. These two subsections had a similar subgrade structure but the strength and stiffness of the cement-treated layer were different, and G~ had a lower strength and stiffness. Settlement of G I was more than twice of that of M e. This case history indicates that an increase in the thickness and stiffness of improved subgrade is efficient for reducing the traffic-load-induced permanent settlement of soft subsoil.

Cement~Lime Mixing Ground Improvementfor Road Construction 0 T--

I

! OO

J

I

I

I

J

I

I

295

I I

9

~ 20

Mf

9

t

~D

E ~D ~D

G} 40

9

60 m

"

m

_

[.-.

80

i 0

Figure 13.

I J I I I I I 200 400 600 800 Elapsed time after open to traffic (days)

J

Comparing the traffic-load-induced settlement of subsections G I and M r (modified from Chai and Miura, 2002).

0

~D =

.~ 2 E ~D

, Cm

9

3

GI

d~ 9

4

o

~D 1 year after open to traffic

.= 5 6

Figure 14.

i

0

i

I

1

i

I

B

i

i

2 3 Vertical strain (%)

I

4

i

5

Calculated plastic strain distribution with depth (modified from Chai and Miura, 2002).

(3) Distribution of traffic-load-induced plastic strain with depth. T h e c a l c u l a t e d plastic strain distribution with d e p t h is d e p i c t e d in F i g u r e 14 for 1 y e a r after o p e n e d to traffic condition. It indicates that for the case c o n s i d e r e d , the significant i n f l u e n c e d e p t h o f traffic l o a d is a b o u t 6 m b e l o w the b a s e o f the e m b a n k m e n t ( p a v e m e n t and b a s e - c o u r s e ) . Also, the plastic strain r e d u c e d v e r y q u i c k l y w i t h i n the u p p e r 2 m.

Chapter 10

296

5. TOTAL COST OF LOW EMBANKMENT ROAD CONSTRUCTION ON SOFT SUBSOIL

5.1.

Concept of total cost

In the case of a road, the total cost mainly consists of construction cost (initial cost) and maintenance cost. It is generally true that the initial cost and maintenance cost is in reversely related, i.e. higher initial cost will require less maintenance cost, and vice versa. The maintenance cost can be divided into direct and indirect costs. The direct cost is the cost for repairing the road and the indirect cost is the effect of repairing the road to social activities. For a low embankment road on soft ground, the repairing cost is directly related to residual settlement and can be divided into two types again, namely, the cost for repairing of the differential settlement and the cost for repairing the cracks and ruts. (1) Repairing of differential settlement. Along a road, the bridges and box culverts are generally supported by piles, which penetrate into stiffer layers, and therefore, have less settlement than the road section directly on soft deposit. The differential settlement will influence traffic and have to be repaired. Figure 15 shows an example of the differential settlement in Saga, Japan. In Japan, it is required that (a) when the different settlement reaches 30 mm, it must be repaired (Japan Road Association (JRA), 1983) and (b) the repaired road must satisfy the required slope for a vehicle speed at 60 km/h (JRA, 1984). Figure 16 shows the scheme/sequence of repairing. (2) Repairing the cracks and ruts. For a low embankment road on soft subsoil, if the subgrade is not strong enough, traffic load can cause cracks and ruts in pavement and they have to be repaired by overlaying of a road with a layer of asphalt.

Figure 15. Exampleof differential settlement in Saga, Japan.

Cement~Lime Mixing Ground Improvement for Road Construction

297

As shown in Figure 17, a lower initial cost implies a higher residual settlement and higher maintenance cost. Ideally, there is an optimum initial investment, which will result in a lowest total cost. The indirect cost includes the social influence of blocking a road for repairing and the uncomfortable feeling of driving through a road with differential settlement. However, it is not easy to evaluate quantitatively.

5.2.

Traffic intensity definition in Japan and estimated direct repairing cost

In Japan, the traffic intensity can be divided into four groups: A to D-traffic (Uchida, 1988). The traffic volumes are as follows: A-traffic: 100-250 vehicles/day/one-way B-traffic: 250-1000 vehicles/day/one-way

Box culvert

[_ I-

"-1

Repairing range Figure

16.

Repairing of differential settlement. (After Miura, 2002; reproduced with the permission of International Association of Lowland Technology.)

I

I

I

I

I

I / 7 /

/ / -

Total cost

_

nee

I

I

I

I

I

I

I

Residual settlement

Figure

17.

Concept of total cost. (After Miura, 2002; reproduced with the permission of International Association of Lowland Technology.)

Chapter 10

298

C-traffic: 1000-3000 vehicles/day/one-way D-traffic: >3000 vehicles/day/one-way Generally, for A- and B-traffic, the width of the road is about 7.0 m (1 lane in each direction) and for C- and D-traffic, the width of the road is 24 m (2-3 lanes in each direction). To repair the different settlement between the crossroad structures (box culverts) and the adjacent road sections, there are different requirements and of course different costs. The strictest requirement is to satisfy the preferable radius (slope) of curvature in longitudinal direction of a road with a value of 0 in Figure 16 of 0.64 ~ for the speed of 60 km/h (will be called the best method). The next less strict requirement is to satisfy the minimum radius of curvature with a value of 0 in Figure 16 of 0.77 ~ (will be called the second best method), and the least strict requirement is to have minimum overlay with a 0 angle of 2.86 ~ (will be called conventional method). As mentioned previously, when the different settlement reaches 30 mm, the repairing will be carried out. Using a unit price of 1400 JP Yen/m2/30 mm, which was the price in Japan in 1995 (Miura, 2002), and assuming that the residual settlement equals the differential settlement (no residual settlement for box culvert structure), Miura (2002) calculated the repairing cost per box culvert as a function of residual settlement (Figure 18). It can be seen that repairing cost increased rapidly with the increase of residual settlement. Also, the difference between the best method and the second best method is not significant, and the difference between the conventional method and the best and the second best method is substantial.

/.~3

.....

I

I

=

9

0

~

20

~

9

l

I

i

i

I

CY-- 9 Best method

W

A []

9 Second best 9 Conventional

_

15-r O d~

10 w

Width 24m

9

Width 7m

0

.9

5 --

r 0 0

Figure 18.

10

40 20 30 Residual settlement (cm)

50

Repairing cost per box culvert structure. (After Miura, 2002; reproduced with the permission of International Association of Lowland Technology.)

Cement~Lime Mixing Ground Improvement for Road Construction

5.3.

299

An example calculation of total cost

For the 10 ground improvement methods adopted for the test section, the initial construction cost has been estimated as in Figure 19 based on the actual price in Japan in 1995. The cost differences between C- and D-traffic and between A- and B-traffic are due to the different requirement on pavement and base-course structure (JRA, 1994). Considering ground improvement scheme C I and C m (highest and lowest initial construction costs as shown in Figure 19), the initial construction cost for a 7-m-wide road (B-traffic) is about 100 and 50 million JP Yen/km, respectively. Figure 12 shows that C I had almost no residual settlement (traffic-load-induced) and C m had about 40 mm. Considering a service time of 20 years, this differential settlement may reach about 50 mm or more. From Figure 18, for a 7-m-wide road (the width of the test road was 7.5 m), the repairing cost per box culvert is about 0.2 million JP Yen for a residual settlement of about 50 mm. Further assuming 10 box culverts/km, the repairing cost will be about 2 million JP Yen/km. For the test road, in about 2 years period, there was no obvious ruts observed and the overall overlay was not conducted and it is considered to be not carried out in the future. Then the total cost will be 100 and 52 million for C~ and C m ground improvement methods, respectively, and C m method is cheaper. Here, the social cost of blocking the road for repairing is not considered. Now let us consider a D-traffic and 24 -m-wide road. From Figure 19, the initial construction costs are about 260 million JP Yen/km for C m method and about 440 million JP

700

I

I

I

I

I

I

1

I O

I I A-traffic B-traffic

m F-I

C-traffic D-traffic

9

600 Width 24 m 500 O

[3

D m

~400 c~ 0

o 300 .,.~

D m -

D

-

m

D n m

[f]

-

m

-

D D m

r~

= 200

0

-

m

-

Width 7m -

.,..~ ..o .,..~

~

= 100 I Ls

Figure 19.

I Es

;~

I I I I I I I Mf C I CII G I G n C m S F Subgrade and base course method

I Ss

Initial construction cost for each method. (After Miura, 2002; reproduced with the permission of International Association of Lowland Technology.)

Chapter 10

300

Yen/km for C I method. To have a realistic estimation of residual (or traffic-load-induced) settlement, two other case histories, Saga airport road and the test section at national road No. 34, Hyogo Machi, Saga, Japan, are briefly introduced. (1) Saga airport road. Saga airport road was constructed from 1990 to 1992 with a total length of 9 km. At the site, there is about 20 m thick, highly compressible and highly sensitive Ariake clay deposit. The total thickness of the road embankment was about 1.1 m, as shown in Figure 20. The material for subgrade was decomposed granite (it is called masado in Japan). Although the lime (instead of cement) was used for stabilizing subgrade, generally, the road structure was close to C m of the test road. The total width of the road is 20 m, in which 11 m (2 lanes) are for traffic and 4.5 m on each side as sidewalk. Saga airport road is the main access road from Saga city to Saga airport. It was used for transporting construction materials (fill materials and others) during the airport construction. The traffic intensity of heavy trucks (200 kN/truck) was about 400 trucks/day (oneway). Settlements were monitored for more than 4 years at three monitoring stations. The measured residual (mostly the traffic-load-induced) settlement was 0 . 2 - 0.3 m (Chai and Miura, 2002). Except the repeated repairing of differential settlement between box culvert structures and adjacent road sections, there was an overall overlay after about 21/2 years of the end of construction. If considering a service time of 20 years and a more intensive traffic condition, the residual settlement can be estimated to reach about 0.4 m or more. (2) Test section at National road No. 34, Japan. At Hyogo Machi, Saga, Japan, the width of national road No. 34 was 12-14 m wide and it had been decided to expand the road to four lanes. To further confirm the effect of some of ground improvement methods, a test section of 135 m long with three different ground improvement methods were constructed in 2001 and opened to traffic in March 2003. One subsection used soil-cement slab-column system, and its cross-section is shown in Figure 21. The ground improvement scheme was close to C I of the test road. The area improvement ratio by soil-cement column was 12.6%. The amount

., ,.~. ~ ~

.,

~.

9

.~

. ,,,~,

..,

~

0.1 m asphalt concrete

,'~

: ~ ~. ~.~,~ :;5..~*..,~ ~ef~%;%~:.;,~.~,;,:~

9 . ~, .. ~ - , . ~ . , . . . . , , ' . .~ ~ ' , ~ . . t , ' , ~ . ~ ' , ~ " . ~ . . ' , . . : . ,~ ".~., ~ , ~ : , . : . ~ . , . . ~ . ~ , ' ~ ' , ~ ~ . . \ ' ~ . ,'.i...~.~^~

.~.

9 ~"

~- ~ ' ~ ; -;., %~:.~ ~ , ~'~'~;:'.," : % S , . ' ~ 5 ",.:~'..~: i .....

.,

'.:

.~'.

~ .... ' r

,~

,.

0.3 m gravel

,? .,~-'

base-course 0.4 m lime treated upper subgrade 0.3 m compacted lower subgrade Figure 20. A typical cross-section of Saga airport road.

Cement~Lime Mixing Ground Improvement for Road Construction 14

21.0

12.0 m ~

301

L = 45.0 m

.!:!:!:!:!:!:!:!:!:!,

N Plane view Pavement

,

........................... s

i:i',l;l::':

l'l: ': ~'| N i;It i ~i':lil~i~~ ,,i~i : Slab 5.5 m !i i:~:,:i, 1[I:','i~::lili:i:~lili:,~l,I w.:i::w.i:ilil::i~:ti]:ii:l~:i[~ :i: lit~i~ :i:. :: 1.0 m thick

A-A section Figure 21.

Plan and cross-sectional view of ground improvement scheme at Hyogo Mach, Saga, Japan.

of the cement used was 1.67 kN/m 3 of soil and the designed unconfined compressive strength of the column was 600 kPa. At the site, the thickness of soft Ariake clay is about 10 m. Traffic intensity has been about 16,000 cars/day, including about 2,800 trucks/day (one-way). One year after open to traffic, the measured settlement (vaffic-load-induced) was 4 mm (Motohara et al., 2004). This test section can be considered as maintenance-free. Considering a road of 24 m wide and residual settlement of 0.4 m (C m ground improvement scheme in Saga area), the maintenance cost for differential settlement between box culverts (10 box culverts&m) and adjacent road sections will be about 150 million JP Yen/km (Figure 18). Just consider one overall overlay (30 mm thick) and the cost will be about 34 million JP Yen/km. The total maintenance cost will be 184 million JP Yen/km. This added to the initial construction cost of about 260 million JP Yen/km, will total to 444 million JP Yen/km. If we adopt a ground improvement scheme of C I, it is almost maintenance-free and the total cost will be 440 million JP Yen/km. It is almost the same as the total cost of the ground improvement scheme of C m. When considering the social impact of blocking the road for repairing, ground improvement scheme of C I should be preferable.

6.

CONCLUSIONS

A road test section of about 1 km long on soft Ariake clay deposit with 10 different ground improvement methods is described, and the field monitored results on settlements in a

302

Chapter 10

period of more than 1 year and BBT and FWD test results are reported. At the test site, the soft clayey layer was about 18 m and the height of the road embankment was about 0.75 m (including the pavement). Both the settlement and deflection test results indicate that an increase in the strength/stiffness of the base-course and the subgrade and/or an increase in the improvement depth of the subgrade of a lower embankment road on soft subsoil can reduce the traffic-load-induced settlement and the deflection of the road significantly. Also, the field data indicate that cement treatment was more effective than lime treatment of soft Ariake clay, and a layer of geogrid in the base-course had a positive effect on reducing deflection of the test road. For a low embankment road, the traffic-load-induced settlement consists of a larger part of residual settlement and controls the maintenance cost of a road. The methods for predicting the traffic-load-induced settlement can be divided into three groups, namely, numerical simulation methods, equivalent static load methods and empirical equations. Comparing with the measurements of some subsections of the test road reported here shows that Chai and Miura's (2002) empirical equation, which consider the number of the traffic load application, strength and compression index of soft subsoil, and the trafficload-induced stress distribution in the ground, is a useful tool for predicting the trafficload-induced settlement. The concept of total cost (construction cost and maintenance cost) is introduced and conceptually there is an optimum combination of initial construction cost and maintenance cost to result in a lowest total cost. It is demonstrated that for a low embankment road on soft subsoil with intensive traffic (D-traffic in Japan), improving the subsoil by soil-cement slab-column system to a depth of 4-5 m from the base of a road embankment is attractive in terms of the total cost.

REFERENCES

Burmister, D.M. (1945) The general theory of stresses and displacements in layered system I, J. Appl. Phys., 16, 89-94. Chai, J.C. & Miura, N. (2002) Traffic-load-induced permanent deformation of road on soft subsoil, J. Geotech. Geoenviron. Eng., ASCE, 128(11) 907-916. Civil Engineering Department, Saga Prefecture (1996) A Study on Predicting the Differential Settlement and Rational Design Method for Road on Soft Subsoil, Research Report, Saga Prefecture, Japan, p 77 (in Japanese). Fujikawa, K. (1996) On optimistic design of low embankment road on soft subsoil by considering the traffic-load-induced Settlement, Dr. of Engineering Dissertation, Saga University, p 199 (in Japanese). Fujikawa, K., Miura, N. & Beppu, I. (1996) Field investigation on the settlement of low embankment road due to traffic load and its prediction, Soils Found. J. Geotech. Soc., 36(4) 147-153 (in Japanese).

Cement~Lime Mk~ing Ground Improvement for Road Construction

303

Hyodo, M., Yasuhara, K. & Murata, H. (1996) Deformation analysis of the Soft Clay Foundation of Low Embankment Road Under Traffic Loading, Proc. 31st Symp. of Japanese Society of Soil Mech. Found. Eng., 27-32 (in Japanese). Japan Road Association (JRA) (1983) Road Maintenance and Repairing Instructions (in Japanese). Japan Road Association (JRA) (1984) Road Structure M a n u a l - Instructions and Practice. pp. 286-297 (in Japanese). Japan Road Association (JRA) (1994) General outline of asphalt pavement (in Japanese). Kitazume, M. (1996) Deep mixing method, Found. Eng. Equip., 24(7) 14-19 (in Japanese). Ladd, C.C. (1991). Stability evaluation during stage construction, J. Geotech. Eng., ASCE, 117(4) 541-615. Li, D. & Selig, E.T. (1996) Cumulative plastic deformation for fine-grained subgrade soils, J. Geotech. Eng., ASCE, 122(12) 1006-1013. Li, D. & Selig, E.T. (1998) Method for railway track foundation design.II:application, J. Geotech. Geoenviron Eng., ASCE, 124(4) 316-322. Mayne, P.W. & Kulhawy, F.H. (1982) Ko-OCR relationships in soils, J. Geotech. Eng. Div., ASCE, 108(6) 851-872. Miura N. (2002) A Field Study Concerning Total Cost of Road Construction on Soft ground, Proceedings. of International. Symposium on Lowland Technology 2002, Eds. Hayashi, S., Araki, H. & Hokao, K., Institute of Lowland Technology, Saga University, Japan, pp 1-10. Miura, N., Fujikawa, K., Sakai, A. & Hamatake, A. (2000) Consideration of the total costs for construction and maintenance of pavements on soft ground based on test road performance, J. Civ. Eng. Jpn. Soc. Civil Eng., No. 659/III-52, 253-263 (in Japanese). Miura, N., Sakai, A., Taesiri, Y., Yamonouchi, T. & Yasuhara, K. (1990) Polymer grid reinforced pavement on soft clay ground, Geotex. Geomembranes, 9, 99-123. Monismith, C.L., Ogawa, N. & Freeme, C.R. (1975) Permanent Deformation Characteristics of Subsoil Due to Repeated Loading, Transp. Res. Rec. No. 537, Transportation Research Board, Washington, D.C., pp. 1-17. Motohara, K., Chai, J.-C. & Miura, N. (2004) Prediction of Traffic-Load-Induced Permanent Settlement of Low embankment Road on Soft Ground, Proceedings. of 59th Annual Meeting of Japanese Society of Civil Engineering, pp 963-964 (in Japanese). Osterberg, J.O. (1957) Influence values for vertical stresses in semi-infinite mass due to embankment loading, Proc. 4th Int. Conf. Soil Mech. Found. Eng., vol. 1. Shen, S.-L. (1998) Behavior of deep mixing columns in composite clay ground. Dr. of Engineering dissertation, Saga University, p 247. Uchida, I. (1988) Road Engineering. Morikita Publisher, Tokyo, Japan (in Japanese).