NEW SOIL TREATMENT METHODS IN JAPAN
H. Miki
Public Works Research Institute, Tsukuba-Shi, Japan
[email protected]
J. Iwabuchi
Japan Association of Representative General Contractors (JARGC),
Central Research Institute for Construction Technology, Tokyo, Japan
[email protected]
S. Chida
Chida Engineering Co., Tokyo, Japan
[email protected]
ABSTRACT
This paper reports of several new soil treatment methods in Japan, which include 1) a
liquefied soil stabilizing method to re-use construction sludge and excavated surplus soils,
2) a high-speed continuous soil treatment system using pipe-line mixers and a light-weight
banking method which utilize in-situ surface soil without purchase and carrying of fill
materials.
Liquefied soil stabilizing method involves adding stabilizer to slurried soil, and enables the
laying of soil with stable quality that requires no compacting at construction sites. The
application of this method has recently been increasing in Japan for backfilling
underground multi-purpose ducts or other underground structures. This method has a
major advantage in that muddy construction sludge, dredged material and various other
types of soils generated from excavation can be effectively used, thus both the waste
disposal cost and the production cost of stabilized soil slurry can be reduced.
An attempt to use lightweight foam mixed soil throughout the entire road embankment
cross-section on a soft ground was shown to be applicable, in which in-situ surface soil
was used to make a high lightweight soil embankment, which means that the in-situ
surface soil is expanded to attain the required volume of the embankment. Pipe-line
mixers were used to produce lightweight foam mixed soil in this case.
KEY WORDS
SOIL STABILIZATION / CONSTRUCTION SLUDGE / SURPLUS SOIL / PIPE-LINE
MIXER / LIGHT-WEIGHT BANKING METHOD / MATERIALS RECYCLING
1. BACKGROUND OF SOIL RECYCLING IN JAPAN
The results of a survey for the fiscal year of 2000 indicate that approximately 208 million
m3 of soil was generated from construction work around the country, of which only about
30% was re-used for construction work. To promote the use of such soils, it is essential to
solve the differences in timing and quality. Particularly, technology to use low quality soil
with high water content needs to be developed.
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Present conditions and issues regarding the effective use of soil generated from
construction work are summarized below.
(a) The amount of soil that needs to be carried out of construction sites is far larger than
the amount of soil used, meaning that there exists a surplus of soil.
(b) On the other hand, large amounts of new materials, such as sand from mountains, are
used, and this has a significant impact on the natural environment.
(c) A large amount of surplus soil is transported to inland reclamation sites, and part of
such soil is inappropriately disposed of.
(d) Proper measures are needed to correct the problems of polluted soil and soil mixed
with waste.
(e) Sludge generated from construction work and polluted soil exacerbate the shortage of
final disposal sites.
(f) One important point when promoting the effective use of soil generated from
construction work is to develop technologies to use low quality soil with low cone index
values, such as mud.
2. LIQUEFIED SOIL STABILIZING METHOD
2.1. Outline
The liquefied soil stabilizing method (LSS) involves adding stabilizer to slurried soil, and
injecting the slurry directly from the transport vehicle or pressure-feeding the slurry from
the plant. This method enables the laying of soil with stable quality that requires no
compacting at construction sites.
The application of this method has recently been increasing; for example, it is used for
backfilling underground multi-purpose ducts, the foundation work for structures, filling
cavities and lightweight embankment. This method has a major advantage in that sludge
generated from construction work, dredged material and various other types of soils
generated from excavation can be effectively used.
Even the muddy sludge generated from construction work can be used as high quality
slurrying material, thus both the waste disposal cost and slurrying cost can be reduced.
2.2. Re-use of muddy soils
According to the soil classification standard in the “Technical Manual for Utilization of
Excavated Soils from Construction Works” issued by the former Ministry of Construction,
muddy soils are defined as those having a cone index of about 200 kN/m2 or less and as
the sludge generated from construction works. When disposing of them, the procedures
specified by the Waste Disposal Law must be followed. Since such excavated soils and/or
sludge contain a large quantity of fine grains, the soils cannot readily be reused as
earthwork materials without any treatment.
For recycling such soils, various technical methods such as addition of sand, solidification
by plaster, lime or cement stabilization, and reuse of the soils as raw materials for cement
have been developed, in addition to the dewatering/drying method. In this report, the
technical background and reasoning for classifying the Liquefied Soil Stabilizing Method
(LSS) as solidification by cement stabilization will be introduced as a technical method of
reuse of muddy soils.
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2.3. Concept and histry of LSS
In order to ensure that the stabilizer may be mixed evenly with cohesive soil containing a
large quantity of fine grains, mixing can be facilitated by turning the cohesive soil into
slurry by increasing its water content. In this case, voids in the mixture thus obtained will
be saturated with water. Therefore, such mixture can not be compacted, but can only be
used to fill spaces closely due to its liquidity, thus achieving its purpose by strength
developed after hardening, like placing concrete into a form. This was the original concept
of developing LSS.
As an extension of this concept, the project “Technical Development of Liquefied Soil
Utilization” was conducted from 1992 to 1995 jointly by Public Works Research Institute,
the former Ministry of Construction and Central Research Institute, Japan Association of
Representative General Contractors as a part of the Comprehensive Technical
Development Project “Development of Techniques for Prevention of Generation and
Reuse of Construction By-products” by the former Ministry of Construction. This project
enabled the Liquefied soil stabilizing method to be generalized nationwide, and a
cumulative total of 3,200,000 m3 of LSS has now been applied. In 2003, LSS was
designated by the Ministry of the Environment as a “Green Procurement” item.
2.4. Required property of LSS
Liquefied soil is used mainly for backfilling narrow spaces between structures and/or
vacant spaces in ground where compaction cannot be used. The liquefied soil must have
sufficient strength for transmitting the structural load on the liquefied soil to the surrounding
ground and the liquidity to fill spaces by itself.
Here, strength means greater shearing strength than that of surrounding ground and
compressive strength to avoid consolidation by overburden pressure, also including the
density effect capable of dispersing the load.
Regarding liquidity, sufficient liquidity to fill narrow spaces is related to the value of the
mortar flow test (JHS A 313-1992 / Photo 1) and this is used as the liquidity index of
liquefied soil to be placed at site. The large-scale model test (Kuno, 1996) as shown in
Photo 2, for example, was conducted to examine the relation between the flow gradient of
the liquefied soil placed and the flow test results. As a result, the relation shown in Table 1
was obtained.
Photo 1 – Flow Test
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Photo 2 – Large-Scale Model Test
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Table 1 – Flow Gradient of LSS
Case 1
Case 2
Case 3 (Linear)
Case 3 (Bend)
Flow value
(mm)
120
160
220
Flow gradient
(%)
11.3
2.3
1.9
2.0
On the other hand, since LSS has high liquidity, the phenomenon of material segregation
in which coarse-grained soils settle in slurry and the bleeding phenomenon in which water
floats up to the surface, may occur. From experience, it is known that there is less material
segregation at the bleeding rate of less than 1% in three hours after placement. In order to
ensure the quality, a bleeding test (JSCE-1986) was conducted to confirm that the
bleeding rate is less than 1%.
Thus, the principle of LSS is to design the mix proportion to be suitable for individual
backfilling specifications while satisfying the strength and liquidity as well as bleeding rate
mentioned above for the given construction sludge.
2.5. Mix design and production of LSS
The method of determining the mix proportion is given here for the case when construction
soil containing a large quantity of fine grains is used as the principal material. First, four or
five slurries of different specific gravity are prepared, to which 100 kg of cement is added.
The bleeding rate, flow value and unconfined compressive strength are determined by soil
tests for each of LSS and the values thus obtained are plotted against the specific gravity
of slurry as shown in Figure 1.
Figure 1 – Determination of Slurry Density
Photo 3 – Placement of LSS
Regarding the specifications for backfilling, the unconfined compressive strength must be
0.15 – 0.3 N/mm2 in consideration of re-excavation, and the flow value for liquidity must be
150 – 250 mm. Then, the specific gravity of muddy water capable of satisfying the
specifications for 100 kg of cement is calculated to range from 1.52 to 1.56.
The sludge and/or excavated soils from the construction site are stirred to ensure that the
specific gravity of slurry falls within this range. LSS satisfying the specifications can be
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produced by adding 100 kg of cement to the slurry and mixing. LSS thus produced is
transported to the backfilling site and placed as shown in Photo 3.
2.6. Application examples
As described above, it is important to control the specific gravity of slurry for LSS. With this
background, two cases of reuse are introduced below.
(1) Reuse of general construction slurry
This plant receives 1,000 to 1,500 m3/day of sludge with no cement contained. The sludge
is generated from shield tunneling works, underground diaphragm wall construction and
various types of pile-driving works. The plant also receives sludge containing some cement,
which is generated from deep ground jet mixing and soil/cement wall construction.
As sludge with consistent properties will yield LSS of better quality, an experienced
engineer inspects the received sludge for any changes in properties. When the place and
type of construction changes, the properties of sludge change and then soil classification
tests are conducted.
A scene of sludge stirring is shown in the left side of Photo 4; during stirring, the specific
gravity and sand content of the sludge are confirmed by measuring instruments. The
machine shown in the top center of this photo is a vibrating screen in which adjusted slurry
and foreign matter of more than 7mm in grain size that have been pumped up by a sand
pump are removed. Under the vibrating screen, there is a slurry storage tank in which twostage stirring vanes rotate to prevent coarse-grained soils from settling. In the top right of
the photo is a sand stock yard from which the required quantity of sand is charged into a
mixer car when producing high-density LSS.
The mixing plant consists of a tank with specific gravity meter for monitoring the condition
of incoming slurry, flow meter to measure the amount of slurry produced, a flow jet mixer
type of mixing device, and a control room in which various quality tests are conducted.
Photo 4 – LSS Plat with Sludge Stirring
Photo 5 – LSS Plant with Shield Sludge
(2) Reuse of shield mud only
This plant receives only high-density sludge obtained by removing water from raw sludge
at the site of shield tunneling works. In case of shield works, since excavation is made
deep underground and the soil strata typically consist of Diluvial silt/clay layers, stable soil
material is obtained. The sludge carried into the plant is high density, having generally
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been concentrated to the density of 1.5 g/cm3. Therefore, the system is designed so that
construction sludge of 1.2 g/cm3 or less in density generated from earth drilling works is
stored for adjusting the high-density sludge and is added whenever necessary.
A general view of the plant is shown in Photo 5. Although the common LSS plants are
equipped with a soil stock yard and a mud stirring facility, this plant has no such facilities
and so appears neatly arranged. As shown, construction sludge is received at the mouth
for high-density sludge and at that for general sludge, then fed to a high-density sludge
tank and a general sludge tank, respectively. In these tanks, there are 5-stage stirring
vanes for suppressing settling, which are rotated in such a manner that the slurry may be
stirred upward. In the liquefied soil production process, the required quantity of slurry is fed
from both tanks to the tank shown in the center of Photo 5 for adjusting the specific gravity.
In case of high-density LSS production, materials for density adjustment are separately fed
into the tank and mixed with the slurry to achieve the required density.
In the top left of Photo 5 is a mixing plant equipped with a 1 m3 biaxial forced action paddle
mixer and a 2 m3 liquefied soil storage tank. The slurry carried in is controlled by computer
in the shipment control room, sent through pneumatic tubes to the mixer, then finally the
product is transported by an agitator car located below the mixer.
3. PIPE-LINE SOIL TREATMENT SYSTEM
3.1. Outline
For the purpose of utilization of dredged mud, pipe-line soil mixing methods are getting
popularized in Japan. The pipe-line treatment system has been developed as a kind of the
pipe-line soil mixing methods. The system is comprised of so called “Kanro Mixer” installed
on the way of dredging pipe line and feeder devices for mixing materials. The system can
be utilized for not only the consolidation of mud but making the foam mixed soil, producing
the grainy soil and so on.
3.2. Process to produce lightweight foam mixed soil
Figure 2 shows the continuous processing approach employed for the construction of the
light-weight embankment.
1. Water is added to the untreated soil material and the mixture is transferred to the mud
conditioning tank.
2. In the mud conditioning tank, the specific gravity of the mud is measured and extra
water added as required.
3. The mud is then transferred to the agitator tank for churning.
4. The mud is pumped at fixed pressure by a pressure pump.
5. The mud is kneaded together with cement milk in the primary line mixer located along
the pump line, then pumped out under pressure.
6. The mud and cement milk mixture is combined with foam in the second line mixer,
then pumped out under pressure.
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Water
Mud
Density control tank
Slurrｙ plant
Agitator
Mud pump
Preparated mud
Pipe line mixer(1)
Foam mixed soil
Cement slurry
Pipe line mixer(2)
Foam
Figure 2 - Production system for foam mixed light weight soil
4. LIGHT-WEIGHT BANKING METHOD USING IN-SITU SURFACE SOIL
4.1. Outline
Foam mixed soil has been used extensively in Japan for road widening and back-filling
projects, but never throughout the entire road cross-section. An attempt to use lightweight
foam mixed soil throughout the entire embankment cross-section on soft ground was done
in which cohesive soil taken in situ from the surface of the ground was used to make a
high lightweight soil embankments, which means that the in-situ soil is expanded to attain
the required volume.
4.2. Concept of a new lightweight banking method
An attempt to use lightweight foam mixed soil throughout the entire embankment crosssection on soft ground was done in which cohesive soil taken in situ from the surface to
the depth of 2m was used to make a 7m high lightweight soil embankments, which means
that the in-situ soil is expanded up to about four times to attain the required volume (Figure
3).
The ground is a soft ground of around ten meters in depth, consisting of alluvial cohesive
soils having an intermediate sand layer (Figure 4).
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8
7
1
22
2
Foam m ixed so i l
2
WL
aHu
Cemen t m ixed so i l
aA s
2
38
2
aA c
aA s
F ig .2 C lo ss sec t ion o f des igned embankmen t
Figure 3 - Cross section of designed embankment
GL
+0 .90
-0 .10
-2 .00
G round su r f ace
ƒÓ
C
Cv
WL
Hasu ike
Sand
( aHu )
( aA s )
-5 .40
A r iake c lay ( aA c )
-9 .80
-10 .90
ƒÁ
Sand
( aA s )
14 .0
-
19 .0
23 .0
15 .0
-
5 .7+1 .61 z
17 .5
20 .0
-
13 .9+1 .18 z 150
-
-
150
-
ƒÁ?F un i t w e igh t ( kN/m3 ) , ƒÓ?F f r ic t ion ang le (?‹ ?j?A C?F cohe s ion ( kN/m2 )
(z=0 a t +0 .9mFigure
GL ) , Cv?F
f f strata
ic ien t omodel
f con soand
l ida tsoil
ion constants
( cm2 / day )
4 -coe
Soil
?} ?] ,P
“y‘w ƒ, ƒf ƒ‹,Æ “y?¿ ’è ?”
4.3. Mix design
The foam mixed with soil design involves choosing a hardening agent that satisfies the
design requirements (specific weight = 8 kN/m3, uniaxial compression strength = 600
kN/m3) using soil from the site, and choosing the type and quantity of the foaming agent.
Table 2 summarizes the characteristics of the soil used. Based on the results of the trials,
the mix ratio shown in Table 3 was adopted.
Table 2 – Properties of site soil (surface soil)
表‐１
現地発生土の土質特性
Grain size distribution (%)
Water content
(%)
Wet Density
(g/cm3)
Sand
Silt
Clay
110
1.42
3.8
55.8
40.4
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Table 3 – Design mix proportion of foam mixed soil
Foaming
agent
Surfactantbased A
Foaming
Target
Target
Added Foam
Dry soil Water
agent
density strength qu
cement content
content
(kg/m3) (kg/m3)
3
2
3
3
(g/cm ) (kN/m )
(kg/m ) (l/m )
(l/m3)
0.8
600
137
441
200
420
1.05
Flow
value
(mm)
160 mm
or
greater
4.4. Construction Method
Foam mixed with soil made with site soil has many benefits apart from reducing the load
on the ground; it also makes purchase and carrying of fill materials unnecessary, and
allows greater flexibility with respect to strength and specific weight values.
In the trial, a high-speed continuous processing system was employed instead of
conventional batch production in an attempt to reduce costs.
The ground is stabilized from a depth of two to four meters. The soil of top two meters at
the surface was used to produce lightweight fill. The embankment was constructed by first
building a trapezium-shaped mound of height 0.7 m around the periphery of the
embankment, then filling it with the foam mixed soil. Ten stages were built in this way to
achieve a total height of seven meters. Geogrid layers were laid at intervals of 2.1 m (three
stages) to add reinforcement to the structure.
The sloping faces are sprayed with a sed-mud-chemical mixture to create vegetation cover.
Figure 5a) shows the construction process in progress while Figure 5b) shows the
completed embankment.
a) Embankment construction in progress
b) Completed embankment (10 stages)
Figure 5 - Construction process in progress
4.5. Monitoring results
Settlement of the embankment measured were only 29 mm in the base plane. Likewise,
lateral displacement was virtually nil, thanks to the shallow stabilization on the ground
surface; the maximum observed lateral displacement was 40 mm.
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5. CONCLUSIONS
Among the technologies for using soil and/or construction sludge generated from
construction work, liquefied soil stabilizing method (LSS) is promising in Japan. This
method has a major advantage in that sludge generated from construction work, dredged
material and various other types of soils generated from excavation can be effectively
used. Even the muddy sludge generated from construction work can be used as high
quality slurrying material, thus both the waste disposal cost and slurrying cost can be
reduced.
For the purpose of utilization of dredged mud, pipe-line soil mixing methods are getting
popularized in Japan. The system can be utilized for not only the consolidation of mud but
making the foam mixed soil.
An attempt to use lightweight foam mixed soil throughout the entire embankment crosssection on soft ground was shown to be applicable in which cohesive soil taken in situ from
the surface of the ground was used to make a high lightweight soil embankments, which
means that the in-situ soil is expanded to attain the required volume.
REFERENCES
Kuno, G., Miki, H., Mori, N., Iwabuchi, J. (1996) Application of the Liquefied Stabilized Soil
Method as a Soil Recycling System. Proceedings of the Second International Congress on
Environmental Geotechnics, pp791-796
Miki, H., Mori, M., and Chida, S. (2003) Trial embankment on soft ground using lightweight
foam-mixed in-situ surface soil, XXIInd PIARC World Road Congress, Durban, CDROM
Miki, H. (2005) Geosynthetic Reinforcement for Marginal Soils and Soft Foundations:
Japanese Perspectives, Geo-Frontiers 2005, ASCE, CD- ROM
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