January 2023 No.97

Japanese Infrastructure Newsletter

IDI

QUARTERLY
Infrastructure

Development Institute—JAPANCONTENTS
Air-Water Separation System for the Vacuum Consolidation Method

Infrastructure Development Institute – Japan (IDI)
Plaza Edogawa-bashi 3F. 1-23-6, Sekiguchi, Bunkyo-ku, Tokyo, 112-0014, JAPAN
Tel: +81-3-5227-4107 Fax: +81-3-5227-4109 E-Mail: idi17@idi.or.jp Website: http://www.idi.or.jp/en/1Infrastructure Development Institute - Japan

Air-Water Separation System for the Vacuum Consolidation Method
Maintaining vacuum pressure at a high level during soft ground improvement using thevacuum consolidation method with air-water separation system
1. Introduction
The Vacuum Consolidation Technology
Association was established in 2000 comprisingconsultants and contractors, with the aim todevelop soft ground improvement by means of
the vacuum consolidation method, which is used
to accelerate consolidation progress to promote
ground settlement and increase groundstability during construction.
Kjellman (1952) was the first person who
introduced this method. The basic components
are vertical drains, a filter drainage layer, anairtight sheet and a vacuum pump. By means ofa vacuum pump, a reduced pressure is createdinside the improved area and distributed
through the vertical drains installed in the softground. The airtight sheet acts as a barrier toseparate the atmospheric pressure between theinside and outside of the improved area. The
difference in atmospheric pressure is a loading
force to consolidate the soft soil.
Vacuum pressure is the key to success inthe vacuum consolidation method. The qualityof ground improvement work can be assured bycarefully checking and maintaining vacuum
pressure under the airtight sheet.
Practically,thevacuum
pressuremeasured under the airtight sheet will bearound 60~80 kPa at the beginning of vacuum
operation. However, it was found that thevacuum pressure measured under the airtightsheet gradually decreased during vacuum
pumping operation. Therefore, the performanceof this method was considered not to meet thedesign load level.
This document presents case histories ofground improvement projects in Japan using
the vacuum consolidation method, the problemof vacuum pressure decrease with its casestudies, and the concept of the air-water
separation system, which was developed tosolve the above-mentioned problem.

projects in Japan, it was found that the vacuum
pressure measured under the airtight sheet wasgradually decreasing during vacuum operationprocess.
Sand blanket

Air-water drainage pipe
Air-water pick up

Airtight sheet

WaterAir
Separation tank
Sheet edge
treatment

Water pump
Soft clay

Vacuum pumpVertical drain

Figure-1 Conventional vacuum consolidation
system in Japan

Two of the case studies from different
locations in Japan during years 2000-2003 wereselected to be presented: One in Kushiro and theother in Noshiro.
Kushiro, located in Hokkaido, Japan, is
where National Highway No. 38 wasconstructed over a soft ground area. Figure-2
shows the field monitoring data from Kushiro.
The vacuum pressure measured at the pump
was about 85~92 kPa during vacuum operation.
The vacuum pressures measured at two
different points under the airtight sheet were
low at the beginning of operation and gradually
increased to 50~60 kPa prior to the start ofembankment operation, as shown in Figure-2(c).
The consolidation settlement induced by
vacuum pressure and embankment loads wasabout 241 cm.
It was found that the decrease in vacuum
pressure somehow relates with the quantity ofground settlement. Figure-2(d) shows thevacuum pressure difference between at thepump and under the airtight sheet. The change
in vacuum pressure based on the elevation headloss calculated from the settlement data, is alsoplotted in Figure-2(d) for comparison. It was
found some agreement in this case.

2. Loss of vacuum pressure
The
schematicofthevacuum
consolidation method developed in Japan shownin Figure-1 is based on the original system
proposed by Kjellman (1952). According to thefield observation data obtained from several
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100
200300(b)80
6040at pumpunder airtight sheet-1
under airtight sheet -2200(c)

100

different pressure at pump-under airtight sheet-1
different pressure at pump-under airtight sheet-2
pressure head loss by settlement80
6040(d)01-Nov-00

01-Aug-00

01-Jul-00

01-Jun-00

001-Oct-00

20(a)Date(b)100
80
60
40at pumpunder airtight sheet-1
under airtight sheet -2200(c)

35
30
25
20
15
10
50
01-Nov-02

100

Different pressure (kPa) Vacuum pressure (kPa)

Settlement (cm)(a)

different pressure at pump-under airtight sheet-1
different pressure at pump-under airtight sheet-2
pressure head loss by settlement(d)01-Apr-03

0.0001-Mar-03

2.001-Feb-03

4.001-Jan-03

6.0

14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0050
100
150
200
250
300

01-Dec-02

8.0

400
Different pressure (kPa) Vacuum pressure (kPa)

Embankment thickness (m)10.001-Sep-00

Settlement (cm)Embankment thickness (m)Infrastructure Development Institute - Japan

Date

Figure-2 Field monitoring data from Kushiro

Figure-3 Field monitoring data from Noshiro

However, it can be seen that the vacuum
pressure difference was smaller than theelevation head loss. This is probably due tosome leakage of vacuum pressure throughintermediated sand layers existing under thesubsoil.
Noshiro is located in Akita, Japan. The
project is a highway bypass construction over asoft ground area in the city. Figure-3 shows thefield monitoring data from Noshiro. As shown in
Figure-3(c), the vacuum pressure measured at
the pump was about 68~88 kPa during vacuum
operation. On the other hand, the averagevacuum pressures measured at two different
points under the airtight sheet were about 60
kPa at the beginning of operation. The vacuum
pressure at the pump slightly decreased during

backfilling embankments up to 6 m and thengradually returned to over 80 kPa. The
consolidation settlement induced by vacuum
pressure and embankment loads was about 237
cm.
Figure-3 (b) and (d) shows the vacuum
pressure difference between at the pump andunder the airtight sheet, and the elevation headloss due to consolidation settlement. It can beclearly seen that the difference between thevacuum pressure difference and the elevationhead loss is smaller than in the case ofKushiro.This could be considered that the lossof vacuum pressure at this site was less than at
Kushiro due to the absence of intermediatedsand layers.
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3. Air-Water Separation System
Based on the aforementioned problem,the concept of vacuum consolidation wasrevised and analyzed.
Figure-4 shows the basic pore pressure
profile during depressurization of atmosphericpressure in the vacuum consolidation system.
Imai (2005) described the use of a vacuum
pump and its limitations based on the use ofhydraulic fluid.
Figure-4 Basic vacuum consolidation system
and water pressure profile (Imai, 2005)

Figure-5 (a) shows the simple schematicof using a vacuum pump to lift up water andFigure-5 (b) shows the pressure diagram
corresponding to the schematic in Figure-5(a),where pa and pv are atmospheric pressure andvacuum pressure, respectively. At point A andpoint D, the pressure is equal to atmosphericpressure. At point B, the absolute pressure isequal to pa-pv.
From point B to point C, there is only air,
therefore, the pressures at point B and point C
are equal when the head loss in the pipe isignored. From point C to point D, the absolutepressure linearly increases to atmosphericpressure. Therefore, the maximum height ofwater lifted up by means of the vacuum pumpis equal to the value in the following inequality.
Figure-5 Reason and limitation of water lift-up
P
Pby vacuum pump (Imai, 2005)H  v  a(1)ww
Hence, water cannot be discharged through the vacuum pump if the elevation head at B-D ishigher than H. This limitation can be solved by discharging water below point C by the water pump,as shown in Figure-5 (a).
Based on the this principle, the conventional vacuum consolidation system was modified to beequipped with air-water separation system embedded under the airtight sheet.

Figure-6 shows the pressure diagramcomparison for the vacuum consolidation
system without and with air-water separation
system. In a static condition, the hydrostaticpressure is equal to Line 2. After atmosphericpressure is depressurized, the hydrostatic line
will change to Line 1 or Line 3 for the vacuum
consolidation system without and with airwater separation system, respectively.
It can be seen that the vacuum pressure
at the vacuum pump to the water pump is thesame, no matter how much is the elevation
head difference between the vacuum pump andthe improved area.

Figure-6 Water pressure diagram for vacuum consolidation
with air-water separation system (Imai, 2005)
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Infrastructure Development Institute - Japan
4. Implementation of Air-Water Separation System

Figure-7 Tsuruga soil profile

The
Maizuru-Wakasa
Expressway
diverges from the Chugoku Expressway at
Yokawa JCT and passes Fukuchiyama,
Maizuru, Obama City and ends at Tsuruga JCT,
connecting to the Hokuriku Expressway, with atotal distance of about 162 km. Soft ground is
distributed along the construction area with adistance of 50 km from Obama-Nishi to Tsuruga.
Wakasa Bay has a kind of irregularsubmerged coastline called ria and many
drowned valleys exist around Mikata-gokolakes.
The Mukasa area is a typical drownedvalley. As a geological feature, the valley ispartially covered with humic and sandy soils. At
both ends of the valley, humic soil predominateswith the depth of 30 to 40 meters.
Tsuruga’s soil profile is shown in Figure-7.
Humic soils and cohesive soils are found to beformed in alternate layers. Cohesive soils
predominate at the center part of Tsuruga’s soil
profile.

Originally, prefabricated vertical drains(PVDs) with the surcharge method wereselected for ground improvement work. PVDs
were installed up to 34 m deep. In this method,the embankment construction rate was limited
to 3 cm per day by the design criteria.
Later on, it was found that theconstruction time would be limited. Increasing
the rate of embankment construction wasrequired to accelerate the construction process.
Therefore, the vacuum consolidation method
was selected to apply because it can provide ahigher stability during construction, resulting
in increasing the embankment construction
rate.
Moreover, vacuum pressure was also used
as a surcharge load in place of the surcharge
load created by embankment material.
Therefore, the vacuum consolidation systemwith air-water separation system (CVC) wasselected to replace PVDs with the surcharge
method.

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Infrastructure Development Institute - JapanThe schematic of the CVC
system is illustrated in Figure-8.
To construct the CVC system,PVDs (7 mm thick) were installed up
to 20 m deep between the existing
PVDs (3 mm thick), which werepreviously installed up to 34 m deep,in a square grid pattern of 1.2 m x 1.2m.
It should be noted in advancethat the top of the existing PVDs (3mm) were not directly connected tothe CVC system.

Figure-8 CVC System

The constructed improvement area was
about 9,000 m2, as shown in Figure-9.
The embankment height including
surcharged backfill was designed to be 14 mhigh.

Figure-10 shows the completion of theembankment.

Figure-9 Vacuum Consolidation Area

Figure-10 Completion of Embankment

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Figure-11 Field monitoring data from Tsuruga
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7. Contact Information

5. Monitoring Results
The field monitoring instruments areshown in Figure-11. According to the results,
the amount of consolidation settlement during
construction was extraordinary about 11 m.
Vacuum pressure measured under the airtightsheet was maintained constantly at about 80
kPa along with consolidation settlement.
By using the air-water separation
system, it is clearly seen that vacuum
pressure measured under the airtight sheet
does not gradually decrease along withconsolidation
settlement
unliketheconventional vacuum consolidation system,
where vacuum pressure decreases by 10 kPafor every 1 m of consolidation settlement. This
result confirms the effectiveness of thevacuum consolidation system equipped withan air-water separating tank in the CVCsystem.

【記載事項】
Vacuum Consolidation Technology Association

Secretariat
Director
Takashi Suzuki
EMAIL: nandhoffice@gmail.com
TEL: +81-3-5828-8115
HP: http://www.nandh.jp

6. Conclusion
The Vacuum consolidation method is atechnique for soft ground improvement byutilizing the difference in atmosphericpressure.
The
conventional
vacuumconsolidation system had a limitation topreserve the vacuum pressure inside theimproved
area
duringconsolidation
settlement.
To resolve this problem, the concept ofvacuum consolidation was analyzed in order toinvestigate the cause of the problem. It was
found that the decrease in vacuum pressureduring consolidation settlement was related tothe elevation head difference between thevacuum pump and the improved area.
The concept of air-water separation wasproposed to resolve the problem and developed
to be used in practice. The vacuum
consolidation system equipped with air-water
separation system has been successfully
utilized in many ground improvement projects
in Japan as well as overseas.

7About IDI and IDI-quarterly
Infrastructure Development Institute-Japan (IDI) is a general incorporated
association operating under the guidance of Ministry of Land, Infrastructure,
Transport and Tourism of Japanese Government.
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