1030401 (1)

International Journal of

Advanced Structures and Geotechnical Engineering

ISSN 2319-5347, Vol. 03, No. 04, October 2014

IJASGE 030401 Copyright © 2014 BASHA RESEARCH CENTRE. All rights reserved

Soft soil improvement by cement column

MD. KAMRUL AHSAN, MD. ISTIAQ HOSSAIN, MASUM SHAIKH, MUHAMMED ALAMGIR

Department of Civil Engineering, Khulna University of Engineering & Technology, Khulna-9203, Bangladesh

Email: [email protected], [email protected], [email protected],

[email protected]

Abstract: Due to scarcity of land for rising population in a country like bangladesh, it is necessary to improve

the soft soil to face the challenges of this problem. Soft soils are generally labeled as ‘problematic’ because of

poor resistance to deformation and very low bearing capacity. Thus, improvement of the weak properties of soft

soil is required, which can be achieved by adopting cement column as one of the soil improvement techniques.

This paper aims to define the effect of cement column in improving soft soil and their installation technique by

laboratory investigation through small scale test. To check the degree of improvement of soft ground due to the

installation of cement column is the main objective of the study. A mixing machine, fabricated locally as a part

of this study, is used here to provide cement column in the soft soil. Astm d2166 is used to determine the

unconfined compressive strength of the reconstituted soil, which is used further to determine the bearing

capacity of the soil media. The “universal testing machine” is used for determination of load-settlement

behavior of the cement column improved ground. Finally, from the experimental investigation it was observed

that the bearing capacity of soft ground can be increased significantly through the installation of cement column.

Keywords: Soft soil, Bearing capacity, Cement column, Reconstituted soil, Unconfined compressive strength

1. Introduction:

The rapid growth in the infrastructure of urban and

metropolitan areas in most countries of the world has

resulted in non-availability of suitable locations.

Accordingly, the marginal ground and reclaimed land

with poor soil conditions; especially in coastal

regions and low land areas are becoming more

attractive for development. Weak deposits are very

common along the coastal region. Most of the

marine deposits are of recent origin and have not

undergone much consolidation. As a result, they

have low shear strength or high compressibility.

Even some of the land deposits, particularly alluvial

deposits along the river belt have loose silt/sand to a

large depth. Even man made deposits such as mine

back-fill or land reclaimed by filling can have

inadequate strength properties requiring ground

improvement.

Deep mixing columns using cement mixed in-situ

with soft soil to stabilize soft clay and organic soil

are commonly used in Sweden, Finland, Norway and

Japan. These methods of soil stabilization have

gradually been improved since 1967 when Mr. Kjeld

Paus patented the method and subsequently new

techniques were invented. In Malaysia, soil

improvement is very important as the country is

abundant with weak soil that is unsuitable for

construction works. Weak soil such as Alluvium

Clayey is abundant throughout Malaysia, constituting

70% of 5000km of the country’s coastline ranging

between 20 to 40 meters in soil thickness. Thus,

ground settlement is an issue due to low bearing

capacity of soft clay material which can cause

problems such as low stability and excessive

settlement (Nur et al., 2011). Not only those countries

but also in Bangladesh it is a problem because it is a

land of delta formation with alluvial deposition. The

method has gradually been improved in Scandinavian

countries since the 1970s (Broms 1999a). Since the

end of the 1990s, it has been the most commonly

used method in Sweden for stabilizing soft soils.

According to Swedish practice, stability calculations

are based on the assumption that the columns and

surrounding soft soil behave as a composite material

(Carlsten & Ekström, 1995). Excavation support

using deep soil mixing technology evolved from the

early 1970’s Japanese practice, in which single soil-

cement columns were created to support excavations

and act as cutoff walls. The behavior of cement

column by deep mixing method has been investigated

experimentally by Miyake et al. 1991, Hashizume et

al. 1998 and Kitazume et al. 1999. The increase in

strength with time of surrounding clay adjacent to

soil-cement columns was experimentally and

numerically studied by Miura et al. 2001 and by Shen

and Miura 1999. The factors controlling in-situ

strength of soil-cement columns have been

investigated by a full-scale test (Horpibulsuk et al.,

2000). The recent laboratory investigation on the

strength development in cement admixed clay at

various conditions of cement content and water

content is presented by Miura et al. 2001 and

Horpibulsuk and Miura 2001. Horpibulsuk et al.

2001 have proposed interrelationship among water

content, cement content, curing time and strength of

cement admixed clays. The application of deep

mixing technique to reduce settlement of an

embankment in Thailand was successfully done by

Bergado et al. 1999.

In this study, the degree of improvement of soft

ground due to the installation of cement column is

investigated in the laboratory. It was observed that

the bearing capacity of soft ground can be increased

significantly through the installation of cement

column.


International Journal of Advanced Structures and Geotechnical Engineering

ISSN 2319-5347, Vol. 03, No. 03, July 2014, pp 310-315

2. Soil cement column technique:

This method is commonly known as deep mixing

method or admixture method. Lime or cement

columns, where quicklime or dry cement are mixed

in situ with soft soil as shown in Figure 1, are

common in Sweden and Finland, to stabilize soft clay

and silt as well as organic soils. The method has

gradually been improved and new applications have

been found. Lime or cement columns have mainly

been used to increase the stability and to reduce the

settlements of road and railroad embankments and to

increase the stability of trenches for sewer lines,

water mains and heating ducts. New efficient

machines have been developed for the installation of

the columns. The diameter and the length have

gradually increased and the time required for the

installation of the columns has been reduced

significantly as well as the costs.

Figure 1: Installation of lime or cement column

Numerous projects have incorporated deep mixing

for excavation support and reduce settlement. One of

the first major applications of cement column as deep

soil mixing for excavation support in the United

States was the Wet Weather Storage Basin for the

East Bay Municipal Utility District (EDMUD)

project in Oakland, California constructed in 1990

(Taki and Yang, 1991). One of the largest projects in

the United States involving deep soil mixing

technology is the Boston Central Artery and Tunnel

(CA/T) project (O’Rourke and O’Donnell, 1997a and

1997b; O’Rourke et al., 1998; O’Rourke and

McGinn, 2004). Yang, 2003; states that the improved

engineering properties of the stabilized soils are

governed by a number of factors including soil type,

slurry properties, mixing procedures and curing

conditions.

The main areas of soil mixing applications are as

follows, with the countries in parentheses indicating

their most extensive use so far:

Foundation support (Japan, Scandinavia, US,

France, Poland)

Retention systems (Japan, US, China, Southeast

Asia, Germany)

Ground treatment (Japan, US, Finland, Sweden,

Southeast Asia)

Liquefaction mitigation (Japan, US)

Hydraulic cut-off walls (Japan, US, Germany,

Poland)

Environmental remediation (US, UK).

Figure 2: Mixing tools of the DJM method. (a) Construction scheme (DJM Association,

2002); (b) Recently used single mixing tool of 1.0m diameter




Figure 3: Selected mixing tools of the Nordic method: (a) SD 600 mm; (b) modified SD

600 mm; (c) PB3 600 mm; (d) peat bore 800mm (courtesy of LCM)

In the deep soil mixing process, admixtures/binders

are introduced into the in-situ soils throughout the

treatment depth and mixed thoroughly using large

diameter single or multiple-shaft mixing tools to form

columns or panels of improved material (Figure 4).

The mix-in-place columns can be up to 1m or more

in diameter. Typical admixtures are cement and lime,

but slag / flyash and/or other additives can also be

used.

Figure 4: Schematic showing overall process of Deep Soil Mixing

For both wet as well as dry Deep Soil Mixing, quality

control during execution is important to ensure

uniform improvement of the soil and to ascertain the

required amount of binder has been mixed uniformly

over the entire depth of treatment. For this purpose,

the mixing units are equipped with automated

computerized recording devices to measure the real-

time operating parameters such as depth of mixing

tool, volume or weight of binder used, flow rate of

grout, rotation speed and rate of penetration and

withdrawal. After allowing for sufficient curing

period (typically, 3 to 4 weeks), the mixed columns

can also be tested using single/group column plate

load tests, unconfined compressive strength tests on

cored/backflow samples, visual examination of

exposed columns, etc.




3. Materials and methodology:

3.1 Properties of Clay (With and Without

Cement)

Table 2: Properties of foundation soils

3.2 Formation of Clay Bed

To form clay bed the following steps were taken

(Figure 5)-

Clay sample was collected from KUET.

A 18'' diameter and 20'' height circular drum was

made.

The bottom part of the drum was made hole for

drain out water.

A geo-jute was in the bottom part of the drum that

only permit to flow water not clay.

Then full fill the drum with clay sample to get a

circular clay bed.

Make a circular slab to distribute the surcharge

load uniformly to the clay bed.

Now kept it 28 days for drainage with surcharge.

Finally clay bed was prepared.

Figure 5: Formation of clay bed

3.3 Installation of Cement Column

For installation of cement column in clay soil the

following steps were done-

A mixing device was prepared which is shown in

figure 6.

After that the mixing device injected into the clay

bed by hand rotating.

In this time the cement was poured in the clay by

the device and mixed with the clay.

The diameter of the column is 6” in diameter

which is one third of the clay bed.

Only 7% cement of the total column is used and

mix with the soil.

Finally the mixing device is lift from the bed by

reverse rotating.

Figure 6: Mixing Device for Installation of Cement

Column

Figure 7: Load Settlement Measurement

The clay bed was tested in “Universal Testing

Machine” to find out the settlement against load for

both clay bed i.e; without cement column and with

cement column (Figure 7). Then series of data were

plotted in a graph to find out the load settlement

relationship and finally compared this graph for

improvement.

4. Results and discussions:

4.1 Stress-strain behaviour of foundation soil

From the graph (Figure 8) it was found that the value

of deviator stress for sample without cement column

is 18 kPa. Whereas, with cement column the value

increased upto 80 kPa. From these values the

undrained shear strength was found to be 8.75kPa

before cement column and after the value was

40.00kPa.

Property

Value

Without

Cement With Cement

Liquid Limit 31% 36%

Plastic Limit 21% 26%

Moisture Content 35% 25%




Figure 8: Unconfined Compressive Test with and without cement

Figure 9: Load Settlement Curve

4.2 Load Settlement of Foundation:

From the load settlement curve shown in figure 9 it

was found that the settlement curve for sample with

cement column is above than the other one without

cement column. It indicates that the bearing capacity

of soft soil is increased significantly by using cement

column.

From graph it was observed that the value of load

intensity for cement column is 822.28kPa whereas in

case of without cement column it is only 172.38kPa.

And the percentage of cement in column is very low,

only 7% of volume of soil in cement column.

5. Conclusions:

The modern development of foundation practices,

namely ground improvements techniques, to

overcome the limitations of the conventional

foundation system has been proved to be both

technically and economically feasible for the

improvement of the marginal sites. Amongst the

various ground improvement techniques for

improving soft ground conditions, cement column is

considered as one of the most versatile and cost

effective method. This ground improvement

technique has been used in many difficult foundation

sites throughout the world to increase the bearing

capacity, reduce settlement, and increase the stability.

Based on the laboratory investigation the followings

can be concluded-

The cement column can be applied upto a suitable

depth.

The bearing capacity of soil is improved after

installing cement column.

Also study gives that improved soil by cement

column will fail with a greater settlement.

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1030401 (1)

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