The Behaviour of
Very Soft Soils
Improved by
Vibro Replacement
Dr.-Ing. V. R. Raju
Keller (M) Sdn. Bhd.
Kuala Lumpur, Malaysia
Presented by
Keller Grundbau GmbH
Kaiserleistr. 44
D-63067 Offenbach
Tel. 069 / 80 51 - 0
Fax 069 / 80 51 - 244
Ground Improvement Conference,
London, June 1997
E-mail [email protected]
www.KellerGrundbau.com
Technical paper 12-64E
The Behaviour of Very Soft Soils Improved by Vibro Replacement
The Behaviour of Very Soft Soils Improved
by Vibro Replacement
Dr.-Ing.V. R. Raju
Keller (M) Sdn. Bhd., Malaysia
ABSTRACT
Vibro replacement (stone columns) has generally been used to treat poor soils ranging from
loose silty sands to soft silts and clays having an undrained shear strength in the order of
15 kPa or more. The suitability of this method for very soft cohesive soils having undrained
shear strengths less than 10 kPa has been questioned in the past. The paper reports on the
behaviour of such very soft soils in terms of settlements and consolidation rates when
improved using vibro replacement.
INTRODUCTION
Large coastal stretches of south east Asia are covered with very soft marine clay deposits. In
addition, very soft cohesive deposits resulting from extensive tin mining activities can be found
in west Malaysia. Rapid industrial growth has necessitated the construction of infrastructure facilities
such as highways and railway lines on these soft deposits. The fast track nature of these projects
implies that very limited time is available for the consolidation of the very soft clays and silts.
The new Shah Alam Expressway which connects the coastal city of Klang to the capital city of
Kuala Lumpur passes over several such very soft deposits. Extensive ground treatment work
using vibro replacement has been carried out to ensure that the performance of the embankments
is as per the requirements of the Malaysian Highway Authorities.
Vibro replacement has been carried out at seven interchanges and overpasses to treat the soils
underneath approach embankments and reinforced earth walls. In all over 900,000 linear metres
of stone columns have been installed in predominantly very soft soils. In the following, the
performance of the treatment is assessed based on measured settlements at the Kinrara and
Kebun interchanges.
–1–
V.R. Raju
SOIL CONDITIONS
Based on origin, the very soft cohesive soils encountered at the Shah Alam Expressway can be
broadly subdivided into tin mine tailings (slimes) and marine clays. The mine tailings are generally
clayey silts with a fine sand content of about 15 %. These soils are found extensively in the Kuala
Lumpur area and vicinity. The marine clays are found along the coast line in the vicinity of the
port city of Klang.
The Kinrara interchange is located at a site where extensive tin mining activity has taken place.
In fact the highway passes directly over a former mine tailings pond where the maximum thickness
of the slime is 16 m. SPT N values are equal to zero over the depth of the slime. Figure 1a shows
the result of a Dutch cone test carried out in the slime pond at Kinrara. Tip resistances range
between 0.15 MPa and 1.0 MPa. Shear strength values are as low as 6 kPa at the top and gradually
increase with depth at the rate of 2 to 3 kPa per meter.
The Kebun interchange is located near the city of Klang on the west coast of Malaysia. The
upper soils in the coastal region are predominantly extremely soft marine clays having thicknesses
of upto 35 m. Figure 1b shows the results of a Dutch cone test carried out at Kebun where the
very soft clay deposit is 11m thick. Tip resistances range between 0.1 MPa and 0.3 MPa in the
soft clay deposit. Undrained shear strengths as low as 5 kPa and increasing at the rate of about
1 kPa per meter in depth have been measured.
A practical feeling for the softness of the soils can be obtained from the fact that an approx. 1m
high embankment on untreated soil has failed. Table 1 summarizes some of the typical values for
b) Kebun
0
0
0
0
-5
-5
-5
-5
-10
-10
-10
-10
-15
-15
-15
-15
-20
-20
-20
-20
-25
Depth [m]
Depth [m]
a) Kinrara
-25
-25
0
1
2
3
qc [MPa]
4
0
1
2
3
4
-25
0
1
2
3
qc [MPa]
FR [%]
4
0
1
2
3
FR [%]
Figure 1: Results of Dutch cone penetrometer tests carried out at (a) Kinrara and (b) Kebun showing tip
resistance qc and friction ratio FR with depth
–2–
4
The Behaviour of Very Soft Soils Improved by Vibro Replacement
moisture content (w), liquid limit (w l ), plastic limit (w p ), plasticity index (PI), clay, silt and sand
fractions, the sensitivity values (St ) and the coefficient of consolidation (c v) for the soft soils
encountered. The samples were taken at a depth of about 2.0 m below ground.
Site
Kinrara
Kebun
Soil type
Mining slime
Marine Clay
w
wl
wp
PI
Clay
Silt
Sand
St
cv
[%]
[%]
[%]
[%]
[%]
[%]
[%]
[-]
[m2/yr]
60
100
60
100
30
40
30
60
40
50
45
45
15
5
2-3
4-5
4.0
1.0
Table 1: Typical results of laboratory tests carried out on the soft soil samples
GROUND TREATMENT
The treatment was designed to the specifications of the Malaysian Highway Authorities. Briefly
stated, these specifications prescribe limits on total settlements which are not to exceed 10 % of
the total theoretical primary consolidation and secondary settlement (on untreated ground) in
the first seven years of service. The differential settlements are not to exceed 1 in 500 in regions
adjacent to piled structures and 1 in 1000 in regions remote from piled structures. The factor of
safety for slope stability and stability of walls against ground failure is to be greater than 1.5.
The stone column layout was based on the design method of Priebe [1995]. The diameter of the
columns was typically 1.0 m, but in some cases as large as 1.2 m in the extremely soft soils.
The columns were installed on a regular grid with spacings ranging between 1.5 m and 2.5 m.
Columns were generally installed down to the depth where a dense or stiff layer was found.
Figure 2 shows a schematic view of the treatment. The first step in the treatment was to place a
sand blanket to form a stable working platform for the construction machinery. This platform
also serves the purpose of a drainage blanket for the consolidation and assists in the transfer of
the embankment load to the columns.
The columns were installed using the Keller vibro technique. Here a depth vibrator connected
to extension tubes penetrates the ground by the vibratory action and is assisted by air or water
jets placed at the tip of the vibrator. In this manner a bore hole is created to the required depth.
Stone is fed to the tip of the vibrator either through a special stone tube (dry method) or through
the annular space between the vibrator and the borehole (wet method). Using an up and down
motion, the stone at the tip of the vibrator is laterally displaced into the soil and simultaneously
compacted. In this manner, a column of well compacted stone is formed in small regular steps.
For further details on the vibro technique, the reader is referred to
Moseley and Priebe [1993] and to Raju et. al. [1997] for details on the works carried out at the
Shah Alam Expressway.
–3–
V.R. Raju
Figure 2: Schematic view of ground improvement using vibro replacement
The stability and load carrying capacity of the columns is a function of the lateral support offered
by the surrounding soil. Considering the extremely soft nature of the soil, special care and
experienced operators are necessary for the installation works to ensure that the borehole does
not collapse and the already built column remains intact. It was found that a monitoring system
which continuously reports the various parameters of the column installation process to the
operator is essential to ensure that columns can be built in a reliable manner.
At the Kinrara interchange treatment was carried out to support embankments ranging in heights
from 3 m to 10 m. At Kebun treatment was carried out to support embankments with heights
ranging between 2 m and 5 m and following treatment and embankment construction, a 1.0 m
high surcharge was placed to speed up the consolidation process and avoid possible problems
related to secondary settlements.
SETTLEMENT PERFORMANCE
Extensive settlement monitoring was carried out using settlement gauges installed at the level of
the top of the stone columns. Over 40 gauges were installed at each site.
Figure 3 shows the results of the settlement measurements from one gauge each at Kinrara and
Kebun, where larger settlements were measured. At the location of the settlement gauge in
Kinrara, 1.2 m diameter columns were installed at 1.8 m centre to centre down to a depth of
17 m. The soil conditions are as shown in Figure 1a. At Kebun 1.1m diameter columns were installed
at 2.2 m centre to centre to a depth of 12 m. Soil conditions are as shown in Figure 1b.
–4–
The Behaviour of Very Soft Soils Improved by Vibro Replacement
10
Kinrara
Height [m]
8
6
4
Kebun
2
0
0
30
60
90
120
150
180
210
240
270
300
330
360
330
360
Settlement [cm]
10
8
6
Kinrara
4
Kebun
2
0
0
30
60
90
120
150
180
210
240
270
300
Time [days]
Figure 3: Embankment height and settlement versus time plots at
Kinrara (CH 34.475) and Kebun (CH 15.450)
At the measurement point in Kinrara where the embankment reached a maximum height of
7.5 m, the settlement measured was in the order of 25 cm. Most of this settlement took place
during embankment construction and a period of 2 - 3 months thereafter. At the measurement
point in Kebun where the embankment height is 2.6 m (including a 1.0 m preload) a settlement
of about 40 cm has been measured (it should be kept in mind that the soil and stone column
layout is different at the two sites and the settlement magnitudes cannot be directly compared).
Here, only 25 % of the settlement has taken place during embankment construction. The remaining
settlement has taken place over a period of almost 8 months thereafter.
In areas not treated with vibro replacement, settlements over 1.0 m were measured for comparable
embankment heights and soil conditions.
CONSOLIDATION
The treatment was designed such that 90 % of the settlements take place during a period of
about 6 months after embankment construction. A method based on Taylor’s square root of
time fitting method has been used as a guide to assess the degree of consolidation of the treated
soil. Figure 4 shows the plots for Kinrara and Kebun with square root of time on the x axis and
settlement on the y axis.
From the plots it can be inferred that at Kinrara, 90 % consolidation has taken place after about
175 days which is 90 days after completion of embankment construction. At Kebun, it is estimated
that 90 % consolidation has taken place after 225 days which is about 180 day after completion
of embankment construction.
–5–
Settlement [cm]
V.R. Raju
0
0
-10
-10
-20
-20
-30
-30
-40
-40
Kinrara
Kebun
-50
-50
0
5
10
15
20
Sqrt of Time [/days]
0
5
10
15
20
Sqrt of Time [/days]
Figure 4: Estimate of 90 % consolidation time using Taylors square root of time fitting method
DISCUSSION
The measured settlements which are in the order of 25 cm at Kinrara and 40 cm at Kebun as
compared to values of over 1.0 m in untreated areas imply an improvement factor of 4 and 2.5
respectively. This clearly indicates that it is possible to improve such soft cohesive soils using
vibro replacement.
A comparison with the values as predicted by the method of Priebe show that in general the
predicted values are larger than those measured on site. An exception would be the case where
too large a spacing is used and the soft material is squeezed out between the columns.
An improvement in shear strength can be assessed by the fact that embankments having heights
of upto 10 m have been built on the very soft soils after treatment. No signs of slope failure
have been noticed. A factor of safety of 1.5 can of course be only shown theoretically.
The settlement behaviour of the two types of very soft soils with comparable shear strengths
has been distinctly different. The slime has consolidated very quickly. Over 75 % of the settlement
has taken place during embankment construction and no significant settlements have been
measured after a period of about 3 months thereafter. In comparison, the marine clay has taken
much longer to consolidate. Over 75 % of the total settlement has taken place after completion
of embankment construction. A period of about 6 months was required for the settlements to
stabilize. This behaviour is understandable when one looks at the values of the coefficient of
consolidation and the highly plastic nature of the marine clay. It is recommended that preloading
be used to avoid the possibility of long term settlements in the soft marine clays.
–6–
The Behaviour of Very Soft Soils Improved by Vibro Replacement
CONCLUSIONS
It is possible to install stone columns in very soft cohesive soils with undrained shear strengths
less than 10 kPa. Following this treatment, highway embankments having heights of upto 10 m
have been built and settlements maintained with tolerable limits. However, it is not a routine
process and very close attention has to be paid by the operator and the Engineer on site.
Automated monitoring systems which provide information on the installation process are essential. The nature of the soft soils has to be carefully assessed when estimating consolidation
periods.
ACKNOWLEDGEMENT
The author is grateful to the concession holders of the Shah Alam Expressway, KESAS Sdn. Bhd.
and the main contractors, Gamuda- Percon JV for their support in carrying out the works
described in this paper. Credit goes to members of the Keller team who carried out the
treatment works in very difficult soil conditions.
REFERENCES
Moseley, M.P. and Priebe, H.J. (1993). Vibro techniques
Ground Improvement, Edited by M. P. Moseley, Blackie Academic & Professional, pp.1-19.
Priebe, H.J. (1995). The Design of Vibro Replacement.
Ground Engineering, December, pp. 31-37.
Raju, V. R., Masud, A., Ha, P. (1997). Vibro Replacement - a technique for extensive ground
improvement works in very soft cohesive soils at the Shah Alam Expressway in Malaysia.
To be publ. at the Darmstädter Geotechnik Kolloqium, Darmstadt, Germany, March 97.
–7–