Estimating Geotechnical Design Pararmeters of Improved Soil by the Preloading Method Using Instrumentation Results and Numerical Approach- a Case Study (In Persian)

Abstract

In his paper, back-analysis using results from instrumentation by finite element software for design of preloading method by embankment with prefabricated vertical drains (PVDs) was used, and the results were compared with each other. Hence, the basic geotechnical parameters obtained by laboratory and field experiments have been modified using this method. In this regard, the Mahshahr oil storage project has been used as a case study. All of the methods that are used for improving soil must ultimately result in increased soil resistance parameters, reduced compressibility, and reduced soil permeability. The soil improvement technique using preloading was used in Mahshahr where there are soft soil layers beneath higher than sub-surface water. The control performance of the embankments, some instruments such as Settlement Plates and Piezometers have been used. The results obtained from the analysis shows that settlement parameters obtained from the instrument data from back-analysis were less than primary consolidation settlement parameters based on laboratory tests. Finally, soil settlement estimation of the oil tank was compared before and after preloading, and it was found that using this method for soil improvement can be very efficient in the current project.

Full text

Journal of Ferdowsi Civil Engineering, Vol. 30, No. 1. 2017.
Estimating Geotechnical Design
Pararmeters of Improved Soil by the
Preloading Method Using
Instrumentation Results and Numerical
Approach- a Case Study
Kh. Mehrshahi
1
H. Alielahi
2
*
1-Introduction
Before construction of engineering structures on
compressible soft soils, it is inevitable to improve
and modify these soils to prevent large unpredictable
settlements resulting in damage to the structure.
“Preloading” is a method widely used in soil
improvement that dates back to the 1930s and earlier.
It is a simple and economic method of increasing the
strength parameters of saturated fine-grained soft
soils. Easy implementation, monitoring and
measuring the settlement of the ground using
instrumentation and checking the behavior of this
method during the procedure are among the
advantages of this method. The preloading approach
can be applied using radial drainage to enhance
consolidation settlement rate, and without radial
drainage by either embankment or vacuum. In
general, soft clayey soils require a long time for
settlement consolidation due to their low
permeability. To increase the consolidation rate in
these soils, radial drains are installed beneath the soil.
These drains cause artificial drainage paths under
clay soils that increase the rate of the consolidation
process by curtailing the drainage path, which in turn
will rapidly increase the strength of the soil,
increasing the capacity of bearing new load on the
soil.
In this regard, in order to improve subsurface soft
saturated clayey layers under the oil storage tanks in
the Mahshahr project, the preloading method is
assessed in this paper. In this case,we used
embankment along with prefabricated vertical
drains (PVDs) with a triangular pattern
Considering the different layers of soil and
subsurface conditions at the project site of Mahshahr
oil depot and compressible layers located in
relatively large depths, the improvement extent has
been high to modify soil characteristics in order to
avoid soil settlement and failure due to the
application of high loads from the tanks. The
inaccuracy of embankment settlement estimates and
the prolonged preloading operations are among the
challenges of soil improvement using preloading.
Therefore, proper selection of soil parameters
including effective parameters in consolidation
settlement values (Cs, Cc and Pc) and soil
1
Graduate, Department of Civil Engineering, Zanjan
Branch, Islamic Azad University, Zanjan, Iran.
2
*Corresponding Author: Assistant Professor,
Department of Civil Engineering, Zanjan Branch,
Islamic Azad University, Zanjan, Iran.
Email: h.alielahi@iauz.ac.ir
consolidation time (Kh and Kv) can address
preloading as a viable and practical option for soil
improvement.
In this paper, back-analysis results of instrument
data have been compared using Plaxis 3D software
for EM-2B embankment (Figure.1), and the initial
effective geotechnical parameters obtained from
laboratory and field experiments have been modified
using this method. Using the modified results of this
study can help usto successfully evaluate and control
the design of the mentioned project.
Fig 1. EM-2B embankment
2-Methodology
In order to perform numerical modeling, the Plaxis
3D software based on Finite Element Method has
been used. This software was used to examine
consolidation settlement according to Biot’s theory
formulations. For three-dimensional modeling of the
embankment, the dimensions of both sides of the
embankment must be set so that the actual ground
conditions are considered with the least impact on the
general behavior of the model. The embankment
height of EM-2B is 14.6m. In order to determine
the boundary conditions, three times the base of
the embankment is considered from both sides of the
model, and for the height of the geotechnical region
influenced by the creation of the embankment
which has been modeled in the program, a height
equivalent to 60m has been considered based on
bore holes data. Figure 2 shows the numerical
modeling of the EM-2B embankment in the Plaxis
3D sofware. Besides, for the soil behavior the Soft
Soil Creep is taken into account.
Fig 2. Three-dimensional modeling using Plaxis 3D
Software
EM-2B

Kh. Mehrshahi - H. Alielahi
In order to simulate drains, the Chai et al. method has
been used. In this method, the equivalent
permeability for drain zones is considered. Hence,
based on back-analysis results, the value of Cf (the
permeability ratio of site to laboratory) reaches 8. In
addition, (Cc) and (Pc) are modified with with 0.12
and 190 kPa values in the software, respectively.
Finally, the modified soil parameters are presented in
Table 1.
Table 1. Back-analysis results of consolidation
parameters of EM-2B
Kh (CL-2)
(m/day)
Kv (CL-2)
(m/day)
Cf (CL-2)
Cc (CL-1)
Pc (CL-
1-1)
(kPa)
Plaxis 3D (After
Preloading)
0.021
0.0104
8
0.12
190
Initial Design
Parameters
(Before
Preloading)
0.0026
0.0013
…..
0.17
180
According to the initial designs, for example,
recorded value of the settlement in the center of
the EM-2B settlement is approximately 122.2cm,
while the settlement calculated in the center of
the EM-2B embankment has been 132cm.
Therefore, initial values of calculated settlement
based on assumed parameters were higher than
the measured values of settlement.
Lower values of measured settlements relative to
calculated settlements can be attributed to the
conservative determination of geotechnical
parameters for settlement calculations. Besides, soil
settlement induced by oil tanks has been compared
before and after preloading based on modified soil
parameters. The obtained simulation results indicated
that after removing the embankment and construction
of the oil tank during 30 days, the final settlement
value reached about 9 cm.
3- Concluding Remarks
In this research, the numerical modeling of soil
consolidation has been discussed using the pre-
loading method with radial drainages in the
Mahshahr oil depot as a case study. In this
regard, back-analysis using instrumentation results
was conducted by using the Plaxis 3D software
based on the finite element method, and the
results were compared with each other. The basic
geotechnical parameters obtained by laboratory and
field experiments have been modified using the
above mentioned method. The results obtained from
the analysis indicate that settlement values from the
instrument data were less than the results obtained
from back-analysis. Indeed, the effective laboratory
parameters intended for primary calculations of
consolidation settlement values of the soil (Cc and
Pc) were more and less than the actual measured
values, respectively, and the effective laboratory
parameters intended for time of soil consolidation
calculations (Kh and Kv) were lower than the actual
measured values. Finally, soil settlement indued by
oil tanks was compared before and after
preloading, and it was found that using this
method for soil improvement can be very efficient in
large-scale projects.




DOI: 10.22067/civil.v1i30.52137










 






PVDs









Estimating the Geotechnical Design Pararameters of Improved Soil by
Preloading Method Using Instrumentation Results and Numerical Approach- a
Case Study
Kh. Mehrshahi H. Alielahi
Abstract In his paper, back-analysis using rsults from instrumentation by finite element software for
design of preloading method by embankment with prefibricated vetical drains (PVDs) was used, and the
results were compared with each other. Hence, the basic geotechnical parameters obtained by laboratory
and field experiments have been modified using this method. In this regard, the Mahshahr oil storage
project has been used as a case study. All of the methods that are used for improving soil must ultimately
result in increased soil resistance parameters, reduced compressibility, and reduced soil permeability.
The soil improvement technique using preloading was used in Mahshahr where there are soft soil layers
beneath higher than sub-surface water. control performance of the embankments, some instruments such
as Settlement Plates and Piezometers have been used. The results obtained from the analysis shows that
settlement parameters obtained from the instruments data by back-analysis were less than primary
consolidation settlement parameters based on laboratory tests. Finally, soil settlement estimation of the
oil tank was compared before and after preloading, and it was found that using this method for soil
improvement can be very efficient in the current project.
Key Words Preloading Method, Radial Drains, Consolidation Settlement, Numerical Approach
    .

 Email: h.alielahi@iauz.ac.ir 



  























[2]



Prefabricated Vertical Drains
 



2




[3]





[4]

Smear
Zone


[5]

Mandrel



[6]



Moruya







 
[7]

PVD


[8]

ABAQUS–
Modified Cam-Clay
Tianjin









-

[9]
Chittagong Sea Port

Ch/Cv


Kh/Kv
[10]


Surabaya






-
.
[11]




-


[12]
Bangkok

PVD


 











PVD




  

Plaxis 3D
















.




Tank 403 EM-2B











PVD





EM-2B


EM-2B


PVD
Colbonddrain CX1000[2]





 
 



EM-2B
CL
BH-2BH-4
SPT
CPTu
SPT









CL-1
CL-2





EM-2B

SPTEM-2B
0
10
20
30
40
50
60
70
050 100 150
Depth (m)
SPT Value
BH2 (Before)
BH4 (Before)



  
EM-2B


(m)

ν
(sat)
Cc
Cs
Pc
(kPa)
γd
(kN/m3)
γ
(kN/m3)
γsat
(kN/m3)
Kh
(m/day)
Kv
(m/day)
Kh/Kv
e0
0 – 10
(CL-1)
0.45
0.17
0.03
70
15.7
20.16
20.59
0.0086
0.0043
2
0.85
10 – 22
(CL-1-1)
0.45
0.17
0.03
180
15.7
20.16
20.59
0.0086
0.0043
2
0.85
22 – 38
(CL-2)
0.45
0.16
0.03
350
16.87
20.64
20.95
0.0026
0.0013
2
0.6
38 – 48
(CL-2-1)
0.45
0.16
0.03
450
16.87
20.64
20.95
0.0026
0.0013
2
0.6
48 – 60
(CL-2-2)
0.45
0.16
0.03
580
16.87
20.64
20.95
0.0026
0.0013
2
0.6








Settlement plates

Piezometers

Inclinometers



EM-2B



""S
""P


EM-2B

EM-2B



EM-2B



 


EM-2B




 


 



EM-2B

EM-2B
0
2
4
6
8
10
12
14
16
 m

EM-2B
0
0.2
0.4
0.6
0.8
1
1.2
1.4
010 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170


S2B-1
S2B-2
S2B-3
S2B-4
S2B-5



  

EM-2B

 
 EM-2BPlaxis 3D


Plaxis 3D
 
[14]


EM-2B






EM-2B
Plaxis 3D


 Soft Soil
Creep[12]

0
5
10
15
20
25
30
35
020 40 60 80 100 120 140 160 180
kPa

Z=5m
Z=16 m
Z=26 m
EM-2B



 
 
[15]










v
)k
v
kh
k
2
e
μD
2
L5.2
1(
ve
k


w
q3 h
k
2
L2π
4
3
(S)ln
s
kh
k
)
S
n
(lnμ



l
)
v
(k
f
C
f
)
v
(kor
l
)
h
(k
f
C
f
)
h
(k 


kve
kv

De

 kh
 L 
n
dw
qw

ks
Fs

 S
kh)f

kh)l
Cf



PVD





[13]

dw
(cm)
ds
(cm)
kh/ks
n
S
L
(m)
qw (m3/year)
μ
kh/kv
kh
(m/day)
Kv
(m/day)
Kve
(m/day)
5.18
28.49
3
30.38
5.5
25
12.096
6.14
2
0.0086
0.0043
0.6880
Cx1000 Colbonddrain[2]

a
mm

bmm

Discharge capacity qw (ml/s)

kf (mm/s)
Opening size O90 (m)

 
100
3.6
140
70
75
3%

 EM-2BPlaxis 3D



  


EM-2BPlaxis 3D




kh

EM-2B





Plaxis 3D











Kh








Cs ,Cc , Pc–




EM-2B
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
-6 -4 -2 0 2 4 6


Layer CL-1
Layer CL-2
-40
-30
-20
-10
0
10
20
30
40
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
cm

Cc layer CL-1
Cc Layer Cl-2
Cs Layer CL-1
Cs Layer CL-2
Pc Layer CL-1
Pc Layer CL-1-1
Pc Layer CL-2
Pc Layer CL-2-1
Pc Layer CL-2-2
EM-3
)فلا(
)ب(



 
 
Kh
CL-2


 Cc

CL-1





















N
1K k
U
2
)
k
U
N
1K k
(U
Error

UK 
U′k

 N


EM-2B
 



EM-2B
CcPc
CL-1 CL-1-1

CL-2



 PcCc
CL-1-1
EM-2B
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
160 170 180 190 200 210 220 230

kPa 
PcCL-1-1
0
0.05
0.1
0.15
0.2
0.25
0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18

Cc
CcCL-1



  
 kh
CL-2EM-2B

EM-2B
Kh (CL-2)
(m/day)
Kv (CL-2)
(m/day)
Cf (CL-2)
Cc (CL-1)
Pc (CL-1-1)
(kPa)


0.0026
0.0013
…..
0.17
180


0.021
0.0104
8
0.12
190

Plaxis 3D



Cf

CL-2










 EM-2B










Plaxis
3D



EM-2B



0
0.05
0.1
0.15
0.2
0.25
0.3
0.001 0.005 0.009 0.013 0.017 0.021 0.025 0.029 0.033 0.037 0.041

Khm/day
KhCL-2



 
 EM-2B


EM-2B 









Tank 403EM-2B


Plaxis 3D







EM-2B










EM-2B




Tank 403



Plaxis 3D-

 
Tank 403


-







0
0.2
0.4
0.6
0.8
1
1.2
1.4
020 40 60 80 100 120 140 160 180




Plaxis 3D
Instrumentation Data
0
5
10
15
20
25
020 40 60 80 100 120 140 160 180
kPa

Plaxis 3D
Instrumention Data



  
Tank 403Plaxis 3D
EM-2B Plaxis 3D



0
20
40
60
80
100
120
140
020 40 60 80 100 120 140 160 180 200 220 240 260
cm

After Preloading
Before Preloading
Swelling
Tank 403



 






PVD


Plaxis 3D









 EM-2B 


EM-
2B 




Cf


EM-2B




Cc

Pc


(Tank 403
EM-2B



























  



2. Federal Highway Administration (FHWA), "Prefabricated Vertical Drains, Vol. I, Engineering
Guidelines" Report No. RD-86/168, pp.1-128 (1986).
3. Bhushan, K., Carlos V., Saaty, R., "Soil Improvement by Precompression at a Tank Farm Site in
Central Java, Indonesia", Journal of Geotechnical Engineering, Vol. 3, pp. 101-108, (2000).
4. Indraratna, B. and Redana, I W., "Laboratory determination of smear zone due to vertical drain
installation", J. Geotechnical and Geoenvironmental Eng., ASCE, Vol. 125, No. 1, pp. 96-99, (1998).
5. IndraratnaB, Rujikiatkamjorn C, Sathananthan I, Shahin M andKhabbaz H., "Analytical and
Numerical Solutions for SoftClay Consolidation using Geosynthetic Vertical Drains withSpecial
Reference to Embankments",

Proceedings of the 5th

International Geotechnical Engineering
Conference, Cairo, Egypt, pp. 55–86, (2005).
6. Sathananthan, I. and Indraratna, B., "Laboratory Evaluationof Smear Zone and Correlation between
Permeability andMoisture Content", Journal of Geotechnical and

Geoenvironmental Engineering
(ASCE), Vol.132, No. 7, pp. 942–945, (2006).
7. Saowapakpiboon, J., D.T. Bergado , Youwai, S., Chai, J.C., Wanthong, P., Voottipruex, P., "Measured
and Predicted Performance of Prefabricated Vertical Drains (PVDs) with and without Vacuum
Preloading", Geotextiles and Geomembranes, Vol. 28, No. 1, pp. 1-11, (2010).
8. Rujikiatkamjorn, Ch., Indraratna, B., and Chu, J., "Numerical Modelling of Soft Soil Stabilized by
Vertical Drains, Combining Surcharge and Vacuum Preloading for A Storage Yard", Can. Geotech. J,
Vol. 44, No. 3, pp. 326–342, (2007).
9. Dhar, AS., Siddique, A., and Amen, SF., "Ground Improvement using Preloading with Prefabricated
Vertical Drains", International Journal of Geoengineering Case Histories, Vol. 2, No. 2, pp. 86,
(2011).
10. Tedjakusuma, B., "Application of Prefabricated Vertical Drain in Soil Improvement", Civil
Engineering Dimension, Vol. 14, No.1, pp. 51-56, (2012).
11. Bergado, D.T., Voottipruex, P., Lam, L.G., Hino, T., "Back-analyses of Flow Parameters of PVD
Improved soft Bangkok Claywith and Without Vacuum Preloading fromSettlement Data and
Numerical Simulations", Geotextiles and Geomembranes, Vol. 42, No. 5, pp. 457-467, (2014).
12. Gia Lam, L., Bergado, D.T., Hino, T., "PVD Improvement of Soft Bangkok Clay with and Without
Vacuum Preloading using Analytical and Numerical Analyses", Geotextiles and Geomembranes,



 
Vol.43, No. 6, pp. 547-557, (2015).
13. Plaxis 3D Foundation (version 1.6) user's Manual. Edited by Brinkgreve, R.B.J. Delf University of
Technology and PLAXIS b.v., The Netherlands, (2005).
14. Biot, M., "General Theory of Three-dimensional Consolidation", Journal of Applied Physics,Vol. 12,
pp.155-164, (1941).
15. Chai J.C, Shen S.L, Miura N.and Bergado D.T., "Simple Method of Modeling PVD Improved
Subsoil", Journal Journal of Geotechnical Engoneering, ASCE, Vol.127, No.11, PP. 956-972, (2001).



  