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

Authors:
  • Islamic Azad University of Zanjan (IAUZ)

Abstract and Figures

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.
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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)
Pc (CL-
1-1)
(kPa)
Plaxis 3D (After
Preloading)
0.021
0.0104
8
190
Initial Design
Parameters
(Before
Preloading)
0.0026
0.0013
…..
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.
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DOI: 10.22067/civil.v1i30.52137
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PVDs
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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
   .

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











[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-2BH-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)
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
70
15.7
20.16
20.59
0.0086
0.0043
2
0.85
10 22
(CL-1-1)
0.45
180
15.7
20.16
20.59
0.0086
0.0043
2
0.85
22 38
(CL-2)
0.45
350
16.87
20.64
20.95
0.0026
0.0013
2
0.6
38 48
(CL-2-1)
0.45
450
16.87
20.64
20.95
0.0026
0.0013
2
0.6
48 60
(CL-2-2)
0.45
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 
Uk

 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

Khm/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 


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Cf
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EM-2B
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


  


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. IndraratnaB, Rujikiatkamjorn C, Sathananthan I, Shahin M andKhabbaz H., "Analytical and
Numerical Solutions for SoftClay Consolidation using Geosynthetic Vertical Drains withSpecial
Reference to Embankments",
Proceedings of the 5th
International Geotechnical Engineering
Conference, Cairo, Egypt, pp. 5586, (2005).
6. Sathananthan, I. and Indraratna, B., "Laboratory Evaluationof Smear Zone and Correlation between
Permeability andMoisture Content", Journal of Geotechnical and
Geoenvironmental Engineering
(ASCE), Vol.132, No. 7, pp. 942945, (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. 326342, (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 Claywith and Without Vacuum Preloading fromSettlement 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).
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
  
... In order to improve subsurface soft saturated clayey layers under the oil storage tanks in the Mahshahr project [25], the preloading method is assessed. The preloading approach can be applied to enhance consolidation settlement rate, by embankment. ...
... The results of the settlements have been plotted, the embankment construction was continued for 108 days, and during the 53 days (waiting time), the soil under the embankment reached 95% of its consolidation. At this stage, the final settlement of the soil 122.2 cm is reached, and the interstitial water pressure has reached its minimum value, which is a sign of the end of the soil consolidation period [25]. At this stage, the soil is slightly inflated and, by removing the embankments, the amount of final settlement has been reduced, the soil settlement has reached 102 cm, indicating that the soils flows from the removal passage of embankment. ...
Article
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Preloading is an improvement technique for compressible soils, and has been applied at DjenDjen port in Jijel province, Algeria, as part of its development and expansion. In addition, this treatment to eliminate the risk of wharf caissons instability. Decent number of research and development works of the preloading process in different countries by several authors have been cited, with the aim of justifying our research and results. The objectives are to understand and apprehend the coastal soil preloading method and its application in terms of the sensitivity of the intervening factors on its achievements, and their effect on the behavior of the soil and the marine structure during and after its implementation. Furthermore, a numerical simulation of the real test of the method of treatment is carried out, by the plaxis 2D code in finite elements, also respecting the actual construction phasing of this structure, in order to compare the calculation results with in-situ measurements to validate the numerical models and to check the stability of the harbor structure. A matrix of consolidation process during pre compression is proposed.
... The empirical correlation can be considered quite logical, since the liquid limit and the shrinkage limit are important parameters controlling the compressibility behaviour. The liquid limit is the extreme limiting water content, above which the forces of the interaction between the particles become sufficiently weak so as to allow the easy movement of the particles relative to each other [74]. The void ratio at the shrinkage limit can be taken as the limiting void ratio, below which the volume change would be insignificant. ...
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In geotechnical engineering, the consolidation and settlement of structures are among the major problems an engineer must deal with. An appropriate estimation of a soil’s settlement is of significant importance, since it directly influences the performance of buildings and infrastructures that are built on soil.The compressibility characteristics of soils form one of the most important parameters required in the design of foundations. The compressibility behaviour of soils islargely dependent on the compression index, the properties and the parameters of the soil. A number of empirical correlations have been developed in the literature that aresupposedto connect the compression index to other soil parameters. The main objectives of this research were to study the relationships between the compression index (Cc) and the swelling index (Cs), and to investigate the effects of the natural void ratio (e0) and the over-consolidation ratio (OCR) on Cc and Cs, in order to combine them with the pre-compression stress (Pc), the consolidation duration (Tc) and the settlement (Su). Consequently, aconsolidation matrix and a consolidation circle are proposed, which gives us a new method and model to facilitate the calculations of the parameters involved in the soil consolidation, so as to summarize the consolidation phenomenon.
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Note from the Editor: Although the use of Prefabricated Vertical Drain (PVD) in soil improvement is not new, this paper is interesting since it gives the full spectrum from preliminary design stage; trial embankment and pilot test to final soil improvement. The final installation of the PVD was based on the soil investigation report and the results of instrumentation monitoring. Finally, using back analysis, vertical and horizontal coefficients of consolidation and compression index can be determined, which can be applied to predict a more accurate prediction of settlement.
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On a macroscale, the effect of installing prefabricated vertical drains (PVDs) in a subsoil is to increase the mass hydraulic conductivity of the subsoil in the vertical direction. Based on this concept, a simple method for modeling PVD improved subsoils is proposed, in which an equivalent vertical hydraulic conductivity kve for the PVD improved subsoil is explicitly derived. With the proposed simple method, analysis of PVD improved subsoil is the same as that of the unimproved case. The theoretical verification of the simple method was made under 1D condition. The calculated average degree of consolidation and excess pore pressure distri- bution in the vertical direction using the simple method are compared with existing theoretical solutions (com- bination of Terzaghi's consolidation theory and Hansbo's solution for PVD consolidation). It has been proved theoretically that, in terms of average degree of consolidation, in the case of one layer and ignoring the vertical drainage of natural subsoil, the maximum error of the proposed method is 10% compared with Hansbo's solution. For the case of one layer or multilayers and considering both vertical and radial drainages with the parameters adopted here, the maximum error of the proposed method is 5%. The multilayer case was analyzed by FEM method, and the proposed simple method is compared with that of using 1D drainage elements. Then, 2D finite- element analyses were conducted for three case histories of embankments on PVD improved subsoils. One case is discussed in detail. The analyses using both the simple method and 1D drainage elements, were conducted. It is shown that for all three cases, the simple method yielded results as good as those using 1D drainage elements.
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This paper presents a finite element analysis of a case study of a combined vacuum and surcharge load through prefabricated vertical drains (PVD) at a storage yard at Tianjin Port, China. The top 15 m of soil at this site was very soft to soft, and needed to be improved using preloading surcharges of more than 140 kPa. To avoid any stability problems associated with a high surcharge embankment, 80 kPa vacuum pressure combined with fill surcharge was applied (40 and 58 kPa for Sections I and II, respectively). A plane strain analysis was performed using equivalent permeability and transformed unit cell geometry. The converted (equivalent) parameters were incorporated in the finite element code ABAQUS, using the modified Cam-clay theory. The performance of a trial embankment at the site of the storage yard is predicted on the basis of a constant vacuum pressure applied on the soil surface and distributed along the length of the drain. The predictions of settlement, pore water pressure and lateral displacement were compared with the available field data, and an acceptable agreement was found based on this numerical approach. The combination of vacuum and surcharge load can effectively shorten the preloading period, reduce the height of the embankment and counterbalance excessive lateral displacements.
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In this study, the extent of the smear zone and the reduction of permeability and water content within the smear zone were investigated using a large-scale consolidometer. The installation of vertical drains by means of a mandrel causes significant disturbance of the subsoil surrounding the mandrel, resulting in a smear zone. The extent of the smear zone for Moruya clay New South Wales, Australia was estimated on the basis of normalized permeability and the reduction of water content by taking undisturbed samples horizontally and vertically at different locations. This study reveals that a significant reduction in water content and horizontal permeability takes place towards the drain, whereas the variation in the vertical permeability is negligible. The smear zone for Moruya clay was found to be 2.5 times the equivalent radius of the mandrel with the horizontal permeability varying from 1.09 to 1.64, an average of 1.34 times smaller than that of the undisturbed zone. Finally, a correlation between the permeability decrease and water content reduction within smear zone is proposed.
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This paper presents the performance of improved soft Bangkok clay with prefabricated vertical drains (PVDs) combined with embankment preloading (conventional PVD method) and vacuum preloading (vacuum PVD method). The performance was evaluated in terms of settlements and flow parameters using analytical methods and numerical simulations in the ABAQUS software. The horizontal coefficient of consolidation (Ch), the ratio (kh/ks) between the horizontal hydraulic conductivity in the undisturbed zone (kh) and the horizontal hydraulic conductivity in the smeared zone (ks), and the final settlement (Sf) were back-calculated using the measured data. The sensitivity analysis was performed by varying the values of kh/ks. The vacuum PVD method was confirmed to have a higher rate of settlement than the conventional method. In particular, Ch increased from 4Cv to 5Cv, kh/ks decreased from 8 to 7, and the consolidation time required to obtain a settlement of 1.30 m decreased from 300 days to 100 days. In addition, the calculated results from both the analytical method and FEM simulations for the conventional PVD agreed with the measured data. However, the results from the vacuum PVD method demonstrated that the FEM simulations yielded more reasonable results compared with the corresponding results obtained from the analytical methods.
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Prefabricated vertical drains (PVDs) with embankment preloading (conventional PVDs) and with embankment combined with vacuum preloading (Vacuum-PVDs) are examined using the field data obtained from the site of the Suvarnabhumi Airport, Thailand. The flow parameters were back-analyzed by comparison of measured and predicted or simulated data. The flow parameters were illustrated in terms of the horizontal coefficient of consolidation (Ch) and the ratio between the horizontal hydraulic conductivity in undisturbed zone (kh) and the horizontal hydraulic conductivity in smear zone (ks) or (kh/ks). Numerical simulations using one-dimensional FEM PVDCON software with equivalent vertical permeability, kev, to determine the appropriate Ch and kh/ks of PVDs with conventional embankment preloading and with embankment combined with vacuum preloading schemes were made. Furthermore, numerical simulations using axisymmetric FEM by ABAQUS software, incorporating horizontal (kh) and vertical (kv) permeabilities, to determine the appropriate kh/ks based on back-calculated Ch of conventional PVD and Vacuum-PVD schemes were also done. The Vacuum-PVD scheme indicated faster rate of settlement than conventional PVD scheme by about 1.7–1.8 times with slight reduction of the kh/ks ratios. For conventional PVD, it was demonstrated that the increase in kh/ks ratios reduced the simulated rate of settlement.
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The settlement of soils under load is caused by a phenomenon called consolidation, whose mechanism is known to be in many cases identical with the process of squeezing water out of an elastic porous medium. The mathematical physical consequences of this viewpoint are established in the present paper. The number of physical constants necessary to determine the properties of the soil is derived along with the general equations for the prediction of settlements and stresses in three‐dimensional problems. Simple applications are treated as examples. The operational calculus is shown to be a powerful method of solution of consolidation problems.
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This paper is mainly concerned with a laboratory study to investigate the effect of smear due to vertical drain installation. The extent of the smear zone around a vertical drain was studied utilizing a large-scale consolidometer apparatus. The test results reveal that a significant reduction in the horizontal permeability takes place toward a central drain, whereas the vertical permeabilty remains relatively unchanged. The radius of the smear zone was estimated to be a factor of four to five times the radius of the central drain (mandrel), and the measured ratio of horizontal to vertical permeability approached unity at the drain-soil interface. The laboratory measured settlements are subsequently compared with the predictions based on the theory of Hansbo and the finite element method. It is of relevance to note that the inclusion of the correct variation of permeability ratios of the smear zone in the plane strain finite element analysis improves the accuracy of settlement predictions.
Soil Improvement by Precompression at a Tank Farm Site in Central Java, Indonesia
  • K Bhushan
  • V Carlos
  • R Saaty
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).