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Investigating Performance of Soil Improvement Project using Preloading Method by Back-Analysis Results Based on Numerical Simulation and Instrumentation data - a Case Study (in persian)

Authors:
  • Islamic Azad University of Zanjan (IAUZ)

Abstract

In order to improve subsurface soft saturated clayey layers under the oil storage tanks in Mahshahr project, the preloading method by embankment along with prefabricated vertical drains (PVD) with a triangular pattern has been used. To control performance of the embankments, some instruments such as Settlement Plates and Piezometers installed throughout the embankment have been used. In this paper, back-analysis using instrumentation results was conducted by Settle 3D software based on finite difference method, and the results were compared with each other. The basic geotechnical parameters obtained by laboratory and field experiments have been modified using mention method. The results obtained from the analysis showed that settlement values from the instruments data were less than those back-analysis results. 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. Also, parametric studies have been performed on the spacing and depth of vertical drains. Finally, soil settlement induced 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.

-






        
            
     
PVD

   
    
Settle 3D


Cc
Pc

Kh
Kv 





h.alielahi@iauz.ac.ir
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
 
"
"
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

   

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

PVD

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. Embankment
3
. Vacuum
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. Prefabricated Vertical Drain
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. Bhushan
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. Indraratna
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. Redana
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. Smear Zone
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
1
. Mandrel
2
. Sathananthan
3
. Moruya
4
. Rujikiatkamjorn
5
. ABAQUS
6
. Modified Cam-Clay
7
. Tianjin
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

PVD




        Ch/Cv

          Kh/Kv



     
      

        
 

   
  PVD

            
      
    




  
      
1
. Saowapakpiboon
2
. Dhar
3
. Chittagong Sea Port
4
. Tedjakusuma
5
. Surabaya
6
. Hydrostatic test
7
. Bergado
8
. Vacuum PVDs
Downloaded from jeg.khu.ac.ir at 23:30 IRDT on Monday August 14th 2017

-
       
PVD 


    
 

      
PVD
              




 
 Cs , Cc
PcKh
Kv 

 
Settle 3D




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

     


vh
(1-U)=(1-U )(1-U ) (1)
UUv
Uh
       

Uv
Uh

40 . 5
U% Ct
π
vv
= , T =
v
0.1 7 9 2
10 0 H
2 .8
4
1+ π
v
v
T
T










(2 )
-8T Ct
v
h
U =1-e xp , T =
h2
hFn de








( 3 )
 Uv Tv
 Cv
  Ht 
 Uh
 Th
Ch
 de 
(S)

F(n)


22 d
n 3n -1 3 e
F(n) = ln(n)- ln(n)- , n =
22
4d
n -1 4n w
(4)
1
. Carillo
Downloaded from jeg.khu.ac.ir at 23:30 IRDT on Monday August 14th 2017

nde

dw

Band-shaped PV draindw


 
2
d=
wab



(5)
a/b


..


%8
1 exp
100
UT
hh
F(n) F Fr
s
 


 ( 7)
Fs

 
F = k k -1 ln(d d )
w
s s s
h(8)
ds
 kh 
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

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.Fr

h
rw
k
F=π z (L - z) ( )
q(9)
   z     qw
  L 

Fr

F(n)Fs



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
Tank 403 EM-2B Tank 601,602 EM-3
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PVD
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   
PVDColbonddrain CX1000  

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

CL
ML

EM-2BEM-3
BH-2BH-4BH-8
BH-10BH-6BH-9

"BH"CPTu""

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

CL-1
CL-2EM-3
 
CL-1ML
                                          CL-2



 EM-2BEM-3  



BH-2 BH-9EM-
2BEM-3 

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EM-2B




(m)


ν
(sa
t)
Cc
Cs
Pc
(kPa)
γ
(kN/m3)
γsat
(kN/m3)
Kh
(m/day)
Kv
(m/day)
Kh/Kv
e0
0
10
(CL-
1)
0.4
5
0.1
7
0.0
3
70
20.16
20.59
0.0086
0.0043
2
0.85
10
22
(CL-
1-1)
0.4
5
0.1
7
0.0
3
180
20.16
20.59
0.0086
0.0043
2
0.85
22
38
(CL-
2)
0.4
5
0.1
6
0.0
3
350
20.64
20.95
0.0026
0.0013
2
0.6
38
48
(CL-
2-1)
0.4
5
0.1
6
0.0
3
450
20.64
20.95
0.0026
0.0013
2
0.6
48
60
(CL-
2-2)
0.4
5
0.1
6
0.0
3
580
20.64
20.95
0.0026
0.0013
2
0.6



(m)

ν
(sat)
Cc
Cs
Pc
(kPa)
γ
(kN/m3)
γsat
(kN/m3)
Kh
(m/day)
Kv
(m/day)
Kh/Kv
e0
0 - 6
(CL-1)
0.45
0.17
0.035
60
19.68
20.11
0.0069
0.00345
2
0.7
6 - 12
(CL-1-
1)
0.45
0.17
0.035
120
19.68
20.11
0.0069
0.00345
2
0.7
12 -
24
(ML)
0.3
0.17
0.03
200
20.4
21.28
0.00864
0.00432
2
0.75
24 -
38
(CL-2-
1)
0.45
0.17
0.03
350
20.82
21.13
0.00172
0.00086
2
0.57
38 -
70
(CL-2-
2)
0.45
0.17
0.03
550
20.82
21.13
0.00172
0.00086
2
0.57
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

          

 
 




 
      
    
EM-2B

""S
""PEM-3


EM-2B EM-2B
EM-2BEM-3
S2S3EM-2B
EM-3     
1
. Settlement plates
2
. Piezometers
3
. Inclinometers
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

P1P2EM-2BEM-3

   

EM-3 EM-3
EM-2B EM-3

 S2EM-2B
S3EM-3
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
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
P1
EM-2BP2EM-3

Settle 3D

 
Pc
CcCs
EM-2BEM-3

EM-2B
EM-3





EM-2B EM-3Settle3D
1
. Finite Difference Method

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

EM-2BEM-3

PVD


. EM-2BEM-3Settle 3D
.Cx1000 Colbonddrain


Settle 3D 

            

a
mm
b
mm

Discharge capacity qw (ml/s)

kf (mm/s)
Opening
size
O90









EM-3
EM-2B
EM-3
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
             

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Kh




Cs ,Cc , Pc


 EM-2BEM-3
kh


EM-2B
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kh


EM-3
2B (EM- Kh

CL-2
   Cc

CL-1 
      
(EM-3 kh
       
CL-2 Cc

CL-1        





 EM-2BEM-3
 

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
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

EM-2BCc
PcCL-1 CL-1-1

 Cc

PcCL-1-1EM-2B
 kh
CL-2
EM-2B
Kh

Settle 3Dm/day
EM-3ML
m/day  CL-2   
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
 Cc
CL-1ML
    
Cc
ML
Cc
CL-1 
Kh
ML
EM-3Kh

CL-2EM-3
 Cc
ML
Cc
CL-1

        

Cf
CL-2
CL-2ML



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
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Day
Instrumentatio n Data (EM -3)
Settle 3D (EM -3)
Instrumentatio n Data (EM -2B)
Settle 3D (EM -2B)



  





 EM-2BEM-3

EM-2BEM-3


    EM-2B 
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Settle 3D
EM-2B
EM-
3
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
 EM-3



  


EM-3
Kh
(ML)
(m/day)
Kv
(ML)
(m/day)
Kh
(CL-2)
(m/day)
Kv
(CL-2)
(m/day)
Cf
(ML)
Cf
(CL-2)
Cc
(ML)
Cc
(CL-
1)
 










Settle 3D









EM-2B
Kh (CL-
2)
(m/day)
Kv (CL-2)
(m/day)
Cf (CL-
2)
Cc
(CL-1)
Pc (CL-
1-1)
(kPa)
 






Settle 3D






EM-2BEM-3

         
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

EM-2B
EM-3
EM-2B
   EM-3        

          
      
   
        

EM-2B 

       EM-2B   
EM-3
EM-2BEM-3 
EM-2B EM-3 Settle 3D

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



     Tank 403   
EM-2B
  Tank 601Tank 602  
EM-3

Settle 3D



EM-2B


Tank 403Tank 601 Tank 602Settle 3D
   
        

Tank 403
Tank 601, 602
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
EM-2B


          Tank 403

Settle 3D
EM-3

EM-3
EM-2B
 EM-3 Settle 3D

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
             


EM-3Tank
601 Tank 602
   

 
Tank 403
Tank 601 Tank 602






PVD

Settle 3D




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
1.  EM-2B EM-3

         EM-2B 
 EM-3   


2.             Cf

EM-2B EM-3


Cc

Pc     
         

3. 

     






4. 
Tank 403EM-2B
Tank 601
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
Tank 602   EM-3      

       
   



5. 



6. 

           
          



    


""

2. Federal Highway Administration (FHWA), "Prefabricated Vertical Drains,
Vol. I, Engineering Guidelines", Report No. RD-86/168 (1986).
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Article
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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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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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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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This paper presents a case history on determination of effective depth of prefabricated vertical drains (PVDs) under embankment loading on a very soft clay deposit at central China, near Jiujiang, Jiangxi Province. The height of the embankment is 5.3 m and construction time was about one year. The PVDs were installed to a depth of 8.5 m at a spacing of 1.5 m in a triangular pattern. Field observations and the finite element method (FEM) were employed to analyse the performance of the soft deposit during embankment construction. The influential depth of the embankment loading was evaluated based on settlement, excess pore pressure, and stress increase in subsoil, both from the observed data and FEM analysis. The effective PVD depth was determined in the following ways: (i) the depth of 5% subsoil settlement of surface settlement, (ii) vertical stress increase in subsoil of 25% in-situ stress, and (iii) consolidation time-PVD depth relation by FEM. Based on the analysis, the effective depth of PVDs is determined to be between 10 and 12.8 m for this field case.
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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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This paper presents a case history of the performance of two full-scale test embankments constructed on soft clay deposit in the eastern coastal region of China. One embankment was constructed on natural subsoil and the other was constructed on prefabricated vertical drain (PVD) improved subsoil. The thickness of the soft clay deposit without PVD-improvement was 19 m and with PVD-improved case was 23 m. The PVDs were installed to a depth of 19 m with spacing of 1.5 m in a triangular pattern. Field performance of the two embankments was analyzed using the finite element method. The following influential factors: (i) hydraulic conductivity of subsoil in field and (ii) discharge capacity of PVDs were investigated numerically. The back-analyzed results for the embankment on natural subsoil showed that the hydraulic conductivity ratio (Cf) of field to laboratory values is about 6. To analyse the PVD-improved subsoil, a simple approach using the equivalent vertical hydraulic conductivity of PVD-improved subsoil was employed. The analytical results show that PVDs increased the bulk vertical hydraulic conductivity of soft subsoil by about 30 times compared to the original non-treated subsoil. The discharge capacity of PVDs in this field case is 79–100 m3/a, which is consistent with the findings from laboratory tests and other reported values in literature.