piles supported embankment.pdf

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Pile-Supported Highway Embankment 5 - 1

5 Pile-Supported Highway Embankment

End-bearing piles can be used to support highway embankments constructed over soft foundationmaterials. This method of support can reduce the potential for excessive deformations and failure

during the undrained stage of construction when excess pore pressures are induced in the foundationmaterials by the embankment loading.

This example presents a FLACanalysis of the initial (undrained) construction stage for a highwayembankment built over soft saturated foundation clay and muck, using timber piles to supportthe embankment.* The piles extend through the soft materials and into underlying silty sands.The embankment includes foamed concrete engineered fill as part of the embankment materials.The light-weight foamed concrete is placed in lifts of approximately 0.6 m thickness. The firstlift is placed over a wire mesh directly in contact with the top of the timber piles. Earth fill andpavement material are placed as cover over the foamed concrete. The analysis also includes atraffic surcharge of 11,500 Pa (240 psf). Figure 5.1 shows a section view of the embankment andfoundation materials. The groundwater surface is at the top of the foundation materials.

Figure 5.1 Half-section view of foamed concrete embankment on timberpiles

* This analysis is based on information provided by K. J. Kim of the North Carolina Department ofTransportation on the design of a foamed concrete embankment supported on timber piles for theU.S. 64 widening project in Tyrrell County, North Carolina.

FLAC Version 4.0
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The properties assumed for the foundation and embankment materials are listed in Tables 5.1and5.2. Note that both saturated and dry densities are shown for the foundation materials. Theembankment materials are assumed to remain dry. TheFLACsimulation is an undrained analysisusing the groundwater configuration mode. Consequently, the drained material bulk modulus and

strength properties and the dry mass densities are input for this calculation mode, because the effectof water is incorporated in the FLACcalculation.

Table 5.1 Properties for Foundation Soils

muck very soft clay silty sand

Saturated unit weight (N/m3) 11,100 13,560 18,840

Porosity (%) 90 80 30

Dry density (kg/m3) 231 582 1620

Drained Youngs modulus (MPa) 0.3 0.5 15.0

Drained Poissons ratio 0.49 0.45 0.3

Drained Bulk modulus (MPa) 5.0 1.67 12.5

Shear modulus (MPa) 0.1 0.17 5.77

Drained Cohesion (Pa) 3500 5000 0

Drained Friction angle (degrees) 0 0 32

Dilation angle (degrees) 0 0 0

Horizontal permeability (m/day) 0.003 0.0003 2.4

Vertical permeability (m/day) 0.001 0.0001 0.8

Table 5.2 Properties for Embankment Materials

foamed concrete earth fill

Dry density (kg/m3) 640 1920

Porosity (%) 30 30

Drained Youngs modulus (MPa) 600.0 10.0

Drained Poissons ratio 0.15 0.3

Drained Bulk modulus (MPa) 286.0 8.33

Shear modulus (MPa) 261.0 3.85

Drained Cohesion (Pa) 50,000 2400

Drained Friction angle (degrees) 0 30

Dilation angle (degrees) 0 0

Horizontal permeability (m/day) 1.2 1.2Vertical permeability (m/day) 0.4 0.4

Treated timber piles are located on a 2.5 m by 2.5 m rectangular spacing beneath the embankmentmaterials. The length of each pile is 12.8 m (42 ft), and the average pile diameter is 0.3048 m (12in). The properties of the timber piles are listed inTable 5.3.

FLAC Version 4.0
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Table 5.3 Properties for Treated Timber Piles

Elastic modulus (GPa) 10.0

End-bearing capacity (KN) 250.0

This analysis is performed as a parametric study to compare the deformation of an unsupportedembankment to that of a pile-supported embankment. In both cases, we first determine the initialequilibrium state of the saturated foundation soils. Then, for the unsupported case, we add theembankment materials and monitor the vertical displacement along the foundation surface directlybeneath the embankment. For the pile-supported case, we install the timber piles and then add thelayers of embankment materials while monitoring the vertical displacements in the same locationsas those for the unsupported case.

The model is created using FLACs graphical interface, the GIIC. Upon entering the GIIC, thegroundwater flow option and structural elements are activated in the Model Options dialog. TheSave Project Asmenu item is then selected from the Filemenu in order to set up a project fileto save the model state at various stages of the simulation. We click on ? in this menu dialog toselect a directory in which to save the project file. A record of the FLACcommands used to createthis model is saved after the analysis is complete, using the File/Export Recordmenu item. Alisting of the record created for this model is given inExample 5.1.

We generate the grid using the Build/Block tool to create a two block by two block grid. Then, we usethe Alter/Shape tool to generate lines defining the excavation slope and the excavation and foundationmaterial boundaries. The embankment is 3 m high and the pavement half-width is 12 m. Theresulting grid is shown inFigure 5.2and coincides with the half-section shown inFigure 5.1. Thegrid before alteration is saved as state P1.SAV, and after alteration as P2.SAV.

FLAC Version 4.0
http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/


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FLAC (Version 4.00)

LEGEND

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Figure 5.3 Groups defined for embankment and foundation materials

After all the material groups are assigned, the foamed concrete lifts and earth fill groups are exca-vated using the Material/Cut&Fill tool. The initial stress state for the saturated foundation soils is thencalculated. We use the ININV.FIS FISHfunction provided in the FISHLibrary (see ININV.FISinSection 3in the FISHvolume). This function automatically calculates the pore pressures and

total stresses that are compatible for a model containing a phreatic surface. The groundwater densityand water bulk modulus are specified before applying this FISH function. We use the Settings/GW

tool to set the groundwater density to 1000 kg/m3 and the groundwater bulk modulus to 10,000Pa (to speed convergence to steady-state flow). We then use the Utility/FishLib tool to access theININV.FISFISHfunction. We enter the phreatic surface elevation (wth= 0) and the Ko ratios(k0x = 1.0 and k0z = 1.0) in the dialog, and press OK . TheFISHfunction is called intoFLACandexecuted. The pore pressure distribution and total stress adjustment are then calculated automati-cally. We now solve for the new equilibrium state, using the Run/Solve tool and running in coupledmechanical-groundwater flow mode. The pore-pressure distribution is shown inFigure 5.4. Themodel is saved at this state as P4.SAV.

FLAC Version 4.0
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FLAC (Version 4.00)

LEGEND

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Cons. Time 2.1748E+06

-5.513E+00


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Figure 5.5 Addition of first embankment lift foamed concrete1

Vertical displacement histories are recorded at four locations along the base of the embankment, at(x=0,y=0), (x=5,y=0), (x=11,y=0) and (x=16,y=0). The displacements are monitored throughoutthe embankment construction; the results are shown in Figure 5.6. The extent of the displace-ments induced by the unsupported construction is shown in Figure 5.7. The maximum vertical

displacement beneath the embankment is approximately 0.6 m (2 ft).The displacements are associated with excess pore pressures that develop in the muck and verysoft clay. This is evident from the pore pressure histories recorded along the centerline of theembankment at y=0 and y=-6 (in the muck) and at y=-10 (in the very soft clay). The plots ofpore-pressure histories are given inFigure 5.8.

FLAC Version 4.0
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FLAC (Version 4.00)

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HISTORY PLOT

Y-axis :

Y displacement( 1, 41)




X-axis :

Number of steps

10 20 30 40 50 60 70

(10 )+03

-5.000

-4.000

-3.000

-2.000

-1.000

0.000

(10 )-01

JOB TITLE : .

Itasca Consulting Group, Inc.

Minneapolis, Minnesota USA

Figure 5.6 Vertical displacements along base of embankmentfor unsupported embankment construction

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FLAC (Version 4.00)

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HISTORY PLOT

Y-axis :

Grid-point pp ( 1, 40)



X-axis :

Number of steps

10 20 30 40 50 60 70

(10 )+03

0.200

0.400

0.600

0.800

1.000

1.200

(10 )+05

JOB TITLE : .



Figure 5.8 Pore pressures beneath center of embankmentfor unsupported embankment construction

The pile-supported embankment construction is simulated by first installing pile elements in theFLACmodel. The model state at P4.SAV (the initial equilibrium state) is restored, and sevenpiles of 12.8 m length are positioned at a 2.5 m spacing within the foundation soils. Before thepiles are placed in the model, the first foamed concrete lift is added. This is done so that the top ofthe piles can be connected to the embankment materials. Then, the piles are positioned as showninFigure 5.9.

In order to represent the three-dimensional effect of the 2.5 m pile spacing, we scale the pileproperties by dividing by the pile spacing. It is only necessary to scale the elastic modulus (to4 GPa) and the end-bearing capacity (to 100 KN) to account for the spacing. (Note that in thisanalysis, we neglect the weight of the piles; the density should also be scaled if this were included.)

The properties of the pile coupling springs are selected to simulate an end-bearing capacity andzero skin friction. The cohesive strengths of the shear coupling springs at the top and bottomelements of each pile are set to 130,000 N/m, while all other shear and normal coupling-springstrength values are set to zero. The value for cohesive strength is derived from an axially loaded

single-pile simulation to produce an end-bearing ultimate capacity of 100 KN in the silty-sandfoundation material. The value for coupling-spring shear stiffness is selected at approximately tentimes the equivalent stiffness of the stiffest neighboring zone. By doing this, the deformability atthe pile/soil interface will have minimal influence on both the compliance of the total model andthe calculational speed (seeSection 4.4.1in Theory and Background). The properties used forthe pile elements in this model are summarized in Table 5.4. The model is saved at this stage asP11.SAV.

FLAC Version 4.0
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Figure 5.9 Location of piles in FLAC model

Table 5.4 Properties for Pile Elements

middle segments top & bottom segments

Elastic modulus (GPa) 4.0 4.0

Radius (m) 0.1524 0.1524

Perimeter (m) 0.976 0.976

Shear coupling spring stiffness (GN/m/m) 0.0 1.0

Shear coupling spring cohesion (N/m) 0.0 130,000

Shear coupling spring friction (degrees) 0.0 0.0

Normal coupling spring stiffness (GN/m/m) 0.0 0.0

Normal coupling spring cohesion (N/m) 0.0 0.0

Normal coupling spring friction (degrees) 0.0 0.0

FLAC Version 4.0


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Pile-Supported Highway Embankment 5 - 1 1

The embankment construction steps are now performed following the same sequence as for theunsupported case. Each of the pile-supported stages are saved as separate save states in P12.SAVthrough P17.SAV.

The vertical displacements are monitored as before; the histories are shown inFigure 5.10. Themaximum vertical displacement beneath the embankment is approximately 0.03 m (1 in).

Also, we note that for this case there is an insignificant change in pore pressures in the muck andvery soft clay, as seen inFigure 5.11.

FLAC (Version 4.00)

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step 71593


HISTORY PLOT

Y-axis :





X-axis :

Number of steps

10 20 30 40 50 60 70

(10 )+03

-2.500

-2.000

-1.500

-1.000

-0.500

(10 )-02

JOB TITLE : .

Itasca Consulting Group, Inc.Minneapolis, Minnesota USA

Figure 5.10 Vertical displacements along base of embankmentfor pile-supported embankment construction

FLAC Version 4.0
http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/


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FLAC (Version 4.00)

LEGEND

15-Sep-01 16:11 step 71593


HISTORY PLOT

Y-axis :




X-axis :

Number of steps

10 20 30 40 50 60 70

(10 )+03

0.200

0.400

0.600

0.800

1.000

(10 )+05

JOB TITLE : .



Figure 5.11 Pore pressures beneath center of embankmentfor pile-supported embankment construction

We are also interested in the axial loading that develops in the piles. We note that the axial forcescalculated in the piles are scaled values; the actual forces are found by multiplying the scaledvalues by the pile spacing. We create a FISHfunction, ACTUAL PILE LOAD.FIS, to calculatethe actual pile loads from theFLACscaled loads. This function is created in the Fish Editorwhichis accessed from the Show menu.Figure 5.12shows the function we have written in the Fish Editor.We use the STR.FIN file (available in the FLAC\FISH\FIN directory) to access the structuralelement data structure (seeSection 4in the FISHvolumefor information on .FIN files). Thisfunction is executed by pressing the Run button in the Fish Editor, and the scaled values for pileaxial forces are overwritten by actual values. We can now create a plot of the actual axial forces inthe piles, as shown inFigure 5.13. (The pile numbers shown in the plot legend correspond to thetop, middle and bottom pile segments, which are assigned different material property numbers.)The maximum pile loading is approximately 210 KN. Please note that care must be taken whenusing thisFISHfunction. The results that are now stored in the structural element array for axialforces are not the correct FLACvalues, and subsequent simulations should not be done from thisstate.

Finally, we note that this project can be re-created by importing the data file PEMBANK.DATlisted inExample 5.1, using the File/Import Recordmenu item. After the record is imported totheGIIC, each save state can be created by first clicking on that state in the Recordpane and thenclicking on the restore state button at the top of the pane. The commands associated with that statewill then be called into FLAC. Note that theProject Tree Recordformat must be enabled (from the

Model Options dialog) to import this record.

FLAC Version 4.0
http://f2d495.pdf/http://f2d401.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d495.pdf/http://f2d401.pdf/http://f2d401.pdf/http://f2d495.pdf/http://f2d495.pdf/


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Figure 5.12 Fish Editor showing ACTUAL PILE LOAD.FIS function

Figure 5.13 Actual loads in piles for pile-supported embankment construction

FLAC Version 4.0


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Example 5.1 PEMBANK.DAT

;Project Record Tree export

;Title: Piled Embankment

new

;Branch 1: p1.sav

config gwflow

grid 60,45

gen 0.0,-40.0 0.0,-15.0 30.0,-15.0 30.0,-40.0 rat 1.0,0.9 i 1 31 j 1 16

gen 0.0,-15.0 0.0,3.0 30.0,3.0 30.0,-15.0 i 1 31 j 16 46

gen 30.0,-40.0 30.0,-15.0 80.0,-15.0 80.0,-40.0 rat 1.02,0.9 i 31 61 j 1 16

gen 30.0,-15.0 30.0,3.0 80.0,3.0 80.0,-15.0 rat 1.02,1.0 i 31 61 j 16 46

model elastic i 1 60 j 1 45

save p1.sav

;Branch 2: p2.sav

gen line 11.75,3.0 20.75,0.0gen line 0.0,-9.0 20.0,-5.0

gen line 0.0,-12.0 20.0,-10.5

mark i 21 61 j 41

mark i 20 61 j 32

mark i 20 61 j 23

mark i 1 20 j 41

model null i 21 j 41 45





model null i 16 j 44 45model null i 15 j 44 45

model null i 14 j 45

model null region 45 43

gen line 0.0,2.5 11.0,2.5

ini x 12.543801 y 2.0714378 i 13 j 44

ini x 13.681461 y 1.5236754 i 14 j 43

ini x 15.619698 y 0.84950686 i 16 j 42

ini x 14.187088 y 1.3551335 i 15 j 43

ini x 16.336002 y 0.63882875 i 17 j 42

save p2.sav

;Branch 3: p3.sav

group silty sand region 55 5

model mohr group silty sand

prop density=1620.0 bulk=1.2499999E7 shear=5770000.0 cohesion=0.0 &

friction=32.0 dilation=0.0 tension=0.0 group silty sand

group very soft clay region 59 29

model mohr group very soft clay

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prop density=582.0 bulk=1669999.9 shear=172000.0 cohesion=5000.0 &

friction=0.0 dilation=0.0 tension=0.0 group very soft clay

group muck region 58 36

model mohr group muck

prop density=231.0 bulk=5.0002132E7 shear=100066.7 cohesion=3500.0 &friction=0.0 dilation=0.0 tension=0.0 group muck

group foamed concrete1 i 1 17 j 41

model mohr group foamed concrete1

prop density=640.0 bulk=2.85999968E8 shear=2.61E8 cohesion=500000.0 &

friction=0.0 dilation=0.0 tension=0.0 group foamed concrete1













group earth fill i 20 j 41

model mohr group earth fill


friction=30.0 dilation=0.0 tension=0.0 group earth fill





















FLAC Version 4.0


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group earth fill i 14 15 j 43model mohr group earth fill



group earth fill notnull i 13 14 j 44 45




group earth fill i 1 12 j 45




prop por=0.3 k11=2.83E-9 k22=9.44E-10 group foamed concrete1




prop por=0.3 k11=2.83E-9 k22=9.44E-10 group earth fill

prop por=0.3 k11=2.83E-9 k22=9.44E-10 group silty sand

prop por=0.8 k11=3.54E-13 k22=1.17E-13 group very soft clay

prop por=0.9 k11=3.53E-12 k22=1.18E-12 group muck

save p3.sav

;Branch 4: p4.sav

model null group earth fill

model null group foamed concrete4




fix x y i 1 61 j 1

fix x i 61 j 1 41

fix x i 1 j 1 41

set gravity=9.81

water bulk 2e4

water density=1000.0

set echo off

call Ininv.fis

set wth=0.0 k0x=0.5 k0z=0.5

ininv

history 999 unbalanced

solve

save p4.sav

restore p4.sav

;Branch 0: p5.sav

FLAC Version 4.0


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initial saturation 0.0 i 1 18 j 41 42

fix saturation i 1 18 j 41 42set flow=off

water bulk=2.0E8

set =large

fix x i 1 j 41 42

history 1 ydisp i=1, j=41




history 5 gpp i=1, j=40



solve

save p5.sav

;Branch 1: p6.sav




fix x i 1 j 42 43

fix saturation i 1 16 j 42 43


solve

save p6.sav

;Branch 2: p7.sav




fix x i 1 j 43 44



solve

save p7.sav

;Branch 3: p8.sav




fix x i 1 j 44 45



solve

save p8.sav

;Branch 4: p9.sav

FLAC Version 4.0


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fix x i 1 j 45 46

fix saturation i 1 19 j 42 46initial saturation 0.0 i 1 20 j 42 46

solve

save p9.sav

;Branch 5: p10.sav

apply pressure 11500.0 from 1,46 to 12,46

solve

save p10.sav

restore p4.sav

;Branch 0: p11.sav




struct node 1 1.251,0.01

struct node 2 1.25,-12.8








struct node 10 11.25,-12.8

struct node 11 13.751,0.01

struct node 12 13.75,-12.8

struct node 13 16.251,0.01

struct node 14 16.25,-12.8

struct pile begin node 1 end node 2 seg 10 prop 3001







struct prop 1001

struct prop 2001

struct prop 3001

struct prop 3001 e 4e9 radius 0.1524 perimeter 0.976 cs ncoh 0 cs nfric 0 &

cs nstiff 0 cs scoh 0.0 cs sstiff 0.0 cs sfric 0

struct prop 3002 e 4e9 radius 0.1524 cs scoh 1.3e5 cs sstiff 1e9 per 0.976

struct prop 3002 cs ncoh 0 cs nstiff 0

struct chprop 3002 range 70 70

FLAC Version 4.0


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struct chprop 3002 range 20 20struct chprop 3002 range 10 10








water bulk 2e8

set flow off

fix x i 1 j 40 42

save p11.sav

;Branch 1: p12.sav





history 5 gpp i=1 j=40



set large



solve

save p12.sav

;Branch 2: p13.sav;






solve

save p13.sav

;Branch 3: p14.sav




fix x i 1 j 43 46



solve

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save p14.sav

;Branch 4: p15.sav



friction=0.0 dilation=0.0 tension=0.0 group foamed concrete4fix x i 1 j 43 46



solve

save p15.sav

;Branch 5: p16.sav




fix x i 1 j 43 46



solve

save p16.sav

;Branch 5: p17.sav

apply pressure 11500.0 from 1,46 to 11,46

solve

save p17.sav

FLAC Version 4.0

piles supported embankment.pdf

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