2D Numerical Simulation of Stabilized Soil Wall by Nailing and Anchorage Methods

2D Numerical Simulation of
Stabilized Soil Wall by Nailing and
Anchorage Methods
Farshad Rashidi1, Amin Torabipour2
1
Graduate student, Department of Civil Engineering, Kharazmi University,
Tehran, Iran.
Email address: [email protected]
2
Graduate student, Department of Civil Engineering, Kharazmi University,
Tehran, Iran.
e-mail address: [email protected]
ABSTRACT
One of the most important issues in geotechnical engineering is using suitable method for stabilizing
the deep excavations. Among widely used methods, nailing and anchorage methods are being used
more in order to stabilizing soil walls in deep excavations recently. Applying prestressing force is one
of the differences between nailing method and anchorage one ,which causes less deformations and
displacements, while in nailing method, the movement of soil mass makes force to the wall. In this
paper, the data of stabilized wall by Briaud and Lim (1999) were used for verifying numerical
simulations. Parametric analyses were performed on nine stabilized walls with different properties and
deformations and settlements of the walls were determined. The main purpose of this paper is
analyzing the performance of soil nail wall in comparison to anchorage method which are simulated in
Plaxis 2D (finite element software) and compared to obtained deformations of the wall according to
these two mentioned methods.
KEYWORDS:
Numerical Modelling, Pile & Anchorage, Concrete Block & Anchor, Nailing,
Plaxis 2D
INTRODUCTION
Due to urban developments, land size limits in cities and increasing the population, excavations
for residential and commercial building become important. Nailing and anchorage are the most
operative methods for stabilizing deep excavations in urban areas. These two mentioned methods are
more applicable in urban stabilizing excavations and engineering practice because of decreasing the
occupied area less than the other methods.
Nailing method is one of the newest operative approach in permanent and temporary stabilizing
of trenches, slopes and soldier walls. Nailing is an in-situ earth retaining technique. For increasing the
strength of soil wall using nail in soil; because the nails are not prestressed, they are called passive
elements. While the wall intends to move forward, the nails act like tensile elements. Permanent or
temporary concrete facing is performed on stabilized wall at the end. Anchorage is also one of the
stabilizing method for reducing the deformations of adjacent structures. The main difference between
anchorage and nailing method is the induced prestressing force to anchors for reducing displacements
in comparison to nailing method. The anchorage method is always performed with piles or concrete
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266
blocks for controlling deformations and maximum values of stress. The differences of concrete blockanchor and pile-anchor systems are in the implementation of these two systems and pile is also a
continuous element in wall height but concrete block is not. Zhang et al. (1999) analyzed the effect of
some parameters like soil wall height, nail length and horizontal distances between nails in slopes
stability by numerical simulation. Fan and Luo (2007) carried out numerical analyses about the effect
of angle and nail arrangement on slope stability of soil walls. The horizontal and vertical distances of
nails should be equal for better performance of nailing in seepage control (Mittal, 2006). Nailing
causes high value in horizontal displacement of the wall and maximum axial tensile force of nails also
increased and safety factor decreased in this method (Singh and Babu, 2009). Using anchorage
method for slope stability indicates the stiffness and prestressing feature of anchors, the performing
time of anchors into the wall will also have effect on lateral displacements of stabilized soil wall
(O’Rourke, 1981). Briaud and Lim (1999) determined the effects of prestressed force anchors, free
length of anchors and the soldier piles fixed end according to numerical simulation with finite
element methods. Lee and Yoo (2008) analyzed the deformations of berlin wall and ground surface
settlement under the surface of excavation. Muntohar and Liao (2013) accomplished some numerical
simulations with Plaxis 2D for analyzing the effect of soil parameters according to two structures in
Taiwan.
In this study, the numerical modelling was verified according to the result of stabilized wall by Briaud and Lim
(1999). Some parametric analyses were performed on nine soil walls with different properties then
displacements and settlements of walls were determined. According to FHWA manual, the safety factor was
determined for analyzed walls and the stabilized soil walls were designed by Slope/W (Geostudio software),
finally the stress-strain analysis and modelling were simulated by finite element software (Plaxis 2D).
NUMERICAL MODELING VERIFICATION
The stabilizing method for a wall with 7.5 meters height and 60 meters length is pile-anchor. The center to
center space of two H-shaped piles is about 2.44 meters, also two rows of anchor with angle of 30 degrees and
12.35 meters length is performed. The soil type of this site is sand and the unit weight of this is 18.5 kN/m3, and
the underground water level is also on 9.5 meters depth under the ground surface. Table 1 indicates the
properties of soil, anchor and pile.
Table 1: Parameters of numerical model
a) Soil
0.93
(kN/m3)
21
1200
(m)
(m)
7.3
5.05
Lock-off
load 1
(kN)
182.35
(m)
9.15
(m)
1.65
Diameter
(mm)
250
272
b) Anchor
Lock-off
load 2 (kN)
160
c) Pile
Spacing
(m)
2.44
0.65
C (kPa)
0
Tendon
stiffness 1
(kN.m)
19846
Tendon
Stiffness 2
(kN.m)
19479
EI
(kN. )
11620
EA
(kN)
1.47E+06
(deg)
32
Figure 1 shows the variation of horizontal displacement versus the depth of excavation and there
is good agreement between the model results and measured data.
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267
Figure 1: Horizontal displacement along wall height
2D NUMERICAL MODELING
As mentioned, numerical models of stabilized wall with both pile & anchorage and concrete & anchorage
methods were created in Plaxis 2D. In this software, the hardening behavior of (HS) was applied to soil clusters.
The soil behavior model was assumed to be nonlinear elastoplastic with isotope hardening that is appropriate for
static analyses of soil material. Simulation of confining stress effects on deformation modulus of soil, plastic
strains, failure, separation of loading and unloading are another ability of this behavior model. Soldier pile,
nail, concrete block and shotcrete were modelled by using plate element in Plaxis based on two dimensional
modelling, an equivalent stiffness was computed for the length of the wall, the unbond length of anchors
(section between fixed end part of anchor and equivalent point on the pile) was also modelled considering with
Node to Node element. According to tensile behavior of anchor and insignificant moment value on this, the
modelling of fixed end section of anchors was done by Geogrid element with elastic behavior in Plaxis 2D and
equivalent stiffness was considered along the length of wall. Prestressing force were also divided to horizontal
distance of anchors.
The boundaries were considered according to Figure 2 in accomplished analyses. According to this Figure,
in order to ensure that boundaries of models do not affect analysis results, the length of ground surface behind
the soil wall (Be) was assumed to be 3 times of the value of soil height (H), and the length of ground surface in
front of soil wall (We) is also equaled to the value of wall height (H). According to model, defining appropriate
boundary conditions is required for stability of model and determining stiffness matrix; so the boundaries of
model was locked horizontally and both horizontally and vertically at two sides and base, respectively.
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268
Figure 2: Boundary condition and dimensions of model
Triangular elements with 15 nodes were used to mesh the soil cluster in modelling process. The
mesh dimensions around the stabilized wall and specially around the anchors must be fine in order to
reduce the modelling errors and high amount of stress-strain variations, and also to reduce the
calculating time, the mesh dimensions for other areas were considered medium size. Figure 3 shows
the dimensions of mesh around the stabilized wall and soil cluster for one of the 9 models.
Figure 3: The mesh of pile-anchor system model
The analysis phases must be defined including excavation soil in front of the wall with specified
height and activating the anchor or nail elements in each executive phases. According to FHWA
manual, the recommended allowable safety factor for stabilized wall considering to anchorage or
nailing method are 1.3 and 1.5, respectively. Figure 4 illustrates two cases of limit equilibrium
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269
analyses in Geostudio (Slope/W) to define the arrangement of nails or anchors and the length of them
based on FHWA manual.
Figure 4: Modelling in Slope/W, based on FHWA manual, a) Nailing method, b) Anchorage
method
Two types of soil were used in analyses. Table 2 indicates the properties of these two soil types.
Table 2: Properties of two soil types I and II
Soil
type
I
II
E
(MPa)
80
80
20
20
0.3
0.3
C
(kPa)
30
10
(deg)
36
36
6
6
Tables 3&4 indicates the propertise of models for both nailing and anchorage methods.
Table 3: Properties of designed anchored wall
Stabilizing
system
Pile-Anchor
BlockAnchor
Model
No.
Soil
type
L
(m)
Angle
(deg)
Prestressing
Strand
number
Surcharge
1
I
10
( )
65.5
1
( )
0
4
II
10
112
1
0
7
I
10
101
1
40
2
I
10
76
1
0
5
II
10
121.5
1
0
8
I
8 to
14
8 to
14
8 to
14
8 to
14
8 to
14
8 to
14
10
110
1
40
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270
Table 4: Properties of designed nailed wall
Stabilizing
system
Model
No.
Soil
type
L
(m)
Angle
(deg)
Diameter
(m)
Surcharge
( )
Nailing
3
6
9
I
II
I
5 to 14
6 to 17
5 to 16
10
10
10
28
34
34
0
0
40
The type of performed pile for pile-anchor system and the dimensions of concrete block for
block-anchor system were doubled IPE200 and 0.8×0.8×0.35, respectively, and the diameter and area
of strand cables were also 15 mm and 142 mm2 respectively and the bond length of anchors and the
thickness of shotcrete were considered 4 meters and 10 cm, respectively.
The equivalent Young’s modulus for the part of bond length of anchors and nails which is
included reinforced element and grout injected could be expressed as follows:
Eq = En (
Ag
An
) + Eg ( )
A
A
(1)
In Eq. (1), En and Eg are the Young’s modulus of reinforced element with steel and grout,
respectively, and A, An and Ag are also the total cross section, the area of reinforced element and the
grout area, respectively.
NUMERICAL SIMULATION RESULTS
As it is obvious in Figure 5 (a) the horizontal displacement of stabilized wall with pile-anchor
system was less than other methods. The obtained horizontal displacement of nailed wall was the
greatest, the calculated horizontal displacement of stabilized wall with concrete block-anchor system
was also between those of two other mentioned methods. In pile-anchor system, the maximum
horizontal displacement occurred at the final one third of stabilized wall height while for two other
methods, the maximum horizontal displacement occurred at the crest of stabilized wall. A similar
analysis were done on soil type II and as it can be apparently seen in Figure 5(b) the maximum and
minimum values of horizontal displacements of stabilized wall between three methods were similar to
those of models with soil type I; but the values of horizontal displacements of models with soil type II
are greater than horizontal displacement values of models with soil type I.
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271
Figure 5: The variation of horizontal displacements in depth of stabilized wall, a) Soil type I,
b) Soil type II
Figures 6 (a-b) indicate the variation of ground surface settlement at a distance from the edge of
stabilized wall. About 70 percent of considerable ground surface settlements of models occurred at
the distance which was about half height of soil wall, the rate of ground surface settlement also
decreased and became about 1 mm over this distance which was three times the height of wall. In
Figures 6 (a-b), the minimum ground surface settlement of the stabilized wall was obtained in pileanchor method, the nailed wall also gave the maximum ground surface settlement and the obtained
ground surface settlements in concrete block-anchor were somewhere between the results of two
other mentioned methods.
Figure 6: The variation of ground surface settlement at a distance from edge of stabilized
wall, a) Soil type I, b) Soil type II
In next models (7, 8 and 9) with 40
surcharge, as shown in Figure 7, the horizontal
displacement of stabilized model in pile-anchor system became minimum values due to continuous
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272
element of pile and great stiffness in comparison to other mentioned methods, the maximum
horizontal displacement of stabilized wall with pile-anchor system occurred in the last one third of
wall height in comparison to two other methods. The maximum values of horizontal displacement
were obtained in nailed wall in these three methods.
Figure 7: The variation of horizontal displacement in depth of stabilized wall (soil type I)
The ground surface settlement of stabilized wall in pile-anchor system is less than two other
methods, the maximum ground surface settlement also occurred in nailed wall as shown in Figure 8.
Figure 8: The variation of ground surface settlement at a distance from edge of stabilized
wall (soil type I)
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273
CONCLUSION AND SUMMARY
The purpose of this paper was investigating the operation of stabilized walls using anchorage and
nailing in finite element method software Plaxis 2D, according to obtained results from numerical
models and comparing these for 3 methods of excavating, results were achieved as follows:
According to minimum safety factor recommended by FHWA manual in nailing and anchorage
methods, the obtained horizontal displacement considering to anchorage method is less than nailing
method for both soil type I and II.
The horizontal displacements of stabilized wall in concrete block-anchor system were greater
than this in pile-anchor stabilized system due to continuousness and more stiffness in element of pile.
Ground surface settlements behind the stabilized wall have direct relationship with horizontal
displacements and in this case the minimum values also occurred in the pile-anchor stabilized system
comparing to two other methods.
The maximum deformations of stabilized wall occurred in nailing method, the obtained
deformations of soil wall into concrete block-anchor system were also greater than pile-anchor
system.
According to existence of surcharge on wall, the performance of pile-anchor stabilized system
can control horizontal displacement and settlement better than two other methods. The horizontal
displacement in concrete block-anchor method is less than this in nailing method.
Generally it can be concluded that using pile-anchor method in some parts of excavation wall,
where is more sensitivity, is more effective in decreasing excavation deflections significantly. So this
method would be applicable in some excavations with high sensitivity.
REFERENCES
[1]
Briaud, J. L and Lim, Y. (1999) “Tieback walls in sand: numerical simulation and
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[2]
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[3]
Mittal, S. (2005) “Soil nailing application in erosion control–an experimental study,”
Geotechnical and Geological Engineering, 24, 675-688.
[4]
Muntohar, A. S and Liao, H. J. (2013) “Finite element analysis of the movement of the
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[7]
Singh, V. P and Babu G. L. (2009) “Plaxis practice-simulation of soil nail structures
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[8]
Singh, V. P and Babu G. L. (2010) “2D Numerical simulations of soil nail walls,”
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Yoo, C and Lee, D. (2008) “Deep excavation-induced ground surface movement
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© 2017 ejge
Editor’s note.
This paper may be referred to, in other articles, as:
Farshad Rashidi and Amin Torabipour: “2D Numerical Simulation of
Stabilized Soil Wall by Nailing and Anchorage Methods” Electronic
Journal of Geotechnical Engineering, 2018 (23.03), pp 265-274.
Available at ejge.com.