Numerical Analysis of Soil Nail Walls under Seismic Condition in 3D

Form Excavations

Siavash Zamiran1, a, Hadi Ghojavand 1,b, Hamidreza Saba2, c

1 Department of Civil Engineering, Science and Research Branch, Islamic Azad University, Iran

2 Assistant professor, Amirkabir University of Technology, Tehran, Iran

azamirans@gmail.com,

bhadighojavand@gmail.com,

chr.saba@aut.ac.ir

Keywords: Soil Nail Walls, Dynamic Analysis, Numerical Modeling

Abstract. Typically, temporary soil nailing systems are not required to provide for design level

earthquake occurrences consistent with the building or structure being constructed inside the

excavation. However, the seismic response of the permanent soil nail walls during the earthquakes

should be evaluated. On the other hand, evaluation of 3D response of soil nailing walls have some

strange manners that should be considered in the numerical analysis.

In this paper, numerical simulations of soil nail walls under vibrational input have been carried out,

and the results are compared with the function of soil nail walls under ordinary statistical loading. The

behaviour of geometry of nails are mentioned under static and seismic analysis. After that some

investigations are carried out to find respond of soil nailing walls in some 3D excavation forms. The

analysis is performed with finite difference software called FLAC3D.

The results are prepared as lateral displacement of the walls and normalized maximum tensile forces

for nails. These results can demonstrate the behavior of external and internal resistance of soil nail

walls under seismic and static analysis. The deformation of wall under the static and dynamic manner

varies in a wide range. On the other hand, tensile loads that are produced in nails under the static

manner are namely 50% less than the dynamic manner.

Introduction

In areas of high seismic activities, earthquake has a wide effect on retaining walls like soil nail walls.

A review of researchers shows the soil nail walls perform much better than classical retaining walls

like gravity walls. When cable takes effect in foundation, it retains the deformation of soil and

reinforces the foundation from failure by transferring its tensile force based on the interaction of soil

and cables. After reinforced by cables, the parameters and stress state of soil mass is improved.

In the present paper, firstly the cable elements build; then numerical models are founded by

FLAC3D. Seismic analysis is carried out to the models. And then deformation and stress responses

are obtained as well as the mechanical response of nails during calculation. Results can show the

mechanism of soil nail walls and their load transferring mode under seismic condition.

For verification of the soil nail walls in FLAC3D, a compression is carried out with Thompson and

Miller’s (1990) research. They described the design, construction and performance of Seattle’s first

nailed walls.

Verification of the finite deference model

Thompson and Miller (1990) investigated the Seattle’s first nailed walls. For this vertical soil nail

wall, nails were mostly installed at 1.8m spacing horizontally and vertically. The nail length is 10.7m,

except for the length of the top row which is 9.8m. The diameter of the drilled holes is 203mm. Nail

bars are installed at an inclination of 15 degrees, though the first row on the high wall was installed at

20 degrees to avoid utilities. A typical section of the high wall is shown in Fig. 1. In this research, the

stability of this soil nailed vertical cut is analysed by FLAC3D. [1]

Applied Mechanics and Materials Vols. 204-208 (2012) pp 2671-2676Online available since 2012/Oct/26 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.204-208.2671

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.250.144.144, University of Melbourne, Melbourne, Australia-28/09/13,13:55:53)

Fig. 1) Typical section of Seattle’s wall

Thompson and Miller measured the highest nail forces by inclinometer and calculated the values

by the finite-element method. A comparison of maximum nail forces in measured values and

finite-element calculations in each row is shown in Figure 2 with the values that gain from finite

deference method, FLAC3D. The results show reliable convergence among calculated values, FEM

and FDM.

Fig. 2 and Fig. 3 shows the reliable convergence of the places that maximum tensile loads occur

through the nail lengths.

Fig. 2) Comparison of maximum nail forces

Fig. 3) The places that maximum tensile loads

occur through the nail lengths.

Numerical model

In this research, an excavation is carried out to the depth of 8m. Model is fixed in normal direction

for the side boundaries. Fixed in three dimensions is done for the bottom boundary. The procedure of

excavation is done by four steps to simulate the ordinary staged construction in the sites. Before

excavation, only gravity is loaded, and calculation is done to make the system become in an

equilibrium state. Then the displacement caused by gravity is eliminated. Afterwards the excavations

are done while the cable elements are placed in the model to simulate the soil nail walls.

Soil mass is modelled in the software by popular constitutive model, Mohr-Coulomb. The

calculation parameters for soil mass are shown in Table 1.

Table 1. Geomechanical parameters for soil mass

parameter value unit (SI)

Young’s modulus 70 Mpa

Poisson’s ration 0.3

Shear modulus 27 Mpa

Bulk modulus 58 Mpa

Internal friction angle 22 degree

Cohesion 25 kPa

2672 Progress in Industrial and Civil Engineering

The length of cable is 7m, with 10º inclination angle and 1m vertical and horizontal spacing. The

first layer of the cables is 1m to the top of the wall in perpendicular directions. The cable system is

considered as the elastic model.

The axial behavior of reinforcement systems may be assumed to be governed entirely by the

reinforcing element itself. Because the reinforcing element is slender, it offers little bending

resistance and is treated as a one-dimensional member with capacity to sustain uniaxial tension.

In evaluating the axial forces that develop in the reinforcement, displacements are computed at

nodal points along the axis of the reinforcement, as shown in Fig. 4. Out-of-balance forces at each

nodal point are computed from axial forces in the reinforcement as well as shear forces contributed

through shear interaction along the grout annulus. [2], [3]

According to some researchers like Wei and Cheng, a thin layer of material with a thickness of 4.0

mm surrounding the nail is used to model the shearing zone between the nail and the soil that is shown

in Fig. 5. [4]

The shear stiffness of the shear zone (grout), Kg, can be estimated in Eq. (1).

Fig. 4) Mechanical representation of fully bonded reinforcement

Which accounts for shear behaviour of the grout annulus.

2

10ln(1 2 )

GKg

tD

(1)

Where G is shear modulus of the shear zone and is identical to shear modulus of soil; can obtain

from Table 1. D is drilled hole diameter and t is annulus thickness of the shear zone and is considered

equal to 0.004 m. The shear zone cohesive strength (slider) per unit length can be estimated as Eq. (2).

(2)

Where c is cohesion of soil. Shear zone friction angle is nearly equal to friction angle of

surrounding soil. The calculation parameters for cable reinforcing are shown in Table 2. These

parameters are similar to the later study and can resist the wall against failure during the earthquake.

[5]. The shotcrete structure is simulated at the face of the wall with shell elements in FLAC3D. The

thickness of shotcrete is 20cm. The shell system is considered as the elastic model. The calculation

parameters for shotcrete are shown in Table 3.

Fig. 5) Idealization of the soil nail system.

( 2 ) cg c D t

Applied Mechanics and Materials Vols. 204-208 2673

Table 2. Cable parameters for simulation of nail

parameter value unit (SI)

Nail diameter 20 mm

Drill hole diameter 100 mm

Young’s modulus of nail 200 GPa

Young’s modulus of grout 22 GPa

Young’s modulus of grouted nail 29.12 GPa

Annulus thickness 0.004 m

shear zone cohesive strength 6.28 kPa

Shear zone friction angle 31 degree

shear stiffness of the shear zone 219.6 MPa

Compressive yield strength of the grouted nail 30 MPa

Tensile yield strength of the grouted nail 30 MPa

density of grouted nail 2200 kg/m3

Table 3. Structural parameters for shotcrete

parameter value unit (SI)

Young’s modulus 24.5 Gpa

Poisson’s ration 0.3

Density 2200 kg/m3

Comparison between static and dynamic behaviour of the wall

To demonstrate the deference between static and seismic behavior of the soil nail wall static and

dynamic analysis is carried out on a same model. The dynamic loading input is based on Kojur

earthquake accelerogram of 28 May 2004, Iran. [6]

During the earthquake, axial force in nails varies in a wide range and after the earthquake a final

value remains in the nails. Figure 6 shows the proportion of static maximum nail forces to dynamic

highest nail forces for three nail rows. The results show the nearest value of static and dynamic

maximum nail force happen in the mid nail row at the mid of the wall (53%).

Fig. 7 shows the proportion of displacements of the wall in static to dynamic analysis. The most

deformation of the wall happens at the top of the wall in the dynamic analysis. Other displacement

values are compared to displacement of the top of the wall in the dynamic analysis. The results show

that sidelong displacement of the wall in seismic condition has 60 to 95% larger than lateral

displacement of the wall in ordinary static condition.

Fig. 6) Proportion of static tensile force to

dynamic tensile force

Fig. 7) Lateral displacement of the wall to

normalized depth

2674 Progress in Industrial and Civil Engineering

Investigation of the direction of earthquake input load to 3D excavation forms

Two excavation types are considered in this study. In these two models, the excavated planes have

90 and 270 degree with each other. Figure 8 shows the 3D form of these two types of excavation.

The lateral earthquake input is impressed into one direction. Therefore, two vertical walls in each

model behave differently during the earthquake. The walls are either perpendicular to earthquake

direction or parallel to that. Figure 9 shows these two deferent manners of wall under earthquake in 90

and 270º excavation types.

Figure 10 shows sidelong wall displacement in 90 and 270º excavation types to normalize depth of

the walls. In 270º excavation, the lateral displacement of the parallel wall to earthquake direction is

20% than the perpendicular one. On the other hand, in 90º excavation type the lateral displacement of

the parallel wall to earthquake direction is 33% than the perpendicular one. The perpendicular walls in

90 and 270º excavation types can move stronger than parallel. To give a reason for this result we can

mention on more motivation of earthquakes to the perpendicular wall than parallel one. FLAC3D 3.00

Itasca Consulting Group, Inc.Minneapolis, MN USA

Step 11993 Model Perspective18:31:29 Mon Jan 23 2012

Center: X: 1.100e+001 Y: 1.200e+001 Z: 1.050e+001

Rotation: X: 40.000 Y: 0.000 Z: 310.000

Dist: 9.561e+001 Mag.: 1Ang.: 22.500

Block Groupsoil

SEL Geometry Magfac = 0.000e+000

FLAC3D 3.00

Itasca Consulting Group, Inc.Minneapolis, MN USA

Step 9693 Model Perspective18:28:53 Mon Jan 23 2012

Center: X: 1.100e+001 Y: 1.200e+001 Z: 1.050e+001

Rotation: X: 30.000 Y: 0.000 Z: 320.000

Dist: 9.561e+001 Mag.: 1Ang.: 22.500

Block Groupsoil

SEL Geometry Magfac = 0.000e+000

Fig. 8) Two types of excavated wall: 90 and 270º type

Fig. 9) The walls perpendicular and parallel to earthquake

Direction in 90 and 270º excavation types

Fig. 10) Lateral wall displacement in 90 and 270º

excavation types to normalize depth of the walls

Applied Mechanics and Materials Vols. 204-208 2675

Conclusions

A series of numerical simulations is reported using FLAC models to predict the dynamic load and

displacement response of walls. FLAC can simulate the interaction behavior of soil-grout-nail

appropriately. The Mohr-Coulomb constitutive model is used to simulate the soil behavior and nail

and shotcrete demonstrate as the elastic manner.

This paper establishes that earthquakes have great influence on the lateral displacement of the soil

nail walls. During the earthquake, bottom and top nail rows motivate more axial forces than the mid

nail rows.

In 270º excavation, the sidelong displacement of the parallel wall to earthquake direction is 20%

than the perpendicular one. On the other hand, in 90º excavation the lateral displacement of the

parallel wall to earthquake direction is 33% than the perpendicular one. Thus, the perpendicular walls

in 90 and 270º excavation types can move stronger than parallel.

References

[1] Thompson, S. R., and Miller, I. R ., (1990) “Design, Construction and Performance of a Soil

Nailed wall in Seattle, Washington”, Design and Performance of Earth Retaining Structures,

Geotechnical Special publication, No. 25, ASCE, pp. 629-643

[2] “Structural Elements”, FLAC, Fast Lagrangian Analysis of Continua, (2004) ITASCA

Consulting Eng, Minnesota, p31-32

[3] QING Du-gan et. al. (2009) “Numerical simulation for the interaction between soil and cable in

deep foundation pit”, International Conference on Computer Modeling and Simulation, IEEE

[4] W.B. Wei, Y.M. Cheng, (2010), “Soil nailed slope by strength reduction and limit equilibrium

methods”, Computers and Geotechnics journal

[5] Zamiran. Siavash, Saba. Hamidreza, Ghadimi Aroosmahalleh. Fereydoun, (2012), “Numerical

Investigation of Seismic Behavior of Soil Nail Walls”, 4th International Conference on Seismic

Retrofitting, Tabriz, Iran

[6] Building and Housing Research Center (BHRC), The ministry of housing and urban development,

Tehran, Iran, www.bhrc.ac.ir

2676 Progress in Industrial and Civil Engineering

Progress in Industrial and Civil Engineering 10.4028/www.scientific.net/AMM.204-208 Numerical Analysis of Soil Nail Walls under Seismic Condition in 3D Form Excavations 10.4028/www.scientific.net/AMM.204-208.2671