Journal of Environmental Treatment Techniques 2013, Volume 3, Issue 3, Pages: 163-169

163

Investigating the Effect of Wide Surcharge and Inclination Angle of

Nails in Excavation (A Case Study: Central District of Isfahan City)

Rassoul Ajalloeian, S. Fazllolah Hashemi*

Department of Geology, Faculty of Science, University of Isfahan, Isfahan, Iran

Department of Civil Engineering, Islamic Azad University, Najaf abad Branch, Iran

Received: 28/06/2015 Accepted: 12/09/2015 Published: 30/09/2015

Abstract

Maintenance of excavation, especially in urban areas, is essential and crucial in order to avoid risks resulted from the

excavation on the adjacent buildings. So, this study tries firstly to investigate various methods used to maintain and stabilize the

excavations, then deals with the factors affecting the stability of excavations trussed by nailing system. Applying the existed

numerical methods, the study performs a modelling of excavation behaviours in the central district of Isfahan city, and then a

comprehensive analysis of such structures’ performance. Thus, the factors influencing the excavation are studied, gathering the

data related to geo-technique studies on the central district of Isfahan city. The various factors such as inclination angle of nails,

wide surcharge over the excavation are studied more exactly by performing the reliability coefficient analysis on extended

models of excavations.

Key words: Trussed excavation, Nailing system, EEM, Reliability, Wide surcharge

1 Introduction

In most of masonry projects, it is necessary to excavate

the land before implementing the main structure.

Conservation systems for excavation are usually retaining

(support) structures providing the vertical or nearly vertical

excavations. Because of the limitation of land dimensions

in urban areas, most of excavations are implemented

vertically and more often the stable slope is not considered

for excavation walls. Although inclined excavations are

possible in suburbs or outskirts of a city, however the

vertical excavation encounters the problem of the walls’

stability. So, it is necessary to consider some strategies to

prevent the walls from being collapsed, such considerations

are called the retaining (support) structures. This study aims

to evaluate the behavior and performance of such structures

affected by various parameters, using the case studies of

excavations implemented in the central district of Isfahan

city. To do that, it is offered firstly a summery of various

methods applied to stabilize the excavations, and finally the

effective factors, such as the inclination angle of nail to the

horizon and the increase in the mat surcharge over

excavation, on the stability of excavated structures retained

by the nailing system. To do that, it is evaluated suitably

the stratification of soil in the studying district, using the

geo-technical studies. A set of analyses can be achieved by

determining the profiles of the soil in the district, applying

two dimensional models by the software Plaxis2D. Using

Corresponding author: S. Fazllolah Hashemi, Department

of civil Engineering, Islamic Azad University, Najafabad

branch, Iran. E-mail: hashemi_fe@yahoo.com

the finite element method, these analyses are performed to

evaluate the factors affecting the excavation performances.

The results from the numerical analyses and the

corresponding field (desert) measures are indicating the

fairly accepted compatibility between them. Being assured

of the accuracy of the models’ performance, it is considered

the impact of other factors, such as inclination angle of

nails, mat surcharge over excavation.

2 Geo-Technical Properties of the Soil in the

Region (Central District of Isfahan City)

It is essential to gain access to the geo-technical data

and properties of the local soil in order to study precisely

the performance of excavations reinforced by nailing

system in the central district of Isfahan city. Studying the

present geo-technical report on the local soil, the data was

evaluated and based on such data and using the software

methods (Rockwork) and statistical techniques, it was

achieved a proper stratification profile of the local soil. In

the geo-technical process, subsurface off-takes were

performed by drilling several machined boreholes in

determined places in concordance with the project

employer. The depth of boreholes is approximately 30

meters, and considering the high number of boreholes in

this region, the depth of boreholes altogether was 630

meters. It could be achieved a three-dimensional

representation from the soil layers in the considered route,

using both geo-statistical techniques and the geo-technical

data related to the drilled boreholes in the square Imam Ali

and its surrounding streets. It was applied some kind of

geo-statistical technique (KRIGING). The various profiles

Journal web link: http://www.jett.dormaj.com

J. Environ. Treat. Tech.

ISSN: 2309-1185

Journal of Environmental Treatment Techniques 2013, Volume 3, Issue 3, Pages: 163-169

164

were determined in different places to specify the soil

layers. Finally, the data needed for soil layers were

obtained and summarized in table (1), using proper geo-

technical profile.

Table 1: Geo-technical properties of the soil from the

studied district

parameters

Fine soil layer

Coarse soil layer

E (kPa)

2 10 kPa

2.5 10 kPa



0.4

0.3

C (kPa)

11 kPa





wet



18.2

Table 2: Parameters of utilized nail and concrete

Concrete

Nails

2×10

EA (kN/m/m)

138×10

EA (kN/m/m)

1067

EI (kN/m

/m)

7.29

EI (kN/m

/m)

1.96

(kN/m/m)w

0.072

(kN/m/m)w

0.2



0.15



(kN/m/m)

1.710

(kN/m/m)

(kN/m)

277

(kN/m)

3 Geometric Properties of Excavation and

Elements Used in Trussing a Nailed

Excavation

The purpose of the present study is to investigate the

various factors affecting the stability of the walls, studying

the wall behaviours of excavation in the central district of

Isfahan city. So, the walls of excavations in the districts

were investigated, using the data on geo-technique

properties from the central district of Isfahan city and

numerical methods from modelling the soil of the

considered region. Since the models analyses are of

parametric ones, one of the present parameters in soil, nails,

or model geometry in each part of the studies will be in

certain and defined range. Thus, the effect of such changes

on the stability of excavations trussed by nailing is

investigated. Also, the amount of tensile forces in nails,

which is of important factors in such excavations, is

considered in studying the static behaviour of structures.

The effect of surcharge is considered as a parameter

affecting the tensile forces in nails and their inclination

angle. In the studies, the slope angle of excavation is

perpendicular toward the horizon. The excavation and then

the nailing processes are performed in six stages. In other

words, to model an excavation and then trussing with

nailing system, an elevation of six meters is excavated in

six steps, and in software modelling, elimination of soil

elements are performed in six one-meter steps in each

stage.

4 Allowable Depth and Slop of Excavation

Before initiating the excavation process, it should be

assured that the depth of excavation would be so that the

relative stability of the soil mass of the excavation wall is

warranted. Accordingly, it is essential to evaluate the

allowable depth of excavation without the needed

reinforcing system, considering the type of local soil, the

width of excavation, and the relevant methods. Thus, this

paper investigates a case study on the stability of an

implemented excavation in the square Imam Ali in order to

determine the allowable depth and slop of excavation,

considering geo-technical properties of the local soil.

Considering the geo-technical data from the soil, there are

various methods such as Limit Equilibrium, which are used

to determine the allowable slope of excavation, as well as

so-called numerical methods to find out the reliability

coefficient for the stability of slops. To analyse the

stability, the software Mathematica was used by the limit

equilibrium method. In such application, limit equilibrium

equations related to the given rupture wedge were written

by programming language, and also, based on the

equilibrium of forces on the wedge, the reliability

coefficient was achieved for the stability of excavation

slop. Several models were created for some excavations by

the software PLAXIS3D- Tunnel and the reliability

coefficient was calculated for the stability of different

widths of excavations. It is necessary to state that it was

modelled two different widths of excavations to investigate

the effect of sensitivity of the excavation width on the

results from the reliability coefficient for the stability of

excavation.

5 Parametric Analyses

5.1 General steps in creating analytic models in PLAXIS

software

A primary sketch is needed to be drawn of the

excavations retained by the nailing system before

modeling, which is performed through the common limit

equilibrium methods.

Regarding the general stability of the excavation wall,

the sliding soil mass is assumed as solid block on which the

forces and anchors should be equilibrated, according to the

principles of limit equilibrium (two-dimensional) methods.

The forces and anchors exerted on the soil mass are

calculated, considering the soil properties (including

internal friction angle, specific gravity, and soil viscosity),

the nail properties (including rupture force and nail

dimensions), and the excavation dimensions (excavation

elevation). Thus, the stability of the excavation walls is

investigated in order the resulted reliability coefficient to be

approached to the critical amount, considering the desirable

different rupture levels. Then, regarding the conditions of

load transition from the soil mass supported by nailing

system, the length needed for the nails is calculated and

their dimensions are studied. Also it is assumed that the

present stresses (tensions) are created because of the weight

of soil mass supported by nailing system and there is no

surcharge over excavation.

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5.2 Effect of inclination angle of nails

In order to perceive the excavations supported by nailing

system more precisely, here is examined the effect of

inclination angle of nails on the performance of a nailing-

supported excavation in the central district of Isfahan city.

So, a series of finite element analyses are conducted on

some of excavation walls reinforced by the nails in order to

study the effect of the inclination angle of nails to the

horizon and the general stability of the excavation

reinforced walls. The other geometric properties of the

excavations reinforced by nailing system including the

inclination angle of excavations’ slop and also, the length

of nails are assumed invariable in such examinations.

Considering the fact that the axial performance of nails is

always very determining in stabilizing the excavations, so it

is applied only the nails sustaining the axial forces in such

examinations. Thus, these elements have only the normal

(axial) solidity, with no considerable bending solidity; as a

result, they wouldn’t tolerate any bending movement and

shear forces.

Fig. 1) The effect of the inclination angle of nails to horizon on the stability

of excavation walls reinforced by nailing system

As shown in the figure (1), the reliability coefficient of

reinforced excavation is increasing by the increase in the

inclination angle of nails to horizon from 5○ to 15○, but the

decreasing trend is initiating by passing the optimized

inclination angle, i.e. the inclination angle between 15○ to

20○. According to such observations and analyses, it can be

concluded that the inclination angle between 15○ to 20○

would be the best inclination angle for the nail to possess

the maximum stability.

Fig. 2) Distribution of tensile forces extending in the first nails row for the

different inclination angles of nails to the horizon.

Fig. 3) Distribution of tensile forces extending in the second nails row for

the different inclination angles of nails to the horizon.

Fig. 4) Distribution of tensile forces extending in the third nails row for the

different inclination angles of nails to the horizon.

Fig. 5) Distribution of tensile forces extending in the fourth nails row for the

different inclination angles of nails to the horizon.

According to the figures 2 to 7, it is seen that the tensile

forces extended in the nails are increased to the optimized

level by increasing the inclination angle of nails to the

horizon, and then they will decrease, passing the optimized

level. According to such fluctuation in the tensile forces

extended in the nails, it is justified the increase and

decrease in the stability of the reinforced excavation wall.

On the horizontal displacement of the excavation mask

Figure 8 shows the changes in the maximum tensile forces

in the nails in the depth of excavation to the changes in the

inclination angle of nails. Figure 9 shows also the changes

in the horizontal excavation mask for the different

inclination angles. As shown, until when the nails serve as

a factor in increasing the stability of excavation wall, the

changes in the horizontal places of the mask will decrease

considerably.

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166

Fig. 6) Distribution of tensile forces extending in the fifth nails row for the

different inclination angles of nails to the horizon.

Fig. 7) Distribution of tensile forces extending in the sixth nails row for the

different inclination angles of nails to the horizon.

Fig. 8) Effect of the inclination angle of nails to the horizon on the

maximum of tensile forces extended in the nails

Fig. 9) Effect of the inclination angle of nails to the horizon

The decrease in the changes in horizontal places of the

wall mask will continue to the optimized inclination angle,

and then the stability of excavation will decrease while the

inclination angle of nails increases, also the changes in

horizontal places of the wall mask will increase

considerably. In other words, because the nails don’t serve

in stabilizing and optimizing the performance of reinforced

excavation wall and have insignificant effect in

strengthening it, it can be seen in very sharp inclination

angles of nails that the changes in horizontal places of the

excavation mask will increase.

5.3 Effect of mat surcharge over excavation

In this section, the effect of mat surcharge over the

excavation is investigated. If it is assumed that the weight

of surface unit for a ranch house (a one-story building) is

equal to 10 KN/M2, then it will be equal to 60 KN/M2 for a

six story building. Considering the prevailing trend for the

excavations in the urban districts, it is essential to

investigate the effect of dead surcharge from the adjacent

buildings on the excavation performance. Therefore, it is

considered three surcharges of 20, 40, and 60 KN/M2 on

the excavation performance in this study. In fig. (10), it is

shown the model used to investigate the effect of mat

surcharge over the excavation on its performance.

Fig, 10) Model used to investigate the effect of mat surcharge over the

excavation.

Fig. 11) Distribution of tensile forces extending in the first nails row for the

different mat surcharge over the excavation.

Journal of Environmental Treatment Techniques 2013, Volume 3, Issue 3, Pages: 163-169

167

Fig. 12) Distribution of tensile forces extending in the second nails row for

the different mat surcharge over the excavation.

Fig. 13) Distribution of tensile forces extending in the third nails row for the

different mat surcharge over the excavation.

As it is considered in the figures 11 to 16, the increase

in the mat surcharge over excavation results in the tensile

forces mobilized in the nails. Considering that the increase

in the mat surcharge serves as a factor increasing the

driving force in the rupture occurred in the soil mass, so

more tensile forces are needed to overcome the shear

tension (stress) developed in the soil mass. Thus increasing

the mat surcharge will result in the increase in the tensile

forces in the nails.

The reliability coefficient of walls is decreasing due to

the increasing mat surcharge over excavation, and as it is

considered, the highest reliability coefficient is related to

the state where the inclination angle of nails are between

15○ to 20○. Figures 22 to 25 show the rupture surface

developed in the models created with different inclination

angles for nails.

Fig. 14) Distribution of tensile forces extending in the fourth nails row for

the different mat surcharge over the excavation.

Fig. 15) Distribution of tensile forces extending in the fifth nails row for the

different mat surcharge over the excavation.

Fig. 16) Distribution of tensile forces extending in the sixth nails row for the

different mat surcharge over the excavation.

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Fig. 17) Effect of the mat surcharge on the maximum of tensile forces

developed in the nails with the inclination angle of zero to the horizon

Fig. 18) Effect of the mat surcharge on the maximum of tensile forces

developed in the nails with the inclination angle of ten to the horizon

Fig. 19) Effect of the mat surcharge on the maximum of tensile forces

developed in the nails with the inclination angle of 15 to the horizon

It is shown in figures 17 to 20 the effect of the mat

surcharge over excavation on the maximum of tensile

forces developed in the nails for different inclination

angles. As it is considered, the increased surcharge over

excavation will increase the tensile forces in the nails due

to the increase in the inclination angle of nails to the

optimized amount. The trend of the increasing tensile

forces in the nails is considered till the optimized

inclination angle of such excavation, i.e. an angle in the

range of 15○ to 20○. After that angle, the increase in the

inclination angle of nails will reduce the tensile forces.

Fig. 20) Effect of the mat surcharge on the maximum of tensile forces

developed in the nails with the inclination angle of 20 to the horizon

Fig. 21) Changes in the reliability coefficient of reinforced excavation to the

inclination angle of nails with the increase in mat surcharge over excavation

Fig. 22) Rupture surface developed in the model used in examining the

effect of surcharge of 60 KN/M2 on performance of excavation reinforced

by horizontal nails

Fig. 23) Rupture surface developed in the model used in examining the

effect of surcharge of 60 KN/M2 on performance of excavation reinforced

by nails with inclination angle of 10

○

Journal of Environmental Treatment Techniques 2013, Volume 3, Issue 3, Pages: 163-169

169

Fig. 24) Rupture surface developed in the model used in examining the

effect of surcharge of 60 KN/M2 on performance of excavation reinforced

by nails with inclination angle of 15

○

Fig. 25) Rupture surface developed in the model used in examining the

effect of surcharge of 60 KN/M2 on performance of excavation reinforced

by nails with inclination angle of 20

○

6 Conclusions

The investigation of the inclination angle of nails

concluded that by the increase in the inclination angle of

nails to the horizon, the performance of reinforced

excavation wall is firstly improved and its stability is

increased, but passing the optimized amount, the increase

in the inclination angle of nails to the horizon results the

wall performance to be weakened and its stability to be

reduced. In other words, by increasing the inclination angle

of nails to the horizon, at first, the reliability coefficient

will increase and then decrease by achieving its optimized

amount. The results from the analyses showed that by

increasing the inclination angle of nails to the horizon by

0○ to 15○, the reliability coefficient of the reinforced

excavation walls will increase, but by passing the optimized

amount, the inclination angle, i.e. the inclination angle

between 15○ to 20○, the reliability coefficient of

excavation is beginning to decrease.

The effect of surcharge over excavation was

investigated and it was considered that the increase in the

mat surcharge over excavation will increase the tensile

forces mobilized in the nails. Considering the fact that the

increased mat surcharge is serving as a factor increasing the

driving forces for the occurrence of the rupture in the soil

mass, so it is needed more tensile forces to overcome the

shear tension (stress) developed in the soil mass, therefore,

the increased mat surcharge will increase the tensile forces

in the nails. Moreover, further investigations showed when

the nails, applied to retain the excavation, are inclined

relative to the horizon, they mobilize more tensile forces in

them by the increased mat surcharge. The trend of

increasing tensile forces in the nails was observed till the

optimized inclination angle, i.e. an angle of 15○ to 20○.

After that, by the increasing inclination angle of nails, the

amount of tensile forces will reduce.

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