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Comparing the Results of Numerical Analysis and Monitoring about the

Behavior of Cracks Occurred Nearby Soil-Nailing Walls

Mehran Akhavan

Department of Civil Engineering, Islamic Azad University – Central Tehran Branch, Tehran, Iran

e-mail: mehran100a@yahoo.com

Soheil Ghareh

Department of Civil Engineering, Payame Noor University, P.O. Box 19395-4697, Tehran, Iran

e-mail: ghareh_soheil@pnu.ac.ir

Ma’ssoumeh Bemani Naeini

Islamic Azad University- Mashhad Branch, Iran e-mail: mbemani_2000@yahoo.com

ABSTRACT Soil-nailing system is a widely-used technique for retaining and stabilizing in-situ soil of trenches and slopes. The highly secured and economical design of retained walls under lateral loads, particularly in urban areas, is of special significance. Hence, designing such a system requires very complicated procedure to accurately analyze the lateral displacements of the wall in terms of shear and bending capacity as well as soil-structure interaction. To this end, this case study selected Hengam (Yas) official-commercial project in Tehran to run a numerical study on the behavior of the cracks and displacements occurred during the excavation stages and construction of the soil-nailing system. Considerable displacements have been monitored on the north side of the site, adjacent to 3-4 storey buildings and a street with medium traffic flow. Also, several cracks have been observed on the nearby earth surfaces and walls. Owing to the recent developments in technology, this study benefits from PLAXIS 2D software in order to study the behavior and predict the development of the cracks adjacent to the soil-nailing walls. More specifically, to evaluate the lateral displacements of soil-nailing walls as well as the development of plastic nodes formed in the soil mass behind the walls, the software was used for numerical modeling in terms of the statement of the mechanism of soil nailing stabilization, optimized nail patterning and bound length of the nails. The modeling was done under vertical and lateral loads for an excavated cutting having a depth around 28 meters, by the means of plan strain and staged-construction method in order to meet actual conditions. Finally, the comparison of the results obtained from the numerical modeling and those from site monitoring shows a very good accordance as could be mentioned that modeling such retaining systems by PLAXIS2D, using H-S soil Model could be reliable.

KEYWORDS: Soil-nailing, numerical study, PLAXIS 2D, Stabilization of excavated cuts

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INTRODUCTION

In the last few decades, Geotechnical engineers and consultants have shown interest in the use of mechanical stabilized earth wall systems to construct soil retaining walls. Among all these systems are: soil-reinforcement, soil-nailing walls, and soil-anchored walls which are all based on the notion of stabilizing the soil through optimizing shearing strength [1]. One of the advantages of soil-nailing is the relatively low cost of performance due to the fact that it includes simple components like drilling of the boring holes, preparation of nails for the purpose of retaining and reinforcing the soil, grout injection, and armored concrete for the wall facing. In addition, this method makes it feasible to make use of the whole excavation area for the structure because there is no need for diaphragm walls, concrete piles, soldier beams or truss systems which normally occupy considerable space. Furthermore, the amount of materials, such as rebar and concrete, used in this system is much less, as compared with those methods which require diaphragm walls, concrete piles and bearing beams. So, using lighter kinds of materials and equipment, this system fits best for the urban areas in which shortage of space is a big problem [2]. In line with more efficient use of the system of soil-reinforcement, the use of recent methods such as soil anchoring and soil nailing are prevalent. Such methods are designed based on the mechanism of load transfer through cohesion and interaction in the interface of soil as well as reinforcing material [3]. Such structures function as a result of complicated interaction of soil with other elements of the soil-nailing walls such as soil mass in the excavation, nail, shotcrete facing and bearing plates. In order to study the complicated interaction between soil and structure for determining the behavior and stabilization of soil-nailing walls, numerical techniques based on analyzing the elements are usually used. Thus, it is of critical importance to use the most appropriate and accurate analytical method to evaluate the behavior of the soil-nailing walls [4]. Over the last decade, it has been the current trend as how to design and analyze soil-nailing walls through a combination of shearing capacity and bending capacity of the nails in such structures. This study aims at investigating the efficiency of bending strength of the nails in the expansion of tension and the amount of horizontal displacement, as compared to geogrids which only possess Axial Stiffness (EA).

OBJECTIVES The recent advancements in technology has remarkable paved the road for evaluating the

behavior of the soil-nailing walls and reinforced soil mass, using two-and three-dimensional modeling based on finite element method. This method is capable of delimiting many of the limitations imposed by other methods. Following the previous studies on the evaluation of the horizontal displacement of the soil-nailing walls under both surface and lateral loading, attempts have been made to predict the potential expansion of the cracks on the neighboring sites. To this end, plastic nodes created in the northern site of the project as a result of numerical modeling were evaluated. This study proposes an appropriate method for evaluating the behavior of the soil-nailing walls in the soil. Using Plaxis2D, the behavior ofthe soil-nailing walls has been modeled under vertical and lateral loads for an excavated cutting with a depth of about 28 meters. The modeling was done under the condition of plan strain due to the rather long length of the walls, as compared to the lateral drifts. The project where the present study was conducted belongs to Hengam (Yas) commercial-official plan in Tehran. The results of the numerical analysis have been compared with those obtained from the evaluation of the behavior and monitoring of the various levels of soil-nailing walls, by length. Also, the numerical data have been compared with the rate of fractures, by width. Further, attempts have been made to present a model for investigating and analyzing the

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cracks appeared in the neighboring sites to the excavation in terms of the location and expansion of the plastic nodes created on the surface of the ground.

INTRODUCING THE PROJECT YasCommercial Complex, in an area of 22,000 square meters is located in Hengam Street,

around Resalat Square, in the east of Tehran. The project site is neighboring Zamzam Factory to the east and some parts of the south. It is located next to residential areas by the northern line. The satellite image of the project as well as the plan showing the trapezoid shape of the site are presented in Figures 1-a and 1-b, respectively.

(a) Project Site Plan (b) Satellite image of the site

Figure 1: The trapezoid shape of the site plan [9]

Providing the area needed for the parking space would require the surrounding excavation of an average of 28 meters depth (i.e. about 25 to 29 meters, depending on the topographic level changes in the soil trenches). Subsequently, the protection of the excavated walls needs to be taken into account, as well. In order for making the surface excavation feasible and providing for performing the foundation structure, retaining systems like soil-reinforcement are needed to stabilize the soil, limit the wall drifts, and protect the neighboring structures. As for the retaining walls, this project has used soil-nailing walls, planned for temporary application. The soil-nailing walls consist of 15 rows of nails of rebar AIII with a diameter of 32 millimeters and length of 8 to 14 meters which have been installed along the wall within a distance of about1.5 to 2 meters from each other. To perform the system of soil-nailing, the method of “Drilled and grouted nails” have been used. In cases where there was the possibility of very loose soil, self-drilling nails have been used. In order to optimize the economical function of the system and add to the efficiency of nail patterning, three types of rebar

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patterning with various lengths and in different spots of the site have been used, depending on the existing surcharge at the high levels and the surrounding area (Figure 1-b).

To carry out the plan, the designated area is first excavated for a depth of 1.5 to 2 meters. Then, having installed a metal mesh, the surface of the ground is shotcreted for about 10 centimeter thick to assure stabilization of the excavated panel. Next, using drill wagons, the boring holes with diameters of 10 centimeters and with the determined angles are created at the soil trenches for the rebar nails to be planted. Finally, grout is evenly injected into the holes. Figure 3 illustrates a general view of the situation of soil-nailing system performed at the northern site of the project. It should be mentioned here that in order to have control over the potential displacements of the retaining walls, 54 reflectors have been installed on different sides of the walls and on various levels. For example, 15 of these reflectors have been installed on the northern side (i.e.7 of which on the upper level and the other 8 on the middle level) (Figure 2).

Figure 2: A Reflector installed on the north side wall of the project

The phase of monitoring has been continued for about 6 months after the construction of the retaining walls and all the displacements occurred due this time have been recorded and evaluated [10]. Using surveying tools such as Total station (TSP), the reflectors were read over intervals of 10 days. According to the bench marks made in the surrounding area, the coordinate of each reflector were recorded onto related forms and tables so that any replacement could be evaluated in comparison with the former position of the reflector.

Having gone through the processes of excavation and soil-nailing stage by stage, when we get to the level of 16 meters from the surface, we observe some cracks on the ground of the street, in the north site, which were made about 9.5 to 14.5 meters away from the excavation wall. These cracks have been made parallel to the project line on the surface of the 42nd Street, on the sidewalk, at the joint line of the neighboring buildings and the sidewalk, and on the facing of masonry walls. Figures 4 and 5 show a general view of these cracks. As a whole, a total of 9 cracks have been monitored in the north side, 2 of which have been made on the surface of 42nd Street; 2 on the stone facing of the neighboring building and the rest have been observed on the sidewalk nearby. Since the incident of the cracks, follow-up inspections and special monitoring have been carried out on daily regular basis.

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Figure 3.Accomplished Soil-nailing wall on the north side (about 20 meters down from the surface)

Figure 4: Crack made on the surface if the 42nd street [11]

Drilling boring holes by Drill-Wagon

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Figure 7: The location of marked cracks adjacent to the north side wall [11]

The reason for the differences of the amount of fractures among the cracks within short distances from each other can be attributed to the texture and hardness of the materials used in adjacent masonry walls. For instance, comparing the cracks made at the joint points between the walls of the project and those of the newly constructed building (J7) and the older building (J6), the fracture of J7 is narrower than that of J6, in spite of the fact that the new building is taller with more stories. This is because of the use of harder materials and following the national Building guidelines. Based on the quantitative data presented in Table1, the width of fractures increases as the process of excavation goes further down to the bottom. Also, after the stage of monitoring, as the reported fracture widths show, the widest fractures are corresponding to the middle of the soil-nailing wall by length that is related to the maximum lateral displacement of the wall.

Table 1: Measurement of opening for the cracks by location and

distance from excavation [11] R

ecor

d N

o.

Crack ID J1 J2 J3 J4 J5 J6 J7

Location 42nd St. 42nd St. 42nd St. 42nd St. 42nd St. Hoorian St.Hoorian St.

Distance from excavation (m)

14.70 14.70 14.60 14.60 14.70 22.50 22.30

1 Crack width(mm)0.00 0.00 0.00 0.00 0.00 0.00 -

2 Crack width(mm)0.44 0.36 0.76 0.64 0.18 1.7 -

3 Crack width(mm)0.62 0.46 1.78 1.54 0.36 2.22 0.00

4 Crack width(mm)0.86 0.66 2.10 2.02 0.50 3.16 0.32

5 Crack width(mm)0.92 1.74 2.16 2.64 0.90 3.22 0.39

6 Crack width(mm)1.24 1.90 2.42 2.78 0.92 3.24 0.40

7 Crack width(mm)1.62 2.10 2.58 2.80 1.08 3.68 0.44

8 Crack width(mm)1.66 2.12 2.62 2.92 1.08 3.96 0.76

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GEOTECHNICAL PARAMETERS OF SOIL Geotechnical parameters and features of soil are said to be the most important and influential

factor in designing retaining walls to be considered for such a project. Therefore, it is of crucial importance to have careful and accurate geotechnical investigation before practically starting to excavate the site. This way, the process of performing the plan will surely proceed on logical basis. The geotechnical features used for the present study are as follows: Mechanically Drilled borings of 30 meters depth and 10 boring pits 20-25 meter deep; doing field experiments and Plate Loading Tests in depths of 5 and 6 meters due to the susceptibility of the obtained amounts to Elasticity Module E; doing laboratory experiments such as estimating the unit weight, determining the percentage of humidity, Atterberg Limits, Direct shear test and Tri-axial test Based on the standards of ASTM [9]. Table 2 presents a summary of the experiments results in terms of various depths.

Table 2: Geotechnical Parameters of soil

Poisson’s ratio

Young modules (kN/m2)

Internal angle friction

(o)

Cohesion(kN/m2)

SPT (NSPT)

Unit Weight (kN/m3)

Depth(m)

Soil Layer

0.30 200 24 5 15 17 0-5 Shallow soil

0.30 400 34 12 35 18 5-20 Dense silty Sand with

gravel

0.30 500 35 15 >50 19 20-30Very dense silty gravel

with Sand

SOFTWARE USED FOR MODELING THE PROJECT This project used Plaxis2D as the software doing numerical modeling. It is a combination of

vigorous programs for the problems of geotechnical engineering. Being designed based on Finite Element Method, this software is capable of solving a wide range of geotechnical problems in engineering from simple linear analyses to complicated non-linear modeling. It is specifically used for the analysis of the effect of soil-structure interactions. This software provides the user with the modeling of different kinds of soils and the related elements in the grounds of geotechnical and soil mechanics, using simple graphical context. The modeling is done based on staged-construction. Thus, the situations of numerical modeling are more likely to be congruent with reality. Finite element Method is a vigorous and reliable tool for analyzing the interactions among nail, soil and structure. Among the advantages of this method are: capability of analyzing the elements of soil-nailing walls and their interactions with soil as well as simultaneous consideration of the parameters involved. Also, this method enables engineers to evaluate any kind of soil by means of plan strain in a two-dimensional situation. One of the main reasons making this method a success in analyzing soil is the use of various behavioral models of soil. Thus, one may use Finite Element Method to investigate the soil behavior in a wide range of strains, the effects of the speed of loading, decrease of hardness, so on and so forth [8].

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NUMERICAL MODELING Using PLAXIS 2D, this study has made an attempt to make a model of the Soil-nailing walls

performed in the northern site of Hengam (Yas) project. The purpose of such modeling is to evaluate the behavior of the system through making a comparison between the obtained results of the numerical analysis with those of monitoring. In addition, the modeling intends to analyze the stabilization and realization of the problem considering the effects of shotcrete, soil-nail interaction as well as predicting the position of potential cracks. The modeling has been done in the situation of plan strain based on staged-construction in accordance with the situation of the soil, the amount of surcharge, and two-dimensional nail plotting. The numerical model consists of soil, shotcrete facing and nail. In order to reduce the bordering effects, soil mass has been used in the shape of cubic rectangular of the size of 100m×70m×1m in which a space was made fitting the situation of the soil-nailing wall. Figure 8 shows a general view of the geometry of the numerical model consisting of 15 rows of nails and shotcrete facing before the excavation. The element of plate has been used for modeling nails and facing in order to take into account the bending stiffness, especially at the joint points to the shotcrete surface; thus, increasing the rate of axial forces in the nails up to 2%, as compared to geogrids. As for controlling the accuracy of the numerical modeling, the obtained results have been compared and calibrated with those of a sample model as published in Bulletin, No. 25 (2009) about Plaxis software.

Figure 8: General view of the geometry of the numerical model

In order to analyze the model made with Finite Element Method, it is required to first mesh the geometry of the model. To do so, PLAXIS software makes use of 15-node triangle elements so that the whole geometry of the model is divided into erratic adjunct triangles. The reason for using triangle elements is simple application and proper adjustment to the irregular borderlines in various geometrical shapes. In order to optimize the accuracy of numerical modeling and assure the accuracy

Nails installed in 15 rows

Silty gravel with Sand

Silty Sand with gravel

Shallow soil layer

Shotcrete facing

Surcharge equivalent to 12m Wide Street

Surcharge equivalent to residential buildings

Silty gravel with Sand

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of the output results, the areas around the nails including the shotcreted facings are refined because of the soil-structure interaction. This will make the divisions of triangle-shaped elements smaller. It should be mentioned that the size of meshes depends on the extent to which the project gives importance to and expects accuracy from the output results of modeling. It also depends on the power of the processing system as well as the amount of time allotted to this matter. In order to optimize the accuracy of numerical modeling, care has been taken to be congruent with the actual performance; i.e. the modeling was also done based on 15-staged excavation in 31 phases.

This study has used the hardening soil model (H-S), assuming homogeneity of soil, for creating soil modeling. As for creating model for the nails, the element of plate with both bending and shearing stiffness has been used. Equivalent elasticity module of Eeq has been considered for the rebar and grout injected cross section. Although the use of E50 module is common in the problems of geotechnical materials, when it comes to matters of unloading such as building tunnels and excavation, module Eur should be used, instead because the lower layers of soil are harder than the layers on the surface; thus, the overall rate of hardness is much more in cases when we are involved in situations of both unloading and reloading. That is why the present study has used H-S soil model rather than Mohr-Coulomb (M-C) for modeling the soil behavior. H-S model is advantageous over M-C not only because of the use of tension-strain curve with the hyperbolic functions, but also because this model is capable of controlling the level of the dependency of hardness to the imposed tension, using parameter m (Equation 1). In addition, this model has applied plasticity relations rather than elasticity relations [7]. Using M-C soil model, the user is required to use a fixed quantity as the module of hardness (E) throughout the whole layer of the model soil, while such quantity for E depends on the rate of tension imposed on the element of soil. It can be said the amount of E changes as the analysis proceeds in the numerical modeling.

E = E ccosφ − σ sinφccosφ + P sinφ (1)

Using Geogrid element, which only possesses the Axial stiffness (EA), for numerical modeling of nails means ignoring the concentration of shearing stresses and bending moments at the joint point of nail and shotcreted facing. Therefore, in order to assure more accurate behavior of the wall, this study has used PLAXIS, using the element of plate, for modeling the nails in the soil-nailing wall. The modeling has been carried out in two states, based on the depth of the excavation. The first state deals with the evaluation of the relevant elements in the depth of 16 meters, while the second state is concerned with those in the whole depth of the excavation; i.e. down to the bottom (28 meters).

Knowing that the first cracks appeared on the surface of the ground when the excavation reached the level of +40.0 (i.e. 16 meters deep), the first state of modeling has considered the excavation down to level +40.0 and evaluated the position of the plastic nodes on the surface (see Figure 9). According to the results of the first modeling, plastic nodes are formed on the ground within a distance of 5 to 10 meters from the edge of the northsoil-nailed wall (with more expansion) , and in a distance of 14 to 18 meters from the edge of the wall (with less expansion). Comparing the results obtained from the evaluation of the behavior and monitoring (Table 1) and those of the numerical modeling of the north wall, the occurrence of the first cracks within the distances of 8.5 and 14.5 meters is predictable and congruent with the results of the numerical modeling.

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Figure 9: Finite element model showing plastic point zones after 16m-Deep excavation

The position of plastic nodes changes in the numerical model as the excavation proceeds down to deeper points. The second state of modeling is concerned with the evaluation of the plastic nodes on the surface when the excavation process reached to the bottom; i.e. 28 meters deep. Figure 10 shows the results of modeling in the second state. According to the results of numerical analysis, the plastic nodes have been formed on the surface within a distance of 40 meters from the edge. In terms of the expansion, those plastic nodes occurred in a distance of 20 meters are expanded in a wider area (e.g. J6 & J7).

Figure 10: Finite element model showing plastic point zones after 28m-Deep excavation

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Area where cracks appeared

28m-Deep excavation

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(5). As a whole, it can be said that the general process of forming cracks and displacements was expectable and the rate of displacements and the width of cracks occurred match the position and expansion of the plastic nodes occurred on the surface and behind the soil-nailing walls.

(6). Comparing the results of numerical modeling and those obtained from the monitoring in terms of the lateral displacements of the wall, in spite of the complicated soil profile and numerical modeling limitations, the findings are congruent with each other in terms of the lateral displacements occurred during the excavation.

(7). As there is a proper congruency between the results obtained from numerical modeling and those of monitoring, this research study found H-S as an appropriate model for predicting the behavior of soil-nailing walls.

(8). Within the scope of this study, the software of PLAXIS 2D has been found to be an appropriate tool for evaluating the behavior of the soil-nailing walls.

ACKNOWLEDGMENT Hereby, the authors express their deep gratitude to the managing director of “Soil and Water

Engineering Corporation” for his cooperation. This study also owes to the executive agents in the site for their sincere help for gathering data.

REFERENCES 1. Liew Shaw-Shong , (2005), "Soil Nailing for Slope Strengthening" ,Gue and partners

SdnBhd, Kualalampur Malaysia, Geotechnical Engineering Journal.

2. Nadir Ansari, and Carol Domitric, (1992), “Soil Nailing earth shoring system- A ten year update”, Isherwood Associates.

3. FHWA-SA-96-069R, (1998)," Manual for Design and Construction Monitoring of Soil Nail Walls”, U.S. Department of Transportation ,Federal Highway Administration.

4. Shiu, Y. K. and Chang, G. W. K. (2006). “Effects of inclination, length pattern and bending stiffness of soil nails on behavior of nailed structures.” GEO Report No.197. Geotechnical Engineering Office. Hong Kong.

5. Lazarte, C. A., Elias, V., Espinoza, D. and Sabatini, P. J., (2003)," Soil Nail Walls", Office of Technology Application Federal Highway Administration, U.S. Department of Transportation, Geotechnical Engineering, Circular NO. 7, Report No. FHWA0-IF-03-017, Washington D.C.

6. Plaxis (2002). “Simulation of Soil Nail Structures using PLAXIS 2D.” Plaxis Bulletin, Spring 2009, No. 25, pages 16-21.

7. PLAXIS Ver.8, (2006), “Plaxis Reference manual”, Delft University of Technology &Plaxis by The Netherlands.

8. Ghareh, S., (2011). “A Numerical Study on the behavior of piles under lateral loads”. Paper presented in the 6th National Conference of Civil Engineering, Semnan, Iran.

Vol. 16 [2011], Bund. Q 1252

9. Rahvar-Pazhohesh-Omran Consultant Engineers, (2009). “Supplemental Geotechnical Report”, Construction of retaining walls for Hengam(Yas) Official-Commercial Project.

10. Rahvar-Pazhohesh-Omran Consultant Engineers, (2010). “Analytical Report on Reflector Records”, Construction of retaining walls for Hengam(Yas) Official-Commercial Project.

11. Rahvar-Pazhohesh-Omran Consultant Engineers, (2009). “Analytical Report on the Cracks occurred adjacent to the north excavation area”, Construction of retaining walls for Hengam(Yas) Official-Commercial Project.

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