Vol-7, Special Issue-Number2-April, 2016, pp1018-1023 http://www.bipublication.com
Case Report
Investigation and evaluation of optimum nailing angle in
stabilization of building excavations
Vahid Tayaripoor Ahmadi1,* and Adel Asakereh2
1MSc Student of Geotechnical Engineering,
Department of Civil Engineering, Hormozgan University, Hormozgan, Iran. 2
Assistant Professor, Department of Civil Engineering, Hormozgan University, Hormozgan, Iran,
Corresponding author. E-mail addresses: [email protected]
ABSTRACT
Nailing is one of the stabilization procedures of excavated walls, which due to its speed and flexibility, has gained popularity in civil projects recently. This procedure can be economically efficient if the nails are placed optimally. Soil nailing retaining structures is one of the most practical approaches of stabilization of excavation walls and slopes in urban environments. Lots of research is conducted on determination of optimum placement angles of nails in the soil, and each one announced different angles based on the case conditions. However, some of the announced angles are not practically operational. So, to increase the safety factor of excavation stability, optimum angle of nail placements, extracted from previous researches, are evaluated and investigated. The obtained results, revealed that placing nails in angles about 20 degrees provide the highest safety factor for excavation stability, and the safety factor decreases with the increase in placement angle.
Key words: nail, excavation, orientation, Soil Nailing
1. INTRODUCTION
In In many structural projects, it’s required to excavate the soil such that the walls are vertical or nearly vertical. Lateral pressures on these walls are due to soil drift, arising from its own wait or probable overloads on the soil nearby the excavation site. These overloads include the weight of the soil above the horizontal excavation edge, nearby buildings, loads due to operation of nearby passageways, etc. Collapse of excavation walls and adjacent buildings during excavation is one of the main obstacles in major civil projects. Unfortunately despite the recent civil engineering developments, the news of excavation wall collapse and nearby buildings and its huge financial and life losses, are spread occasionally. So, considering the ever increasing growth of modern municipal construction, the need for safe
nails is transferred to the soil by the friction between them. It’s noteworthy that, this system allows the engineers to use the soil itself as a retaining structure [3]. The nailing procedure has lots of advantages over common retrofitting procedures such as retaining walls. Moreover, it costs less and so more engineers are willing to get use of it day by day [4].
1.1. Nailing
The soils usually have a relatively high compressive, strength and low tensile and shear strength. Nailing reinforces the soil and increases it’s tensile and shear strength. Building of a reinforced soil nailing wall includes reinforcing the soil during excavation operations using a set of reinforcing bards called “nails”. Nails act in tension and are mostly parallel and in the most practical cases, have a low slope of about 15 degrees downward. Nailing system functionality is based on the transfer of generated tensile force from the soil to nails through shear stress of their common surface [5].
2. STUDY AND DISCUSSION 2.1. Flexural stiffness and slope of nails
Hong and Chen investigated the effects of flexural stiffness and slope of nails on performance of soil nailing walls, using 2D finite difference software. The soil model was nonlinear, had a hyperbolic pre-yield stress-strain relationship, and Mohr-Coulomb yield function. The behavior of a 7m high nailed excavation, having three granular type soils (weak E=30 MPa, relatively dense E=50 MPa, and dense E=100 MPa), consisted of step by step excavation, nailing and applying the facing, was investigated. Beam element was used to investigate the flexural stiffness of the nails. So, flexure and shear are also generated in the nails. Their studies showed that increasing the flexural stiffness, would not increase the normalized tensile force significantly, and decreasing the slope of nail placement, increases the generated force in the nails. This increase is smaller in weak and relatively dense than the dense soil. Moreover, horizontal displacement of wall
increases by increasing the placement angle of nails [6].
The small effect of flexural stiffness and neglecting it are well observed in Shafiee’s investigation. They concluded that if the nails are placed horizontally, the maximum tensile force distribution in the height of the wall is the same for flexible and rigid nails, and the flexural stiffness would be effective, only when the nails are placed with large slope angles like 30 degrees [7].
2.2. Displacements and nail slopes
In a previous research, soil nailing walls were modeled and studied in numerical and limit states software’s. It was observed that with an increase in length and diameter, and decrease in placement angle of the nails respect to horizon, and using one or two rows of restraints in upper levels of excavation wall, displacements of the wall can be reduced significantly. Increase in placement angle of nails leads to increased displacement of the wall, except the 0 and 10 degrees slope that have almost the same wall displacement. The increase in horizontal displacement of the wall due to increase of the nail slope, has a constant rate and its gradient is linear. If L=0.5H, the wall would undergo large displacements, and the soil will reach its brink of failure. If the nail length is increased by 0.2H, the displacements are halved. If the length of the nails is increased again to reach 0.9H, the displacements are reduced more, but increase in nail length from 0.9H to 1.2H would not have a significant effect [8].
7 meters high, and 5.7 wide. Shot Crete was used as the facing, and the nails were placed in horizontal distances of 1.15 meters, and vertical distances of 1 meter, and angle of 10 degrees with respect to horizon. In front of this wall, there’s a backfill that is excavated step by step and the nails are placed again. Figure 1 illustrates the section of the wall [9].
Fig. (1) Geometric profile of the soil–nailed wall in the CLOUTERRE project [9]
As it can be seen in the following picture that with increase in nail placement angle in slopes of about 90 degrees, the factor of safety decreases and vice versa. While in slopes of less than 90 degrees, the effects of placement angle of nails on factor of safety have various peaks.
Fig. (2) Influence of nail orientation on the factor of safety of soil–nailed slopes with a horizontal back slope [9]
2.3. Graphs theory in nail placement angle
Optimum design results of soil nailing slope stabilization using graphs theory in MATLAB software revealed that while placement of the nails in larger relative distances increases the
length of each nail, it leads to decrease in total required nail length for wall stabilization. Based on the numerous analyses on various input data in MATLAB software, it was revealed that the most optimum model for change of nail length in height of the wall, is the placement of longer nails in the mid-height of the wall and shorter nails in the upper and lower 1/3 height of the wall. Despite that this arrangement leads to using nails of longer length, the total volume of required reinforcing bars in soil nailing wall system, is reduced. This phenomena is in agreement with potential failure wedge shape for the wall. Modeling results (Figure 3) revealed that optimum placement angle of the nail, is a low angle with respect to horizon and somehow perpendicular to the soil nailed wall. Also, the upper fill angle has a negligible effect on optimum nail placement angle. Only for slopes with nail placement angles of more than 15 degrees, safety factor is less than walls with horizontal upper fills [10].
Fig. (3) Geometry of the soil–nailed wall
2.4.Effects of changing the nail placement angles
In another research, using FLAC2D software,
stabilization of an excavation in Tohid
angle of 3 degrees below the horizon is used, while a desirable safety factor is attained,
grouting would be possible too [11].
Table (1) Changes in safety factor due to changes in nail placement angle
factor of safety orientation of the nails
(degree)
1.33 -3
1.29 0
1.26 3
1.2 6
1.11 8
Also, another research was conducted to investigate the behavior of reinforced soil nailed excavation. Based on the results presented in Figure 4, by the change of nail placement angle in the range of 5 to 25 degrees, safety factor changes from 1.55 to 1.94. Based on the obtained graph, the best placement angle of the nails is between 15 to 20 degrees [12].
Fig. (4) Influence of nail orientation on the factor of safety of soil–nailed slopes with a horizontal back slope.
3. MODELING AND RESULTS EVALUTION
To investigate the effects of nail placement angles on safety factor of soil nailed slopes, finite element model was created using Plaxis 2D and Geo Studio. The soil of the slope had a density of
19 kN/m3, elasticity modulus of 30’000 kN/m2,
cohesive strength of 50 kN/m2, and internal
friction angle of 30 degrees. The geometry of the model generated in slope/w software is presented in Figure 5.
In continuation, In these analyses, the slope is inclined 90 degrees with respect to horizon, and the nail placement angle is considered 0, 10, 20, 30, 40, and 50 degrees with respect to horizon (Figure 5). After assigning material properties and meshing, initial stress conditions is applied to the model. The model is analyzed and stability safety factor of the slope is obtained. The results extracted from each analysis are presented in the following table.
Fig. (6) Influence of nail orientation on the factor of safety of soil–nailed slopes with a horizontal backslope.
As it can be seen in the graph of Figure 7, the obtained results are in good agreement with the results of Ashrafi et al. (2014) model, and it shows that in placement angles about 20 degrees, nails have higher safety factor and therefore better performance. Though, with an increase in placement angle, safety factor decreases.
Fig. (7) Comparison between results of current and Ashrafi and Besharat (2014) investigations
Compared to results of Bakhshi and Haddad (2013) research, based on the following graph, the obtained results for safety factors, for placement angles of 20 degrees or more are in good agreement and for angles lower than 20 degrees, a difference is observed which can be due to the fact that the modeled excavation in Bakhshi and Haddad (2013) research has a back slope on the ground level, while is horizontal in the current research. After all, it can be said that for reinforced soil nailed vertical excavations, slope stability safety factor is higher for the case that nails form a 20 degrees angle with the horizon.
Fig. (8) Comparison between results of current and Bakhshi and Haddad (2013) investigations
4. CONCLUSION
agreement with similar previous conducted researches in this field.
5. REFERENCES
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[2] Wan-Huan ZHOU,(2008). Experimental and Theoretical Study on Pullout Resistance of Grouted Soil Nails. A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy.
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[4] Sohail gharah et al., 1996. Seismic and quasi-static dynamic evaluation of Mashhad Narges Hotel nailing wall under seismic loads, The first national conference on earthquake disaster management and vulnerability and Lifeline sites.
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