Slope stability analysis is a common design problem in geotechnics and thus, several slope stability methods using classical limit equilibrium analysis are available. In these methods, applied surcharge load and corresponding stresses are distributed in the soil medium often using Boussinesq theory. This theory assumes elastic, homogenous, isotropic soil medium that extend laterally to infinity. Thus, to account for the non-existing laterally extended soil condition (i.e., presence of slope), the stresses calculated by Boussinesq solution are multiplied by a Stress Adjustment Factor (SAF) of two. In this study, which was published first in the GAP 2019 transportation engineering conference proceedings, Hadi Nabizadeh, Raj Siddharthan, Elie Hajj, Mohamed Nimeri, and Sherif Elfass examine the applicability of using an SAF of two for a pavement loading condition (i.e., layered medium and impact load). A full-scale experiment with a sloped edge pavement and a control experiment which had no sloped pavement shoulder were conducted.
1. INTRODUCTION TO SLOPED SHOULDER INVESTIGATION
The rural low volume roads (LVRs) are typically narrower than high volume highway roads and vehicles travel close to the pavement edge, and therefore slope failure is one of the major concerns for such roads. Slope failure can become more critical in no-shoulder or narrow shoulder rural roadways with side slopes. Slope failure causes safety problems and traffic delays and it is often difficult to repair quickly.
During the past decades, several factors such as regional change in economic growth patterns, changing manufacturing and farming practices, etc. have raised the need for the movement of unconventional farm machinery and superheavy load (SHL) vehicles in the LVRs (Douglas, 2016; Siddharthan et al., 2005). SHL vehicles are much larger in size and weight compared to the standard trucks and they may involve gross vehicle weights in excess of a few million pounds. These vehicles require specialized trailers and hauling units and they do often travel at much lower speeds (Hajj et al., 2018). Although it is recommended to keep SHL vehicles as far away as possible from the pavement edge to avoid slope failure, it is not always feasible when a wide heavy vehicle travels along a narrow roadway.
Slope stability analysis is a common design concern in geotechnical practice. Such an analysis encompasses domain, geometry, failure planes, and shear strength parameters, etc. (Das and Sobhan, 2014; Berg et al., 2009; U.S. Army Corps of Engineers, 2003). Classical slope stability methods are based on the limit equilibrium analysis of a mass of soil bounded between assumed possible slip surface(s) and slope surface. Failure is investigated by comparing the driving and resisting forces and moments. In order to estimate the resultant horizontal force due to surcharge load, Boussinesq theory is commonly used in geotechnical practice. This theory assumes elastic, homogenous, isotropic soil medium that extend laterally to infinity. Thus, the calculated horizontal stress using Boussinesq solution is doubled to account for the non-existing laterally extended soil condition (i.e., presence of slope). In other words, the stresses calculated by Boussinesq solution are multiplied by a Stress Adjustment Factor (SAF) of 2.
This SAF is routinely used in retaining wall design where the lateral force on the wall need to be
estimated (Das and Sobhan, 2014; Berg et al., 2009; U.S. Army Corps of Engineers, 2003). However, the use of Boussinesq theory along with the SAF of two may not be applicable when a layered pavement structure with distinctly different strength and stiffness properties is subjected to a moving load.
2. SLOPED SHOULDER RESEARCH OBJECTIVE
In this study, the applicability of using SAF of two for a pavement loading condition (i.e., layered medium and impact load) is evaluated. To this end, a full-scale experiment with a sloped edge pavement (i.e., sloped experiment) and a control experiment which had no sloped pavement shoulder were conducted at University of Nevada, Reno. A careful comparison between the two experiments identified the role of a sloped edge in the stress distribution within a typical pavement structure. The current study is part of a Federal Highway Administration (FHWA) project on “Analysis Procedures for Evaluating Superheavy Load Movement on Flexible Pavements” (Hajj et al., 2018).
3. EXPERIMENTAL PROGRAM
As illustrated in Figure 1, Experiment No. 3 that is considered as the control experiment (no side slope) consisted of 5 inch of asphalt concrete (AC), 6 inch of crushed aggregate base (CAB), and 66 inch of subgrade (SG). In Experiment No. 4 (sloped experiment), a similar pavement structure with a side slope of 1:1.5 (33.7 degrees with the horizontal) was constructed (see Figure 2). It should be noted that effort was made to use same materials and apply similar compaction practices in both experiments. A detailed discussion regarding the construction procedure, instrumentation, material properties can be found elsewhere (Nimeri et al., 2018; Nabizadeh Shahri, 2017).
In both experiments, Falling Weight Deflectometer (FWD) at various load levels (~ 9,000, 12,000, 16,000, 21,000, 25,000 lb) were applied at the pavement surfaces. As illustrated in Figure 2, the FWD loads in Experiment No. 4 were applied at 12, 24 and 36 in. from the edge of the pavement slope (herein referred to as Loc12, Loc24, and Loc36, respectively). As shown in Figure 1 and Figure 2, surface deflections using Linear Variable Differential Transformers (LVDTs) at the center of loading plate and different locations away from the center of the plate were measured. In addition, several Total Earth Pressure Cells (TEPCs) with 4 in. diameter were installed in the base and subgrade layer to capture the load induced vertical stresses during the load application.
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4. COMPARISON OF STRESS MEASUREMENTS
First, it deemed necessary to ensure that the pavement layers in both experiments were compacted to a similar and desired level of densities. Figure 3 shows the measured deflection basins from FWD tests in Experiment No. 3 and Experiment No. 4 at Loc36 (i.e., 36 in. from the edge of the pavement slope). This figure implies that when the surface loads were applied far enough from the slope edge in Experiment No. 4, the measured surface deflections were similar to the deflections measured in Experiment No. 3 (control experiment), indicating that the density and stiffness properties of the pavement layers in these two experiments were reasonably similar. Consequently, experiment No. 3 can be treated as a control experiment so that any difference in stress measurements at the same location in both experiments can be attributed to the sloped edge.
To determine the SAF using the conducted experiments, the measured vertical stresses at the location of Total Earth Pressure Cells (TEPCs) in Experiment No. 4 were compared to the corresponding measured stresses in Experiment No. 3. In other words, the stress distributions in these two pavement structures were compared by monitoring the TEPCs measurements that are located at the similar positions relative to the applied surface load. Figure 4 and Figure 5 show the measured vertical stresses in the subgrade on the nonslope side of the pavement structure with respect to the location of the applied surface load. Compared to the corresponding measured stresses in experiment No. 3, the stress distribution in the nonslope side of the pavement structure was not affected by the sloped edge.
Figure 6 and Figure 7 depict the load-induced vertical stresses measured by the TEPCs that were installed directly under the centerline of the load at different depths in the subgrade. These figures reveal the noticeable increase (about 70%) in the measured vertical stresses in Experiment No. 4 compared to the Experiment No. 3 (i.e., control experiment). Figure 8 represents the measured vertical stresses at the middle of the base layer and centerline of the load. It may be noted that, the TEPC installed at this location failed measuring vertical stresses at the load levels of 21,000 and 25,000 lb. This figure also implies that the lack of lateral support due to the presence of slope gave rise to 40% increase in the load-induced vertical stresses in the middle of the base.
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Figure 9 to Figure 11 show the load-induced vertical stresses in Experiment No. 4 measured by the TEPCs that were located in the slope side with respect to the location of applied surface load. In other words, the TEPCs were closer to the slope edge relative to the surface loads and therefore, these figures represent the stress distribution adjacent to pavement slope.
As shown in Figure 9, when the surface loads were applied at 12 and 24 in. from the pavement edge, the effect of side slope and the lack of lateral support resulted in 80% increase in the induced vertical stresses. Figure 10 and Figure 11 depict the induced measured stresses as the surface loads were applied at Loc36. These figures represent 40% to 80% raise in the measured vertical stresses.
Based on the above presented observations obtained from these full-scale experiments, it can be concluded that the side slope and the lack of lateral support significantly influence the stress distribution in flexible pavement structures. Furthermore, the impact of slope on the stress distribution becomes more critical as the surface load moves closer to the slope.
5. CONCLUSIONS
The low volume roads (LVRs) are extensively used by heavy trucks and unconventional farm machineries and they provide access to the high-volume transportation system. The movements of superheavy load vehicles on these roads have become more common over the years. These vehicles are very large in size and weight (may involve gross vehicle weights in excess of a few million pounds) and they often travel at much lower speeds. Since the rural LVRs are narrower than high-volume highway roads, the vehicles frequently travel close to the pavement edge. Thus, slope failure has been always considered as one of the major concerns for LVRs.
Slope stability analysis is a common design problem in geotechnics and thus, several slope stability methods using classical limit equilibrium analysis are available. In these methods, applied surcharge load and corresponding stresses are distributed in the soil medium often using Boussinesq theory. This theory assumes elastic, homogenous, isotropic soil medium that extend laterally to infinity. Thus, to account for the non-existing laterally extended soil condition (i.e., presence of slope), the stresses calculated by Boussinesq solution are multiplied by a Stress Adjustment Factor (SAF) of two.
In this study, the applicability of using SAF of two for a pavement loading condition (i.e., layered medium and impact load) was evaluated. A full-scale experiment with a sloped edge pavement (i.e., sloped experiment) and a control experiment which had no sloped pavement shoulder were conducted. Table 1 summarizes the comparison between load-induce vertical stresses measured in Experiment No. 4 (i.e., slope experiment) and Experiment No. 3 (i.e., control experiment). It was found that the side slope can significantly affect the stress distribution within the pavement structure. The influence of side slope on the stress distribution resulted in 40% to 80% increase in the load-induced measured stresses. Accordingly, a SAF of 1.6 is recommended when stress distribution near a sloped pavement shoulder is of interest.
ABOUT THE AUTHORS
Hadi Nabizadeh is with the consulting group Applied Research Associates, Inc. Raj V. Siddharthan, Elie Y. Hajj, Mohamed Nimeri, and Sherif Elfass are with the University of Nevada – Reno, Department of Civil and Environmental Engineering.
REFERENCES
Berg, R.R., Christopher, B.R. and Samtani, N.C. (2009). Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes. Report No. FHWA NHI-10-024. FHWA, U.S. Department of Transportation. Washington, D.C.
Das, B.M., and Sobhan, K. (2014). Principles of Geotechnical Engineering. Cengage Learning, Stamford.
Douglas, R.A. (2016). Low-Volume Road Engineering: Design, Construction, and Maintenance. Taylor & Francis Group, LLC.
Hajj, E.Y., Siddharthan, R.V., Nabizadeh, H., Elfass, S., Nimeri, M., Kazemi, S.F., Batioja-Alvarez, D., and Piratheepan, M. (2018) Analysis Procedures for Evaluating Superheavy Load Movement on Flexible Pavements, Volume I: Final Report. Report No. FHWA-HRT-18-049. FHWA, U.S. Department of Transportation. Washington, D.C.
Nabizadeh Shahri, H. (2017). Development of a Comprehensive Analysis Approach for Evaluating Superheavy Load Movement on Flexible Pavements. University of Nevada, Reno, ProQuest Dissertations Publishing.
Nimeri, M., Nabizadeh, H., Hajj, E.Y., Siddharthan, R.V., Elfass, S., and Piratheepan, M. (2018) Analysis Procedures for Evaluating Superheavy Load Movement on Flexible Pavements, Volume II: Appendix A: Experimental Program. Report No. FHWA-HRT-18-050. FHWA, U.S. Department of Transportation. Washington, D.C.
Siddharthan, R.V., Sebaaly, P.E., El-Desouky, M., Strand, D., and Huft, D. (2005). “Heavy Off-Road Vehicle Tire-Pavement Interactions and Response.” Journal of Transportation Engineering, 131, 239-247.
U.S. Army Corps of Engineers. (2003). Slope Stability. Department of the Army, Washington, D.C.
