Geotechnical and Seismic Considerations in the Design of Highway Embankments

GEOTECHNICAL AND SEISMIC CONSIDERATIONS IN THE DESIGN OF HIGHWAY EMBANKMENTS

Roy Anthony C. Luna, MSCE 1, 2 Benjamin R. Buensuceso, Jr., D.Eng.2

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¹ AMH Philippines, Inc., University of the Philippines Diliman, Quezon City, Philippines Institute of Civil Engineering, University of the Philippines Diliman, Quezon City, Philippines

Abstract : The paper presents the various geotechnical considerations in the design of high embankments and focuses on the most common stability analysis procedure. Seismic design or earthquake-resistant design is discussed as it applies to slope and highway embankment stability. Various references in selecting seismic coefficient for pseudo-static analysis are cited. A sitespecific approach using an attenuation relation adopted in the Philippines is presented. Examples were shown, emphasizing the importance of selecting a rational seismic coefficient, with the ultimate aim of coming-up with cost-effective design. Keywords : Embankment, Slope Stability, Limit-Equilibrium Method, Pseudo-Static Analysis, Peak Ground Acceleration 1

INTRODUCTION

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EMBANKMENT STABILITY ANALYSIS

Recent and ongoing expressway and highway projects, particularly those along flatlands, have to be constructed on high embankments because of hydrologic and hydraulic considerations. Consequently, embankment slope stability becomes a primary concern.

The main purpose of undertaking slope stability analysis of high embankments is to contribute to the safe and economic design of road and highway projects. To accomplish this, an adequate understanding of geology, hydrology and soil mechanics, including slope stability principles, is essential.

This paper presents the various geotechnical considerations in the design of high embankments, current practice on stability analysis, and approaches in selecting an appropriate seismic design parameter.

Analyses must be based upon a model that best represents (1) site subsurface conditions, which can be established by a reasonably comprehensive geotechnical investigation; (2) the properties of engineered fill, which is dependent on the borrow source, the method of construction and degree of compaction; and (3) applied loads, including inertia forces due to ground movement. Judgments regarding acceptable risk or factors of safety must also be made to evaluate the results of the analyses.

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GEOTECHNICAL CONSIDERATIONS

An exhaustive geologic assessment and geotechnical investigation is imperative for a cost-effective embankment design. From the results of the investigation, the stability of the side slopes, as well as the bearing capacity and compressibility of the natural ground, shall be assessed. Liquefaction analysis, particularly if the site subsoil is characterized by saturated, non-plastic soils, is also warranted. With relevant and reliable geotechnical data, an optimized embankment design, or if necessary, stabilization measures, can be formulated. Stabilization measures may involve ground improvement techniques, or if right-of-way considerations may warrant, the use of retaining structures. Depending on the subsoil condition, implementation of staged embankment construction can be a cost-effective approach. By this approach, immediate settlements (for sand layers) and consolidation (for clay layers) are allowed to take place, improving the strength of the soils underlying the highway embankment. Otherwise, rapid construction may trigger base failure or induce cracks or zones of weaknesses within the embankment.

Given the slope geometry, geotechnical properties of engineered fill and ground conditions, the stability of a slope can be assessed using either published chart solutions or computer analysis. Stability charts, such as those of Taylor, Spencer, Janbu, and Bishop-Morgenstern, can be useful for preliminary analysis and design. For detailed engineering analysis and design, slope stability analysis using computer programs, which is usually based on limit equilibrium approach for a twodimensional model, is typically utilized. More advanced programs using the finite element or boundary element methods are also available. However, such analysis methods require a complete model of the subsoil (and embankment) and their constitutive parameters obtained through an extensive program of laboratory tests.

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SEISMIC ANALYSIS

4.1 Earthquake-Resistant Design The development of structural design codes in the last couple of decades placed emphasis on earthquake-resistant design. The American Association of State Highway and Transport Officials (AASHTO), in its Standard Specifications for Highway Bridges – Division I-A (Seismic Design), articulated this design philosophy as follows: • • • • • • • •

Hazard to life be minimized; Bridges may suffer damage but have low probability of collapse due to earthquake motions; Small to moderate earthquakes should be resisted without significant damage; Realistic seismic ground motion intensities and forces are used in the design procedures; Large earthquakes should not cause collapse. Where possible, damage that does occur should be readily detectable and accessible for repair and inspection; Function of essential bridges to be maintained; Ground motions used in design should have low probability of being exceeded during normal life-time of the bridge; and Ingenuity of the design not to be restricted.

By definition, bridges are extension of roads and highways over waterways or obstructions. In this context, it is appropriate to adopt the same philosophy in the seismic analysis and design of embankments for highways – particularly, expressways and major arterial roads. 4.2 Pseudo Static Method of Seismic Analysis Earthquake ground motions can induce considerable destabilizing inertial forces in slopes. Seismic analysis is therefore essential in evaluating the long-term stability of slopes and embankments. Generally, the pseudo-static method is the simplest and most common method in evaluating stability of slopes during earthquakes. In this method, the earthquake’s inertial forces are simulated by the inclusion of a static horizontal and vertical force in a limit equilibrium analysis. Typically, the seismic force is assumed to act in a horizontal direction only, inducing an inertial force khW (where kh is the horizontal seismic coefficient and W is the weight of the potential sliding mass), in the slope 4.3 Selection of Ground Motion Parameters As earlier presented, the objective of earthquake-resistant design is to come-up with a structure that can withstand a certain level of shaking without excessive damage. This level of shaking is described by a design ground motion. The specification of design ground motion parameters is one of the most important problems in geotechnical earthquake engineering. In slope stability analysis, this translates to the selection of an appropriate seismic coefficient kh and the value of an acceptable factor of safety (FOS).

Table 1 presents a summary of typical seismic coefficients adopted in practice. Table 1 Typical Seismic Coefficients kh 0.10 0.15 – 0.25 0.05 – 0.15

Remarks US Corps of Engineers Japan State of California

0.15

Seed (1979)

½ PGA

Hynes-Griffin and Franklin (1984)

FOS > 1.0 FOS > 1.0

FOS >1.15 and a 20% strength reduction FOS >1.0 and a 20% strength reduction

PGA is Peak Ground Acceleration. It is the maximum value of acceleration reached at any instant during the ground shaking. PGA is given in relation to the acceleration due to gravity (g) or in absolute values m/s² unit. A very conservative assumption in selecting seismic coefficient is to assume PGA = kh, but this will result in an uneconomical design. Evidently, a rational approach in the selection of seismic coefficients is essential for the cost-effective design of slopes. 4.4 PGA and Ground Attenuation Relation The magnitude of the seismic coefficient to be adopted for a particular site or project should adequately simulate the expected earthquake forces. The computation of PGA can be a rational (and site-specific) approach in establishing the seismic coefficient for slope stability analysis. Generally, the following important factors are considered in determining the PGA: ƒ ƒ ƒ ƒ

Probabilistic or deterministic ground motion hazard model Distance from earthquake generator (fault line or trench) Geologic conditions Attenuation relation model

4.4.1 Attenuation Models Because of the lack of strong motion data for large earthquakes, the Philippines still does not have its own ground attenuation model. The Philippine Institute of Volcanology and Seismology (PHIVOLCS), in its studies, adopted the model by Fukushima and Tanaka (1990) of Japan. The use of this attenuation relation model was considered appropriate since Japan and the Philippines have similar tectonic setting (subduction zone) and are both island arcs. Figure 1 presents the Tectonic and Geologic Map of the Philippines, showing the various trenches and active faults within and around the archipelago.

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Figure 2. Plot of Fukushima and Tanaka Attenuation Model After the 1990 Luzon Earthquake, a joint study by the United States Geological Survey (USGS) and PHIVOLCS led to the publication of the Estimates of Regional GroundMotion Hazards in the Philippines. Utilizing the Fukushima Tanaka Attenuation Model, peak horizontal ground accelerations that have a 10% probability of being exceeded in 50 years were estimated for rock, medium soil and soft soil site conditions. Figure 3 is the map showing peak horizontal acceleration amplitude/s in medium soil. Contours are in terms of acceleration due to gravity (g). Figure 1. Philippine Tectonic and Geologic Map (MGB) Presently, the Philippines is in the process of gathering strong motion data with the ultimate objective of eventually developing its own attenuation relation. 4.4.2 Fukushima and Tanaka Attenuation Model The Fukushima and Tanaka attenuation model is basically a function of the surface-wave magnitude, as well as the shortest distance between the site and fault rupture (earthquake generator). The general functional form of this attenuation relation is given by: log(Am)=0.41 M – log(R + 0.032*10 0.41M) - 0.0034R + 1.3 ƒ Am: mean of peak acceleration from two horizontal components (cm/s2) ƒ M: surface wave magnitude ƒ R: shortest distance between site and fault rupture (km) The above relation is considered valid for magnitude ranges of 4.5 to 8.2 at a distance range of 10 km to 300 km. Furthermore, the model yields ground motion at the surface. Thus, correction is required in order to predict the peak ground acceleration at a particular type of foundation. The average peak accelerations for rock, hard soil, medium soil and soft soil sites are 60, 107, 87 and 139 percent, respectively, of the predicted PGA from the formula.

Figure 3. Acceleration in Medium Soil (Thenhaus et.al.)

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EXAMPLE

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To illustrate the importance of the selection of an appropriate seismic coefficient in stability analysis and embankment design, a 10meters-high road embankment section is analyzed. Two (2) slope geometries were considered. Figure 4 and Figure 5 show 1V:1.5H and 1V:2H slope sections, respectively. Typical intermediate berms, 1.0meter-wide, were provided.

ru=0 1.8

ru=0.1 ru=0.2

1.6 Factor of Safety

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ru=0.3 ru=0.4

1.4 1.2 1 0.8 0.6 0.4 0

0.1

Figure 4. 1V:1.5H slope

0.2

0.3

0.4

kh

Fig. 7 Plot of FOS and kh for 1V:2H slope

6 Figure 5. 1V:2H slope Typical strength properties of engineered fill (corresponding to medium dense to dense soils) were adopted. A computer program (using Bishop Method) was utilized to facilitate the calculations of factors of safety (FOS) at varying porewater pressure condition and seismic coefficient. A value of kh (horizontal seismic coefficient) ranging from 0.05g to 0.4g was considered. The maximum kh = 0.4g illustrates the case where kh = PGA (see Figure 3), assuming the site of the example road project is underlain by medium soil. The case where kh = 0 was also included to establish the factor of safety at static condition. Figure 6 and Figure 7 present the plots of factor of safety at varying seismic coefficient (and porewater pressure ratio). From the plots, critical acceleration (with moderate porewater pressure ratio of 0.2) is 0.1g for 1V:1.5H slope, and around 0.2g for 1V:2H slope. 1.6

Factor of Safety

ru=0 1.4

ru=0.1

1.2

ru=0.2 ru=0.3 ru=0.4

1

0.8

0.6

0.4 0

0.1

0.2

0.3

kh

Fig. 6 Plot of FOS and kh for 1V:1.5H slope

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CONCLUSION

A site-specific approach in selecting seismic coefficient for slope stability analysis has been discussed. It is clearly evident from the examples that variations in seismic coefficient have tremendous impact on the slope geometry, and consequently, on the construction cost. Further research, testing, monitoring of existing roads on high embankments, and performance evaluation are highly encouraged. Collaboration of geologists, seismologists and civil engineers are vital in achieving earthquake-resistant and economical design of road and transport infrastructures. ACKNOWLEDGMENT The authors acknowledge the assistance provided by Engr. Martin Luther L. Cocson of AMH Philippines, Inc., as well as the valuable discussions with Dr. Ramon D. Quebral of the Mines and Geosciences Bureau. The examples presented were based on projects undertaken by the authors. REFERENCES Abramson, L.W., Lee, T.S., Sharma, S., and Boyce, G..M. (2002). Slope Stability and Stabilization Methods. John Wiley and Sons, Inc., New York. American Association of State Highway and Transportation Officials (1996). Standard Specifications for Highway Bridges. AASHTO, Washington D.C. Bautista, B.C., et.al. (2000). A Deterministic Ground Motion Hazard Assessment of Metro Manila, Philippines. Philippine Institute of Volcanology and Seismology (PHIVOLCS), Quezon City. Daligdig, J.A. and Besana, G..M. (1993). Seismological Hazards in Metro Manila. Philippine Institute of Volcanology and Seismology (PHIVOLCS), Quezon City. Kramer, S. L. (1996). Geotechnical Earthquake Engineering. Prentice-Hall, Inc., New Jersey. Thenhaus, P.C., et. al. (1994). Estimates of Regional Ground Motion Hazard in the Philippines. United States Geological Survey (USGS) and Philippine Institute of Volcanology and Seismology (PHIVOLCS).

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