**8. Step-by-step seismic risk identification of bridge embankments**

In order to facilitate the application of the proposed ranking methodology to prioritize bridge embankments, a complete flowchart has been generated. The flowchart provides a useful tool that promotes achieving the final goal of the study. The flowchart in its current form and sequences ensures a minimal effort from the engineer/researcher to apply the specified ranking methodology. Parameters of each embankment including its geometry, material, seismic event, upper level of bedrock, level of natural ground line, soil type, and anticipated failure types are taken into consideration during the development of the flowchart. All considerations, assumptions, calculations and required checks are arranged in a defined order in the flowchart. The loops of the flowchart, shown in Figure 3, allow relative ranking of bridge embankments. Titles are provided to identify the different sections of the flowchart including geometry, materials, seismic event, soil type, analysis, ranking parameters, category identification, and final ranking/prioritization. Notes to explain the steps of the methodology are numbered consecutively, listed in Table 4, and need to be considered along with the flowchart during the seismic risk prioritization of bridge embankments in a designated region.

have (*C/D)min.* ratio greater than or equal to 1.0.

bridge embankments in a designated region.

**7. Ranking and prioritization** 

An embankment in category *B* meets one of the following two criteria: (1) moderate liquefaction potential; or (2) an anticipated (*C/D)min.* ratio less than 1.0, along with a displacement of less than 10 centimeters (4 inches). An embankment in category *C* shall

After classifying the bridge embankments to category *A*, category *B,* or category *C* in accordance with the criteria listed in Table 2, a prioritization within each category is carried out based on the significance of the three ranking parameters. For instance, the higher the displacement of an embankment in category *A*, the higher its seismic risk, and thus it is assigned a higher priority or ranking. The same applies for the prioritization of the embankments in category *B*. On the other hand, the lower the (*C/D)min.* ratio of an embankment in category *C*, the higher its seismic risk, and thus it is assigned a higher priority or ranking.

Having completed the classification and categorization of all embankments in a certain region due to an anticipated seismic event, the embankment prioritization in each category becomes a feasible task. This proposed ranking model is useful for a quick sensitivity assessment of the effect of various site conditions, earthquake magnitudes, and site geometry on possible movement of a designated embankment. Since the intent of the provided ranking model is to compare the seismic risk of the several embankments, regardless of the existence of highly accurate input data in the ranking model, it is the authors' recommendation to further conduct detailed assessments of the behavior of those at-risk embankments. In such detailed assessments, accurate data from sub-soil explorations is to be incorporated. Eventually, a priority list for the seismic risk of all the considered embankments can be prepared, which enables decision makers to take appropriate actions.

**8. Step-by-step seismic risk identification of bridge embankments** 

In order to facilitate the application of the proposed ranking methodology to prioritize bridge embankments, a complete flowchart has been generated. The flowchart provides a useful tool that promotes achieving the final goal of the study. The flowchart in its current form and sequences ensures a minimal effort from the engineer/researcher to apply the specified ranking methodology. Parameters of each embankment including its geometry, material, seismic event, upper level of bedrock, level of natural ground line, soil type, and anticipated failure types are taken into consideration during the development of the flowchart. All considerations, assumptions, calculations and required checks are arranged in a defined order in the flowchart. The loops of the flowchart, shown in Figure 3, allow relative ranking of bridge embankments. Titles are provided to identify the different sections of the flowchart including geometry, materials, seismic event, soil type, analysis, ranking parameters, category identification, and final ranking/prioritization. Notes to explain the steps of the methodology are numbered consecutively, listed in Table 4, and need to be considered along with the flowchart during the seismic risk prioritization of

Bridge Embankments – Seismic Risk Assessment and Ranking 217

**Table 4.** Complimentary notes to Figure 3 "Flowchart for seismic risk assessment and ranking of

**Figure 4.** Predicted "Peak Ground Acceleration" (PGA) of all counties in the Commonwealth of

multiple bridge embankments"

Kentucky during a 250-year seismic event

**Figure 3.** Flowchart for seismic risk assessment and ranking of bridge embankments


**Figure 3.** Flowchart for seismic risk assessment and ranking of bridge embankments

**Table 4.** Complimentary notes to Figure 3 "Flowchart for seismic risk assessment and ranking of multiple bridge embankments"

**Figure 4.** Predicted "Peak Ground Acceleration" (PGA) of all counties in the Commonwealth of Kentucky during a 250-year seismic event

## **9. Bridges in the commonwealth of Kentucky**

Bridges in the western region of the Commonwealth of Kentucky are located near the New Madrid seismic zone, which is potentially one of the most destructive fault zones in the United States. It extends through the Mississippi River Valley and encompasses 26 counties in western Kentucky in the area of its strongest influence. Studies have shown that the probability of an earthquake with a 6.3 magnitude on the Richter scale to hit this area within the next 50 years exceeds 80%.

Bridge Embankments – Seismic Risk Assessment and Ranking 219

bridge inventory for McCracken County is presented in Table 5. Similar inventories for Livingston, Marshall, Lyon, Trigg, Caldwell, and Christian counties are shown elsewhere

*Characteristics of I-24 Bridge Inventory in Western Kentucky:* Eighty-one bridges are located on I-24 and 45 bridges are constructed over I-24, resulting in a total of 127 bridges either on or over the interstate in western Kentucky. Of the 127 bridges, many bridges were designed without following stringent seismic design guidelines, and may not withstand severe seismic events. Lyon and Marshall Counties are located approximately 115 Kilometers (72 miles) and 96 kilometers (60 miles) northeast of the center of the New Madrid seismic zone, respectively. McCracken County, located approximately 72 kilometers (45 miles) northeast of the center of the New Madrid seismic zone, has the largest number of bridges among all other counties with an average of two bridges per mile. The 127 bridges are categorized based on several characteristics, including: structural type, number of spans, maximum span length, skew angle, construction materials, and bearing types. Eighty three percent of the bridges are skewed, of which, 13% have a skew angle exceeding 40 degrees. McCracken County includes the largest number of bridges (38 bridges), followed by Lyon County (27 bridges), Marshall County (21 bridges), Christian County (20 bridges), Trigg County (11 bridges), Livingston County

The geometry of each bridge embankment on or over I-24 in western Kentucky is taken from the bridge plans. The geometry of the 127 studied embankments is classified into five types (Figure 5*a*-5*e*). An embankment has either a single slope or double slopes separated by a perm. The inventory of I-24 bridge embankments in western Kentucky shows that a given slope has one of three possible inclinations (1:1, 2:1, or 3:1), where the first number of the ratio represents the horizontal unit and the second number represents the vertical unit. The drawings shown in Figure 5*a*-5*d* are for cases where the feature crossed by the bridge is either a highway or a railway. The drawing shown in Figure 5*e* is found when the bridge crosses a waterway. The embankment slope geometry is identified by its height (*H*) and the idealized inclination (*b*) (Figure 6). The analysis is carried out on both ends of each bridge and the most critical embankment slope at either end; whichever analysis results in a lower seismic slope stability *C/D* ratio is considered in the seismic

Accurate identification of the soil characteristics requires detailed site-specific subsurface exploration. This approach is expensive, and such data is not available for the majority of the bridge embankments along I-24 in western Kentucky. Pflazer [14] reported on the use of existing geo-technical data to supplement site investigations. Another approach to specify the soil type and its properties is to use existing geological and agricultural maps. The source of soil data is dependent on the *NGL* (Figure 5*f*-5*g*). The *USGS* and the *USDA*  are used to identify the soil type underneath an embankment. The way by which either

(seven bridges), and Caldwell County (three bridges).

**10. Embankment properties** 

vulnerability ranking.

[11].

Passing through seven counties in western Kentucky, I-24 is considered a vital transportation link for the commonwealth of Kentucky. I-24 passes through McCracken, Livingston, Marshall, Lyon, Trigg, Caldwell, and Christian counties in western Kentucky (Figure 4). The objective of this part of the Chapter is to investigate the seismic risk of all bridge embankments on or over I-24 in western Kentucky.

In order to achieve the study objective, a means of accessing which embankments qualify as "most critical" is required. The methodology presented earlier in this Chapter is applied to assess the seismic vulnerability of I-24 bridge embankments. The embankment geometry, materials, type and properties of underlying soil, elevation of the natural ground line, and upper level of bedrock are estimated for each embankment. The minimum seismic slope stability capacity/demand, (*C/D)min* ratio, embankment displacement, and liquefaction potential of each bridge embankment are calculated. Bridge embankments along I-24 in western Kentucky are assigned one of three possible categories to represent their seismic failure risk. A final priority list of the embankments with the highest seismic risk is generated for the 127 bridges on or over I-24 in western Kentucky.

**On-Site Inspection of I-24 Bridges in Western Kentucky:** On-site inspection of the bridges, including photographing different structural components of each bridge, was carried out. The on-site inspection records form an invaluable source that assists in pre-earthquake evaluation studies as well as post-earthquake inspection.

**I-24 Bridge Inventory in Western Kentucky:** One objective of the on-site inspection is to have an informative source of accurate and updated bridge records, which are required for most assessment studies including the current study of seismic ranking and prioritization of I-24 bridge embankments in western Kentucky. Another objective of the on-site inspection is to provide engineers and transportation officials with information delineating the current bridges' conditions in order to facilitate future comparisons with post-earthquake conditions immediately after future earthquakes. Through these comparisons, significant changes can be reported and further studies can be carried out. All the bridges and embankments along I-24 in western Kentucky were visually inspected, photographed and the records were stored in a database. The on-site inspection represents a significant supplement to the "asbuilt" bridge plans. A comprehensive inventory of the bridges was compiled by review of the "as-built" bridge plans, construction and maintenance records, and on-site inspection forms. The inventory provides an essential data record, which is utilized for risk assessment of I-24 bridges and embankments in western Kentucky. A one-page sample of the I-24 bridge inventory for McCracken County is presented in Table 5. Similar inventories for Livingston, Marshall, Lyon, Trigg, Caldwell, and Christian counties are shown elsewhere [11].

*Characteristics of I-24 Bridge Inventory in Western Kentucky:* Eighty-one bridges are located on I-24 and 45 bridges are constructed over I-24, resulting in a total of 127 bridges either on or over the interstate in western Kentucky. Of the 127 bridges, many bridges were designed without following stringent seismic design guidelines, and may not withstand severe seismic events. Lyon and Marshall Counties are located approximately 115 Kilometers (72 miles) and 96 kilometers (60 miles) northeast of the center of the New Madrid seismic zone, respectively. McCracken County, located approximately 72 kilometers (45 miles) northeast of the center of the New Madrid seismic zone, has the largest number of bridges among all other counties with an average of two bridges per mile. The 127 bridges are categorized based on several characteristics, including: structural type, number of spans, maximum span length, skew angle, construction materials, and bearing types. Eighty three percent of the bridges are skewed, of which, 13% have a skew angle exceeding 40 degrees. McCracken County includes the largest number of bridges (38 bridges), followed by Lyon County (27 bridges), Marshall County (21 bridges), Christian County (20 bridges), Trigg County (11 bridges), Livingston County (seven bridges), and Caldwell County (three bridges).
