Minimizing Roadway Embankment Damage from Flooding (2016)

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8 CHAPTER TWO FAILURE MODES 1. Overtopping erosion 2. Softening by saturation 3. Underseepage and piping 4. Through-seepage (internal erosion) and piping 5. Wave erosion 6. Lateral sliding on foundations 7. Other failure modes including culvert failures and pavement failures. Each of the failure modes listed ranked as most frequent in at least one state (Figure 3). Other failure modes, includ- ing failure related to culvert clogging or erosion and pave- ment failure, are also discussed. OVERTOPPING Overtopping of roadway embankments occurs when the headwater (still water or waves for coastal environments) reaches the point on the embankment crest with the low- est elevation. Overtopping flow mechanisms can occur in coastal and riverine environments as follows: Case I: Overtopping resulting from an increase in a river’s headwater level as it reaches the crest (Figure 4a). As identi- fied by Hughes (2008), USACE states that this flow mechanism can also occur in coastal environments if the still-water level slowly increases with time as a result of tide action, time-vary- ing hydrograph, or long-period seiching (the development of a standing wave in an enclosed—e.g., lakes or reservoirs—or partially enclosed—e.g., harbors or seas—body of water). Case II: Overtopping resulting from wind-generated waves, even though the still-water level has not quite reached the embankment crest (Figure 4b). This case is most promi- nent in coastal environments. Case III: Overtopping resulting from a combination of Case I and Case II (Figure 4c) where the still-water level exceeds the embankment crest elevation and wind-generated waves provide a pulsing, unsteady component to the over- INTRODUCTION This chapter uses the survey results and literature review to present roadway embankment failure modes commonly encountered during flooding. These failure mechanisms involve hydrostatic and hydrodynamic forces that result from overtopping, seepage forces, and the lateral pressure caused by headwater elevation. Overtopping forces can lead to surficial erosion. Seepage forces can lead to saturation, internal erosion, and piping. Lateral pressure can cause the embankment to slide on its foundations. Relevant design parameters and available calculation methodologies will be discussed further in chapters four and five. The following references pertain to levees and other flood control structures; in some cases, the structures exhibit fail- ure mechanisms similar to embankments. • Chen and Anderson (1986), Methodology for Estimating Embankment Damage Due to Flood Overtopping • Chen and Anderson (1987), Development of a Methodology for Estimating Embankment Damage Due to Flood Overtopping • Clopper and Chen (1988), Minimizing Embankment Damage During Overtopping Flow • Richardson et al. (2001), River Engineering for Highway Encroachments • Seed et al. (2005), Investigation of the Performance of the New Orleans Flood Protection Systems in Hurricane Katrina • Douglas and Krolak (2008), Highways in the Coastal Environment, HEC-25, Vol. 1 • Bonelli et al. (2013), Erosion in Geomechanics Applied to Dams and Levees • Douglas et al. (2014), Highways in the Coastal Environment: Assessing Extreme Events, HEC-25, Vol. 2. COMMON FAILURE MODES As part of the survey, DOT engineers were asked to rank fail- ure modes based on the frequency of occurrence in their state from the most common (5) to the least common (1). The ranks were summed for each failure mode to produce a score. The modes are presented based on their scores from highest (most frequently occurring) to lowest (least frequently occurring):

9 topping flow. This case is the most problematic, and gener- ally occurs during hurricane events in coastal environments. These mechanisms are further explained in the following sec- tions based on available studies (generally limited to Case I). FIGURE 4 Overtopping mechanisms (modified after Hughes 2008). Flow Patterns The flow patterns associated with overtopping can take differ- ent forms. Kindsvater (1964) differentiates between them as fol- lows: (1) free plunging, (2) free surface flow, and (3) submerged flow. The development of these flow patterns depends on the tailwater condition, but the most common is free surface flow. A free plunging flow occurs when the falling water produces a submerged hydraulic jump on the downstream slope under the surface of the tailwater. A free surface flow occurs when the water follows the contour of the downstream slope rather than plunging into it. A submerged flow occurs when the depth of water on the downstream side rises and the discharge becomes controlled by both the upstream and downstream heads. In the most common case of free surface flow, three overtopping flow zones can be identified (Clopper and Chen 1988; Figure 5): FIGURE 5 Hydraulic flow regimes and overtopping flow zones (after Clopper and Chen 1988). FIGURE 3 Common failure modes in participating states.

10 • Subcritical flow over the embankment crest (Zone 1), which is characterized by the critical depth (Yc) that is approximately equal to two-thirds of the overtopping head (H1). The water surface elevation is drawn down by one-third H1 because of the velocity head. The loca- tion of the critical depth is dependent on the embank- ment geometry and the overtopping head. • Supercritical flow over the embankment crest (Zone 2), which occurs across the stretch of the crest down- stream from the critical depth. The corresponding velocity is a function of gravitational acceleration (g) and the critical depth (Yc). • Supercritical flow on the downstream embankment slope (Zone 3) which occurs along the downstream slope of the embankment. It is described as supercritical owing to the steep downstream embankment face and turbulence from the surface roughness. Corrections are made for the slope angle in flow depth calculations and for air entrainment in the unit weight of the air entrained water. The corrected parameters are then used in relevant velocity and shear stress calculations. Another factor that could be considered in design, par- ticularly for protection systems, is the zone of negative atmospheric pressure that may develop as shown in Figure 6 (Clopper and Chen 1988). This pressure develops on the downstream embankment slope and could lead to separation between the protection technique and the embankment sur- face if not included in the design considerations. This will be further discussed in chapter four. FIGURE 6 Profile of water nappe and theoretical subatmospheric pressure zone (after Clopper and Chen 1988). Erosion Modes Erosion mechanisms depend on the flow characteristics. Chen and Anderson (1987) differentiate between two mech- anisms as follows: • Freefall condition: In this case, erosion starts at the embankment toe and proceeds upstream in a head-cut as shown in Figure 7. In time, this failure mode would lead to a serious breach because the head-cutting at the toe affects the overall embankment stability and induces erosion losses. • Submerged condition: In this case, erosion begins at the downstream slope and propagates both upstream and downstream as shown in Figure 8. FIGURE 7 Progressive stages of unprotected embankment erosion under freefall flow condition (after Clopper and Chen 1988). FIGURE 8 Progressive stages of unprotected embankment erosion under submerged flow condition (after Clopper and Chen 1988). Damage from overtopping can be mitigated by using the different protection and mitigation techniques discussed in chapter six. In general, the downstream (landward) slope would be protected against overtopping by either waves, continuous flow, or a combination of waves and continuous flow. In coastal environments, both the seaward and land- ward slopes would have to be protected because erosion can take place on the seaward face as the storm recedes. Mitigation Measures Douglas and Krolak (2008) present a number of mitigation strategies for coastal roads that over-wash. These strategies include adequate selection of the road location, consideration

11 of the road elevation, construction of sand dunes, and armor- ing of shoulders to prevent back erosion. For both riverine and coastal embankments, different protection options for the downstream slope are discussed further in chapter eight. References that describe the design and applicability of different protection options on the downstream slope of overtopped embankments include HEC-23 (Lagasse et al. 2009a and b), NCHRP Report 568 (Lagasse et al. 2006b), HEC-14 (Thompson and Kilgore 2006), HDS-6 (Richardson et al. 2001), and HEC-11 (Brown and Clyde 1989). SOFTENING AS A RESULT OF SATURATION Saturation occurs when water seeps into the embankment, yet it is not associated with erosion that leads to a breach. Saturation can occur if the embankment is subjected to prolonged durations of rainfall or flooding. The saturation impacts the effective stress within the soil that makes up the embankment. The effective normal stress σ` is given by σ` =σ - auw (Eq. 1) Where: σ is the total normal stress on the same plane in lb/in2, α is the water area ratio often taken equal to the degree of saturation as a first estimate, and uw is the water stress in kPa (psi). The effective stress directly impacts the strength and compressibility of the soil because it directly relates to how hard the soil grains are pushing against each other. Several cases can occur: 1. If the embankment soil has a degree of saturation less than 100%, the soil is unsaturated and the water in the soil pores is in tension. This tension results from the attraction between water and the minerals making up the soil particles. Any wetting of the soil will increase the degree of saturation, decrease the water tension, and decrease the effective stress. 2. If the embankment soil has a degree of saturation equal to 100% but the soil is saturated by capil- lary action, again the water in the soil is in tension. Any wetting of the soil will increase the water con- tent, swell the soil, decrease the water tension, and decrease the effective stress. 3. If the embankment soil has a degree of saturation equal to 100% and the soil is partially underwater, the water is in compression. Any further rise in the water level will increase the water compression and decrease the effective stress. Any one of these cases will lead to a decrease in strength and a decrease in soil stiffness, which is exemplified by the following example of the impact of water content and associated water tension on the modulus of a soil during a Proctor Compaction Test. A standard compaction test was conducted at different water contents on a silty sand. For each water content, the dry density was measured and the modulus was determined with a tool called the BCD (Briaud 2013). As Figure 9 illustrates, the drop in modulus is very significant on the wet side of the optimum water content, while the dry density does not reflect this drastic loss of stiff- ness. Such loss of stiffness will lead to increased compres- sion and deflection of the pavement under traffic loading. FIGURE 9 Impact of water content on soil modulus (Briaud 2013). After the flood ends and as the water level recedes, another issue can arise. If the embankment soil is a fine-grained soil, the water stress inside the embankment may remain locked in at the flood condition level for some time. In this case, the shear strength of the soil remains low as it is the one associ- ated with the low effective stress. But since the water level dropped, the horizontal water pressure on the upstream side is no longer there to provide lateral support to the upstream slope. This is called a rapid drawdown condition and it is the worst condition for the stability of embankment slopes (Chen and Anderson 1987). UNDERSEEPAGE Underseepage can occur particularly if the foundation mate- rial supporting the embankment is pervious. Underseepage results from the difference in total head between the two sides of the embankment and is likely to occur if the embank- ment itself is less pervious than the foundation soil on which it rests. In this case, it is easier for the water to travel through the embankment foundation soil, as shown in Figure 10. The water flow may erode the foundation material, creat- ing a void under the embankment that would weaken the underlying support. Eventually, the embankment would fall into the void and be washed away. The embankment could also fail as a result of downstream slope instability, because

12 the effective stress is very low at the exit face of the flownet associated with the underseepage, which weakens the toe of the downstream slope (Chen and Anderson 1987; Seed et al. 2005; Bonelli 2013). FIGURE 10 Underseepage (after Seed et al. 2005). The USACE manual Design and Construction of Levees presents a number of seepage analysis and mitigation meth- ods. Relevant mitigation structures include cutoffs, riverside blankets, landside seepage berms, pervious toe trench, and pressure relief walls. WAVE EROSION Waves produce hydraulic shear stresses which are quite different from those created by unidirectional flood flows. Wave erosion here refers to erosion of the upstream (sea- ward) slope due to wave action as shown in Figure 11. This type of erosion is different than the wave erosion that would occur on the downstream (landward) side of an embank- ment, in case the latter was overtopped by water flow or run- up waves. FIGURE 11 Wave erosion (after Seed et al. 2005). To mitigate the resulting soil erosion and pavement dam- age, different types of revetments are presented in HEC-25 (Douglas and Krolak 2008) that could be incorporated into the design. THROUGH-SEEPAGE AND INTERNAL EROSION Through-seepage refers to the seepage of water through the embankment. This type of seepage is particularly accentuated if the embankment soil is more pervious than the underlying foundation soil. Internal erosion through the embankment, as shown in Figure 12 (thick lines), may occur when the local hydraulic forces become large enough to wash away particles within the embankment. Such an internal erosion phenome- non can also occur at boundaries and discontinuities between the embankment soil and hard structures such as culverts within the embankment mass. Bonelli (2013) identifies the following mechanisms of erosion associated with seepage through the foundation and embankment. FIGURE 12 Internal erosion (after Seed et al. 2005). Exit seepage erosion and piping: This mode occurs if the seepage flow becomes sufficient to increase the exit gradient either at the slope face or at the toe. As the flow increases, the hydraulic forces increase; once these forces exceed the erosion resistance of the material, the erosion process begins. The flow net then converges on the whole and fur- ther increases the local gradient and the rate of erosion. As a result, the erosion can rapidly “eat back” a tunnel (or “pipe”) beneath the levee, hence the name “piping.” Internal seepage erosion: This mode occurs either within the embankment or in the foundation soil. Internal erosion can result from the washing out of finer soil particles as the water flows, which can eventually lead to the collapse of the embankment. Four phases progressively occur throughout the piping process: (1) initiation of erosion, (2) continuation of ero- sion, (3) progression to a pipe, and (4) initiation of a breach (Bonelli 2013). The USACE manual Design and Construction of Levees includes such possible mitigation measures as horizontal drainage layers, inclined drainage layers, or toe drains to prevent seepage from emerging on the landside slope. Seep- age damage can be mitigated through adequate materials selection and other mitigation measures discussed in chap- ter four. LATERAL SLIDING ON FOUNDATIONS Lateral sliding on foundations occurs when the horizontal water push resulting from rising water equals the lateral resistance of the embankment on its foundation soil. The water pressure on the upstream side has a triangular distri- bution, which results in a horizontal push. As the water rises on the upstream side, this horizontal push increases and may become equal to the friction force at the interface between the embankment and its foundation soil. At this point, the embankment can start to slide toward the downstream side until the water force decreases to less than the friction force. Lateral sliding can be prevented if adequate friction is provided at the embankment-foundation interface to resist

13 the imposed hydrostatic lateral forces. Such resistance can be achieved by keying the embankment in the foundation soil to generate a passive resistance situation in the founda- tion soil mass. OTHER MODES OF FAILURE: PAVEMENTS AND CULVERTS Compilation of relevant available information on the fail- ure of the pavement covering the crest of the embankment shows that pavement damage and deterioration can occur in both riverine and coastal embankments due the following mechanisms: • Undermining of the pavement cover because of back erosion, which generally starts on the downstream side for riverine embankments and on the landward side in coastal embankments. • Undermining of the pavement cover resulting from wave action, which generally occurs on the seaward side in coastal embankments. • Undermining due to flow running parallel to the embank- ment as the storm recedes in coastal environments. • Rafting, floatation, or highway over-washing as a result of uplift forces created by the penetration of the upstream (seaward) head beneath the pavement. • Post-flooding pavement deterioration resulting from saturation as explained in the “Softening Due to Saturation” section of this chapter. Further information is available in chapter five (“Pavement Degradation and Failure”). Based on the survey replies and follow-up interviews, a number of practices have been adopted to minimize pave- ment damage during flooding. Such practices include select- ing rockfill material for the subbase to decrease erosion potential, using an underdrain, keeping the subbase level above the design slope level, and armoring the slopes. Based on the available information, culverts could cause damage through the following mechanisms: • Directly, through presenting a weak spot at which ero- sion initiates (see “Damage in Canyon Environments, Colorado,” in chapter three) • Indirectly, through being clogged, which leads to over- topping and eventually failure. Examples can be found in the in chapter three section “Overtopping Erosion of a Riverine Highway, Wyoming.” In general, most DOTs have unique culvert installation methodologies. Relevant literature includes a number of proce- dures to limit the scour at the inlet and outlet of the culvert, thus delaying erosion. Such guidance can be found in FEMA (2006) and Fell et al. (2008) (discussed in the chapter five section on “Culvert-Related Problems,” and “Culverts” in chapter nine). SUMMARY This chapter presented the failure modes in roadway embankments caused by flooding. The chapter presented seven failure modes and possible mitigation measures. The failure modes are overtopping, softening by saturation, underseepage and piping, through-seepage and piping, wave erosion, lateral sliding, and other modes that include culvert and pavement failures. The following chapter presents case examples of these failure modes that were gathered through the survey and follow-up interviews.