Association of State Dam Safety Officials, 2009 Proceedings, Hollywood, Florida
PREFERRED RISK REDUCTION ALTERNATIVE FOR REACH 1A OF HERBERT HOOVER DIKE Jacob R. Davis1, Erich D. Guy2, Ronald L. Nettles3
Abstract Herbert Hoover Dike (HHD) encircles the 720 square mile (143 mile perimeter) Lake Okeechobee in south central Florida. Original Congressional authorization for construction of HHD followed catastrophic damages and massive losses of life during overtopping events produced by hurricanes of 1926 and 1928. Flood damage reduction has been accomplished for more than 75 years; however, concerns related to seepage instability have existed at HHD since the early 1980’s. They have since become more pronounced during high lake events and through project evaluations, and are generally due to the lack of hydraulic controls and the characteristics of embankment and foundation materials. Following thorough evaluation of numerous alternatives to address stability concerns, the preferred comprehensive risk reduction alternative for (the 4.9 mile long) Reach 1A of HHD requires the construction of two primary components: a partial-depth seepage/piping barrier through the embankment and near-surface consolidated strata, and a landward seepage berm, which is coupled with a relief well system for blanket aquifer foundation condition regions. This paper offers an overview of HHD seepage-related aspects, alternatives considered for addressing Reach 1A stability concerns, and design approaches for the preferred risk reduction alternative. A summary of developed and implemented interim risk reduction measures for Reach 1A is also provided.
Introduction Herbert Hoover Dike (HHD) encircles Lake Okeechobee (the second largest fresh water lake wholly within the United States) in south central Florida (Figure 1). The HHD was originally authorized in response to hurricanes of 1926 and 1928 which resulted in the combined loss of about 3,000 lives, and was constructed in two general phases over the course of more than 30 years. Construction was carried out by a combination of dipper and hydraulic dredges with blasting required in some areas (Figure 2). Unfortunately, no engineering or construction records can be located from the original 1930’s construction, so some uncertainties remain regarding decisions that were made. The second phase of construction was performed during the 1960’s utilizing construction techniques similar to those of the first phase. The original dike was raised and the remaining lake perimeter was enclosed, resulting in the current HHD configuration (final length of 143 miles). Outlet works having sufficient capacity to pass the Standard Project Flood (lake elevation of 25 feet NAVD88, and ~935-year return period) were not built during either phase of construction. Concerns due to seepage at HHD have been documented since the early 1980’s and they became more pronounced during the mid to late 1990’s when two nearly back-to-back high water events served to demonstrate the need for an immediate rehabilitation effort. During these events, the lake crested at about 17 feet (~30-year return period), and numerous sink holes, seeps, pipes, and boils were observed (Figure 3) primarily along the ~50 miles that comprise Reaches 1, 2, and 3; these were subsequently addressed with interim remedial measures.
Embankment and Subsurface Conditions The HHD embankment generally consists of a heterogeneous mixture of lightly compacted to dense, fine to medium carbonate, quartz, clayey, and silty sands, shells, organic soils, and peat. Other materials encountered are limestone and sandstone gravels, cobbles, and occasional small boulders. As an exception, pockets of high concentrations of limestone cobbles and boulders can be found within the embankment. These coarse pockets vary in length and thickness, and can either have voids between the cobbles or can be filled with a matrix of sand and gravel; filled pockets are known to be highly pervious. Typical dimensions of the HHD embankment in Reach 1A are a crest width of 14 feet, a base width of 250 feet, a lakeside slope of 1V:6H, a landside slope of 1V:3H, and a crest height above ground of ~25 feet (at an elevation of 36 feet). The HHD soils are highly variable in the vertical direction, generally consisting of a (often-present) light-weight continuous blanket of organic-rich materials at the surface overlying foundation sands with variable amounts of silt, and several limestone and sandstone strata with variable thicknesses and extents (Figure 4). Beneath this sequence is the Hawthorn Group, which consists of detrital clays, silts, and mudstones; regionally it serves as the Intermediate Confining 1
Civil Engineer, USACE Jacksonville District, Email:
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Unit that separates the surficial aquifer system from the upper zones of the Floridan Aquifer System. In Reach 1A, consolidated strata are present extensively between elevations of 5 and -20 feet. They (along with surrounding sands) were exposed in the lake bottom during initial HHD construction dredging. This exposure created a direct conduit for seepage into the foundation and decreased the effective seepage path distance beneath HHD. Seepage entrance conditions, the highly variable and relatively pervious embankment and foundation, the low weight and relatively impervious landward surficial blanket, and the lack of seepage control/cutoff features result in HHD seepage concerns.
Figure 1. Satellite view of south central Florida (A), and a map view of the Herbert Hoover Dike project reaches (B). Reach 1A (4.9 miles long), is located between Port Mayaca and Sand Cut and is within the 22.5 mile long Reach 1.
Figure 2. Historical (taken during the 1930’s) and present day photographs: (A) and (B) Before and after completion of construction views (looking towards the northeast) from the top of the Pahokee water tower; note the toe ditch landward of the dike embankment in (B); (C) Photo looking south over Reach 1A (focus of this paper), and (D) Photo looking southeast over Pahokee, note arrow pointing to same water tower from which photos (A) and (B) were taken.
Remedial Design Criteria and Methodologies Due to its history of excessive seepage during normal hydrologic events, potential for loss of life, and lack of seepage control/cutoff features, HHD was ranked as an urgent and compelling project by the U.S. Army Corps of Engineers (USACE) Screening Portfolio Risk Assessment process (Halpin and Ferguson, 2007). The following sections discuss HHD seepage-related aspects, alternatives considered for addressing stability concerns along the 4.9 mile long Reach 1A (which is within the 22.5 mile long Reach 1 authorized for major rehabilitation), and design approaches for the preferred alternative.
Figure 3. Representative photographs of prior seepage distress during events having a ~30-year return period: (A) Sinkhole formation in crest; (B) Heave of downstream toe caused by upward seepage flow - survey rod was easily embedded 4 feet deep; (C) Piping at downstream toe of dike; and (D) Saturation of landward toe and embankment slope. Seepage Design Criteria Project-specific seepage and slope stability deterministic remedial design criteria were developed and approved for HHD. Traditional USACE seepage guidance describes a Factor of Safety (FS) based on the use of a maximum allowable exit gradient of 0.5 for saturated unit weights of blanket material greater than or equal to 110 pounds per cubic foot (pcf). However, since blanket materials that surround HHD are much lighter than 110 pcf, use of the cited maximum allowable exit gradient for remedial designs at HHD would lead to unconservative designs. HHD project-specific remedial design criteria require for the Standard Project Flood (lake elevation of 25 feet), minimum steady-state seepage (effective stress uplift) FS values of 3.0 at the dike toe, 1.6 at a distance 150 feet landward of the dike toe, and 1.1 at a distance 300 feet landward of the dike toe and beyond. Also required for the Maximum Surcharge Pool, are minimum transient seepage FS values of 1.5 at the dike toe and at a distance 150 feet landward of the dike toe, and 1.1 at a distance 300 feet landward of the dike toe and beyond. This event is simulated as a static lake elevation of 25 feet rising to an elevation of 30 feet (due to storm-related wind setup) over a 12 hour period. For risk management and decision-making purposes, design FS values are also calculated and reported for the Maximum (or Extreme) Pool, which corresponds to a static elevation of 30 feet. At this elevation overtopping would begin to occur on the western side of Lake Okeechobee; an on-going Hydrologic Frequency Analysis study is in progress to determine the Probable Maximum Flood (PMF) for Lake Okeechobee. Design evaluations have determined that resultant head distributions (for blanket aquifer foundation conditions) are quite similar for the Maximum Surcharge and Maximum Pools; therefore, when design criteria are met Maximum Pool FS values are greater than unity. Seepage Design Properties To assist in the selection of hydraulic design properties for the Reach 1A foundation materials, a constant rate pumping test (Davis et al., 2009) was performed during 2008, when the lake was at a constant elevation of 13.9 feet. Different theoretical drawdown curves were attempted to be matched to all drawdown data to estimate average hydraulic
properties (with foundation transmissivity being the parameter of most interest for Reach 1A seepage analyses). It was concluded that the drawdown data were best modeled by fitting the (“lake recharge”) Hantush curve to acquired drawdown data. The calculated average transmissivity value was adjusted (using the Kozeny approach) to account for partiallypenetrating characteristics of the well network to yield an average foundation transmissivity of 7481 ft2/d. This transmissivity equates to an average foundation hydraulic conductivity of 42.5 ft/d (or 0.015 cm/s), which compares well with a previously published range for the area of 10.0 to 60.0 ft/d (USGS, 1977). Seepage deterministic design values of saturated density for Reach 1A were generally selected based on 33percentile values for fine-grained soils found mostly in blanket zones and on 50-percentile values for coarse soil and rock found mostly in aquifer zones. A smaller density value is more conservative (than a larger value) for uplift computations in blanket zones. The densities of inorganic materials encountered in Reach 1A are similar to typical inorganic materials at most other projects; however, the peat (Pt) and organic silt (OH and OL) which often comprise much of the blanket zone material are much less dense at HHD than inorganic materials for which most seepage analysis design guidance documents were written. Such low tolerance for excess uplift pressures in lightweight organic blanket materials has been documented in two other known case histories (Marsland, 1961; IPET, 2007). Seepage Design Analyses Seepage and slope stabilities for base and remedial alternative conditions were evaluated through the use of twodimensional SEEP/W and SLOPE/W (GEO-SLOPE International, 2004) models, and when applicable, through the use of a custom program developed for relief well system design which addresses three-dimensional aspects relevant to (laterally noncontinuous) wells. This program was developed in agreement with USACE well system design guidance (USACE, 1992), and validated against hand calculations, and plan view SEEP/W modeling results for fully penetrating well system cases. As further detailed previously (Davis et al., 2009), soil profiles representative of conditions for Reach 1A were constructed perpendicular to the dike and geologic cross-section (Figure 4). Model profiles include idealized strata and material zones, with material properties assumed to be homogenous within each stratum or material. Properties assigned to idealized strata were composites based on the thicknesses, classifications, and properties of associated component layers. Reach 1A models were used to calculate head distribution, seepage quantities, and FS values against initiating heaving of the downstream blanket and piping of foundation materials. In the context of a continuum model for seepage-induced failure discussed in recent literature (Halpin and Ferguson, 2007), such initiation would generally be followed on an event tree by erosion continuation (erosion movement up gradient toward the water source), erosion progression (seepage quantity increase and erosion feature enlargement), and breach formation (massive erosion followed by pool loss). Although the abovementioned HHD design criteria do not require failure probability calculations, it can generally be stated that for the base condition in Reach 1A, it is expected that once erosion initiates, continuation will occur since foundation soils are erodible, filters or a cutoff to arrest erosion are not present, and materials capable of acting as a pipe roof are present. The load cases modeled for each soil profile in Reach 1A included events associated with the highest historical lake level (elevation 17 feet), the Standard Project Flood (elevation 25 feet), the Maximum Surcharge Pool (elevation 30 feet) represented by transient wind setup on top of the Standard Project Flood (as from a hurricane), and the Maximum Pool (elevation 30 feet). With the lake level at elevation 17 feet, calculated FS values against subsurface erosion initiation for the base condition were above unity at the dike toe and landward of the toe ditch, but approximately unity in the toe ditch. These results (obtained for the analysis cross-section subsequently in more detail) agree with past observations for the record pool elevation, in that seepage and related erosion initiation were not observed at the dike toe or landward of the toe ditch, but were periodically observed (where dense vegetation did not prevent observation) in the toe ditch.
Figure 4. Generalized geologic cross-section (constructed using a representative sample of borings from a larger database) along the 4.9 mile long Reach 1A at HHD.
Risk Reduction Alternatives and Preferred Alternative To address embankment through-seepage concerns and assist towards addressing under-seepage concerns, a partially-penetrating seepage/piping barrier (“cutoff wall”) through the embankment and near-surface consolidated strata is currently a required component of all remedial alternatives. This feature won’t restrict all groundwater flow, but is intended to “cutoff” pre-existing piping pathways and internally unstable materials within the embankment, as well as in near-surface, consolidated and often quite pervious foundation strata. A second required component of all remedial alternatives currently involves the in-filling (with a granular material) of the existing landward toe ditch. Numerous components were coupled with the partially-penetrating seepage barrier and toe ditch in-filling, and were evaluated through finite element modeling and spreadsheet computations to determine their potential for remedying HHD seepage deficiencies. These components (Davis et al., 2009) included constructing a landward seepage berm of various landward extents, a 100 foot long (or ~4 times the dike height) seepage berm in conjunction with a pumped toe drain, vertical sand drains, a pervious relief trench (comprised of sand, or sand with a gravel core), relief wells, and an upstream relatively impervious blanket. Generally, analyses and engineering judgment have indicated that non-blanket aquifer conditions where present, will require only the construction of the landward seepage berm (along with the partially-penetrating seepage barrier and toe ditch in-filling) to meet remedial design criteria. Where blanket aquifer conditions exist, the thickness and relatively impervious nature of the continuous often organic-rich blanket does not allow enough dissipation of foundation head with increasing distance downstream such that the relatively light weight of the blanket can provide adequate resistance to remaining uplift pressure beyond the berm toe. Therefore, for such conditions a pore pressure relief system is required in conjunction with the seepage barrier, toe ditch filling, and seepage berm. Three categories pressure relief systems were determined to provide the potential in theory for adequately reducing uplift pressures: a laterally continuous toe drain, a laterally continuous relief trench, and a relief well system. For the toe drain alternative to be effective in practice, it would need to be actively pumped to maintain specified phreatic elevations, and for the relief trench to be effective in practice, it would need to be installed to depths as great as 90 feet and constructed of sand with a gravel core. Based on such general difficulties associated with these conceptual remedies and additional factors considered, the preferred remedial alternative for obtaining required pressure relief where needed in Reach 1A of HHD is the installation of relief wells. Shown in Figure 5 is analysis section 1568+00 which represents profile 1AG in Figure 4. The soil profile and conceptual seepage model for the static lake elevation of 30 feet are shown, with the preferred remediation alternative implemented, including a partial-depth seepage/piping barrier through the embankment and near-surface consolidated strata, a 100 foot long seepage berm, and a system of fully penetrating relief wells (displayed as a laterally continuous feature on the profile). To determine the appropriate and most desirable depth and spacing combination for the wells to meet design requirements, three-dimensional aspects relevant to (laterally non-continuous) wells were considered utilizing a developed relief well design and analysis program. The determination of input variable properties for deterministic design analyses for this section was completed as generally discussed above (and as presented in more detail in Davis et al., 2009). Affects of the partial-depth seepage/piping barrier to be constructed on gradients (e.g. estimated effective seepage entry distance) were incorporated through calculations outside of this program. Calculations were also performed outside of this program to ensure projected heads generated between the well line (at the toe of the seepage berm to be constructed) and the dike toe by this program were acceptable. The input property values as shown in Figure 6 are all expected values, and not the occasionally conservative design values (e.g. 33-percentile saturated densities for fine-grained blanket soils), as what is presented in Figures 6 and 7 is an analysis process which followed deterministic design in order to consider uncertainty and ensure tolerable design reliability. The reliability module essentially employed the Taylor’s series expansion using the minimum FS resulting from the well system design (e.g. just downstream of the well line, which is modeled at the landward margin of the seepage berm to be constructed), and ran the deterministic well design and analysis program repeatedly at plus and minus 1 standard deviations for the (random) variables which typically have a significant impact on well system design. Using the variance calculated from these repeated runs, a distribution of the FS for the given well system design was calculated along with probability of heave and reliability index (beta) values. The relative contribution of individual variable uncertainties to the probability of heave is apparent from Figure 7, which indicates that uncertainty and variation of blanket weight and thickness, and the prevailing top of ground elevation within profile 1AG have the most significant affect. A detailed description of how all statistics for reliability analysis were developed is beyond the scope of this paper, but it should be noted that the standard deviation for tail water elevation was deliberately taken as zero as shown in Figure 7. Although the anticipated tail water elevation is not a certain value, analyses demonstrated that the critical tail water elevation with regard to producing the minimum FS occurred at the expected value. Since the FS was found to improve at plus and minus one standard deviations (this is because the FS is a function of both net head on the well system, as well the uplift resistance and other factors), the tail water standard deviation was set to zero so as to not incorporate an inaccurate amount of variance. Considering the results of deterministic and reliability analyses, and employing engineering judgment as to what well system depth and spacing combination provided an appropriate degree of reliability in addition to meeting design requirements, the
well system designed for this subreach consists of fully-penetrating wells on 100 foot centers. The required depths and spacing of relief wells vary across Reach 1A depending on subreach subsurface conditions and their range of uncertainty.
Elevation, ft NAVD88
HHD - R1A STATION 1568+00 40 20 0 -20 -40 -60 -80 -100 -120 -140 -160 -180 -700
Highway
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Railroad
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Cutoff Wall
Relief Well Berm Swale
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Lake = 30.0 ft (Steady State)
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Borrow Pit (in-filled)
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40 20 0 -20 -40 -60 -80 -100 -120 -140 -160 -180 500
Offset, ft
Figure 5. Profile 1AG (Figure 4) soil profile and conceptual seepage model for the static lake elevation of 30 feet. The preferred remediation alternative for this profile is shown, which includes a partial-depth seepage/piping barrier through the embankment, a 100 foot long seepage berm, and a system of fully penetrating relief wells (on 100 foot centers, as determined by a separate 3D analysis). Resultant factors of safety for this alternative satisfy design requirements.
Figure 6. Deterministic relief well system design and analysis program inputs and results for profile 1AG (Figure 5); the program includes a reliability analysis module (Figure 7). Deterministic analysis results coupled with consideration of analysis uncertainty, result in the preferred remedial alternative design (satisfying design requirements) for this profile of fully-penetrating wells on 100 foot centers. Affects of the partial-depth seepage/piping barrier to be constructed on gradients (e.g. estimated effective seepage entry distance) were incorporated through calculations outside of this program. Calculations were also performed outside of this program to ensure projected heads generated between the well line (at the toe of the seepage berm to be constructed) and the dike toe by this program were acceptable. Figure is continued on following pages.
Figure 6 (continued). Refer to prior page for figure caption.
Figure 6 (continued). Refer to prior page for figure caption.
Figure 7. Reliability analysis module of the deterministic relief well system design and analysis program (Figure 6), showing design analysis results for profile 1AG (Figures 5). As reported in Figure 6, deterministic analysis results coupled with consideration of analysis uncertainty, result in the preferred remedial alternative design (satisfying design requirements) for this profile of fully-penetrating wells on 100 foot centers.
Interim Risk Reduction Measures Following the assignment of HHD as a DSAC-I (urgent and compelling) dam, measures were taken to immediately reduce risk until construction completion of the preferred remediation alternative. Interim Risk Reduction Measures (IRRM’s; USACE, 2007) consist of both structural and non-structural means of reducing the risk of project failure. During 2007-2008, nearly 2.5 miles of toe ditch focus areas were in-filled with granular material, providing a filtered exit where none had previously existed. These focus areas were identified as being problematic due to their geometry and recurring seepage distresses. Fortunately, a sustained regional drought has occurred during the period of 2006-2009 within the southeastern U.S. which effectively provided for a natural pool reduction; a new lake regulation schedule was also approved during this time which allows water managers greater flexibility in managing releases. Nearly 30 acres of exotic trees and woody vegetation were removed from within 75 feet of the dike toe, allowing for improved observations and inspections in these areas. Removal of exotic trees and vegetation actually resulted in a slight environmental credit. Shown in Figure 8 are general photographs of structural IRRM’s completed to date. Additionally, as a required feature of the overall project remediation, construction of the partially-penetrating cutoff wall commenced during 2008. As of August 2009, total cutoff wall emplacement amounted to over 14,300 linear feet and continues ahead of schedule.
A
B
Figure 8. General photographs of completed structural interim risk reduction measures: (A) Removal of exotic trees, and (B) Toe ditch in-filling.
Conclusions Analyses have been performed to assess existing conditions and formulate solutions for Reach 1A of HHD, which are in compliance with approved technical guidelines and site-specific (seepage and slope stability) design criteria. The discussion of details from results of all design analyses is beyond the scope of this paper. The currently preferred risk reduction alternative requires the construction of two primary components: 1) a partial-depth seepage/piping barrier (“cutoff wall”) through the embankment and near-surface consolidated strata, and 2) a landward seepage berm coupled with a pore pressure relief system where necessary. In general, non-blanket aquifer conditions require a landward seepage berm, whereas blanket aquifer conditions require a relief well system in addition to the seepage berm. The landward seepage berm will have a nominal width of 4 times the dike height (e.g. a length of 100 feet where the dike is 25 feet tall), and will have nominal thicknesses of 5 feet and 2 feet at the dike and berm toes, respectively. Required depths and spacing of relief wells where required vary across Reach 1A depending on subsurface conditions. For the example cross-section analyses presented in this paper, the required spacing of fully-penetrating relief wells located at the seepage berm toe is 100 feet; locating the well system at the seepage berm toe rather than at the dike toe allows a slight increase in the required spacing, and more significantly allows discharge to be more effectively conveyed (e.g. immediately to a drainage swale rather than through a collector system installed through the berm). With implementation of the preferred risk reduction alternative, a robust instrumentation system will be installed to allow real-time monitoring of performance conditions and further verification of design assumptions during future high water events. The final design report for Reach 1A is scheduled for 2009 completion, with environmental policy compliance and construction scheduled to be phased over the following several years. Coincident with design work, construction of the partial-depth cutoff wall is underway, and interim risk reduction measures have been implemented. Work towards development and implementation of additional interim measures continues.
References Davis, J. Guy, E., and Nettles, R., 2009, Herbert Hoover Dike Seepage Remedial Design Concepts, U.S. Society on Dams Conference Proceedings, 25 p. GEO-SLOPE International, 2004, Seepage Modeling with SEEP/W, 398 p., and, Stability Modeling with SLOPE/W, 332 p. Halpin, E., and Ferguson, K., 2007, USACE Dam Safety: Program Status and Lessons in Transitioning to Risk Informed Approaches, The Journal of Dam Safety, v. 5, pp. 24-36. Interagency Performance Evaluation Task Force (IPET), 2007, Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System, v. 5, app. 8. Marsland, A., 1961, A Study of a Breach in an Earthen Embankment Caused by Uplift Pressures, Int. Soil Mechanics Foundation Engineering Conference, v. 2, pp. 663–668. U.S. Army Corps of Engineers (USACE), 1992, Design, Construction, and Maintenance of Relief Wells, EM 1110-2-1914. USACE, 2007, Interim Risk Reduction Measures for Dam Safety, EC 1110-2-6064. USGS, 1977, Hydraulic Conductivity and Water Quality of the Shallow Aquifer, Palm Beach County, Florida, WaterResources Investigations 76-119.
