John P. Ballegeer, Jie Yu, Keith A. Ferguson, and Terrance I. Nago present a study in the lifecycle costs for structural repair of the Waikoloa Reservoir No 2 in the US
On 15 October 2006, a 6.7-magnitude earthquake occurred in Kiholo Bay just off the Big Island of Hawaii. This earthquake caused extensive damage to buildings, roadways and infrastructure on the Big Island, and also threatened one of Hawaii County’s critical water collection and storage facilities – Waikoloa Reservoir No. 2.
Waikoloa Reservoir No. 2 was constructed in the mid-1970s, and is classified as a high-hazard facility according to the National Inventory of Dams database (NID 2002), a classification that was confirmed based on pre-earthquake inspections by the US Army Corps of Engineers and the Department of Land and Natural Resources, State of Hawaii (USACE 2006). After the earthquake, the University of Hawaii at Manoa (UHM), the US Bureau of Reclamation (USBR), and the Federal Emergency Management Agency (FEMA) performed post-earthquake inspections. The inspectors identified embankment movements, extensive cracks on the upstream concrete liners, and seepage and boils at the downstream embankment toe. Concerns about dam safety prompted the Hawaii County Department of Water Supply to drain the reservoir and take it completely out of service.
Based upon post-earthquake inspections, FEMA prepared a project worksheet for restoration of the reservoir to pre-earthquake conditions. The project worksheet proposed limited repairs on the existing concrete liner and installation of a conventional polypropylene synthetic liner to cover the concrete liner. In order to resume normal operation of Waikoloa Reservoir No. 2, as well as to address serious dam safety concerns, the Department of Water Supply decided to rehabilitate the reservoir and retained Kleinfelder, a nationwide engineering and project management firm, to evaluate site seismicity, embankment seepage, stability and associated dam safety concerns. Kleinfelder then developed rehabilitation alternatives and prepared construction plans and specifications for the selected alternative.
Waikoloa Reservoir No. 2 is located near the town of Waimea in Hawaii County’s District of South Kohala. The reservoir consists of a circular shaped pool with a normal storage capacity of 50 million gallons. The earth embankment is approximately 33ft (10m) high and 2000ft (609.6m) long with an approximately 20ft-wide (6m) crest and 2H:1V upstream and downstream slopes. The bottom and upstream slope of the reservoir was lined with 5-inch thick concrete (shotcrete) panels reinforced with wire mesh.
The embankment’s internal drainage system includes a 2ft-thick (0.6m) cinder drain that extends along the base of the embankment from 20ft (6m) downstream of the embankment centerline to the rock toe, where the cinder drain rides over the rock toe. The internal drainage provision is only presented in areas where the embankments are greater than 24ft (7.3m) in height.
• The UHM inspection noted that the earth embankment may have displaced laterally, as evidenced by the disturbance of fill on the crest and adjacent to the top of the concrete panels. They noted cracks up to 4 inches wide reflecting relative movement and that absolute displacement may have been even larger. They also noted that the observed damage indicated a strong motion component in the east-west direction and that the embankment at the downhill side of the mountain saw the least amount of damage.
• The USBR inspection noted cracking and displacement in the concrete panels and wave wall along the upstream slope and numerous longitudinal cracks, lateral movement, and surface bulges along the crest. The USBR inspection also noted two seepage boils along the southeast toe of the reservoir with estimated flows up to 10 gal/min and minor amounts of fines being transported.
• The FEMA inspection noted that the boils on the southeastern side of the reservoir were near the outlet works pipe. FEMA also noted voids behind the concrete panels.
Embankment and Subsurface Characterization
Kleinfelder performed field exploration, nondestructive geophysical survey and laboratory testing to characterize the embankment and subsurface soil and bedrock conditions. The results of field and laboratory investigations indicate the embankments generally consist of up to 29ft (8.8m) of clayey silt fill overlying native foundation soils. The compacted fill in the embankment is composed primarily of highly plastic, inorganic silt derived from local allophane soils (soils derived from weathering of volcanic ash and rock). The native foundation soils are described as highly plastic, inorganic silt with occasional basalt cobbles and boulders. The bedrock below the embankment is generally described as vesicular basalt and was encountered at depths ranging from 7 to 40ft (2.1-12.2m) below the base of the embankment. Groundwater was generally encountered in the native foundation soils at elevations ranging from 5 to 22ft (1.5-6.7m) above the top of bedrock.
The allophone soils exhibit unique characteristics such as in-situ dry unit weights typically lower than the unit weight of water, moisture contents typically exceeding the soil’s liquid limits and void ratios generally above four, indicating the volume of voids in the soil exceeds the volume of solids. Characterizing the on-site allophone soil and selecting soil-engineering parameters, such as shear strength and permeability for seepage and stability analyses, proved to be a challenging task. The laboratory testing results on the allophone soil samples indicated that conventional empirical relations between soil type, index properties and engineering properties were not valid for this type of material. Kleinfelder researched and found technical papers published by international scholars and a project with similar soil conditions in Indonesia to support the strength and permeability parameter selection. Additionally, sensitivity analyses were performed to demonstrate that shear strength parameter selections were reliable and reasonable.
The nondestructive geophysical surveys conducted on selected concrete panels indicated that significant areas of voids or poor support likely existed underneath the concrete panels.
Seepage and stability analyses
Seepage and stability analyses were performed to evaluate exit gradients and potential seepage quantities of the embankment and to calculate the minimum factors of safety for upstream and downstream embankment slopes.
In areas where the embankment has no internal drainage provisions or in areas where the drain is potentially contaminated or plugged, the steady state phreatic surface in the embankment would exit along the downstream slope with the relatively high exit gradient ratio of approximately 0.5, and the potential for embankment piping is considered likely. The seepage analysis also indicates that a seepage discharge through the full width of the modeled section under normal operating conditions, whether or not the internal drainage system was functioning, would be much less than the observed 10 gal/min. A reasonable conclusion is that the concentrated seepage discharge is evidence that there exists a shortened seepage pathway between the reservoir and the embankment toe area. Any opening in the foundation or embankment would shorten the seepage path between the reservoir and the downstream embankment toe. Such an opening could be associated with a defect in the foundation or an opening along a penetration through the embankment. Foundation defects are possibly associated with fractures or columnar jointing typical of basalt formations. Openings along embankment penetrations are likely associated with the outlet works conduit or a similar conduit. The analysis indicates that any opening through the embankment or foundation soils with an orifice opening even as small as 0.11 square inches would result in a seepage discharge equal to the observed 10 gal/min.
Additionally, the observed seepage boils appear in areas where no internal drainage provisions exist in the embankment that could collect and concentrate the seepage. They also appear in areas where there are embankment penetrations, such as the outlet works, or in areas where the upstream side of the embankment is excavated into bedrock at the bottom of the reservoir, creating the potential for foundation defects to be in contact with the reservoir.
The stability analysis indicates that under steady state seepage conditions and without internal drainage provisions, the minimum factor of safety for the existing embankment downstream slope will be approximately 1.29, which is less than the recommended minimum factor of safety of 1.5 by the Department of Land and Natural Resources, State of Hawaii.
Seismic deformation and soil-structure interaction analyses
Seismic embankment deformation and soil-structure interaction analyses were performed using FLAC software developed by Itasca. In addition to the embankment, the upstream concrete liner also was modeled. The FLAC model was calibrated using the unscaled time-histories of the 2006 M6.7 Kiholo Bay earthquake measured near the site. The analysis yielded results that appear similar to the actual observed reservoir performance.
Using the Kiholo Bay earthquake ground motion time history that was scaled to design earthquake response spectra, the calculated maximum permanent deformations at the end of shaking are approximately six inches in both the x and y directions. The downstream crest had an approximate vertical deformation (loss of freeboard) of 1.49 inches. The moments and shears developed in the concrete liner section indicate that serious damage would occur at the toe of the upstream slope. Axial forces would cause the liner slabs to impinge on one another, resulting in more damage at the joints. Cracks at mid-slab are also likely to occur due to bending moments generated by the horizontal movements of the embankment during earthquake loading.
After extensive field exploration, site characterization, and engineering analyses, Kleinfelder proposed a comprehensive rehabilitation approach that would consist of upstream liner mitigation and downstream embankment modification.
Upstream liner mitigation
The reservoir upstream liner provides the first line of defense against piping and stability issues. Kleinfelder and the Department of Water Supply evaluated three upstream liner mitigation alternatives including repair of the existing liner, replacement of the existing liner with a new concrete liner, and installing a new synthetic liner over the concrete liner.
The repair alternative would focus on rebuilding the conjunction of the concrete panels with adequate reinforcements and joints set up to minimize potential seismic damages at this location. Based on the FLAC analysis, the conjunction of the slope and the bottom concrete panels appears to be the weakest link of the entire concrete liner system and would likely experience serious damages during the design earthquake event. Additionally, the repair alternative would include utilizing a low-pressure grouting technique to fill voids below the concrete panels, cleaning and re-establishing control and expansion joints between concrete panels, and repairing cracks and spalls in the concrete liner,
The replacement alternative would include demolishing the entire existing concrete liner system, and replacing it with all new concrete panels that have adequate reinforcement and joint details to withstand the design earthquake event.
The new synthetic liner alternative would include installing a new synthetic liner over the existing concrete liner. The existing concrete liner would be rehabilitated to support the new liner by cutting or grinding sharp edges, cleaning and sealing joints and cracks, and filling voids beneath the concrete panels. Several synthetic liner materials were evaluated for the rehabilitation alternative, including polyvinyl chloride (PVC, trade name carpi), polypropylene (PP), high density polyethylene (HDPE), chlorosulfonated polyethylene (CSPE, trade name Hypalon), ethylene-propylene-diene monomer (EDPM), and a polyurea spray-on elastomer. Due to cost factors, NSF 61 certification, unit weight of synthetic liner, and established track record for applications similar to the Waikoloa Reservoirs No. 2, the CARPI, HDPE, EDPM, and spray-on elastomer materials were not included in the final selection analyses.
Due to the unique location of this project, cost for transportation and disposal of the demolished concrete panels is significant. Based on the engineer’s cost estimate, installation of a synthetic liner system is the most cost effective alternative for upstream remediation.
Downstream embankment modification
Based on the seepage and stability analyses, potential piping and internal erosion of the embankment are the most significant dam safety concerns at the site. An internal filter and drainage system was designed to arrest potential piping and internal erosion by providing drainage to reduce seepage gradients and to act as a filter to stop the migration of soil particles out of the embankment. The system extends as high up into the embankment as is practical and includes a seepage collection system to allow seepage monitoring. The system also includes seepage diaphragms across embankment penetrations to intercept any seepage pathways along these through-going conduits. The internal filter drainage system is shown on Figure 1.
Another dam safety issue is associated with potential slope instability along the downstream embankment slope where no internal drainage provisions were provided. A seepage-stability berm was considered to address this concern. The seepage-stability berm would consist of a trench drain underneath an earth berm. The berm buttresses the downstream slope and the trench drain lowers the phreatic surface in the embankment to improve the stability. Additionally, the drain would reduce exit gradients at the toe of the embankment, which would reduce the potential for piping and boils along the downstream slope. The schematic of the seepage-stability berm is shown on Figure 2.
Based on Kleinfelder’s analyses and recommendations, the Department of Water Supply evaluated all remediation alternatives and decided to approach the rehabilitation design with two major design components, including the construction of the internal filter-drainage system and seepage-stability berm to address dam safety concerns according to State of Hawaii Regulations, as well as installation of a synthetic liner system on the reservoir bottom and upstream slope to minimize seepage flow.
Synthetic liner selection
The synthetic liner is one of the major design components for the Waikoloa Reservoir No. 2 rehabilitation project. The cost for the synthetic liner and its installation comprised approximately 40% of the entire construction budget.
After initial evaluation, the Hypalon liner (CSPE) and polypropylene liner (PP) were considered for the upstream rehabilitation. The polypropylene liner has a lower initial investment and a shorter installation history. The Hypalon liner demonstrated superior quality and the longest track record of success, but its initial cost is approximately 50 percent more than the polypropylene liner.
The Department of Water Supply had concerns about the long-term reliability and performance of these synthetic liners. The prospect of frequent repairs and/or replacement of the synthetic liner would result in significant operational impacts associated with lost reservoir capacity. Some operational impacts can be quantified in terms of energy costs, however, the inconvenience to operations and to customers are of equal concern but less tangible.
In order to compare long-term reliability and performance of both liners, a life cycle cost analysis was performed. The analysis included comparison of initial investments, maintenance and repair costs, replacement costs, warranty values, residual costs, etc. from a long term cost prospective. The life cycle cost analysis demonstrated that the Hypalon liner has a significant higher initial cost but a lower life cycle cost than the polypropylene liner based on a 25 year service life. The results of this life cycle cost analysis are summarized in Table 1.
A preferred synthetic liner alternative was selected using a simple decision analysis method. The decision analysis method was based on a matrix of individual decision criteria developed to differentiate liner alternatives. The major decision criteria used in the decision analysis included cost criteria, performance criteria, and physical properties criteria. The cost criteria included both the initial investment and life-cycle costs. The performance criteria included the available warranty as well as the reliability, performance history, constructability, and repairability. The physical properties criteria included ultraviolet resistance, specific gravity, reinforcement, dimensional stability, puncture resistance, flexibility, and color choice. Each decision criteria was assigned a weighting factor based on the importance of the decision criteria to the overall decision.
Once the decision criteria and weighting factors were developed, each liner alternative was assigned a score under each weighted decision criteria. In order to emphasize differences between the materials, a score of either 0 or 1 was used. The weighting factors were multiplied by the score and summed to calculate a total weighted score. The total weighted score was used to rank each liner alternative. A summary of the liner alternative decision analysis and the results of the ranking are shown on Table 2.
Based upon the liner alternative decision analysis, the Hypalon liner clearly demonstrated its superior overall and long-term value to the project. FEMA originally recommended and budgeted for the polypropylene liner in its post earthquake project worksheet. After review of the Department of Water Supply and Kleinfelder’s synthetic liner evaluation report, FEMA supported the use of the Hypalon liner and subsequently agreed to provide additional funding for the synthetic liner substitution.
With the joint efforts of Kleinfelder and the Hawaii County Department of Water Supply, analysis and design for Waikoloa Reservoir No. 2 Earthquake Rehabilitation has been completed; and the design documents have been reviewed and approved by the State of Hawaii Department of Land and Natural Resources. The Department of Water Supply is confident that the design for the rehabilitation work will result in the reservoir being much improved over its pre-earthquake condition. Construction for this rehabilitation project began in March 2009 and was completed in April of 2012.
John P. Ballegeer, PE, PG, Senior Geotechnical Engineer ; Jie Yu, PE, Project Manager; and Keith A. Ferguson, PE, Vice President and National Surface Water Director, Kleinfelder, Inc. Terrance I. Nago, PE, Project Engineer with the Department of Water Supply