The Flood Control District of Maricopa County (FCD) operates and maintains 22 singlepurpose flood control dams in central Arizona. Portions of two of these dams, White Tanks Flood Retarding Structure (FRS) No. 3 and McMicken Dam, are located in earth fissure risk zones. FCD is currently designing and/or constructing modifications to rehabilitate both dams to mitigate the risk posed by potential earth fissures at the two dam sites. The two dams are under the jurisdiction of the Arizona Department of Water Resources (Department), and both authors were deeply involved in the application review process for both dams. These two case histories illustrate the regulator’s dilemma related to embankment dams in earth fissure risk zones and identify the variables that the regulator, owner and design engineer had to consider in developing a safe rehabilitation of two dams located in fissure risk zones.
White Tanks FRS No. 3
White Tanks FRS No. 3 is a single-purpose flood control dam located in western Maricopa County. The dam is a homogeneous embankment designed and built by the Soil Conservation Service and has a length of approximately 7700 feet. The dam is located on the lower reaches of an alluvial fan on the eastern flanks of the White Tank Mountains. The foundation profile consists of deep unconsolidated alluvial deposits. Unconsolidated alluvium, primarily consisting of fine sandy silt, silty sand, with lesser amounts of clayey sands, sandy clays of low plasticity, and relatively clean sands extend to depths from approximately 100 feet on the southwest end of the dam to 500 feet on the northeast end of the dam (ADWR, 1998). Highly erodible Holocene soils extend to a typical depth of 10 feet. The underlying Pliestocene soils are generally lightly cemented with smaller zones of low to high cementation.
Subsidence & Earth Fissure Risk
Since construction of the dam in the mid 1950s, nearly 4 feet of differential subsidence has occurred over the length of the embankment, with nearly 4.5 feet of total subsidence at the north end of the dam, and less than a foot of subsidence at the south end. The subsidence was caused by drop in groundwater levels. The differential settlement of the north end of the dam relative to the south end has been attributed to changes in lithology along the length of the dam. AMEC Earth & Environmental Inc. (AMEC, 2003) performed studies to assess fissure risk as this site. The AMEC (2003) study identified a moderate to high risk of earth fissure development along a 2500-foot section of the embankment. During a risk assessment workshop, experts on ground subsidence and earth fissures opined that there was an 80 percent likelihood that earth fissures would develop at the site during the 100-year life of the project. It is important to note however, that no earth fissures have been identified at the site to date.
Proposed Remedial Design
FCD has chosen to remediate the section of embankment within the moderate to high fissure risk zone, where to date extensive investigations have not identified the presence of any fissures. A typical cross section of the proposed remedial design is shown in Figure 6. The proposed design (URS, 2005) includes excavating the Holocene surface soils to a depth of 8 to 10 feet to expose Pleistocene-age soils, constructing a new soil-cement embankment directly upstream of the existing earthfill embankment, and protecting the new soil-cement embankment with two 30-foot deep cutoff walls, one each at the upstream and downstream toes of the new soil cement embankment. The soil-cement embankment is to be buried under earthfill that will be landscaped to improve the aesthetic quality of the project.
Figure 6: White Tanks FRS No. 3 – Typical Cross Section (from URS, 2005)
The design philosophy is that removal of the Holocene soils removes the upper very erodible and potentially collapsible soil section. Early modeling indicated an upstream blanket of soil cement was less effective than the cutoff walls in reducing the potential erosion along a fissure in the upper Pliestocene soils. The fine-grained Pliestocene soils appear to be somewhat less erodible below a depth of about 30 feet. The two cutoff walls will divert seepage along a fissure below the base of the cutoff walls into the more erosion resistant strata. The walls also increase the flowpath length, reducing the initial flow gradient and associated erosive forces. The combination of increased soil erosion resistance and reduced erosive force is expected to limit the eroded width of the fissure below the cutoff walls, and limit the rate of uncontrolled discharge from the reservoir in the event a fissure forms at the dam site. The soil-cement embankment is a key structural element of the project that is provided to bridge across an eroded fissure and to prevent erosion along any cracking that may occur in the dam in association with a foundation fissure.
The Regulator’s Dilemma(s)
Well into the regulatory review and approval process, the Department staff maintained concerns about both the adequacy of the subsurface profile and the characterization of the erodibility of the foundation soils being input to the fissure erosion model. We believed that some of the more erodible layers within the profile were not reflected in the small set of test data. Also we believed additional borings were required to increase the confidence in the subsurface profile being input to the erosion model. The designer agreed that erodible layers existed in the subsurface profile, but indicated they believed these layers were interbedded with areally-extensive erosion-resistant layers that would “throttle” the seepage and limit erosion along the fissure. The Department remained unconvinced, and requested the owner perform additional field investigation and laboratory testing to confirm the subsurface profile and erodibility data input to the model. In-situ field investigative techniques such as developing a borehole erosion test method and drilling large-diameter shafts to allow access for direct visual inspection of the subsurface soils were explored. Ultimately, the owner performed a supplementary field investigation using triple tube coring, and downhole geophysical logging of an agreed upon borehole program.
The Department’s first dilemma was that the existing dam is located in a predicted high risk fissure zone. However, an extensive site investigation did not discover any indications of past or present fissures. Given this, could the Department withhold a permit to rehabilitate and significantly increase the safety of the existing dam?
The Department’s second dilemma was we were not convinced that the present state of technology for the model or soil characterization truly simulates the actual erosion along a fissure. However, we believe that the proposed design protects against these unknowns to a large degree. We do believe the model allows an assessment of the positive impact of structural elements as well as the negative impact of increasing width of a potential fissure. The modeling performed by the owner’s design engineer convinced the Department that the dam design must include a core of soil-cement and that the foundation requires cutoff walls. The modeling also convinced the Department that any fissure must be detected and remediated at very small crack widths (1/4 to 1/2 inches).
Since the model results indicated it was essential to detect very small fissure widths, the third dilemma for the Department related to whether monitoring of the dam for fissure development was feasible. The Department had to be convinced that the state of technology had advanced to the point where very accurate ground strains could be measured as a precursor to fissure development and that small-width fissures could be detected when there were no surface expressions. If this was not possible, we believed it would not be safe to rehabilitate this dam in a fissure risk zone. Through evidence obtained during several site investigations at FCD dams in Arizona and the experience of one of the owners’ design engineers, the Department was convinced that there was a high degree of confidence that a well planned monitoring program could detect both the ground strain occurring as a precursor to fissure development and also any small-width fissures which may be developing below the ground surface.
The Department believes the proposed dam design and the conditions of approval listed below will result in a safe dam that provides the greater public the required long-term flood protection.
- At the time of foundation preparation, carefully examine and map the foundation, and perform a seismic refraction survey to confirm the absence of earth fissures at the dam site today.
- Design and implement a detailed instrumentation and monitoring plan to detect the onset of tensile strains that might serve as a precursor to fissure formation. The monitoring program for ground strains will include high-precision GPS monitoring, installation of a TDR cable, periodic review of interferograms, and low-sun-angle aerial photography. Actual fissure cracks will be searched for using the surface geophysical techniques developed by Rucker (2000), discussed earlier in this paper. Ground truth will include test trenching and geological logging at selected limits of ground strain and/or when fissure signatures are obtained using the geophysical survey lines.
- As part of the monitoring plan, design and implement a detailed site investigation plan to detect fissures when the ground strain limits are reached. The plan will include surface geophysical techniques developed by Rucker (2000) and test trenches.
- Should a fissure be detected at the site, design and implement remedial measures that would isolate the fissure from water in the impoundment.
- Recognizing that the state-of-the-art and -practice in terms of fissure detection, remediation, and general dam engineering will advance with time, review the design every ten years to confirm that the remedial measures implemented today continue to meet current dam safety practices and standards.