Subsidence & Earth Fissuring
Mechanics of Subsidence
Ground subsidence is known to occur in the alluvium-filled valleys in Arizona. This land subsidence is a gradual settling of the earth’s surface caused by subsurface movement of earth materials. The United States Geological Survey (USGS, 1999) estimates that over 80 percent of the subsidence reported in the United States is caused by over-drafting of groundwater resources. Studies indicate that subsidence in Arizona is also largely attributed to excessive groundwater withdrawal.
Historically, water needed for agricultural activities in the semi-arid Southwest has been obtained by pumping of groundwater wells installed within the very deep, alluvium filled valley aquifers. Groundwater withdrawal at rates in excess of the natural replenishment of these aquifers leads to the lowering of the groundwater table. At some locations in Arizona, groundwater levels have been lowered by over 300 feet. Dewatering of the alluvium results in an increase in inter-granular (effective) stress on lower layers, causing the alluvium to consolidate. This consolidation manifests itself as a subsidence bowl at the ground surface. The magnitude of subsidence is directly related to the subsurface geology, the thickness and compressibility of the alluvium, and the magnitude of groundwater declines. Ground subsidence due to groundwater overdraft is essentially irreversible; however, the rate of subsidence can be reduced or arrested by reducing or halting declines in groundwater levels (Bouwer, 1977).
Information pertaining to the major subsidence bowls in Arizona is summarized below:
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Subsidence in the area of a flood control dam typically lowers the dam crest, decreasing the storage capacity of the reservoir and the available embankment freeboard for the design flood, producing an unsafe overtopping potential. The loss of freeboard develops over a period of years (if not decades) and rarely leads to an overtopping dam failure, providing the owner has an active crest monitoring and repair program. A more insidious impact of subsidence on embankment dams, however, is the formation of foundation earth fissures. Even with specific monitoring activities, earth fissures are more difficult to detect than subsidence itself. A rapid reservoir filling during a flood event combined with the presence of earth fissure in the foundation and potential associated cracking in the earth dam can trigger catastrophic erosion of the fissure and failure of the dam.
Mechanics of Earth Fissure Development
Earth fissures develop near the margins of subsidence bowls and/or in the vicinity of a buried rock pediment edge, where differential subsidence induces tensile stresses in the ground that exceed the tensile strength of the unconsolidated alluvium. While fissures are generally associated with ground water declines in excess of 300 feet, earth fissures have also been identified in areas of lesser groundwater declines. The location of earth fissures is primarily controlled by the configuration of the buried bedrock surface, variation in the basin fill stratigraphy, and the location and characteristics of the subsidence area.
Figure 1 depicts a generalized sequence of earth fissure development. Generally, it is believed that the fissure is initiated as a relatively narrow crack that is formed when the induced tensile stresses exceed the tensile strength of the soil. Published information indicates that uneroded fissures range from “hairline” to 2 inches in width, and are typically less than 1-inch wide. Figure 2 shows a typical earth fissure in the early stage of development, where the fissure is still hidden below the surface. Subsequently, seepage from surface infiltration erodes the sides and top of the fissure until it reaches the surface”. Figure 3 shows the initial surface manifestation of an earth fissure which typically includes small depressions, potholes, and surficial cracks oriented along a generally liner alignment. Runoff into these surface features causes significant and rapid enlargement of the fissure through a combination of erosion and slumping of the fissure walls. Finer grained soils are washed vertically into the fissure that theoretically extends to the groundwater table. Generally, the depth of erosion at a fissure has been limited to several tens of feet, which may be due to larger gravels and cobbles plugging the fissure or due to an increase in the resistance of the soils to erosion with depth. Figure 4 depicts a mature fissure.
Figure 1: Stages in Fissure Development (from Larson and Pewe, 1986)
Figure 2: Uneroded Fissure
Figure 3: Surface Potholes and Cracks along a Fissure
Figure 4: Mature Earth Fissure
The surface erosion features associated with mature fissures are known to exceed 20 feet in depth and 30 feet in width. The exposed lengths of fissures at the ground surface are typically less than 1 mile, but one fissure in the Picacho Basin in South Central Arizona is more than 9 miles in length. Generally, the great length of the mature eroded fissure results from areal runoff into the fissure. However, at least one case of extensive lateral erosion of a fissure has been observed (Weeks, 2005) for hundreds of feet in each direction from a relatively small point source of water discharging into the fissure.
Earth Fissures & Potential Dam Failure Modes
The primary failure modes in embankment dams related to earth fissures results from seepage erosion along the fissure in the dam foundation and uncontrolled release of the reservoir. A second failure mode is the uncontrolled release of the reservoir due to erosion of the embankment where the expression of the fissure results in a corresponding crack in the dam. A third failure mode is overtopping of a section of dam due to slumping of the dam into the eroding foundation.
Observations at fissure sites in Arizona have shown that an open fissure can “accept” hundreds of cubic yards of soil generated by erosion of the sidewalls, prior to plugging of the fissure. Consider a fissure through the foundation soils at an embankment dam where the fissure extends into the dam reservoir, providing an access point for the impounded water to intersect the fissure. Water flowing along the fissure under hydraulic reservoir head most certainly results in significantly greater erosion than observed where surficial runoff is the primary mechanism of erosion.
For the condition where the dam embankment retains its integrity and is able to span the eroded fissure, the erosion forms a “tunnel” under the dam. Depending on the geotechnical characteristics of the embankment soils, a portion of the dam may also erode. If the embankment dam is unable to span the eroded foundation fissure, significant cracking and slumping of the upstream face and crest of the dam occurs. Reservoir water flowing along these cracks may lead to piping and breach of the dam in addition to the erosion breach of the dam foundation. Furthermore, depending on the freeboard available and the depth of impoundment, the crest of the dam may slump, resulting in overtopping and subsequent breaching of the dam.
An extensive review of the literature has not identified any conclusive case histories related to impacts of earth fissures on the safety and long-term performance of embankment dams. However, the failure of Picacho Dam in Arizona may have been triggered by an earth fissure. Figure 5 shows the failure tunnel eroded in Picacho Dam (ADWR; file photo).
Figure 5: Picacho Dam Breach