Investigative & Analytical Tools
Where known earth fissures, or the significant risk of an earth fissure zone are identified at the site of an existing dam, and the continued use of the dam is the alternative of choice, the regulator’s first goal must be to have the owner attempt to detect the fissure and implement remedial measures before the fissure erodes due to exposure to surface runoff or hydraulic head. In addition, the owner must also demonstrate that in the event that the fissure is not detected, the geotechnical conditions and embankment characteristics at the site are such that erosion of the fissure will not result in a catastrophic release of the reservoir. With a strong responsibility to safety of the public, the regulator must be convinced, typically through the efforts of the owner and the owner’s design engineer, that the investigative and analytical tools used to develop and evaluate the proposed designs meet both current state-of-the-practice and state-of-the-art at the time of design and construction.
As the entity responsible for review and approval of the proposed designs, the dam safety regulator must be aware of the current state-of-the-practice and technical advances in the fields of fissure detection, geotechnical site characterization, and erosion modeling. This section of the paper discusses some investigative and analytical tools that have been used at dam sites. The existence and application of some of these tools have been known to designers for some time now but may have a modified application in the field of fissure studies. This section also presents some new tools that have been developed exclusively for the study of earth fissures.
Surface Seismic Refraction Surveys
Rucker (2000) has developed a method to detect the presence of a fissure in the subsurface soils using seismic refraction techniques. The presence of an earth fissure or other discontinuity in the soil manifests itself as an anomaly in the time record for the geophones. The anomaly may be a delay in the arrival time of the compression wave, and/or attenuation in signal strength as the energy traverses the subsurface discontinuity. This technique has recently been used very effectively by Rucker to investigate cracks at two FCD dams in Arizona. This technique holds significant promise for fissure detection in that relatively large areas or alignments can be surveyed rapidly. The results of the surface seismic refraction survey is utilized to focus intrusive subsurface investigative techniques such as trenching to visually confirm and describe an existing fissure.
Thorough and detailed geological and geotechnical characterization of the site is considered to be a critical element of evaluating the feasibility of embankment dam rehabilitation in a fissure risk zone. Site investigations play a crucial role in identifying earth fissures (if present), and also provide relatively undisturbed samples for evaluating the erosion and other geotechnical characteristics of the foundation soils. Accurate location and proper backfilling of test trenches and boring must be included within the investigation. This section presents a brief discussion on some of the investigative tools used in fissure risk zones.
Test trenching provides an opportunity for direct visual observation of the subsurface soils and represents the best available technique to study earth fissures, especially those fissures that have no, or limited, surface expressions. In areas where the risk associated with earth fissures is high, continuous trenching within the fissure risk zone may be warranted. At other locations, the scope of trenching may be reduced through use of seismic refraction techniques discussed above. It is imperative that test trenches be carefully logged and documented by an engineering geologist with experience and expertise in evaluating ground subsidence and earth fissures.
Hollow Stem Auger Borings
Hollow stem auger borings can be used to explore and collect samples for general geotechnical characterization of the subsurface, but have very limited value in actual fissure detection. Furthermore, depending on the site-specific subsurface characteristics, it is generally difficult to obtain undisturbed soil samples for laboratory testing.
Soil coring using a track-mounted drill rig equipped with a HQ triple tube coring system has been used successfully to core unconsolidated alluvial soil deposits. The HQ system obtains a 2.4″ core sample and drills a 4″ hole. This method was successfully used to obtain relatively undisturbed samples at four embankment dam sites in Arizona. Core recovery was generally 100 percent, except in zones of relatively clean, uncemented sands, where the soils appear to have been washed away by the drilling fluid.
Modified Pitcher Sampling
A modified Pitcher sampling technique has been used to collect relatively undisturbed samples of alluvial soils. The sampling system consists of an outer barrel fitted with a carbide cutting bit. A spring-mounted Shelby tube is housed inside the barrel. The cutting bit trims the soil and the soil sample is slowly pushed into the Shelby tube. This drilling technique works well when the soils are fine-grained, but recovery is hampered when the subsurface soil contains medium to coarse gravels.
All potential failure modes related to an earth fissure underlying an embankment dam are associated with seepage along the discontinuity, and erosion of the walls and base of the earth fissure. Therefore, evaluating the erodibility of the foundation soils, both in terms of threshold of erosion and rate of erosion are critical when evaluating the safety and integrity of embankment dams in earth fissure risk zones. This section provides a brief discussion on some tests used to evaluate the erodibility of soils.
Seismic Refraction Studies
EHI (2005) attempted to use excavatability information from typical surface seismic refraction surveys to evaluate the erodibility of alluvial soils. The method combines Kirsten’s rippability index approach (1982) with site specific seismic refraction data to evaluate the energy needed to initiate erosion in a soil. The method assumes that in general, a greater resistance to excavation and ripping as estimated from Kirsten’s (1982) approach, the greater the resistance of the soil to erosion. The authors believe that the EHI approach has two drawbacks. Firstly, the method fails to account for the fact that many alluvial soils in the Southwest slake and soften when exposed to water. Secondly, as with most empirical methods, Kirsten (1982) developed his excavation relationship based on a conservative interpretation of his data set. This approach is certainly valid when evaluating excavation conditions; however, when using seismic velocity data to assess erodibility, the very assumptions that make Kirsten’s approach conservative for excavation and rippability, result in unconservative interpretations of erodibility. Seismic velocities provide some general qualitative information on erodibility of alluvial soils, but additional confirmation and perhaps modification of the EHI relationship is required to correlate seismic velocities to quantitative thresholds and rates of erosion.
Vertical Jet Index Tests
The test procedure consists of a water jet impinging on the in-situ soil to be tested. Periodically, a pointer and gage mechanism mounted within the jet tube is used to measure the depth of erosion. The data collected in the field is reduced to estimate the threshold of erosion and the rate of erosion. The principal advantage of this test is that the test can be performed in the field on the soil in in-situ conditions. However, the test requires direct access for men and equipment to the soil strata to be tested and is usually limited to test pits.
Erosion Function Apparatus Test
The Erosion Function Apparatus (EFA) test was developed at Texas A&M University, College Station, Texas. Water is driven through a rectangular pipe by a pump. An undisturbed soil sample is gradually introduced into the flowing water until the soil protrudes 1 millimeter (mm) into the rectangular pipe. The time needed to erode the 1 mm of soil by the flowing water is recorded (Briaud et al. 2001). The process is repeated for increasing flow velocities. Soil disturbance, macro structure and scale effects can significantly affect the results and their interpretation using this test procedure. However, the authors believe this test procedure offers the most promise in quantifying soil erosion on a fissure.
Hole Erosion Test
The hole erosion test (HET) was developed at the University of New South Wales, Australia, and is documented in Wan and Fell (2004). A 6-millimeter hole is drilled along the longitudinal axis of the undisturbed sample to simulate a concentrated leak. The upstream hydraulic head is varied, and the flow rate is used as an indirect measurement of the diameter of the eroded hole, and the rate of erosion. The test needs to be conducted for a sufficiently long time in order to account for the effects of slaking (if any). The authors were also concerned that the shape of the test hole (circular) tends to mitigate the adverse impact of any soil macro-structure or scale effects that may result in higher erosion rates on a fissure.
Fissure Erosion Modeling
EHI (2003) described a MathCAD model developed to evaluate erosion along an earth fissure. The maximum extent of the breach and the time to breach completion are dependent on the magnitude and spatial distribution of the erosive power of water, and on the erosion characteristics of the soil. The model calculates variation in the erosive power of the water in a fissure as a function of time due to changes in water surface elevation within the reservoir (potential energy of the water), and as a function of space due to changes in the dimensions of fissure in general, and the width-to-depth ratio of the fissure in particular.
To the best of the authors’ knowledge, this is the only model available to assess erosion along an earth fissure. The model can be used to evaluate the existing dam configuration, as well potential mitigation measures such as cutoff walls and aprons. This model is a useful tool to study the potential eroded width of an earth fissure at a specific dam site. One drawback of the model is that it incorporates a constant gradient such as used in porous media flow. The model also simplifies the erosion process along the fissure using a velocity-erosion relationship and does not include any erosion related to soil slumping from mechanical failures.
The authors envision that during the initial stages of seepage along an earth fissure, the flow of water is predominantly downward at a relatively high gradient. The soils would be exposed to relatively high erosive forces under this vertical flow. At some point, based on the fissure characteristics and the soil grain-size distribution, the fissure will plug, diverting the high gradient flow laterally along the fissure. This erosion mechanism could lead to rapid lateral migration of a void along a fissure extending below a dam.
Fissure Risk Identification
FCD and their consultant AMEC Earth & Environmental Inc. developed a protocol to qualitatively evaluate fissure risk at a given location. The process includes collecting and reviewing interferograms, low sun angle aerial photographs, selective ground truthing, and seismic refraction surveys to identify zones of low, moderate and high fissure risk. To the best of the authors’ knowledge, this is one of the first attempts to systematically evaluate sitespecific conditions, and assign formal, qualitative fissure risk zonation along an embankment dam. This is a very useful tool in rehabilitating dams in areas potentially prone to earth fissure development in that the embankment can be “segmented” and prioritized for rehabilitation. Depending on stakeholders’ risk aversion and funding capabilities, zones of high, or moderate and high risk may be immediately rehabilitated, while the remainder of the embankment can be monitored and rehabilitated as required at a future date.