How to Assess the Strength of Coal Combustion Residuals

Stability failures of impoundments involving release of Coal Combustion Residuals (CCRs; commonly known as coal ash,) like the one that occurred in Dan River Coal Ash Spill in 2014, have recently highlighted the importance of assessing the stability of CCR impoundments. CCR materials contain pollutants like Mercury, Cadmium and Arsenic. Without proper management, these contaminants can be released, resulting in pollution of waterways, ground water, or drinking water.

Collapsed coal ash impoundment with closed power plant in the background

Failures in CCR slopes can occur suddenly with little time for corrective action to occur. It is hence crucial to assess CCR slop stability with realistic strength properties of the CCR materials to help avoid this undesirable performance.

What are these CCR materials? How are these deposited? And more importantly, how do we determine the strength of these materials to use in calculating the factor of safety for stability?

What are CCRs?

CCRs are produced from burning of coal in coal-fired power plants. They include various by-products such as fly ash, bottom ash, boiler slag or flue gas desulfurization material.

Coal Combustion Residuals:

  • Usually classify as non-plastic or very low plasticity silt, silty sand, or sandy silt
  • Are much lighter than typical soil with a specific gravity as low as 2.0
  • May have water content as low as 20% when dry stacked or as high as 80% when loosely placed

How are CCRs deposited?

CCRs are placed by various methods. The two most common ways are:

  • Ash stacks: CCRs are “dry stacked” and compacted. Dry stacks of CCR are most commonly unsaturated and compacted to some degree.
  • Ash ponds: CCRs are loosely placed by hydraulic sluicing and receive no compaction. These sluiced materials are generally very loose, saturated and therefore susceptible to liquefaction.

CCRs can exhibit considerable variability in deposition over short distances.

Example x-ray: undisturbed tube of ash

How do we determine the strength of CCRs?

Engineers typically perform stability assessments of CCR impoundments which have been in place for some time by assuming they are in a steady state seepage condition. That means they are considering a “drained” failure mode where excess pore water pressures do not develop during shear. Undrained stability with accompanying excess pore pressures generated by shear has generally not been considered a potential failure mode unless the impoundment is still under construction, or some significant alteration to its condition has been recently done.

This common approach for stability assessments overlooks the potential for an undrained failure to be triggered by an event that causes a rapid increase in mobilized shear stress or a rapid decrease in effective stress, such as and earthquake or heavy rainfall. Undrained strength of loose granular materials is less than the drained strength so the undrained failure mode can be more critical. In these instances, if a contractive saturated material (susceptible to liquefaction) is present, an undrained failure can be triggered. An undrained behavior in contractive materials can trigger liquefaction (the CCR material transitions to behave like a viscous fluid) and result in a flow slide. 

Such a failure can be evaluated by means of an undrained stability analysis using undrained shear strengths for the contractile materials. Saturated, contractive CCR materials tend to produce shear strength ratios ranging from 0.12 to 0.35. Common drained strength friction angles for CCR materials range from about 28 to 45 degrees. The associated strength ratios for these friction angles range from 0.53 to 1.00. The difference in strength ratios for an undrained contractive CCR clearly shows the undrained condition to be more concerning.

It is a challenge to obtain a meaningful assessment of the strength of materials like CCRs. They are highly variable and very sensitive to sample disturbance which alters their mechanical properties. In our experience, we find we must use both field and laboratory methods to obtain reliable strength parameters.

Field Testing

These tests require mobilization of drilling equipment to the site and monitoring of the work by trained engineering staff. In addition to the described tests, disturbed and undisturbed samples are generally recovered and returned to a laboratory for further testing. Field tests are accompanied by in-situ measurements of pore pressures in CCR stacks using vibrating wire piezometers that are installed in various depths as well as spatially. We recommend using grouted-in-place pressure transducers with several installed in each borehole to measure total head changes in the vertical and horizontal directions. Flow in most stacks is almost never hydrostatic as assumed by many engineers. Measured pore pressures can help understand the flow regime and the saturation conditions within the deposited CCR.

Standard Penetration Tests (SPTs) are the traditional tool used by geotechnical engineers to characterize granular materials. The so-called “blow count,” for blows per foot required for a 140 lb hammer dropping 30 inches to drive the sampler 12 inches into the soil, gives an indication of the relative density of the penetrated material and returns a disturbed sample. However, as typically performed by drillers, where drilling mud is not used, this measurement can give misleadingly low values. To be meaningful, the hole must be advanced with heavy drilling mud with the mud level always kept above the top of the hole. Otherwise, the SPT value can be quite low and lead to a wrong assessment of the material’s condition. SPT values obtained strictly following ASTM D1586 and D6066 can provide valuable data to help assess the strength behavior of CCR materials.  

Cone Penetration Tests (CPTs) provide the tip resistance, side shear and excess pore pressure developed when pushing a 10 cm2 probe into the material at a constant rate. It can provide lots of data on relative density of the CCR quickly and inexpensively and is a good tool to capture the material variability. However, the measurement is an index test which must be converted to strength through semi-analytical, empirical correlations. Such correlations are reasonably well understood for natural clays and sands, but for CCRs, well established correlation data are limited. Developing an understanding of the strength characteristics of these unique materials requires advanced laboratory testing.

Cross-Hole Seismic Tests can provide a profile of shear wave velocity measurements throughout the CCRs, providing information on the variability within the deposition of the CCRs and the low strain stiffness. Shear wave velocity measured in the field when compared with the corresponding laboratory shear wave velocity measurements can help with assessing the sample disturbance and guide targeted laboratory testing to determine the strength of the CCRs.

Advanced Laboratory Testing

Since disturbance alters the properties of CCR materials, laboratory tests to measure mechanical properties are generally performed on undisturbed samples recovered from the field investigation. Since no sample obtained in the field can be truly undisturbed, testing technique and test interpretation must seek to minimize the effects of sample disturbance on the test results. The advanced tests should be accompanied by index tests (e.g., material classification, moisture content, Atterberg limits, specific gravity) from the same sampling tube to support the knowledge of the tested material and interpretation of the test results.

Laboratory Shear Wave Velocity Measurements is a very useful and quick test to determine the level of sample disturbance and whether the test specimen is representative of field conditions. Piezo-ceramic plates also known as bender elements can be embedded in any type of strength test apparatus and used to measure the shear wave velocity of the specimen at stress levels similar to the in-situ stress conditions. Shear wave velocity gives an indication of the relative density of the test specimen.

Direct Simple Shear Test (DSS) measures the strength of a test specimen in a simple shear mode (distorting the specimen similar to shifting a rectangular cross section to a parallelogram).  A few advantages of DSS are:

  • It is an undrained test
  • The shearing in DSS is closer to the mode of shearing for global stability under CCR embankments which is different from the mode of shearing induced in triaxial loading
  • The size of the specimen is short (1” in height) allowing many more tests from a sampling tube
  • The total duration of the test is much shorter since there is no saturation phase for the specimen and the duration of the consolidation phase is reduced with the short height of the specimen


Dealing with unique materials like coal combustion residuals may require using a series of field and laboratory techniques to assess both their strength and variability. The combination of an appropriate field and laboratory testing program can help reduce unnecessary conservatism and accurately evaluate the stability of a CCR embankment. Actual field pore pressures are also important as they directly affect the effective stresses within the impoundment barrier which in turn directly affect the material strength.  We strongly recommend measuring actual pore pressures in the field at multiple locations vertically and horizontally rather than assume certain flow conditions.

Our practice is to use the most appropriate tools and methods for both field and laboratory testing to obtain a reliable picture of how the CCR material is likely to behave for all the potential failure modes of an impoundment. This comprehensive approach helps us avoid the unnecessary conservatism used when data quality for CCR strength are incomplete or unreliable. This approach takes more engineering effort up front, but it usually pays for itself many times over by avoiding unnecessary conservatism in the stability assessment, reducing, or removing the need for expensive corrective actions, and providing a firm basis to help manage risk.

Seda Gokyer Erbis is a Project Engineer/Assistant Project Manager for Geocomp’s Massachusetts Consulting Group. She has been with Geocomp for six years, holding a doctoral degree in Geotechnical Engineering. She has been leading the project management and technical efforts for one of Geocomp’s largest consulting projects on seismic assessment of coal ash impoundments. She has over seven years of experience in geotechnical earthquake engineering especially in advanced laboratory testing. She has authored and co-authored several publications in peer reviewed ASCE and ASTM journals and conference proceedings.

Published by Geocomp Blog

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