3 important keys to monitoring for stability during ash pond closures

Faced with new regulations, power utilities across the country are closing old impoundments of fly ash and other coal combustion residuals (CCRs). There are two primary methods for closing these facilities: closure-by-removal, which requires excavating slopes into wet ash deposits, and closure-in-place, which involves spreading cover materials across the ash surface. Some of these projects include stacking coal combustion materials on top of saturated ash.

In all these cases, the stability of the ash foundation and constructed slopes is a paramount concern. Slope failures can be disastrous. Massive slope movements may release CCR materials. Smaller slope failures can delay projects, undermine adjacent infrastructure, and bury construction equipment. Most worrisome is the potential danger for construction workers.

Owners expect engineers and contractors to develop stable designs and construction plans that avoid such incidents. Predicting the behavior of wet CCR deposits is difficult and the risks are significant. Projects can be built safely, but construction over saturated fly ash requires diligent planning and monitoring to detect signs of instability.

Here are three important lessons based on our experience on CCR closure projects:

1. Instrument monitoring is essential

Well-planned monitoring programs are needed for successful construction over fly ash. An array of instruments, such as piezometers and inclinometers, should be installed at the start of construction. The type and location of instruments should be selected based on the site conditions and potential failure modes, such as slope instability and bearing capacity.

Instrumentation should be viewed as a key component of the larger monitoring program. However, there are limitations. Subtle changes in the instrument readings, which may indicate deteriorating stability, can be easily overlooked. If a piezometer or inclinometer is not in exactly the right place, the data may not catch a dangerous condition. Even a dense array of instruments may not pick up a developing instability.

2. Set alarm limits

Every instrument should have preset alarm limits for elevated readings that could indicate an emerging structural failure. Selecting the correct alarm limits can be challenging. If set too high, elevated readings may be overlooked. Limits that are set too low can trigger excessive false alarms, resulting in a lack of trust in the system, complacency, and ignored alarms. Alarm limits selected for one site are usually not applicable to other sites or projects.

Following the practice commonly used in dam safety monitoring, we like to set two alarm limits for each instrument:

• Threshold Limit. Construction can continue with routine monitoring when the instrument readings are below the threshold limit. When a reading exceeds the threshold, the engineering team should be notified and, at a minimum, more frequent monitoring should be implemented.
• Action Limit. When an instrument reading exceeds an action limit, construction in that area should be stopped automatically. Stability of the slope should be evaluated by the engineering team and appropriate restrictions or changes in construction implemented.

Professional engineers should be designated to evaluate each alarm and review the data for signs of instability. As construction progresses, alarm limits may need adjustment to better represent changing site conditions.

3. Monitor data trends

To ensure the safety and stability of the CCR construction site, a written monitoring plan should be developed and followed. This plan should outline the frequency of instrument readings and identify the team of professionals charged with collecting, processing, and plotting the data. The monitoring team should compare readings to pre-determined alarm limits.

It’s not uncommon for unsatisfactory conditions to develop before triggering an alarm. Increasing pore pressures, accelerating ground movements, and other sharp changes in the field data may be indicators of developing failures. Corrective actions may be needed even when the readings are below the preset alarms. Hence, the field data should be regularly plotted, reviewed, and assessed by responsible professional engineers.

Successfully facing the challenges

Every CCR closure project presents unique challenges. Owners, engineers, and contractors must work together to ensure the safety of construction crews and successful project completion. Rigorous instrumentation monitoring programs are a key component in these efforts.