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Designing for the Cycle: PSH Hydrodynamics and Stability

From a geotechnical and materials engineering perspective, a PSH reservoir is a brutal environment for a geomembrane. The daily fill-and-drain cycle is a relentless, high-frequency assault of hydrodynamic forces that will exploit any weakness in the lining system over its 50-year service life.

Your core challenge is to engineer a liner solution that can handle this environment, and to do this, you need to really appreciate the various forces at play. In this chapter, we break down the primary challenges you must design for:

Immense Hydrostatic Pressure: The constant force exerted by the deep water column.

Rapid Drawdown and Uplift Forces: The dangerous negative pressures that can develop behind the liner as the reservoir empties.

Cyclic Strain and Material Fatigue: The cumulative effect of thousands of stress/relax cycles on the liner material.

Overall Slope Stability: The analysis required to ensure the entire multi-layer system remains stable on steep slopes under all of these conditions.

Challenge Number 1: Immense Hydrostatic Pressure

The sheer, unrelenting weight of the water itself is the first and most constant challenge you must address. After all, hydrostatic pressure increases with depth, and in a deep PSH reservoir, it is enormous.

As a rule of thumb, water pressure increases by 0.433 PSI for every foot of depth. In a reservoir just 100 feet deep, the pressure on the liner at the bottom is over 43 PSI, or more than 6,200 pounds per square foot. At its peak, that’s the equivalent of parking a heavy-duty pickup truck on every square foot of the liner. This immense load is then removed and reapplied every single day, creating a massive pressure cycle that lasts for the life of the facility.

This immense pressure has two primary implications for your design:

  • It will quickly find and exploit any weakness or flaw in the system, whether it’s an imperfect field seam, a tiny pinhole in the liner, or a weak spot in the subgrade. This is why material quality and installation QA/QC are absolutely critical.
  • The pressure tests your subgrade by transferring directly through the liner to the ground beneath it. If the subgrade isn’t perfectly firm and smooth, this force will push the liner into voids or against hidden sharp objects. In fact, this is the primary mechanism for subgrade-related punctures.

To meet the challenge of hydrostatic pressure, every single component—the subgrade, the underlying layers, the liner, and the seams—must be perfectly suited to resist it.

Challenge Number 2: Rapid Drawdown and Uplift Forces

This is the defining engineering challenge of a PSH liner system. While a standard reservoir deals with mostly static loads, a PSH reservoir is subjected to powerful dynamic forces every time it empties. This means that understanding the physics of the rapid drawdown is absolutely critical.

The Physics of Uplift

Here’s the chain of events:

  • When the reservoir is full, its immense weight raises the natural groundwater level in the subgrade, thoroughly saturating the ground beneath and adjacent to the liner system.
  • During a rapid drawdown, the water inside the reservoir is removed in a matter of hours, but the elevated groundwater in the subgrade cannot recede that quickly.
  • This creates a dangerous imbalance: you have little to no water pressure on top of the liner, yet significant hydrostatic pressure from the trapped groundwater behind it, pushing it outwards.

The outward force is called uplift pressure. It’s a powerful hydraulic force that can lift the entire liner system off the slope, creating large bubbles or “whales” that could trigger a catastrophic failure.

Engineering for the Uplift

Your design has to counteract this hydrostatic force, and you should tackle it from both ends: relieve the pressure from behind and hold the liner down from the front.

Your Active Defense: A  High-Capacity Underdrainage System

Here’s where the drainage system you’ve built into the subgrade itself (as discussed in Chapter 3) shines. The system’s whole job is to relieve hydrostatic pressure by providing a fast, easy path for trapped groundwater to escape before it can build to dangerous levels.

Your Passive Defense: A Heavy Ballast Layer

This is your brute-force solution. The sheer physical weight of your overlying protective cover holds the liner system down, counteracting whatever uplift pressure the drainage system cannot relieve. This cover system (which we’ll cover in detail in Chapter 7) typically consists of a thick geotextile cushion placed directly on the RPE liner, which is then covered by a heavy layer of soil, aggregate riprap, or articulated concrete blocks. Your design must calculate the maximum potential uplift force and then specify a cover layer that is demonstrably heavy enough to resist it.

Challenge Number 3: Cyclic Strain and Material Fatigue

The large-scale forces of pressure and uplift are fairly dramatic strains on your system, but the cumulative effect of thousands of small movements within the liner material itself is no less substantial. Every time the reservoir is filled, the liner is loaded and stretches slightly, and every time it’s drained, it relaxes. Over a 50-year lifespan, this can add up to over 18,000 stress/relaxation cycles—the very definition of cyclic strain.

The Risk of Fatigue

The primary risk of such constant cycling is material fatigue. Just like a paperclip that will break if you bend it back and forth enough times, a geomembrane can be weakened by repeated stretching, even at the microscopic level. Any material with poor fatigue resistance will eventually lose its elasticity, become brittle, and develop cracks, particularly in high-stress areas like corners or around penetrations.

The RPE Advantage: Flexibility and Reinforcement

This is where the composite structure of RPE provides a significant advantage.

Flexibility

The flexible LDPE coatings on RPE are designed to handle this kind of repeated strain without becoming brittle.

Reinforcement

The internal HDPE scrim provides a strong, stable backbone that prevents the material from permanently stretching or “creeping” out of shape over time. The combination of flexibility and stability makes RPE exceptionally well-suited to resist the long-term fatigue caused by relentless PSH cycles.

Challenge Number 4: Overall Slope Stability

Your final and most complex design challenge is ensuring that the entire liner system—from the subgrade to the protective cover—remains stable and does not slide down the steep slopes of the reservoir. For this challenge, you’ll rely on a geotechnical stability analysis performed by a qualified engineer.

Understanding Interface Friction

As we’ve seen, a liner system isn’t a single homogenous sheet; it’s a stratified structure of several distinct materials. Since the structure’s stability relies on the friction between each layer, the stability analysis will consider the friction angles at several key interfaces:

  • The GCL against the subgrade soil.
  • The secondary geomembrane against the GCL.
  • The geonet against the geomembranes.
  • The primary RPE liner against the geonet.
  • The protective geotextile cushion against the RPE liner.
  • The final cover material (soil or rock) against the cushion.

Analyzing the Forces

An engineer will use specialized software to model these layers and analyze their stability under the worst-case conditions. This includes the forces at work when the reservoir is full (high gravitational load) and during a rapid drawdown (high uplift pressure combined with the weight of the cover). The analysis will verify that the friction between the layers is sufficient to prevent any part of the system from sliding. If the study shows the calculated factor of safety against sliding is too low (meaning the friction is insufficient to resist the driving forces), you’ll need to modify the design.

Here are a few typical options:

Change Materials

The simplest solution might be to specify a different geosynthetic material with a higher interface friction angle. For example, using a textured geomembrane instead of a smooth one significantly increases the friction between the liner and adjacent layers (like the geotextile cushion or geonet).

Alter the Geometry

You could reduce the driving forces by making the slope angle shallower. This is often undesirable as it increases the overall footprint and earthwork volume, but it’s a fundamental way to improve stability.

Add Internal Reinforcement

In some cases, incorporating geogrids into the cover soil or aggregate layer can increase tensile strength and help stabilize the overlying mass, reducing the stress on the underlying liner interfaces.

The best solution depends on the factor of safety and the predicted critical sliding surface. The engineer will run iterations of the analysis, adjusting these variables until achieving an acceptable factor of safety (typically 1.5 or higher).


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