Vault Stabilization at Hardin: How We Ensure Long-Term Security

Building and Securing the Vaults


In 2023, CSI constructed and stabilized four large wood vaults at the Hardin site. Each vault was carefully excavated, filled with woody biomass, and sealed with thick layers of native clay. After major rain events, our team used reserved clay to fill in any surface depressions, then compacted and smoothed the surface to maintain a tight, weather-resistant seal.

Adapting to Weather and Nature

The Texas weather put our design to the test, with over 6 inches of rain in May alone. Thanks to the flexibility and abundance of our native clay, any settling or erosion was quickly addressed. By late 2023, the excess clay was sculpted into smooth mounds, and a gas monitoring system was installed to track vault integrity.

Monitoring and Results


Dry, warm weather sometimes caused surface cracks in the clay, but these remained shallow-never deeper than 10 inches. Below 16 inches, the clay stayed moist, dense, and structurally sound, confirming the effectiveness of our sealing process. In March 2024, all vaults were measured and confirmed to be covered by the required clay thickness, with ongoing gas monitoring showing the vaults remain secure and stable.

How Our Carbon Vaults Work

Engineered for Permanent Carbon Storage

At CSI, we design and build large underground pits-typically about 300 by 300 feet and over 12 feet deep-to securely store woody biomass that would otherwise be burned, releasing carbon dioxide into the atmosphere. Instead, this waste wood is tightly packed into the pit and immediately covered with thick, impermeable clay sourced directly from the site. This native clay acts as a natural seal, keeping out oxygen and water, and eliminating the need for plastic liners-a win for both durability and the environment.

Key Features of Our Pit Design:

  • Native Clay Seal: The dense, high-plasticity clay from the Lissie Formation creates a watertight and oxygen-free vault, preserving the buried wood for centuries and preventing greenhouse gas release.

  • No Plastics: We rely solely on local clay, not synthetic liners, for a sustainable, all-natural solution.

  • Dynamic Layout: Our pits are not monotone rectangles; we shape them to fit the land and include nature corridors, supporting habitat connectivity and future revegetation.

  • Cell Construction: For large vaults, the pit is divided into cells. Each cell is filled and capped in stages to minimize wood decay and maximize operational efficiency.

  • Weather Adaptation: Construction and stabilization are timed to minimize soil damage and compaction, and after significant rain, we recompact the clay to maintain a tight seal.