🌧️ Rain causes strange reversal of underground air flow

A strange observation has long puzzled engineers at the Sanford Underground Mine in South Dakota: during heavy rainfall, the airflow in certain shafts would reverse. This unexpected behavior has now found an explanation, with notable implications for mine safety.

This facility, the Sanford Underground Research Facility (SURF), is no longer in active mining, but requires constant ventilation for safety. Jason Connot, a mining engineer, oversees this system. Shortly after his arrival in 2019, he and his team noticed a strange behavior during storms: the fan in shaft 5 became uncontrollable, and airflow in some areas slowed down or even reversed.


To understand this phenomenon, finer measurements were needed. Airflow sensors were installed, and Steve Gabriel, a professor of science, designed monitors with his students that captured a key event during a test of the sprinkler system on the 4850 level. "We felt an increase in airflow, that triggered everything," explains Jason Connot.

The cause was surprising: rainwater directed into shaft 5 to prevent flooding acts like a giant syringe. As it falls, it pushes the air ahead of it, forcing an abnormal circulation through the ventilation network. Jason Connot found equations in the literature explaining this phenomenon in sewers, and with the help of colleagues from the South Dakota School of Mines, adapted them to the site. The calculations matched the observations perfectly.

This discovery goes far beyond mere curiosity. In the event of an underground fire, engineers often pour water down a shaft. "Knowing that this can change the airflow is essential information," says Jason Connot. Thanks to this work, the team can now anticipate these variations and configure the ventilation accordingly, improving safety.

Jason Connot's research, published in the journal Mining, Metallurgy & Exploration, has been praised by his peers. Andrea Brickey, his advisor, commends his curiosity: "He identified a phenomenon that affects ventilation systems and sought to predict it. He succeeded."

The hydraulic syringe principle

The mechanism is based on the principle of a hydraulic syringe. When you push the plunger, air exits through the needle. In a mine, water falling down a shaft acts as the plunger. The descending column of water compresses the air below it, creating a pressure difference that forces air to move through the tunnels.

This mechanism is well-known in urban sewer networks, where large water drops can propel air through pipes. However, applying it to a mine requires precise calculations taking into account the shape of the shafts, the volume of water, and the characteristics of the ventilation system.

The discovery by SURF engineers shows that even small amounts of water, such as rain, can have a significant effect. This paves the way for predictive models to anticipate these airflow changes, which is essential for safety.

In the case of SURF's shaft 5, excess water is discharged into a deep cavity. Falling over several hundred feet (hundreds of meters), it gains enough speed to move a large mass of air. Engineers measured that airflow could double or more during a storm.