The Duke's sappers had been repelled, but they had left behind a subterranean void that threatened the structural integrity of the Iron Road. The soil beneath the Sinking Trestle was now a honeycomb of unstable shafts and bleeding silt. Kael's math was brutal: the next 50-unit Axle Train would exert enough downward force to "punch" the timber piles through the hollowed earth, causing a catastrophic derailment into the marsh. To save the bridge, Kael had to execute a Grouting Blast—a high-stakes engineering gamble to collapse the enemy's tunnels and chemically fuse the marsh clay into a solid foundation.
Kael did not dismantle the captured sulfur charges. Instead, he augmented them. He ordered the production of several barrels of "Liquid Lime-Slurry"—a byproduct of the kiln scrubbers. By pumping this slurry into the Duke's tunnels and then detonating the sulfur charges, the resulting heat and pressure would trigger a rapid pozzolanic reaction. The lime would react with the volcanic ash in the silt and the heat of the blast to create a crude, "flash-baked" concrete—a stone plug.
The "grit" of the operation was the placement. Drax and his team had to return to the black, wet tunnels they had just fought for. They worked in ankle-deep sludge, hauling the heavy iron slurry-pipes and wiring the charges to the Galvanic Line. The air was thick with the smell of wet lime and stagnant methane. Kael monitored the pressure from the surface, his fingers on the copper switches of the battery bank.
"We only get one shot at the timing," Kael signaled via the acoustic pipe. "If the slurry isn't under pressure when the charges blow, we just make a bigger hole. We need the Hydraulic Lock."
He utilized the aqueduct's high-pressure line to "prime" the tunnels. By opening the sluice gates into the slurry-pipes, he forced the lime-wash into every crack and crevice of the Duke's excavation. The pressure was so intense that the ground around the trestle began to "heave" and "bleed" white lime-water through the silt.
Socially, the barony was held in a state of suspended animation. The "Information Citizens" on the Gray Fang stood ready to signal the Imperial capital that the shipment was delayed, or—if the blast succeeded—that the road was open. The original citizens watched the marsh, seeing the strange, milky-white water bubbling up around their bridge. To them, Kael was literally trying to turn the mud into stone.
"Trigger the sequence," Kael commanded.
The explosion was muffled by the weight of the marsh. It was not a roar, but a series of deep, subterranean thumps that sent a ripple through the trestle. For a terrifying second, the bridge settled two inches, the timber piles groaning under the sudden shift. Then, the ground went silent.
The heat of the sulfur blast, contained within the pressurized, lime-filled tunnels, had performed a miracle of "Instant Masonry." The tunnels had collapsed, and the slurry had solidified into a jagged, reinforced mass of chemical stone.
The first test was the "Rumble Audit." Kael had an empty ore wagon rolled across the trestle. The seismic monitors in the guardroom didn't show the chaotic "jitter" of shifting soil; they showed a clean, sharp vibration. The foundations were no longer a void; they were a solid anchor.
However, the "Stone Plug" had a final, thermal consequence. The chemical reaction was exothermic—it generated a massive amount of internal heat. The ground around the bridge was steaming, and the marsh water was beginning to boil at the pile-bases.
"The piles are wood, Baron," Hektor warned, pointing to the rising steam. "If that heat stays trapped, the timber will char from the inside out. The stone will hold, but the bridge will rot."
Kael didn't hesitate. He initiated the Post-Blast Cooling. He reversed the flow of the aqueduct's secondary line, pumping cold river water directly into the grout-zone. The hiss of the steam was deafening, a white curtain that draped the Sinking Trestle for hours. It was the final act of the "Stone Plug" project—tempering the very ground they had just forged.
Fifty Imperial Axles, the completion of the debt, rolled smoothly over the trestle. The bridge didn't move an inch. Kael stood on the "Stone Plug," feeling the warmth of the cooling earth through his boots. He had successfully stabilized the frontier's mud.
"The road is solid, Elms," Kael said, watching the wagons head south. "We've paid the Imperial debt. But we've also left a permanent scar of white stone in the marsh. The Duke's engineer will see that grout-wash from his towers. He'll know we can turn his own tunnels into our foundations. And he'll know the next war won't be fought with shovels."