Pouring concrete in the dry was simple carpentry.
Pouring concrete inside eight feet of roaring mountain river current was an exercise in high-pressure warfare.
At first light, Giles Croft stood on the timber footbridge spanning the Crag-Foot gorge. Below him, the river thundered through a narrow granite chute, throwing plumes of white spray against the mossy rock walls.
The river was running cold and swift, swollen by early snowmelt from the high peaks.
At this crossing point, four of the aqueduct’s thirty-two-foot piers had to be anchored directly into the underwater riverbed. Without these four central pillars, the two-mile aqueduct would remain a broken dream, cut in half by fifty yards of boiling white rapids.
“You can’t put stone in that water, my lord,” Master Duncan Stone-Splitter yelled over the roar of the clouded eye.
Duncan leaned over the rough spruce railing, his leather apron soaked with spray.
“The river will wash the mortar away before it touches bottom,” Duncan continued. “It’s like trying to build a stone chimney inside a waterfall. The water will dissolve the lime, scatter the sand, and leave you with nothing but drowned gravel.”
“We’ll not pour in open water,” Giles replied, shouting into the wind. “We’ll build cofferdams. Caleb, position the pile-driver.”
Captain Kenneth Drake paced the timber narrow high walkway, his ten-foot iron-shod pacing rod balanced across his shoulders. He looked down into the froth with an old sapper’s tactical math.
“If we plant four solid stone piers across this channel, my lord,” Kenneth observed, “we also choke the only natural ford within three leagues. An enemy marching down the canyon floor would be forced into a single file between the stone pillars.”
“Tactical chokepoints are a useful backup consideration,” Giles said. He pulled his slide rule from his pocket. “Our immediate priority is water-flow balance. If the foundations scour out during spring thaw, the entire raised viaduct collapses like a line of dominoes.”
Toby Karr stood near the railing. He leaned out with a sounding line made of braided hemp cord weighted with a lead plummet.
“Water depth at Pier Twenty-Two is seven feet four inches, Lord Giles!” Toby called, checking the red wool markers tied along the cord. “Current speed looks like five knots at the center!”
“Seven point four feet of static head generates over four hundred and sixty pounds of water-pressure sideways push per square foot on the lower timbers,” Giles noted. “Caleb, double the cross-bracing.”
Master Caleb Thorne-Wood had built a floating pile-driver upon two flat-bottomed timber barges. The vessels were lashed together with heavy hemp cables, anchored across the current by iron chains hooked into the rock banks.
In the center of the catamaran rose a twenty-foot timber derrick with upright guide rails of seasoned ash.
Inside the rails hung an eight-hundred-pound block of cast iron.
Master Borin Bronze-Lung had poured the casting the last evening. It melted down worn iron plowshares and broken cart axles in the foundry small roof-dome. The dwarf stood on the barge deck now, adjusting his sight fire gauge around his neck while wiping coal soot from his copper-braided beard.
“Hard gray iron,” Borin barked, slapping the drop-weight with a heavy leather mitten. “Poured hot and chilled in green sand. It will drop like an anvil from heaven and drive those larch stakes straight to the center of the earth.”
A thick hemp rope ran over the top pulley to a double-handled hand-winch manned by four burly quarrymen.
“Timber sheet piles ready, Lord Giles!” Caleb shouted from the barge deck.
Each sheet pile was an eight-inch-thick plank of green larch, ten feet long, carved with a linked tongue-and-groove joint down its edges. The lower tip was beveled into a chisel point, shod with a forged iron shoe crafted by Borin’s smiths.
Lady Millicent stood on the riverbank terrace, her tally-book resting on a dry cedar stump.
“Forty-eight larch logs burned up,” Millicent reported, checking off the tally marks with her iron pen. “Sixteen pounds of forged iron shoes. The saw-pit has met its delivery quota with three hours to spare.”
“Position Pier Station Twenty-Two,” Giles commanded, checking the centerline with his plumb rod. “Drop the first sheet.”
The crew hoisted a larch pile into the upright guide.
“Trip the hammer,” Giles shouted.
The winch dog released. The eight-hundred-pound iron weight plummeted twenty feet through the guide rails.
THWACK-BOOM!
The iron blow drove the timber pile three feet into the riverbed gravel. It sent blast waves through the water.
THWACK-BOOM!
For two days, the rhythmic thud of the pile-driver echoed down the canyon like the heartbeat of an iron giant.
Pile after pile was driven into the gravel. The tongue-and-groove joints locked together, forming four watertight timber boxes—cofferdams—each twelve feet wide and eighteen feet long, enclosing the foundation sites.
To seal the seams between the planks, Caleb’s apprentices drove caulking of hemp oakum soaked in hot pine tar into every joint.
The timber boxes were enclosed. Then, Giles had two Archimedes screws mounted on the barge rails.
The hand-cranks were turned in relays by refugee laborers from the centuriation grid.
Within three hours, the churning brown water inside the first cofferdam was pumped out, cascading over the timber lip back into the main river in a foaming froth.
The riverbed within the box was exposed.
It was bare, scoured solid mountain stone, glistening with wet silt and polished river pebbles.
“Clear the muck down to clean rock,” Giles ordered, pointing into the deep enclosure. “Every speck of silt must be scrubbed with wire brushes. If concrete is poured over mud, the bond fails in shear.”
Duncan and six masons climbed down ladders into the dry bottom of the river, armed with iron scrapers and brooms.
They worked under the shadow of the larch walls. Meanwhile, the river pounded against the outside of the planks with muffled, rhythmic thuds.
Borin Bronze-Lung climbed down beside Duncan, holding his brass lantern close to the dark rock.
“Look at this seam,” Duncan called up. He pointed his pick at a fissure in the rock floor. “Bedrock cleavage runs northeast. If we don’t notch it, the river could lift the heel.”
“Aye, granite shears along the grain when water gets under the skirt,” Borin agreed. He tapped the fissure with his iron-shod tongs. “Give it a good tooth. If stone doesn’t bite stone, the river will spit it back at us.”
“Chisel a three-inch shear key along the centerline,” Giles instructed from above. “That locks the concrete plug against sliding friction.”
Duncan swung his heavy splitting pick, hacking shallow key-trenches across the bedrock to give the concrete a clockwork interlock.
Then came the volcanic cement concrete.
On the riverbank above, two large wooden mixing troughs were working continuously.
Red fiery ash from the Old Fire Pit, freshly slaked burning lime powder from the upright shaft kiln, sharp river sand, and broken dark rock aggregate were mixed into a thick, dense paste.
“Lower the bottom-dump skips,” Giles instructed.
Caleb’s crew used a timber boom crane to swing heavy wooden boxes of wet concrete over the cofferdam.
When the skip reached the rock floor, a trip cord opened the hinged bottom, depositing half a ton of volcanic cement concrete directly onto the bedrock without dropping it through air or washing away the paste.
“Tamp it!” Giles called down. “Drive the paste into every fracture and fissure in the granite.”
Duncan’s men worked with heavy oak tampers, packing the stiff gray mass until air bubbles ceased to rise.
Layer upon layer, the concrete rose within the timber box.
The high-pressure lime and fiery ash reacted with the moisture, generating heat that warmed the damp enclosure. Within twelve hours, the foundation block had cured into an artificial granite monolith six feet thick, bonded immovably to the bedrock of the world.
By the fourth evening, all four river piers were poured up to water level.
Duncan climbed out of the fourth cofferdam, his hands coated in gray lime crust.
He leaned over the rail, staring at the square stone pillars rising straight out of the rushing current.
“The water beats on it,” Duncan murmured, shaking his head in quiet wonder. “The river roars and throws spray, but the stone doesn’t budge a hair. It’s living rock that drinks the river to grow iron bones.”
Giles pulled his pocket rule. He measured the posture of the anchor bolts set into the pier tops.
“Crushing strength is on schedule for four thousand four hundred pounds of pressure,” Giles noted. “Pier bending under current drag is zero. Caleb, strike the cofferdam sheet piles and salvage the larch planks for the upper arch forms.”
Kenneth Drake stood on the narrow high walkway beside them, leaning on his ten-foot rod.
His eyes were not on the river piers. His gaze was fixed on the high jagged peaks at the northern end of the gorge.
A sharp whistle cut through the roar of the river.
Maeve Swift-Ear, the vulpine scout, came leaping down the northern talus slope. She held her large brass ear-trumpet tucked under one arm, her fox ears pinned flat against her copper-red hair.
“Lord Giles! Captain Drake!” Maeve cried, skidding to a halt on the wooden walkway planks. “Listen to the high gorge!”
She held up the brass horn, pointing it toward the northern cloud bank.
Kenneth leaned close to the horn’s narrow mouthpiece. A dull, underground thrumming shook through the metal. A terrifying rumble that sounded like a thousand iron wagons rolling across a hollow wooden bridge.
“The cloudburst,” Maeve gasped. “A cloudburst broke over the high shale basins three leagues north. A lake of rainwater got trapped behind an ice-slow scree, and the scree gave way.”
Duncan stiffened, his dwarven ears picking up the shaking through the bedrock. “Flash flood. A mountain surge.”
The sky above the mountain crests turned a bruised, violent purple. Black clouds, swollen with heavy water, tumbled down the crags like an avalanche of soot.
The wind shifted instantly, bringing a wall of cold, damp air that smelled of torn spruce boughs and crushed shale. Down the canyon, the roar increased tenfold.
Around the bend of the river, two hundred yards upstream, a brown wall of water ten feet high came tearing through the gorge.
The flood carried uprooted hemlock trees, mud, and boulders the size of ox carts, churning with furious foam.
The torrent bore down directly upon the newly poured concrete piers!