I'm doing engineering in the instance.

Chapter 18 The Language of Pipes

Old Zhao stopped at the 310-meter mark.

It wasn't because anything was blocking him; he stopped on his own. Xie Chengzhou was about two meters in front of him when he felt the footsteps behind him disappear. He turned around and saw Old Zhao standing there, switching the thermos to his left hand, pressing his right hand against the pipe wall, and closing his eyes.

Xie Chengzhou didn't speak. He waited.

Old Zhao stood there for about fifteen seconds, his hands pressed against the concrete wall, his eyes closed, his expression focused, as if he were listening to something. Then he opened his eyes.

"There's a fork in the road ahead," he said.

Xie Chengzhou shone the flashlight forward. The pipe was straight in his line of sight, without any branches. "How did you know?" he said.

"Listen," Old Zhao said, "the sound of water."

Xie Chengzhou placed his hand on the wall, closed his eyes, and listened.

The sound of water, the echo, the low-frequency resonance unique to pipes. He layered the sounds in his ears, listening deeper and deeper—his footsteps, Old Zhao's footsteps, the basic sound of the water flow, the reflections from the walls. Then he heard: beneath that layer of uniform water sound, there was a slight frequency difference, like two water sounds superimposed, but not perfectly synchronized, with a very small phase difference, originating about thirty to fifty meters ahead, then propagating backward in the pipe, superimposing into the water sound he heard.

Two streams of water. In two directions.

"Forking," Xie Chengzhou said.

Old Zhao opened his eyes. "Yes," he said, "about thirty meters ahead, the water flows in from two directions, which means the pipe branches there, or there's a branch pipe connecting to it." He paused. "Is it on your map?"

Xie Chengzhou opened the memo and reviewed Rule 2 again: "End condition of the journey: Reach the main control room at the end of the pipeline and close the flow control valve." There was no map, no route instructions, nothing at all.

"No," he said.

Old Zhao switched the thermos cup from one hand to the other. "That's troublesome," he said, his tone calm, as if to say, "I've encountered this situation before. It's difficult to handle, but I can manage it." "If you choose the wrong direction in a branching pipe, you might end up in a dead end or a branch pipe with no exit."

Xie Chengzhou went over the question in his mind. "You've worked in pipelines for thirty years," he said, "how do you determine the direction of the main trunk when it branches out?"

Old Zhao didn't answer immediately. He placed the thermos on the water, let it float for a moment, and observed the direction in which it drifted.

The thermos cup is tilted to the left.

Xie Chengzhou paused for two seconds on this detail, "The water flowed to the left," he said.

"Yes," Old Zhao said, "but that's not enough." He picked up the thermos from the water, tightened the lid, and explained, "The main pipe's water flow direction is correct, but there's also water flowing in the branch pipes, and the direction could be forward. Just looking at the floating debris can only rule out the one where the water is flowing in the opposite direction; it can't confirm the main pipe."

"How do we determine that?"

Old Zhao put his hand back on the wall. "Listen," he said, "not to the sound of water, but to the sound of the pipes."

Xie Chengzhou also pressed his hand against the wall and waited.

"The water flow in the main pipe is flat," Old Zhao said. "But in the branch pipes, where the water flows into the main pipe, there's a drop—even if it's only a few centimeters. That creates a slight turbulent sound, higher in frequency than the main pipe's sound, like a faint hiss, superimposed on the low-frequency sound of the main pipe." He paused. "Now listen, to the left and right, which side has that sound?"

Xie Chengzhou pressed his ear close to the wall and listened again.

Low frequency, low frequency, low frequency—

Then he heard it. To the right, within that uniform sound of water, there was a faint, higher-frequency superimposed sound, discontinuous and intermittent, as if the water had encountered some tiny obstacle somewhere, then bypassed it, creating a subtle disturbance.

"The right side," he said.

"Yes," Old Zhao said, "the right side is the branch pipe, where water flows in, creating a drop and a turbulent sound. The left side is the main pipe, where the water flow is flat and there's no such sound." He removed his hand from the wall. "Go to the left."

Xie Chengzhou noted this method down in his memo: "Methods for determining pipe branching: ① Use floating objects to test the water flow direction (excluding reverse flow). ② Listen to the turbulence sound—when a branch pipe merges into the main pipe, there is a drop in elevation, producing a high-frequency turbulence superposition sound; the main pipe does not have this sound. Determine the direction of the main pipe where there is no turbulence sound. Source: Lao Zhao, a plumber with thirty years of experience."

He paused on the line for "Source".

He used instruments, data, and blueprints on the construction site. This method has no instruments, no data, no blueprints, only the feel of concrete against his skin and the auditory discrimination ability accumulated over thirty years of experience. He doesn't have that ability. He has now learned its principles, but he knows he can't do what Old Zhao just did—fifteen seconds, close his eyes, and then say, "There's a fork ahead."

That was something he didn't have.

They reached the fork in the road, and Xie Chengzhou shone his flashlight to both sides, confirming Lao Zhao's assessment: the diameter of the pipe opening on the left was the same as the main pipe, while the diameter of the pipe opening on the right was slightly smaller, about two meters, which was the standard size for a branch pipe. They turned left.

After walking about twenty meters, Xie Chengzhou stopped, placed his hand on the wall, and listened for a moment.

There was no turbulence sound.

The main branch is on the right track.

He added a line to his memo: "Bifurcation Judgment and Verification: Entering the left-hand pipe, there is no turbulence sound on the wall, confirming it as the main branch direction. Old Zhao's method: Effective."

Then he started walking again, keeping his pace at the same level as Lao Zhao's, stopping every thirty meters to listen to the water pressure against the wall to confirm it was stable before continuing.

Old Zhao walked behind him for about five minutes without saying a word.

Xie Chengzhou noticed that he kept looking at the pipe wall, not casually, but purposefully. His gaze would linger on certain spots for a second or two before moving on. Xie Chengzhou followed his gaze several times but didn't find anything special. However, he didn't interrupt Lao Zhao. He knew what this "experienced person looking at something they were familiar with" meant—let him look, and wait for him to speak.

"The construction quality of this section of pipe is different," Old Zhao said.

Xie Chengzhou stopped. "What do you mean?"

Old Zhao held the beam of his flashlight close to the wall. "Look here," he said, "this is where the construction joint is."

Xie Chengzhou shifted his gaze to the wall. The construction joints were the joints left when the concrete was poured in sections, with one every two meters or so. He had noticed this when he entered the pipeline in Chapter Fifteen; it was a standard sectioned construction practice.

"Here," Old Zhao pointed, "the width of the construction joint is wrong."

Xie Chengzhou held the flashlight close to the construction joint and took a closer look.

Old Zhao is right. This construction joint is about five millimeters wide, about two millimeters wider than the other construction joints he had noticed before. The difference is very small, almost invisible to the naked eye under normal light, but under the direct light of a flashlight, the shadows magnify it, making it discernible.

"What does a wide construction joint indicate?" Xie Chengzhou asked.

"This means that this section of the pipeline was added later," Lao Zhao said. "The original pipeline didn't have this section. Later, for some reason, a section needed to be added. When it was added, different batches of concrete were used, and the shrinkage rate was different, so the joint was wider." He paused for a moment, "The structural integrity of the added section is generally worse than that of the original. When encountering changes in water pressure, this section is more likely to have problems."

Xie Chengzhou noted in his memo: "Supplementary section - discovered - construction joint width approximately 5mm (normal is approximately 3mm) - Lao Zhao's judgment: the supplementary section has lower structural integrity than the original section. Risk point when water pressure changes: this location."

Then he added a line below: "Old Zhao's observation ability: Differences in wall construction quality. Coverage: Beyond my assessment framework."

He closed the memo and continued walking.

After walking about forty meters, Xie Chengzhou discovered the third sensor protrusion.

The protrusion was on the right wall of the pipe, about 1.2 meters above the ground. It was oval-shaped and about 5 centimeters in diameter, almost identical in shape to the first sensor protrusion he had found in Chapter Sixteen. He stopped in place, not approaching, and looked at it for ten seconds. He felt a slight, mild, and intense stress, just like with the first sensor.

"Here's one," he said, "a sensor."

Old Zhao took a step back. "I've got it," he said. "I won't touch it."

Xie Chengzhou marked the location in his memo, then went around it and continued walking.

After walking about twenty meters, Old Zhao said behind him, "How did you know it was a sensor?"

Xie Chengzhou thought for a moment about how to explain. "I can feel it," he said, "a kind of...stress concentration, in the back of my neck." He paused, "not a very precise description."

"No need to be precise," Old Zhao said, "I understand."

Xie Chengzhou turned and glanced at him. "You feel the same way?"

“It’s different,” Old Zhao said. “It’s my feet.” He stomped his foot in the water. “After working in the pipeline for thirty years, my feet are very sensitive to water pressure. When the water pressure is wrong, my feet will feel a tightness, not pain, but tightness, like the ground is pushing up.” He paused. “I guess the feeling in the back of your neck is your body sensing a structural abnormality, but you’re using a different sensory system.”

Xie Chengzhou went through this statement in his mind.

"A sense of stress concentration"—that's how he recorded it in his memo, the name he gave to this feeling using engineering terminology. But he never thought that this feeling was "the body sensing structural abnormalities"; he thought it was just an intuition, something he couldn't quantify.

Old Zhao's explanation provides a framework: it is a sensing system, the same type as Old Zhao's foot pressure sensing, only the object of sensing is different.

In his memo, under the entry for "stress concentration sensation," he added a line: "Analogy: Old Zhao's perception of water pressure on the soles of his feet. Mechanism: The body's system for perceiving structural anomalies, not random intuition, but possibly with a physical basis. Further analysis is needed."

"Old Zhao," he said, "when did you start developing sensation in the soles of your feet?"

"After working in pipelines for about ten years," Old Zhao said, "it didn't happen at first, but then it did. The longer you do it, the more your body learns on its own."

Xie Chengzhou swallowed his words and did not respond immediately.

He worked on construction sites for twelve years. He couldn't pinpoint exactly when his "sense of focused attention" appeared; he only knew it started at some point and became increasingly accurate. He never considered that it was "the body learning"; he thought it was luck or something he couldn't explain.

Now he has another explanation.

He gripped the flashlight tightly, walked forward, mentally processed the information from the conversation, and stored it in his memory, planning to write it down in his memo when it was safe to do so.

The pipes continued to extend ahead of them, the sound of water echoing against the walls. The emergency lights of the second maintenance room were not yet in sight, but he knew they were somewhere ahead, waiting for them.