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ecuscino | Created: 17 Dec 2024 | Updated: 19 Dec 2024
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THE HARDPAN BOYS: THE CLUE OF THE TILTING WALL

One brisk spring day, famous teenage detectives and civil engineering students Frank and Joe Hardpan went for a walk around the city of Bayport. The youths had scarcely begun their stroll when a segmental retaining wall at the corner of Main and Oak Streets caught their eyes.

“Gee whiz, Joe!” Frank exclaimed. “That wall is really tilting out of alignment!”

“I’ll say, Frank,” Joe answered alertly. “Don’t you think this could be a really swell civil engineering mystery?” He pulled out his trusty cell phone and began taking some pictures.

 

Photo of retaining wall

 

Photo of a retaining wall

 

Photo of a retaining wall

 

“Absolutely,” Frank replied, “but we may already have the answer to it. Do you remember,” he queried, “how we talked about retaining walls and their failure modes in our Introduction to Geotechnical Engineering course?”

“Holy smokes, that’s right!” Joe responded. “I think we talked about them in our Foundations and Earth Pressures course, too.” He scratched his head, his countenance puzzled. “But I can’t remember what precisely we learned from Professor Gwyer.” Mack Gwyer was one of the most esteemed professors in Bayport College’s Civil Engineering Department and had taught the Hardpan brothers in their Intro to Geotech and Foundations courses.

“Well,” Frank Hardpan declared, “Professor Gwyer taught us at length about how retaining wall problems often occur due to the build-up of water behind walls.”

“Say, wait a minute, Frank,” Joe chimed in, snapping his fingers, “look at these weepholes!” He pointed to several drainage pipes protruding from between the wall’s concrete masonry units. “Only part of the wall has them, and that section is perfectly vertical. Yet the portion of the wall without weepholes –”

“– is the part that’s overhanging!” Frank excitedly completed the thought. “Joe, I’m almost sure we’re looking at a case of water build-up behind a retaining wall. This is something Professor Gwyer should know about.”

“You bet!” Joe assented, grabbing one last photo. At once, the Hardpans were off to the college’s Engineering Center, where Professor Gwyer was in his office eating lunch.

“Well, if it isn’t two of my favorite students – the Hardpan brothers!” Mack Gwyer cried. “How can I be of assistance, lads?”

Frank and Joe sat down with Professor Gwyer at his office table and reviewed the facts of the case and Joe’s pictures with him. The seasoned engineer listened patiently to the lads’ story and reviewed their photos with keen interest. After the boys had finished and the last picture had been examined, the Professor leaned forward, pressing his index fingers together. “Well, chaps,” Mack Gwyer stated with a grin, “I believe your theory is right. It looks to me like you’ve cracked the case.”

“Thank you, Professor,” said Joe with polite embarrassment, “but we have just one question more. As you’ve seen, our pictures show that the wall has been backfilled with free-draining gravel. Why, then, would there be a water buildup occurring behind the wall, even along sections without weepholes?”

“Good question,” Professor Gwyer noted, cocking an eyebrow. He pulled a faded geotechnical report from his battered rolltop desk and showed it to the Hardpan brothers. “One possibility is that the part of the wall without weepholes is backfilled with natural soil, not gravel. This report is from when the college built its Rec Center several years ago, just a few blocks from this wall.”

“Oh,” murmured Frank, glancing over the site’s boring logs, “I see what you mean, Professor.  The natural soils at the Rec Center site are primarily silts and clays. Perhaps the wall site has similar in situ conditions, and the weephole-less section of the wall is backfilled with natural soil. In that case, the soil’s rather impermeable nature likely prevents the free drainage of water that builds up behind that part of the wall.”

“Correct, Frank,” Mack Gwyer observed. “Plus, even if that part of the wall has gravel backfill, it may be getting fouled.” The professor noted the boys’ perplexed looks and elaborated, “Fouling is when the void spaces of normally free-draining materials such as gravel become clogged with fine particles. The gravel could be breaking into smaller pieces on its own; topsoil could be washing down into the void spaces; underlying silts and clays could be migrating upward into the gravel voids. Regardless, the result is the same. The voids get clogged, the gravel is no longer free-draining, and water builds up behind the wall.”

Professor Gwyer reached across his table and reclaimed the geotechnical report. “Well, fellows, I need to go to coach the GeoWall team’s practice. Good work in solving this little caper.”

“Thank you, Professor. We’re lucky to have your technical guidance close at hand,” Joe Hardpan replied as he and his brother headed out the office door.

“It’s just fortunate,” Frank turned and added with a grin, “that both Joe and I wanted to go for a constitutional today. Otherwise, we might never have noticed The Clue of the Tilting Wall.”

 

Watch your bookstores for upcoming HARDPAN BOYS adventures, The House on the Unstable Cliff and While the Slope Slipped!

Member-author G.O. Teck also writes as Michael Bennett, P.E., M.ASCE.