SAN ANTONIO — Construction crews moved dirt to shore up a group of houses precariously perched on a crumbling hill in San Antonio on Monday as engineers tried to determine why the land below was shifting, causing dozens of homes to evacuate.

Gaping crevices, some 15 feet deep, cut across several yards as dirt cascaded into a towering stone retaining wall that nearly split in half. Fences crumpled like accordions as crews packed dirt under one home and around its exterior after part of its foundation was exposed.

One soil expert said the cause of the landslide appeared to be the result of poor retaining wall design, and a city official said the nearly 1,000-foot-long wall in the upper-middle class neighborhood of sprawling two-story homes was built without a permit.

No one has been injured, but about 80 homes were evacuated on Sunday after a resident in the northwest side subdivision reported that his backyard was sliding down hill. By Monday afternoon, residents in about 55 of those homes were allowed to return after inspections and soil monitoring found them to be safe, said Valerie Dolenga, a spokeswoman for Pulte Homes Inc., the parent company of the neighborhood’s builder, Centex Homes.

One neighbor who was among the first homebuyers in the subdivision set among rolling hills on the outskirts of San Antonio said he was initially told no homes would be built on the crumbling ridge because it was too steep.

Romeo Peart, 32, said one retaining wall failed several years ago before the current one was built and homes were constructed above it.

“They can keep the view now,” Peart said, shaking his head as heavy equipment stuffed dirt beneath an exposed foundation. “And they paid an extra $10,000 for those lots.”

The development, which was started in 2004, has nearly 750 homes with others under construction. The neighborhood, with houses selling for $250,000, is one dozens that have sprung up on hilly former ranch land as San Antonio grew to be the nation’s seventh largest city.

The near-vertical retaining wall likely failed under the weight of the area’s clay soil that readily expands when drenched with heavy precipitation as it was last week, said Sazzad Bin-Shafique, an assistant engineering professor and soil expert at the University of Texas-San Antonio who went out to the site on Monday. Steep, tall retaining walls can hold up if built correctly, he said.

“It’s safe, honestly. We have engineering solutions, but sometimes we do something because we want to reduce costs,” Bin-Shafique said. “Many times, it will be OK, but sometimes, it will not.”

Roderick Sanchez, the city’s planning and development director, said the builder built the retaining wall without a permit. The city was still waiting for verification that the wall was designed by a certified engineer and built to specifications, Sanchez said.

Dolenga said the city approved construction plans for the subdivision including the retaining wall, though she said the company was investigating the permit allegation. She said she didn’t know if the street with the now-jeopardized homes was added later to the subdivision’s plans, though developments are usually built in phases.

“We’ve been building out there a long time. This is an unusual circumstance,” she said.

Engineers spent Monday assessing each of the structures in the evacuated area, while fire officials escorted some families to retrieve belongings from the neighborhood. At least seven homes would likely remain vacant for an extended period, said Fire District Chief Nim Kidd, who is also the head of the city’s emergency management office.

Kenny Crawford, 32, asked fire officials to be allowed to retrieve his car and some belongings on Monday, but because his home is directly below the disintegrating wall, he and his girlfriend were told it was too dangerous.

“They really haven’t given us any info,” Crawford said. “We don’t know what’s going to happen. Of course, property values are going to fall.”

Dolenga said geologists and engineers were looking for a cause of the slide and monitoring for any additional movement of the dirt that was sliding at a rate of 4-inches-an-hour on Sunday. She did not know if there was additional movement on Monday.

Utilities were cut off in the area, and construction crews were moving dirt to shore up the homes on the hill and to protect those below the retaining wall.

Resident Lakeika James, 41, said she had noticed odd noises over the three years she has lived in her house.

“I would hear, laying in my bed at night, grumbling and vibrations. A few nails popped out lately,” she said.

She said she hadn’t planned on staying in the house long-term, and now after the landslide, the mother of a 5-year-old girl wants out.

“I’m just going to be uncomfortable and worried for my family,” she said.