Big delivery trucks will be banished this year from most curbsides in Seattle’s historic Pioneer Square neighborhood, leaving bars and restaurants to restock some other way.

The Seattle Department of Transportation (SDOT) announced the policy Tuesday, to forbid vehicles heavier than 10,000 pounds from parking or driving in the outer lanes next to potentially fragile underground passageways built in the late 1800s.

Vehicles such as box trucks and minibuses must keep to the interior lanes on four-lane roads, starting with First Avenue South. By midsummer they won’t be able to turn right from some blocks, where the right lane is off-limits. Restricted blocks will be marked by new signs.

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First Avenue between Marion Street and Railroad Avenue will be restricted as soon as May 6. Curbside weight limits would extend to sections of 21 city blocks by early September, the city says. SDOT will treat June and July as a transition period, by opening about 16 new legal load zones while repealing others.

The new limits are meant to protect “areaways,” the hollow passages beneath Pioneer Square sidewalks created after the 1889 Great Seattle Fire, when settlers rebuilt streets one story higher than the original roads. They serve as basements, storage cellars and corridors for the city’s famed underground tours.

If soils under the outside lane are overloaded by heavy vehicles, the force could push weak areaway walls into the hollow passages and toward buildings, city engineers fear. That hasn’t happened so far, but some spots are marked for ongoing measurements, said Lorelei Williams, SDOT capital-projects director.

The walls are nearly 130 years old, beyond the common 75-year life span of modern, sturdier infrastructure.

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“Obviously, the areaways are very old, and we’re learning that they’ve been in challenging condition for years,” said Williams, who intends to develop a long-term strengthening plan.

SDOT engineers studied the neighborhood’s areaways in preparation for this year’s King County Metro Transit bus detour through First Avenue, until the Alaskan Way Viaduct is demolished and new waterfront bus lanes are built.

The truck bans are a spinoff of their transit-detour analysis.

When the bus detour began mid-February, Metro instructed its operators to stick to the interior lanes. Some bus drivers travel curbside anyway, to cope with heavy traffic. In March the city added a curbside bus stop on northbound First Avenue approaching South Jackson Street — a rare spot with a strong rebuilt areaway — at the behest of commuters.

Some longtime residents have asked why make the change now, given that Metro ran some bus lines along First Avenue in past decades, before permanently shifting those to Third Avenue.

Williams explained, “It’s 20 to 30 years later, so you have 20 to 30 more years of deterioration on the walls. The more we continue to load the walls in the same way, the more they will deteriorate.”

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Several walls were constructed of brick-and-mortar, others from early unreinforced concrete. Mortar flakes off with the swipe of a finger, said Matt Donohue, city roadway-structures manager. A few curbsides are sturdier, such as those around the Colman Building at First and Columbia, where SDOT recently filled one areaway and rebuilt another for $2.4 million total, he said.

Lisa Howard, executive director of the Alliance for Pioneer Square, said “it’s hard to say whether the city should or should not” impose loading limits, unless she sees more data about vulnerability of specific blocks.

Howard called on SDOT to accelerate further technical studies, in hopes some of the 21 blocks can be approved for heavy loads. “I don’t think it’s realistic that it’s permanent,” she said.

Instead the city should strive to make the 10,000-pound limits temporary, she said, by finding money to repair, reinforce, rebuild or fill the areaways on a case-by-case basis.

Outreach begins this week to affected businesses. Signs will be installed soon. Enforcement plans are unclear, but state law sets a $48 citation fine for improper parking.

A leading option will be to conduct deliveries in the area with lightweight Sprinter vans, said city spokeswoman Emily Reardon. United Parcel Service last fall launched electric-bicycle delivery wagons downtown.

Howard also expects more deliveries in alleys, or on approved curbsides where the goods must be rolled a bit farther to the client’s door.

The restrictions may alter the Downtown Seattle Association’s blue Waterfront Shuttle minibuses, where one street path and one stop might have to shift slightly in late summer.

Charter tour buses should be fine, Howard said, because they already use a solid parking area on Main Street next to Occidental Park. Personal cars, SUVs and pickups can be driven and parked as usual.

“It’s still safe to come down and support the small businesses that are here,” she said.