Walmart has relied for years on more than 140,000 warehouse workers to unload trucks, forklift pallets of merchandise into warehouses, sort and store the goods on towering racks, then reverse the entire flow to send them out to stores and homes.

It isn't an easy job. A worker might walk 20,000 steps a shift. It also isn't all that complicated -- unless you try to program robots to do it.

Consider the seemingly simple task called "breakpack" -- removing items from cardboard boxes to send out individually. It's a no-brainer for Walmart's humans. But it turned into one of the many challenges bedeviling the company's decadelong, multibillion-dollar quest to automate its warehouses.

Walmart and the robotics companies it hired to help searched in vain for a way to choreograph robots to sort merchandise into bins while zipping past warehouse workers, testing technology that either didn't work or cost too much. Eventually, they designed small robots on wheels with motion and obstacle sensors and are now installing them in more than a dozen warehouses.

"We are kind of in this peak complexity," said Rob Montgomery, head of supply-chain operations for Walmart U.S., who is overseeing the automation transition now under way at most of Walmart's roughly 200 U.S. supply-chain buildings.

Problems have multiplied. The cardboard shipping boxes Walmart has used for years are too big for the automated systems. The miniature robots that shuttle goods break down too often. The systems can't accommodate some oversize products, such as big bags of dog food.

What makes the automation campaign so tricky, Montgomery said, is that Walmart is re-engineering its warehouses without taking them out of service.

Many of the nation's largest retailers are struggling through similar automation transitions as pressure grows to boost sales and ship merchandise to homes faster without spending a fortune on additional workers or facilities. The rapid growth of Amazon -- which is ahead of its retail rivals on automation and earlier this year surpassed Walmart to become the largest U.S. company by annual revenue -- has dialed up the urgency.

"In many cases, investments in automation are an arms race" between retailers, said Rueben Scriven, senior warehouse automation research manager at market-intelligence firm Interact Analysis. "And then the Amazon effect is the big elephant in the room."

Retailers without the capital to invest heavily through a long period of troubleshooting are at a disadvantage. Last fall, Kroger said it would take a $2.6 billion charge to close three automated e-commerce fulfillment centers as a result of "the automated fulfillment network not meeting financial expectations." Kroger said it would use stores more often to fulfill online orders and focus on "capital-light" automation efforts.

Walmart expects spending on remodeling stores and rolling out supply-chain automation to peak this year and next. Company executives have said the automation push means it eventually will need fewer warehouse workers, and more of the jobs that remain will involve managing or fixing automated systems rather than lifting boxes.

Because turnover is high in warehouse jobs, the executives said, automation won't necessitate layoffs. "We still need people that unload trailers," said Montgomery, adding that overall sales growth could require as many warehouse workers as today.

Walmart executives said the automation should allow the company to process more inventory, faster, and with more precision, reducing the frequency of out-of-stock goods and reducing costs. They envision a time when a choreographed and predictable cadence of trucks arrive at stores loaded with pallets of goods needed at that moment. The pallets will be organized so store workers can pull them to a single aisle, and the items can be unloaded in the order they are stocked on shelves. That would make store worker schedules more precise and predictable.

First, the company needs to work out lots of kinks.

Technical challenges

For more than two decades, Walmart has used the same kind of cardboard box to shuttle many goods between warehouses and stores, stamped with a reminder that they cost the company an average of $1 and should be reused. Walmart plans to throw them all out in coming months. The new ones are 11/2 inches shorter and need to be taped closed, an added step that will cost money.

"It's a small change," said Phillip McIlrath, the manager of an Indiana apparel warehouse using the new boxes. "But associates doing the work down here, it's changing their daily routine quite a bit."

The new boxes, said Montgomery, will reduce stocking time at stores, and hence labor costs.

Walmart's supply chain feeds 5,200 U.S. stores that sell everything from groceries to clothing to tires, both off the shelves and online. The system, Montgomery said, needs to "handle almost anything that we could put into it, from any supplier and at any time. That has been a challenge."

Many of the nonperishable goods on store shelves are first delivered to warehouses that consolidate shipments from suppliers, then sent to 42 distribution centers. Dozens of separate grocery warehouses handle cold and frozen items. Still others handle many of Walmart's U.S. e-commerce orders, as do the backrooms of stores.

Most of Walmart's supply-chain buildings were built in the 1980s and '90s, so about 80% of its automation construction projects are retrofits. The company also has built several new automated grocery and e-commerce warehouses from scratch.

Different automated systems are needed to handle different items at different locations, often built by different companies. One system is designed to handle full cases of goods sent to Walmart by suppliers, another to sort individual items from their shipping cases. Other systems handle fresh and frozen food or e-commerce orders.

Walmart started trying versions of the systems more than a decade ago, often in a 1.6 million-square-foot warehouse in Brooksville, Fla., that processes nonperishable goods. Those systems are made by Symbotic, a Wilmington, Mass.-based robotics company in which Walmart has a 12.6% stake.

Walmart and Symbotic struggled to come up with a system that worked well enough to install in other locations. In 2019, they ripped one version out of Brooksville and rebuilt it from scratch. In 2022, Symbotic announced that it had struck a deal with Walmart to expand its systems to all 42 warehouses for nonperishable goods by around 2030.

The system stores goods in floor-to-ceiling, open-air metal structures where robots shuttle goods between truck unloading areas and storage racks, and other robots with arms that build 10-foot-tall pallets that are loaded onto trucks bound for stores.

For Symbotic, it was difficult to hit Walmart's goals, primarily the desired "through put," the rate at which goods are processed, said people familiar with the situation. Symbotic tracks the performance of its equipment on an electronic board inside its Massachusetts headquarters, hoping to snuff out problems quickly.

Automating Walmart's warehouses "takes time, hard work, and a strong partnership defined by trust," said Symbotic CEO Rick Cohen in an emailed statement.

Robot issues

A persistent focus has been the frequency with which Symbotic's product-shuffling robots break down or need maintenance. The robots can be thwarted by something as simple as dust buildup, and stoppages can create a cascade of other problems, said some of the people familiar with the situation.

Last year, under 10% of the robots were being fixed at any given time, said a Symbotic spokesman. The percentage has dropped to about 5% in recent months, he said.

From the start, one of Walmart's primary automation goals has been building a neat, tightly wrapped pallet in the exact order that it needs to be unloaded at stores, which would save on labor costs. Walmart and Symbotic haven't achieved the goal yet, but have made progress, said executives at both companies.

As more products move through automated warehouses, the Tetris-like task of building pallets should become more precise because software will have more items to choose from, said Montgomery.

Automation is driving up some costs. The monthly electric bill at Walmart's automated grocery facilities can be $800,000 or more during peak cooling or warming months, up from about $250,000 a month when operations were more manual, said a person familiar with the situation. The mechanical and electrical engineers needed to run the operations cost more than unskilled warehouse workers.

Earlier this year, Walmart was behind on the automation targets it laid out in 2023, when investors and journalists toured its Brooksville automation test warehouse. At the time, executives said by the end of January of this year, about 65% of stores would be serviced by warehouses using automation and 55% of e-commerce warehouse volume would be automated. By August, Walmart was close: More than 65% of stores were receiving some automated freight and over 50% of e-commerce inventory was moving through automated systems.

Brooksville is now nearly "fully automated," said Montgomery, which means about 80% of inventory moves that way. The other 20% is trickier. Items such as large-screen TVs and jumbo bags of dog food will need manual assistance for the foreseeable future, he said. Overall, he said, the company now has a handle on the technology it needs to automate.

Some Walmart suppliers need to adapt their products to work in the automated systems. A few years ago, during the run-up to the Thanksgiving holiday rush, workers in one of the new automated California grocery warehouses realized that its robotic arms fumbled large frozen turkeys. Walmart asked its supplier to reinforce their shipping boxes. Amid a last-minute spike in demand, nearby grocery warehouses were used to ship enough turkeys to stores.

Earlier this year, Walmart started quickly informing suppliers when and how their packaging presents problems.

Walmart hasn't figured out the best way to automate online orders assembled inside stores, not warehouses. It has invested nearly $1 billion since 2016 trying.

Automating e-commerce in stores is hard because space is tighter. And snafus can be maddening for customers who might be waiting inside cars in the parking lot or expecting home delivery at a specific time.

Walmart introduced a system built by Alert Innovation, another Boston-based robotics company, to 20 stores. But Alert struggled to meet Walmart's benchmarks on through put and other measures and faced a cash crunch, according to people familiar with the matter. In 2023, Walmart bought Alert for $400 million.

Walmart then sold the company to Symbotic last year at a loss for $200 million. Symbotic is currently installing a new test version in a Dallas Walmart and running the existing 19 locations, but doesn't expect to move forward with a workable model until 2028, Symbotic executives said on a recent conference call to discuss earnings.

Montgomery said Walmart's leadership team expects there will be messy moments, but top executives and the board view automation as a long-term project that's worth it.

"We know that if we wait till perfect," he said, "it'll take too long."