"It was probably an unlicensed contractor doing work illegally on Hawai'i Loa Ridge who was too lazy to take it anywhere else," Wiseman said. "Since it was construction material, it tends to give a bad image to our industry. But we stand up for green building practices and recycling and taking things properly to the dump and paying the dump fees."

Whoever dumped the material into the city's sewer system violated local and federal environmental laws, said Jim Baginski, acting assistant chief of the division of collection system maintenance for the city's Environmental Services Branch.

Baginski believes someone lifted a remote manhole cover — which can weigh up to 150 pounds — in a discreet location on Hawai'i Loa Ridge. They then poured in plastic material, plywood and three 3-foot-long, 4x4 wooden posts, Baginski said.

"The only possible way to get this into the system is to open a manhole and throw it in," Baginski said. "These lids are pretty heavy. It would be tough to open them with a screw driver. We use a manhole pick and they're still cumbersome to remove."

The debris flowed through the system until it backed up Saturday in a 12-inch-diameter sewer main near Kawaiku'i Beach Park along Kalaniana'ole Highway.

The clog caused an estimated 100,000 gallons of untreated sewage to overflow, which was the largest of six spills over the weekend that totaled 160,000 gallons.

Cleaning up the jammed sewer line took half-a-dozen workers 12 hours on overtime, Baginski said.

Last year, city crews responded to 14 similarly plugged sewer lines that were clogged with such things as bicycle frames and boulders, Baginski said, adding that the number of reports has recently dropped from a high of 30 in 2000.

"I guess people have nothing better to do than plug up the sewer system," he said. "Any place it goes in, it's going to cause a spill."