For the last 10 days, the homes in Riverside Estates just south of here have been jolted twice daily by dynamite blasts at a nearby Howard County sewer construction site.

Homeowners said they could tolerate the noise of the explosions, which came punctually at 4 and 4:05 p.m., but on Monday, they said, more than 30 softball-sized stones showered their houses and the cul-de-sac where their children play. The neighbors who live on Cleos Court became more than frightened.

They got angry and complained to county and state officials, who immediately ordered a temporary halt to the $2.5 million sewer project along the banks of the Middle Patuxent River.

"I know it was an accident, but this isn't the first time it happened," said David Pulford, who said his garage roof was pierced by a stone.

Last month, an explosion at a construction site north of Pulford's home sent a stone through the kitchen roof of a house on Vista Road, west of Rte. 29 north of Cleos Court, Pulford said. No one was injured, although Yogi Barrett, the homeowner, was sitting in a nearby room at the time, Barrett said recently.

"I thought the house was falling down when I heard the crash yesterday," said Bonnie Thomas, whose family lives next door to Pulford. The 4 p.m. blast hurled a wedge-shaped stone roughly 550 feet over a stand of tall trees and onto the Thomases' roof, where it glanced off a gable, Thomas said.

The Thomases and Pulfords said they later discovered more than two dozen stones in their cul-de-sac where neighborhood children frequently play. No one was injured.

On Wednesday, representatives of The Marbro Co., a Beltsville construction firm and the chief contractor for the Middle Patuxent sewage project, are to meet with James M. Irvin, acting director of Howard's public works department, and present a plan to prevent such incidents.

"We never felt something like this would happen--what with the trees there and the blasting being done 500 to 600 feet from the houses," said Jim E. Thompson, Marbro's general manager.

The Marbro employe who set the charges Monday was a demolitions expert who had an "excellent" record with the company, said Thompson, adding that the employe was later reprimanded.

Thompson said that work crews apparently had taken all necessary precautions before setting off Monday's explosion, which was designed to excavate an 18-foot-deep trench for sewer pipe. "We're still not exactly sure what went wrong," Thompson said.

Irvin said he expects the incident to delay the 4-mile sewage line's completion by at most three days.