REPAIR ON RAIL OVERPASS SET TO BEGIN

CHAFFEE -- A spokesman for the Missouri Highway and Transportation Department at Sikeston said work on the earthen approaches to the Highway 77-St. Louis Southwestern Railroad overpass north of Chaffee should begin around mid-May.

General contractor for the project is the bridge and grading division of Robertson, Inc., of Poplar Bluff.

Highway department engineers hope the project will end an earth slide problem that has damaged the four sides of the earthen ap~proaches to the bridge during the past ten years. Two previous attempts to stop the subsidence failed.

A ground-breaking ceremony was held at the overpass Friday to mark the start of the project, but Bob Simpson, highway department district construction engineer, said the contractor won't begin work until there is prolonged dry weather.

"The contractor will be putting up concrete barriers and safety signs, but work won't actually start until we are assured of long periods of dry weather," said Simpson. "We don't anticipate that happening until at least mid-to-late May.

Weather permitting, Simpson said the work should be completed by early October.

Simpson said concrete barriers will be placed on both sides of the highway, along with speed limit signs. "With the type of work going on, and the potential danger involved to the workers and motorists, we are asking motorists to slow down," he said. "There will be no shoulders on either side of the road. There will be about a 45 degree drop off on each side of the pavement. If you would happen to crash through one of the concrete barriers, it will be about a 25-foot drop to the ground."

Simpson said the earth subsidence is caused by the type of soil used to build the overpass in the early 1960s. The soil was taken from two borrow pits located on the west side of the overpass.

Simpson said during dry weather, the soil dries out, leaving large cracks. When it rains, the water pours into the fissures where it is absorbed by the soil which then softens into a clay-like texture. As the soil continues to absorb moist~ure, it expands and begins to slide.

In 1984, the state injected a slurry of lime-fly ash (cinder) into the northeast quadrant of the earthen slopes to try to harden the soil and make it impervious to moisture.

Simpson said the it seemed to work, so the other three quadrants were injected with the lime-fly ash slurry mix in 1986 at a cost of $347,000. Then, engineers waited to see if the problem had been resolved. It wasn't.

For a while, the earth held. But during the past three years, the subsidence has started again, this time on all four sides.

"It appears we were not able to inject enough of the lime-fly ash slurry into the earth to stop the sliding," Simpson said.

This time, the state has taken a different approach to solving the problems. "What we plan to do is to actually take off just about all of the slopes, leaving only the central core that carries the roadway to the bridge," Simpson said. "Instead of the present 3.1-to-4 slope ratio, we're going to cut the slope back to a 1.5-to-1 ratio. All of the dirt that is removed from the slopes will be stockpiled near the bridge.

"As we rebuild the slopes, pure, dry lime will be mixed in with the dirt by a heavy equipment tiller. At the same time, we'll install a layer of sand against the 1.5-to-1 slope near the roadway, and another layer of sand at the bottom of the fill."

Simpson said studies indicate the earth mixed with lime will change the characteristics of the soil, making it more resistant to moisture and expansion.

"The layer of sand will act as a chimney to funnel surface water directly to the bottom of the slopes, where it will drain into nearby ditches," he said. "The idea is to divert the water away from the treated soil."

Total cost of the project is estimated at $857,431. But Simpson said that's still cheap compared to what it would cost if all of the earthen approaches to the overpass were taken out and new ones constructed. "We certainly hope that it won't come to this," he said.

"If that happened, we would have to close the highway to all traffic for a long time."

Simpson said when the overpass was built in the 1960s, very little was known about the characteristics of the soil when it is wet and dry. "Obviously, we've learned a lot since then," he said. "For example, when we built the Rockview railroad overpass, we did not use the soil from that area. We took earth from the hills to the east of Rockview to build the approaches, and let them compact for nearly a year before we built the highway."

Simpson said there have been other similar subsidence problems with earthen approaches to bridges along Interstate 55 in the Bootheel, and a bridge on Highway 62, near Howardville.