City to conduct sinkhole investigation

This is a press release courtesy of the City of Portland

The Portland Office of Transportation responded to a sewer backup early Tuesday afternoon on SE Oak Street between 18th and 19th Avenues. As maintenance crews were trying to locate a manhole from which to clear the blockage, the street gave way at SE 16th and Oak. A high pressure sewer cleaning machine fell in a large sink hole, rupturing a natural gas line and breaking a water main. Two Transportation Maintenance employees were taken to Emanuel Hospital. They were released Tuesday night and are recovering at home from minor injuries.

The cause of this highly unusual event is unknown. City crews will conduct an investigation to determine what caused the event, what the problems are below the street, and what repairs are necessary. What is known is that voids or cavities like these are caused by some unusual water condition underground.

The investigation is expected to take a week or longer. There are a number of variables to look at and a number of utilities involved. Because the sewer line is the deepest, it will be repaired first. Then the water and gas lines will be repaired. After the hole and all cavities are backfilled, the street will be repaired.

The sinkhole measures approximately 20 feet by 15 feet at the opening, 20 feet deep, and 30 feet by 40 feet or larger below. The city is in the process of determining how to stabilize the hole and street to complete the investigation. Beginning as soon as possible, crews will be working within blocks of the sink hole to clean and videotape the sewer line. Meanwhile, sewage appears to be contained and flowing in what is left of the pipe.

The affected sewer line segment, built in 1924, stretches 324 feet from SE 16th Avenue and Oak Street into the Buckman playing field. An inspection in July 2000 revealed the sewer line to be in very good condition. Given the results of that inspection, an event like this was totally unexpected. Therefore, city engineers believe this is a fairly recent phenomenon and are looking at all utilities and possible sources of the problem that caused the hole to form and the street to collapse.
Sink holes this size are a rare occurrence; the city sees one every 10 or 12 years. A normal event is a shallow hole between 6 and 12 inches deep. It starts to form beneath the street, the asphalt softens and dips, and there are visual clues that something is wrong. City engineers believe that whatever caused this event likely occurred quickly.

The intersection of SE 16 Avenue at Oak Street is closed and will remain closed for a week or longer. Motorists are advised to stay away from the area so that crews can safely conduct their investigation and get the street back together. The city will work to minimize disruption to residents and others needing access to that part of the neighborhood.