This just in from our Public Works Administrator:
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Latest update on the situation with the Royal Hills Sewer line.

Internal TV inspection has revealed that the second sink hole in Royal Hills Dr. was caused by a large hole in the pipe line a few feet upstream of the portion that was replaced on June 8. The sink hole has been filled with crushed rock, and the entire width of the street continues to be covered with 20-foot wide steel plates above the pipe. One lane of the street will be closed to traffic and barricaded over the weekend, the other lane will be open to traffic.

The TV inspection has revealed that the entire section of pipe between the MH next to Royal Hills Dr. and the downstream MH in the Royal Hill Apt. complex (about 200-feet of line) is in a near failing condition and has to be replaced soon. The preliminary results of the TV inspection indicate that the pipeline going up the hill to Tiffany Park and the pipeline downstream of the 200-foot section that is failing are in better shape, and may not require immediate attention.

Two methods of replacement of the 200-foot section are being evaluated and priced: replacement with a more shallow line about 4 to 5 feet deep with transition manholes at both ends, or pipe bursting, which involves inserting a device in the failing line which bursts the walls of the pipe and allows a high density polyethylene pipeline to be pulled thorough the passage to serve as a replacement pipe. We will obtain cost estimates for these two alternatives early next week.

Because of the need to proceed with replacement soon, this may constitute an emergency situation. Declaration of an emergency would allow construction without the need for competitive bidding, which would save time. This also will be evaluated early next week.

No construction work on the line is planned for this weekend. However, a call out maintenance worker will visit the site every 8 hours to check the pipeline trench for possible widening, or for development of other sinkholes. If a problem develops, a crew will quickly be mobilized to deal with it. The bypass pump will remain in place on Index Ave. up the hill in Tiffany Park, but will not be running unless an emergency happens over the weekend. If a problem is identified, sewage can once again be quickly bypassed around the deteriorated pipe.

No sewer overflows from this line have been noted since June 2.

Gregg

Gregg Zimmerman, P.E.
Public Works Administrator
City of Renton

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While the Renton sinkholes were extremely dangerous, they did not come close to the startling hole that opened up in Guatemala last week.

In Guatemala, a horrendous 300 foot deep pit opened up, killing several people and swallowing a building.

Guatemala Sinkhole which opened up this week during record floods. This could be a very difficult situation to repair.

(Of course, the circumstances that created the Renton sinkhole and the Guatemala sinkhole are different. The Renton sinkhole is man-made, as a break in a sewer line started sweeping away soil supporting the road. The Guatemala hole came about from the regions volcanic geology which allowed the creation of a subterranean void that became exposed during record flooding. Sinkholes are usually about water in some way. They typically develop where water has moved soil or dissolved soluble rock over a period of time. There can be man-made contributing factors beyond the presence of leaking pipes. For instance, the presence of underground mines –which we have in parts of Renton, or excessive changes in underground water conditions –which we work to monitor in Renton as we pump water from our aquifer.)