Hillside slide dangers

Letter to the Editor

Posted: Monday, April 08, 2002

A frightening development has occurred in the Planning Commission deliberations regarding South Franklin Street. They are going to discuss it again this Tuesday night. There is something they do not understand which must be clarified.

The project that threatened to cause a landslide south of the House of Russia was "monitored" for hillside movement. Movement did occur, but the monitoring mechanisms did not catch it. That was because no engineer connected with the city or the project thought that the movement would take place high up on the hill.

Despite warnings from citizens and another engineer, residences near Gastineau Avenue experienced slippage of the hillside out from under them. Naturally, many people, including the Planning Commission, are concerned that this may happen again.

The frightening problem is that the wrong conclusion has been drawn from the monitoring failure. Expanding the monitoring to include the upper part of the hill, which has been proposed, is not safe.

What the Planning Commission and the developers do not seem to understand is that monitoring cannot stop a massive slope failure. It is very possible that movement may start which cannot be stopped.

Not only is the slope the result of past landslides off the mountain, and thus very loose, but it contains slide planes. Between the layers of past slides, there are layers of compressed organic material. These organic layers are slick. The hill can slide easily on these planes.

This was brought home to me while watching a TV program about Mount St. Helens. It was mentioned that there were small eruptions going on while the pressure built up inside. Finally, a slide plane developed which catastrophically took off a slice of the mountain.

In our case, the vibrations of driving sheet piling and installing anchors have already weakened our hill. It is proposed to do this again much larger and much closer to the apartments. The weakened hill might form a slide plane on one of these organic layers.

A slide on such a slide plane would be unstoppable. It might be so disastrous as to take out South Franklin Street itself, according to one engineer I have spoken to. It would certainly destroy the apartments.

Monitoring is not a safe option in this radical situation. As an engineering department representative told the Planning Commission, "There are no guarantees."

Page Bridges


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