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Comparables:
Sitios Superfund

Distress_to_Structures_on_Loose_Ash_and-1.jpg

SUMMARY

This 1988 conference paper by Messrs. Gouda (the lead engineer for the Logan Triangle study), Lippincott, and Professor Raghu investigates why the homes in Philadelphia’s Logan section kept sinking for decades after they were built.

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THE PROBLEM

Logan covers 17 city blocks and 997 rowhouses, all completed by 1920. They were built on a filled-in valley of the old Wingohocking Creek. The creek was diverted into a 17-foot brick storm sewer, and the valley was filled with up to 40 feet of loose, uncompacted ash and cinders. The houses sat on shallow foundations directly in that fill. A 1986 city-commissioned investigation found:

  • 2–3 feet of total settlement

  • 1–2 feet of differential settlement

  • Settlement still ongoing

As a result, 100 homes were declared imminently dangerous and evacuated, 110 more were declared dangerous, and the rest were moderately damaged.

 

WHAT THE TESTING SHOWED

The engineers drilled 38 borings down to bedrock (Wissahickon schist). They ran lab, model, and field tests on the fill and found:

  • The fill was very loose. Blow counts were under six per foot, and in-place density was about 55–60 pcf, close to its minimum of 55 pcf.

  • It behaves like a loose, non-plastic sand and gravel. It is highly dispersive when loose and partly soluble in weak acid.

  • Water makes it collapse. In a tank test, flooding shrank the fill’s volume by 29%. In a field plate load test, adding a foot of water made the plate drop four inches instantly, and the fill lost its ability to support load.

  • It washes easily into pipes. Water flowing past a perforated pipe buried in the fill left the pipe half full of ash within an hour.

 

THE DIAGNOSIS

Ordinary consolidation would have caused only 4–12 inches of settlement, finished within about two years, and left the fill denser, yet it was still loose. So the authors ruled consolidation out as the cause of the continuing settlement. The real cause was piping: water carrying the ash and cinders away through broken pipes. They proposed this sequence:

  1. Early settlement cracked the cast-iron and terra-cotta utility lines.

  2. Water leaking from those lines weakened the fill.

  3. The weakened fill settled further and broke the pipes more.

  4. Eventually the cracks were wide enough for the fill itself to wash into the city sewers.

Camera inspections confirmed this, finding large amounts of ash in the sewers. In dye tests, water flushed from some toilets never reached the sewer at all and drained straight into the ground under the houses.

 

CONCLUSIONS

The authors expected the subsidence to continue indefinitely unless the piping routes were closed. They acknowledged that repair would probably cost more than the homes were worth. Still, they recommended:

  • Rebuilding the house utility lines and the city sewers

  • Adding street drain inlets to reduce flooding and water seeping into the ground

  • Underpinning any rehabilitated house with pin piles down to bedrock

They noted that the houses could be stabilized but never made level again.

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