Rate of Settlement of Silt Alluvium at Trojan

by Derek H. Cornforth, (F.ASCE), Cornforth Consultants, Inc, Portland, United States,

Document Type: Proceeding Paper

Part of: Vertical and Horizontal Deformations of Foundations and Embankments


Settlements of 1.83 m have been observed at the switchyard of the Trojan Nuclear Power Plant over a 22-year period since 1970. Rockfill had been placed on the site, a flood plain of the Columbia River, to a relatively uniform depth of 7 m. The fill bears directly on 33 m of soft, compressible silts and clays which are underlain by relatively incompressible sand deposits. Because the fill covers a very wide area (380 m times 750 m), the loading conditions offer a rare opportunity to observe the rate of settlement behavior of a deep stratum of layered alluvium under one-dimensional (vertical) settlement. The results show that the alluvium essentially follows the Terzaghi theory of one-dimensional consolidation, i.e. the layered alluvium behaves as if one `composite' material is being loaded. The practical significance of this result is that it demonstrates that the Terzaghi theory can be used to monitor settlements of alluvium in the field as, for example, in preloading construction sites. The settlement analysis also showed that the total settlement observed to date, at the end of the primary consolidation, is in good agreement with the calculated settlement from laboratory consolidation tests.

Subject Headings: Soil settlement | Soil analysis | Consolidated soils | Alluvium | Silt | Power plants | Clays | Soil compression

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