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Technical paper: Surcharging a deep opencast backfill

By D Burford. This paper was first published in GE’s September 1991 issue.


A 10ha area of restored opencast fill has been preloaded prior to the construction of a housing estate. The 24m deep backfill was temporarily preloaded with a 50m square by 7m high surcharge of earthfill to improve its load carrying characteristics. Some 300mm of surface settlement occurred, the amount of settlement decreasing with depth of 611; at 10m below ground level only 10mm of settlement was recorded. Most of the settlement occurred during placement of the surcharge. Because creep settlement was very small, it became unnecessary to leave the surcharge in place for more than one month. The surcharge was moved progressively around the site, and, as it was removed, a small amount of heave of the surface of the surcharged backfill took place. Plate loading tests after surcharging showed immediate settlements of no greater than 7mm at loads of 80kN/m . 

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