Summary

The choice of the public for a route for the Dalton By-pass lay through an extensive area of well-worked and abandoned haematite mine workings. The geology of the area and the method of mining are described, and the reasons are given for the selection of the preferred route.

The problems associated with the route are discussed, together with the detailed planning necessary to organize an extensive complex site investigation, involving almost 700 boreholes in a roadway length of 4½ km. A typical cutting is examined in some detail to illustrate the contractual and technical problems connected with the required excavation of a cutting slope directly over a major iron ore sop; these problems include the removal and treatment of the water-filled collapse cone over the rubble-filled cavern left by the ore extraction. The relative costing of an elevated structure and an embankment founded on collapsed workings are discussed.

Some engineering conclusions are drawn from the site investigation regarding the suitablty of embankment fill, the rate of settlement of embankments on alluvium, and the stability of a rock slope.

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