Mining - Foundations for Mill Construction on Clay and Permafrost

PROPERTY of Malartic Gold Fields Ltd. is situa-ated in the great clay belt in the northern part of the Province of Quebec. This belt represents the floor of the glacial lake Barlow-Ojibway. The clay blanket has a gently rolling, almost flat surface, with here and there projecting knobs and ridges that outline the large irregularities of the surface on which the clay was deposited. At irregular intervals glacial eskers dominate the landscape. The ore-bearing country rock is covered by strata of clay, sand, and silt to a depth varying to 40 ft. There are serious structural problems when it is necessary to place loads of unequal intensities on this surface to support buildings and machinery for a modern mining plant. Earlier mining companies and their engineers paid little attention to the science of soil mechanics, partly because they believed it unnecessary and partly because they knew little of the subject. The results are evident in some of the tottering structures in older camps. Careful analysis of soil problems is well worth-while, since cost of foundations may vary from 20 to 50 pct of the final cost of a structure. Low operating and maintenance costs can be achieved only in this way. If it is definitely established that a plant will be in operation for only a few months, this limited time factor will probably influence the design of the foundation, and rightly so. If sensitive and complickted equipment is to be mounted on these foundations and the plant is to operate for several years, there should be a thorough study of foundations and a very careful design. A familiar problem is presented by the difficulty of operating a permanent plant originally designed for a few months' work. The engineer must beware of these hazards and disassociate himself from them if he cannot prevail on the promoter to spend the proper amount for a thorough foundation study. Laws of certain provinces and states hold the engineer equally responsible with the contractor for any 'damage that may be caused to building through inadequate study-ignordnce of the facts is no excuse in the eyes of the law. Types of Soil: The engineer should be familiar with such terms as silt, clay, peat, and varved clay, as well as perpafrost, especially if he is to work in the far northern reaches of the country. He should also be familiar with soil attributes such as porosity, texture, consistency, permeability, compressibility, consolidation, and shearing resistance. The design of any foundation requires an accurate knowledge of the physical properties and location of the soils involved. Older textbooks on the subject outline various empirical methods to determine the carrying power of soil. Tables are given in reference books. The modern ehgineer should be wary of these modes of thinking. He should analyze his problem carefully, calculate to what depth of soil the loads may be transmitted, and then check the bearing value of all soils to that or greater depths. If the strata are water-carrying, he must determine the possibility of this water being accidentally re-