Modern Gasworks Practice

42 MODERN GASWORKS PRACTICE foundation pressures can be adopted. In such cases long experience is the principal base to work from, and even then considerable latitude is left to the personal factor and eyen th.e Imagination of the engineer. There have been cases, though. rare, in which. the borings have been carelessly and inaccurately made and wrongly de-scribed, and sometimes the difficulties of putting down borings are so great tliat hope of information, from that source must be abandoned. Before the excavation proper for foundations is commenced steps must be taken for the adequate control of all surface and sub-surface water ; in fact, it may in abnormal cases be found profitable to lay down a drainage system which. is capable of dealing with water to a deptli somewhat below that of the proposed excavations. This procedure is, however, decidedly costly, and should only be countenanced in extreme instances. FOUNDATIONS When the depth of an excavation exceeds a few feet, some method of timbering must usually be employed, whilst when the depth is greater than 10 feet, considerable precaution is necessary, so much so that the Board of Trade prescribe that in such cases an expert should be called in. The excavation of ground for gasholder tanks calls for particular care and experience. In general, foundations are classed under the two headings of natural ” and “ artificial,” according as to wh.eth.er the ground requires special treatment other than shallow excavation. In this country the four most common subsoils to be met with are (a) ordinary earthy material, (ö) hard and soft clay, (c) gravel, and (d) chalk; and it is the compress-ible soils of the soft clay and earthy types which demand the greatest amount of attention. It may be said that as a general rule (particularly where foundations for retort bench.es are concerned) it will be profitable to put down a solid block of concrete, so long as the total deptli does not exceed 5 feet. Beyond this the procedure be-comes expensive, and one of the various “ artificial ” methods should be resorted to. The cost of excavation will, of course, largely depend upon the material which is bemg taken out;. but if a good ganger is in charge, he will often find some means of selling the débris, or of getting it carted away by a local contractor free of charge. W hen the conditions of the substrata necessitate a comparatively deep foundation, or when the soil is more or less compressible, the following methods may be employed :—: (a) Concrete piers. (b) Floating foundation. (c) Piles, either wood, metal, or reinforced concrete. Particulars of the pier methods are sliown in Kg. 2, from which. it will be seeu that the complete area is first excavated to a deptli of a few feet, whilst at intervals shafts are sunk through the soft soil on to the stable ground. The dimensions of these shafts vary with. the purpose for which the foundation is required, but they will usually run about 2 feet in width, and extend laterally the entire breadtli of