THE ART OF PREPARING FOUNDATIONS, WITH PARTICULAR ILLUSTRATION OF THE "METHOD OF ISOLATED PIERS," AS FOLLOWED IN CHICAGO. By FREDERICK BAUMANN, Architect. Revised by G. T. POWELL, A. and C. E. WITH NINETEEN WOODCUTS. The art of constructing foundations comprises two distinct but inter-dependent parts: FIRST, the art of treating the ground; and SECOND, the art of building the base. FIRST PART. The Art of Treating the Ground. - All ground, from the nature of things, is compressible-will yield under pressure. This is owing to three different natural causes; FIRST, general compressibility of matter, which is so slight that in practice it causes no concern; SECOND, imperfect packing of the constituent parts and incipient fluidity, which induces to study and care, though positive artificial treatment be not needed; THIRD, semi-fluidity, which in most cases calls for positive artificial treatment. Accordingly, I shall consider the different building-grounds under the head of three distinct classes: solid grounds, compressible grounds, semi-fluid grounds. Class I. Solid Grounds. This class comprises rock, gravel, dry sand, in their natural beds, and of sufficient thickness of strata. The treatment is very simple, and in most cases alike. Excavations must be made to remove loose deposits and expose the natural bed. Surfaces must be made level, because bases should not be started upon inclined planes. In this manner the most common engineering routine will ever attain good results as to foundations. The ground being, for all ordinary practical purposes, next to incompressible, differences in the weights of the various parts of the superstructure produce no manifest defects. Neither is there any considerable manifestation of piers or corners deviating from the line of the perpendicular, though, perchance, such piers or corners were not centrally supported. Concrete or no concrete, inverted arches or no inverted arches, random work or work rightly considered, the result is practically ever the same; the slight deviations from the true lines, which may occur, pass unnoticed; the builder has nought to think on the subject; his common every-day routine suffices him in all his cases, and he remains in ignorance as to the proper principles by which the true art of preparing foundations is governed. Their practice was upon ground of the first class, which prevails in most of the large cities of the country, and taught them nothing to the point; nor could they avail themselves of the experience of others, inasmuch as, beyond this present treatise, there is (as far as at present known) nothing in print even pretending to give information. The evolution of the "method of isolated piers" is but the result of modern wants as to the construction of mercantile buildings. Class II.-Compressible Grounds. This class comprises clay and watery sand, and mixtures of the two, a whole scale of grounds, from the border of the first class downward to semi-fluidity. The sucessful erection of any ordinarily heavy structure upon such ground involves the consistent application of two well known (and often, though loosely mentioned) principles: FIRST, the areas of base must be in proportion to the superincumbent loads; SECOND, the centers of these areas of base must coincide with the axis of their loads. These principles are self-evident, well known, and often loosely mentioned, yet so seldom observed. It is, indeed, needless to prove that ten square feet of bearing surface, cæteris paribus, will bear more weight than will two square feet, or four, or nine. It is superfluous to specially make clear the fallacy of placing the axis of any load upon or near the edge of a base, or in any measure away from its very center. The natural result of such foolish proceedings would be that, as the ground yields, the base assumes an inclined position, and the axis, which must retain its original angle with the base, is thrust out of its perpendicular line, as represented by Fig. 1. It is not then these simple principles Fig.1 d that will occupy me; it is rather their varied and manifold application in the practice of this difficult "art of building," in which economy, rightly understood, is a principal factor, nay, in fact, the factor, which really renders it a science, which can only be attained by one who has acquired a manifold experience, and who previously has had such discipline of mind as to enable him to systematically collect, and assimilate with himself, the mental fruits of his labors. First Rule.-Resolve the building, upon its ground plan of the lower story, into isolated parts, and independently apportion to each its proper share of foundation. The first part of this is of old standing, and often applied in exceptional cases for instance, a church with a massive tower. But the mere keeping the tower separated from the other parts is of no avail, unless the latter part of the rule is observed, by special intent or by chance of circumstances, as the case may be. It is this matter of resolving a complex building into isolated parts, a task requiring experience and sagacity. Scarcely are there any two buildings alike in this respect, and the question ever arises, where shall I stop? With some buildings it may be simple, so that the old every-day routine may suffice. Second Rule.-Estimate the weights of all those (really and ideally) isolated parts, in order to apportion to each its due share of foundation. To this end it is required to know the bearing capacity of the particular ground, and also whether or not, and in what ratio, the load may be increased in proportion to the area of base. If it were found, for instance, that the medium bearing capacity (reduced to a convenient unit) is, say two tons per square foot-meaning that under such proportionate load the ground will be compressed in a limited known ratio-and if it were further known (approximately so at least) that this ratio holds good for any amount of load, the task is at once simple. A pier weighing 120 tons must receive a base pressing an area of 60 square feet; a pier weighing 20 tons must press upon an area of only to square feet, and so on in this proportion. It will be found, however, that the proportion varies with the nature of the ground. Ground least fluid and most solid (dry clay) will thus give too much support to the lesser loads; ground approaching semi-fluidity will give them too little. In each case, therefore, where the properties of the ground are not fully known in advance, tests must be instituted for their ascertainment, and the apportionment made accordingly. Third Rule.-Determine, upon the ground section, centers (and center lines) of all (isolated) parts, which in upright section will be the axis (and axial planes) of these parts, and place the (masonry) bases so that the centers of their areas of contact will coincide with the first centers. It means that foundations must be made to support their loads centrally. The observation of this rule is of the utmost importance, for upon it will depend the perpendicularity of all the walls and the corners of the structure. Let all parts have central foundations, and no inherent tendency will exist to disturb this perpendicularity. There will in such case be no particular need of any anchors, except for temporary use, while in the contrary case the strongest and best applied anchors will not suffice to preserve the exact normal position of the walls and corners. Have the bottom right, and all else will come right without many further precautions. I comprise the above three important rules under the head of "Method of Isolated Piers," which I advance as a scientific method in opposition to the old random method of continuous foundations. I am aware that isolated foundation-piers are of old date. Such isolation of piers has been, however, the exception, not the rule. Its origin is from chance and circumstance, not from logic. I, on the other hand, advance a principle which makes isolated piers the rule in all cases, and continuous foundations the exception, where, for instance, piers of uniform weights are so close to each other that the bases will interconnect. Objection might be raised to this new method, on the ground that any building-ground may not be everywhere of the same uniform density. This circumstance will but, seldom occur, and if and wheresoever it does so, the greater difficulty should be a spur to greater care and perseverance. It would in such case be requisite to make the most careful survey of the ground, to determine the degrees of variations in density, and map the same, in order to obtain a correct basis for estimation and apportionment. The Building-ground of Chicago. The subsoil throughout is of blue clay, covered by sand and loam, which, below the level of ground-water, become “quicksand" and "blue muck." In the central part of the city the clay is found at a depth of about five feet from the original surface, which now is about eight feet below the established grade of streets. This clay-bed is more or less permeated by water, which enters through a network of fine gravelly veins, and through the river channel; it is, therefore, varying in its bearing capacity in proportion to its state of humidity, the driest clay of course being the hardest, and therefore the best for purposes of foundation. In the central part of the city the clay-bed has a distinct surface, covered with a scattered stratum of boulder-gravel, and is termed "hardpan." It approaches the surface to within five feet. Throughout the West Division the clay is equally near to daylight, though it has no distinct surface, the loam gradually changing into clay. From State street eastward, the dip of the clay-bed is so steep that already within one block it becomes ordinarily |