“We must then endeavour to distinguish a virgin soil from ‘made’ or disturbed ground; and to do so, some acquaintance with elementary geology is indispensable. Thus, the crystalliform rocks, granites, gneiss, and crystalline schists remain in the condition in which the cooling of the globe and the upheavals of its crust have placed them. The sandstones, the calcareous rocks, the marls, the gravels, even the clays deposited by water under an enormous pressure, are stratified—that is to say, deposited in layers, like the courses of a building, and present an excellent foundation. The hill there on the right, in whose direction your sister’s wood extends, presents, as you see from this point, escarpments laid bare by the waters of the brook we are going to cross; observe that the stone, which seems denuded, presents itself in almost horizontal layers. It is an oolitic limestone, excellent for building, and on which you may confidently rely as a foundation also. In these strata, therefore, we may excavate cellars, and make use of what we have taken from the excavations to raise the walls. Here we are walking on sandy clays, intermingled with millstone grit. This also forms a good and incompressible foundation. It is otherwise with pure clays; not that they are compressible, but, if they are not secured—if, for instance, they lie on a declivity—they are liable to slip in consequence of the infiltration of water between their layers, and the house built on them goes down with them. And thus you may sometimes see whole villages built on clayey declivities, descending into the valley. Great attention, therefore, must be paid to the method in which you build in clays, if you would avoid these dangers. Sometimes also, when they are greatly compressed by a heavy building, the clays sink down under the weight, and rise proportionally at a little distance, in see-saw fashion. Marine sands, pure, fine or gravelly, are well adapted to receive foundations, because the sand settles naturally, however slightly moistened it may be. To such a degree is this the case, that we can form an artificial foundation if needful by depositing good beds of sea-sand on a questionable soil, and moistening these beds thoroughly. The finer the sand is and the freer from clay the better, for its small, hard, equal grains leave only very slight intervals between them and touch on several points. If the weight compresses the layer of sand, and forces it to settle down, the settling down is regular, and consequently harmless. The building settles thus to the extent of some fractions of an inch, according to its weight; but it does not dislocate, because it settles uniformly. The alluvial deposits formed by slowly-flowing waters, such as rivers or lakes, also compose good foundations, because the layers of gravel or mud have been gradually deposited, and are closely heaped together by the liquid that transported them. It is quite otherwise with marshy soils, for the water, having no current, has allowed vegetables to grow in its bed. These vegetables on dying are annually replaced by others. Successive layers of detritus are then formed under very trifling pressure, leaving between them innumerable cavities, just like a heap of rotten hay. These deposits are called peat-bogs. Nothing can be safely placed on these deposits, for they sink down under the lightest burden. Stop! here we are near the stream, at a point which exhibits this phenomenon. Stamp on this closely-turfed soil. You perceive that the ground sounds hollow, and shakes beneath the shock. Sometimes these peat-beds reach to such a depth, through the accumulation of vegetable detritus, that the bottom can scarcely be reached. If you build upon these, your construction will gradually sink, often unequally, on account of the inclination of the sub-soil, so that the building will lean to one side. It is thus that at Pisa and at Bologna, in Italy, there are towers which inclined thus while they were being built, until the turf was completely compressed under their weight. When these soils occur, the turf must be removed, the rock or gravel must be reached, or piles must be driven in very close to each other, until they can be forced no deeper. Then, on the heads of these piles is placed what is called a raft, a kind of wooden framing, between the spaces of which concrete is poured, and on which the first courses of masonry are placed. Whole cities are built thus. Venice and Amsterdam rest only upon forests of piles driven in mud, which is spongy, because it was formed under a shallow sheet of water which had not power to compress it.
Fig. 6.
“But it is not enough to know the nature of the soil on which a building is to be erected; we must also examine the subjacent water-courses, and how the rain-water flows off on the surface of the ground, or beneath it. The presence of a bed of clay, however thin, between strata of limestone, grit or sand, is a most important fact to the builder; for such beds being impervious—that is, not allowing the rain-water to penetrate them—give rise to currents or sheets of water, which may occasion most disastrous consequences to the foundations. Examine this greenish layer just here, along the escarpment;—it is of clay; it is very thin, and cannot retain water; but suppose it were 20 inches thick. The rains, which will easily penetrate the gravel placed above, will be arrested by this layer of clay, and pursue their course along its plane of inclination, and they will gradually form cavities like small grottoes, and a concealed current. If you build a cellar wall or a foundation descending below that accumulation of water, it will reach your wall and penetrate it, in spite of your efforts, and will fill your cellars. It will consequently be necessary at the outset to divert this accumulation of water by collecting it in a drain to keep it away from your buildings. Give me your note-book, that I may show clearly what I mean by a sketch—(Fig. [6]). Let A B be the stratum of clay, C D the pervious stratum of gravel or sand. A sheet of water running from E to F will be formed after every shower. This sheet will be arrested by the foundation or cellar wall G H, and will soon permeate it, since it cannot reascend nor penetrate the clay. We must, therefore, provide, at I, a transverse drain, with openings on the upper side, through which water will find its way into the channel shown in sketch K. This drain will take the water thus collected wherever you like, and leave the wall G H perfectly dry. You understand, don’t you?
“But if you have to lay your foundations entirely in clay, you must adopt much more serious precautions: for, as I told you just now, the whole bed of clay may chance to slip.
Fig. 7.
“Banks of clay are apt to slip, especially when they present such a section as I have drawn—(Fig [7]). Let A be a bed of rock, B a bed of clay. Rain-water falling on the upper side from D to C, will pass at C below the bed of clay; and if the rain is persistent, it will form from C to E a soft, slippery, soapy stratum, so that the clay bed C B E will slide over it by its own weight, but especially if at G you have burdened it with a building.
“How, then, can we guard against the danger? First, by collecting the water at C into a sewer, or a dry stone drain, so that it may not pass under the clay bed,—in case the latter is very thick. Secondly, if it is only a few yards thick, by getting down to the rock or gravel for the foundation wall, and placing a collecting sewer at I, as above. Then the triangular bed of clay, C I K, will not be able to slide, being kept up by the firmly-planted and loaded wall. The part of the clay lying below, not being moistened from above, will not slip. But this wall, H, and its drain, I, must be thick enough to resist the pressure of the triangle C I K.
“You perceive, then, how important it is to understand the soils on which you have to build; and how essential it is for an architect to have some acquaintance with geology. Remember this well, for the architects of the preceding generation have shown a contempt for these studies, and have relied on their contractors in many instances where that knowledge was required.