The architect should in all cases make every endeavor to discover the nature of the soil upon which his building is to be built before he makes his foundation plan. For most buildings, a sufficient idea of the nature of the soil may be gained by inquiry amongst builders who have put up buildings on the adjacent lots. Many soils, however, vary greatly, even in a distance of 100 feet, owing to the strata having a decided dip, and on all such soils much trouble and annoyance may often be saved by having borings made with a post auger, showing the composition of the soil at different strata. If two borings made on different sides of the site show about the same depth and character of soil it may be assumed that other borings would give the same result, but if the soil brought up by the first two borings show a difference in the character of the soil, or indicate that the strata have a decided pitch, then borings should be made all around the foundations.
For ordinary buildings borings to the depth of 8 or 10 feet are generally sufficient, although a 6 or 8-inch auger may be driven to the depth of 20 or 25 feet by two men using a lever. In soft soils a pipe must first be sunk and the auger worked inside of it. A smaller auger will answer in such cases.
When borings are deemed necessary the owner should be advised of the fact, and his authority obtained for incurring the expense, which should be defrayed by him.
5. Different soils have not only different bearing or sustaining powers, but also various peculiarities which must be thoroughly understood and considered when designing the foundation.
An architect who, as a draughtsman, has had several years' experience in one locality before practicing for himself, will naturally have become acquainted with the peculiarities of the soil in that vicinity; but should his practice extend beyond his own city he should carefully study the nature and peculiarities of the soil in each different locality where he may have work, and also obtain all the information possible, bearing on the subject, from local builders, as otherwise he may fall into serious trouble.
No part of a building is more important than the foundation, and more cracks and failures in buildings will be found to result from defective foundations than from any other cause; and for any such defects, resulting from the neglect of usual or necessary precautions, the architect is responsible to the owner, besides the damage which inevitably results to his own reputation.
The following observations are intended as a general guide in preparing foundations on different soils, although they should be supplemented by the experience of local builders wherever possible.
Rock, when it extends under the entire site of the building, makes one of the best foundation beds, as even the softest rocks will safely carry more weight than is likely to come upon them.
The principal trouble met with in building on rock is the presence of water. As the surface water cannot readily penetrate the rock, it collects on top of the ledge and in the trenches, so that some arrangement for draining away the water should be provided. If the ledge falls off to one side, a tile or stone drain may be built from the lowest point of the footings to near the surface on the slope. If in a sewer district, the water may be drained into the sewer, using proper precautions for trapping and ventilation. If there is no sewer and the rock does not fall off, a pit should be excavated at the lowest part of the cellar to collect the seepage, and an automatic arrangement provided for raising the water into a drain laid above the surface of the rock.
To prepare the rock for the footings, the loose and decayed portions should be cut away and dressed to a level surface. If the surface of the rock dips, or is irregular in its contour, the portion under the footings should be cut to level planes or steps, as shown in Fig. 2. In no case should the footings of a wall rest on an inclined bed.
7.If there are fissures or holes in the rock, they should be filled with concrete, well rammed; or, if the fissure be very deep, it maybe spanned by an arch of brick or stone. In building on rock it is very desirable that the footings shall be nearly level all around the building, and whenever this is not the case the portions of the foundation which start at the lower level should be laid in cement mortar and with close joints, otherwise the foundations will settle unequally and cause cracks to appear above.
8. Should it be absolutely necessary to build partly on rock and partly on soil, the footings on the soil should be made very wide, so that the settlement will be reduced to a minimum. The footings resting on the rock will not settle, and the least settlement in those resting on the soil will be sure to produce a crack in the superstructure, and perhaps do other damage.
Building on such a foundation bed is very risky at best, and should always be avoided if possible.