Improving the Bearing Power of the Boil

IMPROVING THE BEARING POWER OF THE BOIL. When the soil directly under a proposed structure is incapable, in its normal state, of sustaining the load that will be brought upon it, the bearing power may be increased (1). by increasing the depth of the founda tion, (2) by draining the site, (3) by compacting the soil, or (4) by adding a layer of sand.

Increasing the Depth.

The simplest method of increasing the bearing power is to dig deeper. Ordinary soils will bear more weight the greater the depth reached, owing to their becoming more condensed from the superincumbent weight. Depth is especially important with clay, since it is then less liable to be displaced laterally owing to other excavations in the immediate vicinity, and also because at greater depths the amount of moisture in it will not vary so much. However, occasionally the soil grows more moist as the depth increases beyond a moderate distance, in which case increasing the depth is undesirable. For example, in Chicago the clay grows softer after a depth of about 12 to 14 feet below the sidewalk is reached.

In any soil, the bed of the foundation should be below the reach of frost. Even a foundation on bed-rock should be below the frost line, else water may get under the foundation through fissures, and, freezing, do damage.

Drainage.

Another simple method of increasing the bearing power of a soil is to drain it. The water may find its way to the bed of the foundation down the side of the wall, or by percolation through the soil, or through a seam of sand. In most cases the bed can be sufficiently drained by surrounding the building with a tile drain laid a little below the foundation.

In more difficult cases, the expedient is employed of covering the site with a Layer of gravel—the thickness depending upon the plas ticity of the soil, the gravel serving the double purpose of distrib uting the concentrated loads of the footings to a larger area of the native soil and of improving the drainage of the bed of the founda tion. In extreme cases, it is necessary to inclose the entire site with a puddle-wall to cut off drainage water from a higher area.

Springs. In laying foundations, springs are often met with and sometimes prove very troublesome. The water may sometimes be excluded from the foundation pit by driving sheet piles, or by plugging the spring with concrete. If the flow is so strong as to wash

the cement out before it has set, a heavy canvas covered with pitch, etc., upon which the concrete is deposited, is sometimes used; or the water may be carried away in temporary channels, until the con crete in the artificial bed shall have set, when the waterways may be filled with semi-fluid cement mortar. Below is an account of the method of stopping a very troublesome spring encountered in laying the foundation of the dry-dock at the Brooklyn Navy Yard.

"The dock is a basin composed of stone masonry resting on piles. The foundation is 42 feet below the surface of the ground and 37 feet below mean tide. In digging the pit for the foundation, springs of fresh water were discovered near the bottom, which proved to be very troublesome. The upward pressure of the water was so great as to raise the foundation, however heavily it was loaded. The first indication of undermining by these springs was the settling of the piles of the dock near by. In a day it made a cavity in which a pole was run down 20 feet below the foundation timbers. Into this hole were thrown 150 cubic feet of stone, which settled 10 feet during the night; and 50 cubic feet more, thrown in the following day, drove the spring to another place, where it burst through a bed of concrete 2 feet thick. This new cavity was filled with concrete, but the precaution was taken of putting in a tube so as to permit the water to escape; still it burst through, and the operation was repeated several times, until it finally broke out through a heavy body of cement 14 feet distant. In this place it undermined the foundation piles. These were then driven deeper by means of followers; and a space of 1,000 square feet around the spring was then planked, forming a floor on which was laid a layer of brick in dry cement, and on that a layer of brick set in mortar, and the foundation was completed over all. Several vent-holes were left through the floor and the foundation for the escape of the water. The work was completed in 1851, and has stood well ever since." Consolidating the Soil. The bearing power of a soft or com pressible soil may be improved in any one of several ways, viz.: (1) by adding a layer of sand, (2) by driving wood piles, (3) by using sand piles, or (4) by the compressol system.