18. From the above description of the various kinds of soil and other materials met with in foundation beds the following practical deductions may be made:

1. It is generally safe to build on bed rock any structure that may be required, providing the foundation beds are kept level.

2. Gravel, even when mixed with small boulders, can also be considered perfectly reliable for any ordinary structure, under usual conditions.

3. Sand will carry very heavy loads, provided it is confined; but great precautions must be taken to properly confine it, and also to keep water, especially if running, from it, as the action of water on the sand would very soon wash it away.

4. Clay, when compact and dry, will carry large loads, but water should be kept from it, both under and around the structure, the foundations of which might otherwise give way, due to the difficulty of retaining the pasty or semiliquid mass formed.

5. A thick, hard, or compact stratum, overlying a much softer one, even silt or quicksand, will often carry a considerable load, the hard stratum floating upon the soft as a raft floats on the water. It is usually better not to break through this hard stratum, as it serves to spread the base and distribute the pressure over a large area. Most of the large buildings in Chicago are built on soil of the above description. The proper way to build on such material will be treated under the heading "Spread Foundations."

6. The silt, slush, and decayed vegetation contained in the marshy lands, especially in the Southern states, are not fit to build on without piling.

19. In all cases, the base of the foundations should be so spread out as to keep the pressure per square foot of base or footings within the safe limit.

Table 1 gives the safe loads that different kinds of earth, rock, etc. will bear. By calculating the weight of a building, from tables and data to be given, the bearing power of the soil it is to be placed on can readily be found.

These calculations for the bearing power of earth, etc. are safe loads, and may be used with confidence.

Table 1. Bearing Power Of Soils, Etc. IX Tons Per Square Foot

Kind of Material.

Minimum.

Maximum.

Rock, hardest in native bed.........

300

-

Rock, equal to best ashlar masonry .....

25

40

Rock, equal to best brick............

15

20

Clay, dry, in thick beds.............

4

6

Clay, moderately dry, in thick beds.........

4

6

Clay, soft..........................

1

2

Gravel and coarse sand well cemented......

8

10

Sand, compact and well cemented. . . .

4

6

Sand, clean dry...................

2

4

Quicksand, alluvial soils, etc...................

0.5

1