38. Bearing Power of Piles

When driven in sand or gravel, or to hard pan, piles will carry to the full extent of the crushing strength of the timber,- providing the depth of the pile is sufficient to secure lateral stiffness.

"There are examples of piles driven in stiff clay to the depth of 20 feet that carry from 70 to 80 tons per pile : there are many instances in which piles carry from 20 to 40 tons under the above conditions. After a pile has been driven to 20 feet in sand or gravel, any further hammering on the piles is a waste of time and money, and injurious to the pile itself."*

Piles driven from 30 to 40 feet in even the softest alluvial soils should carry by frictional resistance alone from 10 to 12 tons.

For the safe working loads on piles driven in different soils, the following table, compiled from the Engineering News formula, may be used with safety. The values are for minimum lengths of spruce piles and average penetration for last five blows of a 1,200 pound hammer falling 15 feet. When heavier loads than these must be carried, or the penetration is much greater, the actual bearing power of the piles should be determined by testing, unless it is already known from actual experience.

Table II. - Bearing Value Of Piles

SOIL.

PILE LENGTHS.

AVERAGE DIAMETER

PENETRATION.

LOAD IN TONS.

Ft.

Ins.

Ins.

Silt..........................

40

10

6

2

Mud..............................

30

8

2

6

Soft earth with boulders or logs..............

30

8

1

7

Moderately firm earth or clay with boulders or logs.......................

30

8

1

9

Soft earth or clay.............

30

10

1

9

Quicksand..................

30

8

12

Firm earth..................

30

8

12

Firm earth into sand or gravel..............

20

8

14

Firm earth to rock.....................

20

8

0

20

Sand......................

20

8

0

20

Gravel......................................

15

8

0

20

When the penetration is less than that given above, for soft soils the safe loads may be increased according to the Engineering News formula given in the next paragraph.

*" A Practical Treatise on Foundations."

There have been several formulae proposed for determining the safe working loads on piles. Of these, the latest, known as the Engineering News formula, is generally considered to be the most reliable. It is claimed for this formula that it sets "a definite limit, high enough for all ordinary economic requirements, up to which there is no record of pile failures, excepting one or two dubious cases where a hidden stratum of bad material lay beneath the pile, and above which there are instances of both excess and failure, with an increasing proportion of failures as the limit is exceeded."

The formula is :

Safe load in lbs. = 2 w h / s + 1 in which w = weight of hammer in pounds; h, its fall in feet;. s, average set under last blows in inches.