If the penetration is at an uneven rate, it is probable that the pile is passing bowlders or logs. If the penetration is practically zero, it is probable that the pile is against an impenetrable stratum or is already crushed. When the penetration has reached a small amount, say, } or inch per blow, and the hammer rebounds con siderably, it is safe to conclude that the limit of safe driving of that pile has been reached. The penetration to be used in the formula should not be taken unless it has been at a reasonably uniform or uniformly decreasing rate. Of course, the apparent penetration due to the brooming of the head, or to the crushing of the body of the pile, or to the bruising of the point should not be used in the formula for computing the bearing power.
Care should be taken that the test blow is struck on sound wood, as otherwise the computed bearing power may be greatly in error (see paragraph 4 of 4 773). . A slightly broomed head may absorb half to three quarters of the energy of the blow, and increase pro portionally the computed supporting power. This shows how unscientific it is to prescribe a limit of the penetration without specify ing the accompanying condition of the head of the pile, as is often done. Piles driven close together in quicksand or semi-fluid soil will sometimes rise a little when other piles are subsequently driven near but usually this phenomenon need cause no anxiety, as the lower material is already solidly in contact with the piles, and therefore the piles have as great bearing power as the nature of the soil makes possible.
Of course, in making the test blow the hammer should drop vertically.
It is not certain that the bearing power of a pile when loaded with a con tinued quiescent load will be the same as that during the very short period of the blow. The friction on the sides of the pile will have a greater effect in the former case, while the resistance to penetration of the point will be greater in the latter. This, and the fact that the supporting power of piles sunk by the water jet can be determined in no other way, show the necessity of experiments to determine the bearing power under a steady load.
Unfortunately no extended experiments have been made in this direction. We can give only a collection of as many details as pos sible concerning the piles under actual structures and the loads which they sustain. In this way, we may derive some idea of the sustaining power of piles under various conditions of actual practice.
At Philadelphia in 1873, a pile was driven 15 ft. into "soft river mud, and 5 hours after 7.3 tons caused a sinking of a very small fraction of an inch; under 10 tons it sank I of an inch, and under 16.8 tons it sank 5 ft." The above load is equivalent to 360 lb. per sq. ft. of surface of contact. This pile is No. 2 of the table on page 394.
"In the construction of a foundation for an elevator at Buffalo, N. Y., a pile 15 inches in diameter at the large end, driven 18 ft., bore 25 tons for 27 hours without any ascertainable effect. The weight was then gradually increased until the total load on the pile was 371 tons. Up to this weight there had been no depression of the pile, but with 371 tons there was a gradual depression which aggre gated of an inch, beyond which there was no depression until the weight was increased to 50 tots. With 50 tons there was a further depression of of an inch, making the total depression 11 inches. Then the load was increased to 75 tons, under which the total depres sion reached 31 inches. The experiment was not carried beyond this point. The soil, in order from the top, was as follows: 2 ft. of blue clay, 3 ft. of gravel, 5 ft. of stiff red clay, 2 ft. of quicksand, 3 ft. of red clay, 2 ft. of gravel and sand, and 3 ft. of very stiff blue clay. All the time during this experiment there were three pile-drivers at work on the foundation, thus keeping up a tremor in the ground. The water from Lake Erie had free access to the pile through the gravel." * This is equivalent to a frictional resistance of 1,850 lb. per sq. This is pile No. 7 of the table on page 394.
In the construction of the dock at the Pensacola navy yard, a pile driven 16 feet into clean white sand sustained a direct pull of 43 tons without movement, while 45 tons caused it to rise slowly; and 46 tons were required to draw the pile. This is equivalent to a frictional resistance of 1,900 lb. per sq. ft.
In making some repairs at the Hull docks, England, several hundred sheet piles were drawn out. They were 12 by 10 inches, driven an average depth of 18 feet in stiff blue clay, and the average force required to pull them was not less than 35.8 tons each. The frictional resistance was at least 1,875 lb. per sq. ft. of surface in contact with the soil.f Summary. A summary of the ultimate frictional resistance developed in the preceding five examples is as follows: The last is the value obtained in pulling a pile.
At the Hull docks, England, piles driven 16 ft. into "alluvial mud" sustain at least 20 tons, and according to some, 25 tons; for the former, the friction is about 800 lb. per sq. ft. The piles under the Royal Border Bridge "were driven 30 to 40 ft. into sand and gravel, and sustain 70 tons each,"—fhe friction being about 1,400 lb. per sq. ft.