In calculating the weight a pile should support, it is not customary to take into account the assistance it receives laterally from the ground into which it is driven ; and the number of piles, and the load to be brought upon them are ascertained upon the supposition that they must be able to support that load if they were really isolated ; or iu other words, the longer the pile the less must be the weight it is made to bear. Rondelet, in his `Art de Batir,' gives a series of tables of the respective weights which may safely be brought upon piles of variable ratios between the diameters and height; and it may suffice here to say, that when that ratio does not exceed 16 times the diameter, Rondelet considered that the load brought upon the pile permanently, might safely be carried to between 430 and 500 lbs. per inch superficial of the sectional area. Morin, after verifying Mr. Eaton Hodgkinsou's tables of the resistance of columns to compression, increased the value of the resistance of piles ; and the proportionate (permanent safety) loads to be brought upon them are considered at the present day to be nearly as follows :— When piles are being driven, they should be made to advance so far into the earth that the effort exercised upon their heads, or the driving weight, should virtually exceed the statical effect of the intended permanent load ; and when this is attained, the pile is said to have attained its refusal. There are no recorded experiments upon which to found a theory for ascertaining the conditions regulating the descent of piles; and therefore, the rules for fixing their refusal must be con sidered as being simply empirical. It is, however, customary to consider that a pile has attained a satisfactory refusal, if it be intended to support 25 tons, and will not descend more than iths of an inch, under a series of 30 blows from the monkey of a ringing engine, weighing 12 cwt., and falling a distance of 4 feet ; or under a series of 10 blows from a monkey of the same weight falling a distance of 12 feet ; if the weight be reduced to 12} tons per pile, the refusal is diminished to 'ths of an inch ; and if to 5 tons, to 2 inches. At the
bridge of Neuilly, where the piles support a weight of 52 tons each, Perronet adopted a refusal of 1-5th of an inch for a series of 25 blows with a 12 cwt. monkey falling 4 feet 8 inches. It is important to observe, in all these cases, that the piles may often appear " to refuse," after having been struck by frequent blows succeeding one another rapidly, perhaps on account of the superinduction of a state of vibra tion in either the piles themselves, or in the ground ; and that if the driving be recommenced, after an interval long enough to allow the vibration to cease, the pile will begin to descend again. In practice, therefore, no pile should be considered to have attained its-refusal, until the trial has been repeated at the interval of at least one day after the first observation upon its conditions of descent.
In regularly piled_ foundations, it is now the custom to surround the heads of the piles with concrete, to a depth of three feet below tho timber platform, and to extend the concrete at least three feet beyond the outer line of the piles. The platform is usually made of two courses of planks, the lower one tenonting upon the pile-heads, and the upper one halving upon the lower. The whole of this timber is again buried in concrete to a depth of two feet above its uppermost surface. It is sought, in cases where these precautions are taken, to render the whole of the foundation homogeneous, and to guard against any lateral displacement by the piles turning over on their lower external edges, an accident which often occurs when a lofty quay wall is built upon a compressible subsoil ; and although the precautions may be costly, the result of the works executed in many of our own, and in many foreign, sea-ports, proves that they cannot safely be dispensed with.