A Treatise On The Principles And Practice Of Harbour Engineering

PILING. 8 i The following data1 relating to piles driven at Portsmouth Dock Yard extension constitute a typical record of ordinary experience in driving piles into very firm ground. The ground consisted of a regularly stratified, argillaceous sand, containing perhaps half its bulk of pure clay. The beds were fine, some of them not being half an inch thick, wholly impervious to water across the strati- fication, and very slightly, if at all, pervious in the direction of stratification. All the piles were of fir 17 feet 4 inches long and 13| inches square. Six of them were driven by hand by five men with a monkey weighing 15 cwts., and six by steam with a monkey weighing 22 cwts. The maximum fall in the first case was 21| feet, and in the second case 14{ feet. Pile. Time in Driving. No. of Blows. Max. Fall. Final Depression. Total Penetration. Remarks. 1 2 3 4 5 6 7 8 9 10 11 12 hrs. 14 12 154 15 181 14 31 2 14 24 2 31 228 137 179 169 270 181 265 255 280 180 228 130 ft. ins. 19 0 20 1 21 6 21 10 21 9 21 7 7 8 14 3 9 5 10 9 10 8 9 10 ins. 4 i 4 A i 4 V ft. ins. 12 9 11 8 12 3 12 4 13 11 13 2 10 6 < 14 3 14 8 14 0 13 9 11 0 - Steam machine. Hand machine. Piles in all cases ex- cept two driven to the stage of pro- nounced or incipient splitting. In cases 9 and 12, as much as 3 feet and 4 feet 6 inches respectively had to be cut off the top of the piles from this cause. Formulæ professing to give the exact sustaining power of piles are numerous, radically different in form, and conflicting in results. They are to be found in all engineering pocket-books, and little advantage would be derived from quoting them here. Some are extremely complex, embodying elements which have little or nothing to do with the capabilities of a pile to sustain an imposed load. When all has been said, it must be evident that the true test of sustaining power is the resistance offered to the final blow. The leugth, weight, and modulus of elasticity of the pile, are factors possessing no practical value, and a simple formula, linking up the weight of the ram and its fall with the resulting depression, should give all that is required. Major Saunders’s formula is certainly based on these lines, but, unfortunately, it does not adapt itself to all cases. Thus, with a depression of ^ inch under the last blow of a 2000-lbs. ram falling 9 feet, the safe load becomes 270,000 Ibs. To sustain such a load, a pile, in the author’s estimation, should be not less than 15| inches square, and, by Rankine’s rule, 16| inches. Any pile, therefore, of less dimensions would be incapable of supporting so heavy a load with reasonable regard to safety, whereas, as a matter of fact, many 11- and 12-inch piles have been driven to comply with the standard stated in the formula. The values given by the formulæ of other authorities for similar conditions are as follows :—Haswell, 30,000 to 60,000 Ibs. ; Weisbach, 26,800 to 28,000 Ibs.; Wellington, 32,727 Ibs.; Trautwine, 32,460 to 97,380 Ibs., 1 Min. Proc. Inst., C.E., vol. xliv. p. 204. 6