. Journal of the New England Water Works Association . el Piling and Rock. Due to the porosity of the material and the low rainfall, the groundwater level was very considerably lowered during the fall and winter of1921-22. In general it remained about 20 ft. above water level in thetrench. The result of this was to appreciably reduce the pressure on thetimber tracing. After a few timber sets had been put in and the trenchexcavated about 40 ft. deep the bracing began to show strain; one wale PRATT. 463 cracked longitudinally in about the middle, some of the bracing cut intothe wales as much as
. Journal of the New England Water Works Association . el Piling and Rock. Due to the porosity of the material and the low rainfall, the groundwater level was very considerably lowered during the fall and winter of1921-22. In general it remained about 20 ft. above water level in thetrench. The result of this was to appreciably reduce the pressure on thetimber tracing. After a few timber sets had been put in and the trenchexcavated about 40 ft. deep the bracing began to show strain; one wale PRATT. 463 cracked longitudinally in about the middle, some of the bracing cut intothe wales as much as § in. and some of the braces split ;it the ends. Acloser vertical spacing was considered, but the only change actually madewas to substitute oak for pine bearing plates at the ends of the braces andto frame the timber with even greater care than before so as to be sureof a bearing over the entire 144 sq. in. section. As the trench was deepened the ground water dropped and the press-ures apparently never again reached those which obtained in the more. Bottom of Deep Core-Wall Contact Between Steel Sheet Piling and Rock. shallow trench. Most of the braces when removed were sawed and cut outwith no Teat difficulty and some were pulled out with a cable from a derrickhoisting engine without any cutting. In general, the contact of the steelpiling with the rock surface was found to be most satisfactory but in avery few places the piling had encountered rock fragments near the bottomand had been twisted out of its interlock. There were a few piles thathad been overdriven and fishhooked. A small pile hammer was riggedon a derrick set upon the berm of the sloped excavation and any piles notshowing a tight contact were redriveD as was found necessary. Turned up piles were burned off. For a stretch of about 20 ft. onone side in the bottom of the deepest section of the trench an additionalset of short piling was driven inside of the original set. This was the onlyplace where a
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