239. Subraergecl Materia ls , - Prsssures due to subrierged fills may 

 be calculated by substituting for w in the preceding equations the 

 uni-t weight of the rdaterial reduced by buoyancy, and adding to the 

 pressures so calculated, the full hyd.rostatic head of water. Note 

 that for surcharge loads this buoyed unit weight of the material must 

 be increased as shoT-m in the preceding paragraph* 



2li0» Uplift* - For design corputations, uplift pressures should 

 be considered as full hydrostatic riressure for walls whose bases are 

 below sea level or for coinputations involving satiirated baclcfill, 



ICE FORGES 



2lilj The coronion forms of ice are usually classified by the use of 

 terms wliich indicate the manner of formation or the effects produced* 

 Usual classifications include, sheet ice, shale ice, slush ice, frazil 

 ice, anchor ice, and agglor.ierate ice» 



2l!.2i The sjaount of e>X)ansion of water in cooling from 39 Ft to 

 o 

 32 Fj is 1,32 hundredths of 1 percent, whereas in changing from water 



at 32 F» to ice at 32 F^ the amount of e:5cpansion is about 9*^$ percent 



or 68^ times as great.. It has been found that a change of structure to 



denser form talces place in the ice when, I'jith a teirperatu-re lot-jer than 



-8 Fi, it is su.bjected to pressures greater than about 30,000 iDounds 



per square inch* Sscessive pressure, x/ith temperature above -8 F., 



causes the ice to melt* •■Jith the tenperatu.re below -8 F,, the chan-ge 



to a denser form at high pressure results in shrinlcage which relieves 



pressure. Thus, the probable maximujn pressure which can be produced 



by water freezing in an inclosed space is 30,000 pounds per square inch. 



2li3* Designs for da^'is include allowances for ice pressures varying 

 from no special, allowance to as much as Li^jOOO to 50,000 pounds per 

 linear foot. The crushing strength of ice has been found to be about 

 !|.00 pounds per squ-are inch and the thj^ust per linear foot for various 

 thiclmesses of ice as about 28,800 pounds for 6 inches, 57,600 poujids 

 for 12 inches, etc^ Structures subject to blows from floating ice 

 should be capable of resisting from 10 to 12 tons per square foot 

 (139 to 167 pounds per square inch) on the area exposed to the greatest 

 thickness of floating ice^ Ice also e^cpands when warmed from teiiiperatuj*es 

 below fi'eezing to a temperature of 32 F, i-jithout melting. 



2hh. Assuming a lake surface to be free from snow, i-rith an average 

 coefficient of expansion of ice between -20 F. and 32 F« equalling 

 ^0000281!., the total e^^jansion of a sheet of ice a mile long for a rise 

 in terrperature of $0 F; woLild be 3 = 75 feet. ITormally, shore structures 

 are subject to vxave forces cojrp arable in magnitude to the m^id-mvim probable 

 pressujre that might be developed by an ice sheet. As the maximum wave 

 forces and ice thrust cajinot occur at the same time, usually no special 

 allowance for overturning stability to resist ice tlmist is made. However, 



122 



