233 » Unit Weights and Internal Friction Angles . - The unit weight 

 (w) of typical materials and their internal friction angle (0) are 

 given in Table 12, These are average or normal values of w and (2f. 



Table 12 - Unit Meiglibs and Internal Friction Angles. 









I'feight Natural 



Weight 



Internal 



Ifeterial 



Weight Dry 



Drained 



Submer-ged 



Friction Angle 





lbs. / cu.fto 



lbs, / cu.ft. 



lbs. / cu.ft. 



degrees 



Claj, Soft 



60 - 90 



100 - 120 



35 - ^S 



10 



Clay, Coitpact 



90 - 115 



120 - 135 



^S - 70 



20 ~ 25 



Gravel 



105 - 120 



115 - 135 



65- 75 



liO - 50 



Silt, Compact 



85 - 105 



115 - 130 



^s - e^ 



25 - ko 



Silt, Loose 



e^ - 85 



105 - 115 



iio - 50 



20 - 30 



Sand 



90 - 105 



105 - 120 



^b - 65 



25 - IlO 



Sand-Clay, 











Coup act 



115 ~ 130 



135 - iltO 



70 - 80 



iiO " 50 



23li» The resultant pressure is inclined from the normal to the back of 

 the wall by the angle of wall friction 0, Values for 0^, can be taken from 

 Table 13, but should never exceed the internal friction angle of the back- 

 fill material. The vertical corrponent of the earth pressure (P) need not 

 be considered in the stability analysis unless it has considerable effect 

 on the structural design. 



Table 13 - Coefficients and Angles of Friction. 



■■' 









Elnd of Surface 



Coefficient of 

 Friction f 



Angle of ^ 

 Friction ''^1 



Granite, Limestone^ Marbles 









Soft dressed on soft dressed 



0.70 



K 



00' 



Hard dressed on hard dressed 



0.55 



28o 



50' 



Hard dressed on soft dressed 



0.65 



33" 



00' 



Stone, brick, or concretes 









I'lasonry on masonry 



o»65 



33o 



00 » 



Masonry on wood-with grain 



0,60 



K 



00 » 



Masonry on wood-cross grain 



0,50 



2^0 



1;0' 



I^asonry on dry clay 



Oo5o 



18° 



1(0' 



Masonry on wet or moist clay 



0,33 



20f 



Masoniy on sand 



0,1j.0 



21° 



50' 



Masonry on gravel 



0,60 



K 



00' 



Soft stone on steel or iron 



0.iiO 



K 



50' 



Hard stone on steel or iron 



0.30 



16° 



liO' 



120 



