5 replies to this topic

[geo pe]

I am having difficulty understanding which pmax equation to use for Problem 85, data given in the question classify soil as SC (provides both phi and c), which equation would be used, one for clay or one for sand? Solution uses one for sand!! Any thoughts... Thanks

[civilized_naah]

I am having difficulty understanding which pmax equation to use for Problem 85, data given in the question classify soil as SC (provides both phi and c), which equation would be used, one for clay or one for sand? Solution uses one for sand!! Any thoughts... Thanks

Conservative thing would be to treat as pure sand, since it lacks the stickiness (cohesion) that clay has and therefore does not 'hold back' in pushing laterally against bracing system.

[chess5329]

I am having difficulty understanding which pmax equation to use for Problem 85, data given in the question classify soil as SC (provides both phi and c), which equation would be used, one for clay or one for sand? Solution uses one for sand!! Any thoughts... Thanks

What problem is this one, for what you're saying this apparently is a (peck envelope problem). If this is the case you have to check first, stiffness of the soil and then it will tell you what case or formula to use to solve for pmax. I can't tell unless you provide more information about the problem?

my 2 cts.

[geo pe]

Thanks for your response, I was talking about Prob 85 in 6 mim : "A 6' wide by 20' deep trench is being excavated for utility installation. The soil profile parameters are phi = 27 deg, gamma = 115.6 lb/ft3, c = 250 lb/ft2. If struts are spaced every 30' what is the most nearly expected maximum lateral force required". In this question, the soil is miz of sand and clay and I was wondering which equation to use for pmax.

[chess5329]

Thanks for your response, I was talking about Prob 85 in 6 mim : "A 6' wide by 20' deep trench is being excavated for utility installation. The soil profile parameters are phi = 27 deg, gamma = 115.6 lb/ft3, c = 250 lb/ft2. If struts are spaced every 30' what is the most nearly expected maximum lateral force required". In this question, the soil is miz of sand and clay and I was wondering which equation to use for pmax.

The key for this type of problem is the symbols for soil class (SC), this is sand with silt because the c=250 you can consider because the problem doesn't provide more information that the soil has a low content of silt, so you have to solve for the predominant soil which is for sand.
pmax=.65 kaYH=.65(.38)115.6(20')=571

then the problem is asking for the total lateral force Pa=pmax(spacing)(H)=571(30)(20)=342,640lb=342kips.

[geo pe]

thanks!