Geotechnical Question

I am trying to solve the following question: The unit weight of soil is 14.9 KN/M3, The moisture content of the soil is 17% when the degree of saturation is 60%. Determine a. saturated unit weight b. void ratio c. specific gravity of soil solids. Ans. a. 16.33 KN/M3 b. 0.58 c. 2.05. I have spent two hours on this problem today and could not get the answers. I am getting different but very close answers which has lead me to think that the answers are wrong. please help.

thanks

sandiego2010 said:

I am trying to solve the following question: The unit weight of soil is 14.9 KN/M3, The moisture content of the soil is 17% when the degree of saturation is 60%. Determine a. saturated unit weight b. void ratio c. specific gravity of soil solids. Ans. a. 16.33 KN/M3 b. 0.58 c. 2.05. I have spent two hours on this problem today and could not get the answers. I am getting different but very close answers which has lead me to think that the answers are wrong. please help.
thanks

Click to expand...

you have to review very.... very.....but very well and be familiar with "Phase of soil relationships"

find Yd=14.9/1+w=12.74...this will give you Ww=14.9-12.74=2.16...find Vw=Ww/Yw=2.16/9.81=.22m3...then find Vv=Vw/s=.22/.6=.367 m3

now use relationship Vt=Vs+Vv..solve for Vs=1-Vv= 1-.367=.633

now use formula e=Vv/Vs= .367/.633 this give you the void ratio e=0.58.

I think you can develop and find the rest.....if not let me know.

chess5329 said:

sandiego2010 said:

I am trying to solve the following question: The unit weight of soil is 14.9 KN/M3, The moisture content of the soil is 17% when the degree of saturation is 60%. Determine a. saturated unit weight b. void ratio c. specific gravity of soil solids. Ans. a. 16.33 KN/M3 b. 0.58 c. 2.05. I have spent two hours on this problem today and could not get the answers. I am getting different but very close answers which has lead me to think that the answers are wrong. please help.
thanks

Click to expand...

you have to review very.... very.....but very well and be familiar with "Phase of soil relationships"

find Yd=14.9/1+w=12.74...this will give you Ww=14.9-12.74=2.16...find Vw=Ww/Yw=2.16/9.81=.22m3...then find Vv=Vw/s=.22/.6=.367 m3

now use relationship Vt=Vs+Vv..solve for Vs=1-Vv= 1-.367=.633

now use formula e=Vv/Vs= .367/.633 this give you the void ratio e=0.58.

I think you can develop and find the rest.....if not let me know.

Click to expand...

The trick to this answer is to determine the void ratio (e) and specific gravity (SG) before solving for the saturated unit weight. Don't get caught into solving each component of the problem in the order that they're given. Most equations can be solved once e and SG are calculated. Also, you must understand that a soil can have 3 phases of unit weights: dry, moist, saturated. When the question gives you the unit weight, it is assumed it's the moist unit weight.

Good luck!

I THINK THIS ONE IS EASY

s=(wxG)/e ======= e=1.66x.17xG =========e=0.28G

v=(GX9.81x(1+w))/(1+(0.28G)

14.9=(GX9.81x(1+w))/(1+(0.28G) ==========G=2.04=========e=0.57========Vs=(9.81(G+e))/(1+e)=========Vsat=16.30

boo said:

I THINK THIS ONE IS EASY
s=(wxG)/e ======= e=1.66x.17xG =========e=0.28G

v=(GX9.81x(1+w))/(1+(0.28G)

14.9=(GX9.81x(1+w))/(1+(0.28G) ==========G=2.04=========e=0.57========Vs=(9.81(G+e))/(1+e)=========Vsat=16.30

Click to expand...

thanks to ahll who helped. I got it.

There's so many equations that can be written in various ways for phase relationships but I agree, finding the void ratio and specific gravity first makes things a lot easier. Das has a great book on soils (blue book) which I used in college which has great tables and examples for soils. Some examples will be very helpful.