Simple Geotech Question

[stlaggies]

I seem to be stumped on this question.

Soil weighing 600.3 lbs is removed from a test pit. It takes 307.0 lbs of water to fill the test pit. A lab sample of this same soil weighing 105.5 grams is dried in an oven and its weight after drying is 97.2 grams. The max weight of the soil attained in the lab is 115.2lbs/ft^3 and the min is 102lb/ft^3. The SG of the soil solids is 2.7.

Find wet unit weight and dry unit weight:

How I'm trying to solve it: First i'm gonna find volume of solids and volume of water. I take the weight of solids to be .214 lbs and weight of water to be .018lbs. Using the formula I get .00127 for Vs and .000288 for Vw, which gives me something like 137 pcf for dry unit weight. Anyone help with where I made a mistake? My answer choices are 102, 112, 115, 122

 

[sac_engineer]

I seem to be stumped on this question.
Soil weighing 600.3 lbs is removed from a test pit. It takes 307.0 lbs of water to fill the test pit. A lab sample of this same soil weighing 105.5 grams is dried in an oven and its weight after drying is 97.2 grams. The max weight of the soil attained in the lab is 115.2lbs/ft^3 and the min is 102lb/ft^3. The SG of the soil solids is 2.7.

Find wet unit weight and dry unit weight:

How I'm trying to solve it: First i'm gonna find volume of solids and volume of water. I take the weight of solids to be .214 lbs and weight of water to be .018lbs. Using the formula I get .00127 for Vs and .000288 for Vw, which gives me something like 137 pcf for dry unit weight. Anyone help with where I made a mistake? My answer choices are 102, 112, 115, 122

I got 122 lbs/ft3 for the wet density and 112.4 for the dry density. If those the correct answers, I'll can explain how I got them.

 

[djgint]

My answers are the same as sac_engineer.

First get the volume of test pit

Volume = 307 lb/62.4 pcf

Volume = 4.92 ft^3

Find the moist or wet unit weight of the soil

Wet unit weight = Weight of soil / Volume of test pit

Wet unit weight = 600.3 lb / 4.92 ft^3

Wet unit weight = 122 pcf

Find the moisture content

Moisture content = Wet soil - dry soil divide by dry soil

Moisture content = 105.5 g - 97.2 g / 97.2g * 100%

Moisture content = 8.5 %

Find the dry unit weight

Dry unit weight = wet unit weight / (1+moisture content decimal)

Dry unit weight = 122 pcf / 1+0.085

Dry unit weight = 112 pcf

 

[stlaggies]

Thanks guys, That helped.

4.92ft^3 is considered the total volume correct? Would I find volume of voids using Vt-ms/(SG*pw) where ms = 600, SG=2.7 and pw=62.4?

 

[boo]

Thanks guys, That helped.
4.92ft^3 is considered the total volume correct? Would I find volume of voids using Vt-ms/(SG*pw) where ms = 600, SG=2.7 and pw=62.4?

But 4.92 is V of water not V of total soil??????? can you please explain ?

 

[djgint]

ms is not your total weight of sample. It is the weight of solids. Calculate the weight of water first mw.

mw = 600.3 lb * 0.085

mw = 51.03 lb

Now calculate the weight of solids ms.

ms = total weight - weight of water

ms = 600.3 lb - 51.03 lb

ms = 549.27 lb

Now you can calculate the volume of voids vv.

vv = vt-ms/(SG*pw)

vv = 4.92 ft^3 - 549.27 lb/(2.7*62.4 pcf)

vv = 1.66 ft^3

 

[djgint]

boo

The volume of test pit and the volume of water are the same assuming the water won't escape. Its the same as you pour water in the glass (full), weigh the water in lb then divide the weight of water with density of water you'll get ft^3 which is also the volume of the glass.

 

[boo]

boo
The volume of test pit and the volume of water are the same assuming the water won't escape. Its the same as you pour water in the glass (full), weigh the water in lb then divide the weight of water with density of water you'll get ft^3 which is also the volume of the glass.

Thanks

 

[stlaggies]

How would I find the relative density of this?

 

[djgint]

Dr = 1/Drymin - 1/Dry / 1/Drymin - 1/Drymax * 100%

Dr = 1/102 pcf - 1/112 pcf / 1/102 pcf - 1/115.2 pcf * 100%

Dr = 77.9 %