# Geotech Problem

4 replies to this topic

### #1 passpepasspe

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Posted 11 October 2010 - 07:34 PM

A moist soil consists of 10 ft of sand resting on bedrock, with the groundwater table 4 ft below the surface. The unit weight of the saturated soil is 130 lbf/ft3, and the moist soil has a unit weight of 125 lbf/ft3. The earth pressure coefficient for both the saturated and moist soil is 0.40. What is most nearly the total vertical stress at a depth of 10 ft? The given answer is 1280 lbf/ft^2 but i get 1654 lbf/ft^2.

Here is how I am solving this:

Total Vertiical Stress = Effective STress + Pore Water Pressure

PORE water pressure = 6 * 62.4 = 374.4

Effective Stress: 125 (4) + 130( 6) = 1280

total vertiiclal stress: 374+ 1280 = 1654 lbf/ft^2

Not sure how the solution gives 1280 lbf/ft^2

let me know what I am doing wrong.

### #2 sac_engineer

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Posted 11 October 2010 - 07:48 PM

QUOTE (passpepasspe @ Oct 11 2010, 12:34 PM) <{POST_SNAPBACK}>
A moist soil consists of 10 ft of sand resting on bedrock, with the groundwater table 4 ft below the surface. The unit weight of the saturated soil is 130 lbf/ft3, and the moist soil has a unit weight of 125 lbf/ft3. The earth pressure coefficient for both the saturated and moist soil is 0.40. What is most nearly the total vertical stress at a depth of 10 ft? The given answer is 1280 lbf/ft^2 but i get 1654 lbf/ft^2.

Here is how I am solving this:

Total Vertiical Stress = Effective STress + Pore Water Pressure

PORE water pressure = 6 * 62.4 = 374.4

Effective Stress: 125 (4) + 130( 6) = 1280

total vertiiclal stress: 374+ 1280 = 1654 lbf/ft^2

Not sure how the solution gives 1280 lbf/ft^2

let me know what I am doing wrong.

Typical problem where more information is given than what is necessary to solve it.

You have 4 feet of moist soil sitting on top of 6 feet of saturated soil. The total vertical stress (not effective) at the bottom of the layer is the sum of the two vertical stresses above and below the water table:

125 x 4 = 500
130 x 6 = 780

Total = 1280

### #3 passpepasspe

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Posted 11 October 2010 - 08:54 PM

QUOTE (sac_engineer @ Oct 11 2010, 12:48 PM) <{POST_SNAPBACK}>
QUOTE (passpepasspe @ Oct 11 2010, 12:34 PM) <{POST_SNAPBACK}>
A moist soil consists of 10 ft of sand resting on bedrock, with the groundwater table 4 ft below the surface. The unit weight of the saturated soil is 130 lbf/ft3, and the moist soil has a unit weight of 125 lbf/ft3. The earth pressure coefficient for both the saturated and moist soil is 0.40. What is most nearly the total vertical stress at a depth of 10 ft? The given answer is 1280 lbf/ft^2 but i get 1654 lbf/ft^2.

Here is how I am solving this:

Total Vertiical Stress = Effective STress + Pore Water Pressure

PORE water pressure = 6 * 62.4 = 374.4

Effective Stress: 125 (4) + 130( 6) = 1280

total vertiiclal stress: 374+ 1280 = 1654 lbf/ft^2

Not sure how the solution gives 1280 lbf/ft^2

let me know what I am doing wrong.

Typical problem where more information is given than what is necessary to solve it.

You have 4 feet of moist soil sitting on top of 6 feet of saturated soil. The total vertical stress (not effective) at the bottom of the layer is the sum of the two vertical stresses above and below the water table:

125 x 4 = 500
130 x 6 = 780

Total = 1280

given this probelm, isnt there also pore pressure on the 6 ft of saturated soil coz there is water present? maybe i am not reading the problem correcty.. have I drawn the chart below correctly?

___________________

125 unit weight

_________V________4 ft (groundwater)

130 unit weight

__________________ 10
bedrock

### #4 Samuel

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Posted 11 October 2010 - 09:28 PM

QUOTE (passpepasspe @ Oct 11 2010, 04:54 PM) <{POST_SNAPBACK}>
QUOTE (sac_engineer @ Oct 11 2010, 12:48 PM) <{POST_SNAPBACK}>
QUOTE (passpepasspe @ Oct 11 2010, 12:34 PM) <{POST_SNAPBACK}>
A moist soil consists of 10 ft of sand resting on bedrock, with the groundwater table 4 ft below the surface. The unit weight of the saturated soil is 130 lbf/ft3, and the moist soil has a unit weight of 125 lbf/ft3. The earth pressure coefficient for both the saturated and moist soil is 0.40. What is most nearly the total vertical stress at a depth of 10 ft? The given answer is 1280 lbf/ft^2 but i get 1654 lbf/ft^2.

Here is how I am solving this:

Total Vertiical Stress = Effective STress + Pore Water Pressure

PORE water pressure = 6 * 62.4 = 374.4

Effective Stress: 125 (4) + 130( 6) = 1280

total vertiiclal stress: 374+ 1280 = 1654 lbf/ft^2

Not sure how the solution gives 1280 lbf/ft^2

let me know what I am doing wrong.

Typical problem where more information is given than what is necessary to solve it.

You have 4 feet of moist soil sitting on top of 6 feet of saturated soil. The total vertical stress (not effective) at the bottom of the layer is the sum of the two vertical stresses above and below the water table:

125 x 4 = 500
130 x 6 = 780

Total = 1280

given this probelm, isnt there also pore pressure on the 6 ft of saturated soil coz there is water present? maybe i am not reading the problem correcty.. have I drawn the chart below correctly?

___________________

125 unit weight

_________V________4 ft (groundwater)

130 unit weight

__________________ 10
bedrock

The question asked for total pressure and not for effective pressure. The water water pressure is not considered in this case so the answer is 1280.

### #5 palvarez83

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Posted 19 July 2011 - 03:15 PM

QUOTE (Samuel @ Oct 11 2010, 10:28 PM) <{POST_SNAPBACK}>
QUOTE (passpepasspe @ Oct 11 2010, 04:54 PM) <{POST_SNAPBACK}>
QUOTE (sac_engineer @ Oct 11 2010, 12:48 PM) <{POST_SNAPBACK}>
QUOTE (passpepasspe @ Oct 11 2010, 12:34 PM) <{POST_SNAPBACK}>
A moist soil consists of 10 ft of sand resting on bedrock, with the groundwater table 4 ft below the surface. The unit weight of the saturated soil is 130 lbf/ft3, and the moist soil has a unit weight of 125 lbf/ft3. The earth pressure coefficient for both the saturated and moist soil is 0.40. What is most nearly the total vertical stress at a depth of 10 ft? The given answer is 1280 lbf/ft^2 but i get 1654 lbf/ft^2.

Here is how I am solving this:

Total Vertiical Stress = Effective STress + Pore Water Pressure

PORE water pressure = 6 * 62.4 = 374.4

Effective Stress: 125 (4) + 130( 6) = 1280

total vertiiclal stress: 374+ 1280 = 1654 lbf/ft^2

Not sure how the solution gives 1280 lbf/ft^2

let me know what I am doing wrong.

Typical problem where more information is given than what is necessary to solve it.

You have 4 feet of moist soil sitting on top of 6 feet of saturated soil. The total vertical stress (not effective) at the bottom of the layer is the sum of the two vertical stresses above and below the water table:

125 x 4 = 500
130 x 6 = 780

Total = 1280

given this probelm, isnt there also pore pressure on the 6 ft of saturated soil coz there is water present? maybe i am not reading the problem correcty.. have I drawn the chart below correctly?

___________________

125 unit weight

_________V________4 ft (groundwater)

130 unit weight

__________________ 10
bedrock

The question asked for total pressure and not for effective pressure. The water water pressure is not considered in this case so the answer is 1280.

130 #/cubicft is the weight of saturated soil. That includes the water weight.

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