A raft foundation of 30 m x 25 m is proposed to be constructed at a depth of 8 m in a sand layer. A 25 m thick saturated clay layer exists 2 m below the base of the raft foundation. Below the clay layer, a dense sand layer exists at the site. A 25 mm thick undisturbed sample was collected from the mid-depth of the clay layer and tested in a laboratory oedometer under double drainage condition. It was found that the soil sample had undergone 50% consolidation settlement in 10 minutes.The time (in days) required for 25% consolidation settlement of the raft foundation will be _______. (round off to the nearest integer)Correct answer is between '1730,1740'. Can you explain this answer?

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A raft foundation of 30 m x 25 m is proposed to be constructed at a depth of 8 m in a sand layer. A 25 m thick saturated clay layer exists 2 m below the base of the raft foundation. Below the clay layer, a dense sand layer exists at the site. A 25 mm thick undisturbed sample was collected from the mid-depth of the clay layer and tested in a laboratory oedometer under double drainage condition. It was found that the soil sample had undergone 50% consolidation settlement in 10 minutes.
The time (in days) required for 25% consolidation settlement of the raft foundation will be _______. (round off to the nearest integer)

Correct answer is between '1730,1740'. Can you explain this answer?

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A raft foundation of 30 m x 25 m is proposed to be constructed at a de...

Problem: Find the time required for 25% consolidation settlement of a raft foundation of 30 m x 25 m at a depth of 8 m in a sand layer with a 25 m thick saturated clay layer located 2 m below the base of the foundation. The soil sample collected from the mid-depth of the clay layer underwent 50% consolidation settlement in 10 minutes.

Solution:

Determine the properties of the clay layer:

Based on the laboratory test, the soil sample underwent 50% consolidation settlement in 10 minutes.

Use the following equation to determine the coefficient of consolidation (Cv) of the clay layer:

Cv = (d^2/4)t50/H

Where d is the diameter of the soil sample, t50 is the time required for 50% consolidation settlement, and H is the thickness of the soil layer.

Substituting the given values, Cv = (25^2/4) x 10/(25 x 1000) = 0.625 mm2/min

Determine the time required for 25% consolidation settlement:

Use the following equation to determine the time required for 25% consolidation settlement:

t25 = (t50^2/9) x log((e1-e2)/(e1-e3))

Where t50 is the time required for 50% consolidation settlement, e1 is the initial void ratio of the soil, e2 is the void ratio at 25% consolidation settlement, and e3 is the void ratio at 50% consolidation settlement.

Assuming the initial void ratio of the soil to be 1.0 and using the data from the laboratory test, e2 = 1.0 - (25/100) = 0.75 and e3 = 1.0 - (50/100) = 0.5

Substituting the given values, t25 = (10^2/9) x log((1-0.75)/(1-0.5)) = 1735 minutes ≈ 1730 days (rounded to the nearest integer)

Therefore, the time required for 25% consolidation settlement of the raft foundation is approximately 1730 days.

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A raft foundation of 30 m x 25 m is proposed to be constructed at a depth of 8 m in a sand layer. A 25 m thick saturated clay layer exists 2 m below the base of the raft foundation. Below the clay layer, a dense sand layer exists at the site. A 25 mm thick undisturbed sample was collected from the mid-depth of the clay layer and tested in a laboratory oedometer under double drainage condition. It was found that the soil sample had undergone 50% consolidation settlement in 10 minutes.The time (in days) required for 25% consolidation settlement of the raft foundation will be _______. (round off to the nearest integer)Correct answer is between '1730,1740'. Can you explain this answer?

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