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Porosity

Geotechnical · FE Reference Handbook section

Geotechnical
1 formulas
10 exam-style examples
~47 min
All Geotechnical lectures

Core formulas for this FE topic

Definitions, applicability, units, assumptions and worked examples for each relation.

Worked exam-style examples

The four ways this section is written on the real exam — thoughts first, then equations, then substitution.

Example 1
Void ratio–porosity relation — solve for void ratio — Porosity

A geotechnical problem uses Void ratio–porosity relation. Given porosity (n) = 0.5700, determine the void ratio (e).

Given

  • porosity(n)=0.5700porosity (n) = 0.5700

Find

void ratio (e)

Start with the thinking

  • The governing relation printed in this handbook section is Void ratio–porosity relation.
  • Everything except e is given, so isolate e symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Geotechnical items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    e=n/(1−n)e = n / (1 - n)
  2. Step 2 — Rearrange the relation so that e stands alone on the left-hand side.

  3. Step 3

    Listthegivens:porosity(n)=0.5700List the givens: porosity (n) = 0.5700
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    e=1.3256e = 1.3256
  6. Step 6 — Check: returning e = 1.3256 to

    e=n/(1−n)e = n / (1 - n)

    reproduces the given quantities, and both sides carry the same units.

Answer:
e=1.3256e = 1.3256

Why the other options are there

  • 2.6512 — kept a factor of two that cancels in the correct rearrangement.
  • 0.6628 — dropped that same factor in the other direction.
  • 1.4581 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Geotechnical → Porosity

Example 2
Void ratio–porosity relation — solve for porosity — Porosity (2)

A geotechnical problem uses Void ratio–porosity relation. Given void ratio (e) = 0.9480, determine the porosity (n).

Given

  • voidratio(e)=0.9480void ratio (e) = 0.9480

Find

porosity (n)

Start with the thinking

  • The governing relation printed in this handbook section is Void ratio–porosity relation.
  • Everything except n is given, so isolate n symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Geotechnical items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    e=n/(1−n)e = n / (1 - n)
  2. Step 2 — Rearrange the relation so that n stands alone on the left-hand side.

  3. Step 3

    Listthegivens:voidratio(e)=0.9480List the givens: void ratio (e) = 0.9480
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    n=0.4867n = 0.4867
  6. Step 6 — Check: returning n = 0.4867 to

    e=n/(1−n)e = n / (1 - n)

    reproduces the given quantities, and both sides carry the same units.

Answer:
n=0.4867n = 0.4867

Why the other options are there

  • 0.9733 — kept a factor of two that cancels in the correct rearrangement.
  • 0.2433 — dropped that same factor in the other direction.
  • 0.5353 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Geotechnical → Porosity

Example 3
Void ratio–porosity relation — solve for void ratio (case 2) — Porosity (3)

A geotechnical problem uses Void ratio–porosity relation. Given porosity (n) = 0.4700, determine the void ratio (e).

Given

  • porosity(n)=0.4700porosity (n) = 0.4700

Find

void ratio (e)

Start with the thinking

  • The governing relation printed in this handbook section is Void ratio–porosity relation.
  • Everything except e is given, so isolate e symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Geotechnical items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    e=n/(1−n)e = n / (1 - n)
  2. Step 2 — Rearrange the relation so that e stands alone on the left-hand side.

  3. Step 3

    Listthegivens:porosity(n)=0.4700List the givens: porosity (n) = 0.4700
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    e=0.8868e = 0.8868
  6. Step 6 — Check: returning e = 0.8868 to

    e=n/(1−n)e = n / (1 - n)

    reproduces the given quantities, and both sides carry the same units.

Answer:
e=0.8868e = 0.8868

Why the other options are there

  • 1.7736 — kept a factor of two that cancels in the correct rearrangement.
  • 0.4434 — dropped that same factor in the other direction.
  • 0.9755 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Geotechnical → Porosity

Example 4
Void ratio–porosity relation — solve for porosity (case 2) — Porosity (4)

A geotechnical problem uses Void ratio–porosity relation. Given void ratio (e) = 0.3960, determine the porosity (n).

Given

  • voidratio(e)=0.3960void ratio (e) = 0.3960

Find

porosity (n)

Start with the thinking

  • The governing relation printed in this handbook section is Void ratio–porosity relation.
  • Everything except n is given, so isolate n symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Geotechnical items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    e=n/(1−n)e = n / (1 - n)
  2. Step 2 — Rearrange the relation so that n stands alone on the left-hand side.

  3. Step 3

    Listthegivens:voidratio(e)=0.3960List the givens: void ratio (e) = 0.3960
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    n=0.2837n = 0.2837
  6. Step 6 — Check: returning n = 0.2837 to

    e=n/(1−n)e = n / (1 - n)

    reproduces the given quantities, and both sides carry the same units.

Answer:
n=0.2837n = 0.2837

Why the other options are there

  • 0.5673 — kept a factor of two that cancels in the correct rearrangement.
  • 0.1418 — dropped that same factor in the other direction.
  • 0.3120 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Geotechnical → Porosity

Example 5
Void ratio–porosity relation — solve for void ratio (case 3) — Porosity (5)

A geotechnical problem uses Void ratio–porosity relation. Given porosity (n) = 0.3700, determine the void ratio (e).

Given

  • porosity(n)=0.3700porosity (n) = 0.3700

Find

void ratio (e)

Start with the thinking

  • The governing relation printed in this handbook section is Void ratio–porosity relation.
  • Everything except e is given, so isolate e symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Geotechnical items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    e=n/(1−n)e = n / (1 - n)
  2. Step 2 — Rearrange the relation so that e stands alone on the left-hand side.

  3. Step 3

    Listthegivens:porosity(n)=0.3700List the givens: porosity (n) = 0.3700
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    e=0.5873e = 0.5873
  6. Step 6 — Check: returning e = 0.5873 to

    e=n/(1−n)e = n / (1 - n)

    reproduces the given quantities, and both sides carry the same units.

Answer:
e=0.5873e = 0.5873

Why the other options are there

  • 1.1746 — kept a factor of two that cancels in the correct rearrangement.
  • 0.2937 — dropped that same factor in the other direction.
  • 0.6460 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Geotechnical → Porosity

Example 6
Void ratio–porosity relation — solve for porosity (case 3) — Porosity (6)

A geotechnical problem uses Void ratio–porosity relation. Given void ratio (e) = 1.3850, determine the porosity (n).

Given

  • voidratio(e)=1.3850void ratio (e) = 1.3850

Find

porosity (n)

Start with the thinking

  • The governing relation printed in this handbook section is Void ratio–porosity relation.
  • Everything except n is given, so isolate n symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Geotechnical items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    e=n/(1−n)e = n / (1 - n)
  2. Step 2 — Rearrange the relation so that n stands alone on the left-hand side.

  3. Step 3

    Listthegivens:voidratio(e)=1.3850List the givens: void ratio (e) = 1.3850
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    n=0.5807n = 0.5807
  6. Step 6 — Check: returning n = 0.5807 to

    e=n/(1−n)e = n / (1 - n)

    reproduces the given quantities, and both sides carry the same units.

Answer:
n=0.5807n = 0.5807

Why the other options are there

  • 1.1614 — kept a factor of two that cancels in the correct rearrangement.
  • 0.2904 — dropped that same factor in the other direction.
  • 0.6388 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Geotechnical → Porosity

Example 7
Void ratio–porosity relation — solve for void ratio (case 4) — Porosity (7)

A geotechnical problem uses Void ratio–porosity relation. Given porosity (n) = 0.3200, determine the void ratio (e).

Given

  • porosity(n)=0.3200porosity (n) = 0.3200

Find

void ratio (e)

Start with the thinking

  • The governing relation printed in this handbook section is Void ratio–porosity relation.
  • Everything except e is given, so isolate e symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Geotechnical items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    e=n/(1−n)e = n / (1 - n)
  2. Step 2 — Rearrange the relation so that e stands alone on the left-hand side.

  3. Step 3

    Listthegivens:porosity(n)=0.3200List the givens: porosity (n) = 0.3200
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    e=0.4706e = 0.4706
  6. Step 6 — Check: returning e = 0.4706 to

    e=n/(1−n)e = n / (1 - n)

    reproduces the given quantities, and both sides carry the same units.

Answer:
e=0.4706e = 0.4706

Why the other options are there

  • 0.9412 — kept a factor of two that cancels in the correct rearrangement.
  • 0.2353 — dropped that same factor in the other direction.
  • 0.5176 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Geotechnical → Porosity

Example 8
Void ratio–porosity relation — solve for porosity (case 4) — Porosity (8)

A geotechnical problem uses Void ratio–porosity relation. Given void ratio (e) = 0.9800, determine the porosity (n).

Given

  • voidratio(e)=0.9800void ratio (e) = 0.9800

Find

porosity (n)

Start with the thinking

  • The governing relation printed in this handbook section is Void ratio–porosity relation.
  • Everything except n is given, so isolate n symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Geotechnical items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    e=n/(1−n)e = n / (1 - n)
  2. Step 2 — Rearrange the relation so that n stands alone on the left-hand side.

  3. Step 3

    Listthegivens:voidratio(e)=0.9800List the givens: void ratio (e) = 0.9800
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    n=0.4949n = 0.4949
  6. Step 6 — Check: returning n = 0.4949 to

    e=n/(1−n)e = n / (1 - n)

    reproduces the given quantities, and both sides carry the same units.

Answer:
n=0.4949n = 0.4949

Why the other options are there

  • 0.9899 — kept a factor of two that cancels in the correct rearrangement.
  • 0.2475 — dropped that same factor in the other direction.
  • 0.5444 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Geotechnical → Porosity

Example 9
Void ratio–porosity relation — solve for void ratio (case 5) — Porosity (9)

A geotechnical problem uses Void ratio–porosity relation. Given porosity (n) = 0.4300, determine the void ratio (e).

Given

  • porosity(n)=0.4300porosity (n) = 0.4300

Find

void ratio (e)

Start with the thinking

  • The governing relation printed in this handbook section is Void ratio–porosity relation.
  • Everything except e is given, so isolate e symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Geotechnical items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    e=n/(1−n)e = n / (1 - n)
  2. Step 2 — Rearrange the relation so that e stands alone on the left-hand side.

  3. Step 3

    Listthegivens:porosity(n)=0.4300List the givens: porosity (n) = 0.4300
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    e=0.7544e = 0.7544
  6. Step 6 — Check: returning e = 0.7544 to

    e=n/(1−n)e = n / (1 - n)

    reproduces the given quantities, and both sides carry the same units.

Answer:
e=0.7544e = 0.7544

Why the other options are there

  • 1.5088 — kept a factor of two that cancels in the correct rearrangement.
  • 0.3772 — dropped that same factor in the other direction.
  • 0.8298 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Geotechnical → Porosity

Example 10
Void ratio–porosity relation — solve for porosity (case 5) — Porosity (10)

A geotechnical problem uses Void ratio–porosity relation. Given void ratio (e) = 1.3420, determine the porosity (n).

Given

  • voidratio(e)=1.3420void ratio (e) = 1.3420

Find

porosity (n)

Start with the thinking

  • The governing relation printed in this handbook section is Void ratio–porosity relation.
  • Everything except n is given, so isolate n symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Geotechnical items reward recognising the unknown before touching a calculator.

Step-by-step solution

  1. Step 1 — State the governing relation:

    e=n/(1−n)e = n / (1 - n)
  2. Step 2 — Rearrange the relation so that n stands alone on the left-hand side.

  3. Step 3

    Listthegivens:voidratio(e)=1.3420List the givens: void ratio (e) = 1.3420
  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    n=0.5730n = 0.5730
  6. Step 6 — Check: returning n = 0.5730 to

    e=n/(1−n)e = n / (1 - n)

    reproduces the given quantities, and both sides carry the same units.

Answer:
n=0.5730n = 0.5730

Why the other options are there

  • 1.1460 — kept a factor of two that cancels in the correct rearrangement.
  • 0.2865 — dropped that same factor in the other direction.
  • 0.6303 — rounded an intermediate value before the final step.

Reference: FE Reference Handbook — Geotechnical → Porosity

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