Porosity
Geotechnical · FE Reference Handbook section
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.
A geotechnical problem uses Void ratio–porosity relation. Given porosity (n) = 0.5700, determine the void ratio (e).
Given
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that e stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning e = 1.3256 to
reproduces the given quantities, and both sides carry the same units.
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
A geotechnical problem uses Void ratio–porosity relation. Given void ratio (e) = 0.9480, determine the porosity (n).
Given
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that n stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning n = 0.4867 to
reproduces the given quantities, and both sides carry the same units.
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
A geotechnical problem uses Void ratio–porosity relation. Given porosity (n) = 0.4700, determine the void ratio (e).
Given
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that e stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning e = 0.8868 to
reproduces the given quantities, and both sides carry the same units.
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
A geotechnical problem uses Void ratio–porosity relation. Given void ratio (e) = 0.3960, determine the porosity (n).
Given
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that n stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning n = 0.2837 to
reproduces the given quantities, and both sides carry the same units.
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
A geotechnical problem uses Void ratio–porosity relation. Given porosity (n) = 0.3700, determine the void ratio (e).
Given
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that e stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning e = 0.5873 to
reproduces the given quantities, and both sides carry the same units.
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
A geotechnical problem uses Void ratio–porosity relation. Given void ratio (e) = 1.3850, determine the porosity (n).
Given
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that n stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning n = 0.5807 to
reproduces the given quantities, and both sides carry the same units.
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
A geotechnical problem uses Void ratio–porosity relation. Given porosity (n) = 0.3200, determine the void ratio (e).
Given
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that e stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning e = 0.4706 to
reproduces the given quantities, and both sides carry the same units.
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
A geotechnical problem uses Void ratio–porosity relation. Given void ratio (e) = 0.9800, determine the porosity (n).
Given
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that n stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning n = 0.4949 to
reproduces the given quantities, and both sides carry the same units.
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
A geotechnical problem uses Void ratio–porosity relation. Given porosity (n) = 0.4300, determine the void ratio (e).
Given
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that e stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning e = 0.7544 to
reproduces the given quantities, and both sides carry the same units.
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
A geotechnical problem uses Void ratio–porosity relation. Given void ratio (e) = 1.3420, determine the porosity (n).
Given
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
Step 1 — State the governing relation:
Step 2 — Rearrange the relation so that n stands alone on the left-hand side.
Step 3
Step 4 — Substitute the given values into the rearranged relation.
Step 5 — Evaluate:
Step 6 — Check: returning n = 0.5730 to
reproduces the given quantities, and both sides carry the same units.
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