---
title: "True thickness: limestone vertical 600 m"
subject: "Engineering Geology II"
chapter: 7
key: "semester_4/engineering_geology_ii:true-thickness-limestone-600m"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/true-thickness-limestone-600m
site: BCT Engineering
---

# True thickness: limestone vertical 600 m

Engineering Geology II · Chapter 7: Geology and Construction Materials

- Asked in: 2078 Poush
- Video: [True thickness& Vertical thickness](https://www.youtube.com/watch?v=Gk9nBZHexu0) — Geo Nabaa
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/true-thickness-limestone-600m
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-7

## Question

Calculate the true thickness of limestone bed having vertical thickness 600 m at North 75°E/50° NW in an area of 550 km².

## Short answer

True thickness = vertical thickness × cos(dip) = 600 cos50° = 385.7 m.

## Answer

**Principle:** the vertical thickness $t_v$ (as in a vertical hole) is converted to the true thickness (perpendicular to bedding) by
$$
t = t_v\cos\delta
$$
where $\delta$ is the dip of the bed. The strike (N75°E) and dip direction (NW) do not affect the thickness; only the dip amount, $50^\circ$, is needed.

#### Calculation
$$
\begin{aligned}
t &= 600\cos 50^\circ \\
&= 600 \times 0.6428 \\
&= 385.67\ \text{m}
\end{aligned}
$$

**Answer: true thickness of the limestone = 385.7 m.**

#### Volume under 550 km² (for reserve)
$$
V = 550\ \text{km}^2 \times 0.38567\ \text{km} = 212.1\ \text{km}^3 \approx 2.12\times 10^{11}\ \text{m}^3
$$
(taking the 550 km² as the area measured on the bed).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which attitude data affects true thickness?
   A. Only the strike N75°E
   B. Only the dip direction NW
   C. All three equally
   D. Only the dip amount (50°)
   Answer: D. t = tv cos δ depends on the dip alone. (Key point: Which data matter)

2. What true thickness results for tv = 600 m?
   A. About 459.6 m
   B. About 385.7 m
   C. About 600 m
   D. About 933 m
   Answer: B. 600 x 0.6428 = 385.67 m. (Key point: True thickness)

3. What is the volume under 550 km²?
   A. About 330 km³
   B. About 550 km³
   C. About 0.212 km³
   D. About 212.1 km³
   Answer: D. 550 x 0.38567 = 212.1 km³. (Key point: Volume)

## Related in Engineering Geology II

- [Limestone true thickness: A 600 m N28E, C 400 m S10W](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Abh-limestone-true-thickness-a600) — Engineering Geology II, chapter 6, asked 2×
- [Limestone true thickness across stream](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Astream-limestone-true-thickness) — Engineering Geology II, chapter 6
- [True thickness from vertical thickness 150 m](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atrue-thickness-vertical-150-dip-47) — Engineering Geology II, chapter 6
- [Limestone reserve for cement factory](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Areserve-limestone-cement-density) — Engineering Geology II, chapter 7
- [True thickness from apparent thickness 210 m](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Abh-quartzite-thickness-210-attitude) — Engineering Geology II, chapter 6
- [Reserve: limestone dip 30, apparent 20 m](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Areserve-limestone-dip-30) — Engineering Geology II, chapter 7

## Related in other subjects

- [Strike of limestone bed dipping S55°E](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Astrike-of-limestone-bed-dipping) — Engineering Geology I, chapter 5
- [Strike of limestone bed inclined east at 45°](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Astrike-of-limestone-bed-inclined) — Engineering Geology I, chapter 5
- [Strike of limestone bed directed N20°E, dip 45°](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Astrike-of-limestone-bed-directed) — Engineering Geology I, chapter 5
- [Cross drainage: Q 50, HFD 550](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acd-q50-bed30-fsd21-hfd550) — Irrigation and Drainage Engineering, chapter 8, asked 2×