---
title: "Rate analysis: 30 cm compacted gravel subgrade"
subject: "Estimating and Costing"
chapter: 4
key: "semester_7/estimating_and_costing:rate-gravel-subgrade-30cm"
source: https://www.bctengineer.com/q/semester_7/estimating_and_costing/rate-gravel-subgrade-30cm
site: BCT Engineering
---

# Rate analysis: 30 cm compacted gravel subgrade

Estimating and Costing · Chapter 4: Analysis of Rates

- Asked in: 2079 Bhadra, 8 marks
- Video: [How to calculate Granular Sub-Base (GSB) quantity with Compaction Factor](https://www.youtube.com/watch?v=tCs8D_NWFzg) — Civil Field Engineer
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/estimating_and_costing/rate-gravel-subgrade-30cm
- Whole chapter: https://www.bctengineer.com/md/semester_7/estimating_and_costing/chapter-4

## Question

Prepare analysis of rates for providing, laying and consolidation of 30 cm thick compacted gravel for sub grade per square meter.

## Short answer

Compacted volume 0.3 m3/m2, loose factor 1.30, laid in two layers. For 100 m2, gravel 39 m3, unskilled 12 md, skilled 1.5 md, roller 1.5 hr. Rate = Rs 1,588.73 per m2, excluding VAT.

## Answer

#### Basis
Rate for **providing, laying and consolidating 30 cm thick compacted gravel in the sub-grade**, per 100 m², laid and compacted in **two layers of 15 cm** (a single layer thicker than about 15 to 20 cm cannot be compacted properly). Assumptions (illustrative):
- Compacted volume = 100 x 0.30 = 30.0 m³; loose volume = compacted x 1.30 = 39.0 m³.
- Water for compaction = 12% of compacted volume = 3.60 m³.
- Labour 0.4 unskilled md and 0.05 skilled md per m³ of compacted gravel; roller 0.05 hr per m³.

#### Analysis (per 100 m²)
| Description | Qty | Unit | Rate (Rs) | Amount (Rs) |
|---|---|---|---|---|
| Gravel (sub-base grade), loose 39.00 m³ | 39 | m³ | 3,000.00 | 117,000.00 |
| Labour: spreading, levelling (unskilled) | 12 | md | 900.00 | 10,800.00 |
| Labour: skilled (grade checker / roller operator helper) | 1.5 | md | 1,200.00 | 1,800.00 |
| Water for compaction (3.60 m³) | 3.6 | m³ | 500.00 | 1,800.00 |
| 10-t vibrating roller, 1.50 hr | 1.5 | hr | 4,500.00 | 6,750.00 |
| **Sub-total** | | | | **138,150.00** |
| Contractor's overhead and profit | 15% | | | 20,722.50 |
| **Total per 100 m²** | | | | **158,872.50** |

**Answer: Rate = 158,872.50 / 100 = Rs 1,588.73 per m²** (excluding VAT).

Check: the 30 cm rate is three times the 10 cm rate (3 x Rs 529.575 = Rs 1,588.73 per m²), as every quantity scales with the thickness.

## Check yourself

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

1. Why is the 30 cm gravel laid and compacted in two layers of 15 cm?
   A. The roller can only work in 15 cm passes of water
   B. Gravel is supplied only in 15 cm lifts
   C. Two layers double the roller rate for payment
   D. A single layer thicker than about 15 to 20 cm cannot be compacted properly
   Answer: D. Compaction effort does not reach through a thick layer, so thickness is limited to roughly 15-20 cm per layer. (Key point: Layer thickness)

2. For 100 m^2 at 30 cm compacted, what loose gravel quantity is used?
   A. 30.0 m^3
   B. 13.0 m^3
   C. 39.0 m^3 (30.0 x 1.30)
   D. 46.8 m^3
   Answer: C. Compacted volume 100 x 0.30 = 30.0 m^3, multiplied by the compaction factor 1.30 = 39.0 m^3. (Key point: Loose volume)

3. Roller is 0.05 hr per m^3 of compacted gravel. What roller time applies for 100 m^2 of 30 cm?
   A. 0.5 hr
   B. 1.5 hr (0.05 x 30.0)
   C. 1.95 hr (0.05 x 39.0)
   D. 3.0 hr
   Answer: B. Roller time uses the compacted volume 30.0 m^3: 0.05 x 30 = 1.5 hr. (Key point: Roller hours)

4. How does the rate for 30 cm compare to that for 10 cm?
   A. It is three times, since every quantity scales with thickness
   B. It is equal, since the roller hire is fixed
   C. It is nine times, as there are two layers
   D. It is 1.5 times, as only gravel increases
   Answer: A. Gravel, water, labour and roller all scale with thickness, so Rs 1,588.72 is three times Rs 529.58. (Key point: Scaling check)

## Close variants set in other exams

- [Rate analysis: 10 cm compacted gravel subgrade](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Arate-gravel-subgrade-10cm) — Estimating and Costing, chapter 4, asked 2×