---
title: "Field methods for in-situ density"
subject: "Soil Mechanics"
chapter: 2
key: "semester_4/soil_mechanics:field-density-methods"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/field-density-methods
site: BCT Engineering
---

# Field methods for in-situ density

Soil Mechanics · Chapter 2: Solids- Water –Air Relations and Index properties of soils

- Asked in: 2078 Poush, 4 marks; 2077 Chaitra, 2 marks (asked 2 times)
- Video: [Field Density Test (FDT) Methods - Procedure - Comparison](https://www.youtube.com/watch?v=SLKx1KKzEm8) — Iamcivilengineer
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/field-density-methods
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-2

## Question

Describe the different field methods used to determine the in-situ (dry) density / dry unit weight of soil, based on site conditions.

## Short answer

Core cutter (soft cohesive soils) drives a cylinder to obtain volume; bulk density divided by (1+w) gives dry density. Sand replacement (gravelly/sandy soils) uses calibrated sand to measure hole volume. Water displacement, rubber balloon, nuclear gauge, and wax-coated lump are also employed depending on site conditions.

## Answer

The in-situ density is found by taking out a known volume of soil from the ground and weighing it; the water content gives the dry density, $\gamma_d = \gamma/(1+w)$. The method depends on the soil type.

#### 1. Core cutter method (IS 2720 Part 29)
Used for **soft to medium cohesive soils** that stand without crumbling. A steel cylinder (usually 100 mm dia, 130 mm high) is driven into the ground with a dolly and rammer, dug out with the soil inside, trimmed and weighed. Volume = volume of the cutter, so $\gamma = W/V$.

#### 2. Sand replacement method (IS 2720 Part 28)
Used for **gravelly and sandy soils**, hard clay, or where the core cutter cannot be used. A hole is dug, the soil is weighed, and the hole volume is found by filling it with calibrated dry sand (Ottawa/ standard sand) from a pouring cylinder.
$$
V = \frac{W_{sand\ in\ hole}}{\gamma_{sand}}
$$

#### 3. Water displacement method
Used for soils with **irregular holes**. The hole is lined with a thin rubber balloon or plastic sheet and filled with a measured volume of water to find the volume.

#### 4. Rubber balloon method
A balloon density apparatus measures the volume of the hole directly from the water pressed into the balloon.

#### 5. Nuclear density gauge
Gives density and water content quickly by gamma-ray backscatter; used for rapid quality control of compaction.

#### 6. Wax-coated lump (paraffin) method
For **cohesive, hard lumps** of irregular shape; the lump is coated with wax and its volume found by water displacement.

**Selection:** core cutter for soft fine-grained soil, sand replacement for coarse soils, and nuclear gauge for large-scale quick control.

## Check yourself

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

1. The core cutter method is suitable for:
   A. Gravelly soil
   B. Loose dry sand only
   C. Hard rock
   D. Soft to medium cohesive soils that stand without crumbling
   Answer: D. The cutter is driven into soil that holds its shape; volume equals cutter volume. (Key point: Core cutter use)

2. In the sand replacement method, hole volume is found by:
   A. Measuring with a ruler
   B. Weight of calibrated sand filling the hole / unit weight of sand
   C. Gamma-ray backscatter
   D. Wax coating
   Answer: B. V = W(sand in hole)/gamma(sand). (Key point: Sand replacement)

3. After finding in-situ bulk unit weight, dry density is obtained from:
   A. gamma_d = gamma x (1+w)
   B. gamma_d = gamma x w
   C. gamma_d = gamma - w
   D. gamma_d = gamma/(1+w)
   Answer: D. Water content converts bulk to dry unit weight. (Key point: Dry density from field)

4. The nuclear density gauge is used for:
   A. Hard lumps only
   B. Rapid large-scale quality control of compaction
   C. Measuring specific gravity
   D. Laboratory consolidation
   Answer: B. It gives density and water content quickly using gamma-ray backscatter. (Key point: Nuclear gauge)

5. The wax-coated lump method suits:
   A. Loose sand
   B. Liquid soils
   C. Gravel fill
   D. Cohesive, hard irregular lumps, volume by water displacement
   Answer: D. The lump is coated with wax and its volume found by displacement. (Key point: Wax method)

## Related in Soil Mechanics

- [Core cutter field density numerical](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acore-cutter-density-numerical) — Soil Mechanics, chapter 2
- [Theoretical maximum dry density](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atheoretical-maximum-dry-density) — Soil Mechanics, chapter 5
- [Infinite slope FoS, dry cohesive soil](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Ainfinite-slope-dry-cohesive-derive) — Soil Mechanics, chapter 11
- [Embankment excavation quantity (18 kN/m³)](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aembankment-excavated-quantity-2079) — Soil Mechanics, chapter 2
- [Triaxial drainage conditions in the field](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atriaxial-drainage-field-conditions) — Soil Mechanics, chapter 10
- [Infinite dry slope, c = 10 kPa, φ = 25°](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Ainfinite-dry-slope-c10-phi25) — Soil Mechanics, chapter 11

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