Sand Replacement (In-situ Density) Method (IS 2720-Part-28-1974) | Geotechnical engineering

Introduction

This is the most common technique used for the in-situ determination of soil density. In-situ density is the density of the soil determined at its actual depth in the field. It plays a very significant role in determining soil stability, bearing capacity, and also compaction. With regard to alteration in moisture content, usually dry density is reported with moisture content.

Sand Replacement test procedure as per IS code 

This blog post is for a comprehensive overview of the sand replacement method, its applications, and the step-by-step procedure for conducting the test.

Theory

Clearly, in-situ density of soil depends on the moisture content in it. Therefore, results in dry density are generally reported, allowing direct comparison. In-situ density can be applied for assessing the strength of the soil and also to predict settlement rates afterwards and design structures based upon its stability.

Need & Scope

In-situ density of soil is widely applied in civil engineering operations like these:
Soils: Bearing Capacity: The in-situ density helps evaluate the bearing capacity of the soil for foundation design. 
Stability Analysis: Stability of slopes, earth embankments, and retaining structures require knowledge of the in-situ density of the soil. 
Settlement Calculations: The density helps in calculating pressures caused on the underlying layers in underground structures. 
 Soil compaction: soil compaction controls the quality of embankment and earth dam constructions by using the sand replacement method to check soil compaction values.

Apparatus Required

The sand replacement method requires the following equipment:

1. Sand pouring apparatus with a conical base
2. Calibration cylinder to measure the volume of sand
3. Glass plate for calibration
4. Vernier calipers to measure dimensions
5. Standard sand (graded between 300 and 600 microns)
6. Soil tray with a central hole
7. Digging tools (e.g., chisels)
8. Balance (20 kg capacity) for weighing soil
9. Containers for moisture content analysis
10. Sensitive balance accurate to 0.01 gm
11. Oven controlled at 105°C to dry the soil

Procedure

1. Determining the Weight of Sand Occupying the Cone

1. Fill the sand pouring apparatus (with its valve closed) with sand and weigh it (W1).
2. Place the apparatus on a smooth glass plate, open the valve, and allow the sand to fill the conical portion. Once the sand stops flowing, close the valve and weigh the remaining sand (W2).
3. The weight of sand occupying the cone is calculated as W1 - W2.

2. Determining the Density of Sand

1. Refill the sand pouring apparatus (valve closed) and weigh it (W3).
2. Position the apparatus over the calibration cylinder and open the valve to fill the cylinder and cone.
3. Once the sand stops flowing, close the valve and weigh the remaining sand (W4).
4. The weight of sand in the calibration cylinder is determined as (W3 - W4) - (W1 - W2).
5. Calculate the volume of the calibration cylinder (Vc) by either measuring its dimensions or filling it with water.
6. The density of the sand is calculated as (Weight of sand in cylinder / Vc).

3. Determining the Density of Soil

1. Prepare a flat testing surface and place the soil tray firmly in place.
2. Excavate a hole in the soil (diameter matching the tray hole, depth around 10 cm). Weigh the excavated soil.
3. Weigh the sand pouring apparatus before the test (W5) and place it over the hole. Open the valve to fill the cavity with sand.
4. Weigh the apparatus after the test (W6). The weight of sand in the cavity is (W6 - W5) - (W1 - W2).
5. Calculate the volume of the cavity using the sand density and the sand weight filling the cavity.
6. Determine the soil's bulk density using the weight of the excavated soil and the volume of the cavity.
7. Collect soil samples for moisture content analysis.

I) Density of Standard Sand

Particulars	Values
a) Sand Occupying Cone
Weight of sand pouring cylinder + sand before opening shutter (W1)
Weight of sand pouring cylinder + sand after opening shutter (W2)
Weight of sand in cone (W1 - W2)
b) Sand Occupying Calibrating Cylinder
Weight of sand pouring cylinder + sand before opening shutter (W3)
Weight of sand pouring cylinder + sand after opening shutter (W4)
Weight of sand in Calibrating Cylinder Wc = (W3 - W4) – (W1 - W2)
Density of the standard sand (γs) = Wc / Vc

II) In-situ Density using Sand Replacement Method

No.	Particulars	Values
1	Bulk density of standard sand, γs (gm/cc)
2	Weight of sand pouring apparatus + sand before experiment (W5), gm
3	Weight of sand pouring apparatus + sand after experiment (W6), gm
4	Weight of sand drained out (W6 - W5), gm
5	Weight of sand occupying cone (W1 - W2), gm
6	Weight of sand occupying cavity [Ws = (W6 - W5) – (W1 - W2)], gm
7	Volume of cavity (V = Ws / γs), cc
8	Weight of soil scooped out from the cavity (W), gm
9	Bulk density (γt = W / V), gm/cc
10	Moisture content, w%
11	Dry density, [γd = γt / (1 + w)], (gm/cc)

Moisture Content Percent

Container No.	Wt. of container (Wt)	Wt. of container + Wet soil (Ww)	Wt. of container + Dry soil (Wd)	Wt. of water (Ww - Wd)	Wt. of dry soil (Wd - Wt)	Moisture content, w% = (Ww - Wd) / (Wd - Wt) * 100

Average moisture content, w% = _____________

Average dry density of the soil layer (gm/cc) = ___________________

Conclusion

The sand replacement method is an effective, reliable technique for determining in-situ soil density. This method is essential for ensuring the stability of foundations, slopes, embankments, and other critical structures. Accurate determination of dry density allows engineers to evaluate compaction and assess the bearing capacity of soils, contributing to safer and more efficient civil engineering designs.

Reference 

• Indian Standards (IS) 2720 (Part 28): 1974 (Reaffirmed 2010)
• Soil Testing Manual: Procedures, Classification Data, and Sampling Practices by Gregory T. Nimmo
• Soil Mechanics and Foundations by B.C. Punmia, Ashok Kumar Jain, Arun Kumar Jain
• Geotechnical Engineering Principles and Practices by Donald P. Coduto, Man-chu Ronald Yeung, William A. Kitch