Determination of In-situ Density of Soil by Water Replacement Method

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The water replacement test method is used for the determination of field density of soil and rocks (includes materials that commonly contain particles larger than 75mm) in a test pit based on the ASTM D5030.

It is specifically used for the density evaluation of soil in unsaturated condition. The accuracy of the test result would be influenced for soils that disturb easily. It is used in various construction projects such as earth embankments, road fills, and structure backfill.

Water replacement test method is suitable for test pits with a volume between approximately 0.08 and 2.83 m^3. However, it may be used for larger sized excavations if desirable.

Apparatus

1. Balance

Two balances with different readability and capacity are required. A balance of 0.1 g readability, and the other one should have a capacity and readability appropriate to the mass for the specific test pit dimensions within the range of 0.08 to 2.83 m^3 volume.

2. Drying Oven

3. Sieves

No. 4 sieve (4.75-mm) and 3-in. (75-mm)

4. Metal Template

Metal template having circular shape with inside diameters of 0.9 m or more, to serve as a pattern for the excavation.

5. Liners

Liners 4 to 6 mils thick

6. Water-Measuring Device

Such as storage container, delivery hoses or piping, and a water meter.

7. Water-Level Reference Indicator

8. Equipment for Digging Soil

Such as shovels, picks, chisels, bars, knives, and spoons for digging test pit;

9. Containers

For retaining the test specimen without moisture change

Sources of Errors

1. The presence of excessive moisture in the material would influence the calculated density. The influence is paramount in materials with high permeability such as sands and gravels where the bottom of the test hole is close to or below the water table.
2. The evaluation of volume of specimen may be affected by buoyant forces of free water beneath or behind the liner.
3. Rain falls and direct sunlight influence the final result of the test. Therefore, both test area and equipment shall be protected from rain fall, snowfall, and direct sunlight. 

Test Preparations

1. Determine the mass of empty container that contain the excavated material (M1).
2. Estimate the quantity of water to be used. The volume of water required for the test can be calculated by two times the volume of the template (ring) plus the volume of the test pit.

Procedure

The following procedure is used for soils in which no more than 30% of its particles is retained on the sieve 19mm, otherwise different approaches shall be used for estimating the volume of the materials.

1. Specify an area for the test, large enough for metal template. It should be free from large particles that undermine the ring.
2. Prepare the area by removing loose materials, level soil surface.
3. As shown in Fig. 1, place the metal template and use nails or weights to prevent movement. The slope of the template should not exceed 5 %.
4. Place a liner over the metal template. It should extend around 1m outside the template, as shown in Fig. 1.
5. Set the water-measuring device indicator to zero. Discharge the water from the water reservoir into the template until the water level reaches a practical level, Fig. 4.
6. Set the water-level reference indicator, Fig. 5. Record the final reading of the water-measuring device (V1).
7. Make appropriate markings so that the water-level indicator can be placed in the identical position and at the same elevation following excavation of the test pit. Disassemble the water-level reference indicator.
8. Remove the water in the template, and remove the liner.
9. As shown in Fig. 6, excavate the test pit using hand tools like shovel and chisel. Prevent heavy equipment movement around the area to avoid soil deformation.
10. Place entire excavated soil into container and weigh the total mass (M2), and cover them to prevent loss of moisture.
11. The dimensions of the pit shall resemble the metal template. Inward slopes can be used if required.
12. Clean the bottom of test pit and its sides need to be as smooth as possible and free of pockets.
13. Place the liner into the test pit, extend it 1m outside the template after it is placed and shaped within the pit.
14. Set the water-measuring device indicator to zero.
15. Pour the water from the containers into the test pit until the water reaches the water-level reference indicator.
16. After the filling is complete, record the final reading of the water-measuring device indicator (V2).
17. Remove the water from the test pit, and remove the liner.
18. Inspect the liner for any holes that may have allowed water to escape during the test.
19. Loss of water will require another determination of the volume.

Calculations and Results

Step 1: The volume of the test pit is equal to the difference between the volume of water in step 6 and that of step 16:

Volume of Test Pit (V)= V2-V1 Equation 1

Step 2:Calculate the mass of excavated materials which is equal to the total mass of the excavated material and containers (M2) minus the total mass of the containers used to hold the excavated material (M1).

Mass of Excavated Materials (M)= M2-M1 Equation 2

Step 3: Calculate the wet density of the excavated material.

Wet Density= M/ V Equation 3

Where:

M: mass of excavated material in Step 2.

V: volume of the test pit in step 1

Step 4: Obtain a moisture content specimen representative of the excavated material.

Step 5: Calculate and record the dry density of the material.

pd= pw/(1+(w/100)) equation 4

Where:

pd: dry density of excavated material

pw: wet density of excavated material in step 3

w: moisture content of excavated materials in step 4.