DETERMINATION OF FIELD DENSITY OF SOIL BY SAND REPLACEMENT METHOD (IS-2720-PART-28)

Aim

To determine the field density of soil at a given location by sand replacement method

Reference

IS-2720-Part-28-Determination of dry density of soils in place, by the sand replacement method

Apparatus

1. Sand pouring cylinder
2. Calibrating can
3. Metal tray with a central hole
4. Dry sand (passing through 600 micron sieve)
5. Balance
6. Moisture content bins
7. Glass plate
8. Metal tray
9. Scraper tool

Theory and Application

Determination of field density of cohesion less soil is not possible by core cutter method, because it is not possible to obtain a core sample. In such situation, the sand replacement method is employed to determine the unit weight. In sand replacement method, a small cylindrical pit is excavated and the weight of the soil excavated from the pit is measured. Sand whose density is known is filled into the pit. By measuring the weight of sand required to fill the pit and knowing its density the volume of pit is calculated. Knowing the weight of soil excavated from the pit and the volume of pit, the density of soil is calculated. Therefore, in this experiment there are two stages, namely

1. Calibration of sand density
2. Measurement of soil density

Sand Replacement Method

Procedure

Stage-1 (Calibration of Sand Density)

1. Measure the internal dimensions (diameter, d and height, h) of the calibrating can and compute its internal volume, V_c = πd²h/4.
2. Fill the sand pouring cylinder (SPC) with sand with 1 cm top clearance (to avoid any spillover during operation) and find its weight (W₁)
3. Place the SPC on a glass plate, open the slit above the cone by operating the valve and allow the sand to run down. The sand will freely run down till it fills the conical portion. When there is no further downward movement of sand in the SPC, close the slit. Measure the weight of the sand required to fill the cone. Let it be W₂.
4. Place back this W₂ amount of sand into the SPC, so that its weight becomes equal to W₁ (As mentioned in point-2). Place the SPC concentrically on top of the calibrating can. Open the slit to allow the sand to run down until the sand flow stops by itself. This operation will fill the calibrating can and the conical portion of the SPC. Now close the slit and find the weight of the SPC with the remaining sand (W₃)

Stage-2 (Measurement of Soil Density)

1. Clean and level the ground surface where the field density is to be determined
2. Place the tray with a central hole over the portion of the soil to be tested.
3. Excavate a pit into the ground, through the hole in the plate, approximately 12 cm deep (same as the height of the calibrating can). The hole in the tray will guide the diameter of the pit to be made in the ground.
4. Collect the excavated soil into the tray and weigh the soil (W)
5. Determine the moisture content of the excavated soil.
6. Place the SPC, with sand having the latest weight of W₁, over the pit so that the base of the cylinder covers the pit concentrically.
7. Open the slit of the SPC and allow the sand to run into the pit freely, till there is no downward movement of sand level in the SPC and then close the slit.
8. Find the weight of the SPC with the remaining sand (W₄).

Precautions

• If for any reason it is necessary to excavate the pit to a depth other than 12 cm, the standard calibrating can should be replaced by one with an internal height same as the depth of pit to be made in the ground.
• Care should be taken in excavating the pit, so that it is not enlarged by levering, as this will result in lower density being recorded.
• No loose material should be left in the pit.
• There should be no vibrations during this test.
• It should not be forgotten to remove the tray, before placing the SPC over the pit.

Observations and Calculations

Enter all the data as per the table given below and calculate accordingly.

Sl no	Data (Calibration of Unit Weight of Sand)	Trial-1
1	Volume of the calibrating container, V (cm3)
2	Weight of SPC + sand, W1 (g)
3	Weight of sand required to fill the conical portion on a flat surface, W2 (g)
4	Weight of SPC + sand (after filling calibrating can), W3 (g)
5	Weight of sand required to fill the calibrating container, Wc = (W1-W2 –W3) (g)
6	Unit weight of sand, γsand = (Wc)/V   (g/cm3)

Sl. no	Data (Determination of Density of Soil)	Trial-1
1	Weight of the excavated from the pit (W)  (g)
2	Weight of sand + SPC, before pouring, W1 (g)
3	Weight of SPC after filling the hole & conical portion, W4  (g)
4	Weight of sand in the pit Wp = (W1-W4-W2)  (g)
5	Volume of sand required to fill the pit Vp=Wp/γsand  (cm3)
6	Wet unit weight of the soil γwet=W/Vp  (g/cm3)
7	Dry unit weight of the soil γdry=γwet/(1+m)  (g/cm3) (where ‘m’ is the moisture content of soil)