To determine the field density by Sand Replacement (Sand Cone) Method.

OBJECT : To determine the field density by Sand Replacement (Sand Cone) Method.

OBJECTIVE AND SCOPE

The In-Situ density of natural soil is needed for the determination of the bearing capacity of soils, for the purpose of stability analysis of slopes, for the determination of pressures on the underlying strata, for the calculation of settlement, and for the design of underground structures. It is a very quality-controlled test, where compaction is required, in cases like embankment and pavement construction.

STANDARDS

AASHTO T191-86

ASTM D1556-82

BIS1377:part 4

EQUIPMENT

• Sand pouring cylinder
  
• Calibrating can
  
• Metal tray with a central hole.
  
• Dry sand (passing through 600-micron sieve)
  
• Balance
  
• Moisture content bins
  
• Glass plate
  
• Metal tray
  
• Scrapper tool
  

THEORY

Determination of the field density of cohesionless soils is not possible by the core cutter method, because it is not possible to obtain a core sample. In such a situation, the sand replacement method is employed to determine the unit weight. In the sand replacement method, a small cylindrical pit is excavated, and the weight of the excavated soil is measured. The 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 the pit is calculated. Knowing the weight of the soil excavated from the pit and the volume of the pit, the density of the soil can be calculated. Therefore there are two stages in this experiment, namely:

• Calibration of sand density
  
• Measurement of soil density
  

PROCEDURE

Stage 1 (Calibration of Sand Density)

1. Measure the internal dimensions (Diameter d, Height h) of the calibration can and its volume, V_c = πd²h/4
   
2. Fill the sand pouring cylinder with sand with 1 cm top clearance (to avoid any spillover during operation) and find its weight (W₁).
   
3. Place the sand pouring cylinder 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 until it fills the conical portion. When there is no further downward movement of sand in the sand pouring cylinder, 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 sand pouring cylinder, so that its weight becomes equal to W₁ (As mentioned in point 2). Place the sand pouring cylinder concentrically on the top of the calibration can. Open the slit to allow the sand to run down until the sand flow stops by itself. This operation will fill the calibration can and the conical portion of the sand pouring cylinder. Now close the slit and find the weight of the sand pouring cylinder 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 12cm 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 in the tray and weigh the soil (W).
   
5. Determine the moisture content of the excavated soil.
   
6. Place the sand pouring cylinder, 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 sand pouring cylinder and allow the sand to run into the pit freely, till there is no downward movement of sand level in the sand pouring cylinder, and then close the slit.
   
8. Find the weight of the sand pouring cylinder with the remaining san W₄.
   

PRECAUTIONS

1. If for any reason it is necessary to excavate the pit to a depth other than 12cm, the standard calibration should be replaced by one with an integral height the same as the depth of the pit to be made in the ground.
   
2. Care should be taken in excavating the pit so that it is not enlarged by leveling, as this will result in lower density being recorded.
   
3. There should be no vibrations during the test.
   
4. It should not be forgotten to remove the tray, before placing the sand pouring cylinder over the pit.
   

OBSERVATIONS AND CALCULATIONS

Calibration of Unit Weight of the Sand:

Determination of Density of soil

RESULT

The field dry density of the soil comes out to be g/cm³

Review Questions

1. What is Relative Compaction?
   
2. The term FDT stand for?
   
3. What are the specifications of sand used in sand cone apparatus?