Knowing the correct method for field density test calculations is very important for Transportation Engineers and Civil Engineers.
Whether you are about to found the footing of a building on soil, or is working on a dam project where roller compacted concrete is used or you are working on a highway project where subgrade is being prepared; you need a criterion / procedure for quality control of the foundation.
The degree of soil compaction is evaluated at site with the help of field density tests and bulk density of soil as well as degree of compaction is calculated.
The most common methods used at site for this purpose are:-
- Sand replacement method or sand cone method
- Water replacement method
- Core-cutter field density test
- Rubber Balloon Method
- Heavy Oil Method
The basic purpose of field density test calculation is to know the density achieved at site with the help of mechanical compaction equipment.
This density is than compared with the maximum dry density achieved in laboratory by Modified Proctor Test – this comparison is termed as “degree of compaction” in field density test calculations.
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The foundation compaction is either accepted or rejected based on this degree of compaction.
In this post I will elaborate how to calculate a compaction test report in case of sand replacement method as well core cutter method.
“The basic aim of field density test calculation is to find the in-situ weight of the soil that is obtained either by core or by sand metal tray which is divided by the in-situ volume of the excavated hole”
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Field Density test Calculation in Sand Replacement Method
In case of field density test by sand replacement method the steps involved for calculation are as follows:-
- Determine the Bulk Density of Sand
The first step in case of sand replacement method is to calibrate the sand to be used i.e. to determine its bulk density.
This step should be carried out in the laboratory in order to avoid any error or mistake because the sanctity of test calculation is based on this parameter.
The calculation is as follows :-
- Volume of the calibrating cylinder; V = ((Pi) d^2/4) x h =5500 ml. ; where ‘d’ is dia of cylinder and ‘h’ is height of cylinder
- Weight of calibrating cylinder; W1 = 819 gm
- Weight of calibrating cylinder full of sand; W2 = 8650 gm
- Weight of sand: WS = W1 – W2 = 7831 gm
- Bulk Density of Sand ρs = WS / V = 7831 gm/ 5500 ml. = 1.4238 gm/cc = 8 kg/m3
In the field the first step is to excavate the hole by securing the excavated soil. A flat metal plate is placed on the soil. The hole is excavated around the hole of the metal plate by hammering the chisel into the soil up to sufficient depth.
- Determine the weight of the sand in the hole
In this step a hole of at least 150 mm depth is excavated in the soil carefully. The apparatus full of sand is placed on the hole and shutter is opened to fill the hole with sand.
When the sand stops from further going in the hole, apparatus is removed.
But in this process a heap of sand contained in the funnel would also accumulate on the hole whose weight should be subtracted to get the correct weight of sand in the hole.
For this the sand apparatus is placed on a flat glass and shutter is opened just to know the weight of sand in the funnel.
- Weight of apparatus full of sand; W3 = 8650 gm.
- Weight of apparatus with remaining sand; W4 = 8100 gm.
- Weight of sand filling the funnel / cone = W5 = W3-W4 = 8650 -8100 = 550 gm.
Now the apparatus is placed on hole and process is repeated.
- Weight of apparatus full of sand; W6 = 8650 gm.
- Weight of apparatus with remaining sand; W7 = 6704.7 gm.
- Weight of sand filled in soil hole plus funnel heap = W8 = 1945.3 gm.
- Weight of sand in the hole = W9 = W8 – W5 = 1945.3 – 550 gm = 1395.3 gm.
- Calculate the volume of hole
- Volume of hole: Vh = (weight of sand in hole W9) / (bulk density of sand ρs) = 1.3953 kg / 1423.8 kg/m3 = 9.8 x 10-4 m3.
- Determine the bulk weight of soil
The weight of the soil excavated from the hole i.e. secured in the metal plate is then weighed.
W10 = bulk weight of the excavated soil = 1896.5 gm.
- Calculate the in-situ bulk density of soil
Bulk density of the soil ρb= bulk weight of soil W10 / Volume of hole Vh = 1.8965 gm / 9.8 x 10-4 m3 = 1934.6 kg / m3
γb = 1934.6 kg / m3
- Determine the in-situ moisture content of the soil
The excavated sample of soil is placed in a moisture container whose weight is already noted and is than placed in oven for drying and afterwards dry weight is estimated to calculate the moisture content as follows.
- weight of empty container = W11 = 14.6 gm.
- weight of wet soil + container = W12 = 62.9 gm.
- weight of wet soil = W13 = W12 – W11 = 56.6 gm.
- weight of dry soil = W14 = 42 gm.
- moisture content = m = (W13 – W14 )/ W14 x 100 = 15 %
- Calculate the dry density of soil
γd = γb / (1+m/100) = 1934.6 / (1+0.15) = 1682.2 kg/m3
γd = 1682.2 kg / m3
- Determine the degree of compaction by comparing with MDD (Maximum Dry Density)
- Maximum dry density MDD obtained by Proctor Test = 1766.31 kg/m3
- Degree of Compaction (DOC) = (dry density of soil / MDD) x 100 = (1682.2 / 1766.31) x 100
DOC = 95.2 %
According to the IS the acceptable values of degree of compaction for different types of fill is listed below :-
- Base layer-98%
- Sub base layer-98%
- Type 1 layer-95%
- ABC layer-100%
- Road shoulder- 98%
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Calculation of Field Density by Core Cutter Method
The calculations of field density test by Core cutter method are very easy as compared with the sand replacement method. However, the steps involved in the calculation are listed below :-
- Volume of cylinder is calculated ; V = ((pi x d x d ) / 4 ) x h ; where d is internal dia of mould; h is height of the mould.
- Moisture content m of the soil is calculated as stated above
- Bulk density of soil is calculated as : weight of wet soil / volume of cylinder
- Dry density of soil and degree of compaction is calculated in similar fashion explained above.
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I am also preparing an online calculator for field density of soil which I will share here so stay tuned.