Sunday, February 6, 2011

COMPACTION TEST.(IS:2720 – PART – 7 & 8)

COMPACTION TEST.
(IS:2720 – PART – 7 & 8)
INTRODUCTION:
Compaction of soil is mechanical processes by which the soils particles are constrain to be packed more closely together by reducing the air voids. Soil compaction causes decrease in air voids and consequently an increase in dry density. This may result in increase in shearing strength. The possibility of future settlement or compressibility decreases and also the tendency for subsequent changes in moisture content decreases. Degree of compaction is usually measured quantitatively by dry density.

Object:
To determine the compaction test by light / heavy compaction test method.

Apparatus:
a) Cylindrical mould of capacity 1000 cc, with an internal diameter of 10cm and height 12.73 cm or a mould of capacity 2250 cc, with an internal diameter of 15 cm and height of 12.73 cm. The mould is fitted with a detachable base plate and removable collar or extension of about 6 cm high.
b) For light compaction, a metal rammer having 5 cm diameter circular face, and weight 2.6 kg is used which has a free drop of 31 cm. For heavy compaction, the rammer has 5 cm diameter circular face, but having weight 4.89 kg and free drop of 45 cm.
c) Steel straight edge having be leveled edge for trimming the top of the specimen.
d) Other accessories include moisture containers, balances of capacity 10 kg and 200 g , oven, sieves and mixing tools.

Procedure:
Preparation of samples:
For light compaction, about 20 kg of the representative soil is air-dried, mixed pulverized and sieved through 19 mm IS sieve. The fraction retained on 19 mm sieve is not used in this test. If there is note worthy proportion of materials retained on 19 mm sieve, allowance for larger size materials is made by replacing it by an equal weight of material passing 19 mm sieve and retained on 4.75 mm sieve.

For heavy compaction, about 45 kg of the representative soil is air-dried, mixed pulverized and sieved through 37.5 mm IS sieve. The fraction retained on 37.5 mm is not used in this test. If there is note worthy proportion of materials retained on 37.5 mm sieve, allowance for large size materials is made by replacing it by an equal weight of material passing 37.5 mm and retained on 4.75 mm sieve.

For compacting the soil in the mould every time the required quantity will depend on the soil type, size of the mould, moisture content and amount of compaction. As a rough guidance, for each test 2.5 kg of soil may be taken for light compaction and 5.8 kg for heavy compaction. The estimated weight of water to be added to the soil every time may be measured with a graduated jar in cc. Enough water is added to the specimen to bring the moisture content to about 7% less than the estimated OMC for sandy soils and 10% less for clayey soils. The processed soil is stored in an airtight container for about 18 to 20 hours to enable moisture to spread uniformly in the soil mass.

The mould with base plate fitted in is weighed. The process soil-water mixture is mixed thoroughly and divided into eight equal parts.

(i) For light compaction the wet soils is compacted into the mould in three equal layers, each layer being given 25 blows of the 2.6 kg hammer, if 10 cm diameter mould is used. When the 15 cm diameter mould is used, 56 blows are given to each of the three layers by the 2.6 kg hammer.
(ii) For heavy compaction, the wet soil mix is compacted in the mould in five layers each layer being given 25 blows of 4.89 kg hammer when the 10 cm diameter of mould is used. When the 15 cm diameter mould is used, 56 blows are given to each of the five layers by 4.89 kg hammer.

The blows should be uniformly distributed over the surface of each layer. Each layer of the compacted soil is scored with a spatula before placing the soil for the succeeding layer. The amount of soil used should be just sufficient to fill the mould leaving about 5mm to be struck off on the top after compacting the final layer. The collar is removed and the compacted soil is leveled off to the top of the mould by means of the straight edge. The mould and the soil is then weighed. The soil is then ejected out of the mould and cut in the middle and a representative sample is taken in airtight container from the cut surface. The moisture content of this representative
specimen is determined by finding the wet weight, keeping in the oven at 1050 to 1100C and finding the dry weight the next day.

This procedure is repeated five to six times using fresh part of the soil specimen and after adding a higher water content than the preceding specimen every time so that the last compaction is carried out at moisture 7 to 10 percent higher than estimated optimum moisture content.

Calculations:
Let the weight of mould with moist compacted soil  =       W g
                                  Weight of empty mould   =       Wm g
                                               Volume of the mould   =       Vm cc
                                                      Moisture content    =       w %
                                       Specific gravity of the soil    =          G

                                                                                                (W – Wm)  
                                                        Wet density, gm      =                            g/cc
                                                                                                    Vm                                     
                                       
                                                                                gm                                    (W – Wm)
                 Then dry density,  gd    =                          =                                  g/cc
                                                                              w                                        w
                                                                     (1 +       )                    Vm(1 +          )
                                                                               100                                     100

                                                            Vv                 gd
                          Porosity,  n = 100            =  (1 -            ) 100
                                                          V                      G gw

                                                         G gw
                            Voids ratio, e = (            - 1)
                                                          gd

Results:
Points are plotted with moisture content on the X-axis and dry density on the Y-axis and a smooth curve is drawn connecting the points. From this curve, the maximum dry density (MDD) is noted and the corresponding value of moisture content taken as optimum moisture content (OMC) of the soil.

MDD & OMC Graph.


No comments:

Post a Comment

Post a Comment