DETERMINATION OF WATER CONTENT - DRY DENSITY RELATION USING HEAVY COMPACTION.
A. GENERAL
What is Dry Density?: Dry density is the ratio of mass of total solid in a soil sample (Ws) to its bulk volume (V), γ d=Ws / V
What is compaction? : Civil engineering projects like Railway, Road Embankment, Dams, Air port run way, filling of low areas, filling excavation etc. require the use of soils as a fill material. It is generally necessary to compact it in order to enhance its density obtaining improved engineering properties. In fact loosely filled material has a poor shear strength,mostly unsuitable for civil Engineering Project. Compaction on site is usually effected by mechanical means such as rolling, ramming or vibrating, however on proper control in the process of compaction gives best result at a reasonable cost. Laboratory compaction tests provide the basis to control the procedures for implementation at site.
Compaction tests in the laboratory furnish the following basic data for soils:
1. The relationship between dry density and moisture content of representative sample for a standard degree of compactive effort.
2. The moisture content for the most efficient compaction at which the maximum dry density is achieved under that compactive effort.
Effects of proper compaction of soils:
1. Higher shear strength gives Greater stability.
2. Lower compressibility gives less settlement under static load.
3. Higher CBR value gives less deformation under repeated loads.
4. Lower permeability provides less tendency to absorb water maintaining the shear strength in order.
5. To assess the degree of compaction / dry density achieved in the field time to time for comparing with the Maximum dry density obtained in the laboratory of the representative sample under a predefined compaction effort.
Some Important Definition:
Compaction: The process of rearranging and packing soil particles more closely together, usually by rolling or mechanical means, thus increasing the dry density of the soil.
Optimum moisture content (OMC): The moisture content of a soil at which a specified amount of compaction will produce the maximum dry density. Fig-5
Maximum dry density(MMD): The dry density obtained from the relationship graph using a specified amount of compaction at the optimum moisture content Fig-5
Relative compaction: The percentage ratio of the dry density of a soil in situ to its maximum compacted dry density determined by using a specified amount of compaction.
Dry density-moisture content relationship: The relationship between dry density and moisture content of a soil when a specified amount of compaction is applied.The relationship is understood from the graph as shown in a graph.
Compaction Vs consolidation
Compaction of soil is the process by which the solid soil particles are rearranged and packed more closely together by mechanical means, thus increasing the dry density. It is achieved through the reduction of the air voids in the soil, with little or no reduction in the water content.
consolidation of soil is the process in which pore water under pressure is squeezed out slowly under the action of a prolonged static load followed by settlement and change in volume.
B. Scope: IS 2720, Part-VIII deals with the method of Test for determination of water content - Dry density relationship using heavy compaction.
C- Suitability of Test:
This method of test is suitable for cohesive soil but does not suit so well to the compaction characteristics of sand, gravel etc where grains are displaced during ramming. Some nearly cohesionless soils compact satisfactorily in the standard test although in many cases the water density curve is not well defined.
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| Fig-1 |
D- Apparatus:
1.Cylindrical Metal Mould - It shall be either of 100 mm diameter and I 000 cm3 volume or 150 mm diameter, and 2 250 cm3 volume and shall conform to IS : 10074-1982 in Fig-1
2. Sample Extruder ( Optional) - It consists of a jack, lever frame or other device adopted for the purpose of extruding compacted specimens from the mould.
3. Balances - One of 10 kg capacity sensitive to 1 g, and other of 200 g capacity and sensitive to 0’01 g.
4. Oven -Thermostatically controlled, with interior of non-corroding material to maintain temperature between 105% and 110°C.
5. Container - Any suitable non-corrodible airtight container to determine the water content for tests conducted in the laboratory.
6. Steel Straightedge - A steel straightedge about 30 cm in length and having one bevelled edge.
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| Fig-2 |
7. Sieve - 4.75-mm, 19-mm and. 37.5 mm IS sieves conforming to IS: 460 ( Part I )-1978
8. Mixing Tools - Miscellaneous tools, such as tray or pan, spoon, trowel and spatula, or a. suitable mechanical device for thoroughly mixing the sample of soil with additions of water.
9. Metal Rammer - Heavy compaction rammer conforming to IS : 9 189-1979*.
E. SOIL SPECIMEN
1. A representative portion of air-dried soil material so that about 6 kg of soil passes through a 19-mm IS sieve for soils not susceptible to crushing during compaction , or about 15 kg of material passing a 19 mm IS sieve for soils susceptible to crushing during compaction shall be taken (see para 2 below). This portion shall be sieved on a 19 mm IS sieve and the coarse fraction rejected after its proportion of the total sample shall be recorded.
2. The soil should be considered susceptible to crushing during compaction if the sample contains granular material of soft nature. such as soft lime, stone, sandstone, etc. which is reduced in size by the action of the 4.9-kg rammer. The procedure given in para 5.2 of IS 2720 part VIII for soils susceptible to crushing during compaction can be applied to all soils if it is convenient to do so.
3. As per IS 2720 / Part-VIII, para 4.1.1 Aggregations of particles shall be broken down so that if the sample was sieved on a 4.75-m IS sieve, only separated individual particles would be retained.
F- PROCEDURE Soil Not Susceptible to Crushing During Compaction - The procedure is as follows:
1. A 5-kg air dried soil passing the 19 mm IS test sieve shall be taken. As per note-1 in para 5.3 of IS 2720 part VIII, retention of larger quantity on 19mm IS sieve more than 5% will give considerable error on the density obtained compared to the obtainable with soil as a whole when tested.However for soils containing larger proportions of gravel, the use of a bigger mould ( 2250 cc) will avoid major errors.
2. The sample shall be mixed thoroughly with a suitable amount of water The amount of water to be mixed at the commencement of the test depends upon the type of soil under test. In general, with sandy and gravelly soil. a moisture content of 3 percent to 5 percent would be suitable, while with cohesive soil a moisture content about 12 percent to 16 percent below the plastic limit. If plastic limit is 18(say), moisture to be added 18-14(Av)=4%.
3. It is important that the water is mixed thoroughly and adequately with the soil, since inadequate mixing gives variable test results. This is particularly important with cohesive soils when adding a substantial quantity of water to the air dried soil. With clays of high plasticity, or where hand mixing is employed, it may be difficult to distribute the water uniformly throughout the air dried soil by mixing alone, and it may be necessary to store the mixed sample in a sealed container for a minimum period of about 16 hours before continuing with the test.
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| Fig-3 |
7. The compacted soil specimen shall be removed from the mould and placed on the mixing tray. The water content of a representative sample of the specimen shall be determined as in IS: 2720 (Part II )- 1973
8. The remainder of the soil specimen shall be broken up, rubbed and passed through the 19-mm IS test sieve, and then mixed with the remainder of the original sample. Suitable increment of water shall be added successively and mixed into the sample. In general, increments of 1 percent to 2 percent are suitable for sandy and gravelly soils and 2 percent to 4 percent for cohesive soils. To get the better accuracy of the test it is often advisable to reduce the increments of water in the region of the optimum moisture content. The above procedure from para F(4 ) to F(8) shall be repeated for each increment of water added. The total number of determinations made shall be at least five to get a suitable curve. The range of moisture content should be such that the optimum moisture content, at which the maximum dry density occurs, must be within that range.
G- Procedure- for Soil susceptible to crushing during compaction:
1. The soil should be considered susceptible to crushing durlng compaction if the sample contains granular material, containing soft limestone, sandstone, etc. which is reduced in size by the action of the 4.9 kg rammer. The procedure given in para 5.2 of IS 2720 for soils susceptible to crushing during compaction can be applied to all soils if it is convenient to do so, as per para 4.1 Note-1 given in IS 2720 Para-VIII.
2. However, the procedure as per para 5.2 of IS 2720, part-VIII amalgamating the notes as per following- Five or more numbers of 2.5 kg sample of air dried soil passing the 20-mm IS test sieve, shall be taken. However, for the soils containing larger proportions of gravel, more than 5% retained on 19mm sieve there will be major error in the result obtained. However the use of a bigger mould 2250 ml, avoiding major errors, shall be used. The samples shall each be mixed thoroughly with different amounts of water to give a suitable range of moisture contents .At the commencement a moisture content of 3 percent to 5 percent would be suitable for sandy or gravelly soil, while with cohesive soil a moisture content about 12 percent to 16 percent below the plastic limit. i.e if plastic limit is 18, moisture to be added 18-14(average)=4%. The range of moisture content shall be such that , the maximum dry density occurs within that range.
a) Each sample shall be treated as in Para F(4) to F(6) above.
b) Each compacted specimen shall be treated as in Para F(7).
c) The remainder of each soil specimen shall be discarded.
H- Compaction in large size mould
For compacting soil containing coarse material up to 37.5 mm size, the 2250 CC mould should be used, A sample weighing about 30 kg and passing, the 37.5-mm IS sieve is used for the test. Soil is compacted in five layers, each layer being given 55 blows of the 4.9 kg rammer. The rest of the procedure is the same as para F & G above.
I- CALCULATION
Bulk Density – γm in g/cc, of each compacted specimen shall be calculated from the equation: γ m = (m2 –m 1) / Vm,
m1 = weight of mould + Base Plate, as in para F(4) at the start.
m2 = weight of mould + Base Plate + compacted soil as in F(6)
Vm= Volume of the mould in cm3
All values are known, γ m can be determined from the formula above.
Dry density, γ d = 100 γ m / (100+w) in gm/cm3.
w = moisture content, if the value of w is 15%, then w shall be written as 15, not 0.15
The dry densities, γ d obtained in a series of determinations shall be plotted against the corresponding moisture contents w. A smooth curve shall be drawn through the resulting points and the position of the maximum value on this curve is to be located.
A typical Graph Showing numerical of Moisture content (%) Vs Dry Density (Kg/m3) in the fig-5
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| FIG-5 |
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