Identification of Expansive Soils

Greetings!

• Identification of Expansive Soils:

Expansive soils can be identified by studying their mineralogy, which can be identified by the following tests:

1. X-Ray Diffraction test.
2. Microscopic examination.
3. Differential Thermal Analysis(DTA).

There are certain simpler tests to determine the expansive characteristic from engineering point of view.

1. Free swell test 
2. Differential free swell test.

   

   Swelling Index determination

In  free swell test 10 gm of soil passing through 425 mic. IS-sieve if poured in a cylinder containing 100 ml of distilled water, and left undisturbed for 24 hours. The volume of the soil is measured and the free swell Sf is given as  (Vs-Vi)/Vi *100.

Where,

Vi= Initial Volume

Vs= Volume of the swelled soil.

The Bentonite may swell from 1200 to 2000%;  Kaolinite - about 80 % ; Illite - 30 to 80%.

In differential free swell test, two samples of 10 gm dry soil passing through 425 mic. IS - sieve are poured in 50 ml graduated cylinders, one containing non-polarizing liquid(Kerosene) and another water. Both the jars are kept undisturbed for 24 hours and differential free swell (DFS) is given as,

   

Differential Free Swell (DFS) = [ Vs(w) - Vs(k)]/ Vs(k) *100 

Where,   Vs(w) = Volume of soil in water.

               Vs(k) = Volume of soil in kerosene.

A relationship between differential free swell (DFS) and degree of expansion is given in the table below:

         Differential Free Swell (DFS)[%]                 Degree of expansion

1.     <20                                                                     Low
2.     20-35                                                                 Moderate
3.    35-50                                                                  High
4.       > 50                                                                Very High

Holtz and Gibbs(1956), gave the following table to classify the expansive soils into low, moderate, high and very high expansive categories based on volume change(%), Colloidal content(%), Plasticity Index(PI) and Shrinkage Limit(SL).

 

              Property                                                     Property Ranges for

                                                         Low               moderate             High           Very High

1. Volume Change(%)               0-10                10-20                 20-30               >30
2. Colloidal Content(%)             0-15                10-25                 20-35              >25
3. Plasticity Index(PI)(%)           0-15                 10-35                20-45              >30
4. Shrinkage Limit(SL)(%)         12                    8-18                   6-12                10

R.B. Peck, W.E. Hanson & T.H. Thornburn (1974), gave the following relation between the Plasticity Index(PI) and Swelling Potential:

                     Swelling Potential                 Plasticity Index(PI)

1.              Low                                                 0-15
2.              Medium                                          10-35
3.              High                                                20-55
4.              Very High                                       >=35

Gromko, G.J. (1974) has related shrinkage limit(SL) to the linear shrinkage(LS) and degree of expansion:

            SL(%)                      LS(%)                 Degree of expansion

1.   <10                           >8                             Critical
2.   10-12                       5-8                             marginal
3.    >12                         0-5                            non-critical

Sridharan et al. gave another relationship between the swell potential and free swell index.

Thanks for your kind visit!