Hello,

Simply due to the fact that if we use finer materials like slit and clay, the soil is prone to high volume change that may affect the results of  PL LL specifically.

Dear Emad,

Thank you for your response to this question.

I mean why the No.40 sieve? For example, why we do not use No.100 or No.20 sieve? Take into consideration that I know the result of PL and LL will change because of changing the size of aggregate.

Regards

Dear Professor Tsambani,

Thank you for your time and the useful reference you provided.

Unfortunately, I could not find any reason why No.40 sieve is used for Atterberge Limit test. The material pass this sieve are composed of clay, silt and sand. However, Clay mineral are responsible for plasticity of the soil. 

Regards,

Hi Ebrahim,

The reason we use the No. 40 sieve is because that was the sieve size Atterberg used when he first reported his work on the plasticity of clays in 1911. The Atterberg consistency limit tests became formally recognised at the International Society of Soil Science at the Berlin Conference in 1913. The Atterberg tests were then introduced into the field of geotechnical engineering by Karl Terzaghi in the 1920’s. Terzaghi’s assistant, Arthur Casagrande, standardised the Atterberg tests in his paper in 1932 and have been used around the world ever since.

However, it was not until sometime later, in 1948, when the Unified Soil Classification System recognised the No. 200 sieve (75 micrometers) as the upper grain size limit for fine-grained soils. By this time the accepted, and formally recognised, Atterberg consistency limit test procedures of Casagrande using the No 40. sieve had been followed worldwide for over 16 years. No one has sought to change the procedure using the No 40 sieve in 1948, and no one has sought to change the standardised procedure in the subsequent 68 years between 1948 and 2016.

I hope the above is of help. 

Best wishes,

Anne Wilkins

Hi Anne,

That is a comprehensive and interesting answer. Thank you for your time.

Cheers,

It is also important to mention that it is very dificult to separate the fines (diameter equal or less than 0,075 mm) from the fine sand by dry sieving.

Even the hydrometer analysis for the determination of silt and clay fractions of the soil grading curve uses material that passes through the No.40 sieve. Fine sand is deposited almost immediately in the bottom of the glass cylinder and is not measured by the hydrometer. After the hydrometer test, the content of the glass cylinder is passed through sieve No 200; what is retained is dried and then sieved through the smaller mesh sieves.