Effect of Fineness of Soil on California Bearing Ratio Value

Being the bottom- most layer of a road, the sub-grade soil acts as one of the most important load-bearing strata on which the performance and design of a pavement depend essentially. The grain size of soil particles plays a significant role in strength characteristics of the sub-grade as working recommendations vary significantly respecting varying particle sizes. Generally, California Bearing Ratio (CBR) value is widely used for the evaluation of subgrade strengths. Since this test is an expansive and time-consuming one, it is much needed to establish an alternative approach to determine the CBR value from some other physical properties of soils, which are easy, cheap, and low time consuming with limited facilities for laboratory experiments. To confront this, an attempt has been conducted in this study to correlate CBR value of sub-grade with some physical properties of soils, such as Fineness Modulus (FM) along with grain sizes. Six types of fine sands were collected from six notable sites, i.e bed of River Payera in Borguna, River Katcha in Pirojpur, River Shayendha in Barisal, River Kapotakkha in Khulna, River Bhairab in Bagerhat, and one sample was collected from subsoil in Jessore. At the initial stage, the necessary engineering properties of soils, like fineness modulus, grain size distribution, unit weight, dry density, and optimum moisture content, were determined, followed by thirty CBR samples with different FM values at optimum moisture content. After that, a correlation was established in the form of an equation to express CBR value as a function of fineness modulus using regression analysis. From field and laboratory test results, it was found that the six sands are uniformly graded non-plastic fine sand with FM 0.13 to 0.26 and their PI is less than 5 and LL is less than 30. CBR of sands lies in between 11.44% to 12.09% and their swelling is in between 0.58% to 0.69%. The equation of co-relation is y = 0.0496x + 0.1081, where y refers to CBR (%) and x refers to Fineness modulus. From which one can ascertain the probable CBR value of find sand corresponding its FM value. With the help of this study, the CBR value of soil sample can be predicted in an easier, cheap, and less time-consuming manner to classify and evaluate the soil sub-grade rather than conventional CBR test.