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With the rapid growth in the construction industry in Sri Lanka, presently there is a shortage of
natural resources, especially soils. Therefore, finding good quality soil that conforms to the
construction specification is challenging.
Expansive soils are one of the major problematic materials in the road construction industry.
Clay minerals within expansive soils are subjected to significant volume changes when their
environmental conditions are altered from dry to wet. Therefore, prolonged periods of drying
and wetting cycles in the expansive soil can result in surface movements and distress in the road
pavement which may lead to safety issues for road users and high maintenance costs to the road
authorities.
There are various measures used to predict the swell behaviour of soils in the road construction
industry. Some of the measures which regularly used are CBR swell, shrink-swell Index, soil
suction, plasticity index, weighted plasticity index, swelling pressure, linear shrinkage, clay
content and cation exchange capacity. Even though some of the measures used are complex,
CBR swell test is one of the simplest ways of predicting the swell behaviour of soil.
As Sri Lanka is a tropical country, four days of soaked CBR is carried out as a part of the
requirement of pavement design guidelines. CBR swell test is also conducted as a part of the
CBR test. Therefore CBR swell data are readily available. But there is no published information
on relationships between CBR swell with soil parameters for Sri Lankan conditions. Therefore,
it is expected that the relevance of current specifications on quantitative estimation of soil swell
using CBR swell parameter to be in this research. With that concern, the objectives of the study
are to understand the nature of expansive soils and their properties, to identify the parameters of
expansive soil behaviour used by engineers in road construction and then identify and quantify
relationships that may exist in CBR swell with relevant soil parameters.
Particle size distribution, LL, PI, PL, Soil classification, Modified compaction, CBR and CBR
swell data were collected on 61 numbers soil samples and a database was prepared. Then linear
regression analysis and multiple regression analysis were carried out considering PI, LL, MDD,
CBR and percentage passing in the 75μm test sieve as variables in order to obtain correlations to
CBR swell.
Depending on the analysis, the variation of CBR swell is higher for soil types CH, CL, MH and
SM and also, these soil types have shown higher CBR swell values over the other soil types. The
lower CBR swell range for each soil type is less than 0.5 which is independent of whether the
soil is fine-grained soil or coarse-grained soil. As per the analysis, the results from linear
regression analysis, logarithmic regression analysis and multiple regression analysis, a
relationship between CBR Swell and other soil parameters cannot be established. The soils
which could be identified as expansive soil based on the WPI value, were not given evidence to
be identified as expansive soils based on their CBR swell.
The reliability of the relationships between CBR swell with soil parameters is greatly influenced
by the accuracy of the CBR swell test, especially the use of correct filter paper between soil and
perforated base plate of CBR mould. Hence, use of inappropriate filter paper may lead to the migration of fine particles into the soaking tank which will result in low CBR swell
measurements.
Therefore, it is suggested to consider CBR swell along with other swell parameters and/ or verify
the results with a direct swell parameter for future studies related to expansive soil and obtaining
correlations for soil swell. |
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