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Black cotton soils in Myanmar M.Sc. Nyan Myint Kyaw Black Cotton Soil is a well-known expansive soil in Myanmar. These soils are easy to identify by their high potential for swelling and shrinkage under changing moisture conditions. The climate of most parts of Myanmar is divided into three seasons. Rainy season starts in May and ends in September. Cold season follows the rainy season up to February and is followed by hot season. Cold season and hot season can be classified as dry season with very low precipitation. Difference of moisture content in soil can be very high in some areas due to fluctuation of ground water table. This changing moisture conditions mainly cause volume changes in black cotton soils. In the dry season, these soils become very hard but excessive shrinkage due to low moisture content causes deep and wide cracks. In the wet rainy season, these soils become soft and swelling occurs due to high moisture content. The buildings resting on black cotton soil areas suffer from non-structural and structural damages due to the effect of high soil volume changes under changing moisture contents. Most problems are recognized in central Myanmar where annual precipitation is lower than 500 mm. Due to financial problems and lack of technical knowledge, most buildings are not properly designed and constructed. In most cases, foundation soil exploration is not carried out systematically or sometimes ignored. Most engineers and contractors must do their job only with past experience. Local practice is to place foundation in black cotton soil at an economical depth where effect of weathering is assumed to be minimum. Foundation soil inspection with the help of some test pits is main source of information for most engineers. When damages occur, no clear reason can be concluded because of several weaknesses both in superstructure and foundation. Only important buildings are constructed properly and some useful technical data can be studied from these projects. Some of these buildings are not free from damages although they are believed to be designed and constructed properly according to local experiences.From the inspection of a damaged building, it is found that soil movement is probable cause of all non-structural and structural damages. The plain concrete floor slabs resting on ground are unequally uplifted by supporting soil. It results in several cracks and unevenness. Subsequently, the partition walls directly resting on floor slab are distorted. Although structural members are designed to withstand applied loads, it is found out that the differential foundation movement due to volume changes of foundation soil was not properly considered or underestimated. Drainage systems are found to be in bad condition. Soil samples from that site normally contain clay content higher than 50%. The actual moisture content at the time of soil testing was near the plastic limit and clay was classified as stiff clay. Liquid limit between 75 and 90, plasticity index between 45 and 65 are recorded. It can be concluded from the above mentioned problem that knowledge about volume changing behavior of black cotton soil is not satisfactory in our country. Most engineers consider only the problems resulting from excessive shrinkage. Recognizing this problem, a laboratory testing program was carried out to study the expansive behavior of black cotton soil. Some useful results were produced from these tests. We need to study more to understand the behavior of black cotton soil. It can be concluded that black cotton soil is well-known but its behaviors are not enough understood in Myanmar. |
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