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Sub-ballastDate: 2015-10-07; view: 390. Script (Listening) Step 4: Compare your answers with those of the other students. Step 3: Do the interview. For the answers use the information from the texts and Internet resources you searched while preparing your questions. Step 2: With the whole group, make an interview form to use for the interview. Where the foundation consists of free draining materials such as rock fill or sand a sub-ballast may be more appropriate than an impermeable capping. Sub-ballast is not impermeable and is designed as a filter material to limit fouling by subgrade material and to prevent itself from fouling the ballast. Its purpose is to: − Spread vertical load from the ballast. − Prevent migration of fines from the subgrade. − Permit migration of moisture. The grading of the sub-ballast is important and must be specifically designed for a particular subgrade and ballast. Laboratory testing of both the subgrade and the ballast is required to be able to design a grading for sub-ballast. The performance of the sub-ballast is not destroyed by saturation as is that of the capping and it is much more easily compacted. It may provide a better solution than capping in areas where the subgrade is constantly wet due to, say, a spring or in soft rock formation situations.
6) What is the main function of sub-ballast? 7) What are the other purposes of sub-ballast? 8) The grading of sub-ballast needs special laboratory testing, doesn't it? 9) Is capping or sub-ballast much more easily compacted? 10) What cases does sub-ballast provide a better solution than capping?
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