Sub-ballast

Date: 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?