Sub-ballast/Subgrade

Question 1

What affects the behaviour of any granular material, and what affect does it have?

Answer 1

All granular materials are affected by there boundary conditions.
confined ballast is much stiffer than non-confined.

Question 2

compare ballast on a stiff support to ballast on a soft support.

Answer 2

Stiff support - ballast appears stiffer than exactly equivalent ballast on soft support

Question 3

What should be added to track if the subgrade soil is soft

Answer 3

sub-ballast layer should be used.

Question 4

what does a sub-ballast layer do

Answer 4

Acts as a sacrificial layer, a separator between ballast and soil

Question 5

What alternatives are there for a sub-ballast layer

Answer 5

A Geogrid - locks stones into position, preventing sideways movement – increased strength/stiffness properties.

Bound foundation - really stiff layer - cement/lime/asphalt. On new track, this is easy to implement.

Question 6

What is the key reason for a sub-grade layer?

Answer 6

Makes ballast receive excellent support - so deterioration of the track occurs much slower.

Question 7

How can we improve existing track?

Answer 7

Much tougher job.

‘Dig out and replace’ - time consuming and expensive operation, avoided if possible.

‘Mini-piling’ - Drill between sleepers, pump cement/lime in; enhance stiffness of ballast/sub-grade.