Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 243

The general intention should be shorter maintenance intervals or preventative mainte-
nance measures in order to prevent consolidation or too much growth of the deposits.
5.4.4.2 Cleaning of drainage systems
In the course of maintenance work, the pipes must be cleared of deposits without damage
to the pipes. In the high-pressure flushing-suction process, pressures of 15 to 40 MPa at
the pump or 10 to 20 MPa at the flushing head are used with total water quantities of 300
to 600 l/min in order to remove the loosened deposits. Very high-pressure cleaning with
jet pressures of over 40 MPa should not be used for the maintenance of tunnel drainage,
but rather for repair works.
When flushing drains, the local flow direction of the drain should be observed to rule out
flushing against the flow direction. The flushing pressure should be suitable for the pipe
material and the use of chains or hammer jets should generally be avoided unless cleaning
is impossible without these methods. The maximum flushing distance should be restricted
to two runs between manholes in order that the flushing pressure does not exceed a reason-
able value due to the long distance.