Retrospective Evaluation of Cured-in-Place Pipe - (NEPIS)(EPA ...

However, the figures published by OFWAT cannot be taken at face value in terms of total rehabilitation
effort since they refer to rehabilitation activity in terms of structural enhancement. For example, a length
of condition Grade 4 or 5 sewer lined to improve condition to Grade 3 or better is deemed a valid
rehabilitation output and may be included in the volume of activity, whereas a joint sealing by CIPP in a
Grade 1 or 2 sewer may improve serviceability, but not structure and accordingly will not be counted.
The length reported may also refer to the whole section of sewer, whereas the repair may be a simple
CIPP patch repair. So the volumes of work reported as undertaken by OFWAT may be at variance with
actual lengths of lining installed by preferred contractors. The CIPP contractors themselves were
contacted for estimates of work done for TW and, on this basis, from equally fragile data, it seems more
likely that the actual volumes of CIPP undertaken for TW annually may be only about 25 to 30 km (16 to
19 mi) over the period since privatization.
The bulk of TW’s experience since 1971 has been with polyester resin and felt lining, inverted with water
and hot water cured. Currently, TW’s preferred lining methods include traditional polyester resin and felt
for structural improvement, epoxy resin and felt for leak tight lining (mainly for pipes 150 mm in
diameter), woven hose lining (Brawoliner) for small diameter pipes with multiple bends and UV cured
glass reinforced polyester linings for critical locations such as adjacent to rail tracks where time pressures
require rapid installations.
TW confines its work to experienced contractors, particularly Onsite and Waterflow, having had
unsatisfactory work completed by small contractors. In its experience, construction defects, if present,
become apparent within 1 to 2 years of installation. Its experience overall has been good, though some
deficiencies in older works have been identified through routine and emergency CCTV surveys where
local municipalities acting as TW agents may not have been as informed on acceptance standards and
where acceptance criteria have been improved over the years in the transition from WRc IGN 4.34.04
Issue 1 (April 1986) to the current BS12566 Part 4:2002 (see Table C-3).
Table C-3. TW Sewer Renovation Acceptance Criteria and Relevant Standards
WRc IGN 4-34-04
Criterion Issue 1 1986
Appearance Smooth, generally free
from wrinkles, degree
of wrinkles may be
agreed by purchaser
and manufacturer
Thickness Minimum specified
and up to 15%
thicker. May be
greater where felt
Tensile
Strength/
Modulus/
Elongation
Flexural
Strength/
Modulus/
Elongation
layers overlap
Not less than
25 MPa
1,700 MPa
1.5%
Not less than
50 MPa
2, 600 MPa
1-2.5%
WRc IGN 4.34.04
Issue 2 1995
Surface irregularities less than
6mm or tabulated value, % of
diameter, normally
longitudinal 2% in the invert
and 5% above the flowline,
circumferential 2% anywhere
Minimum specified and up to
15% thicker
C-4
BS EN 13566
Part 4:2002
Surface irregularities not more
than 2% of diameter or 6 mm
Mean thickness not less than
design thickness and minimum
not less than 80% of design
thickness or 3 mm
NA Not less than
Declared value < 15 MPa
N/A
Not less than
Declared value < 25 MPa
Declared value
Declared value < 0.75%
Declared value < 0.5%
Not less than
Declared value < 25 MPa
Declared value < 1,500 MPa
Declared value < 0.75%