Revised SRM Design Method for Sewer Rehabilitation
Presenter: Nick Orman, WRc
New SRM Design Method
The first edition of the Sewerage Rehabilitation Manual (SRM) was one of the earliest published design methods for sewer renovation systems and followed an extensive programme of research. The method is used both in the UK and internationally. Although the SRM has been revised on a number of occasions since then, the sewer renovation design method is largely the same as it was in 1983. Since that time there has been further research into the structural design of renovation systems, and improved materials. In addition, structural design generally has adopted a limit state design approach in line with the principles of EN 1990.
Alignment with the principles of limit state design according to EN 1990
Use of lower characteristic values (95% fractile values) for key design parameters. Importantly for instability limit states such as buckling the elastic modulus should be a lower characteristic value rather than the mean value.
Guidance on manufacturers’ declared values in the light of using EN 1990 principles. New guidance on QC taking account of the variability of the materials. This needs to take into account the use of short-term values in QC as a surrogate for the long-term values.
Type I Design
Traffic surcharge loadings updated in accordance with EN 1991 in alignment with BS 9295:2020.
Composite section fully taken into account using the equivalent area method (elastic theory).
Calculation of bending moment using method of Moore & Doherty for circular liners or BS 9295:2020 for egg-shaped liners.
Limiting strain in the grout to strain at first break will significantly reduce the stress that can be taken by liners made from materials with substantially different strain properties to the grout (e.g. GRP).
Type II Design
Buckling calculations now use the Glock/Boot method which gives better consideration of the effect of the gap where the gap is small. The Timoshenko method retained for grouting of circular liners where gap is large.
For non-circular liners a separate check on tensile stress is carried out.
The new edition will be published as a printed book and like its predecessors will make use of design charts to simplify the approach for the user.