State of Technology Report for Force Main Rehabilitation, Final ...

head loss to maintain flow capacity. This cost needs to be factored into the decision process when
considering alternatives.
Future expenditures could include the cost of repairing a major break in a main, or carrying out some
renewal on the main in future years. These costs, especially the timing and cost of a break, can be
difficult to quantify, but some estimate is needed for a life-cycle cost comparison.
All else being equal, the renewal alternative with the lowest present value based on a life-cycle cost
analysis would be the selected option. However, there are usually other considerations that must also
come into play and have a bearing on the choice of renewal technologies as discussed below.
4.2 Capacity
Some consideration must be given to the capacity requirements of a system whenever a sewer force main
asset reaches its end-of-life and a decision is to be made on either renewing or replacing the asset. This
often requires hydraulic modelling of the wastewater system in combination with population density
forecasts. This model will help to determine if the existing force main is of adequate size, whether it
could be downsized to allow for certain renewal technologies, or if upsizing is needed to handle predicted
future flows from growth.
Sliplining of a sewer force main is going to result in a loss of cross-sectional area and therefore capacity.
With a 5% reduction in inside diameter, which is essentially the minimum that could be sliplined, the
corresponding loss of capacity with no change in friction factor is 10%. A 10% inside diameter
reduction, which is more normal, will result in a 19% loss of capacity. Unless that type of capacity loss
can be tolerated, or cost-effectively accommodated with pump upgrades, sliplining is going to fall by the
wayside as an effective option. A pump upgrade to maintain or improve capacity is an expensive option,
but still merits consideration.
Most of the PVC and PE close-fit liners and CIPP lining products will result in a very modest 0.5% to 3%
reduction in inside diameter of the pipe. The improved flow characteristics of these smooth liners,
usually with Hazen & Williams flow coefficients of 145 or higher, compensate for the slight reduction in
cross-sectional area. These solutions will generally work when the present system offers sufficient
capacity.
If future growth dictates that greater capacity will be required, then the options for rehabilitation narrow
to either offline replacement with a new, larger diameter pipe or pipe bursting with upsizing. The
diameter limit on upsizing is generally limited to one or maximum two pipe diameter sizes. Larger
upsizings have been successfully completed, but should be carefully evaluated as to the equipment
capabilities and the effect on nearby structures.
4.3 Accessibility
Accessibility will affect both the cost to renew a sewer force main, as well as the chosen technologies.
Fully deteriorated pipelines in rural areas, with no environmentally sensitive areas to cross, and not likely
to inconvenience the general public, will be more cost-effective to fully replace using conventional open
cut construction as opposed to the use of a structural lining in the existing pipe. That comparison may
change in the future as new structural spray-on linings become available.
Conversely, pipelines in congested areas with traffic and underground utilities to contend with, either
partially or fully deteriorated, are ideal candidates for some form of either online replacement or
rehabilitation. Pipe bursting needs to be carefully controlled if the main is in close proximity to other
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