Upsizing by pipe bursting: how far to go and what it costs the ground

01 /What does upsizing mean in pipe bursting?

Upsizing is replacing a pipe with a larger one along the same path. The PPI Handbook of Polyethylene Pipe defines a one-size upsize as moving to the next standard size, for example 8-inch to 10-inch, and a two-size upsize as skipping one, for example 8-inch to 12-inch. Replacing with the same nominal size is called size-for-size or in-line replacement.

Upsizing is bursting's signature advantage. The EPA fact sheet says it typically yields the largest increase in hydraulic capacity of any trenchless rehabilitation method, because lining methods reduce the inside diameter of the existing pipe. The same fact sheet adds that, from a value-engineering standpoint, going at least one size larger is often considered advisable on public mains, to leave capacity for future loading.

02 /How much soil does an upsize push aside?

The head does not cut or remove soil; it compresses it outward. The volume displaced per metre is the area of the expander's circle minus the area of the old pipe's outside circle. The Upsizing & Displacement Calculator does exactly that sum: expander diameter equals new outside diameter times one plus the oversize, and litres per metre equal the ring area in square metres times 1,000.

Two things stand out. First, even a size-for-size burst displaces soil, because the expander is always larger than the host. Second, the ratio climbs quickly when a small host receives a much larger pipe: going from a 115 mm outside diameter to 168 mm pipe nearly doubles the displacement of the default example and crosses the calculator's 1.5 threshold, where it adds a note to ask the installer how ground movement, pulling force and nearby utilities will be managed.

03 /How many sizes up is realistic?

The PPI handbook says bursting is commonly performed size-for-size or one size up, that upsizes of up to three sizes have been successful, and that the larger the upsize, the more energy is needed and the more ground movement results. It also summarizes a NASSCO difficulty classification in which size-for-size or one-size upsizes fall in the routine class, two-size upsizes in the moderately difficult to challenging class, and three or more in the challenging to extremely challenging class, alongside depth, diameter, length and soil.

The EPA adds that the amount of upsizing possible depends on the compactness of the old trench backfill and the native soil, because a large upsize has to burst through the surrounding soil as well as the pipe. A lateral laid in loose backfill in a wide trench accepts upsizing far more easily than one in dense, undisturbed clay till.

04 /Does a bigger pipe actually carry more?

Yes, and by more than the diameter change suggests, because smooth polyethylene also flows better than rough clay or concrete. The PPI handbook calculates that for gravity flow, a 15 percent increase in inside diameter combined with PE's smoother wall can roughly double capacity compared with old clay or concrete, and that for pressure pipe a 41 percent increase in inside diameter doubles the bore area and the flow. Those are the manufacturers' figures under stated assumptions, but the direction is right.

For most single-family homes, capacity is not the problem. A properly sloped 100 mm lateral carries a house's flow comfortably, and a clay lateral that backs up is usually failing from roots, offsets or a sag, not from being too small. Upsizing earns its cost when fixtures have been added, a secondary suite or second dwelling now drains through the same line, the municipality requires a minimum size on replacement, or on public mains where growth is expected.

• Confirm with your municipality whether a replacement lateral must meet a minimum size or material.
• Match the new pipe to the city-side connection; a larger private line feeding a smaller city stub needs a proper reducer and gains little.
• Remember that frost cover and slope, not only diameter, decide whether a line freezes or holds solids.

05 /Why does the expander oversize matter as much as the new pipe size?

Because the soil only ever sees the expander. Two installers pulling the same 168 mm pipe through the same 150 mm host can displace quite different volumes: at a 5 percent oversize the calculator gives about 6.8 litres per metre, at 15 percent about 11.6 litres. Across a 20 m lateral that is roughly 135 litres against 233 litres of ground compressed around the pipe.

A larger overcut is not simply careless. The PPI handbook explains that an overcut of about half an inch around the pipe, together with bentonite or polymer lubrication, is one of the main ways to cut friction and keep pulling stress on the new pipe within its safe range, especially on longer runs. The trade is more displacement for less pulling load. On a short, shallow residential run close to a slab, an installer may lean toward a tighter expander; on a longer, deeper run in stable ground, a larger overcut can be the safer choice for the pipe.

Ask which way your installer is leaning and why. The answer tells you whether they have thought about both the pipe and the ground.

06 /What should I ask an installer about upsizing?

A good upsizing conversation is specific. Ask for numbers, not reassurance.

1. What are the outside diameters of the existing pipe and the proposed new pipe?
2. What expander diameter will you use, and why that oversize?
3. How deep is the pipe at each end and at the shallowest point?
4. What lies within roughly two to three pipe diameters of the route, and will it be exposed first?
5. Is the goal capacity, or is the upsize simply the pipe size you stock?
6. Would a size-for-size burst reduce ground movement enough to matter here?

The EPA fact sheet flags three conditions as critical for ground displacement: a shallow pipe where movement is directed upward, much larger replacement pipe, and deteriorated utilities within two to three diameters of the pipe being replaced. Those three belong in every upsizing discussion. More on clearances is in pipe bursting risks and problems.

Try your own numbers in the Upsizing & Displacement Calculator, then run the Pipe Bursting Fit Check, which adds a point for upsizing two or more sizes. When you are ready to compare installers, send the details.

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