Plate B — Joint offset.
A joint offset is what happens when one section of bell-and-spigot clay pipe has settled vertically — or shifted laterally — relative to the next. Under Buffalo's pre-1940 housing, the most common cause is differential settlement under the foundation step where the lateral exits the basement wall, and the second-most-common cause is consolidation under the sidewalk and curb cut where vehicle load was added long after the pipe was laid.
Plate B · Profile, joint offset at foundation step
RKT-MOR-B- A
- 1908 limestone foundation, typical Buffalo pre-1925 construction
- B
- Settled bedding zone under foundation step
- C, E
- 6" clay tile, upstream and downstream of the offset joint
- D
- Vertical offset, here ≈ 17.5% of nominal diameter
§ I · The PACP severity ladder
NASSCO PACP v8 grades joint offsets on the percentage of nominal diameter the offset represents:
- JOS — joint offset, small. ≤ 10% of nominal. Severity 1–2. CIPP-treatable; the liner bridges the offset cleanly.
- JOM — joint offset, medium. 10–25% of nominal. Severity 2–3. CIPP-treatable up to about 15%; pipe-bursting required above that.
- JOL — joint offset, large. > 25% of nominal. Severity 4–5. Trenchless rehabilitation is no longer viable; the joint is functionally a discontinuity and the bursting cable cannot reliably track the bore.
The 6"-ID lateral that is the dominant Buffalo housing-stock geometry tolerates 0.6" (10%) of vertical offset cleanly, and 0.9" (15%) marginally. The example above, at 1.05" (17.5%), is right at the edge of CIPP viability and would in our shop be a pipe-bursting candidate.
§ II · Why offsets happen
Three causes account for almost all the offsets we see in Buffalo:
- Foundation-step settlement. The basement wall sits on a footing on undisturbed soil. The lateral exits the basement through a hole in the wall, but its bedding immediately outside the wall is backfill — soil that was excavated to dig the foundation, then replaced. Backfill consolidates over decades; original soil does not. The result is a step in the support profile right at the wall, and the joint immediately outside the wall settles into it.
- Curb-cut consolidation. The original 1908 plumbing crew laid the lateral expecting horse-drawn carts at the curb. The 1948 driveway slab, the 2010 SUV, and the 2023 oil-truck delivery all add load that the lateral never planned for.
- Frost heave + thaw cycle. Buffalo sees about 80 freeze-thaw days a year on average, per NOAA's Buffalo NWS station.1 A lateral at marginal depth (< 4 ft) is vulnerable to seasonal frost heave; over decades, the cumulative cycling shifts joints.
§ III · What we recommend
- Up to 10% offset (JOS): CIPP lining. The liner conforms to the host bore including across the offset.
- 10–15% offset (JOM low): CIPP lining, with explicit acknowledgment that the liner will have a small wrinkle at the offset that may catch fibrous waste over decades. Customer's call.
- 15–25% offset (JOM high): Pipe bursting. The host pipe is replaced entirely, the bedding can be improved during pit excavation, and the new HDPE line is continuous through the former offset zone.
- > 25% offset (JOL): Open-cut excavation. We sub the dig out to a regional excavator (typically Mark West Utilities); we do the plumbing.
References
- NOAA National Weather Service, Buffalo NY office. Climate normals — freeze-thaw days. weather.gov/buf
- NASSCO PACP v8 — JOS/JOM/JOL coding. nassco.org/programs/pacp
- U.S. Army Corps of Engineers. EM 1110-2-2906 — Bedding and backfill of buried pipe. publications.usace.army.mil
- City of Buffalo, Department of Public Works. Streetscape and curb-cut standards. buffalony.gov