Gravity Mains, Laterals and Manholes —
Graded Right the First Time

Everything that goes down a toilet, sink, floor drain, or grease trap leaves the building through a lateral, joins a gravity main — eight inches and up — and travels to the treatment plant without a single pump, purely on fall. Manholes are the round concrete access shafts placed at every bend, size change, grade change, and intersection, and they exist so crews can inspect and clean the line. Where the ground runs out of fall, you need a lift station: a wet well with pumps that lifts the wastewater and pushes it through a pressurized force main until gravity can take over again. Sanitary is never combined with storm drainage — during heavy rain a combined system overflows raw sewage into the creek, which is why new ones have not been permitted in generations.

New buildings and new pads are obvious. The one that catches owners out is a change of use: a retail shell becoming a restaurant, or a warehouse becoming manufacturing, triggers a fresh capacity calculation and impact fee even when the pipe is already in the ground. The single biggest budget surprise on rolling foothills ground — and that is most of what we work — is elevation. If the lowest fixture on your site cannot drain by gravity down to the tie-in manhole’s invert, you need a lift station, and a lift station is not a line item you absorb quietly. It brings pumps, a wet well, a control panel, standby power, a force main, and a permanent operating obligation with it. After that it comes down to whose rules you are under, and they are not interchangeable. OJRSA out in Oconee County, for instance, requires service lines larger than four inches to connect at a manhole rather than tap the pipe, and multi-family over four units to take a minimum eight-inch service. ReWa has its own list, and so does every authority across the North Carolina and Georgia lines. Read the right one before you draw the profile.

Depth and rock decide almost everything on a gravity sewer. Eight-inch PVC in open ground at normal depth is your baseline number; go deeper than twelve feet, hit rock, or work under existing pavement and the per-foot figure multiplies, because rock is priced by the cubic yard and it is the most expensive dirt on any site. Manholes price by depth — a standard four-foot structure under ten feet is routine work, while a deep one brings shoring, dewatering, and a crane with it. A duplex submersible lift station with generator, controls, and site work is a project inside your project, not an accessory to one. Budget the authority’s fees separately, because every one of them prices capacity its own way. OJRSA, in Oconee County, assesses by equivalent residential unit — 300 gallons per day each — or by the gallon for other uses; ReWa and the Greenville-area systems publish their own schedules, and so does every county we work in across the state lines. We pull the current numbers for your jurisdiction and build them into the estimate rather than leaving them as a surprise. On schedule: design and permitting is eight to twenty weeks, installation of a 400 to 800 foot run is one to three weeks in normal soil and three to six with rock, and then closeout takes another two to five. One constraint catches nearly every general contractor — the mandrel deflection test cannot be run until backfill has been in place at least thirty days.

Everything we work sits on Piedmont geology — granite and gneiss under weathered saprolite and red clay whose thickness changes dramatically over short distances, and that is as true in Toccoa or Hendersonville as it is behind our shop in Easley. Rock depth is rarely predictable from a few borings, and a gravity sewer needing 0.40% fall cannot simply route around rock it finds sixty feet into a five-hundred-foot run without losing grade. Red clay compounds it: trench walls look stable dry and turn to soup after rain, and clay backfill placed wet consolidates later and pulls the pipe down into a belly. The rules laid over that geology are local, specific, and enforced to the letter. OJRSA is a fair example of how specific: an inspector notified no less than 48 hours before a connection, in-line hammer taps banned outright, manhole taps core-drilled and sealed with a rubber boot and rubberized plaster, mortar and cement explicitly not allowed. ReWa around Greenville has its own version of that list, and the authorities we build under in North Carolina and Georgia have theirs. A contractor who has not read the right one loses a day and a connection.