To set a precast manhole to the correct invert elevation: (1) calculate required bottom-of-base elevation = design pipe invert − base invert channel offset (from the shop drawing); (2) set bedding gravel to that elevation with a level and rod; (3) set the base and verify the invert channel elevation with a direct level shot; (4) stack barrel sections and grade rings to achieve the design rim elevation within ±0.05 ft; (5) log all measured elevations before backfilling.
Manhole Components
- Base slab: Bottom unit with integral benching and invert channel. Pipe stub-outs are pre-cast or core-drilled in the field.
- Barrel sections: Cylindrical rings (typically 48” or 60” diameter) stacked on the base.
- Cone section: Eccentric or concentric reducer transitioning from the barrel diameter to the frame opening (~22–26 inches).
- Grade rings: Short concrete rings used to fine-adjust the frame and cover to the design rim elevation.
- Frame and cover: Cast iron frame mortared on grade rings; cover set in the frame.
Setting the Base to Elevation
Invert channel offset above base bottom (from shop drawing): 0.75 ft
Required bottom-of-base: 98.50 − 0.75 = 97.75 ft
Set the top of bedding gravel at 97.75 ft using a level and rod before setting the base.
After setting the base, verify the invert channel elevation directly with a level shot on the channel floor. Do not trust nominal pre-cast dimensions — manufacturing tolerances vary between suppliers.
Setting Rim Elevation
Top of cone after stacking all barrels: 104.55 ft
Gap to fill: 105.00 − 104.55 = 0.45 ft
Frame height: 0.25 ft
Grade rings needed: 0.45 − 0.25 = 0.20 ft (use 6-in + 2-in rings)
Set grade rings in mortar, plumb, and level. Set the frame in mortar (or per project spec). Verify the rim elevation with a level and rod: must be within ±0.05 ft of design (or per spec) and flush with the pavement surface.
Pipe Connections
Whether pipe connections are pre-cast or core-drilled in the field, verify the pipe stub-out invert elevation before placing bedding and installing the pipe. The pipe invert must match the design invert at both the incoming and outgoing connections at the manhole.
Field Verification Checklist
- Base invert channel elevation: ±0.05 ft of design ✓
- Rim elevation: ±0.05 ft of design ✓
- Manhole plumb: verified with plumb bob or level on barrel ✓
- Pipe connections sealed (rubber boot or mortar joint per spec) ✓
- Joints between sections grouted and watertight ✓
- All measurements logged in Gradelog for as-built documentation ✓
See also: Gravity Sewer Installation: Grade and Alignment Control and Pipe Grade Calculator.