Flood-Head Uplift on a Slab, Hatch, or Manhole Cover

Scopea basement slab when the groundwater rises, a manhole cover when the sewer surcharges -- and the numbers are larger than intuition expects. The uplift pressure is the unit weight of water times the head above the underside, and a concrete slab resists only with its own weight, 12.5 psf per inch of thickness at 150 pcf. A 4 in basement slab with 3 ft of groundwater head beneath it after the basement was pumped dry carries 187.2 psf of uplift against 50 psf of weight: 137.2 psf net upward, and it would lift under only 0.80 ft of head. A 6 in slab floats at 1.20 ft, so slab weight alone is never the answer to a high water table, and this is why basements are pumped down in stages. A 2 ft manhole cover under a 10 ft head takes 1,960 lb -- nearly a ton -- which is why the USACE handbook warns against weighting covers with sandbags and advises ringing the manhole to reduce the head, or pumping it. The handbook's printed 2,060 lb implies about 65.6 pcf; the calculator reports the 62.4 pcf value and names the difference. The calculator gives the pressure, the force, the net, the head at which the element floats, and the counterweight a cover would need. It does not account for reinforcement, connection to walls or footings, pressure relief or underdrains, soil above a buried structure, dynamic surcharge, cover locks, or the slab's spanning capacity under uplift. FEMA P-55, the USACE Flood Fight Handbook, the engineer of record, and the utility govern.

Formula or tableuplift pressure p = gamma_w x h (62.4 pcf fresh, 64.0 salt); force = p x area (round cover pi/4 D^2); slab resistance = 150 pcf x thickness = 12.5 psf per inch; net = p - resistance; floating head = resistance / gamma_w; counterweight = force - own weight when positive.

Source and editionFEMA P-55, Coastal Construction Manual, Volume II, Eq. 8.4 (vertical hydrostatic force), and USACE St. Paul District, Flood Fight Handbook (2016), Section 5.7 (manhole covers under head). The engineer of record and the utility govern.

Free accessFEMA P-55 and the USACE Flood Fight Handbook are free federal publications.

What governsEstimate. AHJ and licensed structural engineer govern.

API field nameshead_ft, water, element, slab_thickness_in, slab_area_sqft, cover_diameter_ft, cover_weight_lb, uplift_psf, resisting_psf, net_psf, floating_head_ft, uplift_lb

• Water unit weight 62.4 pcf fresh, 64.0 pcf saltFEMA P-55 Chapter 8
• Slab weight normal-weight concrete at 150 pcf, 12.5 psf per inchstandard practice
• Hold-down own weight only; no reinforcement, wall connection, soil cover, or relief drainagetile scope