Example Calculations

for Manhole Flotation

Assembly

Assume a 10 ft. high Rhino manhole is installed and backfilled in granular soil. Although it is unlikely to occur, we will assume a worst-case situation, i.e., groundwater table is up to the top of the lid and the manhole is empty. The dimensions of the example unit are (Fig. 2):

B = 49 in. = outside diameter of manhole
Bs = 54 in. = outside diameter of stiffener ring
Bc = 28.75 in. = OD of cylindrical section of top
Hc = 11.50 in. = Height of Cylindrical section of top
Hs = 24.50 in. = Height of spherical section of top
Ht = 36 in. = Total height of top section
He = 24 in. = Height of cylindrical elevation section
Hb = 36 in. Height of base section

STEP No. 1: DETERMINE UPLIFT FORCE, F1:
Hydrostatic pressure at depth 10 ft = 634 lb/sq. ft.
Area of base = 13.1 sq. ft.
Total uplift pressure, F1 = 8172 lbs.

STEP No. 2: DETERMINE WEIGHT, W2, OF MANHOLE
a. Concrete-filled lid = 226 lbs.
b. 3 ft. high top section = 150 lbs.
c. 2 ft. high elev. section = 128 lbs.
d. 3 ft. high base section = 130 lbs.
Total manhole weight = 762 lbs.

STEP No. 3: DETERMINE WEIGHT OF SOIL, W3, ABOVE MANHOLE SHELL:
Use gravel or crushed stone. Unit weight of soil is 125 lb/cu ft. Buoyed unit weight is 62.6 lb/cu ft.
Volume of soil in 49 in OD cylinder.W3, less total volume of top section - 12.54 cu ft. Weight of soil, W3, is 785 lbs.

STEP No. 4: DETERMINE WEIGHT OF SOIL WEDGE, W4, ABOVE LOWER RING:
Use gravel or crushed stone. Unit weight of soil is 125 lb/cut ft. Buoyed unit weight is 62.6 lb/cu ft.
Volume of soil in truncated cone less volume used in Step No. 3 = 464 cu ft.

STEP No. 5: DETERMINE WEIGHT, W5, TO SHEAR OUTLET AND INLET PIPES:
Although the shear force required to shear the outlet and inlet pipes can be several thousand pounds, that force has been ignored for this example.

SUMMATION OF FORCES, RESISTING - UPLIFT
Total of resisting forces, W2 + W3 + W4 + W5 = 30593 lbs.
Total uplift force, F1 = 8172 lbs.
Factor of safety - 3.74

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