FIBERGLASS MANHOLE BUOYANCY CONSIDERATIONS

This paper explains the calculations we have made which demonstrate why flotation is not a problem with these products. It should be noted from the start of this theoretical discussion that, in practical terms, these products have been in service for over twenty five years and not one of the thousands of Manholes or Wetwells which have been installed has ever become displaced due to flotation.

Discussion:
Any object buried in the ground below the normal water table is subject to an uplift force equal to the weight of the water it displaces. The object will tend to move upward if there is not sufficient restraining force to counteract the buoyancy. All buoyant forces are in the vertical plane and, therefore, act only on the surfaces in the horizontal plane. Since the only horizontal plane in the wetwell or manhole is the base slab, all buoyant forces act on the slab, and there are no buoyant forces tending to separate the manhole from the slab.

The net hold-down force (positive if holding down, negative if tending to float) can be calculated based on the following equation:

Ht = Hm + Hc + Hs - B

Where:

Ht = Net hold-down force (lbs.)

Hm = Hold-down force due to cylinder weight (lbs.)
Hc = Hold-down force due to base slab (lbs.)
Hs = Hold-down force to soil column (lbs.)
B = Buoyant force of cylinder (lb.)

For any closed cylinder, such as a manhole or wetwell, the buoyant force (expressed in lbs/ft of cylinder height) equals the volume of the cylinder multiplied by the weight of the displaced water:

Equation 1: B-pi r2hGw

    B = buoyant force of clinder (lb.)
    R = radius of cylinder (ft.)
    H = Height of cylinder (ft.)
    Gw = unit weight of water (62.4 lbs/cu.ft.)

For any buried cylinder, the factors which restrain the cylinder from floating include:

  1. the weight of the cylinder itself,
  2. the weight of the bottom slab to which the cylinder is attached,
  3. the weight of the column of soil over the slab.

The force due to the weight of the cylinder is:

Equation 2: Hm = c (h)

    c = weight of cylinder per ft. of height (lb./ft.)
    h = cylinder height (ft.)
    Hm = Hold-down force due to cylinder weight

The force due to the weight of the slab is:

Equation 3: Hc = (A) (T) (Gc - Gw)

    Hc = Hold-down force due to base slab
    A = Area of the slab (ft.2)
    T = Minimum thickness of the slab (ft.)
    Gc = Unit weight of concrete (150lb/ft3)

The Volume of the soil column is found by projecting the area of the slab upward to the ground surface and calculating this volume, and then subtracting the volume of the cylinder. The weight of the soil occupying the net volume is then calculated. (Note that the soil density is reduced by the unit weight of water to allow for the buoyancy of the soil particles:

Equation 4: Vc = A (h)

    Vc = Volume of projection above slab (cu. ft.)

Equation 5: Vm = pi

    Vm = Volume of cylinder (cu. ft.)

Equation 6: Vt = Vc – Vm

    Vt = Volume of soil column (cu. ft.)

Equation 7: Hs = (Gs- Gw) VT

    Hs = Hold down force due to soil column (lbs.)
    Gs = Unit weight of native soil backfill (a poor soil of 100 lb/cu. ft. is assumed).

 

The following table summarizes the calculations for a 48" diameter Manhole at varying depths:

Burial
Depth
h
(Ft.)		 Manhole
Weight
Hm
(Lbs.)		 Soil
Weight
Hs
(Lbs.)		 Concrete
Weight
Hc
(Lbs.)		 Buoyant
Force
B
(Lbs.)		 Total
Force
Ht
(Lbs.)		 OK
or
FLOAT
3 195.0 2385.6 377.0 -2352.4 605.2 OK
4 260.0 3453.7 502.7 -3136.6 1079.7 OK
5 325.0 4658.1 628.3 -3920.7 1690.7 OK
6 390.0 5998.9 754.0 -4704.8 2438.0 OK
7 455.0 7476.1 879.6 -5489.0 3321.7 OK
8 520.0 9089.7 1005.3 -6273.1 4341.9 OK
9 585.0 10839.7 1131.0 -7057.3 5498.4 OK
10 650.0 12726.1 1256.6 -7841.4 6791.3 OK
11 715.0 14748.9 1382.3 -8625.6 8220.6 OK
12 780.0 16908.1 1508.0 -9409.7 9786.3 OK
13 845.0 19203.7 1633.6 -10193.8 11488.4 OK
14 910.0 21635.6 1759.3 -10978.0 13327.0 OK
15 975.0 24204.0 1885.0 -11762.1 15301.9 OK
20 1300.0 39091.9 2513.3 -15682.8 27222.4 OK
25 1625.0 57389.8 3141.6 -19603.5 42552.8 OK

 

CONCLUSION:
The preceding calculations have ignored a number of factors, all of which tend to increase the hold-down force on the cylinder. The weight of any structures on top of the cylinder, such as the top concrete slab in the case of a wetwell or the grade adjustment and ring and cover in a manhole installation, have not been included. Also ignored is the wedge effect of the soils which, depending on the nature of the soil in which the cylinder is buried, could dramatically increase the weight of the column of the soil over the slab.

It is therefore concluded that, under the assumptions stated in this paper, Fiberglass Manholes and Wetwells are not subject to "float-out". This conclusion is supported by extensive experience with Wetwell and Manhole installations, particularly in the coastal areas of the South and Southeast, which tend to have poor soil conditions and very high water tables.

  

Copyright © 1999 Containment Solutions, Inc.