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Manhole Flotation Resistance
Buried hollow structures, such as plastic and concrete manholes, have a tendency to float out of the ground under high groundwater conditions. Any buried hollow structure must be designed and installed to resist potential flotation forces.
The Rhino manhole, when properly installed can withstand very high groundwater tables (see examples below), primarily due to a unique stiffening/anti-flotation ring that has been incorporated into the unique design.
There are many factors involved in resisting the flotation forces on a buried Rhino manhole. Each of the following forces or weights act together to keep the manhole from floating out of the ground:
1.) Hydrostatic pressure, F1, from the groundwater acts upward on the bottom of the Rhino manhole, attempting to push it out of the ground
2.) The weight of the manhole & list, W2, act downward.
3. The weight of the (buoyed) soil, W3, directly above the dome also acts downward.
4.) Each Rhino manhole component has a stiffening ring around the circumference of the manhole shell. The 2.5-inch wide rings cause each manhole section to behave like an earth anchor, pushing against a wedge of soil, W4, extending outward from the manhole, as shown in Fig. 1. The angle that the wedge makes with the horizontal depends on the type of backfill used. In general, any gravel or crushed stone will result in an internal friction angle of 28(degrees) to 36(degrees). This is valid unless very loose sands or silts are used as backfill. As a general rule, any clean, granular, compacted soil (coarse sand, gravel, crushed stone) will have a friction angle of at least 30(degrees). This analysis is valid for depths of bottom ring not exceeding 4 manholes diameters.
5. The outlet and inlet (not shown) pipes will resist the upward hydrostatic force with a force, W5, against the soil. The maximum value of W5 is the force required to shear or break the pipe.
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