Manhole anchoring
There are three types of anchoring options for manholes. The first relates to installation of manholes below the water table. The second is for industrial applications involving large changes in temperature. When manholes are installed below the water table, the manhole displaces water. The upward lift of the displaced water tries to push the manhole out of the ground.
By extending the bottom of the manhole, a lip is created. The weight of soil on this lip holds the manhole in place. By casting a concrete ring over this lip, additional area and weight are applied and holds the manhole in place.
When a lip is used, bolts can be used to hold the manhole in place. This is the Bolting Option. Another method of holding the manhole in place is to cast concrete above the lip. Rebar should be placed in the slab poured above the lip. This is the Double Pour Option.
Many industrial sewer applications experience large changes in temperature. This change in temperature causes the high-density polyethylene pipe to expand and contract. If the manholes are not designed to handle these forces, the inlet and outlet connections to the manhole may fail.
To prevent forces of expansion and contraction from reaching these connections, a waterstop is welded to the high density polyethylene pipe. This is needed because concrete does notadhere to high-density polyethylene pipe. The waterstop forms a mechanical attachment point, transferring forces away from the manhole.
A third option is to use an Intermediate Anchor Ring. A rebar reinforced concrete slab is casted above the inlet and outlet pipes. Anchor lugs are welded to the body of the manhole to prevent sheer forces from being applied to the inlet and outlet pipes if the slab sinks. Note that anchor lugs can be used as an alternate to oversized bottoms.