A removably insertable apparatus for creating and accessing manholes and a method for rehabilitating manhole structures and the like, using the apparatus. The apparatus comprises two sections, both of which are load bearing composite materials such as reinforced fiberglass. The top unit is a removable, hollow, upwardly extending riser portion, of varying heights, which may contain an external flange on top, to support a manhole cover or other closure. The second section is a base unit with an outer diameter that is smaller than the inner diameter of the top unit. The base unit is embedded into the ground, or into a concrete pad. The top unit fits over the base unit, and any gap between the two units may be sealed with an o-ring type gasket to seal the interior and prevent the introduction of fluid, gas, or other matter into the interior of the units once installed. Both units may be cut on site or pre-cut with holes or stubouts. The method comprises the steps for rehabilitating an existing manhole using the apparatus.