KianSanat Pasargad manufactures manhole risers using a spiral winding process. In spiral winding, a flat
polyethylene extrudate is wound over a rotating mandrel. A small corrugated tube is wound
over first layer of extrudate to enhance stiffness of pipes. Smooth outer surface is achieved by
winding a flat extrudate over stiffener tubes. Additional layers are used to increase the riser’s
wall thickness & stiffness. Spiral winding is an ID-controlled process. The inside diameter
remains fixed regardless of the wall thickness, whereas in conventionally extruded pipe the OD
is controlled and increasing the wall thickness reduces the inside diameter. When designing a
manhole, it is important to recognize that spirally wound manhole risers are ID sized.
Closed profiled structure gives riser wall very high resistance to circumferential forces such
as ring stiffness. Special construction of riser wall also resists longitudinal crushing or buckling
of the riser wall. The hollow stiff core is joined wall to wall and two wall layers are supporting
core from both sides to provide resistance to crushing or buckling.
Manholes risers are classified based on ring stiffness such as 2 KN/sq.m., 4 KN/sq.m., 6 KN/
sq.m., 8 KN/sq.m., etc. The ring stiffness is tested according to ISO 9969.