... the mouth of the Manhole set to U-shaped, with a T-type synthetic rubber, U-shaped slot embedded in the contact ... . Anti-slip: 3mm tread covers convex surface, and thus play a good non-slip effect. 4. Anti-impact: Manhole covers the ... contact with the colossal surface, directly on the T-rubber, and dealt with gently, to eliminate noise. ductile iron ...