... in the drainage of railway,highway ,tunnel HDPE double-wall corrugated pipe using HDPE as main material, ... stiffness(KN/m2)≥6.3flattening testGive pressure in vertical and diameter deformation at 40%,unloading immediately, it ... -30°C, one meter height and free falling.vertical shrinkage ratio (%)≤3water permeable area (cm per linear ... wall should be smooth , no burr. HDPE double-wall corrugated pipe