Pipe Laying # 1970 Southampton
The subject of this paper is the behaviour of large diameter pipes made by the spiral winding process, inwhich a band of molten polyolefine is extruded onto a heated mandrel. The mandrel traverses to produce an overlap of about 10 mm which is compressed by a roller to form a continuous weld. As many passes as required can be made, so that pipes can be built up to any required thickness and in lengths as long as the longest mandrel which can conveniently be handled. The mandrel is partially collapsed before withdrawal.
The process as described has been used to make pressure pipe to DIN. 8074, and can offer economic advantages in the larger diameters, particularly as the process lends itself to the manufacture of sockets.
The spiral winding process is also capable of producing double walled pipes. In this development of the basic process, two or more passes of the mandrel are made, with the layers separated continously or intermittently by feeding in a spacer of flexible spiral metal strip. Typical double walled constructions which can be made by this process are shown in Fig. 1.
Constructions of this type have a larger second moment of area than solid walled pipes with the same material content. They are thus of especial interest for large diameter pipes subjected to external loading , and because of this have already been used extensively as sewers and effluent lines on the Continent in diameters to 1.8 m. It was felt that similar developments in this country would be expedited by practical trials here, and so the work described in this paper was undertaken.
The hehavioclr of solid walled large diameter oioes in H. D. Polythene under external loading has been extensively reported (Gaube (E), Hofer (H) and Muller (W), Kunstoffe 1970 (3) 146-149) and therefore the approach adopted to the problem of assessing the performance of double walled pipes was to attempt to relate it to that of solid walled pipes. The assumption was made that solid walled pipes would exhibit the same behaviour under external loading as double walled pipes with longitudinal sections having the same second moment of area.