These problems are particularly severe when (a) the pipes are large ones (e.g. of external diameter greater than 3 inch, particularly greater than 5 inch), (b) when the wall thickness of the coupler is small compared to its diameter (e.g. the ratio of the internal diameter of the coupler to its wall thickness is at least 5:1, particularly at least 8:1), (c) when no internal support member is used,