confine the resin around the preform and prevent it from being soiled by sand or earth from the trench for this purpose the protective film 12 is carried by a reel 80 which may rotate both about its own axis as shown symbolically by the arrows k in the direction of unwinding and about the preform as shown symbolically by the arrow 1 the resin impregnated section of the preform 9 the preform being protected by a protective film is continuously deposited over its entire length labeled l in the bottom of the trench t as a result of the movement of the mobile assembly 4 5 along the latter next the two ends of the preform 9 are closed off in an airtight manner by means of closure plugs 90 91 as illustrated in fig7 b next a pneumatic compressor 92 is connected to one of the closure plugs in this case the plug 90 as illustrated in fig7 c the preform 9 is inflated with compressed air by means of this compressor 92 the inflation propagates progressively from the plug 90 toward the opposite plug 91 and the preform is made u201c round u201d adopting a cylindrical shape of course the protective film is sufficiently flexible and elastic not to counteract the radial expansion of the preform wall during inflation as already mentioned above the circular cross section thus obtained is well defined and constant from one end to the other because of the inexpansible nature of the filamentary reinforcement a pressure of the order of 1 bar is sufficient for the preform to be correctly inflated over its entire length l in the example illustrated in fig7 d this curing is obtained by electrical means.