is provided with swellable material 7 and its side faces are covered by quadrangular plates 20 one edge dimension h of which corresponding to the width of the section 8 of the sealing device the other edge dimension of the plates 20 exceeds the corresponding dimension of the core element inclusive the swellable material 7 so as to form freely extending portions 21 a space being formed therebetween into which the section 8 of the sealing device 4 may be introduced said extending portions being provided with slots 22 into which the ribs 18 of the section 8 will fit during the jointing conduits 23 are arranged in the core element 19 from which pipe connecting pieces 24 extend through the swellable material 7 the pipe connecting pieces are located so that they will fit into the injection conduits 16 17 of the section 8 when the jointing element here generally designated 25 is mated with the sealing device 4 the conduits 23 in the core element are also connected to pipe connection pieces 26 extending from one of the side plates 20 through these pipe pieces injection medium may be introduced into the conduits 23 and further into the injection conduits 16 17 of the sealing device fig7 and 8 illustrate how a jointing element 25 may be used to form an angular connection between two sealing devices 4 the jointing element is pushed down on the longitudinal edge portion 12 of the horizontal sealing device.