Abstract: A method of providing a substantially leakproof shielding layer in the ground. The method includes the following steps: a) making a plurality of substantially parallel holes (H) in the ground by means of a guided drill head (4) which ejects ground-decomposing liquid mainly in the advance direction of the head, and b) passing through each hole an injection head (6) which ejects injection liquid essentially transversely to the advance direction thereof, so that the injection liquid will penetrate into the ground material and mix therewith and after transition to a solid state, together with injection liquid in an adjacent hole, which has also penetrated into and mixed with the ground material and has been transformed into a solid state, will form part of the shielding layer. The invention further includes a device for performing the method.