Ground anchors consist of steel cables or rods inserted into predrilled holes and subsequently filled with cement grout. The cables are subsequently stressed and locked against a cap slab on the ground surface. This holds back the ground formation against collapse. The method is extensively used for slope stabilization and retaining walls. HD Engineering is specialized in the design and manufacture of anchor drills such as the HD90MKIV, HD110-3 and HD180 which can drill holes at any angle and are able to safely traverse steep slopes. They usually employ hydraulic over burden drifters whereby a hole is drilled using an inner rod and outer casing. Subsequently the inner rod is retracted and the anchor strands are inserted into the casing left temporarily in the hole to hold back the formation against collapse. The casing is later retrieved leaving the anchor in the hole, while grouting is carried out. Also see hydraulic drifter drilling.