A rib for supporting and consolidating an excavation comprises at least a first structural element (5A) and at least one connection element (10) for connecting the said structural element (5A) of the said rib to a corresponding connection element (10) of an adjacent rib (1), the said connection element (10) being capable of engaging through translatory motion with the said corresponding connection element (10) of the adjacent rib.

A rib for supporting and consolidating an excavation comprises at least a first structural element (5A) and at least one connection element (10) for connecting the said structural element (5A) of the said rib to a corresponding connection element (10) of an adjacent rib (1), the said connection element (10) being capable of engaging through translatory motion with the said corresponding connection element (10) of the adjacent rib.

A method and arrangement are proposed for injecting CO.sub.2 into a subterranean aquifer for storage therein. In order to reduce the effects of water evaporation from brine in the aquifer when dry CO.sub.2 is injected into the aquifer, the CO.sub.2 is supplied mixed with a salt-lean fluid, i.e. a fluid that contains a low concentration of ions that can precipitate out as salts. The mixing may take place at the wellhead, with the CO.sub.2 salt-lean fluids supplied via separate low-grade material pipelines. The proportion of CO.sub.2 and salt-lean fluid in the mixture is such as to obtain a CO.sub.2 composition that is saturated with salt-lean fluid at the site of injection into the aquifer. By injecting saturated or wet CO.sub.2, less water is evaporated from the brine and the salt precipitation is greatly reduced, so keeping the pore spaces clear and providing an increased accessible pore volume for CO.sub.2 storage.

A method and arrangement are proposed for injecting CO.sub.2 into a subterranean aquifer for storage therein. In order to reduce the effects of water evaporation from brine in the aquifer when dry CO.sub.2 is injected into the aquifer, the CO.sub.2 is supplied mixed with a salt-lean fluid, i.e. a fluid that contains a low concentration of ions that can precipitate out as salts. The mixing may take place at the wellhead, with the CO.sub.2 salt-lean fluids supplied via separate low-grade material pipelines. The proportion of CO.sub.2 and salt-lean fluid in the mixture is such as to obtain a CO.sub.2 composition that is saturated with salt-lean fluid at the site of injection into the aquifer. By injecting saturated or wet CO.sub.2, less water is evaporated from the brine and the salt precipitation is greatly reduced, so keeping the pore spaces clear and providing an increased accessible pore volume for CO.sub.2 storage.