Complex sentence: Investments included the demolition in the mid- 1950s of the original 1870 Twyn-y-Garreg huts, but the site remained dormant almost until the colliery closed, when the new "Navigation Street" was built.
Split into simpler sentences: Investments included the demolition in the mid- 1950s of the original 1870 Twyn-y-Garreg huts. The site remained dormant almost until the colliery closed, when the new "Navigation Street" was built.