Space-starved Singapore has expanded outwards by building into the sea and upwards by constructing high-rises but planners are now looking underground as they seek new areas for growth.The nation has carefully managed its rapid growth in recent decades to avoid the problems faced by other fast developing Asian metropolises, such as overcrowding and traffic chaos.But with its population of 5.6 million expected to grow steadily in coming years, authorities are now considering how to better use