ElastiCity

Dig a deep pit several kilometres wide, seal it hermetically by vitrification and fill it with water. Stretch a tough elastic surface across it, cover that in infrastructure and gyroscopically supported buildings and street furniture. Some of the infrastructure is helical in form and can therefore expand. People and businesses gradually fill the city, bringing with them urine and waste water, which is treated and discharged via the sewers into the pit. Also, the city collects rainwater. As time goes by, the elastic surface gradually bulges and inflates, moving the buildings further apart, which stay upright due to the gyroscopes. Build more edifices between the older buildings. More people move into the city (or engage in economic activity there), thereby importing more liquid waste and further inflating the city. Ultimately, what was formerly a flat city becomes a hemispherical city of twice the area, through which it's possible to move along spiral and radial roads away from the centre assisted by gravity. Finally, either dig satellite pits or start building down into the pit, allowing people to live surrounded by ancient slurry (best not to think about that bit).