Capitol Hill is home to many historic retaining walls built from brick or stone. Many homes with English Basements have a lower entrance that is sidewalk level and/or feature a raised garden in between the door and the sidewalk.
Hydrostatic pressure often builds behind these walls and, with nowhere to go, can start to damage the structure. Common indications are deflection and loreal destabilization of the wall. An early sign of that the wall may be failing is efflorescence — white stains that build up on the surface of the wall generated by salt deposits as moisture permeates the masonry. If a problem is left unchecked, you might eventually find spalling, where the brick starts to flake apart.
Moisture issues can typically be avoided with the installation of a weep — essentially, a small gap in the masonry structure that allows hydrostatic pressure to drain out. A drain will also alleviate moisture problems. These are typically a perforated drain pipe or tile set into an aggregate stone bed and wrapped with a filter fabric.
The vast majority of Capitol Hill retaining walls were surprisingly built without weeps.