The 340-meter long Islais Creek tunnels are part of the Islais Creek transport/storage project, a series of combined sewers in southeast San Francisco designed to reduce discharges into San Francisco Bay during storms. The project involved tunneling through Bay Mud, a well-known and troublesome weak soil in San Francisco Bay. Further complicating tunnel construction was the presence of piles supporting sewers and a railroad over the tunnel. To address these challenges, jet grouting was used to pre-treat the Bay Mud to allow for open-face tunneling. Jet grouting is a form of ground modification whereby a cement grout is injected into the ground through a rotating drill bit. The soil structure is virtually obliterated by the jets and the original subsurface material is ultimately mixed with grout to form soil-cement columns. This was the first tunnel project anywhere to utilize full-face jet grouting for tunneling in soft clays. The successful completion of the project demonstrated that jet grouting for pre-treatment of soft soils is possible and compatible with open-face tunneling methods. Tunnel inflows and surface settlements above the tunnels were less than conventional theories would predict. This project also showed that ground heave is a critical issue with jet grouting under structures. Maintaining good return of cuttings during jet grouting is necessary to prevent such heave. Precutting and pretreatment of soils above the "design" zone for jet grouting was used to enhance return of jet grout cuttings.