Abstract:

The main tunnels of Line 16 Lot1 of the Grand Paris Express project are underway since 2019 using tunnel boring machines (TBM). They are drilled entirely in permeable strata (fine/silty sands, marls, and limestones) at a depth of 20÷30 m below the groundwater level. Groundwater inflow with high pressure is a major issue at the arrival/departure sections of the TBMs as well as during the excavation work of connection galleries between the main tunnels and maintenance or security shafts. Line 16 Lot1 is located under dense urban areas with the requirement of minimizing the drawdown of the groundwater and water inflow in civil engineering works; the purpose is to avoid excessive settlements on existing facilities and dissolution reactivation process of gypsum-rich strata. In order to minimize these risks, different types of ground improvement technics (jet grouting and cement mixture injection) have been applied, where jet grouting is used in fine sandy soil and cement mixture injection used in fissured marly-limestone. In addition, both technics of improvement have been associated on the same site. From a geotechnical point of view, the sands of Beauchamp are classified as coarse-grained soils (60% SM and 40% ML in the Unified soil Classification System ) and Marly-limestone of Saint-Ouen and deep Marls as fissured soft to medium-hard calcareous rock. This paper presents the geotechnical conditions, the ground improvement methodology and quality control during construction. The control sequence is performed by vertical and then horizontal drilling holes. The method of control work after excavation (horizontal control) allows to confirm the quality of the treatment blocks and specifically the bonding between the treatment block and the diaphragm wall.