Abstract:
A retaining structure (Support of Excavation, SOE) with soldier piles and lagging was designed and constructed for a 30ft (8.90m) deep excavation for tunnel construction of the Ottawa Stage 2 LRT project. The PLAXIS 2D's finite element model was used for the design. To accommodate an existing road crossing, a temporary bridge was used as a detour, connected to the soldier piles and employed as top struts for the SOE. This approach provided a cost-effective solution, however resulting in additional lateral loads from traffic on the bridge. Various challenges surfaced, such as large pile spans and specific lagging designs due to four utilities crossing the excavation. The maximum span between soldier piles reached 20ft (6.08m). Limited clearance under the bridge complicated matters, requiring careful planning for the construction sequence, allowing excavation, mid strut installation, and concrete pouring for the tunnel roof. Another complexity was an existing building supported on spread footings, located approximately 26ft (8m) behind the SOE. Assessing the impact of excavation on the building was completed by PLAXIS models. A detailed geotechnical instrumentation and monitoring plan was implemented during and after excavation to monitor the SOE piles and adjacent structures.