Compensation grouting is the addition of volume to the in situ foundation soils through the pressurized injection of cement-based grouts. The specific intent is to provide reinforcement and/or consolidation of the soil in order to compensate for ongoing structural settlement or to prevent potential settlement from occurring during tunneling operations immediately below sensitive overhead structures or adjacent deep excavation. The operation provides tightly controlled volume replacement to counteract unacceptable structural settlement.
Compensenation grouting is accomplished using a pre-placed Tube à Manchette (TAM) or “sleeve port” pipes installed between the tunnel crown and the structure to be protected and is used where there is no direct access to the structure itself. Strategic grouting is initiated at the tunnel shield if real time monitoring indicates the potential for foundation soil loss. Repeat injections are typically performed at each port to ensure the formation of a supporting network of multiple grouted lenses through the target soils.
- Mitigation of anticipated unacceptable settlement during soft ground tunneling or adjacent deep excavation
- Highly precise incremental results achievable
- Can be applied in a wide range of soils from loose sands to hard clays
- Extensive instrumentation and real time electronic monitoring of both the structure and ground movement is required.
- A high degree of coordination is required between the grouting and structural monitoring teams.
The East Side Access Northern Boulevard Crossing illustrates the success of compensation grouting in controlling settlement of an active subway structure during SEM tunneling through highly complex ground just a few feet beneath.