The effect of foam concrete grouting on urban structure vibrations in Tabriz metro lines 1 and 2

The construction and operation of urban train tunnels, especially beneath dilapidated areas, often cause vibrations that disturb residents. A prime example is the Tabriz metro's lines 1 and 2, where vibrations exceeded permissible limits, leading to significant public dissatisfaction. This necessitated a 20 dB reduction in vibration levels. Consequently, a foam concrete injection plan was laboratory-tested for its quality and performance as a vibration insulation material. The results show that replacing 60% of conventional grout with foam concrete for injection behind segments in EPB (earth pressure balance) tunneling is an innovative approach that leads to a 56.5% reduction in water bleeding, a 71.42% increase in gel time, a 30% increase in viscosity, a 35.8% decrease in compressive strength, and a 48% decrease in wave propagation speed. The one-day uniaxial compressive strength of this mix design is 0.28 MPa, making it suitable for injection behind segments. Ultrasonic test results on foam concrete samples indicate a wave propagation speed of 237.31 m/s. Consequently, among the five tested mix designs, the one containing 60% foam, due to its suitable properties for injection and reduction of wave propagation speed, can effectively reduce urban structure vibrations. Furthermore, to assess the feasibility of foam concrete injection instead of conventional grout, this design was implemented over a 120-meter section of Line 2. Ultimately, the injection results and settlement measurements indicate the successful application of this design.