While temporary timber structures have been extensively used in the underground construction processes, their current design is often conservative and over-engineered. Understanding the real performance of mixed timber-steel supporting systems will be beneficial to reduce both the health and safety risks to workers, and to cut the construction cost and time. In this paper, we introduce a novel technology called Wireless SmartPlank Network (WSPN) to monitor the timber structures in underground constructions. SmartPlank is a wooden beam equipped with a streamlined wireless sensor node, two thin foil strain gauges and two temperature sensors, which enable us to measure the bending strain and temperature experienced by the both sides of the beam, and transmit this information in real-time over an IPv6 (6LowPan) multi-hop wireless mesh network and Internet while the underground construction is taking place. The SmartPlank was carefully tested and calibrated in the lab. Four SmartPlanks were deployed at Tottenham Court Road (TCR) station during the Stair 14 excavation, together with seven relay nodes and a gateway node to provide connectivity to the Internet, and this monitoring will last for one and half years until the Central Line possession in 2015. The captured temperature data shows that the cement hydration rose up to 33 degrees around 10 hours after grouting, while the measured strain data indicates that the planks were not experiencing pure bending, with very large compression on the top surface during grouting but much less strain on the bottom side. The converted vertical earth pressure is much smaller than the design load. Some challenges we encountered on the real site deployment are also highlighted.