Construction monitoring of cut and cover tunnels

Abstract

Over 700 instruments were installed to monitor the performance of the cut and cover tunnels that carry the Channel Tunnel Rail Link through Ashford. Careful monitoring confirmed design assumptions and provided a sound basis for changes to the construction sequence that accelerated the works. This paper highlights key features of the monitoring system. It describes the instruments that were installed, reviews their performance, and identifies practical issues that were addressed during the contract. The results help to quantify toe movements of embedded retaining walls. They identify variations in temporary prop loads that are explained by reference to the method of excavation and changes in prop stiffness during initial loading. Prompt reporting and open access to the monitoring data developed confidence and trust between the project partners, and led to closer interaction and greater efficiency.

Design and performance of retaining walls

“…Over 700 instruments were installed during the contract, and all 14 structures were carefully monitored during construction (Holmes et al, 2005). This paper summarises the maximum wall movements and prop loads that were measured.…”

“…For bottom-up construction cantilever movements at the top of the wall (Roscoe, 2003) governed both deflected shape and maximum movement. Figure 11 plots the maximum wall movements at the end of excavation against retained height H. These measurements include toe movements of about 5 mm (Holmes et al, 2005) as indicated on the figure. Also shown are the linear relationships often used to estimate the movement of embedded walls (Gaba et al, 2003).…”

“…This aspect is particularly important where an observational method is being used and controls are set on the rates of movement (Holmes et al, 2005), and at Ashford results from the first structures to be constructed were used to optimise the subsequent excavations.…”

“…Loads were measured in 36 sets of three props (Holmes et al, 2005). Variations in load within each set were generally within AE25% of the average, and all load measurements in steel props were corrected for temperature effects during processing.…”

“…Detailed monitoring measurements were made during the construction of all 14 structures (Holmes et al, 2005). The paper summarises the wall movements and prop loads that were measured, and compares these with the design calculations.…”