Retaining wall movements, CTRL Contract 430 - Ashford Tunnels

摘要

Detailed monitoring during construction has provided a comprehensive set of data on the behaviour of 14 embedded retaining structures in stiff clays. Wall movements during the initial cantilever stage of excavation are summarised and the sway movements during subsequent excavation has been identified. Within the usual range cantilever movements may be found approximately from retained height using a linear relationship such as δc = 0.4%H but measurements at this site show that movement is better related to a higher power of retained height. The relationship δc = 2.5 x 10~(-3) H~4 /I + 4.5, where δc is in mm, H in m and I in m~4/m gives a reasonable fit to the Ashford data and is proposed as a basis for estimating cantilever wall movements in similar conditions. The amount of sway that occurred was much less than the corrections for sway that were made in moderately conservative pseudo finite element analyses. Sway movements were shown to be less than prop shortening and realistic values of prop stiffness should be used if sway is to be considered. The sway corrections were time consuming but did not improve accuracy. In some cases they predicted critical conditions that did not arise. Sway corrections may reduce the calculated moments and shear forces in the more heavily loaded wall and it is not necessarily conservative to over predict sway. It was agreed during the contract that sway corrections were not needed where the initial out of balance force was less than 20% of the mean prop force. Finite Element analysis that model the behaviour of both walls and the props between them provided realistic output and should be preferred in situations in which sway is a real issue.