Chalk anchorages: exhumation load transfer mechanism and design guidelines

An anchorage test programme carried out on the site of a deep basement construction in Norwich, UK, allowed the determination of anchorage capacities and creep characteristic data, and the exhumation of several anchor bodies in chalk. This paper reports on the performance of the chalk-based anchorages, the observation of exhumed grout body shapes and sizes, the extent of grout penetration into dissolution cavities, pressure-grouted fissures, and the formation of grout cake around the grout body periphery. Knowledge of construction techniques utilised for drilling, flushing, pregrouting, redrilling, pressure grouting and the general methods of control implemented to prevent dissolution cavity collapse has been related to the characteristics of the exhumed anchor bodies. Analysis of these grout bodies has allowed the proposal of a failure mechanism, comprising shaft resistance and shear of grouted fissures. Tests were carried out on both conventional and singlebore multiple anchors (SBMAs). Using short, fixed-length capacities with recently developed efficiency factors allowed accurate evaluation of the ultimate capacity of a typical 10 m fixed-length conventional anchorage. This design approach confirms that the SBMA system mobilised double the capacity of a conventional 10 m fixed anchor. Advancements in SMBA technology have enabled designs of 20 m fixed lengths, safely supporting working loads of 3000 kN.