Exhumation and Design of Anchorages in Chalk

Synopsis An anchorage test programme carried out on the site of deep basement construction in Norwich, UK, allowed determination of anchorage capacities, 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, extent of grout penetration into dissolution cavities, pressure grouted fissures and the formation of grout cake around the grout body periphery. 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 single bore multiple anchors (SBMA). Using short fixed length capacities with recently developed efficiency factors allowed accurate evaluation of the ultimate capacity of a typical 10m fixed length conventional anchorage. This design approach confirms that the SBMA system mobilised double the capacity of a conventional 10m fixed anchor. Advancements in SMBA technology has enabled designs of 20m fixed lengths, safely supporting working loads of 3000 kN.