The Behavior of a Thread-Bar Grouted Anchor in Soils from Local Strain Monitoring

International standards discourage the use of grouted anchors with a fixed length exceeding 10 m. However, grouted anchors with a fixed length between 10 and 20 m are frequently used in Italy to transfer high loads to ground with poor geotechnical properties. This paper presents the results of investigation tests on an anchor with a length of 36 m, of which 18 m is fixed, sloping 40° from the horizontal; the anchor is comprised of a reinforced thread-bar which was instrumented with strain gauges and founded in nonhomogeneous ground, a sand deposit followed by marly clay. The test aimed at investigating the progressive mobilization of the shear strength along the foundation. The results indicate a very low shear strength offered by the sand, probably disturbed by the drilling, and an unusually fast mobilization of the shear strength in the marly clay at the deep end of the anchor. The results are particularly useful to identify the reasons for the observed poor performance of the grouted anchor. In particular, the study once again made it clear how important the influence of the execution details on reaching the expected load capacity may be, and likewise the practice of investigation tests on suitably instrumented test anchors.