Load Transfer in Pressure Injected Anchors

Abstract

Two instrumented pressure injected (grouted) soil anchors were installed as part of tied-back excavation support systems in dense sands. Strain gages were used to determine the distribution of load in the anchors during load testing and subsequent excavation. Relationships between load transfer (slope of the load distribution curve) and anchor displacement are constructed from the strain gage data. The magnitude of anchor displacement required to mobilize high values (10 to 25 kip/ft) of load transfer is very small, on the order of 0.1 in. (2.54 mm). Concentration of load transfer near the front of the anchors results from the dependence of load transfer mobilization on anchor displacement. During unloading, residual loads (and strains) are locked into the anchor by a shear stress reversal near the front of the anchor. Following lock-off, load tends to increase towards the back of the anchor with time. The majority of this load redistribution appeared to be in response to strains induced in the soil mass by excavation, rather than relaxation in the soil-anchor system.