Uplift Behaviour Of Frictional Anchors In Soil

With the development of city construction, buoyancy effects of buildings, such as tall building, subway, underground supermarket and so on, are getting more and more serious; at the same time, anchors are widely employed to withstand uplift forces. During the last many years various researchers have proposed approximate techniques to estimate the uplift capacity of soil anchors. The majority of past research has been experimentally based and, as a result, current design practices are largely based on empiricism. In contrast, very few rigorous numerical analyses have been performed to determine the ultimate pullout load of anchors because of many factors involved in this procedure. To evaluate the uplift behavior of shaft tension anchors in layered soils and frictional anchor groups, a constitutive model for a strain hardening–softening and volumetric dilatancy model was quoted, a series of triaxial CID and CU tests on clay and sand were performed to obtain the parameters needed for the soil model. In addition, filed tests on a single shaft anchor and an anchor group were conducted in layered soils. Thus, study results will prove very useful for future research and design of anchors in practice. The following conclusions can be drawn:As to tension anchor, with the increasing of loading, the head displacement and the force of anchors were also increased. Under the same loading level, the normal forces of the anchor decreased as the embedment depth increased. The uplift load was transmitted from top to the bottom along the bounded length. The load-displacement relations and ultimate loads calculated from numerical analyses produced good agreement with those measured by field tests not only for the single anchor but also for the anchor group.As to the anchor in layer soil, the higher elevations of ground water table the less anchorage capacity. For a vertical anchor which wad anchored on foundation mat, the effect of over consolidation ratio on anchorage capacity was not evident, when depth of excavation was greater than 5m. There is not much distinction on anchorage capacity between an anchor installed in a pure soil and a layer soil, if their average SPT blow counts are equal. However, when an anchors was located in a layer soil, the friction stress occurs discontinue behavior, the double protection should be used in fixed length for keeping long term anchorage capacity. Whether SPT blow count, overburden depth, fixed length or diameter of an anchor increased, the anchorage capacity also increased. Increasing fixed length should be the optimum method to increase the anchorage capacity.When a 2×2 anchor group with spacing of 2D (D: anchor diameter), efficiency of the anchor group was calculated to be only 75%~80%. There is no interference when spacing was 6D, and the efficiency of an anchor group could more than 95%. The yielding zone of the anchor groups increase as overburden depth increase. The efficiency of anchor groups can be eliminated as the anchors spacing are increase horizontally. It can be found that from the numerical results, the efficiency of 3×3 anchor group is greater than that of 2×2 anchor group.