The Effect on the Uplift Resistance of Anchors of Ground Disturbance During Placing

In civil engineering works, anchors are sometimes used to support uplift of tensile forces. The installation of an anchor in the ground will inevitably cause a certain degree of disturbance to the soil around it. From a practical point of view this zone of disturbance does exist to a lesser or greater extent depending upon the types and shapes of anchor used and the surrounding soils. The work described in this thesis was primarily concerned with the effect of installation disturbance on the pullout capacity of a vertical circular plate anchor embedded in sand. As far as the author is aware, this particular anchor problem has not received much attention. Because of this no literature review which had a direct relevance to the present investigation was available. To date only Kulhawy (1985) has proposed tentative guidelines for the design of spread anchors embedded in a soil zone which had a density different from the surrounding soil mass. Nevertheless a brief summary of the available pullout theories on circular vertical plate anchor is presented. To investigate the effect of installation disturbance on the anchor pullout capacity, two types of sand bed were required. Firstly to offer a standard of comparison with disturbed sand beds, homogeneous sand beds were prepared in a sand container in order to carry out pullout tests on anchors with depth/diameter (D/B) ratios ranging from 3 to 15. Three states of homogeneous sand bed of unit weight 17.14, 16.40 and 15.75 KN/m3 of relative densities 92%, 70% and 49% respectively were used in this investigation. These sand beds represented the dense, medium and loose sand states in the context of the research. Three model circular anchors were used namely 25, 50 and 75mm diameter. The pullout tests on the anchors were performed under load-control and displacement-control so that a comparison could be made of the ultimate load on the anchor between these two types of test. It was found that under the load- controlled test the post peak load on the anchor could not be observed. The anchor pullout capacity curves expressed in terms of the breakout factor (Pu /gD) against the depth/ diameter ratio (D/B) between the dense and loose homogeneous sand beds were used as the upper and lower limits of the pullout capacity available. Secondly to simulate the effect of the installation procedure disturbed beds were prepared or formed in such a way that a volume of loose sand in the form of a cylinder was formed above the anchor position within a container filled with homogeneous sand beds of varying unit weights deposited from a spreader. The depths of anchor embedment D, were varied to produce a range of D/B ratios from 3 to 15. The unit weight of the loose homogeneous sand bed i.e. 15.75 KN/m3 was used as a basis for the formation of the cylindrical disturbed zone above the anchor plate. The width of the disturbed zone Bz, was varied in proportion to the anchor diameter B over the range of Bz/B = 1,2 and 3. Results from the pullout tests in the homogeneous sand beds showed that the author's tests were consistent with Fadl's (1981) experimental results. Particular attention was drawn to Fadl's work because his theory was used by the author as a preliminary step to establish his (the author's) theoretical uplift resistance for an anchor in a disturbed zone. From the tests carried out in the disturbed beds, the installation disturbance significantly affected the anchor pullout capacity especially when the ratio of Bz/B was greater than 1. The tests also showed that the value of Bz /B should be kept to a minimum and for a plate anchor the minimum possible value of Bz/B was 1. When the width of the zone of disturbance was increased to three anchor diameters, the results showed that the anchor pullout capacity embedded in a disturbed zone surrounded by a dense homogeneous sand bed, was similar to that anchor as if it were being pulled out from a bed which was wholly disturbed throughout the sand mass. A simple expression for the anchor uplift resistance in a disturbed zone derived from Fadl's equations is presented. The theoretical uplift resistance showed a reasonable agreement with the test results. Conclusions were drawn from the test results and the theoretical analysis. Due to the absence of other theoretical analyses and published works, it was not possible to make a comparison with the existing data. Finally suggestions were made and further works were proposed for the future