BEHAVIOUR OF HELICAL SCREW ANCHORS IN SANDS

Abstract

The design of many engineering structures requires foundation systems to resist both compressive and tensile forces. These types of structures, which may include high rise buildings, chimney towers or transmission towers, are commonly supported by soil anchors. As the uses of anchors increased many-fold in recent years, to support substantial large structure, a greater understanding regarding their behaviour is required. During the last forty years various researchers have proposed approximate techniques to estimate the uplift and compressive capacity of soil anchors. The majority of past research has been experimentally based and, as a result, current design practices are largely based on empirical relationships. In contrast, very few rigorous numerical analyses have been performed to determine the ultimate pull-out and compressive load of anchors. The study of anchors is complicated by the large number of variables that influence its overall behaviour. These include anchor size, shape, embedment depth and orientation. Apart from these anchor properties, large numbers of soil variables are also there which influence the result greatly. All these variables must be considered in an analysis to achieve correct result. In present research work, a comprehensive study into the behaviour of helical screw anchors is presented. Consideration was given to the wide range of parameters that influence anchor capacity. The aim of the present research was to better understand anchor behaviour and to develop rigorous stability solutions for earth anchors that can be used by design engineers. In the present study, the locally available fine sand collected from Solani River bed was used for laboratory investigations. A good number of tests were performed by varying number of anchors (1, 2, 3 and 4), number of screw blades in the anchor (1, 2 and 3) and embedment depth ratio (H/B) (for compressive load H/B = 2, 4, 6 and 8 and for pullout load H/B = 4, 6, 8 and 10). The results obtained from the tests indicate that the bearing capacity in compression is much higher than that in tension. In compression tests, for initial increase in H/B (i.e. for increase in H/B from 2 to 4), the increase in Quc was 56%, while for further increase in H/B (i.e. for increase in H/B from 6 to 8); the increase in Quc was 28% which was quite less than earlier increase. Also, for initial increase in Na; the increase in Quc was 27% (i.e. for increase in Na from 1 to 2), while for further increase

in Na; the increase in Quc was 20% (i.e. for increase in Na from 3 to 4) which was less than earlier increase. Similarly, for initial increase in nb, the increase in Quc was 27% (i.e. for increase in nb from 1 to 2), while for further increase in nb; the increase in Quc was 7% (i.e. for increase in nb from 2 to 3) which was less than earlier increase. It was clear from these test results that ultimate compressive load increases with increases in all these parameters mentioned above but this rate of increase is higher for initial increase than in the later stage. Similar results were observed for pullout tests also. In pullout tests, for initial increase in H/B (i.e. for increase in H/B from 4 to 6), the increase in Qup was 169%, while for further increase in H/B (i.e. for increase in H/B from 8 to 10); the increase in Qup was 81% which is quite less than earlier increase. Similarly, for initial increase in Na; the increase in Qup was 150% (i.e. for increase in Na from 1 to 2), while for further increase in Na; the increase in Qup was 25% (i.e. for increase in Na from 3 to 4) which was quite less than earlier increase. Also, for initial increase in nb, the increase in Qup was 60% (i.e. for increase in nb from 1 to 2), while for further increase in nb; the increase in Qup was 25% (i.e. for increase in nb from 2 to 3) which was quite less than earlier increase. It was clear from the above results that the increase in capacity was prominent with the initial increase of parameters only. There will not be any significant increase in the capacity with further increase in the values of these parameters. A parametric study was also conducted in the laboratory to ascertain the increase in value of angle of internal friction ( ) and improved apparent coefficient of friction (f*) between the anchor and the soil because of insertion of anchors in soil. From this study it was found that the value of angle of internal friction increased from 35o for virgin soil to 48o for soil-anchor system with 4 double helical screw anchors. For compression tests it was found that the value of apparent coefficient of friction increases from 3.73 to 17.58 (increased by almost 371 % with increase in no. of anchors from 1 to 4, embedment depth ratio from 2 to 8 and no. of screw blades in an anchor from 1 to 3). Similarly for pullout tests, the value of apparent coefficient of friction increases from 1.39 to 10.02 (increased by almost 620 % with increase in no. of anchors from 1 to 4, embedment depth ratio from 4 to 10 and no. of screw blades in an anchor from 1 to 3). Theoretical formulation was also done for ultimate pullout capacity of single helical screw anchor. The difference between the theoretical and experimental value came close to 9% for shallow anchors (for embedment depth ratio of 4 & 6) and 4% for deep anchors (for

embedment depth ratio of 8 & 10). By comparison it can be said that the difference between the theoretical and experimental values is minimal. Hence it can be said that the present theory predicts the ultimate pullout capacity of multiple helical screw anchors with sufficient accuracy. PLAXIS 3D Foundation Suite was used to verify the findings from the laboratory tests conducted during this investigation. The results obtained by conducting laboratory experiments were compared with the results obtained by conducting PLAXIS runs on the model similar to the one on which laboratory experiments were conducted. The difference between laboratory test and PLAXIS results was within 10% which can be accepted.