Uplift Behavior Of Shallow Belled Pies In Gobi Gravel

Gobi is a common geological formation in Northwest China. In recent years, theconstruction of electricity transmission systems that span from West to East China has beenplanned in Xinjiang and Gansu Province. Thus, the construction of foundations in Gobi gravelfor transmission towers is inevitable. Transmission line towers experience a mode offoundation loading that is unique among engineering structures, in that uplift loading typicallycontrols the design. Recently, belled piers have been widely applied in Gobi areas. This type offoundation makes use of the natural bearing capacity of the soil and exerts considerableresistance against tensile loading. Nowadays, the study of the uplift bearing capacity has beenmainly focused on the piles and the spread foundations in backfilled soils. There was littleinvestigation on the uplift behaviors of shallow belled piers in Gobi gravel. Based on themeasured data of46field uplift-loaded belled piers at7sites in Gansu an Xinjiang, fourrepresentative interpretation criteria were used to evaluate the uplift load-displacementbehavior. The method to determing the ultimate uplift bearing capacity in tests andcorresponding theoretical method to precdict the ultimate uplift bearing capacity in design weresuggested. Using the confidence limit method, the hyperbolic curves for predicting theproposed design load-displacement curve were analyzed under serviceability limit state (SLS).The main conclusions and characteristics of this study were summarized as follows:(1) The Gobi gravel can be classified as well-graded gravel (GW) soil. The precipitationof the soluble salts may cause salt cementation of the particulate medium in Gobi gravel, andthis kind of salt cementation may change the properties of granular media in natural aridclimate environment. Under drought conditions, Gobi gravel has a high bearing capacity andlow compressibility, which makes it different from other soil-rock mixtures (S-RM).(2) The sequence of the factors to determine the ultimate uplift bearing capacity were theratio of embedment depth to belled diamerter, the extension angle of the bell, and the shaftdiameter. Under tensile load, the uplift load-displacement curves could be schematicallysimplified to a generic load-displacement curve with three distinct regions: an initial linearregion, a nonlinear transition region, and a final linear region. The bearing capacity must beinterpreted.(3) The slope tangent method represented the lower bound, the Chin method representedthe upper bound, and TL2was in the middle. The tangent intersection methods were between L1and L2. The L1-L2method could be used to interpret the load-displacement curve for the belled piers in Gobi gravel.(4) It was assumed that, under the ultimate equilibrium state, the predicted ultimate upliftcapacity of a belled pier was determined by the sum of the dead weight of pier foundation, thedead weight of earth lying above the bottom of truncated cone within the soil sliding surface,and the vertical projection of the shearing resistance acting on the soil sliding surface. Based onthis assumption, a calculation method to predict the ultimate uplift capacity for the belled pierfoundations in Gobi gravel was suggested. Furthermore, the corresponding parameters of thiscalculation method were also provided.(5) The original load-displacement curve for each load test of belled piers in Gobi gravelcould be normalized using the hyperbolic fitting method. Normalizing the load-displacementcurves reduced the scattering in the curves. Explicit hyperbolic curve-fitting constants could beobtained for the axial uplift of the belled piers in Gobi gravel at different confidence levels.Normalized load-displacement curves were obtained, which correspond to the mean and the95%confidence level. In general, the mean curve more closely represents the actual data pointsbecause it is intended to model the actual test data. However, the field tests showed asubstantially stiffer load-displacement response than the data obtained at the95%confidencelevel.(6) The predicting method about the permissible uplift load and allowable upliftdisplacement was obtained by considering the uncertainties of uplift bearing capacity anddisplacement together. This can be used as a convenient design tool to simultaneously considerboth the capacity and displacement of belled piers in Gobi gravel.