Axial Uplift Behaviour of Belled Piers in Coarse-Grained Saline Soils

School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi 710055, China Institute of Geotechnical Engineering, Xi’an University of Technology, Xi’an, Shaanxi 710048, China State Key Laboratory of Road Engineering Safety and Health in Cold and High-altitude Region, CCCC First Highway Consultants Co., Ltd., Xi’an, Shaanxi 710075, China Northwest Electric Power Design Institute Co., Ltd. of China Power Engineering Consulting Group, Xi’an, Shaanxi 710075, China


Introduction
Massive infrastructures have been designed and constructed in Northern China, such as the West-East electricity transmission project, and have to face the problematic soils, e.g., coarse-grained saline soils, especially those distributed in the Gobi Desert or the desert margin in inland areas [1,2].
e coarse-grained saline soils experienced cyclic salification and desalination due to complex processes that salt in soils periodically exists in crystalline or dissolved state, accompanied by a continuing migration and accumulation of dissolved salt, resulting in particular engineering properties [3,4].Experiments prove that this kind of problematic soils exhibits particular behaviours because of that better bearing capacity can be obtained as soils crystallized with good bonding between particles.However, for these soils, dissolved salt may induce wetting collapse in ground, leading to damages in infrastructures [5,6]. is kind of behaviours differs from both cohesive soils with good bonding between particles and coarse soils with better drainage [7][8][9][10].us, the design of foundations involving the coarse-grained saline soil deserves urgent attentions for its bearing characteristics and ground treatment scheme.
e engineering design for the foundations in coarsegrained saline soils lacks a reasonable consideration of the mechanism and characteristic value for bearing capacity, selection of foundation type, and the construction method.For design tasks when such particular geological conditions are involved, the designers often follow the principles that are applicable for conventional soils and conservative plans for design.More importantly, the bearing capacity for this particular material cannot be fully utilized, especially for cases when soils maintain a crystalline state.So far, the belled pier foundation is frequently used in designing a transmission line. is kind of foundation well utilizes the bearing capacity of soils and also has the merit that no redundant abandoned soils and water will be produced during construction [11,12].
e other aspect involved in uplift behaviour of belled pier is the failure mode in foundations.Ikeda et al. [13] conducted numerical analysis of the uplift problems to assess the progressive and localized failure in foundations based on an elastoplastic finite element method, which was verified by centrifugal experiments.e uplift bearing capacity and the load-displacement relationship have been investigated by the scholars [14][15][16]; however, little work has been done on the uplift capacity of the belled pier constructed in coarsegrained saline soils.
Considering the urgent needs of transmission line construction, four groups of model tests were carried out on the belled pier foundations constructed on two-layered soils, coarse-grained saline soil layer above, and conventional coarse soils (CS) layer.Two occurrence modes and three different thicknesses for saline soils were considered.e uplift behaviours of the belled pier foundations including the apparent failure mode, ground displacement, loaddisplacement curves, and ultimate uplift bearing capacity were measured, aiming to provide guidance for the foundation design in coarse-grained saline soils.

Brief Introduction to the Coarse-Grained Saline Soils.
Most of the saline soils are distributed in areas where the climate is arid or semiarid except for the coastal regions in China, with less precipitation but larger evaporation.Affected by the climate, the occurrence of salt in saline soils shows seasonal variations that soils tend to desalinate in summer due to intensive and larger rainfall, while in both spring and autumn, evaporation exceeds the precipitation and thus the salt accumulated in ground surface cannot be leached, inducing a further salification in soils [17,18].e topographic features for saline soils developed are mainly lowland, inland basin, and marsh land, e.g., the Hexi Corridor, inland basin, and the Gobi Desert in Xinjiang Province, China.e salt was migrated from high to low areas with surface and underground runoff and subsequently accumulated in lower land.In addition, the coupling of leaching and evaporation due to both precipitation and runoff generally led to the layered or nestshaped gathering of salt and both the crystalline salt bed and saline lens with thickness of a few centimetres can be easily observed in saline soil regions, called surface aggregation.
e sampling site, i.e., the Multicolored Bay, located in the north of Jimsar County in Xinjiang Province, is a scenic area with coarse-grained saline soils widely distributed.Figure 1 presents the strata feature for coarsegrained saline soils in this area.Figure 2 illustrates the variation of dissolved salt with depth for three test sites in the Multicolored Bay.
e dissolved salt is primarily distributed within the depth of 3.0 m and dissolved salt significantly decreases with depth.Saline soils show discontinuity and inhomogeneity in horizontal directions and most of the dissolved salt gathered along the principal line of torrent in gullies.

Specimen Preparation.
e coarse-grained saline soil samples were taken from three test sites in the Multicolored Bay of Xinjiang Province.e water content of soils is about 4%, and the dry density is within the range of 2.15-2.26g/cm 3 .e coarse-grained saline soils are constituted by three kinds of soils, i.e., gravel, medium sand, and silty loess, with the mass percentages of 55%, 20%, and 25%, respectively.e grain sizes for gravel and medium sand are 2-20 mm and 0.2-2 mm, respectively, while for silty loess the particles with a size smaller than 2 mm take a dominant proportion.e salinity tests for soils from their three sites, five kinds of common ions measured, are conducted as shown in Figure 3.It clearly shows that the soils' sulphate ion accounts for a major proportion of the total dissolved salt content, followed by the calcium and chloride ions, while the contents for both bicarbonate ion and magnesium ion are lower.For the model tests, coarse-grained saline soils were produced by the above three soils and the mass percentage is identical to that in three sites to keep consistency in the composition of particles.Besides, the easily obtained anhydrous sodium sulphate is used in model tests, with the mass percentage of 8%.
e coarse-grained saline soil samples were prepared by mixing the three soils that were calculated and weighted according to the proportions abovementioned.

Test Procedure.
By referencing the detailed design of the South Hami-Zhengzhou transmission line, the test pit and belled pier foundations were designed with a scaling factor of 10%.e dimensions of the test foundation pit were 2.8 m × 2.8 m × 1.5 m, and the load-bearing brick column was heightened to 2.2 m. e fired common bricks of size (240 mm × 115 mm × 53 mm) were used, with cement mortar filled between bricks.
e standard compressive strength of both bricks and cement mortar are 15.0 and 5.0 MPa, respectively.e specific sizes of the belled piers are presented in Figures 4(a) and 4(b).e ratio of depth to diameter, i.e., (H−n)/D, is 2.5, in which, H is the embedded depth of foundations, n is the height of the expanded column, and D is the diameter of the bottom plate.e thickness for CSS, h, is used as a controlling variable to evaluate the uplift bearing capacity of belled pier foundation.Four groups of experiments were carried out in the laboratory, corresponding to four working conditions.e CSS in the crystalline state were utilized in the first three groups, with the thicknesses of 50, 100, and 150 mm, identified as Test I, Test II, and Test III, respectively.While for the fourth group, 100 mm thick coarse saline soils in the dissolved state were used, identified as Test IV. e sodium sulphate solutions with a mass percentage of 8.0% were sprayed on 2 Advances in Civil Engineering  Advances in Civil Engineering coarse-grained soils and were then wrapped with preservative films for 48 hours to ensure a uniform infiltration of solutions.Beneath the coarse saline soil layers, the common coarse soils were placed, compacted by the electric shocking rammer layer by layer, with each layer less than 20 cm.Soils were filled in the test pit, and each layer was compacted until the target height was reached.In addition, the uplift load was directly applied on the top of the belled pier when a 100 mm thick coarse saline soil layer in the dissolved state was filled in the foundation, i.e., the axial uplift loading test initiated.While for the other three groups before testing, the filled coarse saline soil layers with various thicknesses were firstly air-dried for about 48 hours to ensure the crystallization of the sodium sulphate solutions.Figure 4(c) gives the uplift loading system for the belled pier foundations.According to the estimated ultimate bearing capacity of the tested foundations, the loading system includes two I-section steel beams with a length of 3.6 m, load-bearing brick columns, two lifting jacks, connecting bolts, and steel boards for bearing and connecting.
e tests adopted a slow maintenance loading method and an equivalent load, not exceeding 10% of the ultimate bearing capacity.e load was applied step by step, except that the first loading level was taken to be two times the equivalent load.No impact was allowed during the loading processes and thus as the loading was maintained, the amplitude of variation is controlled to be less than ±10% of the equivalent load.e settlement of the pile top was read after the loading initiated at a time of 5, 15, 30, 45, and 60 min, and later the interval was kept to be 30 min.e standard for deformation stabilization is that the settlement of the pile top is less than 0.1 mm within an hour and appeared twice successively after either level of the load exerted for 30 min.
e tests terminated when the load cannot be maintained or the settlement of the pile top at any level of the load is larger than 25 mm.

Failure Modes in Ground Surface.
e belled pier foundation with a 100 mm-thick coarse-grained saline soil layer was taken to evaluate the surface feature of fracture during uplift, as shown in Figure 5. e high-pixel photographs in Figure 5 present the belled pier-centered surface uplifting phenomenon and circular cracks which were gradually formed.Besides, fine cracks emerged in ground surface at higher loads, and fracture rapidly expanded with penetrating cracks as the ultimate bearing capacity is approached.e overall shear failure plane was generated in this case with circular and radial-pattern fractures on ground surface.Visually, a failure cone was completely uplift off the ground.e rectangular coordinate system was set up with the origin at the center of the belled pier and coordinate axis parallel to the brick columns.Based on the measured displacement and crack development during uplift loading, the range of uplift at each loading stage is plotted in Figure 6.Here, for the sake of simplicity, only two working conditions were considered here, i.e., Test II and Test IV.Note that the ring lines in black with the applied uplift load annotated represent the range of crack in the ground surface that stabilized at each level of loading.As the settlement of the pile top stabilized, the range of uplift for coarse-grained saline soil foundations manifested as a circumferential outward expansion with the uplift load increasing step by step. is gives the basic quantitative data for the observed circular fracture of ground surface developed with uplift loading.
Figure 7 presents the relationship between the ground uplift displacement and the distance from the foundation center at each uplift load.It shows that, at larger uplift loads, the displacement for foundations tends to grow and the range where uplift load affects enlarges, which can be regarded as a detailed presentation of how the uplift of ground develops with the applied load.Moreover, as the distance from the center increases, the uplift displacement for the monitored points is relatively smaller than those that are closer to the center, and all the four groups of tests exhibit similar variations.At the initial loading steps, the ground displacement for the points close to the center of the bell pier grows rapidly and the gradient for the displacement is relatively smooth and then it stabilizes at larger loads, while for the adjacent points, the displacement is relatively smaller.e curves all show a softening tendency with a peak value at a displacement of about 20 mm, which can be taken as the ultimate bearing capacity of the foundations.At a given displacement, the thicker the coarse-grained saline soils in the crystalline state, the larger the uplift load during testing, and the ultimate bearing capacity was also included.e foundation with a 100 mm-thick dissolved coarse-grained saline soil layer exhibits lowest ultimate bearing capacity.A further analysis of the measured data indicates that, in the single logarithmic coordinates, three stages can be found from the uplift load and the ratio of load to displacement, as shown in Figure 8(b).e ultimate bearing capacity, i.e., the peak points of the curves, is plotted with the ultimate displacement, as shown in Figure 9. Clearly, the bearing capacity of the foundation significantly increased as the thickness of the crystalline soil layer increased and is larger than that in the dissolved state.Besides, the maximum displacement shows nonlinear changes, with the maximum value appearing at a thickness of 100 mm in the crystalline state, and it is two times larger for the dissolved state.

Load-Displacement Relationship.
is indicates that both ultimate bearing capacity and ultimate displacement are sensitive to the dissolution of coarse-grained saline soils and should be paid more attention: (1) e first stage represents a linearly elastic relationship of ground displacement versus uplift loads. is results from the fact that, at relatively lower uplift loads, soils above the bottom board of the belled pier experienced extrusion forces and were compacted.During this stage, soils exhibit mainly the elastic compressive deformation.

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(2) At higher uplift loads, the displacement of belled pier foundations tends to vary nonlinearly, and the ratio of the uplift load to the ground displacement is signi cantly lower than that in the rst elastic stage, and the deformation of soils includes primarily the plastic deformation, rather than the elastic deformation; in this case, the plastic zone grows in soils at the stresses applied.(3) As the ultimate bearing capacity was approached, fractures on ground surface rapidly expanded and penetrating cracks were formed in this stage, with an overall shear failure plane emerged.Soils in the  6 Advances in Civil Engineering foundations slide along the shear failure plane and the uplift bearing capacity measured decrease considerably, identi ed as a load reduction that was measured by the sensors.

Failure Planes.
Four typical pro les were obtained by measuring the failure cones with an interval of 45 °. Figure 10 illustrates the plan view of the failure cones at the four working conditions.It shows clearly that the planar projection of the failure cones is close to circular, and the di erences may result from the variance of both loading conditions and lling materials in foundations.Moreover, the planar projection for the dissolved state is larger than those in the crystalline state, proving that the range of failure that the uplift loading has caused is relatively larger, implying the fact that the ultimate bearing capacity is lower.e failure curves at each pro le were measured and are plotted in Figure 11, with both ground surface and boundary line separating the two layers sketched.At four working conditions considered here, no matter what the coarse-grained soils are crystallized or dissolved, the failure modes for the belled pier foundations all show linear change from the base to the top.
Based on the morphology of the failure planes of the belled pier foundations, we can approximately use two linear relations to describe the failure curves of the two layers at each pro le, respectively.e following equation was tted by the least square method:

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where x and z are the horizontal and longitudinal coordinates, respectively; m is the tted parameter; and θ is the inclined angle of failure plane during uplifting.Taking the measured data of the third group as an example, the tting data for failure planes are presented in Figure 12.Note that the CSS and CS represent the coarse-grained saline soils and conventional coarse soils, respectively; L and R are the directions considered in both sides of the belled pier.Both values of m and θ for each pro le approach to the constants, indicating that the pro le for the failure cone is approximately identical, which is supposed to happen when lling materials and loads are uniformly distributed in the belled pier foundations.Here, the measured and tted data for the failure curves for the four working conditions were compared and presented in Figure 13.Most of points lie on or close to the line of y x, which shows that the tted curves are reasonable for describing the measured failure planes.Moreover, the tted data for the dissolved coarse-grained saline soils exhibit larger deviation from the measured.
e statistical analysis was carried out on the angle of uplift, θ, for three kinds of strata listed in Figure 14, i.e., common coarse-grained soils and crystallized and dissolved coarse-grained saline soils.e results show that the angles of uplift at the four working conditions all show greater variance as the strata varied.If we take the mean value of the angles of uplift in the preliminary design, the 8 Advances in Civil Engineering angles of 32 °, 34 °, and 18 °, corresponding to the above three strata, and can be used for engineers' reference for the sake of safety.us, the simpli ed failure modes at the four working conditions can also be adopted in engineering design.Note that the precipitation or evaporation of the ground should be carefully considered in engineering design.For example, the angle of uplift for coarse-grained saline soils shows little di erence from that for common coarse-grained soils in the dry season, while the salinity in soils can be dissolved as the precipitation occurs, and in this case, the angle of uplift decreases signi cantly.Note that the ultimate uplift bearing capacity mainly includes three components such as the bearing capacity of the ground shear failure plane and the self-weight of both foundation and uplift fracture cone.e rst two components will be signi cantly reduced at smaller uplift angles, further leading to the decrease in the ultimate uplift bearing capacity.Advances in Civil Engineering pier foundations in most cases originates from the failure of foundation.us, the yield and even failure are not assumed to occur for belled piers.e elastic constitutive equation for RC can be given as

Modelling of Uplift Behaviour Incorporating a Practical Interface Model
where σ ij and ε ij are the stress and strain tensors; E and μ are the elastic modulus and Poisson's ratio; ε m is the mean strain; and δ ij is the unit matrix.e belled pier foundations are in general embedded in shallow layers, and the stress-strain relationship for soils in that case can be simply described by an ideally elastoplastic constitutive equation.As the soils in foundations deform within the elastic limit, the above Equation ( 2) is obeyed, while the components of stress and strain tensors are calculated by the following equation of plasticity when the ideally plastic state is reached: where e ij and s ij are the deviatoric strain and stress tensors, respectively; σ m is the mean stress tensor; and K is the volumetric elastic modulus.e parameter dλ can be calculated by where σ s is the yield limit. e Mohr-Coulomb yield criterion was adopted here to simulate the possible compressive shearing and tensile damage of pile soils during uplifting, as presented in Figure 15.e interface between belled piers and soils around was modelled by interface with zero thickness and can be

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generalized in Figure 16, with the normal and tangential stresses determined by where the subscripts n and s represent the normal and tangential directions; F and u are the force and displacement of the interface; k is the sti ness; and A is the area.e maximum shear stress in the interface was calculated by the Mohr-Coulomb yield limit: where c and ϕ are the cohesion and internal friction angle of the interface, respectively.e following should be satis ed if F s > F smax : where F smax is the maximum tangential stress of the interface; σ n is the normal stress; and ψ is the dilatancy angle.Provided that the tensile failure occurs at both sides of the interface, then F n and F s are assumed to be zero.

Geometric Model and
Parameters. e geometric model based on the borehole data is presented in Figure 17, including three main components such as the reinforced concrete belled pier, coarse-grained saline soils, and conventional coarse soils.
e thickness for the second was controlled to be 50, 100, and 150 mm, while 100 mm for the dissolved state.
e normal displacement was restrained in four sides and the bottom, while the upper is assumed displacement free, with a normal uplift load applied on the top of the belled pier, considered as the z direction.
e parameters in computation are listed in Table 1.

Comparison of Measured and Calculated Results.
Figure 18 illustrates the simulated load-displacement curves at four working conditions.e curves all exhibit similar changes that the ratio of load to displacement declines and is hardening.
e simulated and measured data were plotted in Figure 19, and comparisons prove that the simulated results agree well with the measured data.Most of the data points corresponding to the lower uplift loads are all close to the y x straight line, while for those at higher loads, larger deviations between the two sets of data can be observed.e heterogeneity of lled materials may account for this discreteness that cannot be well modelled by an ideally elastoplastic constitutive equation.e uplift load characterized by an abrupt change in the loaddisplacement curves was taken as the ultimate bearing capacity.e simulated data are plotted in Figure 20 together with the measured.It shows that the two sets of data coincide well, and the ultimate bearing capacity for the foundation in the crystalline state is higher than that in the dissolved state.However, similar changes can be noted as the depth of coarse-grained saline soils increases, proving that the simulation gives a reliable result for the working conditions considered here.e failure planes of the belled pier foundation during uplifting can be determined by the growth of the plastic zone until a critical limit surface is reached, identi ed by a visually observed penetrating range.Figure 21 gives the failure modes of soil mass at the four working conditions.Axisymmetric failure envelops centered on the belled pier were found from the bottom upward.e angle of uplift for foundations with CS is higher than that with dissolved CSS, while it is lower than that in crystalline state, which shows good agreement with the measured data.e failure planes calculated and measured are both plotted in Figure 22.
ese two pro les exhibit similar pattern, with a small discrepancy between the two, and this may be   Advances in Civil Engineering caused by the simpli cation of the stratum feature in computation.
Figure 23 illustrates the development of plastic range for the belled pier foundation with a coarse-grained saline soil layer with a thickness of 100 mm.It clearly shows that the plastic zone rstly emerged from the bottom of the belled pier, implying that the uplift load has been transferred from the top to the bottom and eventually exerted on surrounding soils.e plastic zone enlarged from the bottom upward with the increase in the loading level, and nally the penetrating plastic zone was produced.Also, at the nal stage of uplift loading, the range of the plastic zone at speci ed depths mainly develops horizontally, indicating that the load transferred by the friction between pile and soil is limited.
Visually, the pattern of the plastic zone is quite close to the failure plane obtained based on the displacement isolines.

Conclusions
e result of this result of model tests on belled pier foundations with coarse-grained saline soils of di erent thicknesses and two occurrence modes has been provided.Modelling of uplift behaviour of belled pier foundations was also carried out based on a practical interface.e main conclusions are as follows: (1) Considerable ground uplift phenomena were observed during model tests and the belled Advances in Civil Engineering 13 pier-centered circular cracks emerged, manifesting as that, at larger uplift loads, the cracks tend to enlarge, rapidly extend, and penetrate the ground surface, where the circular and radial-pattern fractures were noted.An overall shear failure plane appeared eventually.
(2) e relationship between uplift load and displacement shows softening characteristics, and when the ultimate bearing capacity is reached, a stage of load reduction was observed.As the thickness of coarse saline soil layer increased, the bearing capacity increased signi cantly while lowered for foundations in the dissolved state.
(3) e failure planes for foundations in both crystalline and dissolved states can be simpli ed as a linear form.Based on the measured data of each pro le at the four working conditions, the angles of uplift for the three kinds of stratums are 32 °, 34 °, and 18 °, respectively, and can be used for design.(4) Comparison of calculated and measured failure planes proves a good coincidence, and axisymmetric failure envelops centered on the belled pier were noted.e angle of uplift for foundations with CS is higher than that with dissolved CSS, while it is lower than that in the crystalline state.Besides, the plastic zone enlarged from the bottom upward with the

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increase in the loading level, and finally, the penetrating plastic zone was produced.

4
Figure 8(a)  shows the relationship between uplift load and ground displacement at the four working conditions.

Figure 6 :
Figure 6: Range of ground uplift at various loads.(a) Test II and (b) Test IV.

Figure 7 :
Figure 7: Ground displacement at uplift loading.(a) Test II and (b) Test IV.

Figure 9 :Figure 10 :
Figure 9: Comparison of both ultimate uplift bearing capacity and displacement.

Figure 13 :
Figure 13: Comparison of predicted and measured curves.(a) Predicted and measured data.(b) Deviation.

Figure 20 :Figure 19 :
Figure 20: Comparison of measured and simulated uplift bearing capacity.

Table 1 :
Model parameters in computation.