Analysis of Rotary Vibration of Rigid Friction Pipe Pile in Unsaturated Soil

Based on the mixture theory and previous work, the governing equation of the rotary vibration of rigid friction pipe pile in unsaturated soil is established. /e analytical solution of this equation can be used to analyze the displacements and the complex stiffness of rotary vibration. /e results show that the contribution to stiffness is as follows: solid< liquid< gas; and the contribution to rotational impedance is as follows: solid> liquid> gas. In addition, when the fluid permeability coefficient decreases, the stiffness decreases and the rotational impedance increases, but the influence is not obvious (especially the gas permeability coefficient). Four different kinds of degradation problems are also presented. Relevant conclusions can provide reference for engineering application.


Introduction
e seismic response analysis of pile foundation is an important problem in foundation engineering, and it is widely used in many engineering fields, such as bridge engineering or hospital construction. ere is much research about the analysis of situation whether pile foundation is under horizontal or vertical dynamic loading. But, superstructure will also rotate under the action of earthquake or some sort of periodic dynamic force (for example, some mechanical or medical equipment with periodic natural vibration), and that will have a great influence on pile foundation which is under abutments, slender transmission towers, or special mechanical equipment. For end bearing pile, since the bottom of the pile is fixed, the rotation of the superstructure will cause torsional vibrations of the pile; for rigid friction piles such as concrete pipe pile, since the modulus of soil is far less than the modulus of the pile (the difference is about 3 orders of magnitude), the rotation of the superstructure will cause rotary vibrations of the whole pile.
For static torsion problem of a single pile, the solution has been found in principle since SaintVenant problem in elastic mechanics had been solved [1]. ere is also much valuable research even when considering the effect of soil surrounding pile. By comparison, the vibration problem of pile foundation under dynamic torsion load is hard to get analytical solution like static problem because of its complexity, but there is also some valuable research work. Novak and Sachs [2] applied the plane strain soil model to study the vibration characteristics of embedded rigid foundation under dynamic torsional loading and derived an approximate analytical solution for torsional vibrations of footings partially embedded in a semi-infinite medium or a stratum. Wang et al. [3] developed an analytical solution to investigate the torsional vibration of an end bearing pile embedded in a homogeneous pyroclastic medium and subjected to a time-harmonic torsional loading. Zhang [4] derived dynamic governing equations of transversely isotropic saturated soil in cylindrical coordinates on the basis of the dynamic equations of saturated soil and the stress-strain relationships of transversely isotropic medium. In experimental study, Fattah et al. [5][6][7] analyzed the dynamic response of pile foundation in dry and saturated sandy soil excited by two opposite rotary machines and found that in dry soil the pile tip load decreased for all (L/d) ratios and operating frequencies.
Whereas in saturated soil, the pile tip load increased for small (L/d) ratios and low operating frequencies. Fattah et al. [8] also studied load sharing and behavior of single pile embedded in unsaturated swelling soil, the results showed that the ultimate skin resistance increased to about 49% when the initial degree of saturation decreased from 90% to 70%. In addition, Chikr et al. [9][10][11] used shear deformation theory to make lots of analysis of the plates on the elastic foundation, and some other scholars [12,13] used Hamiltonian principle or virtual displacement principle to reduce the unknown quantities needed in the calculation of plates.
It can be seen from previous research work that many scholars have done a lot of work deeply in this field. But, on the one hand, these researches mainly simplified the soil around the pile into single-phase soil or saturated soil, and there were few researches on unsaturated soil. On the other hand, the previous researches mainly focus on the dynamic torsion of end bearing piles, and the dynamic rotation of friction piles is seldom studied. e rotary vibration of rigid friction pipe pile in an unsaturated medium is studied in this article, and this problem is proposed by the author of this article first. is dynamic problem involves a pile-soil coupling system. Since solid pile is the special case when the inner radius of pipe pile tends to be 0, the rigid friction pipe pile analyzed in this article can be regarded as the extension of traditional solid pile. Considering the rapid development of prestressed concrete pipe piles in recent years (prestressed concrete pipe pile industry scale in China reached 73.058 billion Yuan in 2018, with a total length of about 336 million meters) and seismic research is an important subject in pile foundation engineering, we hope that this article can provide reference for the engineering application of prestressed concrete pipe piles. e concept of rotary vibration of rigid friction pile is given first. In engineering, single pile foundations can be adopted in some high-voltage transmission towers, wind turbines (including onshore and offshore wind turbines), bridge piers, or some mechanical equipment with periodic dynamic response. When the superstructure is subjected to wind load, wave load or the equipment itself generates periodic dynamic response during operation and the dynamic couple with rotational effect may be generated on the pile top, so as to make the pile itself produce rotational dynamic response. For the end bearing pile, the pile bottom is firmly supported and the torsion may happen to the pile. But, for the rigid friction pipe pile, its bearing capacity is mainly derived from the friction on the side of the pile. As the constraint at the bottom of the pile is weaker than that at the end bearing pile and the stiffness of the pile itself is larger, it is assumed that the phase of rotation of the upper and lower ends of the rigid friction pipe pile can be approximately regarded as the same when torsion occurs (see Figure 1, where u θ is the amplitude of rotational vibration, e is the base of the natural logarithm, i 2 � − 1, ω is the circular frequency, t is the time, H p is the pile length, and R I /R O is the internal/external radius of the pipe pile, respectively). e plane strain condition is approximately satisfied beyond a certain distance from both the ends, it can be regarded as a rotating vibration around the symmetry axis of the pile, and the problem can be regarded as a plane problem.
In addition, for the friction pile group pile foundation, when the ground vibration causes the upper structure rotation, the instantaneous center of rotation of the pile will have similar dynamic response. Because of the irregularity of the rotation, there may be several rotational centers successively in the whole vibration process; this means the rotation may happen to more than one pile. When the superstructure is irregular, the effect will be intensified. erefore, for rigid friction piles, this problem may occur on either single pile foundation or group pile foundation. On this basis, it is assumed that the physical quantities (such as velocity and rotation angle) are all small in the vibration process, and the pile-soil interface does not separate. is is the rotary vibration model of rigid friction pile. Since there are many symbols in this article, we have included a symbol table in the Table 1 for the convenience of readers.

Basic Governing Equations for Dynamic
Problems of Unsaturated Medium is section will refer to the method of Vardoulakis and Beskos [14] to establish the basic governing equations of unsaturated soil elastodynamics and then establish and solve the governing equations of rotary vibration of rigid friction pipe pile. Solids, liquids, and gases are represented, respectively, by the subscript s, l, and g. e relative densities of solid, liquid, and gas are defined as follows: where ρ m (m � s, l, and g) represents the mass density of the corresponding component, n is the porosity, and S is the saturation. Besides, the relationship between mass discharge q m i and volume discharge Q m i is and the relationship between volume discharge Q m i and the velocity of the components in the medium v m i is According to [14], introduce Q l i and Q g i as relative specific discharge: For the convenience, the superscript s can be omitted for the displacement component of the solid. Based on the above definitions and mixture theory [15], the total stress can be defined as follows: where s, l, and g represent the solid, liquid, and gas, respectively, and the solid stress which is the first term on right side, namely, "intergranular stress", is transmitted by the contact of the particles in the earth framework. It can be also represented as τ ij : Assuming that the liquid and gas are ideal fluids, the stress has the following forms: In the above equation, δ ij is the Kronecker symbol, p l and p g are the pressure of liquid and gas, respectively, and there is a minus sign in front since the fluid can only be compressed. By rewriting (10) in the incremental form, we can get According to the Verruijt's and Luger's view [16] which is based on the effective stress, the relationship between particles stress Δτ ij and effective stress Δτ ij ′ is as follows: where Δτ ij ′ depends only on the deformation of solid components. According to the linear elasticity theory, there is where λ and μ are Lamé constants of dry soil and ε ij is the strain tensor of solid. c can be defined as follows: where β s is the compressibility of the solid phase component material and β is the volume compressibility of soil. Besides, unit liquid and gas penetration forces b l and b g are defined as follows: where ] l and v g ,respectively, correspond to the dynamic viscosities of liquids and gases, also called absolute viscosities, k is the intrinsic permeability of solids. Based on equation (14) and the nearly saturation hypothesis, reference [14] established the dynamic governing equation of unsaturated soil: where u i is the displacement of the solid component.

Governing Equation.
Firstly, rewrite the displacement of liquid and gas relative to solid as w i and v i [17]. Suppose that the pile top sustains a simple harmonic circumferential couple M p � Me iωt (M is the peak of the couple). Considering the symmetry of the model and assuming that all the movements are simple harmonic, the circumferential displacement of the solid phase component has the form u θ (r, t) � u θ e iωt and the circumferential displacement of the liquid and gas phase components relative to the solid phase component is w θ (r, t) � w θ e iωt and v θ (r, t) � v θ e iωt (u θ , w θ , and v θ are the amplitude of rotational vibration), and the rest of the components of the displacement are zero. Equations (20)∼(22) can be simplified as follows: where It is the Laplace operator of a central symmetric plane problem; ρ is the density of unsaturated soil: Substitute the latter two equations of (23) into the first equation and rewrite μ as the shear modulus G: where Equation (26) is the basic governing equation of the rotary vibration of rigid friction pile in unsaturated soil to be studied in this article.

Solution of Rotary Vibration Displacement of Rigid
Friction Pipe Pile. Rearrange equation (26) as follows: where Equation (28) is the first-order Bessel equation of imaginary argument, the solution of which is where I 1 and K 1 are the first order of the first and second Bessel functions of imaginary argument, respectively, and C a and C b are arbitrary constants. I 1 and K 1 have the following properties (according to chapter 7 of reference [18]): where z is a complex number whose real part is nonzero. In the rotary problem, the pile modulus (dozens GPa) is much larger than that of soil around the pile (dozens MPa), so the pile can be considered as rigid body. For the pipe pile, assume the inside is also filled with soil and the maximum rotation angle of the pile body is θ p , then the boundary conditions can be taken as follows: where the subscript I represents inside and O represents outside. Naturally, the following boundary conditions shall be provided at the origin and at infinity, respectively: For in-pile soil, arbitrary constants in (30) can be determined by using equations (31), (33), and (35): Similarly, for out-pile soil, arbitrary constants in (30) can be determined by using equations (32), (34), and (36): erefore, for soil inside and outside the pile, the displacement without factor e iωt is as follows, respectively:

Calculation of Complex Stiffness of Rotary Vibration of Rigid Friction Pipe Pile.
In order to calculate the dynamic stiffness of pile, the stress is first calculated. e shear stress is as follows: For soil inside and outside the pile, there are In order to calculate the above two equations, the properties of the first and the second-order Bessel functions of imaginary argument need to be considered. According to chapter 7 of reference [18], where J 1 is the first-order Bessel function of real arguments and H (1) 1 is the first order of the first Hankel function. To simplify the above two equations, when q is calculated according to equations (29) and (27), the real part of q can be nonnegative. In this way, the amplitude angle of the independent variable can be taken between − π and π/2, so that equation (44) can be taken with only the upper half branch. In addition, according to Euler's formula in complex analysis, If θ � π, the equations (44) and (45) can be reduced to Mathematical Problems in Engineering According to chapter 7 of reference [18], the following equations can be established: After substituting equations (47) and (48) into (49) and (50), respectively, we can get Substitute the above two equations into equations (42) and (43), respectively, and we can get Considering that the pile body rotates under the coaction of external load on the pile top and soil around the pile, the dynamic balance equation is as follows: where M p is the simple harmonic circumferential couple which the pile top sustains: where M is the peak of the couple of forces and M s is the circumferential couple of the pile body under the coaction of soil inside and outside the pile: where H is the effective pile length participating in the rotary vibration. By substituting equations (53) and (54) into (57), it can be obtained that In addition, I p is the moment of inertia of pile: where ρ p is the density of the pile.
After substituting equations (56), (58), and (59) into (55), it can be simplified as follows: where k M is the rotary stiffness of the pile body and a complex function. It can be written as follows: where k M1 is the real part, representing the true stiffness of the pile body rotation, and k M2 is the imaginary part, which reflects the dissipation of energy, and it is referred to as the rotational impedance in this article. Finally, it is pointed out that the proposed method can be extended in principle to the saturated porous medium containing any number of fluid components. Since the analysis process is similar, it will not be repeated here.

Degradation of Saturated Medium.
In contrast to unsaturated soil, saturated soil is the special case of unsaturated soils at saturation S � 1. In this case, the pores in the soil particles are all filled with incompressible fluid (liquid). e medium degenerates from a three-phase body to a twophase body. All the effects of the gas phase components on the governing equation can be treated as 0. Equation (29) is changed to the following form (subscript sat indicates saturation): erefore, when S � 1, displacements, shear stresses, and complex stiffness deform as follows: In addition, since liquid phase and gas phase have exactly the same status in equation (27), this degradation mode is also applicable to the situation that the pores in soil particles are completely filled by gas and do not contain water.

Degradation of Single-Phase Elastic Medium.
On the surface, this problem seems very easy. If we start from the equation of the saturated medium in the previous section and just let the porosity n � 0, rewrite μ as the shear modulus G, and use the subscript e to be elasticity, equations (29) and (30) change to the following form: u θe � C ae I 1 q e r + C be K 1 q e r .
However, the following process of determining the unknown coefficients C ea and C eb encountered difficulties. In the equations (31) and (32) referenced before, z needs to be a complex number whose real part is nonzero. In contrast, the right end of equation (64) is a negative real number without an imaginary part. is means that the independent variable q e r in equation (65) is a pure imaginary number. e physical explanation is that the absence of fluid results in a pure elastic reaction of solid components in the process of motion without any phase delay or energy dissipation [19]. In this case, equation (32) is valid. For the soil inside the pile, the displacement formula is as follows: But, equation (31) is not valid.
Since the condition that fluid mass is nonzero may lead to the situation that the degradation is unable to be established, we can make the fluid mass tend to a very small positive value. ere are two methods: to make the porosity tend to be a very small positive value in the saturated medium; to make the fluid density tend to be a very small positive value. e former is equivalent to assuming that the pores between soil particles in saturated soils are very small, and the latter is equivalent to assuming that the pores of soil particles contain only air and that the density of air is very small. Consider the former approach first, since zero porosity will cause q e to change from a complex number whose real part is not zero to a pure imaginary number, the porosity of the saturated medium can then be taken to be a very small, but not zero, positive number (for example, 10 − 10000 ). So, the function I 1 can be eliminated deterministically. But, at the same time, because the value of porosity is extremely small, it means that the value of the second term in the bracket of the right end of equation (62) will be much smaller than the value of the first term, and its effect on the calculation results of K 1 can be ignored. e displacement of soil outside the pile is calculated in Equation (67).
According to equations (65) and (32) and, at the same time, according to the previous discussion let z � q e r, repeating the previous derivation, we can get (69) Finally, the formula for calculating the complex stiffness of the rotary vibration of the pipe pile in the single-phase elastic medium under the simultaneous action of pile top force couple and the shear stress of soil outside and inside the pile is as follows: e results obtained by making the fluid density tend to a very small positive value are identical in form to the above equations.
It must be noted here that there are limits to either reducing porosity or fluid density. e porosity of soils is generally of the order of 0.1, while the density of air at 273.15 K is 1.29 kg/m 3 at one standard atmospheric pressure. But, consider that the density of soil particles is usually 2600∼2700 kg/m 3 and three orders of magnitude larger than air. So, maybe the latter approach is more realistic.

Degradation of Static Problems.
e static problem corresponds to that the rotation frequency ω is equal to 0. According to equation (27), the q used in the dimensionless process becomes 0 directly. us, in the final analytical expressions of physical quantities of displacements and shear stresses, the limit when q⟶0 is directly taken to obtain the displacement and shear stresses, respectively, as follows: e subscript st stands for static. In equation (55), let ω � 0, we get

Mathematical Problems in Engineering
Substitute equations (73) and (74) into equation (57) and substitute ω � 0 into equation (56). By substituting the obtained results into equation (55), the following equation can be obtained: where k M is the rotary stiffness of pile body and is a real number. e fact that there is no imaginary part also indicates that there is no impedance effect in the dynamics of this static problem.

Degradation of Solid
Pile. If the internal radius R I � 0, all equations can be degraded to conclusions for rigid solid friction piles. At this point, the "soil inside the pile" mentioned above does not exist. Since there is no principled difficulty in this degradation, the specific equations are omitted.

Calculation Examples and Analysis
In order to analyze the problem in this article specifically, the following example is constructed. e specific parameters are as follows: ρ p � 2500 kg/m 3 , ρ s � 2600 kg/m 3 , ρ l � 1000 kg/m 3 , It can be seen from the two figures that, within a given frequency range, the stiffness decreases and the rotational impedance increases while the frequency increases. e decrease of stiffness is not very drastic. When the frequency increases from 0 to 100 rad/s, the stiffness decreases from about 5.65 × 10 8 N·m to about 5.35 × 10 8 N·m, only reducing by about 5%. However, in contrast, the change of rotational impedance is more drastic. When the frequency increases from 0 to 100 rad/s, the rotational impedance increases rapidly from 0 to about 2.7 × 10 7 N·m. Moreover, it can be seen from Figure 3 that the rotational impedance grows faster. However, in this example, even the magnitude of the maximum rotational impedance (about 2.7 × 10 7 N·m) is an order smaller than the stiffness value. In addition, the variation law of the stiffness and rotational impedance in this rotary vibration is basically the same as that of the torsional vibration stiffness and impedance in reference [20].
In addition, the influence of other factors on stiffness and rotational impedance can be further analyzed. Figures 4 and  5 show the calculated results of the stiffness and rotational impedance of porosity n � 0.26, 0.28, and 0.30 adjusted, respectively, under the condition of fixed saturation S � 0.5 and other parameters unchanged.
It must be pointed out that porosity has a great influence on soil modulus. In this example, the soil modulus is assumed to remain the same during the slight adjustment of porosity in order to avoid the interaction between variables (the analysis object of this example can also be considered as several different types of soil with the same modulus but different porosities). Because the change in porosity in this example is very small, the difference between the three curves is not significant. However, as can be seen from the numerical results, as for the stiffness, its value will increase with the increase of porosity. However, the rotational impedance decreases with the increase of porosity, and the range of rotational impedance changes is larger than that of stiffness. From the qualitative results, it can be seen that the more solid the soil has, the more obvious its elasticity will be. At this time, the external force needed to maintain the periodic elastic vibration of the system is smaller, that is, the stiffness is smaller.
Please notice that the definition of rotary complex stiffness in this article (k M � M/θ p ) is special. If we apply a similar definition to a periodic natural vibration system without attenuation, the stiffness of the system is zero, and that is because the external force is zero, but the amplitude is nonzero (natural vibration).
Fix porosity n � 0.3, other parameters as before, and adjusted saturation of 0.5, 0.7, and 0.9, respectively, to obtain the law of stiffness and rotational impedance changing with saturation, as shown in Figures 6 and 7.  However, it can be seen from the numerical results that the stiffness decreases slightly with the increase of saturation. is is mainly due to the increase of liquid in the pores, which increases the inertia of the rotary vibration and makes the external force slightly reduced. According to Figure 7, the rotational impedance increases with increasing saturation. is shows that liquid dissipates more energy than gas. But, the increase is very small. Based on the above analysis, the contribution of the three components to stiffness is as follows: solid < liquid < gas; contribution to rotational impedance: solid > liquid > gas (again, please note that the more the solid, the greater the elasticity, the smaller the external force required for periodic vibration, and the smaller the stiffness).
By adjusting the value of unit liquid penetration force b l and other parameters as the first example in this section, the corresponding variation of stiffness and rotational impedance can be obtained, as shown in Figures 8 and 9.
As can be seen from Figures 8 and 9, when b l increases (i.e., permeability coefficient decreases), the stiffness decreases slightly but has almost no effect. Moreover, when its order of magnitude reaches 10 7 , a limit state almost appears, that is, the     further increase of b l makes stiffness almost unchanged. e rotational impedance is basically the same in the figure and only slightly increases in the result value. Physically, reducing the liquid permeability coefficient is equivalent to making the solid easier to maintain natural vibration, similar to the previous analysis of adjusting porosity. erefore, it is equivalent to a reduction in stiffness, which corresponds to an increase in the rotational impedance.
By adjusting the value of unit gas penetration force b g , other parameters as the first example in this section, the corresponding variation of stiffness and rotational impedance can be obtained, as shown in Figures 10 and 11.
As can be seen from Figures 10 and 11, gas permeability has almost no effect on the calculation results. Although it can be seen from the numerical results that the influence trend of gas permeability on stiffness and rotational impedance is similar to the previous results of liquid permeability, from the perspective of engineering, the density of gas itself is too small to affect the dynamic characteristics of the system significantly.
erefore, the effect of gas permeability can be ignored in the analysis of practical engineering.

Conclusion
In this article, the mixture theory under the framework of continuum mechanics is used as the basic tool, and the field equations are established by using, Vardoulakis and Beskos' methods. is part of work is based on the previous work of the other scholars. e specific work and conclusions of this article are as follows: (1) e concept of rotary vibration of rigid friction pile is established. Referring to the analysis method of torsional vibration, the basic governing equations of rotary vibration of rigid friction pile in unsaturated soil are put forward and the model is extended from solid pile to pipe pile. (2) e solution of the rotary vibration problem of the rigid friction pipe pile in unsaturated soil is obtained. For the soil inside the pile and the soil outside the pile, the analytical expressions of such physical quantities as circumferential displacements, shear stresses, and complex stiffness of rotary vibration are given, respectively. (3) Based on the extensiveness of the model in this article, four degradations are given: degradation of saturated soil, degradation of single-phase elastic medium, degradation of static problem, and degradation of solid pile. e relevant formulas have been given. (4) Based on the analysis of the example, the variation laws of stiffness and rotational impedance with frequency, porosity, and saturation are obtained, respectively. e different influences of solid, liquid, and gas components are obtained. Contribution to stiffness is as follows: solid < liquid < gas; contribution to rotational impedance is as follows: solid > liquid > gas. Also, the influence of liquid permeability and gas permeability is analyzed; when the fluid permeability coefficient decreases (fluid penetration force increases), the stiffness decreases and the rotational impedance increases, but the influence is not obvious (especially the gas permeability) ese conclusions have reference significance for the engineering in which pipe piles might be under dynamic loading. In addition, based on the discussion on acceleration in reference [21], the discussion of normal stress of pile and soil can be referred to in reference [22].

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.