Research on Uplift Bearing Performance of Assembled Steel Pipe Pile used in Transmission Lines in Mountainous Terrain

Assembled steel pipe pile, which is a novel pile foundation, is developed in the paper. The ultimate uplift bearing capacity of the pile is proposed, and simulation by Plaxis3D and the corresponding experiment are performed to verify the theory. In the simulation, ultimate uplift bearing capacity of the assembled steel pipe pile and ultimate lateral frictional resistance of the interface of pile-soil increases with the increasing of the strength and stiffness of the interface of pile-soil, and with the increasing of length-diameter ratio, ultimate uplift bearing capacity of the assembled steel pipe pile increases while the ultimate lateral frictional resistance decreases gradually. The ultimate lateral friction is influenced by both of the strength of the soil around the pile and the interface of pile-soil, and the ultimate uplift bearing capacity obtained by simulation and theoretical calculation are close. Long-gauge FBG sensors are used in the experiment for measuring the longitudinal strain of the pile, and the error of ultimate uplift bearing capacity between the results of experiment and theory is less than 10%.

factors of the line corridors such as the lack of land resources, environmental protection leads that the line inevitably go through the hills or mountains which are steep and inconvenient to transport. According to statistics, the foundation construction of transmission line accounts for more than 60% of the construction period, so the foundation construction in mountainous terrain is the most primary factor for restricting the construction period.
At present, most piles used in the foundation construction of transmission engineering in mountainous terrain are bored concrete piles and the production of concrete is in the spot, which consumes lots of manpower and material resources to transport the sand, cement, water and other materials. Compared with the bored concrete pile, steel pipe pile has the advantages of high stiffness, strong ability for resisting the moment and shear, short construction period, and convenient to accomplish, which is widely used in the world since the 1950s. Although the steel pipe pile is convenient to construction in the spot, the diameter and length of the pile is usually large which leads that the transportation of it is inconvenient in mountainous terrain. The assembled structure is convenient to transport and has high production efficiency compared with the integral structure.  9) ACEM2018 and SBMS1 the superstructure and the uplift bearing capacity is the control condition for designing the foundation of transmission line [1] , the uplift bearing capacity of the assembled steel pipe pile is need to be researched.
In the research about uplift bearing capacity of the pile foundation, Chattopadhyay et al. [2] proposed a method for predicting ultimate uplift bearing capacity of the pile foundation in sand by considering the diameter, length and surface of the pile and the character of the soil. Deshmukh et al. [3] established the relation between critical depth ratio of the pile and ultimate uplift bearing capacity in the cohesionless soil and the predicted results are compared with the experiment data. Khatri et al. [4] studied the ultimate uplift bearing capacity of the pile under the vertical load based on the hypothesis that the cohesive force of the cohesive soil increases linearly with increasing of the depth on the undrained condition. Shanker et al. [5] developed a semi-empirical model for predicting the ultimate uplift bearing capacity of the pile in the sand and it is verified by the experiment. Alawneh et al. [6] studied the influence factors about the ultimate uplift bearing capacity of the pile in the dry sand. Shelke et al. [7] modified the lateral soil pressure coefficient by considering the soil arch effect and the modified method is verified by the test. Zhu et al. [8] deduced the elastic solution about the uplift pile based on winkler foundation model and researched the deformation and bearing capacity of the pile. Huang et al. [9] proposed an uniform method for calculating the bearing capacity about different pile length based on finite element modelling. Qian et al. [10] studied the deformation characteristics of the pile through the shear test and numerical analysis. Yu et al. [11] simulated the process of uplifting the large diameter rotary excavated filling pile and the corresponding experiment results are compared to analyze the internal force of the pile.
The researches about bearing capacity of the uplift pile are mainly focus on the filling pile, while the studies about the steel pipe pile on this aspect are less, and the corresponding researches of the assembled steel pipe pile are little. In this paper, the structure of the foundation of assembled steel pipe pile is presented firstly in section 2.

The structure of assembled steel pipe pile
The structure of the pile body in the foundation of assembled steel pipe pile, which is composed of grouting, steel pipe and the filling soil in the pile, is strikingly different from the traditional digging pile. The construction process of the assembled steel pipe pile mainly contains precasting of the pile, installation in the spot, and pressure grouting. Firstly, the pile is precasted in the factory or spot according to the desired size. And then, the pile is put in the foundation pit by sections. At last, the grouting is grouted for filling the gap between the pit wall and the pile body.
The foundation of assembled steel pipe pile is mainly composed of pyramid stents, precasted steel pipe and connection fittings. The steel pipe is connected to the steel tower by pyramid stents, and the steel pipes are connected with each other by flanges. The sketch of the foundation of assembled steel pipe pile is shown in Figure 1.

Ultimate uplift bearing capacity of the assembled steel pipe pile
The main factors for influencing the ultimate uplift bearing capacity contain length and diameter of the pile, property of the soil around the pile, interface property between pile and soil and the size of anchor. Considering the weight of assembled steel pipe pile and the inner soil, the ultimate uplift bearing capacity of the assembled steel pipe pile can be expressed as where U is the pile girth, i and are the interface ultimate friction and the corresponding correction coefficient about the ith layer of the soil respectively, is the thickness of ith layer of the soil, is the weight of assembled steel pipe pile and the inner soil, is the empirical coefficient, which is 0.8 for the permanent anchor and 1 for the temporary one, is the girth of the anchor, ℎ is the length of the embedded section of the anchor, and is the characteristic value of bonding strength between mortar and rock, which can be set according to the GB 50007-2011 [12] .
In equation (1), the parameters can be set according to the engineering experience except . As the result that the pile side frictional resistance is influenced by pile-soil interface strength and soil strength, it need to modify the pile-soil interface strength for obtaining the real value of the pile side friction.

Analysis based on finite element 4.1 Engineering background and geometric model
The transmission line engineering of 220 kV between Yongzhou and Linwu in Hunan is from Lanshan 220 kv substation to Linwu 220 kv substation. The line length of the proposed project is about 52.8km, and the topography along the line is cragged. The main topography is low-hilly, whose elevation ranges from 300m to 500m.
Plaxis3D is used for simulating the assembled steel pipe pile in this paper as shown in Figure Table 1. The soil with the depth ranges from 0 to 3m is hard plastic clay and the soil with the depth exceeds 3m is decomposed rock.

Constitutive relation and hypothesis
The strengths of pile and grouting are greater than the soil, so they seldom be strength-broken. The pile and grouting are simulated by linear elastic model, and they are regarded as a whole with assuming that the interface will not be damaged. The 10-node Mohr-Coulomb model is adopted to simulate the soil, and the yield criterion of Mohr-Coulomb is used. The elasto-plastic Coulomb friction model is adopted as the contact model between pile and soil, which is shown in Figure 3.

Parametric analysis
In order to study the influence of grouting condition and length-diameter ratio on the uplift bearing capacity, the finite element simulation is used to analyse.
Because the grouting condition influences the strength and stiffness of the interface of pile-soil, different values of strength and stiffness are adopted for simulating the different grouting condition. In the paper, four groups of value are included and the corresponding parameters are shown in Table 2. The curve of load-displacement of the pile obtained by simulation is shown in Figure 7.  In the same grouting condition and lateral area, the influence of different length-diameter ratio on the ultimate uplift bearing capacity of the assembled steel pipe pile is studied. The length-diameter ratios are set 27.778, 15.625, 10.000, 6.944 respectively for simulation. The curve of load-displacement of the top of pile is obtained as Figure 9 and the corresponding results are list in Table 3. In Figure 9, the displacement increases quickly after the load exceeds the critical value. From Table 3, the maximum of bearing capacity is 5696 kN when the length-diameter ratio is 16.667 and the minimum value is 4734 kN with the length-diameter ratio 8.333. In the condition of same lateral area, the embedded depth of the pile in the decomposed rock increases with the decreasing of the pile diameter, which lead to that the bearing capacity increases. As shown in Figure 10, when the pile reaches to the ultimate uplift bearing capacity, the lateral frictional resistance of the interface increases firstly and then decreases in the condition that the length-diameter ratio is different. And with the increasing of length-diameter ratio, lateral frictional resistance of the interface decreases gradually.
where is the ultimate uplift bearing capacity of single pile, 1 and 2 are the interface ultimate frictional resistance correction coefficients of hard plastic clay and decomposed rock.
The least square method is used to fit the correction coefficients according to the results in the simulation under different grouting conditions and the correction coefficients are list in Table 4. Then the fit coefficients are used to predict the ultimate uplift bearing capacity of a group of piles. The predicted results are compared with the finite element as shown in Table 5.  In Table 5, the error is less than 10%, which indicates that the formula of ultimate uplift bearing capacity of the assembled steel pipe pile is reliable.

Experiment about the assembled steel pipe pile
The experiment is for testing the uplift bearing capacity of the assembled steel pipe pile at different grouting pressures. The geometric similarity ratio is 1:10 in the scale experiment, and the elastic modulus is consistent with the original structure. The used soil is field remolded, and the rock is strongly weathered sandstone. The box for filling the soil is 1m wide, 1m long, and

Conclusion
The conclusions of the paper are summarized as (1) 9) ACEM2018 and SBMS1 3008 errors are less than 10%, which indicates that the theory is reliable. In the future, the influence of more parameters such as the grouting pressure, water-cement ratio and concrete age on the bearing capacity is need to research.