Keywords

1 Introduction

Pipe-sheet composite wharf structure can be made of steel pipe pile with high stiffness and steel sheet pile with low price to form pipe sheet composite wall [1, 2]. The pipe pile is used as the main force component to withstand lateral pressure such as water pressure and earth pressure, and the sheet pile between pipe piles is used as the soil retaining and linking structure. This new form not only has the advantages of low price of steel sheet pile, but also improves the stiffness of the overall section by using steel pipe pile with large stiffness, which greatly reduces the deformation of the composite wall, and has good force transmission and interlocking through special latch connection. As a new type of sheet pile structure, steel pipe composite sheet pile structure has not been applied in large-scale deepwater wharf. As a new type of wharf structure, the calculation method of pipe-sheet composite structure is not mature, and the calculation method of earth pressure behind it has not been given in the current specification. In terms of calculation and research of pipe-sheet composite structure, Fuqiang and Li [3] proposed through research that the lock in pipe-sheet composite system structure plays a role in transferring load. Since the stiffness of steel sheet pile is far less than that of steel pipe pile, it can be ignored in the force study, and the force of steel pipe pile should be mainly calculated during calculation. Zhang [4] confirmed that pipe-sheet composite piles can be loaded by using the vertical elastic foundation beam method through practical engineering verification. In the past, numerical simulation is often used in the calculation of pipe-sheet composite pile structure, due to the lack of relevant actual monitoring data, if the calculation result is small, it may cause the instability of the structure, and if the value of the safety coefficient is large, it may cause the actual cost to be high. This paper adopts the method of comparing the actual engineering monitoring and the calculation of national norms to confirm the distribution and calculation method of the rear earth pressure of the new type of pipe-plate combination structure, which makes the calculation of the rear earth pressure more close to the actual situation, ensures the stability of the structure and reduces the cost as much as possible, and can provide a reference for the same type of dock structure engineering.

2 Calculation Method of Earth Pressure

2.1 Method in China Standard

As the calculation method of earth pressure in pipe-sheet composite pile structure has not been provided in the Design Code for Wharf Structures (JTS 167-2018), when the ground surface is horizontal and the wall back is vertical, the standard value of active earth pressure horizontal strength generated by the soil itself and the wharf surface under uniform load can be calculated according to the following formula [5]:

$${p}_{ax}=\left(\sum {\gamma }_{i}{h}_{i}\right){K}_{a}{\text{cos}}\delta -2c\frac{\mathrm{cos\varnothing cos}\delta }{1+{\text{sin}}\left(\mathrm{\varnothing }+\delta \right)}$$
(1)
$${e}_{aqx}=q{K}_{a}{\text{cos}}\delta $$
(2)

The standard value of passive earth pressure level strength generated by the soil itself in front of the wall is calculated as follows:

$${p}_{px}=\left(\sum {\gamma }_{i}{h}_{i}\right){K}_{a}{\text{cos}}\delta +2c\frac{{\text{cos}}\varnothing {\text{cos}}\delta }{1-{\text{sin}}\left(\varnothing +\delta \right)}$$
(3)

In this equation, Ka and Kp have active and passive earth pressure coefficients according to Eqs. (4) and (5) respectively, where ϕ is the effective internal friction Angle of the soil, θ is the Angle between the wall and the plumb line, and β is the slope of the ground. The pipe-sheet composite pile is vertically placed on the ground, and θ = β = 0° is taken. δ is the wall friction Angle, take 1/3 \(\mathrm{\varnothing }\) for sandy soil and 2/3 \(\mathrm{\varnothing }\) for cohesive soil.

$${K}_{a}=\frac{{{\text{cos}}}^{2}\left(\varnothing -\theta \right)}{{{\text{cos}}}^{2}\theta {\text{cos}}\left(\delta +\theta \right){\left[1+\sqrt{\frac{{\text{sin}}(\varnothing +\delta ){\text{sin}}\left(\varnothing -\delta \right)}{{\text{cos}}\left(\delta +\theta \right){\text{cos}}\left(\beta -\theta \right)}}\right]}^{2}}$$
(4)
$${K}_{p}=\frac{{{\text{cos}}}^{2}\left(\varnothing +\theta \right)}{{{\text{cos}}}^{2}\theta {\text{cos}}\left(\delta -\theta \right){\left[1-\sqrt{\frac{{\text{sin}}(\varnothing +\delta ){\text{sin}}\left(\varnothing +\delta \right)}{{\text{cos}}\left(\delta -\theta \right){\text{cos}}\left(\beta -\theta \right)}}\right]}^{2}}$$
(5)

2.2 Method in America Standard

The port engineering design standard system in the United States is different from that in China. There is no special national, departmental or local design standard, and the relevant design method is provided by the departments engaged in relevant engineering. This paper takes the Design of sheet pile walls (EM 1110-2-2504) as reference. Active earth pressure and passive earth pressure can be calculated according to the following formula [6]:

$${P}_{a}={p}_{v}{K}_{a}-2c\sqrt{{K}_{a}}$$
(6)
$${P}_{p}={p}_{v}{K}_{p}+2c\sqrt{{K}_{p}}$$
(7)

where, Ka and Kp are the coefficients of active earth pressure and passive earth pressure respectively, which are the same as the Chinese specification and are calculated according to Eqs. (4) and (5) respectively, where δ is the wall friction Angle and there is no clear rule. Generally, 1/2 \(\mathrm{\varnothing }\) is taken and pv is the effective vertical earth pressure at the depth of Z. The valid values of ϕ and c are calculated according to the following formula:

$${\text{tan}}{\varnothing }_{eff}=\frac{{\text{tan}}\varnothing }{{\text{FSP}}\left({\text{FSA}}\right)}$$
(8)
$${c}_{eff}=\frac{c}{{\text{FSP}}\left({\text{FSA}}\right)}$$
(9)

Among them, FSP is the passive earth pressure safety factor, which is generally 1.5 under normal conditions of wharf structure, and FSA is the main dynamic earth pressure safety factor, which is 1.0 in most cases.

2.3 Method in Japan Standard

According to the Japanese code, the earth pressure acting on the building should be calculated according to the soil, sandy soil, clay soil, the type of the building and the state of the force, active, passive, normal force and earthquake force, etc. The active earth pressure and passive earth pressure of sandy soil acting on the wall are calculated according to the following formula [7, 8]:

$${P}_{a}={K}_{a}\left[\sum {\gamma }_{i}{h}_{i}+\frac{w{\text{cos}}\theta }{{\text{cos}}\left(\theta -\beta \right)}\right]{\text{cos}}\theta $$
(10)
$${P}_{p}={K}_{p}\left[\sum {\gamma }_{i}{h}_{i}+\frac{w{\text{cos}}\theta }{{\text{cos}}\left(\theta -\beta \right)}\right]{\text{cos}}\theta $$
(11)

Among them, the active upward pressure coefficient Ka and passive earth pressure coefficient Kp are calculated according to Eqs. (4) and (5) respectively (δ direction is not considered when calculating the passive earth pressure coefficient), δ is the wall friction Angle, the backfill soil has ϕ/2, generally within the range of ±15–20°, and w is the wharf surface load. For cohesive soil, the Japanese code stipulates that the active earth pressure and passive earth pressure strength acting on the wall are calculated according to the following formula.

$${P}_{a}=\sum {\gamma }_{i}{h}_{i}+w-2c$$
(12)
$${P}_{p}=\sum {\gamma }_{i}{h}_{i}+w+2c$$
(13)

where c is the cohesion of the soil. When the active earth pressure of cohesive soil is calculated by the formula, the negative earth pressure is not considered. When the earth pressure is negative, it can be calculated by 0. For clay, the friction Angle δ between the soil and the wall is not considered, that is, δ = 0°.

2.4 Contrastive Analysis

This paper lists the active earth pressure calculation formulas used in the design of sheet pile piers in China, the United States and Japan. From the formulas, it can be seen that the same active and passive earth pressure coefficients are used in the calculation of earth pressure in all countries. Under the calculation conditions of wharf structure, the earth pressure coefficients are only slightly different in the value of wall friction Angle. In the calculation of earth pressure, Japan gives different calculation formulas for sandy soil and cohesive soil according to the soil quality, while the Chinese and American methods do not consider the soil quality, and use the same formula to calculate the earth pressure of different soil. The safety factor method is used to calculate the internal friction Angle Ï• and cohesion force c of passive earth pressure in the United States, which is relatively more conservative.

3 Information of Project

3.1 Project Introduction

The shoreline length of a container terminal project in Guangzhou is 1460 m, and the dock structure is designed according to the berthing of 100,000 ton container ships, the design bottom elevation is −16.0 m, and the width of the berthing water area at the front of the wharf is 92 m. The new type pipe-sheet composite structure of wharf hydraulic structure has a design service life of 50 years, and the structural safety grade is Class II. In order to study the size and distribution form of earth pressure behind the pipe-sheet composite structure, the earth pressure behind the pipe-sheet composite structure is calculated, and the test instrument is buried in the field for verification, which provides a basis and reference for the earth pressure calculation behind the new pipe-sheet composite structure.

3.2 Geological Conditions

The project is located at the intersection of rivers and oceans in the estuary of the Pearl River Delta. Its basement is a deep metamorphic regional metamorphic rock in the Lower Paleozoic, which is covered with strata formed in different Quaternary periods and of different origin. According to the drilling data, there are mainly terrestrial alluvial strata, estuarine alluvial sedimentary strata, terrestrial alluvial sedimentary strata, sea-land interaction sedimentary strata and artificial fill soil layers from the bottom up. The project is mainly for soft soil layer, and the geotechnical parameters are calculated according to the recommended values of geotechnical parameters given in the investigation report, the measured values of standard penetration number and the experience of related projects, as shown in the following Table 1.

Table 1 Physical and mechanical property indexes of main rock and soil layers
Full size table

3.3 Structure Condition

The berth of this project is designed according to the berthing of 100,000 ton container ships,, as shown in the following Fig. 1. and the design bottom elevation is −16.0 m. The single anchor sheet pile scheme with pipe-sheet composite pile at the front wall is adopted. The combination of Φ2032mm steel pipe pile with 24 mm wall thickness and Z-shaped steel sheet pile is adopted at the front wall. The material of steel pipe pile is Q420B, the material of steel sheet pile is S430GP, and the pile top elevation is +2.0 m. The superstructure of composite sheet pile is cast-in-place C40 reinforced concrete parapet with a width of 5.0 m. The yield strength of the tie rod steel is not less than 550 MPa, the tensile strength is not less than 750 MPa, the spacing is 3.35 m, the installation elevation is 0.5 m, and the diameter is Φ140mm. The anchoring structure adopts 1.0 m steel pipe pile combined Z steel sheet pile structure, the top of the pile is set with C40 concrete guide beam, the top elevation is +2.0 m, and the bottom elevation of the anchoring pile is −14.0 m.

Fig. 1
A diagram displays the pipe sheet composite wharf structure. P H C is between 2 walls with sheet pile inside steel pipe pile. Below the right wall are 2 steel pipe piles connected by a sheet pile.

Pipe-sheet composite wharf structure

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3.4 Monitoring Scheme

During the construction period and trial operation stage, a monitoring section is arranged along the shoreline of the pipe-sheet composite structure, and the active and passive earth pressure of the steel pipe pile and steel sheet pile are respectively dynamically monitored and the monitoring frequency is once a week.

4 Monitoring Results and Comparative Analysis

4.1 Comparative Analysis of Measured Results and Theoretical Calculations

According to the relevant calculation formulas, as shown in Fig. 2, the earth pressure calculation methods adopted by China, the United States and Japan in the calculation of pipe-sheet composite piles are all based on the coulomb earth pressure formula, and are modified on this basis. The Chinese and American codes take into account both the cohesion and the influence of the friction Angle on the wall, and the calculation formulas and calculation methods are roughly the same. There are only some differences in the values of some parameters, while the Japanese standard considers clayey soil and non-clayey soil, and there are some differences in the calculation method of the active earth pressure of clayey soil. When calculating active earth pressure, the Japanese method does not use the Coulomb coefficient for corresponding reduction, and the calculation result of active earth pressure is larger and more conservative than that of China and the United States. When calculating passive earth pressure, the American standard will reduce the safety factor of soil parameters, and the calculation result is smaller and more conservative. In terms of earth pressure calculation, China's standard is more radical than other countries, and the calculated dynamic earth pressure is small, and the passive earth pressure is large.

Fig. 2
Two line graphs of, a, active earth pressure and, b, passive earth pressure plotted versus elevation. Calculated earth pressure in China, U S, and Japan, steel pipe pile, and sheet pile are plotted. Both graphs generally exhibit increasing pressure as elevation increases.

Comparison between calculated earth pressure and measured earth pressure

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The in-situ earth pressure monitoring results show that the earth pressure on the pipe pile and sheet pile increases gradually from shallow to deep, and the size and distribution law of the earth pressure distribution on the pipe pile and sheet pile are relatively consistent, and no obvious earth pressure redistribution phenomenon occurs. There is still a large earth pressure on the rear side of the sheet pile. Therefore, when the structural stress calculation is carried out subsequently, the calculation of the sheet pile should not only consider its soil retaining effect, but also need to carry out the structural strength and stability check to ensure the safety and stability of the structure. In terms of active earth pressure calculation, the active earth pressure calculation methods and results in China and the United States are basically the same, and are closer to the actual test results. When the subsequent stress calculation is performed, considering uncertainties such as sudden changes in the soil stratum and construction deviations, a suitable safety factor can be selected according to the actual engineering conditions to ensure that the calculated earth pressure is greater than the actual earth pressure and to ensure the stability of the structure. The calculation results obtained by the Japanese calculation method are relatively more conservative, and the calculation results are obviously larger than the measured results. The adoption of this calculation method may lead to a greater depth of the structural piles into the ground for subsequent stress calculations, and the demand for the structural parameters of the piles, such as strength and stiffness, is also greater, which corresponds to the related project cost will also be higher. In terms of passive earth pressure, the results of passive earth pressure calculations in China and Japan are basically the same and significantly larger than those calculated by the U.S. specification using a safety factor discount. According to the measured passive earth pressure is small, mainly concentrated in the toe of the wall in contact with the soil, mainly because the actual displacement of the combined pipe and sheet pile in this project is small, and the passive earth pressure required to resist the displacement is also small. And the passive earth pressure on the front side of the steel pipe pile is obviously large compared with that on the front side of the steel sheet pile, which is mainly related to the fact that the steel pipe pile mainly bears stress and the soil deformation on its front side is relatively large. Therefore, when calculating the passive earth pressure, the passive earth pressure of the soil within 10 m of the top of the passive zone should be mainly considered, and the passive earth pressure below the depth of 10 m should be appropriately discounted in the calculation to reduce the safety risk. The discount factor of steel sheet pile and steel pipe pile should be considered separately in the calculation of subsequent structural stress and stability by using the calculation of passive earth pressure to reduce the safety risk.

To sum up, when the American code is used to calculate the relevant earth pressure of pipe-plate composite structure, the calculated results have a high consistency with the field measured results, and the American code standards can be selected for calculation and analysis.

4.2 Comparative Analysis of Measured Results and Numerical Calculation

The numerical modeling software was used to simulate the earth pressure behind the pipe-sheet composite structure, and the results were shown in Fig. 3. It can be seen that when the pipe pile and sheet pile were modeled respectively, due to problems such as the finite element module division of the model, distortion often occurred at the junction of soil layer and the junction of structure, and the calculated value of earth pressure was abnormally large. Therefore, the equivalent section replacement method should be used to calculate this situation. The soil pressure distribution behind the pipe-sheet composite pile structure can be well calculated by using the numerical simulation method, and the equivalent conversion method of section parameters can be used to simplify the pipe-sheet composite pile into sheet pile structure during setting, which can simplify the model to avoid distortion and stress concentration. The correct earth pressure results can also be calculated.

Fig. 3
A line graph of elevation in meters versus active earth pressure in kilopascals. Steel pipe pile, sheet pile, steel pipe pile numerical, sheet pile numerical, and equivalent section numerical are plotted. Sheet pile numerical has the highest pressure with around 250 kilopascals at 0 meters.

Comparison between numerical simulated earth pressure and measured earth pressure

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5 Conclusions

By comparing with theoretical calculation and field monitoring, this paper calculates and analyzes the distribution and magnitude of earth pressure behind the new pipe-sheet composite pile structure, and the following conclusions can be drawn.

  1. (1)

    Behind the pipe-sheet composite pile structure, earth pressure is exerted on both pipe pile and sheet pile at the same time, and the magnitude and distribution of earth pressure on pipe pile and sheet pile are consistent, and no obvious earth pressure redistribution phenomenon occurs. There is still a large earth pressure on the rear side of the sheet pile. Therefore, when the structural stress calculation is carried out subsequently, the calculation of the sheet pile should not only consider its soil retaining effect, but also need to carry out the structural strength and stability check to ensure the safety and stability of the structure.

  2. (2)

    The calculation methods and results of active earth pressure in China and the United States are basically the same and closer to the actual test results. The calculation results obtained by the Japanese calculation method are relatively more conservative. The passive earth pressure calculation results in China and Japan are basically the same, but larger than the actual test results, the U.S. specification for the calculation of passive earth pressure using the safety factor method, the calculation results are smaller and closer to the actual results. Comprehensive comparison, the U.S. standard calculation of earth pressure is more in line with the actual, the subsequent calculation of related projects can be prioritized to consider the use of U.S. standards.

  3. (3)

    The method of numerical simulation can effectively calculate the soil pressure distribution behind the pipe-sheet composite pile structure, and the method of equivalent conversion of cross-section parameters can be used to simplify the pipe-sheet composite pile into the sheet pile structure, which can simplify the model to avoid aberration and make the calculation results closer to the actual results.

  4. (4)

    The soil body at the rear side of the pipe pile of tube-sheet combination structure will have a certain degree of soil pressure redistribution due to the arch shape of the pipe pile, and the calculation formulas do not consider the influence of the form of retaining structure at the front side, which can be further investigated in the subsequent study to further refine the calculation results with a view to reducing the cost of the project.