Keywords

1 Introduction

Rock-socketed pile is widely used in high-rise buildings and bridge engineering with high load and high settlement requirement. Soft rock socketed pile is a common type in China. When some scholars study the model test of overlying soil thickness and earth pressure on rock-socking piles in soft rock, the 1 g model test used in the laboratory is generally scaled down, which leads to the reduction of soil layer thickness, so the stress field of the model soil is difficult to reflect the in-situ dead weight stress. In order to reflect the in-situ stress field, it can be realized by the centrifuge model with high cost. When this test condition is not available, some scholars use air bag, pile load or rigid support to load the rock soil around the pile. Airbag load pressure range is limited, more than 400 kPa pressure air easy to stress concentration and damaged, hard to ensure the success rate. Pile load test need space is large, is not convenient operation, low pressure range. Rigid support load applied load can be larger, but exist eccentric load, stress non-uniform problem in geotechnical engineering.

In this paper, the effect of coverage on the bearing capacity of rock-socketed part is discussed by using finite element method, and the equivalent simulation method of overburden pressure is proposed, the data of rock-socketed section is extracted and analysed. The aim is to simulate the in-situ stress field without limiting the free deformation of the soft rock around the pile and to ensure uniform loading. Therefore, how to simulate the in-situ stress field without limiting the free deformation of the soft rock around the pile and ensuring uniform loading is of great significance to reveal the bearing mechanism of the soft rock-socketed pile under different stress states.

2 The effect of Coverage On Bearing Capacity of Soft Rock Socketed Part and Model Testing Method

2.1 FEM Analysis of Effect of Coverage Layer on Standing Capacity of Soft Rock-Socketed Pile

The soft rock socketed piles with different burden thickness and pressure are simulated by finite element method, and the overburden is clay. By comparing the influence of different overburden thickness and overburden pressure on the carrying peculiarity of rock-socketed section, we are able to judge whether the impact of overburden and equivalent overburden pressure on the carrying peculiarity of rock-socketed section is consistent (Table 1).

Table 1 FEM Analysis Scheme of Soft Rock Rock-socketed Piles
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Using axisymmetric model, 15-node triangular elements are adopted for pile [1], soft rock and clay overburden, and the thickness of soft rock below pile bottom is 10 m. The calculation model is shown in Fig. 1. The bottom of the model is a fixed boundary, both sides are fixed in the horizontal direction, and can move freely in the vertical direction.

Fig. 1
3 illustrations. a. A narrow long pile across a layer of overlying soil and soft rock. b. A narrow short pile in the soft rock with overburden pressure on top of the soft rock. c. A broad and long pile in the soft rock.

FEM Model and Meshing of Pile & Rock/Soil

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In this example, the parameters of soft rock are selected with reference to the soft rock simulation sample, which is still in the linear elastic stage when its peak strength is 50%, so the deformation modulus is equal to the initial tangent modulus, instead of the confining pressure in the Janbu formula with the mean stress, the formula for calculating the deformation modulus E is as follows:

$$E = Kp_{a} \left( {\frac{p}{{p_{a} }}} \right)^{n}$$
(1)

where K and n are the test parameters, for this test, 1959.3 and 0.1066, respectively; p is the mean stress; pa is atmospheric pressure, 0.1 MPa.

For the case of overburden pressure or overburden self-weight stress is \(\sum {\gamma h}\), the average stress P of soft rock can be calculated by the formula below:

$$p = \frac{{1 + 2K_{0} }}{3}\sum {\gamma h}$$
(2)

where K0 is the static lateral pressure coefficient, K0 = 1-sin 43.4° = 0.313; and \(\sum {\gamma h}\) is the overlying pressure or the self-weight stress of the overlying layer.

When the overburden pressure is 10,100,300.500 kPa or the overburden thickness is 0.5,5,15,25 m respectively, the deformation modulus of soft rock is 144,184,206 and 218 MPa respectively. The calculation parameters of the soft rock rock-socketed pile model are exhibited in Table 2.

Table 2 Model Parameters
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2.2 Effects of Different Overlying Soil Thickness and Different Overlying Pressure on Carrying Characteristics of Rock-Socketed Section

The calculation results of load-settlement on the top of rock-socketed section of pile foundation with different overburden thickness are exhibited in Fig. 2. The top load of the rock-socketed section is the axial force of the pile, and the top settlement of the rock-socketed part is the pile shaft subsidence of the buried depth. The larger the thickness of overburden, the smaller the settlement under the same load (axial force) on the top of rock-socketed section, and the load corresponding to the yield point of the load-settlement curve increases with the thickness of overburden, and the corresponding settlement decreases. The calculation results of load-settlement relationship on the top of rock-socketed sections with different overburden pressures are exhibited in Fig. 3. The pile top load is the upper load of the rock-socketed part, and the pile top settlement is the top subsidence of the rock-socketed section. With the increase of the overburden pressure, the settlement becomes smaller and smaller under the top load of the same rock-socketed part. And the greater the overlying pressure, the greater the load corresponding to the yield point of the load-settlement curve, and the corresponding settlement is smaller.

Fig. 2
A multiline graph plots the top of the socketed section settled versus the top load of the rock socketed section with 4 decreasing lines for covering layers of thickness 0.5, 5, 15, and 25 meters. The line for a thickness of 15 meters has the highest value of around (17000, 43).

Load-settlement Curve of Soft Rock-Socketed Pile with Different Overlying Soil Thickness

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Fig. 3
A multiline graph plots the top of the socketed section settled versus the top load of the rock socketed section with 4 decreasing lines for overlying pressures at 10, 100, 300, and 500 kilopascals. The highest value of (17000, 67) is for a pressure of 10 kilopascals. Values are estimated.

Load-settlement Curve of Soft Rock-Socketed Pile with Different Overburden Pressure

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2.3 Equivalent Modelling of Overlying Soil Using Overburden Pressure for Piles in Soft Rock

Figure 4 shows the contrast of load-settlement curves at the top of rock-socketed segment of pile foundation with different covering thickness and equivalent overlying pressure. It can be seen that the top load and settlement relationship curve of rock socketed pile with upper coverage is basically the same as that of rock socketed pile without overburden after equivalent coverage pressure, and the load settlement curve after equivalent overburden pressure is slightly higher. This is because the distribution of dead weight load of the overlying soil layer on the rock-socketed top surface under the impact of pile is uneven in a small range, and the equivalent coverage stress homogenizes the upper load, which caused settlement is slightly smaller, but this discrepancy does not exceed 10%. This fully indicates that the effect of coverage on the bearing capacities of rock-socketed part can be equivalent to that of coverage pressure without the effect of deadweight stress of overburden.

Fig. 4
3 multiline graphs plot the top of the socketed section settled versus the top load of the rock socketed section with decreasing lines for soil thickness of 0.5, 5, and 15 meters, and overburden pressure of 10, 100, and 30 kilopascals.figure 4

Comparison of top Load-Subsidence Relationship Curves of Soft Rock-Socketed Pile with Distinct Coverage Thickness and Equivalent Overlying Pressure

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3 Model Test Device of Micro Pile in Soft Rock Considering the Influence of Coverage Pressure

In order to directly understand the bearing mechanism of rock-socketed piles in soft rock, it is necessary to develop a physical model test device for the bearing mechanism of single pile which can be used for CT scanning. In order to simulate the soil stress field around a pile and visualize the deformation distribution, development process and failure mode of a single pile under vertical load, the bearing mechanism of a single pile is studied (Fig. 5). And the vertical bearing characteristics of the rock-socketed pile in soft rock are comprehensively interpreted [5].

Fig. 5
A photograph and a C T scan image. a. A physical model test device with pile top loading system and model pile between a reaction system, pile surrounding soil loading system, and model cylinder. b. C T scan image of pile model test. The initial position of the bottom of the pile is indicated.

Application of Model Testing Technique for Piles in Soft Rock considering Overburden Pressure (Huang et al. 2019 [4])

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4 Conclusions

When the overburden layer is equivalent to the overlying pressure of the relevant self-weight, due to the equivalent of the uniform load, the vertical displacement field and the vertical stress field of the rock-socketed depth of 1/4 ~ 1/3 pile length of the soft rock socketed pile will be different, while the vertical displacement field in the rock-socketed section and the lower part is almost the same as the vertical stress field. For the whole rock-socketed member, it is reasonable that the overburden is equivalent to the uniform overburden pressure, and the difference of the top settlement of the rock-socketed member caused by the uniform load is less than 10%. After the inner wall of the model barrel is treated with butter and foil to reduce friction, there is basically no effect of the edge wall effect. which provides a reliable technical guarantee for carrying out the model test of the carrying peculiarity of the model pile.