The accurate estimate of the ultimate bearing capacity of a single pile in the vertical direction is an important issue in the design of the pile foundation. This paper presents a static test on a single-pile model. The test was performed through a large-scale model casing test equipment that is independently developed. Various factors that affect the different test soil samples have been taken into account. In addition, the test has measured the pile’s internal stress and displacement through the sensors that were installed on the pile. What is more, a series of studies on the settling character of the single pile, pile lateral friction, changing nature of tip resistance, and its development with settling have been carried out. Finally, this paper analyzes the bearing capacity behavior and load transfer mechanism in the compressive static load test on the single pile in the vertical direction. The test results show that, under the same static load, the lateral friction of a pile in the sand is bigger than that in the silty clay, and with the increasing load at the pile tip, the increment speed of tip resistance in the silty clay is much faster than that in the sand, while pile’s bearing capacity in the sand is much bigger than that in the silty clay.
The rapid urbanization has led to huge demands on the infrastructure constructions in city area. Meanwhile, the pile foundation is one of the most common forms of the infrastructure foundations in the constructional engineering. The design concept of the pile foundation with settling control has increasingly been accepted by the academia and engineering community [
Through the indoor test on the large-scale open pile model, this paper studies the settlement mode, bearing capacity, and lateral friction during the static load process of a single pile, as well as the working nature of tip friction, which have revealed the internal mechanism of pile–soil interaction in static load process.
This test adopts the large-scale model casing equipment that is independently developed by the Qingdao University of Technology, with its internal dimension of 3000 mm × 3000 mm × 2000 mm (
The large-scale model casing test equipment.
The pile model used for the test consists of two aluminum alloy tubes with concentric circles, and their inner and outer tubes are connected to the pile shoe, with Poisson’s ratio 0.3 and elasticity modulus 72 GPa. The pile model P1 is of 1000 mm long, 140 mm in diameter, and 13 mm in thickness, while the pile model P2 is of 1000 mm long, 160 mm in diameter, and 13 mm in thickness. Except for the diameter, the pile model P1 and P2 are made in the same structure, as shown in Figure
The structure of double-walled pile.
On the outer wall of the outer tube, a groove is opened for fiber bragg grating (FBG) microsensor to be stuck in and sealed with epoxy resin glue, while the sensor of inner tube is also installed onto its outer wall and placed into the enclosed annular space to protect it from environmental disturbance (see Figure
The inner and outer tubes of double-walled open-ended pile.
Two soil samples are used for this test: one is the sea sand, and the other is the silty clay taken in a Qingdao area. The sea sand used is dried out to reduce the influence from other factors, and the relative density of the sand is
The grain composition of sand sample.
As for the silty clay, its optimum moisture content is
According to inner dimensions of casing model 3000 mm × 3000 mm × 2000 mm (length × width × height), the height of soil sample is 1800 mm, and the sample is filled into the casing model in nine times, and large scraper is used to fill in the soil at 200 mm each time, and each layer of the soil sample is tamped down both manually (two times) and mechanically (one time). Each layer of soil is given at least 12 h for its self-compactness relying on gravity, so as to ensure the homogeneity of test sample. In each layer of soil, eight cutting rings are placed each at 300 mm from the edge to the four faces of the casing model. After the test, take out all cutting rings from each layer to measure the soil compactness and work out the average compactness.
Fill the soil sample into the casing model with layers at a height of 200 mm for each layer, manually (two times) and mechanically (one time), until the soil at each layer is tamped to the required height.
Place an open-ended pile into the casing model after determining its sinking position in the casing model, using a leveling rod to adjust the perpendicularity of the pile, so as to ensure its perpendicularity during pile sinking.
Use a hydrocylinder to slowly press the pile to reach the required height. After sinking the pile into the required position, give the sand sample at least 15 d and the saturated foundation soil at least 28 d before starting the test.
The servo-loading system and the casing model counterforce device are adopted as the loading devices for this test. The pile is applied with net load by the grading of servo-loading system to reach the ultimate load. The loading in the test is in accordance with the slow maintenance loading method as specified in The Technical Code for Building Pile Foundation (JGJ94-94) [
During the static test, net load is applied by the servo-loading system through the loading device to reach the ultimate load. In the sand, the load of pile model in the first level is 8 kN; in the second level, it is 16 kN; in the third level, it is 24 kN; in the fourth level, it is 32 kN; in the fifth level, it is 40 kN or damaged. In silty clay, the load of pile model in the first level is 1 kN; in the second level, it is 2 kN; in the third level, it is 3 kN; in the fourth level, it is 4 kN; in the fifth level, it is 5 kN; in the sixth level, it is 6 kN; in the seventh level, it is 7 kN; in the eighth level, it is 8 kN; in the ninth level, it is 9 kN or damaged. Loading should be stopped if, at a certain level, the settling volume of the pile model under the load effect is five times as much as the settling volume of the previous level, or there is no obvious increase in the load of pile foundation.
Before loading at each level, record the data collected by each pile sensor and measure the displacement at the pile tip. Load for 30 min to repeat the aforementioned operation, and then measure the displacement at the pile tip after each 15 min of loading. The specific test plan is shown in Table
Single-pile static test.
No. | Type | Shape | Outer diameter (mm) | Inner diameter (mm) | Length (mm) | The thickness of inner and outer walls (mm) | Test sample |
---|---|---|---|---|---|---|---|
1 | Single pile | Double-walled open-ended | 140 | 114 | 1000 | 3 | Sand |
2 | Single pile | Double-walled open-ended | 140 | 114 | 1000 | 3 | Silty clay |
3 | Single pile | Double-walled open-ended | 160 | 134 | 1000 | 3 | Silty clay |
According to the data collected from the sensors, we can obtain the axial force of pile shaft through the following formula:
According to the axial force of each section, pile lateral friction can be obtained through the following formula:
According to the lateral friction of soil layer, the unit side friction of such layer can be worked out through the following formula:
According to the data collected by the sensor at the bottom of pile lateral side, the tip resistance can be worked out through the following formula:
At the start of the static test, the increasing load will first compress the upper part of the pile, with part of the load passing down and the other part becoming lateral friction. With increasing load, the compressed upper part of the pile will have a relative displacement, while the lateral side will receive an upward side friction. When all of the lateral frictions have reached the limit and the load is still increasing, the pile tip will bear more load; the tip settlement will also grow and even reach or surpass the allowable deformation, and the pile will be damaged.
The purposes of this indoor large-scale static test on open-ended pile include the following: (1) studying the basic mode of single-pile settlement in the sand and silty clay; (2) investigating the single-pile bearing capacity, tip resistance, operating characteristic of lateral friction, and development of pile tip displacement under the application of static load.
Figure
The tip displacement of pile in different soils along with load change.
Figure
The lateral friction of pile in different soils along with load change.
Figure
The lateral friction of pile with different diameters in soil samples along with load change.
Figure
The tip resistance of pile in different soil samples along with load change.
With the increase of the load, the side friction resistance of the pile body gradually reaches the maximum level. At this time, the load increases again, and the side friction resistance no longer increases, resulting in a larger increase in the pile-end resistance.
The relationship between the tip resistance of the single pile and the pile load is revealed in the curved shown in Figure
The tip resistance of piles with different diameters in various soil samples along with load change.
In this paper, we have studied the settlement mode of a single pile in the static load process, its bearing capacity and lateral friction, and the working properties of its tip resistance. Based on the results, the main conclusions can be drawn as follows. With increase of tip load, the vertical displacement of pile tip is increasing. Applying the same load at pile tip, the bigger the pile diameter, the smaller the settling volume. Under the same load, the settling volume of a pile in the sand is less than that in the silty clay. The bearing capacity of a pile in the sand is far bigger than that in the silty clay. Applying the same load at pile tip, the less the diameter of a pile, the larger the proportion of its lateral resistance, and the bigger the diameter of a pile, the larger the proportion of its tip resistance. With the increase of load application at pile tip, the tip resistance experiences slow to quick increase. Applying the same load at pile tip, the tip resistance of a pile in the sand is smaller than that in the silty clay. From that, we can see that, under the same load at pile tip, the lateral friction of a pile in the sand is bigger than that in the silty clay.
Some or all data that support the findings of this study are available from the corresponding author upon reasonable request.
The authors declare no conflicts of interest.
This research was funded by the National Natural Science Foundation of China (41772318).