According to the force characteristics of the double-row pile supporting structure, two new types of double-row piles are developed: the prestressed strong-constrained double-row piles and the recycling assembled double-row piles. A comparative field test was conducted on the support effects of the two new double-row piles and conventional double-row piles. The test site is located in a deep foundation pit of the Beijing Daxing International Airport Project. The feasibility and reliability of the two new support structures are verified. Field monitoring included section strain and bending moment of the pile body, horizontal displacement of the pile body, and vertical and horizontal displacement of the pile top. The research shows that because of the prestressed anchorage cables in the rear row piles, the prestressed strong-constrained support structure can provide better tensile performance from the rear piles, and the deformation and displacement are minimal. The recycling assembled double-row piles have similar deformation and displacement to the conventional piles. Through the connection of the steel members, the construction time can be effectively shortened. After the backfill of the foundation pit, the steel members can be recycled and the cost can be reduced.
Currently, double-row pile support structures are widely used in slope engineering, hydraulic engineering, road engineering, and other fields. This type of support structure mainly consists of front row piles, rear row piles, crown top beams, and coupling beams [
Schematic diagram of traditional double-row pile support structure.
Many scholars have studied conventional double-row pile support structures based on the unique structural form and stress variation characteristics. Based on the classic earth pressure theory in China, the most representative one is the volume-ratio coefficient method proposed by He et al. [
All of the above studies are based on the traditional double-row pile support structure, and there is less research on the optimization and improvement of the traditional support structure [
The test site is located in a deep foundation pit of the Beijing Daxing International Airport Project, located in Yufa town and Lixian town of Daxing District in Beijing and Guangyang District of Langfang city in Hebei province, as shown in Figure
Location of test sites.
The field test site is shown in Figure
Test site.
The surface layer of the test field is generally covered with an artificial accumulation layer with a thickness of 0.40∼1.80 m, which consists of a clayey silt soil layer, sandy silty soil fill ① layer, and slag soil ①1 layer. The local distribution may be thicker, as influenced by human transformation.
Under the artificial accumulation layer, the recent sedimentary layers consist of the newly deposited sandy silt and clayey silt ② layers, silty sand and fine sand ②1 layers, organic clay, organic heavy silty clay ②2 layers, silty clay and clayey silt ②3 layers, sandy silt and clayey silt ③ layers, organic clay and organic heavy silty clay ③1 layers, fine sand and silty sand ③2 layers, and silty clay and clayey silt ③3 layers.
Under the new sedimentary layer, the quaternary sediments are silty clay and clayey silt ④ layers, silty clay and sandy silt ④1 layers, fine sand and silty sand ④2 layers, heavy silty clay and clay ④3 layers, and fine sand and medium sand ⑤ layers.
The prestressed strong-constrained type support is created by adding a prestressed anchor cable along the pile at the top of the rear row of piles, and the anchors are tied to the inside of the reinforcement cage. After the reinforcement cage is driven into the pile holes and the concrete of the pile has reached the strength requirements, the anchor cable is tensioned. Due to the increase in the tensile strength of the rear row of piles, the length of the front row piles can be appropriately shortened, as shown in Figure
Elevation view of new double-row piles. (a) Prestressed strong-constrained double-row piles. (b) Recycling assembled double-row piles.
The recycling assembled type support uses detachable steel components instead of the traditional crown beams and coupling beams, while the rear row piles use steel pipe piles instead of traditional bored piles, as shown in Figure
The numbers in Figure
The numbers in Figure
In order to fully compare the mechanical properties of the two new types of double-row pile retaining structures and the conventional ones, the three types of piles were all cast with C30 concrete. The compressive strength of concrete was 14.3 N/mm2, and the elastic modulus was 3 × 104 N/mm2. The longitudinal reinforcement of the pile is 14 B20 HRB400 steel bars, and the stirrups are HRB400,
Geometric design parameters of test piles.
Test pile type | Crown beam size (m) | Length of front and rear row pile (m) | Pile diameter (m) | Row spacing (m) | Pile spacing (m) | Depth of foundation pit (m) |
---|---|---|---|---|---|---|
Conventional | 0.8 × 0.8 | 12.5/12.5 | 0.8 | |||
Prestressed strong-constrained | 0.8 × 0.8 | 10.0/12.5 | 0.8 | 2.4 | 2.0 | 8 |
Recycling assembled | — | 12.5/12.5 | 0.8/0.6 |
The purpose of this field test is to determine the mechanical properties of the new double-row pile retaining structures in the excavation process of the foundation pit, explore the displacement and deformation behavior of the new pile types, and comprehensively evaluate the working performance of the new pile types in the field test.
In order to reveal the law of displacement and deformation of the new pile type, the changes of the conventional double-row pile supporting structures after the force deformation are compared. The test focuses on the horizontal and vertical displacement of the pile top and horizontal displacement of the pile body. The pile body deformation was monitored, as given in Table
Test procedure.
Property | Monitoring position | Monitoring quantity | Instrument |
---|---|---|---|
Pile strain | Conventional front and rear row pile | Conventional pile strain gauge: five groups + one branch | Vibrating string embedded strain gauge (BGD-4200) |
Prestressed strong-constrained front and rear row pile | Prestressed strong-constrained pile strain gauge: four groups + two branches | ||
Recycling assembled front row pile | Recycling assembled pile strain gauge: five groups | ||
Horizontal displacement of pile body | Conventional front row pile | Three types of front row pile for a total of six | Clinometer |
Prestressed strong-constrained front row pile | |||
Recycling assembled front row pile | |||
Horizontal displacement and vertical displacement of pile top | Conventional front and rear row pile | One of three types of front and rear piles, a total of 12 measuring points | Remote displacement automatic monitoring system, electronic total station |
Prestressed strong-constrained front and rear row pile | |||
Recycling assembled front and rear row pile |
The field monitoring instruments are shown in Figure
Field test monitoring instruments. (a) Electronic total station. (b) Clinometer. (c) Remote automatic displacement monitoring system. (d) Strain collector BGK408. (e) Strain gauge BGD-4200.
The displacement measurement point plan view of the pile tops is shown in Figure
Pile top displacement measurement point plan view.
Reflective sheet in pile top.
The plan view of the inclinometer pipe is shown in Figure
Inclinometer plan.
Inclinometer pipe site layout.
The site construction process consisted of the following steps and is shown in Figure Behind the pile, a trench with a depth of 1 m, width of 2 m, and length of about 25 m was excavated for laying the steel strands. In order to minimize the disturbance of the overlying soil on the steel strands, aluminum-plastic tubes and iron tubes are sheathed outside the steel strands. At the end of the trench away from the foundation pit, a 1.5 m long steel bar was drilled into the bottom of the pit to firmly connect the steel strand to the exposed bar head as a fixed point The other end of the strand was extended to the top of the crown girder and connected to the cable-type displacement meters of the piles at the top and the rear of each pile, respectively The connections of the whole system were checked, the initial data values were collected, and the groove was carefully filled.
Connection diagram of the remote displacement automatic monitoring system. (a) Relative position of the aluminum-plastic tube, iron pipe, and steel strand. (b) Fixed point connection behind the pile. (c) Connection of guyed displacement gauge on the pile top.
The strain of pile body was monitored by a vibrating wire embedded strain gauge. The strain gauges were bound to the main reinforcement of the steel reinforcement cage symmetrically, and the vertical spacing was 200 mm from top to bottom. The elevation view of the measuring point layout is shown in Figure
Pile strain measuring point layout elevation.
Binding and burial of strain gauges.
When the installation of each monitoring point was complete and the commissioning was correct, the initial values were collected and the excavation of the foundation pit was carried out. The pit was designed for a depth of 8 m, excavated in four steps at a rate of 2 m for each step, and the concrete was sprayed on the wall of the foundation pit after each excavation to prevent runoff between piles. After each excavation step, data acquisition was performed for each monitoring aspect three times/day (morning, afternoon, and evening) to capture the time of the initial changes in the data during excavation. The monitoring continued until the fifth day when the pit excavation was completed. The excavation process of the foundation pit is shown in Figure
Excavation process of foundation pit. (a) Excavation of 2 m. (b) Hanging net between piles. (c) Spraying concrete. (d) Excavation of 4 m. (e) Excavation of 6 m. (f) Excavation of 8 m.
As shown in Figure
Comparison of vertical displacement time history of pile top. (a) Front row pile. (b) Rear row pile.
Due to the summer flood season, after the excavation of the foundation pit, heavy rainfall occurred in the test area for several consecutive days, resulting in sinking of the vertical displacements of the front and rear pile tops, especially for the recycling assembled double-row piles. The sinking of the front row piles was −2 mm, and the sinking of the rear row piles was −3 mm. The main reason for the analysis is that the strata within the site are mainly composed of thick clayey soil, silty soil, and sand soil, and the soil structure is relatively loose, so the engineering properties are poor. Under strong rain, the bottom of the pit is weakened with water and soil. Therefore, the pile body sinks under the tow of the soil around the pile, and since the hollow steel pipe pile is used for the recycling assembled rear pile, the weight of the pile is smaller, and the towing effect is more obvious.
Positive values on the vertical axis in Figure
Comparison of horizontal displacement time history of pile top. (a) Front row pile. (b) Rear row pile.
It can be seen from Figure
Horizontal displacement of front row piles. (a) Conventional front row pile. (b) Recycling assembled front row pile. (c) Prestressed strong-constrained front row pile.
The front row pile of the prestressed strong-constrained pile appears as negative values in the early stage of monitoring, where the maximum displacement reaches −1.82 mm. The prestressed strong-constrained front row piles showed a negative value in the initial period of monitoring, and the maximum displacement reached −1.82 mm. That is to say, displacement of the pile top produces a deviation from the side of the foundation pit relative to the initial value, and then, as the excavation depth increases, the displacement gradually changes from a negative value to a positive value and increases continuously. This phenomenon indicates that the prestressed anchor cables of the prestressed and strongly constrained rear row piles exert a certain anchorage effect on the front row piles. The tension is transmitted to the front row pile top through the coupling beam. Therefore, the displacement change process of the front row pile top gradually converts from a negative value to a positive value, and the final displacement is smaller.
After the monitoring was complete, the horizontal displacement of the pile body was as shown in Figure
The comparison of horizontal displacement for three front row piles.
The variation of strain at the end of the excavation and at the end of monitoring is shown in Figure
Variation of strain for front row piles. (a) Conventional pile. (b) Recycling assembled pile. (c) Prestressed strong-constrained pile.
The strain distributions of the three types of piles in the front row are basically the same, and the effect of pulling and pressing on the front and rear sides of the pile is obvious. The maximum strain is in the middle of the pile, which is about 6 m below the pile top. It shows that the binding effect of the crown beam and soil is obvious. The front row pile above the excavation face is subjected to tensile stress before the pile and compressive stress behind the pile, resulting in tensile deformation and compressive deformation; however, the tension and compression below the excavation face are the opposite. Due to the prestressing of the strong-constrained rear row pile anchors, the strain of the prestressed strong-constrained front row piles is less than that of the other two types of piles, especially at the 0–3 m position of the upper part of the pile where the effect is more pronounced.
It can be seen from Figure
Comparison curve of bending moment.
In order to compare the deformation of the prestressed strong-constrained and conventional rear piles, strain gauges were laid at the same location of the two piles, 2 m below the top of the piles, away from the foundation pit, and the results were compared. According to the results presented in Figure
Comparison of strain at 2 m below the back row pile top.
Before excavation of the foundation pits, initial strain values were collected, and prestressing was then applied to the prestressed strong-constrained rear row pile anchors. Before the first excavation, due to the effect of prestressing, the strain value continues to decrease, indicating that the rear row piles have increased pressure and that the transmission of prestress has a certain time effect; the stress gradually is balanced thereafter. After the first excavation, due to the earth pressure generated by soil unloading, the strain value gradually increases, that is, the prestressed strong-constrained rear row piles change from compression to tension. After each excavation of the foundation pit, the strain value has a certain abrupt change, and the strain value increases gradually, which means that each excavation unloading of the soil exerts a significant tensile stress on the rear pile body. The ultimate strain value of the prestressed strong-constrained rear row pile is
The horizontal displacement of piles in the prestressed strong-constrained double-row piles and recycling assembled double-row piles supporting structures is basically the same as that of the conventional ones. After the foundation pit excavation is completed, the maximum horizontal displacement point appears in the upper part of the pile about 2-3 m. The displacement of the prestressed strong-constrained type in each step of excavation is smaller than that of the conventional and recycling assembled types, and the overall horizontal displacement change trend between the conventional and recycling assembled types is basically the same. For the three types of supporting structure, the horizontal displacement of the front pile top is larger than the rear row pile, which explains that the influence of the active earth pressure on the front row pile is more obvious under the unloading of the foundation pit. The horizontal displacement values of the prestressed strong-constrained front and rear row piles are all smaller than the conventional and recycling assembled ones, indicating that the prestressed strong-constrained type has a better restraining effect on the front and rear rows of pile tops. Recycling assembled piles ensure that the displacement is similar to that of the conventional piles. Steel members are used to connect rear row piles and front row piles, which effectively reduces construction time and speeds up the construction process. After the foundation pits are backfilled, steel members can be recycled to save costs. Due to the existence of prestressed anchor cables in the rear row piles, the rear row piles of the prestressed strong-constrained supporting structure can provide better tensile performance. At the same time, the pulling anchor effect of prestressed anchor cable can restrain the displacement and deformation of prestressed strong-constrained front row piles, and the restraint effect on the top of the front row pile is especially obvious. It shows that the prestressed strong-constrained support structure has better deformation coordination ability.
The data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest.
The research was supported by the Fundamental Scientific Research Business Expenses of Provincial Universities in Hebei Province (JQN2020027) and North China University of Science and Technology Doctoral Research Startup Fund (BS201813).