Based on the characteristics of moso bamboo including high short-term strength, stable performance, and ability to provide temporary support for shallow foundation pits in soft soil, the stress characteristics and supporting effects of the ecological composite supporting system have been explored through model tests and numerical calculation analysis of the moso bamboo micropile-composite soil nailing structure. The results showed that the bamboo pile can effectively control the horizontal deformation of the side wall of the foundation pit and the ground surface settlement, achieving a relatively satisfactory supporting effect. Furthermore, the bamboo pile has visibly bent in middle and lower parts, where the regional shear point is most likely to appear, the axial force of the soil nail is distributed in an oval pattern with a smaller force on both sides and a larger force in the middle part, the maximum axial strain is 447.3
At present, the main materials used in the main part of the foundation support structure in China are cement and steel, but the production of these materials will consume lots of energy and produce much pollution. The support structure is usually temporary. After the completion of the foundation, the cost of disassembling the support structure is high, and it is troublesome to transport these materials. And these processes may affect the progress of the project. But discarding these materials in the soil will leave severe construction difficulties and hidden dangers for the construction of any nearby projects to be built in the future. The pile composite soil nailing structures have been widely investigated owing to their engineering applications. Through the combination of the single soil nail wall-supporting structure and the supporting pile structure, the advantages of the two supporting structures can complement each other, thereby effectively addressing soil sliding between support piles, deformation control of the soil nail wall support, and limited supporting depth. At present, moso bamboo soil nailing wall-supporting structures have been applied in engineering and achieved excellent supporting effect, but their reinforcement mechanisms, load transfer paths, and failure modes need to be further studied [
In recent years, scholars and engineering technicians all over the world have made more research on moso bamboo. Based on the research results of bamboo obtained in Brazilian universities and other institutes around the world, the first specification for bamboo was enacted to determine the physical and mechanical properties of bamboo. The results of the investigations have shown that bamboo can satisfactorily substitute steel. The structural elements developed and studied could be used in many building constructions [
Most of the research on moso bamboo focused on the test of small-sized bamboo pieces. Although there are many studies on the pile-soil nail wall composite support system, the use of moso bamboo in the support system is little. At present, there is no standard for testing the physical and mechanical properties of the whole round bamboo poles in China. Therefore, the use of reasonable support forms and environmentally friendly and energy-saving building materials is an urgent task to solve the current shallow foundation pit support project, and it is also a requirement for sustainable development construction. In this study, the model tests and numerical calculations of the moso bamboo micropile-composite soil nailing structure for wall support have been conducted. The bearing characteristics and supporting effect of the moso bamboo micropile-composite soil nailing system have been discussed in detail from the aspects of surface settlement, internal force and deformation of pile, pressure of the soil around pile, and the soil nail axial force of the supporting structure, to provide a reliable theoretical basis and analysis method for the application of moso bamboo composite-based supporting systems in soft soil shallow foundation pits.
According to the demands of Technical Specification for Retaining and Protecting of Building Foundation Excavation (JGJ 120-2012), the embedded depth of the supporting pile should be no less than 0.4 times the excavation depth of the foundation pit. It is assumed that the embedded depth ld of the bamboo pile is 2 m, the excavation depth is 4 m, and the ground additional loading
Distribution of earth pressure [
Active Earth pressure intensity:
Passive Earth pressure intensity:
Stability coefficient:
When the safety level is first class,
In this test, a steel model box with dimensions 5 m × 2 m × 2 m was used with a plastic film covering the inside to reduce the boundary effect and ensure the integrity of the artificially tamped foundation soil. The original excavation stratum was simulated by layering and tamping the silty clay, and tamping tests were conducted to make sure the number of tamping required has been reached. During the filling process, on-site sampling was conducted to test the volumetric weight and water content. After tamping, the volumetric weight of actual soil was 18.4 kN/m3, and the water content was 14.35%. The physical and mechanical properties of the soil sample are shown in Table
Physical and mechanical properties of the soil sample.
Soil layer name | Density (g/cm3) | Cohesion (kPa) | Friction angle (°) | Bulk modulus (MPa) | Shear modulus (MPa) | Poisson’s ratio |
---|---|---|---|---|---|---|
Silty clay | 1.84 | 13.14 | 22.3 | 5.62 | 1.94 | 0.35 |
The piles used in the model were bamboo piles with an outer diameter of 16 mm and an inner diameter of 12 mm. Strain gauges were attached to the opposite sides of each bamboo pile. The length of the pile was 600 mm, and its Young modulus was 15 GPa. The bamboo piles were numbered from 1 to 14, as shown in Figure
Layout of the foundation pit. (a) Pit plan. (b) Foundation pit profile.
Basic parameters of supporting materials.
Material name | |||||
---|---|---|---|---|---|
Bamboo pile | 12 | 16 | 2.0 | 15 | 0.22 |
Bamboo nail | 10 | 14 | 0.8 | 15 | 0.22 |
The basic similarity ratio of the model test takes the geometric length similarity ratio and the elastic modulus similarity ratio as
Similarity relationship between physical quantities (prototype/model).
Physical quantity | Area | Line loading | Concentration | Stress | Strain | Volumetric weight |
---|---|---|---|---|---|---|
Similarity ratio | 100 | 10 | 100 | 1 | 1 | 0.1 |
Resistance strain gauges were used to measure the deformation of the pile body. The dimensions of the strain gauge base were 7.3 mm × 4.1 mm, and the dimensions of the wire grid were 3.0 mm × 3.1 mm. The strain gauges were arranged on the opposite sides of the pile body every 0.1 m along the length of the pile. The strain gauges on the soil nail were arranged on the opposite sides of the soil nail along the lengths of 0.05, 0.2, and 0.35 m. After the strain gauge attachment was completed, epoxy resin was applied to prevent moisture. Consequently, the bamboo piles and soil nails were calibrated by preapplying an external force on the top and recording the strain value during the loading process with strain gauges.
Because the pile bodies were symmetrically arranged and loaded in the direction of the short side, the dial gauges needed to be arranged only on the top of piles #2, #4, and #7 to collect the displacement of the pile top. Displacement gauges were arranged on the short side of the foundation pit to monitor the foundation pit settlement changes in real time.
The earth pressure on the sides of the pile was measured using TXR-2030 strain-type miniature earth pressure gauge. When the soil was layered and filled, the pressure gauge was preburied in the vicinity of piles #2, #4, and #7 according to the excavation depth of the foundation pit and position of the bamboo pile. An earth pressure measurement point was arranged every 0.1 m along the length of the bamboo pile, as shown in Figure
Layout of measurement points. (a) Schematic diagram of monitoring system. (b) Schematic diagram of on-site monitoring.
This step was divided into three levels: the first level excavation was 0.15 m, then, the first row of soil nails was driven; the second level excavation was 0.15 m, and the second row of soil nails was driven; the third level excavation was 0.1 m, and the total depth of excavation was 0.4 m. Meanwhile, real-time data measurement was performed for each excavation level, and the data values were obtained after stabilized before proceeding to the next level of excavation, as shown in Figure
(a) Excavation and (b) loading of foundation pit.
The loading around the foundation pit was realized by stacking standard mass blocks (the dimensions of each mass block were 0.6 m × 0.2 m × 0.2 m, and the mass was 36 kg), and each level of loading was 3 kPa. Real-time data measurement was performed for each loading, and stabilized values were taken before proceeding to the next level of excavation, as shown in Figure
Considering that the influence of the boundary on the middle piles was small [
As shown in Figure
Distribution curves of stress on the back of the piles. (a) Excavation stage. (b) Loading stage.
As shown in Figure
According to the deformation rules of the pile and the monitoring data, the strain values of piles #2, #4, #5, #6, and #7 were selected for analysis to reduce the influence of the boundary effect. The change in the pile bending moment at each stage of excavation and deformation rules of the pile body in the step loading stage were studied.
The bending moment of each point of the micropile can be obtained by attaching strain gauges to both sides of the pile and using the bending theory calculation in material mechanics. The calculation formula is as follows:
As shown in Figure
Distribution curves of the bending moment of pile body: (a) Pile #2. (b) Pile #4. (c) Pile #5. (d) Pile #7.
To reveal the deformation of the bamboo pile in the loading process, with the symmetry of the supporting structure and loading taken into account, the strain data of supporting piles #2, #4, #5, and #7 were selected for analysis, and the curves of change in pile strain with burial depth were plotted. Figure
Distribution curves of strain on pile bodies. (a) Pile #2. (b) Pile #4. (c) Pile #5. (d) Pile #7.
According to “Technical Regulations for Building Foundation Pit Support” and “Technical Specifications for Foundation Pit Soil Nail Support,” the ratio of the length
The construction environment in the excavation stage is complex, and the collected data greatly fluctuates; thus, it is impossible to analyze the stress of the soil nail. Therefore, only the stress of the soil nail in the loading stage was analyzed in this section. Under the external loading, the soil nail is in a combined deformation state of bending and axial tension. In this study, the stacking method was used for loading, and the load value used was relatively small. The soil nail strain was measured by attaching strain gauges on both sides of the soil nail. The collected data is regarded as soil nail deformation caused by axial tension, and the bending strain of the soil nail was not considered. The upper and lower surface strains of the soil nail were
As shown in Figure
Axial strain distribution curves of the soil nail under (a) 3 kPa, (b) 6 kPa, and (c) 9 kPa.
As the loading level increased, the axial strain of each layer of soil nails showed an increasing trend. The axial strain increase rate of the soil nail near the surface layer was higher than that from the surface layer, and the axial strain of the soil nail on the upper layer was generally greater than the axial strain of the soil nail at the bottom of the foundation pit, indicating that the soil body on the upper part of the foundation pit was relatively more deformed under upper loading. Therefore, as the load increases, the axial force of the soil nail gradually increases. Among all parts of the soil nail, the middle part is obviously stretched. It shows that the soil nails can redistribute the stress of the soil, restrict and strengthen the surrounding soil, limit the deformation of the foundation pit, increase the strength of the foundation pit, and maintain the stability of the foundation pit.
In the test, the displacement variation data of support piles #2, #4, and #5 in each loading stage were collected, and the pile top displacement curves are plotted in Figure
Pile top displacement and settlement. (a) Pile top displacement. (b) Foundation pit settlement.
We deduce that the displacement of pile top and surface settlement are relatively small in the excavation process, unaffected by the step excavation but becomes larger in the loading process. With increased loading, the displacement continuously increases. At a certain stage, the displacement goes through a sudden increase and then gradually stabilizes. The reason lies in the internal damage of the foundation pit, which gradually develops from the shallow layer to the deep layer.
Figure
Foundation pit model.
Material parameters.
Name | Volumetric weight | Cohesive force | Internal friction angle | Poisson’s ratio | Elastic modulus | Element type | Constitutive model |
---|---|---|---|---|---|---|---|
Silty clay | 17.9 | 13.14 | 22.3 | 0.35 | 6.8 | Solid | Mohr-Coulomb |
Moso bamboo pipe pile | 20 | — | — | 0.22 | 1.5 × 104 | Beam | Elastic |
Moso bamboo soil nail | 8 | — | — | 0.22 | 1.5 × 104 | Cable | Elastic |
Connecting beam (C30) | 25 | — | — | 0.22 | 1.5 × 104 | Beam | Elastic |
Generate the initial mesh for foundation pit excavation, and establish the initial calculation model. Apply the displacement constraint boundary condition at the boundaries. Perform iterative calculation under the initial stress and pore water pressure conditions to reach initial stress balance. Construct the supporting structure. Excavate the first layer of soil, and construct the first row of soil nails. Excavate the second layer of soil, and construct the second row of soil nails. Excavate the third layer of soil until the bottom of the foundation pit is reached.
The analysis of the foundation pit displacement field mainly involves settlement analysis (vertical displacement), the horizontal displacement of slope top, and displacement of the side wall layer of the foundation pit in the excavation and step loading stages to summarize the activity pattern of the soil body in the foundation pit.
Distribution curves of foundation pit ground settlement. (a) Excavation stage. (b) Loading stage.
Displacement curves of the pit wall surface layer.
According to the horizontal displacement nephogram of the side wall of the foundation pit at each loading stage in Figure
Horizontal displacement nephogram of the side wall of the foundation pit at each loading stage. (a) 10 kPa, (b) 20 kPa, (c) 30 kPa, and (d) 40 kPa.
Figure
Pile stress distribution curves. (a) Pile #1, (b) pile #3, (c) pile #4, and (d) pile #6.
In the excavation stage, the soil nail in the middle of each row was selected as the research object, and the axial force distribution curves of the moso bamboo soil nail along its full length in different excavation stages were plotted, as shown in Figure
Soil nail axial force distribution curves in the excavation stage. (a) The first, (b) second, and (c) third rows of soil nails.
According to the nephogram of the axial force change of each row of soil nails in the loading stage, the deformation of the middle area of each layer of soil nails is obvious, and the deformation of the soil nails near the long side of the foundation pit is small and the soil nails in the long side direction are less affected by the loading, and the axial force changes are not obvious. As the loading increases, the axial force of each layer of soil nails continues to increase. The increase in magnitude and value of the axial force of the second layer of soil nails is greater than that of the first layer of soil nails. Similarly, the axial force of the third layer of soil nails is greater than that of the second layer soil nails; when loaded to 40 kPa, the maximum axial force is 62 kN. The numerical simulation results are opposite to the laboratory test results. The study shows that the foundation pit model in the model test is small, and the size effect is not obvious. Secondly, the upper loading is small, the sliding zone is formed late, or no sliding zone appears, so the upper region soil nailing is the first. When the force is contacted, the growth is faster, and the simulation is restored to the actual working conditions. As the loading increases, the sliding surface is gradually formed, and the axial force of the soil nail gradually increased. Therefore, it is necessary to strengthen the design and reinforcement of soil nails in the middle area in actual engineering projects. According to the stress and deformation pattern of the soil nails, the influence of different lengths, materials, incident angles, and sizes on soil reinforcement can be taken into account to realize the reinforcement of the foundation pit by divisions and blocks.
In this study, bamboo pipe is used as the main material of the supporting system. To study the working mechanism, stress state and deformation characteristics of the bamboo pile-anchor system in the foundation pit supporting project, indoor model tests, and corresponding numerical simulations were carried out. The main conclusions as follows: Earth pressure around the pile. During the excavation stage, the earth pressure behind the pile above the bottom of the foundation pit gradually decreases with the increase of the excavation depth and then stabilizes; below the bottom surface of the foundation pit, from the excavation to the end, the fluctuation of the earth pressure behind the pile is small. In the loading stage, the earth pressure is proportional to the loading. When the loading reaches a certain level, the earth pressure increases suddenly above the bottom of the foundation pit, and the pile-soil system is completely destroyed. Pile bending moment. The bending deformation of the pile body is similar to the cantilever beam. Above the excavation surface, the pile bending moment gradually increases with the increase in excavation depth, and the variation range is relatively large; during the loading process, the pile bending moment near the bottom of the foundation pit varies greatly, and the pile bending deformation is significant. The middle and lower parts of the bamboo pile are obviously bent; the bending moment transition point is on or near the excavation surface and gradually decreases with the increase in excavation depth. Soil nail axial force. In the early stage of excavation and support, the axial force of soil nail shows great randomness, which rises in the middle stage and fluctuates steadily in the later stage, reflecting the lack of order of foundation pit excavation. During the loading process, the axial strain of soil nail displays an oval rule with a smaller strain on both sides and a larger force in the middle, and the axial force distribution of each soil nail is proportional to the loading. The increase rate of the axial force in the middle area of the same row of soil nails is higher than that of the soil nails adjacent to the long side of the foundation pit, and the change value of the axial force of the upper layer is higher than that of the lower layer. Pile top displacement and settlement. In the excavation process, the variation in the surface settlement is not large; when the side of the foundation pit is loaded, the settlement and displacement of the pile top slowly increase with loading. When the loading reaches a certain level, the settlement is accelerated, and the local horizontal displacement of the side wall of the foundation pit is significant. According to the FLAC3D simulation analysis results, the moso bamboo pile composite supporting system improves the bearing capacity of the supporting structure and has good restraining capacity to control foundation pit settlement, pile top displacement, and horizontal movement of the surface layer. The numerical calculation results and related rules are consistent with the results of the indoor model test, further demonstrating the feasibility and suitability of the novel supporting system as soft soil foundation pits.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
The authors thank the workers, foremen, and safety coordinators of the main contractors for their participation. This work was supported by the National Natural Science Foundation of China (Grant nos. 41672308 and 51878554).