J. Cent. South Univ. (2020) 27: 1861-1869

DOI: https://doi.org/10.1007/s11771-020-4413-z

Model test on vertical bearing capacity of X-section concrete pile raft foundation in silica sand

XU Lai(徐来)¹, PENG Yu(彭宇)^{2, 3}, DING Xuan-ming(丁选明)^{2, 3}, LIU Jia-yi(刘家易)⁴

1. Guangdong Zhonggong Architectural Design Institute Co., Ltd., Guangzhou 510670, China;

2. College of Civil Engineering, Chongqing University, Chongqing 400045, China;

3. Key Laboratory of New Technology for Construction of Cities in Mountain Area, Chongqing University,Chongqing 400045, China;

4. CCTEG Chongqing Engineering Co., Ltd, Geotechnical Engineering Institute, Chongqing 400042, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: To reveal the bearing capacity of the X-section concrete piles pile raft foundation in silica sand, a series of vertical load tests are carried out. The X-section concrete piles are compared with circular section pile with the same section area. The load-settlement curves, axial force and skin friction, strain on concave and convex edge of the pile, pile-sand stress ratio, distributions of side and tip resistance are presented. The results show that bearing capacity of the X section concrete pile raft foundation is much larger than that of the circular pile raft foundation. Besides, compared with the circular pile, the peak axial force of X-section piles under raft is deeper and smaller while the neutral point of X-section concrete pile is deeper. Moreover, the strain on the concave edge is much larger than that on the convex edge of the pile, and the convex edge has more potential in bearing capacity as the vertical load increases. The X-section pile has higher pile-sand stress ratios and load-sharing between side resistance and tip resistance. Above all, the X-section concrete pile can significantly increase the bearing capacity of pile-raft foundations in silica sand.

Key words: X-section pile; pile raft foundation; model test; neutral point; pile-soil stress ratio

Cite this article as: XU Lai, PENG Yu, DING Xuan-ming, LIU Jia-yi. Model test on vertical bearing capacity of X-section concrete pile raft foundation in silica sand [J]. Journal of Central South University, 2020, 27(6): 1861-1869. DOI: https://doi.org/10.1007/s11771-020-4413-z.

1 Introduction

Pile technology has been widely used in ground improvement [1-4], and many special shaped piles are commonly used in geotechnical engineering, such as the rectangular pile, H-section, L-section, Y-section pile, I-section, barrette, tapered pile, belled pile, squeezed branch pile, and pipe pile [5-9]. These unconventional section piles are useful in improving the side friction of piles.

With the construction of infrastructure in China, the requirement on low carbon and harmonious development between man and nature has been emphasized [10, 11]. The X-section concrete pile is a kind of energy saving and emission-reducing pile foundation technology [12, 13], changing the cross-section of piles. The new foundation technology decreases the consumption of concrete, thus reduces the energy consumption in the production of the pile, Therefore, X-section concrete pile composite foundation has been widely used in high-speed railway and other projects of soft foundation reinforcement.

Due to the particularity of X-section concrete pile, the bearing capacity of X-section concrete pile is more complex than that of a circular concrete pile. Therefore, in the past, researchers have carried out a series of tests to study the load transfer mechanism of X section concrete piles [2, 13-15]. The effect of cross section form on the distribution of side friction and tip resistance of the pile is revealed. Full-scale model tests of single piles with X-section concrete pile have been carried out under the vertical static load [16-18]. It is found that the bearing capacity of the X-shaped pile increases by 24% compared with that of the circular concrete pile under the condition of equal cross-sectional area. The full-scale model test of a single pile with X-section concrete pile found that under the same vertical load, compared with circular concrete pile with equal cross-sectional area, X-section concrete pile has better flexural and horizontal load resistance [19]. The uplift resistances of X-section concrete pile have been studied by means of full-scale model test and finite element analysis [14, 20]. The results show that X-section concrete piles have a higher anti-drawing capacity, under the action of upward pull-out force, and there is no large stress concentration in the pile. Furthermore, the property and vertical load transfer mechanism of X-section single pile composite foundation has been studied [7, 21]. The results show that the pile-soil load sharing of X section concrete pile is more reasonable than that of the circular concrete pile.

The above studies mainly focus on the mechanical characteristics of single piles and composite foundation of the pile group, which promotes the in-depth study of cast-in-place X-section concrete pile. However, due to the interaction among pile body, raft plate and surrounding sand, and the vertical load transfer mechanism of X-section, concrete pile raft foundation is quite different from that of ordinary pile foundation. In this study, the model tests of circular concrete pile, single pile and group pile raft foundation are carried out, and the vertical load transfer mechanism of X-section concrete pile raft foundation is revealed according to the strain law of concave arc section and convex arc section of section piles. The study can be used as a reference for the design of X-section concrete pile raft foundation.

2 Model tests

2.1 Model test equipment

The model test apparatus consists of a model tank, a loading system and a measuring system. The layout of piles and apparatus is displayed in Figure 1(a). The size of the model tank is 1 m×0.8 m×0.8 m (length×width×height). The loading system includes a hydraulic pressure pump, a hydraulic jack, and a reaction beam. Figure 1(b) provides the size of model tank and the layout of piles. The measuring system in this test consists a data acquisition system, dial indicators and strain gauges that were pasted on the side of piles. The error of the force applied by the jack is 3%, and the accuracy is 0.01 mm for dial gages. Before the loading of piles, a raft was placed on top of piles. The settlement of piles was measured by the dial indicator and the vertical strain along piles was recorded by the data acquisition system.

Figure 1 Schematic diagrams of experimental setup under vertical load:(Unit: cm)

2.2 Model pile and sand

The section of X model pile is shown in Figure 2. The length of X model pile was 700 mm and the outer diameter was 50 mm. The C32.5 concrete (the mass ratio of cement: sand: stone: water=1: 1.11: 2.72: 0.38) and steel reinforcement cages were used. The steel reinforcement cage consisted of four 12# steel wires and 18# steel wires at 20 stirrups along the pile model. The uniformity coefficient (C_u) and curvature coefficient (C_c) of the silica sand in the model tank were 1.22 and 0.97, respectively. Concrete piles were prefabricated in specific PVC containers. The sand layers were filled in the container by controlling the relative density of about 70%. Sand layers in the model container were filled and compacted artificially using sand- rain method, and sand density was checked by sampling tests. The buried depth of pile was 700 mm. To find the compressibility of silica sand, the settlement of raft has been tested. The vertical load-settlement curve is in a gradual shape under the vertical load, and the calculated compressive modulus E_s is about 19 MPa. Basic parameters of the silica sand used are shown in Table 1. Slow maintain load method was used in this model’s test work. Each vertical load grade was 1/10 of the estimated total vertical load, and the loading time of each loading was about half an hour. The preload was 15% of the estimated maximum vertical load, and the preloading time was 12 h. Load test termination criteria were according to MANIAM [22].

Figure 2 Cross section of X model pile (Unit: mm)

Table 1 Parameters of silica sand used in this model test

3 Model test results and discussion

3.1 P-S curves of circular and X-section concrete pile raft foundation

Relationships between the vertical load and the displacement (P-S curves) for single piles are shown in Figure 3. The increase of displacement is nearly linear as the vertical load increases when the load is smaller than 1.5 kN. The pile displacement increases rapidly when the vertical load is larger than 1.8 kN for the circular concrete pile and 2.1 kN for the X-section concrete pile, respectively. The bearing capacity of the X-section concrete pile is determined in the bi-logarithmic coordinate system. The vertical load capacity of X-section concrete pile is 2.1 kN, white it is 1.8 kN for the circular concrete pile. This result provides the bearing capacity of the X-section concrete pile is a little larger than that of the circular concrete pile when the cross-sectional area is the same.

Curves of the load-displacement for four-pile raft foundations are shown in Figure 4. The displacement increases with the increase of loading, and the load-settlement curve is in a gradual shape.

Figure 3 Vertical load-settlement graph of single pile

Based on the “80% criterion” [23], Table 2 shows that the ultimate load capacities of circular and X-section concrete pile raft foundation both are 15 kN; but the settlements of the circular and X-section concrete pile raft foundation are 13.5 and 5.7 mm, respectively. Based on the bi-logarithmic coordinate system [24], Figure 5 illustrates that vertical load of intersection point (2.54, 2.01) and (2.74, 1.86) is 12.7 and 15.5 kN, respectively; here, settlements of the circular and X-section concrete pile raft foundation are 7.46 and 6.42 mm, respectively. Based on the criterion of s/D₀=10% [26], the load capacities of circular and X-section concrete pile raft foundation are 9.02 and 13.25 kN, respectively. Based on the settlement increment/ load increment (△s/△Q) [26, 27], the breaking points in Figure 6 show that the ultimate load capacities of circular and X section concrete pile raft foundation both are 12 kN. Here, the settlements of the circular and X-section concrete pile raft foundation are 2.85 and 6.33 mm, respectively. The results show the bearing capacity of X-section concrete pile raft foundation is much larger than that of the circular concrete pile raft foundation.

Figure 4 Load-displacement curves of four-pile raft foundations

Table 2 Table of “80% criterion”

Figure 5 Load-settlement curves of four-pile raft foundations in bi-logarithmic coordinate system

Figure 6 Increment curves of load-settlement under different load

3.2 Influence of pile numbers on P-S curves

The pile numbers are controlled by the pile spacing because the area of raft is constant. The effects of the pile numbers on the P-S curves of X-section concrete pile group under 3D₀, 4D₀ and 6D₀ pile spacing are shown in Figure 7. Figure 7 illustrates that the displacement decreases with increasing pile numbers. This is because the pile-pile interaction is improved with the increasing pile numbers. At the same time, the efficiency of the X-section concrete pile raft foundation is larger than single raft efficiency obviously.

3.3 Axial force and skin friction

Figure 8 shows the axial force of the X-section concrete piles and the circular concrete piles. It is shown that the axial force distribution of the X-section concrete pile is similar to that of a conventional circular concrete pile foundation increasing first and then decreasing with depth. When the vertical load applied on the pile top is smaller than 9 kN, both the increasing and decreasing trends of axial force are flat; however, when the vertical load applied on the pile top is larger than 9 kN, the axial force rapidly changes with the increase of vertical load. The phenomenon indicates that the axial force of the X-section concrete pile plays an important role in the bearing capacity as the vertical load increases.

Figure 7 Load-displacement graph of various pile spacing

Figure 8 Distribution of axial force:

To show the difference of axial force distribution between circular concrete piles and X-section concrete piles, axial force distributions under the low vertical load of 6 kN and high vertical load of 15 kN are contrasted, respectively, as shown in Figure 9. When the vertical load is 6 kPa, the peak axial force on circular concrete piles emerging at a depth of 20 cm is 1.02 kN, accounting for 17% of the total vertical load; the peak axial force on X-section concrete piles emerging at a depth of 10 cm is 2.86 kN, which is 47.67% of the total vertical load. The peak axial force of X-section concrete piles under raft is deeper and smaller than that of circular concrete piles. The comparative distributions of axial force under the vertical load of 15 kPa are also displayed in Figure 9. The peak axial force on the X-section concrete pile emerging at a depth of 20 cm is 5.56 kN, accounting for 37.07% of the total vertical load; the peak axial force on circular concrete piles emerging at a depth of 10 cm is 6.10 kN, which is 40.67% of the total vertical load. The peak axial force of X-section concrete piles is only a little smaller than that of circular concrete piles.

Figure 9 Comparison of distribution of axial force under vertical load of 6 and 15 kN

Figure 10(a) provides the skin friction of X-section concrete pile, and Figure 10(b) provides the skin friction of circular concrete pile. The maximum negative skin friction of the X-model pile is 122.83 kPa, and the neutral point of the pile emerges at about 200 mm, which is 2/7 of the length of the X model pile. The maximum negative skin friction of circular concrete pile is 190.73 kPa, and the neutral point of the pile emerges at 150 mm, which is nearly 2/9 of the length of the X model pile. The results in Figure 10 indicate that the effect of skin friction becomes more important to bearing capacity with the increase of loading. The neutral point of X-section concrete pile is deeper than that of the circular concrete pile, which indicates that the skin friction of X-section concrete pile initiates later with the increase of vertical loading.

Figure 10 Distribution of skin friction:

3.4 Strain on concave and convex edge of X-section concrete pile

To show the strain difference of concave and convex edge on X-section concrete pile, strain distributions under various vertical loads are contrasted, respectively, as shown in Figure 11. The figures show that strains of both concave and convex edge on X-section concrete pile rise at the beginning and then reduce with depth. The peak strains on concave and convex edge both emerge at a depth of 20 cm. When the vertical load is smaller than 12 kN, the strains on concave and convex edge both increase sharply with the increase vertical load; however, when the vertical load on the pile top is larger than 12 kN, the strains on concave edge less increase with the increasing vertical load. The phenomenon indicates that the convex edge on the X-section concrete pile has more potential in bearing capacity as the vertical load increases. The reason can be explained that as compared with the concave edge, the convex edge is closer to the center of pile, and the motion tendency between pile and the surrounding sand is smaller; when the bearing capacity of concave edge gets to the largest value, further increased vertical loads can be borne by convex edges of the X-section concrete pile.

Figure 11 Pile strain on X-section concrete pile:

3.5 Pile-sand stress ratio and load-sharing

Figure 12 provides the load-sharing ratio between pile and silica sand. The load-sharing ratio of the silica sand increases with the increase of vertical loading, which indicates that more vertical load distributes to the silica sand with the increase of vertical loading. When the vertical loading increases to approximate the ultimate load, the load-sharing of pile and silica sand is nearly equal. Compared with the circular concrete pile, the X-section pile acts much later. The vertical load-sharing of pile shows that the pile plays a main role in bearing capacity of the pile-raft foundation constantly. The higher vertical load-sharing of X-section concrete pile indicates that the bearing capacity of the X-section concrete pile is better than that of the circular concrete pile due to the larger perimeter. The results indicate that the silica sand under the raft plays a more important role in bearing vertical load with the increase in vertical load. When raft has been pressed in silica sands, the sand under the raft is much denser than that under pile toe.

Figure 12 Load-sharing ratio for pile and silica sand under different vertical loads

Pile-sand stress ratio under the raft is one of the significant parameters to reflect the bearing characteristics of a pile foundation, affecting vertical load transfer and deformation mechanism under the raft. Tests in this study can reveal the pile-sand stress ratio of pile-raft foundations.Figure 13 provides the pile-sand stress ratio of the X-section concrete piles. The figures illustrate that the pile-sand stress ratio decreases with increased vertical load, coinciding with pile-sand load-sharing. The larger pile-sand stress ratio of the X-section concrete pile indicates that the X-section concrete pile is more efficient than the circular concrete pile due to sharing more vertical loads.

Figure 13 Pile-sand stress ratios of circular concrete pile and X section concrete pile

3.6 Distributions of side and tip resistance

The load-sharing between side resistance and tip resistance of the X-section concrete pile and circular concrete pile under pile-raft foundations are shown in Figure 14. The total vertical load of pile consists of side resistance and tip resistance, and the pile-bearing vertical load is developed by side resistance and tip resistance. The load-sharing of side resistance increases with the increase of loading, which indicates that more vertical load distributes to the side resistance with the increase of loading. When the vertical load increases to about 16 kN, the load-sharing of side resistance and tip resistance is nearly equal for the circular concrete pile. Comparing the circular concrete pile, the X-section concrete pile acts much later. With further loading, the tip resistance will play the dominant role on bearing loading, which would increase the settlement rapidly for the X-section concrete pile.

Figure 14 Load-sharing ratio of side resistance and tip resistance

However, the side resistance still plays the dominant role on bearing loading with further loading, which would not increase the settlement obviously. Therefore, the higher load-sharing of side resistance indicates that the bearing capacity of the X-section concrete pile is better than that of a circular concrete pile.

4 Conclusions

The bearing capacity of an X-section concrete pile-raft foundation has been studied on the basis of a series of model tests. By analyzing the results of the tests, the following conclusions can be obtained:

1) Bearing capacity of the X-section concrete pile is only a little larger than that of the circular concrete pile with the same cross-sectional area; however, by replacing circular concrete pile with X-section concrete pile, the increment of bearing capacity in X-section concrete pile raft foundation is much larger than that in single pile.

2) The efficiency of the X-section concrete pile raft foundation is much larger than single raft efficiency. The tip resistance of the X-section concrete pile plays an important role in bearing vertical load as the load increases. Moreover, the peak axial force of X-section concrete piles under raft is deeper and smaller than that of circular concrete piles.

3) The skin friction plays a more important role in bearing capacity with the increase of loading. The neutral point of X-section concrete pile is deeper than that of the circular concrete pile. The strain on concave edge of the X-section concrete pile is much larger than that on the convex edge, and the convex edge has more potential in bearing capacity as the vertical load increases.

4) Compared with circular concrete pile, the X-section concrete pile under raft has higher pile-sand stress ratio and higher load-sharing. The peak side resistance of the X-section concrete pile occurs later to get the ultimate load-sharing, which indicates that the bearing capacity of the X-section concrete pile is better.

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(Edited by FANG Jing-hua)

中文导读

石英砂地基中X截面混凝土桩筏基础的竖向承载力模型试验

摘要：为了揭示X截面桩筏基础在石英砂中的承载能力，将X截面桩与同截面积的圆形桩进行对比研究，开展了一系列模型试验。研究内容包括X截面桩筏基础的荷载-沉降曲线、轴力和摩阻力、X截面桩凹凸边缘的应变分析、桩-土应力比、桩侧阻力和端部承载力的分布。结果表明：X截面桩筏基础的承载力远远大于圆形桩筏基础的承载力。筏板下的X截面桩轴力峰值比圆形桩的轴力峰值小且位置更深；因而，中性点位置也比圆形桩更深。此外，X截面桩的凹缘处应变远大于凸缘处，且随着荷载的增大，凸缘处具有更大的承载力潜能。与圆截面桩相比，X截面桩具有更高的桩-土应力比，更大的侧阻力与端阻力荷载分担比。因此，X截面桩可以显著提高砂土中桩筏基础的承载力。

关键词：X截面桩；桩筏基础；模型试验；中性点；桩-土应力比

Foundation item: Project(51878103) supported by the National Natural Science Foundation of China; Project(2016YFE0200100) supported by the National Key Research and Development Program of China

Received date: 2020-01-20; Accepted date: 2020-04-10

Corresponding author: DING Xuan-ming, PhD, Professor; Tel: +86-13996171067; E-mail: dxmhhu@163.com; ORCID: 0000-0002- 3563-2222