A Full-Scale Field Study on Bearing Characteristics of Cast-in-Place Piles with Different Hole-Forming Methods in Loess Area
This paper presents the results from a full-scale field study on the 3 different types of cast-in-place piles: rotary drilling piles (RDPs), manual digging piles (MDPs), and impact drilling piles (IDPs), for a bridge construction project of Wuqi–Dingbian Expressway, in Shaanxi. The results indicate that under the similar conditions, MDP exhibits the largest bearing capacity (11000 kN) in the loess area, followed by RDP (9000 kN) and IDP (8000 kN). And all tested values exceed the estimated value (7797.9 KN), indicating that the calculation formula of bearing capacity recommended by the Chinese standard is safe and conservative. During the load transfer process, the axial force attenuation rate of the pile body increases with pile side resistance. The average attenuation rate of MDP is the largest (24.2%), followed by RDP (19.72%) and IDP (16.69%). The bearing characteristics of these test piles are mainly pile side resistance, but the manual digging method created the least amount of disturbance to the soil around the pile, and due to its hole wall being rough, this enhances the pile-soil interactions. Hole-forming methods mainly affect the exertion of pile side resistance compared with pile end resistance. In view of pile side resistance and pile end resistance not taking effect at the same time, degree of exertion of these 2 resistances should be considered when designing cast-in-place piles in loess areas, and different partial coefficients should be used.
Pile foundations have been developed for thousands of years since the discovery of early pile foundation sites in the Republic of Chile. Up to now, pile foundations are also the most widely used building foundations or supporting structures [1–4]. When surface soils are too loose (soft) to support the shallow foundation safely and economically, such geotechnical structures can be used to better distribute the loads through the soil (friction piles) or transmit loads to a stronger soil layer at depths (end-bearing piles) [5–7]. As one of the most representative pile forms, cast-in-place piles are widely used in bridge and other engineering fields because of their great advantages (moderate cost, convenient construction, low construction noise, etc.) [8–12].
Loess is widely distributed in Asia (74°N-32°N, based on data from Baidu Encyclopedia), especially in the central and western regions of China, where there is continuous development of China's economy and infrastructures in the promotion of the Belt and Road program; a large number of transportation networks are being built in the loess area, and cast-in-place piles will be widely used in this process. According to incomplete statistics, in China alone, the annual use of cast-in-place piles are at least 1 million or more [13–17]. Loess has strong structural characteristics. The construction of cast-in-place piles in the loess strata will inevitably destroy the structure of loess, which will affect the mechanical properties of loess and pile-soil relationship [18–21]. There are many research studies on the hole-forming methods and loess in the cast-in-place pile foundation engineering, but the influences of hole-forming methods on the bearing capacity of pile foundation are not universal [22–27]. Meanwhile, the influence of hole-forming methods on the bearing capacity of the pile foundation is also related to geological conditions, foundation forms, etc. Therefore, the influences of hole-forming methods on bearing characteristics of cast-in-place piles in the loess are uncertain and need further research [28–30].
Presently, the cast-in-place piles for bridge engineering in the loess area mainly include rotary drilling piles (RDPs), manual digging piles (MDPs), and impact drilling piles (IDPs) [31–34]. There are obvious differences between the piles with different hole-forming methods (as shown in Table 1) [35–38]. Many researchers [39–41] have analyzed the characteristics of these piles for different purposes; however, very few studies in the literature offer the side-by-side comparison. Based on this, combined with the actual situation of Wuqi–Dingbian Expressway test area, static load tests of RDP, MDP, and IDP have been carried out. The influence of the hole-forming methods on the bearing characteristics of the cast-in-place piles is analyzed from the following aspects: (1) pile body settlement; (2) transfer law of axial force for the pile body; (3) distribution of the pile side resistance; and (4) exertion degree of pile end resistance.
2. Structural Characteristics of Loess
Structural characteristics are the intrinsic characteristic of soil. The structure of soil essentially includes the cementation and the composition (as shown in Figure 1). The former reflects the characteristics of soil skeleton connection while the latter reflects the geometrical and spatial characteristics of soil skeleton. Many research studies show that loess has strong structural characteristics [43–45]. Loess deformation is mainly elastic before its original microstructure is destroyed under certain disturbance, and its pore pressure is also elastic pore pressure, which has no effect on the effective stress of soil skeleton, so the strength of soil will not change greatly [46, 47]. Once the structure of the soil has been destroyed, it will cause a series of adverse effects (such as the generation of plastic pore pressure, the decrease of effective stress, and the deterioration of mechanical properties). The construction of piles in loess will inevitably cause great disturbance and destroy the structure of loess, which will affect the mechanical properties of loess and the pile-soil relationship.
3. Project Site and Subsoil Profile
Wuqi–Dingbian Expressway (Figure 2) is located in Yan’an city and Yulin city, Shaanxi Province, China. The starting point of the expressway is located in Zoumatai, east of Wuqi County and ends in Shijingzi, southeast of Dingbian County. The expressway has an approximately total length of 92.22 km and is an important part of the 3rd vertical Dingbian to Hanzhong expressway in the network “2637” planned by Shaanxi Province. The abutments on both sides of the test area are located on the loess Liang-Mao region (the loess hilly area can be divided into 2 types according to its shape: the long strip is called “Liang” and the oval or round shape is called “Mao” © Baidu Encyclopedia), and the topographic relief of the abutment area is small. The ground elevation is 1629.60–1644.59 m with a relative height difference about 14.99 m.
The groundwater in the test area is deeply buried, and there is no groundwater distribution in the depth of drill hole (the depth of drilling is 60 m). According to the drilling results, soils in the test area are all in slightly wet and hard plastic state. Table 2 is the detailed characteristics of the soil in drill hole sampling.
4. Static Load Test
4.1. Bearing Capacity Estimation
To ensure the safety and reliability of the test, the bearing capacity of the pile (as shown in Table 3) was estimated by using formula (1) recommended in Chinese standard  before the test:where is the total pile side resistance; is the total pile end resistance; μ is the perimeter of pile body; is the pile side resistance of the i-layer soil around the pile (the friction resistance is the pile side resistance in friction piles); is the specific penetration resistance near the pile end, and its value can be directly determined in the standard , according to parameters (such as the soil properties, the diameter of pile, and the length of pile); li is the length of the pile passing through each layer of soil; is the correction coefficient of pile end resistance (when the pile length is between 10 and 30, is interpolated between 0.75 and 0.90 according to the standard); and is the pile end area.
4.2. Test Design and Procedure
From Figure 3, it is noted that the roughness of every pile wall is different due to the different hole-forming methods. Compared with impact drilling method, the other 2 methods have less disturbance to the soil around the pile, and spade and rotary drilling rig continuously cut the soil in the hole, resulting in rough hole wall. And compared with IDP, the hole wall of MDP is rougher.
Referring to the Chinese standard , the on-site static load test (Figure 4) was carried out. The results of the test for 3 different types of cast-in-place piles (MDP, RDP, and IDP) have been compared and analyzed. The pile diameter and length of the 3 test piles were all 1.5 m and 25.0 m. The diameter and the length of the anchor pile (AP) were 1.5 m and 30.0 m separately. The test piles were constructed with C30 concrete, and C40 concrete was used to reinforce the part 1.5 m away from the top of the piles. The anchor piles (APs) were all RDP, and all of them were cast by C30 concrete. The rebar used for the test piles was configured according to standard . Meanwhile, 3 rows of rebar strain gauges were installed on the sides of the piles, and 7 rebar strain gauges were arranged in sequence along the pile at 0.5 m, 3.5 m, 6.5 m, 11 m, 15.5 m, 20.0 m, and 24.5 m (Figure 5). The distance between 2 adjacent anchor piles was 7.0 m, and the test piles were arranged in the center of the adjacent 4 anchor piles. The rebar used in the anchor piles was also configured according to the standard . To fully study the bearing characteristics of test piles with different hole-forming methods, all 3 test piles were loaded to failure in field tests.
5. Analysis of the Test Results
5.1. Ultimate Bearing Capacity of Single Pile
The Q-S curves of the 3 test piles, as shown in Figure 6, are overall steeply descending. And according to the standard , for such steeply descending Q-S curve, the inflection point is the ultimate bearing capacity. Therefore, the ultimate bearing capacities of MDP, RDP, and IDP are 11000 kN, 9000 kN, and 8000 kN, respectively. The corresponding settlements are 10.89 mm, 7.22 mm, and 3.35 mm. The bearing capacities of the 3 test piles obtained from the test are all larger than the estimated value (7797.9 kN), indicating that the bearing capacity estimated by the recommended formula (formula (1)) of the Chinese standard is safe and conservative. However, the bearing capacities of these piles are quite different, which further illustrates that different hole-forming methods will have different effects on the improvement of bearing capacity of cast-in-place piles.
The maximum load applied on the MDP, RDP, and IDP is 12000 kN, 12000 kN, and 14000 kN, respectively. Meanwhile, the corresponding maximum settlements are 77.49 mm, 72.86 mm, and 63.9 mm. With the removal of the upper load, the final settlement values of MDP, RDP, and IDP are 63.58 mm, 63.72 mm, and 55.51 mm, where the pile body rebound displacement of MDP is the largest (13.91 mm), followed by RDP (9.15 mm) and IDP (8.14 mm). In the process of unloading on the pile top, the Q-S curves have been shown as a gentle rebound curve (Figure 6), indicating that the rebound value has a close relationship with the load applied on the pile top. The rebounds of settlement are mainly composed of elastic compression of the pile, followed by the back friction caused by the restoration of soil structure during the dissipation of frictional forces between piles and soils [50–53].
5.2. Transfer and Attenuation of Axial Force
By analysis of the field test data, the distribution of axial forces for MDP, RDP, and IDP is shown in Figures 7–9, respectively. Along the pile body, the regularities of distribution for these test piles are basically the same, which gradually decrease from top to bottom. And under the similar load, pile end resistance of IDP is the largest, followed by RDP and MDP. The results suggest that IDP has the best effect of pile end resistance, while, for MDP, pile side resistance exerts the best effect.
Under the action of axial load, the pile top will generate axial displacement (settlement), which is the sum of elastic compression of pile body and soil compression of pile end. When the pile moves downward relative to soil, pile side frictional resistance acting upward on the pile will generate. In the process of load transferring downward along the pile body, the friction must be overcome continuously, which leads to the decrease of the axial force of the pile body with the increasing strata depth. When the axial force is transferred to the pile end, the axial force will be balanced by the pile end soil support. The axial force attenuation rate represents the effect of pile side resistance, and the effect of pile side resistance increases with the axial force attenuation rate.
The attenuation rates of the axial forces of these piles are approximately the same. It is assumed that the axial force attenuation rate of the test pile is :where is the axial force on the i section of pile body; is the axial force on the section of pile body; and is the number of rebar meter.
Formula (2) can be used to calculate the axial force attenuation rates of these test piles, as shown in Figure 10. The variations of the axial force attenuation rates for these test piles are basically the same under all levels of load: all decreasing firstly, then increasing and decreasing lastly. The maximum, minimum, and average attenuation rates of axial force for MDP, RDP, and IDP are 26.13%, 21.22%, and 18.4%; 20.5%, 17.22%, and 14.53%; and 24.2%, 19.72%, and 16.69%, respectively.
5.3. Pile Side Friction Resistance Distribution
5.3.1. Distribution along the Pile Body
Under the similar load conditions, pile side resistances of these test piles level vary in different sections of their pile bodies (Figures 11–13). However, along their pile bodies, these pile side resistances’ variations are roughly the same: all increase firstly and then decrease. Pile side resistances of all test piles reach the peak at the section of 11 m from the top of the pile and then the curve of IDP reaches the second peak at the bottom of the pile body. The peaks of these curves are related to the characteristics of the soil around the pile and pile-soil interactions [54–57].
As shown in Figure 14, when the load applied to the pile top is same, pile side resistance of MDP is the largest, followed by RDP and IDP. The result suggests that the concrete protection wall of MDP is conducive for the development of pile side resistance and enhancement of pile-soil interactions. Meanwhile, IDP will form a 2–3 mm thick mud cake around the pile during the construction process, which reduces the pile-soil interactions and is not conducive to the exertion of pile side resistance.
5.3.2. The Relationship between Pile Side Resistance and Pile-Soil Relative Displacement
As shown in Figure 15, when the pile-soil relative displacements are less than 5 mm, pile side resistances of these test piles increase sharply with the displacements; however, as the relative displacements continue to increase, pile side resistances of these test piles increase slowly. When the relative displacements are the same, the MDP has the largest pile side resistance, followed by RDP and IDP.
5.4. Effect of Pile End Resistance
5.4.1. Pile End Resistance under Various Loads
From Figure 16, the pile end resistances of these test piles increase with the upper loads. And under the similar load, IDP’s pile end resistance is the largest, followed by RDP and MDP. This indicates that during the process of applying load, the ratio of pile end resistance for IDP is larger than the other 2 piles in the total resistance, and pile end resistance has the greatest impact on IDP.
5.4.2. Pile End Resistance under Ultimate Bearing Capacity
From the static load test, all the pile side resistances and pile end resistances of these test piles in the ultimate bearing capacity state are obtained, respectively. As illustrated in Table 4, when every test pile reaches the ultimate bearing capacity, the ratios of pile side resistance to pile end resistance are not the same. MDP has the largest ratio of pile side resistance in total resistance, followed by RDP and IDP, while the ratio of pile end resistance is on the contrary. Although the bearing characteristics of these 3 test piles are mainly depending on the pile side resistance, the degree of exertion of pile side resistance and pile end resistance in the 3 test piles is not the same.
5.4.3. The Change Law of Pile End Resistance with Pile Settlement
Due to the very small compression deformation of the test piles, the settlement of pile top is used as the settlement of piles in this paper. From Figure 17, it is noted that when the settlements of the 3 test piles are the same, pile end resistance of IDP is the largest, followed by RDP and MDP. With the increase of the pile settlements, the pile end resistances of 3 test piles increase continuously. And among them, pile end resistance of IDP increases the most, but pile end resistances of the 3 test piles do not tend to a definite value. It shows that pile end resistance does not reach the limit value during the process of applying the load at the pile top. When the settlements of piles are less than 5 mm, the differences in pile end resistances of the 3 test piles are small. However, when the pile settlements are larger than 10 mm, especially larger than 15 mm, pile end resistance of IDP is significantly larger than that of RDP and MDP when the pile settlements are the same.
5.5. The Relationship between Pile End Resistance and Pile Side Resistance
For 3 test piles under different loads, pile side resistances and pile end resistances are all shown in Figure 18, and the ratios of every single resistance to the total resistance are shown in Figure 19. From Figure 18, it is noted that when the loads are applied at the top of every test pile, the line graphs of pile side resistance and pile end resistance both show an ascending trend. And the continuous increase of the pile side resistance does not tend to a definite value, which suggests that pile side resistance does not reach the limit value after the first loading. In Section 5.1, the ultimate bearing capacity of MDP is 11000 kN, followed by RDP (9000 kN) and IDP (8000 kN), indicating that the pile side resistances of all test piles are not at the maximum state when the ultimate bearing capacity is reached.
From Figure 19, (1) pile side resistance ratio of MDP is the largest, while the ratio of pile end resistance is the smallest. (2) Pile side resistance ratio of IDP is the smallest, while the ratio of pile end resistance is the largest. But the variations of the 3 test piles are basically the same. At the later loading, pile side resistance ratios of these piles all show a decreasing trend, and the ratios of pile end resistance show an increasing trend, but the change trend is not obvious. The results suggest that the 3 test piles in this filed test are all friction piles, and pile side resistances and pile end resistances are not in the limit state at the last loading. During the early loading on the pile top, especially during the first loading and the second loading, the ratios of pile side resistances of the 3 test piles decrease, but the ratios of pile end resistances increase. The result indicates the complexity of the mechanical properties of the soil and the development of pile side resistance requires a process. Therefore, when designing cast-in-place piles in loess areas, it is necessary to consider the development degree of pile side resistance and pile end resistance and then use the different partial coefficients in pile side resistance and pile end resistance [58–63].
Through the full-scale field study, bearing characteristics of the 3 types of cast-in-place piles (MDP, RDP, and IDP) under different loads in loess area are studied in this paper. Based on the analysis of test results, the following conclusions can be drawn:(1)Under the similar conditions, the ultimate bearing capacity of MDP is the largest (11000 kN), followed by RDP (9000 kN) and IDP (8000 kN). All actual bearing capacities are larger than the estimated value (7797.9 kN), which indicates that the pile bearing capacity estimated by the standard is conservative. The final settlement values of MDP, RDP, and IDP are 63.58 mm, 63.72 mm, and 55.51 mm; after unloading, the pile body rebound displacement of MDP is the largest (13.91 mm), followed by RDP (9.15 mm) and IDP (8.14 mm).(2)The attenuation rates of axial force for piles (MDP, RDP, and IDP) are basically the same under the action of all levels of loading, all decreasing firstly and increasing finally. Among them, the rate of MDP is the largest (average is 24.2%), followed by RDP (average is 19.72%) and IDP (average is 16.69%).(3)The bearing capacity of all 3 test piles is mainly dependent on pile side resistances in loess area. Pile side resistance is not evenly distributed along the pile body, which is related to properties of soil around piles and pile-soil interactions. Under the same loads, pile side resistance of MDP is the largest, followed by RDP and IDP, while pile end resistances of the 3 test piles are opposite.(4)Hole-forming methods mainly influence the roughness of hole wall in loess area, which determines the value of pile side resistance. Compared with the impact drilling method, the other 2 methods (manual digging method and rotary drilling), especially manual digging method, have less disturbance to soil around the pile, resulting in rough hole wall and enhancing the pile-soil interactions.(5)Different partial coefficients should be adopted rationally for pile side resistance and pile end resistance when designing cast-in-place piles in the loess area because pile side resistance and pile end resistance are not taking effect at the same time.
Data supporting this research article are available from the corresponding author on request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the financial support by the National Key R&D problem of China (no. 2018YFC0808606) and the Project on Social Development of Shaanxi Provincial Science nos. 2018SF-378 and 2018SF-382. The authors also sincerely thank Rodney Sheldon Garnes who is a native speaker graduated from the University of the West Indies for his help in the language modification of this paper.
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