Experimental Study on the Effect of Pile-End Soil on the Pile Load Transfer Law
Experimental Study on the Effect of Pile-End Soil on the Pile Load Transfer Law
Zhang, Hangyu;Ma, Hailong
2022-06-22 00:00:00
applied sciences Article Experimental Study on the Effect of Pile-End Soil on the Pile Load Transfer Law Hangyu Zhang and Hailong Ma * School of Civil Engineering and Architecture, Zhejiang Sci-Tech University, Hangzhou 310018, China; zhywymail@126.com * Correspondence: mahailonglg@126.com; Tel.: +86-138-5803-5810 Abstract: This paper compares and analyzes the difference in the skin friction between pile-end soilless compressive pile and conventional compressive pile at various stages during loading by the in situ test method. The influence of pile-end soil on the load transfer law of compressive piles in clay-dominated stratified foundations is further investigated. The results show that the overall load–displacement curves of the pile-end soilless compressive pile and the conventional compressive pile both present a slow decline followed by a steep drop. The length of the linear section on the load–displacement curve of the pile-end soilless compressive pile is less than that of the linear stage of the conventional compressive pile. Under the vertical load, the distribution laws and distribution forms of the skin friction ratio of the pile sections of the two piles are more consistent. The pile-end soil of the conventional compressive pile restricts the skin friction of the pile’s middle-lower and lower pile segments when compared to the pile-end soilless compressive pile. This restriction manifests itself as a reduction in pile skin friction, and the weakening effect decreases from bottom to top. Keywords: compression pile; skin friction; load transfer law; weakening effect; in situ test 1. Introduction Citation: Zhang, H.; Ma, H. The working principle of pile foundations is that the pile foundation transfers the Experimental Study on the Effect of upper load to the soil around the pile and the soil at the bottom of the pile through contact Pile-End Soil on the Pile Load between the side and bottom of the pile and soil, thus ensuring that the building meets the Transfer Law. Appl. Sci. 2022, 12, 6347. requirements of foundation stability and the allowable amount of deformation. Therefore, https://doi.org/10.3390/app12136347 pile–soil interaction [1,2] has become a problem that must be considered in the design of Academic Editor: Chin Leo pile foundations. The pile transfers part of the load to the surrounding soil through the Received: 1 June 2022 frictional shearing action between the pile side and the soil, and the rest of the load is Accepted: 20 June 2022 transferred to the pile-end soil through the pile body. Pile section properties [3–5], the Published: 22 June 2022 pile–soil contact surface [6–8], and pile–soil shear mode [9–11] all affect the interaction between the pile and the surrounding soil. Publisher’s Note: MDPI stays neutral The single pile bearing capacity calculation of the existing pile foundation is to sim- with regard to jurisdictional claims in ply add the pile-end resistance and the skin friction; the default pile-end resistance and published maps and institutional affil- pile-side friction resistance do not affect each other. However, a large number of existing iations. research practices show that the pile-end soil affects the exertion of the skin friction, and the measured skin friction of a single pile varies with the strength of the pile-end soil or the pile-bearing stratum. Some studies [12–15] have shown that there is an interac- Copyright: © 2022 by the authors. tion between skin friction and pile-end resistance; it is mainly manifested in the local Licensee MDPI, Basel, Switzerland. strengthening effect of the skin friction at the pile-end. The effect of this strengthening This article is an open access article effect is related to the strength of the pile-end soil. The greater the strength of the soil distributed under the terms and at the pile-end, the more obvious the strengthening effect. Additionally, the mechanism conditions of the Creative Commons of the strengthening effect of skin friction is explained through Meyerhof’s [16] failure Attribution (CC BY) license (https:// mode of pile foundation. Zhang et al. [17] affirmed that the increase of soil strength at the creativecommons.org/licenses/by/ pile-end has a strengthening effect on the skin friction through the measured skin friction 4.0/). Appl. Sci. 2022, 12, 6347. https://doi.org/10.3390/app12136347 https://www.mdpi.com/journal/applsci Appl. Sci. 2022, 12, 6347 2 of 11 of the static load test of super-long piles and used the Mohr–Coulomb theory to analyze the effect of pile-end soil strength on skin friction. By quantifying the test results of the model test, Liu et al. [18] found that there is a relationship between the frictional shear strength of the pile side and the strength of the pile-end soil. Some studies pointed out that there is a weakening effect of the skin friction at the pile-end. Dong [19] and Xiong [20] pointed out that there is a weakening effect of the skin friction at the pile-end when the soil strength at the pile-end is low through the static load test of the grouted pile. The lower the strength of the soil at the end of the pile, the more obvious the weakening effect. Through numerical analysis, Ju et al. [21] found that when the water content of the sediment at the pile-end is large, excess pore water pressure will appear on the pile–soil contact surface near the pile-end, thereby reducing skin friction. Skin friction at the pile-end shows a weakening phenomenon. The existing research on the different performances of skin friction at the pile-end is mainly manifested as the strengthening effect and the weakening effect. The studies are based on the presence of pile-end soils, and there are few quantitative studies of the effect of the presence or absence of pile-end soil on the skin friction of pile. This test compares the load transfer laws of two pile types, pile-end soilless compressive pile and conventional compressive pile, in a layered foundation with mainly clay soil. Our aim was to study the load transfer mechanism under vertical load and the difference in the performance of the skin friction traits to analyze and discuss the quantitative effect of the presence of pile-end soil on skin friction. 2. Overview of In Situ Test 2.1. Geological Conditions of the Test Site The experimental site is located in Changshu City, Suzhou City, Jiangsu Province. The stratum from top to bottom is miscellaneous fill, silty clay, muddy–silty clay, clay, and silty clay. Of which the miscellaneous fill was removed before the start of the test. The soil layer parameters are shown in Table 1. Table 1. Parameters of each soil layer at the test site. Soil Thickness Soil Thickness Ultimate of the Pile-End of the Standard Value Soil Soilless Conventional Soil Layer of Shaft Layer Number Compressive Compressive Resistance Pile Pile q /kPa si h/m h/m 1 Miscellaneous fill - - - 2 Silty clay 1.0 0.9 25 3 Muddy silty clay 2.2 1.9 18 4 Clay 2.2 2.4 65 5 Silty clay 1.0 1.4 55 2.2. Test Pile Preparation and Loading Scheme To comparatively study the effect of pile-end soil on the skin friction of the pile, two test piles were used: a pile-end soilless compressive pile, and a conventional compressive pile. The depth of the two piles into the ground varied slightly due to the thickness of the stratum (Table 1). The length of the pile was controlled according to entering the bearing layer for clay at 0.6 m. The length of the pile-end soilless compressive pile was 3.8 m, and the length of the conventional compressive pile was 3.4 m. In this test, a steel pipe was used to develop the test pile. The elastic modulus of the pile was 206 GPa, the diameter of the pile was 108 mm, and the wall thickness of the pile was 4 mm. In order to obtain the strain at different depths of the pile section under load, the steel pipe was divided into four 0.8 m sections of steel pipe and one 1.4 m section of steel Appl. Sci. 2022, 12, x FOR PEER REVIEW 3 of 11 Appl. Sci. 2022, 12, x FOR PEER REVIEW In order to obtain the strain at different depths of the pile section under load3 of , the 11 s teel Appl. Sci. 2022, 12, 6347 3 of 11 pipe was divided into four 0.8 m sections of steel pipe and one 1.4 m section of steel pipe. After the strain gauge was attached to the inner side of the test pile, the steel pipe was welded. The In order str a to ob in g ta au ing the es were strain BF at di H- ff12 eren 0-3A t deA ptme hs of tal str the pa ile in sec gatuges, the size ion under load ,of the the steel sen si- pipe. After the strain gauge was attached to the inner side of the test pile, the steel pipe pipe was divided into four 0.8 m sections of steel pipe and one 1.4 m section of steel pipe. tive grid was 3 × 2 mm (length × width), the nominal resistance was 120 Ω, the sensitivity was welded. The strain gauges were BFH-120-3AA metal strain gauges, the size of the After the strain gauge was attached to the inner side of the test pile, the steel pipe was coefficient was 2.08 ± 1%, and the accuracy grade was A. Epoxy glue and silica gel were sensitive grid was 3 2 mm (length width), the nominal resistance was 120 W, the welded. The strain gauges were BFH-120-3AA metal strain gauges, the size of the sensi- used on the surface of the strain gauge to prevent the strain gauge from becoming damp sensitivity coefficient was 2.08 1%, and the accuracy grade was A. Epoxy glue and silica tive grid was 3 × 2 mm (length × width), the nominal resistance was 120 Ω, the sensitivity when exposed to water in the ground. The strain gauge arrangement is shown in Figure gel were used on the surface of the strain gauge to prevent the strain gauge from becoming coefficient was 2.08 ± 1%, and the accuracy grade was A. Epoxy glue and silica gel were 1. The d dampawhen ta acqu exposed isition of to water pile sin tra the in g gr aound. uges w The as con strain nected gaugeand c arrangement ollected by is shown the DH3 in 820 used on the surface of the strain gauge to prevent the strain gauge from becoming damp Figure 1. The data acquisition of pile strain gauges was connected and collected by the high-speed static strain data acquisition instrument. This high-speed static strain data ac- when exposed to water in the ground. The strain gauge arrangement is shown in Figure DH3820 high-speed static strain data acquisition instrument. This high-speed static strain quisition instrument had a sampling frequency of 100 Hz, which can accurately acquire 1. The data acquisition of pile strain gauges was connected and collected by the DH3820 data acquisition instrument had a sampling frequency of 100 Hz, which can accurately and record the slow change signal in the test to ensure the accuracy of the test acquisition high-speed static strain data acquisition instrument. This high-speed static strain data ac- acquire and record the slow change signal in the test to ensure the accuracy of the test data. quisition instrument had a sampling frequency of 100 Hz, which can accurately acquire acquisition data. and record the slow change signal in the test to ensure the accuracy of the test acquisition data. (a) (b) (a) (b) Figure 1. Schematic diagram of the position of the strain gauge in the test pile: (a) the pile-end soil- Figure 1. Schematic diagram of the position of the strain gauge in the test pile: (a) the pile-end soilless Figure 1. Schematic diagram of the position of the strain gauge in the test pile: (a) the pile-end soil- less compressive pile and (b) the conventional compressive pile. compr less com essive prespile sive pil ande( and b) the (bconventional ) the convention compr al com essive pres pile. sive pile. The pile tip of the pile-end soilless compressive pile was separated from the pile body, The pile tip of the pile-end soilless compressive pile was separated from the pile The pile tip of the pile-end soilless compressive pile was separated from the pile as shown in Figure 2. After the pile-end soilless compressive pile was pressed to the design body, as shown in Figure 2. After the pile-end soilless compressive pile was pressed to the body, as shown in Figure 2. After the pile-end soilless compressive pile was pressed to the position, it was then pulled up by 50 mm, and the pile tip was separated from the pile body design position, it was then pulled up by 50 mm, and the pile tip was separated from the design position, it was then pulled up by 50 mm, and the pile tip was separated from the to form a pile-end soilless compressive pile. The static load test was carried out after 28 d pile body to form a pile-end soilless compressive pile. The static load test was carried out pile body to form a pile-end soilless compressive pile. The static load test was carried out of pile construction, and the test was carried out by the slow maintenance load method [22]. after 28 d of pile construction, and the test was carried out by the slow maintenance load after 28 d of pile construction, and the test was carried out by the slow maintenance load Load–displacement curves and displacement–time logarithm curves were obtained. The method [22]. Load–displacement curves and displacement–time logarithm curves were method [22]. Load–displacement curves and displacement–time logarithm curves were site was loaded, as shown in Figure 3. Simultaneous recordings of the strain gauge readings obtained. The site was loaded, as shown in Figure 3. Simultaneous recordings of the strain obta wer ined. e carried The sout ite w during as load loading. ed, as shown in Figure 3. Simultaneous recordings of the strain gauge readings were carried out during loading. gauge readings were carried out during loading. Figure 2. Schematic diagram of the pile-end soilless compressive pile. Appl. Sci. 2022, 12, x FOR PEER REVIEW 4 of 11 Figure 2. Schematic diagram of the pile-end soilless compressive pile. Appl. Sci. 2022, 12, x FOR PEER REVIEW 4 of 11 Appl. Sci. 2022, 12, 6347 4 of 11 Figure 2. Schematic diagram of the pile-end soilless compressive pile. (a) (b) Figure 3. Schematic diagram of test pile loading: (a) the pile-end soilless compressive pile and (b) (a) (b) the conventional compressive pile. Figure 3. Schematic diagram of test pile loading: (a) the pile-end soilless compressive pile and (b) Figure 3. Schematic diagram of test pile loading: (a) the pile-end soilless compressive pile and (b) the the conventional compressive pile. conventional compressive pile. 3. Analysis of Test Results 3. Analysis of Test Results 3. Analysis of Test Results 3.1. Load Displacement Relationship 3.1. Load Displacement Relationship 3.1. Load Displacement Relationship According to the test results, the load–displacement curves (Q-s curve) of the pile- According to the test results, the load–displacement curves (Q-s curve) of the pile-end end soilless c Accordin ompressive p g to thile e te and st rethe co sults, the nventi load on –d al co isplacemen mpressiv t cu erves (Q pile are p -s cu lot rve) of th ted as shown e pile- soilless compressive pile and the conventional compressive pile are plotted as shown in end soilless compressive pile and the conventional compressive pile are plotted as shown in Figure 4. Figure 4. in Figure 4. Load Q/kN Load Q/kN Load Q/kN Load Q/kN 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 0 5 10 15 20 25 30 35 40 45 50 55 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 0 5 10 15 20 25 30 35 40 45 50 55 3 4 4 4 6 6 The pile-end soilless The convention The convention The pile-end soilless compressive pile compressive pile compressive pile compressive pile (a) (b) (a) (b) Figure 4. Q-s curve of test pile: (a) the pile-end soilless compressive pile and (b) the conventional Figure 4.com Q-s curv pressive e of test pi pile. le: (a) the pile-end soilless compressive pile and (b) the conventional Figure 4. Q-s curve of test pile: (a) the pile-end soilless compressive pile and (b) the conventional compressive pile. compressive pile. From Figure 4a, it can be seen that the Q-s curve of the pile-end soilless compressive Fr pile omshows a slow–ste Figure 4a, it can ep ty be seen pe. When that the the Q-s load curve is less of the than 13 pile-end .35 kN soilless , the p compr ile-top essive settlement From Figure 4a, it can be seen that the Q-s curve of the pile-end soilless compressive pile shows a slow–steep type. When the load is less than 13.35 kN, the pile-top settlement is small, and the settlement varies approximately linearly with the increase of the load. As pile shows a slow–steep type. When the load is less than 13.35 kN, the pile-top settlement is small, and the settlement varies approximately linearly with the increase of the load. the load increases, the Q-s curve gradually bends, and the settlement rate increases, show- is small, and the settlement varies approximately linearly with the increase of the load. As As the load increases, the Q-s curve gradually bends, and the settlement rate increases, ing a nonlinear change. When the load is greater than 40.05 kN, the settlement rate of the the load increases, the Q-s curve gradually bends, and the settlement rate increases, show- showing a nonlinear change. When the load is greater than 40.05 kN, the settlement rate pile top increases rapidly, and the Q-s curve shows a steep drop point at 45.39 kN. The ing a nonlinear change. When the load is greater than 40.05 kN, the settlement rate of the of the pile top increases rapidly, and the Q-s curve shows a steep drop point at 45.39 kN. cumulative settlement of the pile top is 4.31 mm. Figure 4b show the Q-s curve of the pile top increases rapidly, and the Q-s curve shows a steep drop point at 45.39 kN. The The cumulative settlement of the pile top is 4.31 mm. Figure 4b show the Q-s curve of conventional compressive pile, which is consistent with the trend of the curve of the pile- cumulative settlement of the pile top is 4.31 mm. Figure 4b show the Q-s curve of the the conventional compressive pile, which is consistent with the trend of the curve of the end soilless compressive pile, and the curve shows a slow–steep type. When the load is pile-end soilless compressive pile, and the curve shows a slow–steep type. When the load conventional compressive pile, which is consistent with the trend of the curve of the pile- is less than 21.32 kN, the pile top settlement is small. As the load increases, the Q-s curve end soilless compressive pile, and the curve shows a slow–steep type. When the load is Displacement s/mm Displacement s/mm Displacement s/mm Displacement s/mm Appl. Sci. 2022, 12, x FOR PEER REVIEW 5 of 11 less than 21.32 kN, the pile top settlement is small. As the load increases, the Q-s curve Appl. Sci. 2022, 12, 6347 5 of 11 gradually bends, and the settlement rate of the pile top increases, showing a nonlinear change. When the load is greater than 53.3 kN, the settlement rate of the pile top increases rapidly, and the Q-s curve shows a steep drop point at 58.63 kN. The cumulative settle- gradually bends, and the settlement rate of the pile top increases, showing a nonlinear ment of the pile top is 4.34 mm. change. When the load is greater than 53.3 kN, the settlement rate of the pile top increases The overall performance of the Q-s curves of the two test piles is relatively consistent, rapidly, and the Q-s curve shows a steep drop point at 58.63 kN. The cumulative settlement and the Q-s curves can be divided into three stages: the linear stage, the local shear stage, of the pile top is 4.34 mm. and the failure stage. For the pile-end soilless compressive pile, when the load is in the The overall performance of the Q-s curves of the two test piles is relatively consistent, range of 0–10.68 kN, the Q-s curve is approximately a straight line, showing that the pile and the Q-s curves can be divided into three stages: the linear stage, the local shear stage, body is in a linear stage. When the load is in the range of 13.35kN–40.05 kN, the Q-s curve and the failure stage. For the pile-end soilless compressive pile, when the load is in the changes nonlinearly, the pile top settlement increases significantly, and the soil around range of 0–10.68 kN, the Q-s curve is approximately a straight line, showing that the pile the p body ile g isrin ad aua linear lly ch stage. angeWhe s non nline the a load rly, show is in the ing range thatof th 13.35–40.05 e pile body kN, is the in tQ-s he lcurve ocal shear changes nonlinearly, the pile top settlement increases significantly, and the soil around stage. When the load is in the range of 42.72Kn–48.06 kN, the pile top settlement increases the pile gradually changes nonlinearly, showing that the pile body is in the local shear rapidly, and when the soil around the pile is damaged, the pile displacement will increase stage. When the load is in the range of 42.72–48.06 kN, the pile top settlement increases sharply, and the whole pile slides downward, at which time the pile is in the failure stage. rapidly, and when the soil around the pile is damaged, the pile displacement will increase Similarly, the conventional compressive pile is divided into three stages: When the load sharply, and the whole pile slides downward, at which time the pile is in the failure stage. is 0–15.99 kN, it is the linear stage. When the load is 21.32kN–53.3 kN, it is the local shear Similarly, the conventional compressive pile is divided into three stages: When the load is stage. When the load is 58.63kN–63.96 kN, it is the failure stage. Different from the pile- 0–15.99 kN, it is the linear stage. When the load is 21.32–53.3 kN, it is the local shear stage. end soilless compressive pile, when the conventional compressive pile is in the failure When the load is 58.63–63.96 kN, it is the failure stage. Different from the pile-end soilless stage and the soil at the end of the pile is damaged by compression, the pile displacement compressive pile, when the conventional compressive pile is in the failure stage and the soil at the end of the pile is damaged by compression, the pile displacement will increase will increase sharply, and the whole pile will slide downward. sharply, and the whole pile will slide downward. Based on the measured data, the s-lgt curves of the two test piles under different Based on the measured data, the s-lgt curves of the two test piles under different static static loads were obtained, as shown in Figure 5. loads were obtained, as shown in Figure 5. lg(t/min) lg(t/min) 0 1 2 3 0 1 2 3 10 10 10 10 10 10 10 10 5.34 kN 8.01 kN 10.66 kN 10.68 kN 15.99 kN 13.35 kN 21.32 kN 16.02 kN 3 4 26.65 kN 18.69 kN 31.98 kN 21.36 kN 4 37.31 kN 24.03 kN 42.64 kN 26.70 kN 6 29.37 kN 47.97 kN 53.30 kN 32.04 kN 34.71 kN 58.63 kN 6 8 37.38 kN 63.96 kN 40.05 kN 42.72 kN 45.39 kN 48.06 kN (a) (b) Figure 5. s-lgt curves of test pile: (a) the pile-end soilless compressive pile and (b) the conventional Figure 5. s-lgt curves of test pile: (a) the pile-end soilless compressive pile and (b) the conventional compressive pile. compressive pile. From Figure 5a, after the pile-end soilless compressive pile was loaded to 48.06 kN, From Figure 5a, after the pile-end soilless compressive pile was loaded to 48.06 kN, the pile top settlement suddenly increased to 8.12 mm, and the soil around the pile was the pile top settlement suddenly increased to 8.12 mm, and the soil around the pile was damaged. Combined with Figure 4a, 45.39 kN was the ultimate bearing capacity of the damaged. Combined with Figure 4a, 45.39 kN was the ultimate bearing capacity of the pile-end soilless compressive pile. From Figure 5b, after the conventional compressive pile pile-end soilless compressive pile. From Figure 5b, after the conventional compressive pile was loaded to 63.96 kN, the pile top settlement suddenly increased to 10.13 mm, and the was loaded to 63.96 kN, the pile top settlement suddenly increased to 10.13 mm, and the pile-end soil was damaged. Combined with Figure 4b, 58.63 kN was the ultimate bearing pile-end soil was damaged. Combined with Figure 4b, 58.63 kN was the ultimate bearing capacity of the conventional compressive pile. capacity of the conventional compressive pile. Compared with the ultimate bearing capacity, the linear section of the pile-end soilless compressive pile accounts for about 23.5% before the ultimate bearing capacity, and the Compared with the ultimate bearing capacity, the linear section of the pile-end soil- linear section of the conventional compressive pile accounts for about 36.4% before the less compressive pile accounts for about 23.5% before the ultimate bearing capacity, and ultimate bearing capacity. This shows that the length of the linear section on the Q-s the linear section of the conventional compressive pile accounts for about 36.4% before the ultimate bearing capacity. This shows that the length of the linear section on the Q-s Displacement s/mm Displacement s/mm Appl. Sci. 2022, 12, x FOR PEER REVIEW 6 of 11 Appl. Sci. 2022, 12, 6347 6 of 11 curve of the pile-end soilless compressive pile is smaller than that of the linear stage of the curve of the pile-end soilless compressive pile is smaller than that of the linear stage of the conventional compressive pile, and the conventional compressive pile has better ductility. conventional compressive pile, and the conventional compressive pile has better ductility. 3.2. Analysis of Pile Axial Force 3.2. Analysis of Pile Axial Force Through the strain gauges set on the pile body, the strain at each section of the pile Through the strain gauges set on the pile body, the strain at each section of the pile body under different loads can be obtained, and the axial force of the pile at each section body under different loads can be obtained, and the axial force of the pile at each section can be calculated. The calculation formula is: can be calculated. The calculation formula is: P = A ⋅ E ⋅ε . (1) i p p i P = A E # . (1) i p p i Here, Ap represents the cross-sectional area of the pile. Ep represents the elastic mod- Here, A represents the cross-sectional area of the pile. E represents the elastic p p ulus of the pile. εi is the strain at the pile section i. Pi represents the value of the axial force modulus of the pile. " is the strain at the pile section i. P represents the value of the axial i i at the pile section i. force at the pile section i. The variation curves of the pile axial force magnitude along the depth for the pile-end The variation curves of the pile axial force magnitude along the depth for the pile-end soilless compressive pile and the conventional compressive pile are shown in Figure 6. soilless compressive pile and the conventional compressive pile are shown in Figure 6. Axial force P/kN Axial force P/kN 0 5 10 15 20 25 30 35 40 45 50 55 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 0 0 5.34 kN 8.01 kN 1 1 10.68 kN 13.35 kN 10.66 kN 16.02 kN 15.99 kN 18.69 kN 21.32 kN 21.36 kN 26.65 kN 2 2 24.03 kN 31.98 kN 26.70 kN 29.37 kN 37.31 kN 32.04 kN 42.64 kN 34.71 kN 47.97 kN 37.38 kN 3 3 53.30 kN 40.05 kN 58.63 kN 42.72 kN 45.39 kN 63.96 kN 48.06 kN 4 4 (a) (b) Figure 6. Axial force distribution curve of test pile body: (a) the pile-end soilless compressive pile Figure 6. Axial force distribution curve of test pile body: (a) the pile-end soilless compressive pile and (b) the conventional compressive pile. and (b) the conventional compressive pile. Under the vertical load, the axial elastic compression of the pile causes the relative Under the vertical load, the axial elastic compression of the pile causes the relative displacement of the pile and soil, thereby generating the skin friction, while the vertical load displacement of the pile and soil, thereby generating the skin friction, while the vertical overcomes the skin friction and transmits down the pile body. As shown in Figure 5, the load overcomes the skin friction and transmits down the pile body. As shown in Figure 5, axial force curve of the pile body is roughly linearly distributed. The steepness of different the axial force curve of the pile body is roughly linearly distributed. The steepness of segments reflects the size of the skin friction of the segment; the steeper the curve, the different segments reflects the size of the skin friction of the segment; the steeper the curve, smaller the skin friction, and vice versa, the greater the skin friction. At the initial stage of the smaller the skin friction, and vice versa, the greater the skin friction. At the initial loading, the slope change of the axial force curve on the upper part of both test piles is rela- stage of loading, the slope change of the axial force curve on the upper part of both test tively obvious, which indicates that the skin friction at the pile body is fully exerted. At the piles is relatively obvious, which indicates that the skin friction at the pile body is fully same depth, as the load increases, the slope of the axial force distribution curve gradually exerted. At the same depth, as the load increases, the slope of the axial force distribution decreases unt curve gradually il it is st decrable. Th eases until is shows that the skin friction it is stable. This shows that dethe veloskin ps grfriction adually w develops ith the increase of the load and finally tends to be stable. This is mainly because when the load is gradually with the increase of the load and finally tends to be stable. This is mainly because gradually increased, the compression and displa when the load is gradually increased, the comprcement of the pile body increase and the ession and displacement of the pile body skin friction increases with increase and the skin friction the incr increase of the relative displace eases with the increase of the ment relative until it reaches stability. displacement until it reaches Under di stability ffere.nt loads, the axial force of the two test piles gradually decreases down- Under different loads, the axial force of the two test piles gradually decreases down- ward with the depth, reflecting the characteristics of the skin friction gradually exerting from ward top wit to b h the ottom depth, alon reflecting g the pile b the od characteristics y, and the axiaof l fo the rce dis skin trfriction ibution form gradually s are bexerting asically from top to bottom along the pile body, and the axial force distribution forms are basically similar but also different: For the axial force distribution of the pile-end soilless compres- similar but also different: For the axial force distribution of the pile-end soilless compressive sive pile (Figure 6a), the axial force at the pile-end soilless compressive pile is zero during pile (Figure 6a), the axial force at the pile-end soilless compressive pile is zero during load- loading, which directly reflects the successful fabrication of the pile-end soilless compres- ing, which directly reflects the successful fabrication of the pile-end soilless compressive sive pile. The loads are balanced by the skin friction provided by the pile-side soil, and pile. The loads are balanced by the skin friction provided by the pile-side soil, and the Depth h/m Depth h/m Appl. Sci. 2022, 12, x FOR PEER REVIEW 7 of 11 Appl. Sci. 2022, 12, 6347 7 of 11 the curves are uniformly distributed as the load increases. For the axial force distribution of conventional compressive pile (Figure 6b), the load is mostly borne by the skin friction curves are uniformly distributed as the load increases. For the axial force distribution of during the initial loading period. With the increase of the load, the resistance of the pile- conventional compressive pile (Figure 6b), the load is mostly borne by the skin friction end is gradually exerted. Under the ultimate load, the slope of the axial force curve of the during the initial loading period. With the increase of the load, the resistance of the pile-end pile body becomes smaller, and the axial force at the end of the pile is about 42% of the is gradually exerted. Under the ultimate load, the slope of the axial force curve of the pile pile top load, exhibiting the characteristics of an end-bearing friction pile. body becomes smaller, and the axial force at the end of the pile is about 42% of the pile top load, exhibiting the characteristics of an end-bearing friction pile. 3.3. Analysis of the Skin Friction 3.3. Analysis of the Skin Friction According to the difference between the axial force of the pile body between the two measAccor uring po ding intto s d the ivide dif d b fery ence the pi between le side su the rfaxial ace are for ace of of the the sect pile ion, body the sk between in frictithe on f two i of the section can be obtained. The calculation formula is: measuring points divided by the pile side surface area of the section, the skin friction f of the section can be obtained. The calculation formula is: P − P i−1 i f = (2) i P P i 1 i U ⋅ l f = (2) Ul where Pi–1, Pi represents the axial force values of two adjacent measurement points above where P , P represents the axial force values of two adjacent measurement points above i 1 i and below the pile section i. U represents the pile circumference. li represents the length and below the pile section i. U represents the pile circumference. l represents the length of of the pile section i. the pile section i. From the calculation results, the skin friction distribution curves of the two test piles From the calculation results, the skin friction distribution curves of the two test piles are obtained. The linear stage is a solid line, the local shear stage is a dashed line, and the are obtained. The linear stage is a solid line, the local shear stage is a dashed line, and the failure stage is a dotted line, as shown in Figure 7. failure stage is a dotted line, as shown in Figure 7. Skin friction f/kPa Skin friction f/kPa 0 5 10 15 20 25 30 35 40 45 50 55 60 0 5 10 15 20 25 30 35 40 45 50 55 60 5.34 kN 10.66 kN 8.01 kN 15.99 kN 1 10.68 kN 1 13.35 kN 21.32 kN 16.02 kN 26.65 kN 18.69 kN 31.98 kN 21.36 kN 37.31 kN 24.03 kN 42.64 kN 26.70 kN 2 2 47.97 kN 29.37 kN 53.30 kN 32.04 kN 34.71 kN 58.63 kN 37.38 kN 63.96 kN 40.05 kN 42.72 kN 3 3 45.39 kN 48.06 kN 4 4 (a) (b) Figure 7. Distribution curve of the skin friction: (a) the pile-end soilless compressive pile and (b) the Figure 7. Distribution curve of the skin friction: (a) the pile-end soilless compressive pile and (b) the conventional compressive pile. conventional compressive pile. In Figure 7, it can be seen that skin friction is gradually exerted with the increase of the In Figure 7, it can be seen that skin friction is gradually exerted with the increase of load. Affected by the load, the pile body is compressed and deformed, resulting in the rela- the load. Affected by the load, the pile body is compressed and deformed, resulting in the tive displacement of the pile and the soil around the pile, thus generating skin friction to relative displacement of the pile and the soil around the pile, thus generating skin friction bear the upper load. Overall, skin friction tends to increase first and then decrease and in- to bear the upper load. Overall, skin friction tends to increase first and then decrease and crease as the load increases, and the curve is characterized by “small on both sides and large increase as the load increases, and the curve is characterized by “small on both sides and in the middle”. Both piles decrease skin friction along the pile body after peaking with in- large in the middle”. Both piles decrease skin friction along the pile body after peaking with creasing load, and the rate increasing load, and the rate of decrease is gr of decrease is gr eater for conve eater for conventional ntional p piles iles than for than for pile-end pile-end soilless compressive pile. soilless compressive pile. At the ultima At the ultimate te load, load, the maximum skin friction of the pile-end the maximum skin friction of the pile-end soilless compressive pile is 43.46 kPa, and the maximum skin friction of the conventional soilless compressive pile is 43.46 kPa, and the maximum skin friction of the conventional compressive pile is 42.06 kPa. The soil layers corresponding to the peaks of the skin friction compressive pile is 42.06 kPa. The soil layers corresponding to the peaks of the skin friction of the two piles are silty cla of the two piles are silty clay–muddy y–muddy silt silty y clay clay layers. layers. The maxi The maximum mum skin friction of the t skin friction of w the o two piles is similar in value, indicating that the skin friction performance of the two test piles is similar in value, indicating that the skin friction performance of the two test piles in piles in this soil layer is almost the same. At the pile-end position, the skin friction of the this soil layer is almost the same. At the pile-end position, the skin friction of the two piles two piles are 35.45 kPa and 25.55 kPa, respectively. The skin friction of the conventional are 35.45 kPa and 25.55 kPa, respectively. The skin friction of the conventional compressive compressive pile is about 28% lower than that of the pile-end soilless compressive pile. pile is about 28% lower than that of the pile-end soilless compressive pile. Depth h/m Depth h/m Appl. Sci. 2022, 12, 6347 8 of 11 3.4. Evolution of the Skin Friction The skin friction distribution curve is normalized, and the abscissa is defined as the segment skin friction divided by the maximum skin friction of the test pile at this level of load, which becomes the pile segment skin friction ratio, and the ordinate is defined as the position of the strain gauge divided by the pile length to become the depth ratio. The normalized curve is shown in Figure 8. The pile-end soilless compressive pile are solid lines, and the conventional compressive pile are dashed lines. In order to describe the variation law of the skin friction ratio of the pile segment conveniently, define 0–0.25 L as the upper part of the pile, 0.25 L–0.5 L as the upper-middle part of the pile, 0.5 L–0.75 L as lower-middle part of the pile, and 0.75 L–L as the lower part of the pile; the length of the pile is L. Figure 8 correspond to the distribution changes and comparisons of the pile segment skin friction ratio of the two piles in the linear stage, the local shear stage, and the failure stage. In Figure 8a, in the linear phase, the pile side frictional resistance of both piles increases and then decreases in the direction of the pile body, and the peak value occurs in the middle and upper part of the pile. In the upper area of the two piles, the pile segment skin friction ratio of the pile-end soilless compressive pile decreases from 0.6 to 0.51, while the pile segment skin friction ratio of the conventional compressive pile increases from 0.43 to 0.5. This indicates that the performance of load transfer on the upper part of the pile-end soilless compressive pile is better than that of the conventional compressive pile when the test piles are in the linear stage. In Figure 8b, In the local shear stage, with the increase of load, the distribution trend of skin friction of the two test piles continued to increase first and then decrease, and the location of peak skin friction did not change. In the upper part of the two piles, the lateral friction ratio of the pile section pile segment skin friction ratio of the two test piles increased continuously but more concentrated as the load increased, indicating that the two test piles behaved in a more consistent manner as the load increased and the skin friction provided by the soil around the upper part of the pile increased gradually reaching the limit. In the middle-lower sections of the pile, the pile segment skin friction ratio of the pile-end soilless compressive pile increases from 0.48 to 0.84, while the pile segment skin friction ratio of the conventional compressive pile increases from 0.49 to 0.68. In the lower part of the pile, the pile segment skin friction ratio of the pile-end soilless compressive pile increases from 0.3 to 0.67, while the pile segment skin friction ratio of the conventional compressive pile increases from 0.4 to 0.56. Compared with the conventional compressive pile, the pile segment skin friction ratio of the pile-end soilless compressive pile is more variable. It shows that as the load increases, the friction performance of the soil layer around the lower-middle part and lower part of two piles of the pile-end soilless compressive pile is better than that of the conventional compression pile. That is to say, the skin friction of the lower-middle part and lower part of the pile-end soilless compressive pile is exerted to an increasing degree. A large number of experimental studies have shown that only when the vertical load reaches a certain value does the pile-end resistance gradually come into play, and then the pile-end resistance may have an effect on the skin friction. Therefore, the variation of skin friction under ultimate load is an effective means to study the effect of the presence of the pile-end soil on the pile skin friction. From Figure 8c, when the load is in the failure stage, the skin friction of both piles increases to different degrees. From the upper part of the pile to the middle-lower part of the pile, the skin friction of the pile-end soilless compressive pile is smaller than that of the conventional compression pile. In the lower part of the pile, the pile segment skin friction ratio of the pile-end soilless compressive pile increases from 0.71 to 0.82, while the pile segment skin friction ratio of the conventional compressive pile increases from 0.59 to 0.61. This shows that as the load increases to the failure stage, the skin friction of each pile section and the pile segment skin friction ratio also increases, and the laws of the two piles are relatively consistent. However, from the values of the pile segment skin friction ratio, the conventional compression pile is much smaller than the Appl. Sci. 2022, 12, x FOR PEER REVIEW 8 of 11 Appl. Sci. 2022, 12, 6347 9 of 11 3.4. Evolution of the Skin Friction The skin friction distribution curve is normalized, and the abscissa is defined as the segment skin friction divided by the maximum skin friction of the test pile at this level of pile-end soilless compressive pile, and the skin friction of the conventional compression pile load, which becomes the pile segment skin friction ratio, and the ordinate is defined as the shows an obvious weakening phenomenon. Combined with the skin friction values of the position of the strain gauge divided by the pile length to become the depth ratio. The nor- corresponding pile sections, the skin friction is 30.59 kPa for the conventional compressive malized curve is shown in Figure 8. The pile-end soilless compressive pile are solid lines, pile and 37.27 kPa for the pile-end soilless compressive pile in the middle-lower part of the and the conventional compressive pile are dashed lines. In order to describe the variation pile. The skin friction of the conventional compressive pile is about 18% lower than that of law of the skin friction ratio of the pile segment conveniently, define 0–0.25 L as the upper the pile-end soilless compressive pile. In the lower part of the pile, the skin friction of the part of the pile, 0.25 L–0.5 L as the upper-middle part of the pile, 0.5 L–0.75 L as lower- conventional compression pile is 25.55 kPa, and the pile-end soilless compressive pile is middle part of the pile, and 0.75 L–L as the lower part of the pile; the length of the pile is L. 35.45 kPa. The skin friction of the conventional compressive pile is about 28% lower than Figure 8 correspond to the distribution changes and comparisons of the pile segment skin that of the pile-end soilless compressive pile. This weakening effect exists in the middle friction ratio of the two piles in the linear stage, the local shear stage, and the failure stage. and lower parts of the pile, and the weakening effect decreases from bottom to top. Pile segment skin friction ratio 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 The pile-end soilless 0.2 compressive pile 5.34 kN 8.01 kN 10.68 kN 0.4 The convention compressive pile 10.66 kN 15.99 kN 0.6 0.8 1.0 1.2 (a) Pile segment skin friction ratio 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 The pile-end soilless compressive pile 0.2 13.35 kN 16.02 kN 18.69 kN 21.36 kN 24.03 kN 0.4 26.70 kN 29.37 kN 32.04 kN 34.71 kN 37.38 kN 0.6 40.05 kN The convention compressive pile 21.32 kN 26.65 kN 0.8 31.98 kN 37.31 kN 42.64 kN 47.97 kN 1.0 53.30 kN 1.2 (b) Pile segment skin friction ratio 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 The pile-end soilless compressive pile 42.72 kN 45.39 kN 0.4 48.06 kN The convention compressive pile 0.6 58.63 kN 63.96 kN 0.8 1.0 1.2 (c) Figure 8. Distribution curve of the pile segment skin friction ratio along depth: (a) the linear stage, (b) the local shear stage, and (c) the failure stage. Depth ratio Depth ratio Depth ratio Appl. Sci. 2022, 12, 6347 10 of 11 In general, the skin friction of both piles is basically the same when subjected to vertical load, with the law of top-down gradual action. That is, the skin friction on the upper part of the pile bears the load first, and as the load continues to increase, the skin friction on the lower part of the pile comes into play. The distribution form and distribution pattern of the pile segment skin friction ratio is more consistent between the two piles, while the difference in the magnitude of the pile segment skin friction ratio reflects the difference between the two piles. For the pile-end soilless compressive pile, from the linear stage to the local shear stage to the final failure stage, the skin friction of each pile section reaches the limit, and then the soil around the pile breaks down, and the pile top load reaches the maximum. For the conventional compressive pile, from the linear stage to the local shear stage to the final failure stage, skin friction comes into play continuously, and the end resistance also comes into play gradually as the load increases. Different from the pile-end soilless compressive pile, the skin friction of some pile sections is not kept consistent. This may be due to the presence of the pile-end soil, which provides an end resistance that limits the skin friction, which shows a weakening effect. 4. Conclusions Based on the in situ experiment, this paper compares and analyzes the difference in load transfer laws between the pile-end soilless compressive pile and the conventional compressive pile and draws the following conclusions: 1. The Q-s curves of both the pile-end soilless compressive pile and the conventional compressive pile have obvious inflection points and show a slow–steep drop type. The linear section of the pile-end soilless compressive pile accounts for about 23.5% before the ultimate bearing capacity, and the linear section of the conventional compressive pile accounts for about 36.4% before the ultimate bearing capacity. This shows that the conventional compressive pile has better ductility. 2. During the whole loading process, the peak position of the skin friction of the two types of piles is in the middle-upper part of the pile, and the surrounding soil layers are all silty clay–muddy silty clay layers, and the skin friction provided by the soil layers to the two piles is basically the same. 3. When the two piles are loaded, the skin friction is basically the same as the top-down gradual effect of the law, and the distribution form and distribution law of the pile segment skin friction ratio is more consistent between the two piles. 4. Compared with the pile-end soilless compressive pile, the pile-end soil of the conven- tional compressive pile restricts the skin friction of the middle-lower part of the pile and the lower pile section, and this restriction is manifested as a weakening effect on the skin friction of the pile, and the weakening effect is 18% and 28% in turn. The research conclusion of this paper is based on the steel pile. Due to the great difference between the material of concrete pile and steel pile and the manufacturing method of the pile, the applicability of this conclusion to concrete pile needs further study. Author Contributions: Conceptualization, H.M. and H.Z.; methodology, H.M. and H.Z.; software, H.Z.; validation, H.Z.; formal analysis, H.Z.; investigation, H.Z.; resources, H.M.; data curation, H.Z.; writing—original draft preparation, H.Z.; writing—review and editing, H.Z.; visualization, H.M.; supervision, H.M.; project administration, H.M.; funding acquisition, H.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 51878618. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All data generated or analysed during this study are included in this published article. Appl. Sci. 2022, 12, 6347 11 of 11 Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. References 1. Ghasemzadeh, H.; Tarzaban, M.; Hajitaheriha, M.M. Numerical analysis of pile–soil–pile interaction in pile groups with batter piles. Geotech. Geol. Eng. 2018, 36, 2189–2215. [CrossRef] 2. Wu, Y.D.; Ren, Y.Z.; Liu, J.; Ma, L. 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