Experimental Evaluation of the Bending Behavior of a Drilled Shaft with Partial Casing under Lateral Loads
, Guoliang Dai 2,3,*, Xueying Yang 2,3, Qian Yin 2,3, Wenbo Zhu 2,3 and Fan Zhang 2,3Round 1
Reviewer 1 Report
Figure 3
It is not clear why the diameter of the steel tube is 4 m. An outer diameter of 2m is previously specified.
Figure 5
The same error regarding the diameter.
Equations 1 and 2
It is not clear the calculation of the pilot's bending stiffness. In relation (2) the same notation is used for the rigidity of the armature casing as in the relationship (1) for the casing. In conclusion, it is not clear how was calculated the bending stiffness for the composite section of the pile.
On the other hand, a constant value of the deformation module for concrete is used, or it varies with the deformation. It is not clear whether the authors have taken this into account in their research.
Rows 143-144
A correction is required regarding the meaning of the y-axis in figure 6. It is not clear what (EpIp)measured means. What exactly was measured? Ep's value depending on the efforts? How?
It is not clear why the maximum bending moment is only 240kNm. Figure 4 shows graphs in which the moment reaches values of 6000kNm.
It is also not clear why the analysis in Figure 6 is made for the section located at 0.3 m below the loading point. According to figure 3 there are no strain gauges at the depth of 0.3 m below the loading point. Further explanations are needed.
Figure 9
Same ambiguity regarding the bending moment measured. It was probably calculated from the measurements of strains, but it is not clear how.
Row 283
It requires correction. Incomplete phrase.
Conclusions
The conclusion (1) is true only for a relatively small applied load. It would have been interesting to determine at what level of load the connection between the tube and the pilot's body is no longer firm, and the plane cross-section assumption is no longer valid.
Conclusion (2) indicates that the tube-concrete connection is not important for the load-bearing capacity of the pilot, but this may be true up to a certain load value. As in the previous conclusion, however, the hypothesis corresponds to a relatively low load value.
On the other hand, from figure 9 the differences between the curves exist from relatively small loads, but indeed the results are not conclusive. Perhaps the calculation of the bending moment for the compound section (casing/tube + concrete + reinforcement casing) does not take into account the degradation of the concrete and the entry into the plastic field of the steel at large deformations (including the deterioration of the connection between the tube and the concrete).
General remark
The outer metal tube is in direct contact with the ground. I have not seen anywhere any remarks regarding the corrosion in time and if the authors have taken this effect into account.
Author Response
Comments and Suggestions for Authors 1
Figure 3
It is not clear why the diameter of the steel tube is 4 m. An outer diameter of 2m is previously specified.
Authors’ Response: We would like to thank the reviewer for his/her time and for the thorough review. The related values of the steel tube have been modified as following:
Figure 1 The related values of the steel tube (B-B).
Figure 5
The same error regarding the diameter.
Authors’ Response: The related values of the steel tube have been modified as following:
Figure 2. The Distribution of Strain Along Pile Cross Section
Equations 1 and 2
It is not clear the calculation of the pilot's bending stiffness. In relation (2) the same notation is used for the rigidity of the armature casing as in the relationship (1) for the casing. In conclusion, it is not clear how was calculated the bending stiffness for the composite section of the pile.
Authors’ Response: Very good point. We’ve modified the related content as following:
For the rigid stiffness of steel-concrete piles, the calculation method is as follows:
|
(1) |
where Es is the modulus of elasticity of the steel casing, Is is the moment of inertia of the steel casing, EpIp is the bending stiffness of the pile, and η is the correction coefficient. There are different η values for different criteria, such as Chinese codes, with a value of 1.0. In Japan, this value is 0.2, and in Europe , it is 0.6.
The initial bending stiffness EpIp of the DSPC is calculated as follows:
|
(2) |
where EsIs=Esπ(d14-d4)/64, EpIp=0.85Ecπd(d2+2(aE-1)ρgd02)/32, d1 is the diameter of the outside casing, d1=2.0 m, d is the pile diameter, d=1.8 m, d0 is the net diameter that does not include the thickness of the cover, d0=1.64 m, aE is the ratio of the modulus of elasticity of the steel bar to that of the concrete, aE= Es/ Ec =5.56, ρg is the reinforcement ratio of the pile body, ρg=0.57%, Es=2.1×105 MPa, Ec is the modulus of elasticity of the concrete, and Ec=3.6×104 MPa.
On the other hand, a constant value of the deformation module for concrete is used, or it varies with the deformation. It is not clear whether the authors have taken this into account in their research.
Authors’ Response: Very good point. We didn’t take the deformation into account. The purpose of this paper is to verify the assumed plane cross-section effect on the bending capacity of DSPCs, and the effect of concrete-steel interfaces on the bending stiffness of DSPCs. The bending capacity changing with the loading level is obtained as a conclusion, not as a condition.
In addition, Fig. 6 shows the bending moment-stiffness response of pile section 0.3 below the loading point. it can be found that the bending stiffness of the pile section changes with the loading level. While the reason of this phenomenon is very complex. It probably be caused by the deformation module for concrete, or the concrete-steel interfaces, or both. A correction coefficient, α = 0.937, was proposed in this paper, which related to the loading level, in the calculation of the bending stiffness.
Rows 143-144
A correction is required regarding the meaning of the y-axis in figure 6. It is not clear what (EpIp) measured means. What exactly was measured? Ep's value depending on the efforts? How?
Authors’ Response: Fig. 6 shows the bending moment-stiffness response of pile section 0.3 below the loading point. The x-label is the ratio of the bending moment at the pile section and the maximum value, M/Mmax, where M=Fh, F is the lateral force, h is 0.3 m, Mmax is 240 kNm. The y-label is the ratio of bending stiffness (α) at the pile section and the maxi-mum value, where α = (EI)measured/EI, (EI)measured= M/(dw2/d2z), w is the deflection of pile.
It is not clear why the maximum bending moment is only 240kNm. Figure 4 shows graphs in which the moment reaches values of 6000kNm.
It is also not clear why the analysis in Figure 6 is made for the section located at 0.3 m below the loading point. According to figure 3 there are no strain gauges at the depth of 0.3 m below the loading point. Further explanations are needed.
Authors’ Response: The value in Fig.4(b) may not be the real bending moment, because we are not sure the rigid stiffness of steel-concrete piles, and we merely care about the position where the maximum bending moment appeared. We can assure that the maximum bending moment existed within the depth of 11 m to 14 m. From Fig. 2, the depth of 12 m where the strain gauges were attached is the nearest measurement section in the pile body. Therefore, the distribution of strain along the cross section is given in Fig. 5. Form the distribution, we can conclude that the casing and the concrete body deformed as an assembly at the maximum bending moment condition.
We can be sure the real bending moment below 0.3m of the loading point, where Mmax=Fmax*L=800kN*0.3m=240kNm. The deflection profile was also given in Fig.4(a). The rigid stiffness of steel-concrete piles can be given by (EI)measured =M/(d2w/dz2), w is the deflection of the pile.
Figure 9
Same ambiguity regarding the bending moment measured. It was probably calculated from the measurements of strains, but it is not clear how.
Authors’ Response: We’ve also added the required information in the revised paper. The bending moment in Fig.9 are not from the measurements of strains, it is a simply supported beam under concentrated loads. We obtained the bending moment at the central of the beam by the following beam theory:
Figure 3 The related beam theory (a simply supported beam under concentrated loads).
Row 283
It requires correction. Incomplete phrase.
Authors’ Response: The related content has been modified as following:
On the other hand, in Figs. 11 (c)-(f), the strain of the casing and the concrete is in a nonlinear relationship at the beginning, which indicates that the plane cross-section assumption is no longer suitable.
Conclusions
The conclusion (1) is true only for a relatively small, applied load. It would have been interesting to determine at what level of load the connection between the tube and the pilot's body is no longer firm, and the plane cross-section assumption is no longer valid.
Authors’ Response: Agree, the results from laboratory tests have proved this comment.
Conclusion (2) indicates that the tube-concrete connection is not important for the load-bearing capacity of the pilot, but this may be true up to a certain load value. As in the previous conclusion, however, the hypothesis corresponds to a relatively low load value.
Authors’ Response: Agree, the test pile would become a part of the bridge, and it is impossible to study the pile during a field test until the collapse of the structure or the loss of serviceability, so the interaction between the pile and the casing under such high loads cannot be tested. To completely investigate the impact of the slip between the concrete and casing of piles on the bending capacity of DSPCs, bending tests of pile specimens were implemented in the laboratory tests.
On the other hand, from figure 9 the differences between the curves exist from relatively small loads, but indeed the results are not conclusive. Perhaps the calculation of the bending moment for the compound section (casing/tube + concrete + reinforcement casing) does not take into account the degradation of the concrete and the entry into the plastic field of the steel at large deformations (including the deterioration of the connection between the tube and the concrete).
Authors’ Response: The bending moment in Fig.9 are not from the measurements of strains, it is a simply supported beam under concentrated loads. We obtained the bending moment at the central of the beam by the beam theory.
General remark
The outer metal tube is in direct contact with the ground. I have not seen anywhere any remarks regarding the corrosion in time and if the authors have taken this effect into account.
Authors’ Response: we really appreciate the reviewer for the supportive and creative comments. Considering the corrosion of the outer metal tube is really a good point. We would like to continue our work on this.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
1) I suggest improving the literature review and the introduction
2) Figure 5. If the pile diameter is 2 m, why is the y (mm) from 0 to 4000 mm (along y-axis)?
3) Figure 5. What is µ along the x-axis?
4) Why did you focused on the section below 0.3m the loading point? (Figure 6) In your study it should be better refer to a section in which the bending is higher than 240 kNm, This Mmax (240 kNm) is an extremely low value for a section with a diameter of 2 m. Please add a similar graph referring to a section in which the bending is higher (i.e., at least at a depth between 9.0 - 15.0 m in which you attain larger bending, as shown in Figure 4b)
5) As the results in Figure 6 refer to an extremely low bending moment, maybe be also your conclusions should be rewritten (especially the point (1) of your conclusions).
Author Response
Comments and Suggestions for Authors 2
1) I suggest improving the literature review and the introduction
Authors’ Response: The related content has been improved as following:
- Introduction
Cast-in situ concrete piles (CCPs) have been widely used in bridge projects to bear complex and large loads, especially the lateral load and overturning moment. During the cast-in situ concrete pile construction process, steel casings are widely used. However, in some projects, these tubes cannot be removed before grouting. This will form a drilled shaft with partial casing (DSPC). Fortunately, it was found that these piles are more supportable than traditional CCPs. For example, in the Tai Zhou Bay Super Bridge of China, the lateral force from the upper bridge was so large that it caused a large bending moment in the pile foundation, of which the section strength could not bear, a similar phenomenon has been observed in the Hong Kong-Zhuhai-Macao Bridge. Some research has also been conducted on similar improvements for concrete-filled steel columns [1] and concrete plugs embedded in tubular steel piles [2, 3].
DSPCs have some differences from traditional CCPs, concrete-filled steel columns, or concrete plugs embedded in tubular steel piles [2, 4, 5]. In a DSPC, the steel casing is partially used (Fig. 1); then, a hole is drilled in the casing, and the soil in the tube is removed by the slurry method. This may result in a weak bond between the concrete body and steel tube in the composite structure. Many results have been reported about the bond stress capacity of concrete-filled steel columns. In these studies, the different shapes of column sections were tested [6, 7], and the interface length, interface condition [8], mechanical connectors, and diameter-to-thickness (D/t) ratio were also studied [9, 10, 11, 12]. However, few of them have investigated the assumed plane cross-section effect on the bending capacity of DSPCs, the mud effect on the bond strength, and the mechanical interlocking of concrete-steel interfaces under bending conditions.
Therefore, in this study, a set of experimental programs about the Tai Zhou Bay Super Bridge was introduced. A DSPC with a diameter of 2 m, length of 87.9 m, a steel tube diameter of 2.0 m and a length of 33 m was studied in clay beds. Strain gauges were distributed along the steel rebars in the concrete pile, and at the surface of the steel tube at the same cross sections along different embedment depths. Therefore, the longitudinal strains of the concrete pile and steel tube can be studied separately. Moreover, a set of laboratory experiments were implemented. They are reinforced concrete-filled steel tubular columns (RCFSTCs) under pure bending conditions. In these tests, strain gauges were distributed along the steel rebars in the concrete pile and at the wall of the steel tube at the pure bending section of these RCFSTCs. Different wall thicknesses and mud conditions were considered to study the influence of the bonding quality on the bending capacity.
2) Figure 5. If the pile diameter is 2 m, why is the y (mm) from 0 to 4000 mm (along y-axis)?
Authors’ Response: The related values of the steel tube have been modified as following:
Figure 1. The Distribution of Strain Along Pile Cross Section
3) Figure 5. What is µ along the x-axis?
Authors’ Response: The related content has been modified as following:
In Fig. 5, the x-label is the distribution of strain along the cross section of the DSPC at the depth of 12 m, the values were from sliding microtiters, vibrating-wire strain gauges and inclinometers in Fig.4. The y-label is the height of the pile section.
4) Why did you focused on the section below 0.3m the loading point? (Figure 6) In your study it should be better refer to a section in which the bending is higher than 240 kNm, This Mmax (240 kNm) is an extremely low value for a section with a diameter of 2 m. Please add a similar graph referring to a section in which the bending is higher (i.e., at least at a depth between 9.0 - 15.0 m in which you attain larger bending, as shown in Figure 4b).
5) As the results in Figure 6 refer to an extremely low bending moment, maybe be also your conclusions should be rewritten (especially the point (1) of your conclusions).
Authors’ Response: The bending moment along the pile and the casing is available as shown in Fig. 4 (b). The value in Fig.4(b) may not be the real bending moment, because we are not sure the rigid stiffness of steel-concrete piles, and we merely care about the position where the maximum bending moment appeared. We can assure that the maximum bending moment existed within the depth of 11 m to 14 m. From Fig. 2, the depth of 12 m where the strain gauges were attached is the nearest measurement section in the pile body. Therefore, the distribution of strain along the cross section is given in Fig. 5. Form the distribution, we can conclude that the casing and the concrete body deformed as an assembly at the maximum bending moment condition.
On the other hand, we can make sure that the real bending moment below 0.3m of the loading point, where M=F*L=800kN*0.3m=240kNm. The rigid stiffness of steel-concrete piles can be given, and it can be found that the bending stiffness of the pile section changes with the loading level.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors responded to all the comments.
