Numerical Investigation of the Upside-Down Hanging Well Combined with Curtain Grouting for Strengthening a Flood Control Ancient Levee

Round 1

Reviewer 1 Report

The article is well structured. The problem is correctly defined, and it is explained how the authors obtained the variation of the safety factors of an ancient levee before and after reinforcement during the flooding process. However, the article needs extensive editing in the English language. For example, p2 line 61 is poorly written: "about 600 meters old, more than 1000 years old". To improve the coherence in the document. Avoid informal writing like "100 thousand people." At the end of paragraphs, use "." or ":" as appropriate.

Author Response

Thank you very much for your comments and suggestions. We have made some revisions and statements for some problems according to the reviewing comments in our revision manuscript, marked by blue color. Our response to the comments is given as follows.

 

Comment 1. The article is well structured. The problem is correctly defined, and it is explained how the authors obtained the variation of the safety factors of an ancient levee before and after reinforcement during the flooding process. However, the article needs extensive editing in the English language. For example, p2 line 61 is poorly written: "about 600 meters old, more than 1000 years old". To improve the coherence in the document. Avoid informal writing like "100 thousand people." At the end of paragraphs, use "." or ":" as appropriate.

A: Thank you very much for your comments. We have checked the full text and revised the English expression. The revised abstract is as follows:

The sentence "about 600 meters old, more than 1000 years old" is modified to "At present, there are still about 600 m left, with a history of more than 1000 years".

The sentence "100 thousand people" is modified to "and lots of residents' lives were seriously affected".

The English of other parts of the article has also been modified and displayed in blue font and at the end of paragraphs, the use of "." or ":" has also been appropriately revised.

Author Response File: Author Response.docx

Reviewer 2 Report

 In this paper, combined with the characteristics of river water level fluctuations, the effect of the upside-down hanging well and curtain grouting reinforcement of the ancient levee is analyzed by a numerical method, and the variation law of the levee stability in the flood process before and after reinforcement is explored. The study results show that the flood process significantly affect the pore water pressure of the filling soil between the ancient levee and the well, and has a weak impact on that behind the well, which is conducive to the levee stability. The horizontal displacements of the levee and the fill present the opposite changes law before and after reinforcement. Before reinforcement, the maximum horizontal displacement reached 2.75 cm, and the displacement was toward Lan River. This was caused by the deformation of the soil squeezing the levee after the water level rose, which was extremely unfavorable to the levee stability. After reinforcement, the levee and fill shifted away from the river, mainly due to the hydrostatic pressures caused by the rising water level. The reviewer would like to propose the following comments based on the contents of the manuscript.

1. The Introduction is not good. A good introduction should include current research progress and deficiencies. In this paper, the authors describe many details of the technology scheme of the upside-down hanging well and curtain grouting. The highlights and the scientific contributions of this manuscript need to be emphasized.

2. In section 2, the material and methodology were proposed. The numerical model calculation theories were developed. How do you consider the water migration? This is the most important characteristic of unsaturated soil.

3. In section 3, the result and discussion were presented. The author introduces many contents about the test results. But these are irregular and theoretical analysis. It is lack of innovation. Besides, model validation is needed. It needs to be added
4. In section 4, the conclusions were presented. There is too much to say about the conclusion of the article. It needs to be simplified. What is the most important conclusion?

Author Response

Reviewer #2:

Thank you very much for your comments. We have carefully made some revisions according to your comments. All changes are marked by blue characters in this version. Below is our brief response to the key points raised in the review:

 

Comment 1. The Introduction is not good. A good introduction should include current research progress and deficiencies. In this paper, the authors describe many details of the technology scheme of the upside-down hanging well and curtain grouting. The highlights and the scientific contributions of this manuscript need to be emphasized.

A: Thank your comments. We have supplemented and revised the introduction and added a separate section of project background as follows:

1. Introduction

The phenomena that water level changes trigger bank slope instability and cause disasters often occur during the flood season every year [1 – 3], and this process involves complex transient flow problems of saturated-unsaturated soil [4 – 7]. Especially, the sudden drawdown of the water level has a significant impact on the bank slope stability [8 – 10]. Their stability is mainly affected by the initial water level, water level drawdown rate, drawdown amplitude, seepage coefficients and changes of the groundwater level [11 – 14]. The seepage process alters the soil pore water pressures, and then changes its physical and mechanical properties [15 – 17]. The periodic change of the water level will also bring about irreversible damage to soil properties, which will initiate potential safety hazards [18]. For unsaturated soil, the increasing water content will reduce its mechanical properties, even lead to cracks [19, 20].

Southeast China is rich in water and rainfall, with crisscross river networks, and there are many problems caused by seepage in engineering [21, 22]. In ancient times, the levees built by waterfront in many towns were not only military facilities, but also flood control facilities [23]. They still play a huge role in flood control, and also add a strong cultural heritage to cities and towns, becoming city cards, which is of great significance to local cultural, economic and social development [24 – 28]. However, in recent years, the news of the destruction of ancient levees built along rivers has been reported frequently, which has aroused concern from all walks of life about the protection of ancient buildings. The engineering geological and hydrogeological conditions of the ancient levees are complex, and there are many factors affecting their destructions. Through historical changes, the factors affecting each destruction are different, which adds difficulties to the analysis of their destruction mechanism. It is not only blind to carry out engineering reinforcements when the failure mechanism is unknown, but also the reinforcement measures may not play their due role. Indoor and outdoor tests, on-site monitoring and investigation are very important and necessary for accident analyses and engineering reinforcements. However, the above methods are difficult to meet extreme working conditions of the levees and analyze various working conditions in the process of engineering practices, which not only can not fully reflect the impact of influencing factors on the stability of the ancient levees, but also have the problems of long time and large capital investment. For the ancient flood control levee that affects the safety of flood discharge and is in urgent need of reinforcement, it is necessary to evaluate the influence of the main factors on its stability before and after reinforcement under a certain working condition. It is the best choice to use numerical simulation technology to study it in combination with some indoor and outdoor tests, monitoring and survey technologies. Taking Lanxi ancient levee in the southeast China as an example, this paper discusses the effect of the anti-seepage reinforcement by adopting the upside-down hanging well and curtain grouting technology with a numerical method.

The technology scheme of the upside-down hanging well and curtain grouting is rarely used in the construction of the ancient levee reinforcement, and the impact on the levee stability during the water level change process is not clear. Therefore, this paper intends to select a typical section for in-depth study, and analyze the stability change laws of the ancient levee before and after reinforcement during floods of the Lan River. According to the characteristics of Lan River water level changes, combined with indoor tests and on-site observation, the stability change of Lanxi ancient levee before and after reinforcement is analyzed through the construction of a numerical model by selecting the maximum flood process in recent years, the influence of water level change on the stability of the ancient levee after anti-seepage reinforcement is evaluated, and the evolution law of the stability safety coefficients during the flood process and the reinforcement effect are quantitatively revealed. The study provides a theoretical basis for risk identification, prevention and control, reinforcement measures selection of ancient flood control levees.

2 Engineering background

The ancient levee is located in the upper reaches of Lan River in Lanxi City, Zhejiang Province, China, and the lower reaches of the intersection of Qu River and Jinhua River (Figure 1 a). It was built in the Northern Song Dynasty (At present, there are still about 600 m left, with a history of more than 1000 years) and rebuilt in the seventh year of Zhengde of the Ming Dynasty (1512). The levee is also used as a city wall, and has military defense and flood control functions. As the water level at the intersection of the three rivers changes dramatically, it has been destroyed by water many times in history. Up to now, the levee is still used as an important flood control facility (Figure 1 b), which plays a significant role in local flood control. The surface near the water side of the ancient levee is built with strip stones, the masonry material is local red sandstone, and the bonding material is traditional lime slurry. Behind the stone levee is miscellaneous fill with loose structure, poor uniformity, large seepage coefficient, and the levee body has collapsed for many times (Figure 1 c, d). There are a lot of seepage and stability problems in the levee body and foundation (Figure 1 e), and many sections also appear bulging and cracking (Figure 1 f).

Figure 1. The location and damage characteristics of Lanxi ancient levee. (a) The location of the ancient levee, (b) the ancient levee under floods, (c) the failure of the upper part of the ancient levee, (d) the collapse of the ancient levee, (e) the seepage of the ancient levee, (f) the bulging and cracking of the ancient levee.

In history, Lanxi ancient levee has been damaged and repaired many times. The last serious collapse occurred in the section near the West Gate Tower, and the repair of this section was completed in 2015. During the "6.25" flood on June 25th, 2017, several sections of the ancient levee along the river burst and piping, and some of them collapsed. The situation was critical. Floods poured into the old city of Lanxi, causing flooding in the urban area, and lots of residents' lives were seriously affected. At that time, the flood control standard of the ancient levee at the West Gate Tower section could not withstand the flood with a 20-year return period, thus there were potential safety hazards during the flood season.

From March 2020 to August 2021, the local management department strengthened the ancient levee of the West Gate Tower section by combining the upside-down hanging well and curtain grouting (Figure 2 a). The purpose of the reinforcement is to cut off the seepage passage of the levee body and foundation. The upside-down hanging well is one of the excavation methods of shaft and foundation pit. It is similar to the shaft made from top to bottom. Generally, the upper layer is completed first, then the lower layer is excavated after the support is completed, and then the pouring is carried out from bottom to top. Considering the complex living environment near the ancient levee and the loose filling behind it, the upside-down hanging well excavation method is adopted, which can be used for excavation and support. For upside-down hanging well construction, the upper open trench (Figure 2 b) shall be excavated first, and the concrete longitudinal beam of the straight trench opening shall be poured. The upside-down hanging well shall be built below. It is backfilled with C25 concrete, and the diameter and depth are 1.6 m and 5.0 m (Figure 2 c, d, e and f). Two rows of the cement grouting are used for the anti-seepage reinforcement of the lower foundation of the well, with the row spacing of 0.5 m. The hole spacing at the side close to the levee is 2.0 m, and the second row is 1.0 m. The lower 1.0 m depth is bedrock grouting (Figure 2 g).

Figure 2. The construction process of the upside-down hanging well and curtain grouting. (a) The construction scene of the levee reinforcement, (b) the foundation excavation, (c) the cast in situ concrete formwork support for the upside-down hanging well, (d) the upside-down hanging well wall after pouring, (e) the fabrication of the reinforcement cage in the well, (f) the concrete backfilling, (g) the curtain grouting, (h) the schematic diagram of the anti-seepage reinforcement system.

 

Comment 2. In section 2, the material and methodology were proposed. The numerical model calculation theories were developed. How do you consider the water migration? This is the most important characteristic of unsaturated soil.

A: We thank your comments. Due to the existence of saturated and unsaturated regions in soil during water level fluctuation, this will have a great impact on seepage. In the manuscript, we use the unsaturated soil theory to calculate the seepage. without considering the microscopic migration of waterWe first obtain the mechanical suction through the pressure plate method, and obtain the relationship between the matrix suctions and the volume water contents, namely the soil water characteristic curve (SWCC). Then, the relationship between the permeability coefficients and the matrix suctions is established by Formula (6), and the relationship between the permeability coefficient and the volume water content is established by Formula (7).

 

Comment 3. In section 3, the result and discussion were presented. The author introduces many contents about the test results. But these are irregular and theoretical analysis. It is lack of innovation. Besides, model validation is needed. It needs to be added

A: Thank you very much for your comments. We have modified this part and added a subsection to verify the correctness of the numerical model in ‘Result and discussion’ section as follows:

4.4 Result Verification

To verify the correctness of the above calculation results, the water level monitoring data and the levee foot pavement displacement measured data during the construction period are selected for comparative analyses of the calculation sections of the ancient levee, as shown in Figures 18 and 19.

 

Figure 18. The comparison between monitoring data and calculated values of the water level near the section from Mar. 17 to Apr. 1, 2021

 

Figure 19. The comparison between measured data and calculated values of the horizontal and vertical displacement of the levee foot pavement during the construction period

It can be seen from Figures 18 that the calculated and monitored groundwater levels change with the Lan River water level, and their change laws are consistent, which is in good agreement. According to Figure 19, the measured data of the horizontal and vertical displacements of the levee foot pavement during the construction period are also in good agreement with the calculation results. Since the stability safety factors cannot be verified by tests or monitoring data, it can be seen from the change laws of the stability safety factors with the water level before and after the reinforcement, the calculation results are basically consistent with the actual situation of the project. The above analyses show that the selection of the seepage, displacement and stability calculation methods and parameters is correct.

 

 

Comment 4. In section 4, the conclusions were presented. There is too much to say about the conclusion of the article. It needs to be simplified. What is the most important conclusion?

A: Thank you for your comments and suggestions. We have simplified the conclusion, and the revised conclusion is as follows:

There is a lack of systematic analysis and research on the use of the upside-down hanging well and curtain grouting for the anti-seepage reinforcement of the ancient levee. This paper studies the stability change law of the ancient levee during the flood process before and after reinforcement. The main conclusions are as follows:

(1) After reinforcement, the flood process significantly affects the pore water pressure of the filling soil between the levee and the upside-down hanging well, showing a similar change law as before the reinforcement. The change of the water level has little influence on the fill pore water pressure, which is beneficial to the ancient levee stability.

(2) The horizontal displacement of the ancient levee and the fill presents the opposite change law before and after reinforcement during the flood process. Before reinforcement, the maximum horizontal displacement of the upper part of the levee reached 2.75 cm, and it was toward the Lan River. This was caused by the soil deformation squeezing the levee after the water level rose. After reinforcement, the ancient levee and fill shifted away from the Lan River, mainly due to the hydrostatic pressure caused by the rising water level.

(3) The safety factors of the reinforced ancient levee also fluctuated significantly with flood fluctuation, and the change was basically consistent with the water level variation. Their minimum and maximum values were 1.727 and 2.273, respectively. During the water level fluctuation, the safety factor only decreased by 24.02%, which was half of the change range of the safety factors before reinforcement. The stability of the reinforced levee is largely improved.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments for author File: Comments.pdf

Author Response

Please see the attachment for the reply to the comments

Author Response File: Author Response.pdf

Reviewer 4 Report

In this manuscript, the stability change of Lanxi ancient levee before and after reinforcement is analyzed through the construction of a numerical model by selecting the maximum flood process in recent years, the influence of water level change on the stability of the ancient levee after anti-seepage reinforcement is evaluated, and the evolution law of the stability safety coefficients during the flood process and the reinforcement effect are quantitatively revealed. This research is innovative, and this manuscript can be accepted after minor modifications.

(1)    In the introduction: Please clarify the significance and innovation of this research.

(2)    The seepage characteristics of geomaterials also have great influence on the research results of this paper. Unfortunately, there are no sentences related to seepage characteristics of geomaterials in the introduction. It is suggested that the author add a brief introduction to the research results in this field, and The following documents are recommended:

Experimental study on permeability characteristics of calcareous soil. Bulletin of Engineering Geology and the Environment. 2017, 77(4): 1753–1762. DOI:10.1007/s10064-017-1104-6.

Study of permeability of calcareous silty layer offoundation at an artificial reclamation island. Rock and Soil Mechanics. 2017, 38(11): 3127–3135.

(3)    How to determine Poisson's ratio in Table 1？

(4)    How to get the relationship curve between permeability coefficient and matrix suction in Fig. 7？

(5)    In section 4: Implications for engineering practice should be added

Author Response

Please see the attachment for the reply to the comments

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The revisions are satisfactory to this reviewer. The manuscript is recommended for publication after spell check.

Reviewer 3 Report

I am satisfied with the revised version.