Deformation Law of the Diaphragm Wall during Deep Foundation Pit Construction on Lake and Sea Soft Soil in the Yangtze River Delta

So far, there have been a large number of diaphragm walls in the Yangtze River Delta as engineering examples of deep foundation pit maintenance structures in subway stations, but there is a lack of systematic research and summary on the deformation characteristics of ground connecting walls. This study aimed to clarify the deformation law of the diaphragm wall during the excavation of a deep foundation pit in a soft soil region. Based on the monitoring data of the diaphragm wall of the deep foundation pit of the Hangzhou metro station, the monitoring data of the deep foundation pits of 15 subway stations in Shanghai and Ningbo cities around Hangzhou were considered. Grouping and classiﬁcation methods were used to analyze the similarities and diﬀerences in the deformation characteristics of the diaphragm wall in the three regions. The results indicate the following: the maximum lateral deformation of the diaphragm wall in Hangzhou increases linearly with the relative depth of the maximum lateral deformation. The maximum lateral deformation of the foundation pit in Hangzhou is 0.072% H ∼ 0.459% H, with a mean of 0.173% H. The wall deformation in Hangzhou varies signiﬁcantly with the depth of the foundation pit, but the inﬂuence of the depth of the foundation pit on the wall deformation is considerably less than that in Shanghai and Ningbo. The corresponding position of the maximum lateral deformation in the excavation depth increases linearly with the excavation depth of the foundation pit, and the corresponding position of the lateral deformation of the diaphragm wall in Shanghai is more aﬀected by the excavation depth of the foundation pit. The lateral deformation of the diaphragm wall increases rapidly in the range of 0 H–0.5H, and the maximum lateral deformation occurs at 0.5H–1.1 H.


Introduction
Soft soil has significant structural properties including high compressibility, high water content, high sensitivity, low strength, low permeability, and low bearing capacity [1], resulting in high-risk excavation of the foundation pit in a soft soil region and the occasional occurrence of disastrous foundation pit accidents therein. Soft soil is widely present throughout China's coastal areas, and the rapid economic development and a high degree of urbanization in coastal regions have increased the excavation scale of foundation pits in soft soil areas. erefore, the special engineering properties of soft soil not only lead to a significant construction risk to the excavation of a foundation pit but also provide a wide space for the optimization design of construction personnel.
Owing to the complexity and uncertainty of the site conditions in the excavation of deep foundation pits, the influence of various factors on deformation cannot be considered simultaneously using the existing theory. e on-site monitoring data embodies the comprehensive effect of various factors in the construction process. erefore, an indepth analysis of the monitoring data becomes an effective source to understand the deformation characteristics of deep excavation in soft soil. However, because monitoring data collection is generally a time-consuming process, and some of the foundation pit monitoring data collection does not meet the requirements of the specifications, the use of limited monitoring data to effectively analyze the deformation law of the foundation pit needs to be studied urgently.
Several studies have been conducted to effectively analyze the deformation law of a deep foundation pit. Clough, Long, Boone, Moormann, and others have statistically analyzed the construction process of foundation pits under different geological conditions worldwide; they investigated the deformation characteristics of various foundation pits by category and obtained the deformation laws of the foundation pit, which have been widely used in foundation pit design and construction [2][3][4][5]. I. H. L. Wong, Musuda, Leung, Hsieh, and Pakbaz analyzed and summarized the maximum lateral displacement, settlement, and the maximum deformation position of the foundation pit in Singapore, Japan, Hong Kong, Taipei, Iran, and other regions, respectively, during foundation pit excavation; their proposed rules were widely applied to the engineering practice [6][7][8][9][10]. Wang, Xu, Li, Zhang, Mei, and Elbaz analyzed the deformation characteristics and influencing factors of foundation pits in Shanghai, Beijing, Jinan, Xi'an, Guangzhou, and other regions, respectively; their findings can be used as an effective reference for engineering practice in these regions [11][12][13][14][15][16]. In addition, Alipour, Farzi, Zhang, Hsieh, and Cui analyzed the deformation laws of representative single foundation pit excavation projects in Tehran, Awaz, Singapore, Taipei, and Nanjing; these results enrich the practice of foundation pit excavation in this region and can be used as a useful supplement for the deformation laws of the foundation pit [17][18][19][20][21]. e studies mentioned above provide an effective reference for the analysis of the foundation pit excavation law in a soft soil region for this study; however, the study of the deformation law of the diaphragm wall regarding the deep excavation of the metro station in the Hangzhou region is still insufficient. is study is based on monitoring data of the deep foundation pit in Hangzhou; 15 subway stations in the cities of Shanghai and Ningbo, which are in the proximity of Hangzhou, are also considered. Grouping and classification methods were used to analyze the similarities and differences in the deformation characteristics of the diaphragm wall in the three regions. e results can provide a reference for the targeted design and construction of the diaphragm wall of a station's deep foundation pit in varying soft soil regions. e improvement of the deformation mechanism of a station's deep foundation pit and design theory and the reduction in construction risk are also significant. e main soft soil distribution along the coast of China, which is analyzed in this study, is shown in Figure 1.

Geological Conditions.
e common characteristics of soft soil, such as significant natural water content, high compressibility, and low strength, make the engineering properties of soft soil poor; the soft soil engineering properties vary significantly across China, with apparent regional characteristics. All these factors increase the construction risk of the foundation pit excavation of a subway station in a soft soil area. e urban area of Hangzhou is located south of the Yangtze River Delta, the North Bank of the lower reaches of Qiantang River, and the west end of Hangzhou Bay; having a complex landform, the soft soils in the Hangzhou are mainly Quaternary Holocene marine and lacustrine soft soils, of which silt, silty clay, muddy clay, and clay distributed within 2-20 m below the surface are the main soft soil layers affecting project construction. Shanghai is located in the southeast front of the Yangtze River Delta; the soft soil layer is distributed stably in the coastal area, mainly composed of muddy, silty clay and lower muddy clay, and the thickness of the soil layer generally ranges from 10.0 m to 20.0 m. Ningbo is located south of the Yangtze River Delta; its coastal soft soil is mainly composed of muddy, silty clay and muddy clay, and the typical soft soil thickness is more than 10 m. e physical and mechanical parameters of the soft soil of the three locations are shown in Table 1 [22][23][24][25].

Foundation Pit Overview.
To reduce the number of factors influencing foundation pit deformation, the statistical data in this study include all the deep foundation pits of the metro station; the maintenance structure is the diaphragm wall, and the internal support is the form of the reinforced concrete brace + steel brace. e basic information of the station is shown in Table 2. e variables used in this study are shown in Figure 2

Relationship between the Maximum Lateral Deformation and Relative Depth of Maximum Lateral Deformation of the Diaphragm
Wall. e maximum lateral deformation of a diaphragm wall is related to the relative depth of the maximum lateral deformation [9,12]. Figure 3 indicates the relationship between the maximum lateral deformation (U hmax ) and the relative depth of the maximum lateral deformation (H h /H) of the diaphragm wall. As shown in Figure 3(a), the relative depth of the lateral deformation of the diaphragm wall in Hangzhou is 0.5-2, with a mean value   Advances in Civil Engineering of 1.46; the greater the relative depth of the maximum lateral deformation, the greater the maximum lateral deformation. ere is a linear relationship between the maximum lateral deformation of the diaphragm wall and the relative depth of the maximum lateral deformation. According to previous studies [28,29] and Table 1, the significant change in the relative depth of the lateral deformation of the diaphragm wall in the Hangzhou region is primarily due to the fact that the characteristics of the soft soil at the bottom of the pit are quite different during the excavation process. When the bottom of the foundation pit consists of moderately compressible soft soil, the relative lateral displacement depth of the diaphragm wall will decrease, and the maximum lateral deformation of the diaphragm wall will also decrease. When    the bottom of the foundation pit consists of high compression soft soil, the relative lateral deformation of the diaphragm wall will increase, and the corresponding lateral deformation of the diaphragm wall will also increase. erefore, in order to reduce the lateral deformation of the diaphragm wall, the soil at the bottom of the pit can be properly reinforced by mixing piles and cast-in-place piles to increase the strength of the soil [26].
As shown in Figures 3(b) and 3(c), the relative depth of the lateral deformation of the diaphragm wall in Ningbo is 0.92-1.02, with a mean of 0.98; the relative depth of the lateral deformation of the diaphragm wall in Shanghai is 0.94-1.1, with a mean of 0.99. e relative depth of the maximum lateral deformation of the diaphragm wall in the two locations changes insignificantly, mainly because the soft soil properties at the bottom of the two pits do not change significantly; most of the foundation pit bottom is reinforced [35,36], which also makes the maximum lateral deformation relative depth of the diaphragm wall in the two locations smaller than that in Hangzhou.

Influence of the Insertion Ratio on the Maximum Lateral Deformation of the Diaphragm Wall.
e insertion ratio of the diaphragm wall (the ratio of the embedded depth below the foundation pit bottom of the ground connection wall to the depth of the foundation pit) is an important factor affecting the excavation deformation of the foundation pit. In general, when the wall has sufficient strength and rigidity, the degree of deformation of the foundation pit can be effectively reduced and the coefficient of stability against the uplift of the soil can be improved by appropriately increasing the insertion ratio [15]. Figure 4 presents the relationship between the insertion ratio of the diaphragm wall and the maximum lateral deformation of the diaphragm wall.
As shown in Figure 4, the maximum lateral deformation of the diaphragm wall decreases with an increase in the insertion ratio. As shown in Figure 4(a), the insertion ratio of the diaphragm connection wall in Hangzhou is apparently 0.73-1.1, with a mean value of 0.95; excluding the anomalous data of Group 1, the insertion ratio in Hangzhou has a clear linear relationship with the maximum lateral deformation of the diaphragm wall. As shown in Figure 4(b), the insertion ratio of the diaphragm wall in Shanghai is 0.81-1.02, with a mean value of 0.93; the insertion ratio of Shanghai also has a linear relationship with the maximum lateral deformation of the diaphragm wall, but the correlation during a linear fitting for the coefficient R 2 is excessively small. is is mainly because the statistical sample in Shanghai is small; therefore, there is an accidental error in data fitting, but it has the same deformation trend as Hangzhou. No apparent rules have been found in Ningbo owing to insufficient data. Considering the analysis above, if the maximum lateral deformation of the diaphragm wall is too large in actual projects, it can be limited by appropriately increasing the insertion ratio of the diaphragm wall.

Influence of the Aspect Ratio on the Maximum Lateral
Deformation of the Diaphragm Wall. Studies have shown [15] that the aspect ratio of foundation pit excavation has a significant effect on the stability of the foundation pit. Figure 5 presents the relationship between the aspect ratio of the foundation pit and the lateral deformation of the diaphragm wall.
e maximum lateral deformation of the diaphragm wall increases with the increase of aspect ratio of the foundation pit, as shown in Figure 5. However, the correlation coefficient between the two is apparently not large; in addition, the effect of the ratio of the length to the width on the lateral shape of the diaphragm wall is not clear and requires further study.

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However, the basic trend of the aspect ratio regarding the lateral deformation of the wall is similar to that reported in [13,15].

Relationship between the Depth of the Foundation Pit and the Maximum Lateral Deformation of the Wall.
e relationship between the maximum lateral deformation of the wall and the depth of the foundation pit is shown in Figure 6; the maximum lateral deformation of the wall increases with the increase of the depth of the foundation pit. e maximum lateral deformation of the foundation pit in Hangzhou is 0.072% H∼0.459% H, with a mean value of 0.173% H, and that in Shanghai is 0.171% H∼0.373% H, with a mean value of 0.258% H, similar to the statistical laws of Xu Zhonghua and Yang Min. e maximum lateral deformation of the foundation pit in Ningbo is 0.195% H∼0.327% H, with a mean value of 0.270% H.
Considering the comparison, the wall deformation in Hangzhou varies significantly with the depth of the foundation pit, but the influence of the depth of the foundation pit on the wall deformation is far less than that in Shanghai and Ningbo; the wall in Shanghai and Ningbo is substantially affected by the depth of the foundation pit. As shown in Table 2, there are more types of soft soil in Hangzhou than in the other two locations. Hence the variation of the influence of the foundation pit excavation on the diaphragm wall is significant, which is an important reason why the wall deformation in Hangzhou varies widely with the depth of the foundation pit. erefore, Shanghai and Ningbo should pay attention to the influence of the depth of the foundation pit on the deformation of the walls of the two places during the construction process; reducing the lateral deformation of the wall by increasing the internal support can be considered [37]. Hangzhou can consider reducing the internal support density to improve the engineering economy on the premise of ensuring safety.
Based on the aforementioned analysis, to compare the deformation performance of the support system under different excavation conditions, the statistical data regarding the lateral deformation of the support in other regions are summarized in Table 3.
e results indicate that the relationship between the maximum lateral deformation of the wall in Hangzhou and the depth of the foundation pit is similar to that in Tokyo, and the relationship between the maximum lateral deformation and the depth of the foundation pit in Shanghai and   Advances in Civil Engineering Ningbo is similar to that of Taipei; however, the change range is smaller than that of Taipei. e lateral deformation of the foundation pits in Singapore, Dublin, Hong Kong, Guangzhou, Jinan, and other regions is relatively small, owing to the nature of the soil layer and the construction technique. e foundation pits in Singapore and Jinan were constructed in soft soil, but the support system used bored pile or sheet pile support. e bored pile or sheet pile support structure is believed to have stronger constraints on the lateral deformation of the wall; hence, when the lateral deformation of the wall exceeds the specification, the drilling pile strength, sheet pile support, or combined support can be considered. Figure 7, the greater the excavation depth, the deeper the position corresponding to the maximum lateral deformation. ere is an apparent linear relationship between the excavation depth and the position corresponding to the maximum lateral deformation within the excavation depth. e corresponding position depth of the lateral deformation of the diaphragm wall in Shanghai is more affected by the excavation depth of the foundation pit. According to the statistical law [2,13], the lateral deformation of the diaphragm wall is generally "bow-shaped" during the excavation of the foundation pit. Hence the soil at the maximum lateral deformation of the diaphragm wall will not suddenly reach 0. e excavation of the foundation pit in Shanghai has a wider influence on the diaphragm wall and soil because the corresponding depth of the diaphragm wall is more affected by the excavation depth of the foundation pit. When the excavation depth of the soft soil foundation pit in Shanghai gradually increases, the influence of the soil excavation on the diaphragm wall and the lateral deformation of soil should be considered during the design and construction.

Relationship between Excavation Depth and Maximum
Lateral Deformation within Excavation Depth. Figure 8 presents the trend line of the relationship between the excavation depth and the maximum lateral deformation within the excavation depth; during the excavation of the two foundation pits, as the excavation depth increases, the maximum lateral deformation of the wall in the excavation depth continues to increase, but the deformation rules of the two are different. When the excavation depth of the foundation pit is shallow in Shanghai, the maximum lateral deformation of the diaphragm wall is relatively large. e maximum lateral deformation rate of the diaphragm wall gradually decreases as the excavation depth increases, corresponding to the slope of the fitting curve gradually decreasing in Shanghai. When the excavation of the foundation pit is shallow in Hangzhou, the maximum lateral deformation of the diaphragm wall is small. e maximum lateral deformation rate of the diaphragm wall gradually increases as the excavation depth increases, corresponding to the slope of the fitting curve in Hangzhou gradually increasing. Considering the maximum lateral deformation law of the diaphragm wall during the excavation of the two locations, the wall deformation at the initial stage of the foundation pit excavation in Shanghai should be focused on; increasing the top layer of the internal support to reduce the influence of the larger wall deformation in the early stage can be considered. Hangzhou should focus on the influence of the increase in the wall deformation rate deeper in the excavation of the foundation pit. e deformation law can be appropriately used by considering a small initial deformation and fast deformation rate in the later period by reducing the upper support density and increasing the lower support density. Figure 9 presents the statistical distribution curve of the horizontal lateral displacement along with the wall depth when the diaphragm wall is excavated to the bottom of the pit; the wall deformation changes into a "bow" distribution mode being large in the middle and small at both ends, which is similar to the law summarized in [2,13]. ere is a small range of change for the diaphragm wall in Hangzhou, but that in Shanghai and Ningbo is large, with an insignificant difference between them. A small number of foundation pits have negative lateral displacement values at the top of the wall, generally within 5 mm, which is mainly due to the excessive axial force of the first support or the outward thrust of a gantry crane erected on the crown beam.  In order to further analyze the wall lateral deformation mode, the lateral deformation curve is considered dimensionless, for which the horizontal axis is the ratio of the lateral displacement to the maximum lateral displacement, and the vertical axis is the ratio of the excavation depth to the foundation pit depth. e results are shown in Figure 10.

Lateral Deformation Mode of the Wall.
As shown in Figure 10, although the construction and soil conditions, as well as the construction methods for each foundation pit, are quite different, the upper and lower limits of a 95% confidence interval of the dimensionless cubic polynomial can well envelope the deformation curve of the foundation pit, which is similar to the results of the dimensional analysis conducted by Li Shu [13]. e lateral displacement of the diaphragm wall increases rapidly in the range of 0 H-0.5 H, and the maximum lateral displacement occurs at a position between 0.5 H-1.1 H. e maximum lateral displacement of a diaphragm wall decreases gently in the range of 1.1 H-2.0 H. After 2.0 H, the lateral displacement of the diaphragm wall is negligible. e analysis apparently indicates that the foundation pit must be supported in a timely manner in the early stage due to the rapid growth of the diaphragm wall lateral displacement. When the foundation pit is excavated to half of the depth of the foundation pit, the lateral deformation of the wall essentially reaches its maximum value; strict monitoring should be performed to prevent construction safety problems due to the deformation of the foundation pit exceeding the specified limit.

Discussion
e manuscript systematically compares and analyzes the lateral deformation modes of the Diaphragm wall during the excavation of the soft soil foundation pits of subway stations in different regions of the Yangtze River Delta and points out the differences between their changes. At the same time, the influence of the aspect ratio and the insertion ratio on the deformation characteristics of the ground wall is analyzed.
is provides a reference for guiding the optimization of the excavation design plan and construction plan of the subway station foundation pit in different soft soil areas of the Yangtze River Delta. However, the manuscript is limited by the limited statistical data of Ningbo soft soil area, and it is not comprehensive enough to summarize the deformation characteristics of the ground wall in the Ningbo. e content needs to be improved with the development of engineering construction in Ningbo.    linearly with the increase of the excavation depth of the foundation pit. (ii) When the excavation depth of the foundation pit is shallow in Shanghai, the maximum lateral deformation value of the diaphragm wall is larger, and the maximum lateral deformation rate of the diaphragm wall gradually decreases with the increase of the excavation depth. However, the relationship between the maximum lateral deformation value and the deformation rate with depth during the excavation of the diaphragm wall in Hangzhou is opposite to that in the Shanghai area. Data Availability e data used to support the findings of this study are available from the first author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.