influence of construction cracks on seepage

( 16 )

Influence of Construction Cracks on Seepage Characteristics of River Embankment in Tunnel Crossing Section

施工裂縫對隧道穿越段堤防滲流特性的影響

JIAN WU*

吳　　健College of Water Conservancy and Hydropower Engineering, Hohai

University, Nanjing, China, M.S. Student

LEI GAN

甘　　磊1. College of Water Conservancy

and Hydropower Engineering, Hohai University, Nanjing, China,

2. Research Center for embankment Safety and Disaster Prevention

engineering technology of the Ministry of water resources, Zhengzhou, China

Associate Professor

DAN ZHANG

張　　丹Research Center for embankment Safety

and Disaster Prevention engineering technology of the Ministry of water

resources, Zhengzhou, China, Researcher

ABSTRACT

After the tunnel-shield-driving machine excavated through the north levee of Yangtze River, several transverse cracks appeared in the river embankment, and the accumulative settlement of the embankment was massive. This paper investigates the influence of cracks in the embankment construction of different depths on the seepage stability of the Yangtze River embankment. Based on the three-dimensional saturated - unsaturated seepage finite element method, the three-dimensional finite element seepage analysis model of the embankment in the tunnel crossing section was established. The seepage behavior of the embankment under different crack depths within the highest water level in history is calculated. The results show that the deep mixing pile reduces the water head obviously. The through cracks have great influence on the seepage field of the stratum around the cracks. When the crack depth is less than 8.5 m, the maximum seepage gradient of dam foundation and dam body is 0.181 and 0.206 respectively, which is less than the allowable seepage gradient of corresponding soil layer, indicating the seepage stability of the embankment can meet the requirements.

Keywords: crossing section, river embankment, seepage, finite element method, stability.

摘　　　　　要

某長江隧道盾構機穿越江北大堤，堤身出現施工貫穿式裂縫，且累計沉降較大。本文考慮不同深度堤身施工貫穿式裂縫對長江大堤堤身滲流穩定性的影響，採用三維飽和－非飽和滲流有限元法，建立了隧道穿越段大堤的三維有限元滲流分析模型，計算分析了歷史

* Corresponding author: College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, China / Hohai University, 1 Xikang Road, Nanjing City, Jiangsu Province, China / [email protected]

臺灣水利　第 69 卷　第 2 期民國 110 年 6 月出版

Taiwan Water ConservancyVol. 69, No. 2, June 2021DOI: 10.6937/TWC.202106/PP_69(2).0003

( 17 )

1. INTRODUCTION

In recent decades, a large number of river-crossing tunnels were built in China. When Slurry Shield Tunnel Boring Machine method was used to build the underwater tunnel in the soft soil area, it inevitably went through the embankment (Wu et al, 2011). In view of the construction process of embankment in tunnel crossing section, the construction disturbance, soil excavation and other reasons contribute to the derivation of three key points to be noted in the construction stage. Firstly, the construction disturbance would lead to uneven settlement of the embankment, which would form cracks in the body of the embankment and seriously reduce the impermeability of the embankment, especially when the cracks pass through the embankment body. Secondly, the construction disturbance changes the compactness of the soil layers and causes the increase in permeability coefficient of the original soil layer, which may further lead to the increase in seepage discharge, and even result as seepage damage and contact damage (Pan et al, 2017). Thirdly, the seepage water flows out from the unlined section of the tunnel would result in the increasing seepage hydraulic gradient in the soil layer at the top of the tunnel (Cao, 2016).

During the operation period of the crossing-embankment tunnel, the embankment soil layer and the contact surface of the tunnel are weak, and prone to seepage damage. The seepage damage of the tunnel crossing section not only occurs in the embankment and its foundation, but also in the contact between the soil layer and the structure. The

disturbance of foundation caused by the directional drilling have a little effect on the seepage field of the embankment, but the large amount of mud gaps in the cohesive soil layer on the embankment would reduce the impermeability and anti-floating ability of the original embankment soil (Yu and Hu, 2006). The maximum seepage gradient is likely to occur in the contact surface between two material zones with the high different permeability coefficients (Chen, 2017). After the interception pipeline crosses, the seepage line of the embankment is higher than that before the crossing, and the seepage gradient increased after the crossing of the pipeline (Pan and Wang, 2014; Li et al, 2007). The results show that the project has little impact on the safety of the embankment. The influence of the thickness of the tunnel and the groundwater on the osmotic stability of the embankment project was studied by Zhang (2016). Sun (2012) had studied the influence of excavation engineering on the permeability characteristics of rock and soil. Mao et al (2014) had investigated the influence of the project on the seepage stability of the embankment.

The construction and operation of the tunnel through the embankment would change the original seepage field of the embankment. Construction cracks would increase the permeability coefficient of the embankment and the hydraulic gradient at the top of the tunnel, which would affect the safety of the embankment. This paper presents the investigation on the effect of construction cracks on river embankment in the tunnel crossing section at Yangtze River. The influence of construction cracks and deep mixing pile on the seepage characteristics of the embankment was studied by

最高水位條件下不同裂縫深度下堤防的滲流性態。計算結果表明：深層攪拌樁消減水頭的作用明顯；貫穿式裂縫對裂縫周圍地層的滲流場影響較大，但當裂縫深度小於8.5 m，堤身與堤基的最大滲透坡降分別為0.206和0.181，均小於相應材料的允許滲透坡降值，大堤的整體滲流穩定能滿足要求。

關鍵詞：穿越段，堤防，滲流，有限元法，穩定。

( 18 )

the three-dimensional saturated-unsaturated seepage finite element method, and the stability of the embankment was evaluated.

2. SEEPAGE THEORY

It is assumed that the seepage in unsaturated soil is in accordance with the Darcy Law. The difference with seepage in saturated soil is that the seepage coefficient of unsaturated soil is not a constant, but a function of soil saturation. The basic differential equation for saturated-unsaturated seepage is as follows:

(1)

where: hc is the pressure head, ksi j is the saturated

permeability coefficient tensor, ki3 is the permeability coefficient value related to the third coordinate axis in the saturation permeability coefficient tensor, kr is the relative permeability, which is the ratio of the permeability coefficient of unsaturated soil to that of the saturated soil, in the unsaturated area 0 < kr < 1, in the saturated area kr = 1, C is the specific water capacity, in the positive pressure area, C = 0, β is the saturated-unsaturated selection constant, equal to 0 in the unsaturated area and equal to 1 in the saturated area, Ss is the specific yield or storage coefficient in the unconfined aquifer, Ss in saturated soil is a constant, and Ss = 0 in unsaturated soil, when the soil skeleton and water compressibility are neglected, there is also Ss = 0 for the saturated area, Q is the source term.

The definite conditions for the unsteady saturated-unsaturated seepage differential equation include initial conditions and boundary conditions.1) Initial conditions

hc (xi , 0) = hc (xi , t0), i = 1, 2, 3 (2)

2) Boundary conditionshc (xi , t)|Γ1 = hc1 (xi , t) (3)

(4)

and

hc|Γ3 = 0 (5)

(6)

where: ni is the boundary direction normal direction, t0 is the initial moment, hc1 is the known water head, qn is the known flow, qr(t) is the rainfall infiltration flow, hc(t0) is initial t0 time seepage field head, Γ1 is the known water head boundary, Γ2 is the known flow boundary, Γ3 is the rainfall infiltration boundary, Γ4 is the saturated exit surface boundary.

3. NUMERICAL SIMULATION CASES

3.1 Overview of the embankment

The embankment of a Yangtze River tunnel crossing section is filled with silty clay. The width of the embankment top is 8−10 m wide, top elevation is about 11.70 m. The top retaining wall is about 1.0 m higher than the embankment. The shield machine of the tunnel had led to the settlement of the embankment, which caused two cracks on the embankment surface. The construction period of the project occurred in flood season of the Yangtze River. In order to ensure the seepage safety of the embankment, it is necessary to analyze the seepage behavior of the embankment under the condition of historically highest water level and evaluate the seepage stability of the embankment.

3.2 Numerical model

The coordinate and range of the model are shown in Fig. 1. The calculation model simulates the main structure of the embankment and the geological conditions of the stratum. The finite

( 19 )

element mesh is generated by the super-unit method of the control section (Jiang et al, 2008; Wang et al, 2012). The total number of nodes and elements of the three-dimensional finite element model is 28,342 and 25,304 respectively. The boundary conditions of the model are determined as follows. Five boundaries (X = 0, 120 m, Y = 0, 120 m, and Z = 0 m) are taken as the impervious boundary. The upstream slope is taken as the upstream known head boundary of the model. The part above and below the downstream water level of the downstream slope are taken as the downstream seepage boundary and the downstream known head boundary, respectively.

3.3 Simulation parameters and conditions

Based on the engineering geology and geological exploration results, the permeability coefficient of each material are shown in Table 1. The historical maximum water level (10.22 m) was taken as the upstream water level and the downstream water level is 6.0 m. The depth of the crack at the top of the embankment in the model

would change for different conditions, and the effect of the construction cracks on the embankment and the seepage behavior of the embankment were analyzed. The proposed simulation conditions are shown in Table 2. Based on the equivalent treatment method, the fracture is assumed to be an equivalent continuous permeability medium with a high permeability coefficient (103 cm/s).

4. RESULTS

The results of the seepage exit gradient, the maximum penetration gradient of the embankment and the seepage flow, are presented in Table 3. Here, the seepage flow per unit width means the average flow of unit width along the axis of the embankment within the depth range of the model.

The groundwater level contour distribution of Condition 6 is shown in Fig. 2. According to the contour map of groundwater level, the distribution law of the groundwater level contour of the embankment is clear under different conditions,

Fig. 1. The 3D finite element model (unit: m).

Table 1. Permeability coefficient of each material zones

Material zones Permeability coefficient (cm/s) Material zones Permeability coefficient (cm/s)masonry facing 5E-01 silt 3E-04

mixing pile 5E-07 fine sand 4E-03clay 2E-06 muddy silty clay 6E-06

mucky silty clay 3E-06 plain fill 3E-05

( 20 )

Table 2. Simulation conditions

Condition Crack depth (m) Water level (m) RemarksC-1 0 Upstream water level 10.22,

downstream water level 6.00Without the deep mixing pile

C-2 0 Upstream water level 10.22, downstream water level 6.00

With the deep mixing pile

C-3 5.5 Upstream water level 10.22, downstream water level 6.00









Without the deep mixing pile

Table 3. Seepage results

Conditions Exit gradientMaximum seepage

gradient of embankment foundation

Maximum seepage gradient of embankment

body

Seepage flow per unit width (m3/d/m)

C - 1 0.171 0.129 0.208 7.445C -2 0.071 0.152 0.189 6.618C - 3 0.073 0.153 0.192 6.846C - 4 0.074 0.166 0.195 6.916C - 5 0.074 0.173 0.199 7.015C - 6 0.075 0.181 0.206 7.146C - 7 0.179 0.191 0.376 8.547

Note: The soil layer with the maximum permeable gradient of the embankment foundation is a muddy silty clay layer, and the allowable value is 0.45. The soil layer with the maximum seepage gradient of the embankment is a clay layer, and the allowable value is 0.40. Escape occurs in the plain fill layer, and the allowable value is 0.35.

Fig. 2. Groundwater level contour in the embankment area of Condition C−6 (unit: m)

( 21 )

and most contour lines are evenly distributed from the upstream to the downstream. The potential contour in the deep mixing pile is obviously dense and the saturated surface obviously decreased, which indicates that the mixing pile can effectively decrease the water head.

The water potential distribution of section Y = 60 m is shown as in Fig. 3. The water potential distribution near the cracks is shown in Fig. 4. Fig. 3 and Fig. 4 show that the potential contour at different crack depths rise evenly, occur sudden change at the crack, and protrude upstream. The increase of local seepage gradient has a great influence on the seepage field in this area, which may damage the stability of soil mass, and cause crack expansion or formation subsidence downstream of the crack, and endanger the stability of the embankment.

The seepage flow per unit width in Condition C−2 is 6.618 m3/d/m, and that of Condition C−3, C−4, C−5 and C−6 are 6.846 m3/d/m, 6.916 m3/d/m, 7.015 m3/d/m, 7.146 m3/d/m respectively. Compared with Condition C−2, the seepage flow increased by 3%, 4%, 6% and 8% respectively. The calculation shows that with the increase of crack depth, the seepage flow per unit width increases. After deep mixing pile anti-seepage treatment, as the difference in Condition C−1 and Condition C−2, the deep mixing pile can reduce the saturated surface in the embankment and decrease the effect of construction cracks on the leakage of embankment, and effectively reduce the influence of the penetration crack depth on the seepage flow of the embankment.

As shown in Table 3, from Condition C−2 to Condition C−6, the maximum seepage gradient of the embankment is 0.206, which appears not far from the upstream of the mixing pile. The maximum seepage gradient of the embankment foundation is 0.181, which occurs near the embankment crack. The maximum exit gradient of downstream slope is 0.075. All seepage gradient values are less than the allowable values of the corresponding materials. When the penetration crack depth reaches 11.5

Fig. 3. Water potential distribution of the embankment (section y = 60 m).

( 22 )

m, as shown in Condition C−7, the maximum seepage gradient of the embankment is 0.376, which appears in the plain fill layer, and is higher than the allowable value 0.35. The conclusion can be drawn that once the penetration crack depth exceeds 11.5 m, without the deep mixing pile anti-seepage treatment, seepage failure would occur in the embankment.

5. CONCLUSION

(1) When the initial construction crack depth is less than 8.5 m, the maximum seepage gradients of the embankment body, embankment foundation and the escape section are smaller than the allowable values of the corresponding soil layers, which meet the requirements of seepage stability.

(2) When the depth of the penetrating crack in the top of the embankment changes, the potential contour of the embankment changes suddenly, and the local seepage gradient increases. The cracks may endanger the stability of the embankment. When the construction crack depth reaches 11.5 m, the maximum seepage gradient of the plain fill layer is higher than the allowable

value, which endanger the embankment and may result in the seepage failure.

(3) The deep mixing pile can reduce the saturated surface in the embankment and decrease the effect of construction cracks on the leakage of the embankment. In addition, the deep mixing pile would also control the further development of construction cracks.

ACKNOWLEDGMENTS

This work was supported by the Open Program of Safety and Disaster Prevention Engineering Technology Research Center of the Ministry of Water Resources (2018001), the National Natural Science Foundation of China (51609073), the Fundamental Research Funds for the Central Universities (2018B11514), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (YS11001).

REFERENCES

Chen Hongren, Zhang Guijin, and Li Yi. (2017). “Study on the Influence of River-Crossing Pipeline to the Permeability Safety of Embankment.” Yellow River., 39(02), 34-37. [in Chinese]

Jiang Hang, Shen Zhenzhong, and Qiu Qianyong. (2008). “Improvement of Finite Element Method and Its Validation in 3-D Unstable Saturated-unsaturated Seepage.” Water Resources and Power., 03, 54-56. [in Chinese]

Li Diyuan, Li Xibing, Zhang Wei and Gong Fengqiang. (2007). “Stability analysis of surrounding rock of multi-arch tunnel based on coupled fluid-solid theorem.” Chinese Journal of Rock Mechanics and Engineering., 2007(05), 1056-1064. [in Chinese]

Mao Changxi, Duan Xiangbao, Cai Jinbang and Hu Zhaowang. (2014). “Empirical formulas for important parameters of unsteady seepage of levees.” Journal of Hydraulic Engineering., 2004(01): 52-56. [in Chinese]

Pan Wenwei, and Wang Jianping. (2014). “Impact analysis of pipeline crossing the North River embankment.” Pearl River., 35(06), 93-94. [in Chinese]

Pan Xuezheng, Chen Guoqiang and Peng Ming. (2007). “Risk Assessment on Shield Excavation in Very Large Tunnel Across Qiantang River.” Chinese Journal of

Fig. 4. Water potential distribution around the crack under different conditions (unit: m).

( 23 )

Underground Space and Engineering., 1245-1254. [in Chinese]

Sun Xushu, Li Jianlin, and Deng Huafeng. (2012). “Analysis on Effect of Permeability Characteristics in Rock and Soil Mass by Excavation Engineering.” Journal of Wuhan University of Technology., 34(09), 74-79. [in Chinese]

Wang Dong, Shen Zhenzhong, and Tao Xiaohu. (2012). “Three-dimensional finite element analysis and safety assessment for seepage field of a tailings dam.” Journal of Hohai University (Natural Sciences)., 40(03), 307-312. [in Chinese]

Wu Shiming, Lin Cungang, and Zhang Zhongmiao. (2011). “Risk analysis and control for slurry shield under-passing embankment.” Chinese Journal of Rock

Mechanics and Engineering., 30(05), 1034-1042. [in Chinese]

Yu Jianping, and Hu Weizhong. (2006). “Influence of directional drilling through embankment on the embankment foundation and design of seepage prevention engineering.” Yangtze River., 08, 81-83. [in Chinese]

Zhang Guijin, Liu Liling, and Chen Hongren. (2016). “Study on the influence of the cross-river tunnel to the embankment project.” Journal of Railway Science and Engineering., 13(10), 2018-2026. [in Chinese]

Received: 108/12/02

Revised: 108/12/24

Accepted: 109/01/17

influence of construction cracks on seepage

Documents