岩土工程技术

2024, 38(1): -1--1.

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2024, 38(1): .

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Analysis of Force and Deformation of a Double-row Steel Pipe Pile Cofferdam in a Foundation Pit in a Lake

Zhuang Xuzhe, Yuan Congcong, He Xiaohui, Zeng Chen, Shi Yufeng, Ding Haibin, Meng Fanshi

2024, 38(1): 1-6. doi: 10.3969/j.issn.1007-2993.2024.01.001

[Abstract](184) [FullText HTML] (47) [PDF 1893KB](54)

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Cofferdams need to be used to ensure the dry construction site for foundation pit located in water areas. However, the study on cofferdams deformation is still not perfect. Based on the research background of Aixihu Tunnel project in Nanchang, Midas GTS NX finite element software was used to study the influence of cofferdam width and tie bar position on the deformation law of double-row steel pipe pile cofferdam. The model is proved to be correct by comparing the simulation results with the actual measurements. The results show that the cofferdam width has an appropriate range, too small and too large will cause the cofferdam deformation is too large. In this project, the reasonable width of cofferdam should be 3.5～4.5 m. The change of cofferdam width has little influence on the excavation deformation of cofferdam foundation. Considering the deformation of both sides of the steel pipe pile, the deformation of the tie rod is small when it is set at 0.8 m on the top of the cofferdam.

Research on Conditioning Method of Gravel Soil in Qingdao Metro Shield Construction

Ji Haifeng, Liu Weijing, Zhao Guoliang, Niu Yuzhe, Xiao Fengchun, Jiang Yusheng, Yang Xing

2024, 38(1): 7-13. doi: 10.3969/j.issn.1007-2993.2024.01.002

[Abstract](104) [FullText HTML] (36) [PDF 3544KB](24)

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Shield tunneling in gravel stratum is often faced with many difficulties such as large torque of cutterhead, tool wear, and poor slag. Aiming at the research problem of reasonable conditioning index of gravel soil, the typical gravel soil layer of Qingdao Metro Line 4 is taken as the research object. Firstly, the basic characteristics of gravel soil are evaluated by particle screening, LCPC test, and permeability test. Five groups of gravel soil conditioning schemes were designed based on the traditional conditioning schemes of sand and clay soil. Slump test, permeability test, and direct shear test were carried out to analyze the flow plasticity, permeability and shear resistance characteristics of gravel soil with different conditioning indices. Finally, the best scheme of foam and polymer combination conditioning was verified according to the variation of torque of the shield machine in practical engineering. The results show that the mixture of 7% foam and 5% polymer can effectively reduce the permeability coefficient of gravel soil, maintain certain plasticity, and play a role in reducing the torque of the cutterhead and improving tunneling efficiency.

Reasonable Support Timing of Tunnel Pre-grooving Method and Research on Segment Joint Structure

He Wei, Fan Peishuo, Ge Fei, Xu Lirong, Yang Yongxiang, Zhao Yang

2024, 38(1): 14-21. doi: 10.3969/j.issn.1007-2993.2024.01.003

[Abstract](85) [FullText HTML] (37) [PDF 2237KB](21)

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Pre-grooving method is a kind of over-advance support method for tunnel excavation, which can greatly reduce the amount of surface settlement, and has less impact on the tunnel surrounding rock and soil and surface buildings (structures). Using indoor tests and numerical simulation methods, the research was carried out on the strength of the concrete used for casting pre-grooving duct piece, the reasonable support timing, and the reasonable stubbing method at the joints of the pre-grooving duct piece. The results show that the concrete with C30 proportion has a compressive strength of more than 6 MPa when curing for 1.5~2 h, which meets the requirement of mold removal; the bonding medium has obvious restraining effect on the circumferential displacement of the duct piece, and epoxy resin can be used as an excellent bonding agent for the duct piece; the ideal range of mortise and tenon joint mortise angle α is 55°~70°, and the ideal range of the depth of the mortise and tenon joint is 6~8 cm, taking into consideration of the operability of the pre-grooving process.

Numerical Simulation and Deformation Control of Shield Tunneling Through Soft Soil Layer Pre-reinforcement

Wu Yongzhe, Jin Yijie, Zhang Zhiyu, Wang Yanzi, Yang Ping

2024, 38(1): 22-29. doi: 10.3969/j.issn.1007-2993.2024.01.004

[Abstract](201) [FullText HTML] (35) [PDF 2443KB](29)

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Relying on the shield tunnel project in the Chuang-Ju section of Foshan Metro Line 3, the surface settlement and tunnel deformation under different reinforcement methods were obtained by numerical simulation, and the deformation law of the shield when crossing the full-section soft soil layer was studied. A set of suggestions for shield tunnel safety crossing reinforcement is proposed, and the simulation results are verified by combining with the field measurement. The research shows that when the shield is advancing in the full-section soft soil stratum, the surface settlement, tunnel vault settlement and horizontal convergence value all increase continuously, which mainly occur when the shield passes through and after the shield tail segment is protruded, and it is also the main scope of action of pre-reinforcement; the impact of the leading tunnel on the surface settlement is greater than that of the trailing tunnel; when the shield passes through the unreinforced soil layer, the maximum values of surface settlement, tunnel vault settlement, and horizontal convergence are 27.7 mm, 14.28 mm, and 10.57 mm, respectively. The deformations of the pre-grouting reinforcement are 57.0%, 66.6%, and 61.0% of the unreinforced ones, respectively. When the triaxial stirring piles are used for pre-reinforcement, the deformations are 32.1%, 48.5%, and 43.0% of the unreinforced ones. The pre-reinforcement can effectively control deformation, and the deformation control of the triaxial stirring pile is better than that of the advance pre-grouting reinforcement. When the ground environment in the shield tunnel is complex and the ground reinforcement conditions are not available, it is recommended to use advance pre-grouting reinforcement to control the surface settlement and tunnel deformation.

Effect of Pile Foundation Grouting in Karst Area Based on 3D Numerical Simulation

Xu Zishu, Liu Shuzhuo, Chen Junsheng, Rao Cheng, Liu Xingyu

2024, 38(1): 30-36. doi: 10.3969/j.issn.1007-2993.2024.01.005

[Abstract](164) [FullText HTML] (33) [PDF 2357KB](37)

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To study the grouting effect of the modified expansive grouting material developed according to the actual engineering requirements, the slurry parameters were collected through material performance tests such as compressive strength, and applied in the finite element software Midas GTS NX, and based on the three-dimensional cave model constructed by CATIA. The change of stress and deformation of the soil layer under the pile foundation in different states of the cave and modified slurry filling before and after, and the grouting effect was compared and analyzed by this standard. The study showed that the grouting reinforcement of the cave under the pile foundation can effectively avoid the occurrence of engineering accidents, and the overall strength of the modified grouting material was significantly improved, the deformation settlement and stress were reduced, and the structure was more uniform and dense compared with the ordinary cement slurry, which also proves that the numerical simulation analysis has certain guiding significance to determine the grouting effect of the cave, and provides reference for the design and construction of the project.

Research and Compilation of Technical Standard for Investigation of Geotechnical Engineering in Xisha Coral Island & Reef Area

Yang Yongkang, Yang Wu, Xie Shuling, Wang Ying, Feng Chunyan

2024, 38(1): 37-40. doi: 10.3969/j.issn.1007-2993.2024.01.006

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To adapt to and meet the needs of engineering construction and provide a standard for geotechnical engineering investigation in Xisha coral islands & reef areas, the practical experience and research results of geotechnical engineering investigation projects in Xisha Islands were summarized. Through data collection and analysis, the Technical Standard for Investigation of Geotechnical Engineering in Xisha Coral Island & Reef Area (DBJ 46—060—2022) was compiled. The background, workflow and main contents of the standard are introduced. The content that needs further research is discussed.

Safety Measures and Early Warning Analysis of Tunnel Crossing Existing Section

Zhang Jie

2024, 38(1): 41-46. doi: 10.3969/j.issn.1007-2993.2024.01.007

[Abstract](100) [FullText HTML] (38) [PDF 2021KB](23)

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The crossing between the new subway and the existing subway line is a high-risk working condition. When the new tunnel crosses the existing tunnel in a short distance, the stress state of the existing tunnel structure will inevitably change and the structure will be deformed. Therefore, it is very necessary to formulate necessary safety measures for the crossing existing section. The relevant safety guarantee experience of the Nanjing Metro Line under construction going up and down through the existing operation line was summarized. In the up-crossing condition, take the guarantee measures to strengthen the external conditions, internal conditions and emergency response. At the same time, combined with the numerical analysis, it is concluded that MJS portal reinforcement has an obvious protective effect on the existing tunnel; in the under-crossing condition, the supporting measures of strengthening the stiffness inside and outside the existing tunnel, configuring the test section and operation cooperation were taken. Combined with the numerical analysis, it is found that the circumferential + longitudinal reinforcement in the existing tunnel has a significant protective effect on the existing tunnel, and the change rate of settlement and horizontal displacement of the existing tunnel is the largest within the range of 20 m from the heading face. Finally, based on the existing interval warning situations in engineering practice, the relevant reasons were analyzed and warnings and references were provided for similar projects.

Stability Analysis and Reinforcement Design for Soil-like Slope in Operating Highway

Hou Wenteng, Wang Yukui, Du Xiangbo, Zou Chenhao, Sun Kang

2024, 38(1): 47-51. doi: 10.3969/j.issn.1007-2993.2024.01.008

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The surface layer of a highway soil-like slope is colluvial soil, underlying strongly weathered-weakly weathered clay shale. It was found that the slope showed signs of deformation and failure in highway operation inspection. The development process, causes and stability of deformation and failure of this soil-like slope were analyzed and studied, the finite element strength reduction method and the circular sliding surface method were used respectively. The results show that the failure mode of the slope is a shallow traction landslide. The calculation results show that the slope is in an unstable state under normal conditions, and abnormal conditions I (rainstorm). Through comprehensive analysis combined with stability calculation, the closed slope surface cracks, prestressed anchor pile with beam, and inclined drainage holes are proposed for treatment. The monitoring data after reinforcement verifies the stability of the slope and the reliability of the regulation plan. The research results can provide a reference for the prevention and control of landslide disasters in soil-like slopes.

Optimization of Support System for Deep Foundation Pit with Special-shaped Soil-rock Combination Strata

Ji Yanlei

2024, 38(1): 52-57. doi: 10.3969/j.issn.1007-2993.2024.01.009

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The foundation pit of a certain open-cut tunnel is located in a soil-rock combination layer, with a large depth of about 17.1 m and a deep excavation area of about 27.9 m. The original design of the overall foundation pit used underground continuous walls and 4 reinforced concrete supports, and the inner wall of the pit used drilled cast-in-place piles as vertical support structures. Two steel pipe supports were added to the deep pit area. To save construction period and project cost, a series of optimizations have been made to the original design scheme, taking into account the distribution of strata and geological conditions. Numerical simulation was used to compare and analyze the deformation characteristics of the foundation pit support system and surrounding environment before and after optimization. The results showed that the optimized support scheme met the design and regulatory requirements. The foundation pit was constructed according to the optimized plan and on-site monitoring work was carried out. The monitoring results of the foundation pit showed that: (1) The horizontal displacement of the continuous wall of the foundation pit, surface settlement, support axial force, deformation of surrounding buildings, and uplift at the bottom of the pit were all below the control requirements; (2) The maximum horizontal displacement of the continuous wall occurs at a certain height above the soil rock interface, with a maximum value of approximately 0.025% of the shallow foundation pit depth; (3) The deformation on both sides of the irregular foundation pit is asymmetric, and the maximum surface settlement occurs on the shallow foundation pit side. The impact range of the outer surface of the foundation pit is about 1.0H; (4) The lateral deformation of the foundation pit excavation in the lower bedrock section is small, and it has little impact on the range of surface settlement grooves. The optimization plan saves time and cost and can provide a reference for similar foundation pit projects.

Cause Analysis and Treatment Measures for the Collapse at the Entrance of Diaoyutai Tunnel

Zhang Chao, Wu Hua, Jiang Tonghu

2024, 38(1): 58-64. doi: 10.3969/j.issn.1007-2993.2024.01.010

[Abstract](94) [FullText HTML] (21) [PDF 2778KB](16)

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An in-depth analysis was conducted on the causes of the collapse at the entrance of the Diaoyutai Tunnel on the Chi-Qi Expressway, and corresponding measures for dealing with the collapse were proposed. The effectiveness of the collapse treatment was verified through on-site monitoring and analysis. The research results indicate that the collapse of the Diaoyutai Tunnel entrance is the result of a combination of various factors such as terrain, geology, rainfall, and improper construction measures. Among them, terrain and geological conditions are the internal factors that cause the collapse, while rainfall and improper construction measures are the external factors that induce the collapse. Based on the above influencing factors and combined with the actual situation of the tunnel entrance, a comprehensive treatment plan of temporary support+permanent protection has been proposed, which includes reinforcement of the tunnel arch, backfilling of the tunnel face with back pressure, unloading of the collapsed body, double layer small conduit support at the entrance, extension of the open cut tunnel, and protection of the roof and forward slope anchor rod frame. The on-site inspection and monitoring data indicate that the treatment measures for tunnel entrance collapse are appropriate, and the engineering effect is significant, achieving the expected purpose.

Design and Research of Deep Foundation Pit Close to Ultra High Voltage Power Pipeline in Soft Soil Area

Wei Renjie

2024, 38(1): 65-69. doi: 10.3969/j.issn.1007-2993.2024.01.011

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Taking a deep foundation pit project adjacent to the ultra-high voltage power pipeline in Shanghai as the research object, the retaining form of the foundation pit was introduced, and the corresponding protection measures were provided according to the pipeline protection requirements. Through the comparison and analysis of the theoretical calculation value and the actual monitoring value, it can be seen that: (1) the lateral deformation of the enclosure is in the shape of a "fish belly" parabola, and the deformation resistance of the diaphragm wall is better than that of the cast-in-place pile; (2) The surface settlement can reflect the vertical displacement of the pipeline. The construction conditions before foundation pit excavation will have a certain impact on the surrounding roads and pipelines; (3) The measured deformation is larger than the theoretical calculation because the theoretical calculation only considers the deformation of the retaining structure during excavation.

Design and Practice of Super-large Deep Foundation Pit Adjacent to Subway in Shanghai

Han Xiao

2024, 38(1): 70-74. doi: 10.3969/j.issn.1007-2993.2024.01.012

[Abstract](177) [FullText HTML] (36) [PDF 1670KB](50)

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The construction of foundation pit in soft soil area is very difficult and the risk is high. Based on the super deep foundation pit project in Shanghai core area, combined with the subway protection requirements, through targeted protection design for the deep foundation pit near the subway, and strengthening the process control of the foundation pit construction, the subway structure is well protected while ensuring the smooth implementation of the foundation pit project. Engineering practice shows that a series of safety protection measures adopted in the design are reasonable and reliable, which can provide reference for similar deep foundation pit projects.

Application of Full Convolution Neural Network in Garbage Soil Investigation

Xu Siyi, Zhang Xu

2024, 38(1): 75-77. doi: 10.3969/j.issn.1007-2993.2024.01.013

[Abstract](91) [FullText HTML] (30) [PDF 2116KB](25)

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Due to the difference in resistivity between the garbage and the original soil, the most commonly used garbage soil detection methods are high-density resistivity method and time-domain electromagnetic method. However, the low efficiency of manual interpretation of inversion results and the difficulty in ensuring accuracy still need further study. This research introduces the application of the full convolution neural network in the garbage soil investigation. Through the identification of the garbage soil detection data of the underground buildings of a demolished green space reconstruction project, the garbage soil range was determined, which shows the effectiveness, practicability and reliability of this method. It is the reference basis for waste soil investigation, earthwork calculation and improvement of land properties.

Study on the Pullout Resistance of Hollow GFRP Reinforced Soil Nails

Liu Jun, Shen Jing, Zhang Jianquan, Song Ye, Liu Peng

2024, 38(1): 78-84. doi: 10.3969/j.issn.1007-2993.2024.01.014

[Abstract](71) [FullText HTML] (22) [PDF 1681KB](16)

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Hollow GFRP reinforced soil nail is a kind of hollow thick-walled soil nail, which has the advantages of light weight, easy cutting, and corrosion resistance. At present, there are relatively few studies on hollow GFRP soil nails, and their pullout resistance and pullout resistance mechanisms are still unclear. Relying on the foundation pit project of Beigangzi Station of Beijing Metro Line 12, the field pullout test of hollow GFRP reinforced soil nails was carried out, and numerical simulation in FLAC3D was used to investigate the distribution of stress and displacement along the nail length of hollow GFRP reinforced soil nails with different pullout force. The effects of different inner diameters and lengths on stress distribution and pullout resistance were analyzed, and finally, the pullout resistance mechanism of hollow GFRP reinforced soil nails was summarized. The results show that the displacement and axial force of soil nails in the process of pullout decay exponentially along the length of the nails. The shear stress peaks at 1.5 m from the nail head and decreases to zero at 3.5 m from the nail head; increasing the inner diameter makes the axial force of the soil nail decay faster along the nail length, and at the same time improves its pullout capacity. It is recommended that the inner diameter of the hollow GFRP soil nails with an outer diameter of 32 mm be taken as 15 mm; increasing the length of the soil nails not only improves the pullout capacity to a certain extent, but also makes the axial force of the nails more uniformly distributed; the destructive process of the hollow GFRP soil nails can be divided into the three stages of elasticity, plasticity, and damage, and the damage form is manifested as pullout.

Experimental Study on Mechanical Deformation Characteristics of Gravity Cement Wall in Deep Soft Soil Area

Zhao Dinghong, Qi Jiawei, Song Yongwei

2024, 38(1): 85-89. doi: 10.3969/j.issn.1007-2993.2024.01.015

[Abstract](74) [FullText HTML] (18) [PDF 2586KB](23)

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This research evaluated the sensitive factors affecting the strength of gravity cement earth wall in deep soft soil area, including cement label, water-cement ratio, cement content, curing age, etc. The strength variation law of multi-factor coupling was obtained through the indoor test. Combined with the deformation analysis of cement earth wall, the effective method of deformation control of gravity cement wall was proposed, which is of great significance for further study on the applicability and optimization design of this kind of support structure in deep soft soil area.

Slope Stability Analysis of Polypropylene Fiber Reinforced Expansive Soil

Ren Hao, Zhang Ziyi, Dang Fang, Hao Chenxi, Zhang Yuhang, Nan Yalin, Guo Hong, Yang Kuibin

2024, 38(1): 90-94. doi: 10.3969/j.issn.1007-2993.2024.01.016

[Abstract](135) [FullText HTML] (43) [PDF 1170KB](25)

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To improve the slope stability of the expanding soil and improve the mechanical properties of the expanding soil, the polypropylene fiber was used to modify the expanded soil, and the shear strength change law of the slope of the modified expansion soil with different confining pressure and different polypropylene fiber dosage was calculated through the straight shear test, and then the slope of the expanded soil with different polypropylene fiber content was analyzed based on Geostudio software using the limit equilibrium method. It is found that the polypropylene fiber content can effectively improve the shear strength of the slope of the expanded soil within a certain range, and the 0.14% fiber soil showed good shear resistance in the straight shear test. The analysis results of Geostudio's limit equilibrium method show that the stability coefficient of 0.14% fiber soil slope is 2.431, which is 0.591 higher than that of plain expansive soil slope. It is recommended to use 0.14% fibrous soil to strengthen the slope of expanded soil.

Spatial Variability of Cohesive Soil Parameters Based on Cone Penetration Test and Geotechnical Laboratory Tests

Ma Ming, Ye Dongming, Lu Jun, Ren Shouqin, Chen Zhehui

2024, 38(1): 95-100. doi: 10.3969/j.issn.1007-2993.2024.01.017

[Abstract](98) [FullText HTML] (23) [PDF 974KB](33)

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Spatial autocorrelation distance is a key parameter for analyzing the spatial variability of geotechnical parameters. This study utilizes three methods, including the average distance method, recursive space method, and the curve limit method, to calculate the autocorrelation distance based on side friction (f_s), tip resistance (q_c), and soil parameters. These methods were applied to 246 cone penetration tests (CPT) and 18 soil sampling points obtained from a construction site of cohesive soil in Zhejiang Province. The effectiveness of different methods in computing the vertical and horizontal autocorrelation distances was compared. It’s found that the recursive spatial method is most effective in calculating the autocorrelation distances, with fewer outliers and smaller variation coefficients. The autocorrelation distances based on f_s and q_c are comparable, with vertical and horizontal autocorrelation distances of approximately 2 m and 19 m, respectively. Nonetheless, the outcomes vary somewhat depending on the soil parameters utilized. The typical values of autocorrelation distance for cohesive soil were determined, which provides a foundation for analyzing the spatial variability of soil parameters. This research has important value for laying of sampling points, establishment of random fields, and subsequent reliability analysis.

Experimental Study on Shear Strength of Fulvic Acid-Contaminated Red Clay

Meng Jianping, Li Shanmei, Liao Yongbin, Zhang Hao, Li Junwu

2024, 38(1): 101-105. doi: 10.3969/j.issn.1007-2993.2024.01.018

[Abstract](75) [FullText HTML] (36) [PDF 1941KB](12)

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To investigate the effect of fulvic acid on the shear strength of red clay, the red clay was mixed with different contents of fulvic acid powder to prepare samples and saturated with distilled water. Direct shear tests were carried out on the samples with soaking ages of 7 d, 14 d, and 28 d to analyze the effects of different dosages of fulvic acid and soaking time on the shear strength of red clay samples. The test results show that the addition of fulvic acid will reduce the shear strength of the soil samples. With the increase of fulvic acid incorporation, the shear strength of the samples increases and the internal friction angle decreases, respectively. With the same amount of fulvic acid, the longer the soaking time, the greater the cohesion and the smaller the internal friction angles. The active functional groups of the fulvic acid, such as carboxyl groups, hydroxyl groups, carbonyl groups, and methoxyl groups adsorbed on the surface of soil minerals, wrapping on the surface of soil particles and forming a bonding effect between the soil particles, thus affecting the shear strength indexes of the soil.

Analysis on In-situ Direct Shear Test Results of Unsaturated Silty Sand in the Eastern Part of Beijing

Zhang Xiangying, Zhou Yufeng, Yang Haojun, Feng Jianyang, Cheng Qiushi

2024, 38(1): 106-109. doi: 10.3969/j.issn.1007-2993.2024.01.019

[Abstract](67) [FullText HTML] (14) [PDF 776KB](16)

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For the typical unsaturated silty sand layer in the eastern part of Beijing, in-situ direct shear tests were conducted, and the cohesion and internal friction angle were obtained by the least squares method. The internal friction angle values are increased by 11.11%~24.4% compared to traditional regional empirical values, and by 3.01%~17.4% compared to the values from the indoor direct shear test. The cohesive force of the silty sand layer measured by the indoor direct shear test is 4.8~7.5 kPa, while the value measured by the in-situ direct shear test is up to 9.5 kPa. The research results provide experimental support for the optimization of the shear index parameters of the unsaturated sand layer in this area, save the cost of foundation pit support, and provide the possibility for realizing green construction in the whole process of engineering construction.

Experimental Study and Variability Analysis of Strength Parameters in Passive Zone Reinforcement of Soft Foundation Pit

Xu Wei, Tai Jun, Wen Yongkai, Zhao Chenxi, Yan Bo, Hu Ke

2024, 38(1): 110-115. doi: 10.3969/j.issn.1007-2993.2024.01.020

[Abstract](142) [FullText HTML] (46) [PDF 1747KB](40)

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Passive zone reinforcement is an effective deformation control method, which has been widely used in soft soil deep foundation pit engineering, but there are few reports about the measured strength parameters and its variability analysis of the passive zone reinforcement. Based on a soft clay deep foundation pit in Wuhan area, in-situ cone penetration tests, dynamic penetration tests, and laboratory unconfined compressive strength tests were carried out, and its correlation between the laboratory unconfined compressive strength parameters and in-situ test indexes was established for the passive zone reinforcement. Through the established empirical formula, the continuous in-situ test data were converted into unconfined compressive strength values, and the variability and distribution of strength parameters of the passive zone reinforcement were discussed by statistical analysis. The results showed that there were significant differences between the strength of the passive zone reinforcement and the undisturbed clay, compared with the empty pile with 10% cement content above the bottom of the pit, the strength of the solid pile with 20% cement content below the bottom of the pit was not significantly increased. The strength of the passive zone reinforcement has strong nonuniformity in both empty and solid piles, and the variation coefficient of the empty pile was larger than that of the solid pile. The distribution of cement-soil strength for the empty pile with low cement content was fitted by an exponential distribution, while that of cement-soil strength for the solid pile with high cement content was approximately lognormal.

Influence of Preconsolidation on the Shear Strength of the Soft Soil in Floodplain Facies of Yangtze River

Cui Zhipeng, Yan Gang, Wang Jun, Zhong Ping, Wang Zhenxiang, Wu Qi

2024, 38(1): 116-120. doi: 10.3969/j.issn.1007-2993.2024.01.021

[Abstract](73) [FullText HTML] (27) [PDF 1527KB](18)

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The consolidation fast shear test is an important method to obtain the consolidation and undrained shear strength of soft soil, and the soft soil of the Yangtze floodplain has the characteristics of high compressibility, large pore ratio, high moisture content, low permeability, and long consolidation time, which seriously affects the production efficiency of soft soil consolidation fast shear test. To study the effect of preconsolidation on the shear strength of the Yangtze River floodplain facies soft soil and explore the method to improve the efficiency of fast shear tests for consolidation of floodplain soft soil, a series of consolidation and shear tests were carried out, taking the Nanjing Yangtze River floodplain soft soil as the research object. The results show that the Yangtze River floodplain facies soft soil is a kind of soft soil with rapid consolidation in the early stage and stable and slow consolidation in the later stage. The consolidation degree can reach 95% when consolidating for 25 minutes, while it takes 5～8 hours to fully consolidate and stabilize. The shear strength of soft soil increased with the increase of consolidation degree U, and its growth rate gradually slowed down. The increase of shear strength with the increase of U under each load condition was basically the same. When the degree of consolidation is 70%～80%, its shear strength reaches 90%～95% of that when fully consolidated. This study can provide a reference for improving the fast shear test method of soft soil consolidation.

Field Tests Study on Post-Grouting Cast-in-place Bored Piles in Soft Soil Area of Indonesia

Men Qingbo, Qiao Jianwei, Xia Yuyun, Ma Jiayi, Shao Bingchang

2024, 38(1): 121-126. doi: 10.3969/j.issn.1007-2993.2024.01.022

[Abstract](101) [FullText HTML] (32) [PDF 1093KB](32)

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Post-grouting bored piles are widely used in soft soil engineering in Indonesia, but the theoretical research on the bearing characteristics of post-grouting bored piles still lags behind engineering practice. Based on a project in Indonesia's soft soil region, field tests of bored piles with different pile lengths and post-grouting bored piles were carried out. The results show that the ultimate bearing capacity of post-grouting bored piles is 1.20 to 1.26 times that of bored piles, and the rebound rate and elastic deformation of post-grouting bored piles are correspondingly increased. Under the geological conditions of the study area, the upward flow height of post-grouting cement slurry is 12.4~13.8 m, and the downward penetration depth of cement slurry below the pile tip is 1.3~2.1 m, which is 1.6~2.6 times the pile diameter. The blow counts of the standard penetration test (SPT) of the rock and soil layers below the cement slurry return height of the post-grouting pile and below the pile end are significantly increased compared to those before grouting. The improvement of the pile side friction resistance and pile end resistance caused by post-grouting is the main factor for improving the bearing performance of the bored pile. The research results have reference significance for the design and construction of post-grouting in soft soil areas in Indonesia.

2024 Vol. 38, No. 1