岩土工程技术

Study on the Prediction Method of Pile Foundation Bearing Capacity Based on Finite Medium Cavity Expansion Theory

Mo Pinqiang, Liu Yao, Gao Xinwei, Zhang Zheng

2022, 36(1): 1-9. doi: 10.3969/j.issn.1007-2993.2022.01.001

[Abstract](663) [FullText HTML] (47) [PDF 1142KB](57)

Abstract:
Installation of displacement pile could significantly increase the bearing capacity of pile foundation, and a new analytical method for bearing capacity prediction of displacement pile is proposed based on the elastic-plastic cavity expansion solution in finite soil medium. Mohr-Coulomb yield criterion is used to derive the unified analytical solution for both cylindrical and spherical cavities, assuming the pressure is remained constant at the outer boundary. The solutions are then adopted to analyze the penetration of precast pile, and a mechanical model for pile bearing capacity is constructed. The results show that scale effect is important for cavity expansion. For cylindrical cavities, the cavity pressure and the plastic zone of a finite medium with outer/inner radius ratio greater than 110 is similar to that of infinite medium, whereas the soil expansion curve is comparable for spherical scenarios with outer/inner radius ratio larger than 25. The mechanical model assumes the spherical cavity expansion at the pile end, and pile shaft is taken as cylindrical cavity expansion, which provides the changes of stress field during displacement pile installation and the bearing capacity. The magnitude of pile bearing capacity is compared with existing solution, standard design value, and measured data from field test, validating the proposed novel cavity-expansion based method.

Dynamic Response Analysis and Comparison of Two Different Types of Underground Cavern Structures

Huang Lijing, An Xiaojing, Ren Yali

2022, 36(1): 10-15. doi: 10.3969/j.issn.1007-2993.2022.01.002

[Abstract](95) [FullText HTML] (28) [PDF 1303KB](23)

Abstract:
The stability of surrounding rock is related to the structure of the cavern. The dynamic response of deep buried dome cavern structure and straight wall-arch cavern structure under the explosive loadings was studied by numerical analysis, and the deformation and the failure mode of the two structures were analyzed and compared. The analysis results show that the overall deformation and the ratio of horizontal and vertical deformation of dome cavern structure is better than the straight wall-arch cavern structure. Dome cavern structure has much larger failure load. The conclusions can be used for reference in related projects.

Interaction Analysis of New Shield Tunnel Passing Through Existing Tunnel in Short Distance

Hu Jing, He Zhijian, Wang Yiyang, Luo Xuedong

2022, 36(1): 16-21. doi: 10.3969/j.issn.1007-2993.2022.01.003

[Abstract](214) [FullText HTML] (151) [PDF 2399KB](60)

Abstract:
To study the interaction of the newly-built shield tunnel passing through the existing tunnel in a short distance, Wuhan Rail Transit Line 5 shield construction parallel to the existing line 2 was analyzed. The numerical model of shield tunnel and existing tunnel was established by FLAC3D. The influence law of the new shield tunnel construction on the existing tunnel deformation was studied, and the vault deformation characteristics of the new shield tunnel under the action of train dynamic load was analyzed and verified with the field monitoring data. The results show that: the closer to the tunnel face, the greater the deformation rate of the existing tunnel vault; the farther to the tunnel face, the deformation rate is relatively gentle. The horizontal deformation of both sides of the existing tunnel tends to protrude inward, and the deformation of one side close to the shield tunnel is far greater than that of the opposite side, but both of them are within the warning value range. The results show that the horizontal deformation of the existing tunnel near the shield tunnel increases, and decreases slightly after the completion of the subsequent grouting, and then maintains stable; the deformation of the new shield tunnel vault changes little under the action of the existing subway train dynamic load.

Key Geotechnical Engineering Problems in Loess Excavation and Filling Site Formation

Cai Huaien, Zhang Jiwen, Li Pengjun, Liu Shuai, Liang Xiaolong

2022, 36(1): 22-25. doi: 10.3969/j.issn.1007-2993.2022.01.004

[Abstract](206) [FullText HTML] (56) [PDF 744KB](52)

Abstract:
There are many geotechnical engineering problems in the formation of excavation and filling sites in collapsible loess area, and engineering accidents will occur if the problems are not effectively identified and prevented. In order to avoid the occurrence of related engineering accidents and ensure the safety of engineering construction, the key geotechnical engineering problems and treatment measures based on engineering practice were analyzed. The safety of slope and excavation, the prevention and control of surface water and groundwater, and foundation deformation are the main geotechnical engineering problems. The wetting deformation of filling body is the key and difficult point. Waterproof and drainage are the main measure to avoid engineering problems. The buildings near the top of the slope should be embedded, and the uneven earth pressure on the foundation (basement) should be considered for the buildings near the bottom of the slope. It is easy to form saturated soil on both sides of the excavation and filling interface and around the spring point. The main seepage channels of groundwater are in strongly weathered rock and strongly permeable rock with monoclinic structure. Slope treatment, waterproof and drainage, foundation treatment, excavation engineering, building embedment should be considered systematically.

Practice and Analysis of Fish-Belly Beam Steel Support in Foundation Pit

Yi Zhenghua

2022, 36(1): 26-30. doi: 10.3969/j.issn.1007-2993.2022.01.005

[Abstract](290) [FullText HTML] (211) [PDF 3964KB](73)

Abstract:
Assembled prestressed fish-belly beam steel support is a new type of support structure system for deep foundation pit , which is green and recyclable. It can form a large excavation space and effectively control the deformation of foundation pit. Taking a foundation pit project in Shanghai as an example, the application and implementation effect of assembled prestressed fish-belly beam steel support in deep and large foundation pit engineering are introduced. The monitoring results show that the prefabricated prestressed fish belly beam steel support has a good effect in deep and large foundation pit engineering in soft soil area, and has remarkable effect in engineering safety control and economic benefits, which can be used as a reference for similar foundation pit engineering.

Dynamic Testing Application of Foundation Treated by Preloading in the Water Transport Engineering

Zhang Kaiwei, Nie Qingke, Li Zhiyong, Qi Songsong, Wei Jianming

2022, 36(1): 31-36. doi: 10.3969/j.issn.1007-2993.2022.01.006

[Abstract](148) [FullText HTML] (55) [PDF 1100KB](27)

Abstract:
After the foundation is treated by vacuum stacking pre-pressure, the bearing characteristics, dynamic parameters and characteristics of the foundation are abnormal, which has a great impact on the design of dynamic foundation. Combined with the actual water transport engineering project on the vacuum stack pre-pressure treatment of the rotary jet grouting pile composite foundation and bored pile foundation, dynamic tests were conducted to obtain the stiffness parameters of compression, shear and torsion, the first vibration mode of vertical and horizontal steering, the damping ratio of torsional steering and the total vibration mass of the foundation and pile. Based on the measured data, the vibration conversion coefficients related to the static stress on the bottom surface of rotary jet grouting pile composite foundation and bored pile foundation were calculated, and the foundation compression, shear, torsional stiffness, torsional steering damping ratio and the improvement coefficient of the first vibration mode of vertical and horizontal return steering of the foundation were also calculated. The test and calculation results of various actual parameters show that when the foundation dynamic parameters actually obtained on the site of two different types of foundations are used for the vibration and vibration isolation design of power machine foundation, parameters should be converted into the foundation dynamic parameters adopted in the design according to the actual service conditions of power machine foundation, so as to avoid deviation to the foundation design.

Distribution and Characteristics of the Phreatic Water in Tianjin Binhai Core Area

Wen Weiguang, Lu Qing, Liu Xiaolei, Wang Hui, Song Wenjing

2022, 36(1): 37-42. doi: 10.3969/j.issn.1007-2993.2022.01.007

[Abstract](144) [FullText HTML] (48) [PDF 1169KB](34)

Abstract:
Taking Tianjin Binhai core area as an example, the engineering characteristics of phreatic water in coastal area were studied. The lithological composition, spatial distribution and permeability of the phreatic water-rich aquifer and phreatic aquifer in Binhai core area were investigated, and the status and dynamic change of the phreatic water level were analyzed. The range values of water quality analysis indexes such as Cl⁻, SO₄²⁻ were provided, and their water quality was evaluated. The results show that: (1) The phreatic water level in the Binhai core area is shallow and mainly distributed in silts layers of Q₄^3Nal, Q₄¹al and Q₄²m and other phreatic water-rich aquifer, with a thickness of less than 6 m. The permeability coefficient of phreatic aquifers is generally 0.1～0.6 m/d, indicating weak permeability. (2) The annual dynamics of the phreatic water level in Binhai core area show a certain law with seasonal changes. The annual variation range is generally 0.22～0.72 m in the past five years, and the maximum water level in the past five years has an overall upward trend, which is about 0.3 m. The main indicators of phreatic water quality in the past five years have the trend of decreasing first and then increasing, and the water quality is poor.

Application of Cross Hole Sonic Wave Method in Rock Mass Detection of Dam Site of a Reservoir

Gu Jinyong, Wang Hong

2022, 36(1): 43-48. doi: 10.3969/j.issn.1007-2993.2022.01.008

[Abstract](170) [FullText HTML] (67) [PDF 1562KB](29)

Abstract:
The cross hole sonic wave detecting technology is an important method for concrete inspection. To study the development of joints and fissures, rock properties and distribution of the rock mass in the dam site area of Xiababi Reservoir, three boreholes in each of the two locations in the dam site area were selected. Cross hole sonic wave detection technology was used to test the sonic wave velocity of rock mass. The test results show that the basic rock mass organization structure of the horizontal section of the dam site is partially damaged. The longitudinal wave velocity of the rock mass is generally 2000～4000 m/s. The weathering cracks of the rock mass are developed and relatively broken. There are cracks, broken zones and local weak interlayers. The research results could provide data support for the evaluation of the integrity of the foundation rock mass of the dam site, and provide a detailed and reliable basis for solving the stability and leakage problems of the dam site.

Application of Comprehensive Exploration Method in the Investigation of Sand Gravel Layer

Sun Changqing

2022, 36(1): 49-53. doi: 10.3969/j.issn.1007-2993.2022.01.009

[Abstract](220) [FullText HTML] (118) [PDF 867KB](83)

Abstract:
As the bearing layer of foundation, sand gravel layer has the advantages of low compressibility and high bearing capacity. However, the stability of sand gravel layer is poor, which has the characteristics of no cohesion, strong permeability, high hardness and great difference in gradation, so it is difficult to survey and construct the subway. Based on the first phase project of Beijing Metro Line 19, the comprehensive exploration method of drilling, artificial exploration well, geophysical exploration, in-situ test, indoor test, on-site pumping test and underground water flow velocity measurement and other exploration methods were conducted, which were used to find out the characteristics of sand gravel layer, such as density, particle gradation, sand gravel layer strength and the occurrence of groundwater. This study could provide the basis for the design and construction of tunnel, and also provides a reference for the investigation of sand gravel layer.

Risk Assessment of Foundation Pit During Transition Period Based on Detection Test and Dynamic Monitoring

Li Shu, Zhang Lizhan

2022, 36(1): 54-58. doi: 10.3969/j.issn.1007-2993.2022.01.010

[Abstract](116) [FullText HTML] (53) [PDF 1764KB](35)

Abstract:
The third-party detection and dynamic monitoring data of a certain foundation pit during transition period was analyzed, then the design parameters of supporting structure and stratigraphic parameters were modified to calculate the stability of the foundation pit. Through quantitative and qualitative analysis, the risk assessment of foundation pit during transition period was implemented comprehensively, objectively and precisely. Then some pertinence reinforcement measures are proposed to ensure the reliability of the foundation pit under extended service.

Application of Bamboo Joint Pile in Strengthening Composite Foundation of Variable Cross-Section Screw Pile

Wang Xuemei, Liang Hao, Ding Jihui, Cui Zhipeng, Li Junpeng

2022, 36(1): 59-63. doi: 10.3969/j.issn.1007-2993.2022.01.011

[Abstract](82) [FullText HTML] (60) [PDF 1313KB](26)

Abstract:
The composite foundation with variable cross-section compacted screw pile was conducted in soft soil area. When the stacking height reaching 5～6 m, which was below the design height of 11 m, the ground subsidence occurred and the maximum subsidence was about 1.30 m. The grid frame column had a large deformation, and the maximum horizontal displacement was more than 680 mm, which led to the material shed in an ultra dangerous state. Bamboo joint pile was used to reinforce the composite foundation of variable cross-section compacted screw pile. By reasonably arranging the driving direction of bamboo joint pile, adopting long auger and stress release hole, the influence of soil squeezing effect of bamboo joint pile on original compacted screw pile, side column cap and foundation pile was reduced. After the pile sinking, the characteristic value of the vertical bearing capacity of the single bamboo joint pile was not less than 740 kN, which meet the design requirements. The reinforced bamboo pile composite foundation+ raft meet the design requirements of surcharge.

Variable Stiffness Leveling Foundation Design and Analysis of CFG Composite Foundation

Lu Pingzhen, Qi Wei, Sun Hongwei, Fang Yunfei

2022, 36(1): 64-69. doi: 10.3969/j.issn.1007-2993.2022.01.012

[Abstract](165) [FullText HTML] (61) [PDF 1389KB](55)

Abstract:
Beijing International Culture Silicon Valley Park is located in Chaoyang District, Beijing. The project area is mainly composed of silty clay, silty soil and sandy soil with high groundwater level. In this project the only-underground-building needs anti-floating piles. At the same time, due to large load of the main towers and insufficient foundation bearing capacity, the CFG composite foundation is adopted under the main towers. Thus, CFG composite foundation under the main towers and anti-floating piles under the only-underground-building are used in this project. The differential settlement between towers and only-underground-building and also the core tube and the outer frame column of the tower building is difficult to control. Through the calculation and analysis of differential settlement deformation based on the interaction between geotechnical foundation and structure, the engineering measures for controlling and coordinating differential settlement are verified. The measures include selecting reasonable bearing layer of CFG pile, adjusting the foundation stiffness by using different spacing of CFG piles and cushion thickness, and optimizing the anti-floating piles according to the geotechnical settlement deformation analysis. The variable stiffness between the towers and their skirt buildings as well as the core tube of the tower and the columns of the outer frame is realized. The design is verified by the engineering inspection and settlement measurement.

Application of High Density Electric Method in Exploration of Tunnel Engineering

Dong Yao, Xiao Juan, Liu Yan, Jin Lu, Wang Wei

2022, 36(1): 70-73. doi: 10.3969/j.issn.1007-2993.2022.01.013

[Abstract](171) [FullText HTML] (35) [PDF 2749KB](53)

Abstract:
The tunnel passes through areas with complex terrain, and it is difficult for tunnel exploration and drilling, which causes difficulties for geophysical exploration. It is of great significance to carry out the safety evaluation of engineering exploration quickly and effectively for the later construction. Based on a project and comprehensive utilization of previous geological data, Wenner and Schlumberger devices were used for high-density electrical method. The working principle and application effect of high-density electrical method in different devices were analyzed. The Wenner device has large detection depth, high lateral resolution and obvious reflection of the characteristics of high resistance abnormal body. The Schlumberger device has relatively shallow detection depth, with high vertical resolution, strongly anti-interference ability and obviously reflection of low resistance anomaly body.

In-situ Test Research on Shear Strength of Cretaceous Mudstone in Jiaodong

Tian Lichuan

2022, 36(1): 74-78. doi: 10.3969/j.issn.1007-2993.2022.01.014

[Abstract](85) [FullText HTML] (18) [PDF 2637KB](28)

Abstract:
Cretaceous mudstone is widely distributed in Jiaodong area of China, which has the characteristics of softening and expansion in water, as well as shrinkage and disintegration with water loss. The accuracy of rock shear strength is significant for construction and design of foundation and slope stability. According to the in-situ shear tests and laboratory direct shear tests, the results show that the internal friction coefficient and internal friction angle are basically the same. The cohesion of rock is 10.6 times that of rock mass. The main reasons for differences are poor representativeness, large disturbance, big divergence, size effect, and sample integrity etc. The in-situ shear test can be more truly reflect the actual engineering properties, which is available for engineering design and optimization.

Mechanical Characteristics and Failure Mode of Wasted Polyester Fiber-reinforced and Cement-stabilized Sand

Qiu Anbing

2022, 36(1): 79-86. doi: 10.3969/j.issn.1007-2993.2022.01.015

[Abstract](126) [FullText HTML] (31) [PDF 2706KB](36)

Abstract:
Using a low-cost and environmentally benign method to reinforce cement-stabilized sand is a key issue in geotechnical engineering. Polyester fiber from waste clothes was used to reinforce cement-stabilized sand in this study. The effects of the fiber content and fiber length on the unconfined compressive strength of cement-stabilized sand were examined. The failure modes of the cement-stabilized sand before and after reinforcement were qualitatively analyzed according to the macroscopic damage morphology of a test sample. The impacts of the fiber content and fiber length on changes in the failure mode of the cement-stabilized sand were analyzed according to the brittleness index. The degree of transformation from brittle failure to ductile failure of the cement-stabilized sand was evaluated. The results showed that the optimum reinforcement performance with waste polyester was obtained at a fiber content of 1.0% and fiber length of 9 mm. These conditions realized improvements in the unconfined compressive strength, peak strain, residual strength, and residual strain of the cement-stabilized sand of 43.3%, 18.2%, 276.9%, and 190.9%, respectively. After polyester reinforcement, the failure mode of the cement-stabilized sand gradually transformed from brittle failure to semi-ductile and ductile failure, as reflected by changes in the macroscopic damage morphology. Before reinforcement, the damage morphology consisted of individual cracks extending through the sample in the longitudinal direction, which is typical of brittle failure. After reinforcement, the damage morphology of the sample transformed into unilateral or sparse conjugate cracks representative of semi-ductile failure and dense network-type conjugate cracks representative of ductile failure. Using the optimum fiber content and fiber length yielded the most conjugated cracks on the sample surface, lowest brittleness index, and highest degree of transition from brittle failure to ductile failure. These research results provide a scientific basis for enhancing cement soil with an environmentally benign approach and for analyzing the failure mode of fiber soil.

2022 Vol. 36, No. 1