岩土工程技术

A New Method for Evaluating Collapse Sensibility of Loess and Loess Foundation

Song Xianhua

2019, 33(5): 249-254. doi: 10.3969/j.issn.1007-2993.2019.05.001

[Abstract](46) [PDF 1205KB](8)

Abstract:
The evaluation of loess collapsibility in engineering is confine to the concept of collapse settlement,and the evaluation of collapsible sensibility is not involved in previous research.The evaluation of collapsible sensibility is still at the exploratory stage.Although studies and discussions,as well as numerous evaluation methods have been presented by many scholars,thereis no consensus of opinion proposed,and no unified standard of quantitative evaluation on the collapsible sensibility provided.Based on a large number of engineering practice data,and on the premise of no increasing amount of tests,a new evaluation method of the loess collapsible sensibility and loess foundation collapsible sensibility is presented,and the corresponding quantitative evaluation standard is provided.The method is simple and feasible,and easy to beapplied in engineering.This method has been proved to be effective by practice.Even so,this method still needs to be further improved in engineering practice.

Multi-factor Coupling Analysis of the Influence of KCl Solution on the Expansion of Ankang Expansive Soil

Zhai Zhigang, Lei Shengyou, Yuan Wenzhi, Li Siyu, Chen Yufei, Liu Mingyu

2019, 33(5): 255-258. doi: 10.3969/j.issn.1007-2993.2019.05.002

[Abstract](42) [PDF 1127KB](11)

Abstract:
The free swelling ratio of modified Ankang expansive soil with KCl solution at different concentrations was tested;the effect of KCl solution with different mass fractions on the expansive soil was analyzed;and the swelling mechanism of Ankang expansive soil was further discussed.The free swelling ratio test of the modified expansive soil was conducted with different mass fractions of KCl solution,being 3%,6%,9%and 12%,respectively.The results show that comparing with the plain expansive soil,the KCl solution would reduce the free swelling ratio of theexpansive soil.With theincrease of KCl concentration,the free swelling ratio of the soil samples decreased at first,then increased and finally decreased again,showing an overall fluctuant decreasing tendency.Influenced by KCl solution with concentration of 3%,6%,and 9%,the free swelling ratio of expansive soil decreased and became weak expansion potential soil.The free swelling ratio of soil samples reached the lowest value being 33%due to theimpact of 12%KCl solution,and the expansive soil became non-expansive soil.According to the results,and focusing on the effect of the crystal layer expansion and its inhibition,as well as the concentration change of the salt solution between the pores on the expansion characteristics of the expansive soil,four effects on the fluctuation of the swelling ratio were studied,and the primary and secondary relationships of various effects in different concentration intervals were analyzed emphatically.Based on the theory of crystal layer expansion and the theory of permeability,the expression of the expansion potential of expansive soil was proposed.The partial coefficient which affects the expan-sion potential was discussed in details,showing that the effects of each partial coefficient in different concentration intervals are different.This expression could reasonably explain the change of the free swelling ratio of the expansive soil,which firstly decreased,then increased and finally decreased.It is determined that the fluctuant tendency is caused by the relative strength of the crystal layer expansion and its inhibition,as well as the concentration change of the salt solution between the pores.This theory is expected to make up the deficiency of the theory of the crystal layer expansion.

Void Ratio of the Coral Sands in Maldives and the Preliminary Application of the Electrical Density Gauge

Wang Duli, Wang Lu, Jiang Baikun, Zhang Qichang

2019, 33(5): 259-262,302. doi: 10.3969/j.issn.1007-2993.2019.05.003

[Abstract](30) [PDF 3017KB](5)

Abstract:
The typical characteristics of the coral sands in Maldives are irregular shape and abundant micropores,with irregular opening pores on particle surfaces and closed pores in particle interior,which is very different from the quartz sands.Based on the analyses of the physical and chemistry characteristics,an effective method to measure the void ratio of the coral sands is proposed,and calculation has been done with the actual measurement.Besides,the application of the electrical density gauge in engineering practice has the benefits of increasing the data reliability and improving the work efficiency.

Freeze-thaw Deformation of Compacted Sandy Soil in Yinchuan Plain under Closed System

Xu Hanbao, Cui Zizhi, Shen Supin

2019, 33(5): 263-266. doi: 10.3969/j.issn.1007-2993.2019.05.004

[Abstract](41) [PDF 827KB](7)

Abstract:
Combined with local engineering geology and climate characteristics,L₉(3⁴) orthogonal test was designed.The freeze-thaw deformation characteristics of compacted sandy soil in Yinchuan Plain under the combined action of compaction,saturation and freeze-thaw cycle were studied by using closed system freeze-thaw test method.The results show that compacted sandy soil with saturation not less than 30%is characterized by frost heave,and the volume increases after freezing and thawing,which is dominated by expansion effect of water freezing.Thaw settlement deformation accounts for a large proportion of the frost heave deformation,being about 80%.The freeze-thaw deformation decreases with the increase of compactness,and increases with the increase of saturation,showing a hyperbolic relationship with the number of freeze-thaw cycles.The joint effect of saturation and compaction is obviously greater than that of any one of the factors,but it is not equal to the simple superposition of the results of the two factors acting alone.Saturation and compaction have significant effects on freezethaw deformation of compacted sandy soil,which are the main controlling factors.In order to alleviate the hazards of freezing and thawing,the compaction should be strictly controlled in the construction of compacted sand engineering,and theincrease of saturation should be strictly prevented in operation.The research results have practical significance for the evaluation and prevention of the frost damage of compacted sand engineering in Yinchuan plain winter irrigation area.

The Relationship Between the Effect of Foundation Treatment and the Change of Geological Environment in Loess Saline Soil of Xining

Wang Zhiming, Lian jieming

2019, 33(5): 267-270. doi: 10.3969/j.issn.1007-2993.2019.05.005

[Abstract](77) [PDF 945KB](6)

Abstract:
Based on the construction in loess saline soil of Xining,the common method of foundation treatment in loess saline soil is studied,and the successes and failures in construction are summarized.The results show that the change of geological environment is an important influence factor for engineering properties in loess saline soil which should not be ignored,and it is a key point for the successful application of foundation treatments.This research is a fundamental guidance for adjusting the saline soil foundation treatment method according to local conditions.

Liquefaction Evaluation of Saturated Sand Foundation for Category I Items of Earthquake Resistance in the Survey of Nuclear Facilities

Sun Lichuan, Zhao Zaili, Zhu Yongqiang

2019, 33(5): 271-277,311. doi: 10.3969/j.issn.1007-2993.2019.05.006

[Abstract](42) [PDF 1569KB](7)

Abstract:
A variety of methods are applied to the survey of a nuclear facility.According to the liquefaction evaluation of saturated sand foundation of category I items,the safety of liquefaction evaluation in the current earthquake resistance design specification is not comparable with the serious consequences of the destruction of a nuclear facility.The safety margin proposed by scholars based on the SPT and shear wave velocity liquefaction evaluation method is wider than the current seismic specification.It is suggested that the latest research results of liquefaction evaluation should be used for comparative analysis in addition to the liquefaction evaluation in accordance with the specification.Based on the actual engineering,the liquefaction probability control value of category Iitems for earthquake resistanceis proposed to be adjusted from 4%to 8%,and the nominal anti-liquefaction safety factor is proposed to be adjusted from 1.45 to 1.2.For the liquefaction evaluation curve based on shear wave velocity,there will be abnormal calculation results under certain conditions,and whether the sand is liquefied or not should be determined comprehensively with geological data.

Comparison of Several Discrimination Criteria for Seismic Soil Liquefaction

Hao Bing, Ren Zhishan, Li Congyun

2019, 33(5): 278-283. doi: 10.3969/j.issn.1007-2993.2019.05.007

[Abstract](86) [PDF 3561KB](27)

Abstract:
Seismic soil liquefaction potential evaluation is an important part in seismic geological disasters assessment.Different methods of seismic soil liquefaction potential evaluation lead to different conclusions.Several main methods are sorted out.Through specific engineering test data,evaluations are applied based on these methods,including standard penetration test method,NCEER method,static cone penetration test method and shear wave velocity method,and the conclusions of various methods are analyzed.It is concluded that there are obvious defects in liquefaction potential evaluation applied by shear wave velocity method.For the shallow silt,it is more risky with this method,and for the deep sand,it is too conservative.The results of the other methods,including standard penetration test method,NCEER method and static cone penetration test method,are basically consistent.However,there are obvious differences between the NCEER method and other methods for sandy soil with high content of fine grains.

Practice Research on Deep Foundation Pit Adjacent to Shallow Foundation Pit in Soft-soil Area

Liu Baoshi

2019, 33(5): 284-287. doi: 10.3969/j.issn.1007-2993.2019.05.008

[Abstract](147) [PDF 1862KB](17)

Abstract:
A case of a deep foundation pit project in soft-soil area of Shanghai has been studied.Focusing on the environmental characteristics of a shallow foundation pit adjacent to one side of this deep foundation pit,the design idea of foundation pit retaining structure is proposed,and the brace type with large angle brace are introduced in detail,which is more conducive to the overall deformation control of deep foundation pit.Through the analysis and research of the monitoring results,the safety and rationality of the design are proved.The experience obtained from this case is of reference significance for similar projects.

Hydrogeology Features of Loess Hilly Gully Region in Yan'an

Cai Huaien, Zhang Jiwen, Zheng Jianguo, Zhang Ruisong, Liang Xiaolong

2019, 33(5): 288-292. doi: 10.3969/j.issn.1007-2993.2019.05.009

[Abstract](74) [PDF 1133KB](14)

Abstract:
The urban area of Yan'an is located in the valley area of the loess hilly and gully region.The flat and narrow valley zone cannot meet the needs of the urban development,therefore,it is necessary to expand the urban construction to the surrounding loess hilly and gully areas.In the loess hilly and gully area,the construction of the project is bound to be “cutting slope and filling the ditch” or “cutting the gully and filling the ditch”.However,the hydrogeological conditions of this area are relatively complex.The large-scale construction projects, “cutting slope filling ditch” or “cutting the gully and filling the ditch”,are facing with many geotechnical engineering problems caused by the groundwater.The groundwater types,the permeability characteristics of the strata,the hydrogeological structure under the features of various geomorphology,the water bearing systemof groundwater and the runoff and excretion characteristics are summarized and analyzed based on the hydrogeological survey,drilling and test in the loess hilly and gully region of Yan'an.Also,the hydrogeological conceptual model of the loess hilly and gully area is constructed.The results can provide a reference for the “slope cutting and trench filling” or “trench cutting and trench filling” projects in the loess hilly and gully area of Yan'an and similar geological conditions.

EPS Vibration Reduction Performance under Frozen Soil

Liu Qian, Yang Shihao, Zhang Zhenhu

2019, 33(5): 293-298. doi: 10.3969/j.issn.1007-2993.2019.05.010

[Abstract](46) [PDF 2427KB](6)

Abstract:
Under the effect of the seismic load,pile foundation is prone to deformation and stress concentration at the interface between two soil layers.This phenomenon is particularly obvious at the interface between frozen soil and the other soil layers which can eventually lead to pile failure.The EPS material is effective on stress concentration and pressure-relief,and widely used as roadbed filler.The EPSmaterial was proposed to be the protective layer between pile and frozen soil in the depth of frozen soil.Shaking table modeling test and numerical simulation was conducted to evaluate the EPS material's vibration reduction performance.This effect of plastic material was also discussed in further numerical simulation.It shows that the EPS material can reduce the peak value of strain,shear force and pressure of pile in the frozen soil.The plastic material has stronger capacity to reduce them,and the effect becomes better as the cross-sectional area of material increases.

Measures and Engineering Project Cases of the Tunnel Karst Treatment in the Fuxiu Second Line of the Yuhuai Railway

Jiang Bo

2019, 33(5): 299-302. doi: 10.3969/j.issn.1007-2993.2019.05.011

[Abstract](55) [PDF 4358KB](10)

Abstract:
A large number of karst caves were discovered during the tunnel construction in the Fuxiu second line of the Yuhuai Railway,with high safety risks during the construction.The karst tunnels are classified,and different control measures are taken according to different project conditions,including concrete backfill,steel tube grouting,high pressure jet grouting pile or pile foundation joist(or beam bridge)structure crossing.Well effect of construction is verified according to the site engineering project cases.

Application Study of Unwedge at Construction Geology During Tunnel Excavation

Ding Bin

2019, 33(5): 303-306. doi: 10.3969/j.issn.1007-2993.2019.05.012

[Abstract](86) [PDF 3019KB](5)

Abstract:
During tunnel excavation,geologists should give the geological prediction based on the geological conditions of tunnel,and propose engineering support methods to ensure the safety of tunnel construction.Comparative analysis of stereographic projection and Unwedgeis conducted in the same tunnel for the stability analysis of rockmass.The comparative analysis shows that Unwedge has advantages of better visualization,more convenience for operation,etc.The application of Unwedgeis expected to be promoted in geological construction during tunnel excavation.

Optimization of Construction Scheme for Loess Tunnel with Super Section

Tang Ge

2019, 33(5): 307-311. doi: 10.3969/j.issn.1007-2993.2019.05.013

[Abstract](36) [PDF 1642KB](6)

Abstract:
Through the research of the Fu Jiayao 1^# tunnel for Zhonghe to Shuifu section of the Lanzhou-Qinwangchuan Expressway,the optimization of construction scheme for loess tunnel with super section was proposed from the aspects of technical economy,monitoring measurement,numerical simulation,etc.Mathematical analysis was proceeded based on the data from on-site monitoring measurement of tunnel vault subsidence and peripheral displacement convergence.The force and deformation of the tunnel surrounding rock and the support in the excavation process was summarized.Meanwhile,the threedimensional model of tunnel excavation process was simulated by using finite element software MIDAS/GTS.Comparing the calculated data with the on-site monitoring data,the tunnel is demonstrated to be stability,which shows that the construction schemeis optimized and reasonable.This method is of reference significance for the construction of the same type tunnels currently.

2019 Vol. 33, No. 5