A test method for analyzing the deformation of landslide model

D Lei ,Mingjie Chen ,Zhiho Xu ,Bin Luo,＊ ,Gunlu Jing ,Ki Fn ,Qiho Zhou

a College of Civil Engineering,Sichuan Agricultural University,Dujiangyan,611830,China

b Sichuan Higher Education Engineering Research Center for Disaster Prevention and Mitigation of Village Construction,Dujiangyan,611830,China

c School of Civil Engineering,Southwest Jiaotong University,Chengdu,610031,China

Keywords:Landslide Soil deformation Shaking table test Phosphor bronze strip Two-dimensional contour plot

ABSTRACT In order to study the overall deformation of geotechnical model conveniently,the worksite of landslide bridge foundation reinforced by the front and rear row anti-slide piles in Chenglan railway was taken as an example.On the basis of shaking tabe test of a 1/40 reduced scale model,the landslide deformation caused by vibration waves was monitored through burying self-made phosphor bronze strips in soil.Combined with the horizontal and vertical coordinates of the bending strain points on the phosphor bronze strips,the digital matrix was converted by applying Renka Cline random matrix generation method,and the two-dimensional contour plots were drawn based on it.The results showed that the two-dimensional contour plots reflected the basic law of landslide deformation reasonably,and it revealed the evolution process of landslide deformation and failure.The research conclusions were consistent with the test phenomenon,which met the basic requirements of overall deformation analysis of landslide model.This proposed method can monitor multiple cross sections and was practical for model test.

1.Introduction

The forced deformation law of landslide is a research hotspot in geotechnical engineering.Many scholars have carried out the targeted researches through model tests,but the content of soil deformation is less(Li et al.,2019;Yu et al.,2019;Zhang et al.,2018;Tu et al.,2016).The traditional method study the deformation and stability of landslide model by monitoring the displacement of some feature points (Chen et al.,2016;Kramer S L et al.,1997),and the overall deformation of soil could not be obtained intuitively and vividly,which brings some inconvenience to the mechanism analysis of landslide deformation and failure.Some scholars tried to introduce the emerging technology into the deformation monitoring of landslide models.The ground three-dimensional (3-D) laser scanning technology was proposed by Xu Jinjun in model test.The test results showed that the method could effectively monitor the slope displacement,but could not observe the deformation law inside landslide model(Xu et al.,2010).The method of dial indicators and digital close-range photography was applied to the displacement monitoring of landslide model,and the displacement vector map of two-dimensional(2-D)model was obtained.It provided a new method of analyzing the deformation and failure mechanism of the landslide (Ren and Chen,2005).The algorithm and image acquisition protocol of multi-view photogrammetry technology were studied,and the test results showed that the method can be used to monitor the dynamic displacement and deformation of slope surface (Stumpf A et al.,2015).

Fig.1. Shaking table and rigid model box.(a) Schematic diagram of shaking table structure;(b) Rigid model box.

The existing research results showed that the emerging technology were mainly aimed at the displacement monitoring of slope surface(Wang et al.,2020).Although the monitoring method proposed by Ren Weizhong can obtain the displacement and deformation inside landslide(Ren and Chen,2005),it is only limited to the two-dimensional model.The deformation of multiple profiles of landslide can not be analyzed simultaneously.In this paper,a shaking table model test was designed and completed,the prototype worksite was a landslide bridge foundation reinforced by anti-slide piles in Chenglan railway,and a new deformation test method of landslide model was explored.The self-made phosphor bronze strips were scattered burying in soil,the bottoms and the tops of strips were fixed and free respectively.The soil deformation could be obtained indirectly through the deflection of phosphor bronze strips.Combined with the horizontal and vertical coordinates of the bending strain points on the phosphor bronze strips,the digital matrix was generated by applying Renka Cline random matrix method,and the two-dimensional contour plots were drawn based on it.The basic law of inner deformation of slope body was analyzed in loading vibration waves,which revealed the evolution process of landslide deformation and failure,and this proposed method provide technical reference for test analysis at the similar conditions.

2.Shaking table model test design

2.1.Brief introduction of test equipment

The test was carried out on a unidirectional electro-hydraulic servo shaking table in the high speed railway line engineering laboratory of Southwest Jiaotong University(Gao et al.,2014;Ogawa et al.,2001).As shown in Fig.1(a),the table size is 4 m × 2 m and the working basic parameters of the shaking table are listed in Table 1.In order to be convenient for the construction and observation of the model test,the rigid model box was applied and filled with geotechnical simulation materials(Lin et al.,2006;Zhang et al.,2014).As shown in Fig.1(b),the geometric size of rigid model box is 3.7 m × 1.5 m × 2.1 m (length ×width×height),and the inner wall along the long axis of model box was filled with 8 cm thick polyethylene foam board,rubber pad and other buffering materials(Zhang et al.,2018)to reduce the reflection effect of vibration waves during the test (Lin et al.,2006;Ilankatharan et al.,2010;Lin et al.,2018;Zhou et al.,2019).

Table 1 Performance parameters of shaking table.

Table 2 Similarity criterions of physical quantities.

Table 3 The physical and mechanical properties of soil.

2.2.Similarity relation design

According to the geometry size of rigid model box,the prototype landslide was decreased by 40 times.Under the same gravity field,the similarity ratio of gravity acceleration satisfied the basic condition:Cg=1.Taking length,weight and loading acceleration as the major factors,the similitude relationship of physical quantities was derived according to dimensional analysis (Li et al.,2011;Luo et al.,2016).And C was defined as the similitude ratio of physical quantities between prototype and model.The similitude criterions are shown in Table 2.In fact,the physical parameters did not fully meet the similarity design,such asCT=but it did not affect the analysis of deformation law of landslide model.

2.3.Model filling

The test model was constructed artificially with the geotechnical simulation materials,the fine breccia soil and silty clay were tamped when the soil layer reached 10 cm,so that the density,moisture content,internal friction angle,cohesion achieved the predetermined values.The physical and mechanical properties of the model soil are shown in Table 3,and the parameter values are basically consistent with prototype worksite.The mixture of bentonite and fine sand with a mass ratio of 1:2 was used to simulate the potential sliding surface (Hungr et al.,2014;Gratchev et al.,2007).The bedrock was a stirred mixture of different materials,including red clay,fine sand,cement,and water with a mass ratio controlled as 1:0.55:1:0.25.The layered filling method was adopted for soil to ensure compaction.Through the previous test verification,the density of the rock mass after hardening was 2300 kg/m3,and the elastic modulus reached 16.4 GPa to ensure the necessary strength and rigidity.In order to reduce the reflection of seismic waves,the polyethylene foam board and sponge cushion were filled on front and rear sides of the geotechnical model.The friction effect between the soil and toughed glass was reduced by painting a layer of vaseline on the inside of model box.

According to the actual reinforcement ratio and stirrup ratio of bridge pile foundation and anti-slide piles in the prototype worksite,the reinforced steel framework of the reduced-scale structure was assembled and the micro-concrete was cast to form the model structures as shown in Fig.2a-b.The structural size and strength grade of micro-concrete of the anti-slide piles and bridge pile foundation are shown in Table 4.An iron box was fixed at the top of the pier,a sliding track was welded inside and two counterweights were hung on the track to simulate the inertia force of the adjacent box girders (Ling et al.,2006),as shown in Fig.2c.The weight and spacing of the counterweights were determined according to the similitude relationship.The panorama of the filled test model is shown in Fig.2d.

Fig.2. Test model.(a) Reinforced skeleton frame;(b) Bridge pile foundation;(c) Load simulation of box girders;(d) Panorama of the test model.

Table 4 Structural dimensions of piles and cushion cap.

2.4.Strain key points layout

The research results showed that the phosphor bronze strip had good flexibility and reflected the forced deformation of the surrounding soil reasonably (Yazdandoust,2017;Nakajima et al.,2008).The five phosphor bronze strips used in this test were all buried at the medial axis of the landslide model with a total of 20 strain points,as shown in Fig.3.The 1/2 Bridge circuit was used to test the bending deformation of the phosphor bronze strips.The strip was 0.3 mm thick and 40 mm wide,with an elastic modulus of 102 GPa and a density of 8.8 g/cm3.Strain gauges were pasted on the front and rear sides of the phosphor bronze strips symmetrically,as shown in Fig.4.The bottom of the phosphor bronze strips were buried and fixed in the bedrock,and the tops were free.After the completion of model filling,the connected suspending iron wires were cut off.When the model started to vibrate,the phosphor bronze strips and the surrounding soil produced deflection deformation together,and the deformation state of the soil was indirectly obtained accordingly.

Fig.3. Strain key points layout Meitug

Fig.4. Phosphor bronze strips.(a) Schematic diagram of strain gauges pasting;(b) Schematic diagram of phosphor bronze strips burying.

Fig.5. Loading steps of vibration waves.

2.5.Vibration wave loading

The sinusoidal waves with frequencies of 3 Hz and 10 Hz were used for loading,and the peak acceleration increased by 0.1ggradually.The loading steps of vibration waves are shown in Fig.5 and the duration of vibration wave loading was 10.23 s.Before the start of sinusoidal waves loading,the excitation test was applied with 0.08gGaussian white noise wave,and the natural vibration frequency of the slope was measured to be 20.4 Hz.

3.TEST result analysis

3.1.Test phenomena

As shown in Fig.6,the silty clay slipped slightly along the sliding surface in loading sinusoidal waves,and the shear deformation near the sliding surface increased.As the silty clay contained a considerable ratio of cohesive particles,the integrity of the soil layer was much better,which was conducive to the reflection of vibration waves on the slope and the formation of complex vibration wave field.After the test,there was no significant deformation and failure in landslide,and the antiseismic retaining structure played a good role on the landslide reinforcement.

Fig.6. Model test phenomenon.(a) Silty clay sliding downward;(b) Small deformation of landslide.

Fig.7. Flow chart of two-dimensional contour plots of landslide deformation.

Fig.8. Contour plots of landslide deformation under loading of 3 Hz sinusoidal waves.(a)Loading 0.1 g 3 Hz sinusoidal wave;(b)Loading 0.2 g 3 Hz sinusoidal wave;(c) Loading 0.3 g 3 Hz sinusoidal wave;(d)Loading 0.4 g 3 Hz sinusoidal wave;(e) Loading 0.5 g 3 Hz sinusoidal wave;(f) Loading 0.6 g 3 Hz sinusoidal wave;(g)Loading 0.7 g 3 Hz sinusoidal wave.

Fig.9. Contour plots of landslide deformation under loading of 10 Hz sinusoidal waves.(a)Loading 0.1 g 10 Hz sinusoidal wave;(b)Loading 0.2 g 10 Hz sinusoidal wave;(c)Loading 0.3 g 10 Hz sinusoidal wave;(d)Loading 0.4 g 10 Hz sinusoidal wave;(e)Loading 0.5 g 10 Hz sinusoidal wave;(f)Loading 0.6 g 10 Hz sinusoidal wave;(g) Loading 0.7 g 10 Hz sinusoidal wave.

3.2.The contour plots of landslide deformation

For different loading conditions,the horizontal and vertical coordinates of test points were acted as the first and second columns in table,the bending strain values of key points on the phosphor bronze strips were extracted as the third column.On this basis,the matrix was generated by applying Renka Cline random method through the drawing software ORIGIN,and the two-dimensional contour plots of landslide deformation were generated automatically from the matrix.The simplified procedure was shown in Fig.7.

As shown in Fig.8 and Fig.9,the shear deformation of landslide increased gradually as the peak acceleration of the sinusoidal waves enlarged.Compared the loading conditions with different frequency and same peak acceleration,the landslide deformation was relatively larger in loading 3 Hz sinusoidal waves as the soil inertia force was stronger.The model deformation was focused on the slope surface of sliding section and the sliding surface around rear row anti-slide piles.As the sinusoidal waves with the different frequencies were reflected on the slope surface of silty clay,the incident waves and reflected waves formed a complex vibration wave field together,which increased the deformation near the slope surface of sliding section,and the shear strain of the soil accumulated gradually from the upper to the lower,the peak value of shear deformation occurred on the slope surface behind the rear row antislide piles.Due to the cementation of clay particles in sliding mass,there were no tension cracks on the slope surface during vibration.The movement of silty clay produced shear deformation near sliding surface,and the deformation growth in sliding section was rather bigger than anti-slide section.There was a integral slip trend of silty clay along sliding surface as shown in Fig.6(a).The landslide thrust was mainly resisted by the rear row anti-slide piles,which balanced the forced deformation of bridge foundation and anti-slide piles.Meanwhile,the spacing between the bridge foundation and rear row anti-slide piles was very small,and there was stronger dynamic interaction which conducted soil compression between them.The bedrock was approximated as a rigid body with small strain.Due to the limited number of phosphor bronze strips and strain key points in the test model,the sample error during the generation of contour plots was amplified,and the forced deformation of some bedrock was larger slightly.

4.Conclusions

In this paper,the overall deformation of landslide model was tested by using the phosphor bronze strips.Combined with the horizontal and vertical coordinates of the bending strain points,the two-dimensional contour plots were drawn based on the digital matrix,and the overall deformation law of the shaking table test model was analyzed,the conclusions were drawn:

(1) As the phosphor bronze strips had better flexibility,the strips and nearby soil deformed synchronously during vibration.According to the bending deformation distribution of the phosphor bronze strips in landslide,the overall deformation law of the shaking table test model can be obtained indirectly and met the basic requirements.

(2) The two-dimensional contour plots can be used to analyze the forced deformation of geotechnical model and monitor the multiple cross sections.This method was suitable for indoor test analysis in similar conditions.

(3) The drawing accuracy of two-dimensional contour plots was related to the number of measurement points and burying scheme,and the sample error need to be controlled.

Acknowledgement

This study is supported by the Major Project Program of the Scientific and Technological Research and Development Plan of the Ministry of Railways,China(Z2012-061)