Inverse Computation of In‐situ Strength Parameters on Unstable Slope

Construction Engineering Volume 3, 2015 www.seipub.org/ce Doi: 10.14355/ce.2015.03.001

Inverse Computation of In‐situ Strength Parameters on Unstable Slope Yashen Zhang1,3 , Naiwu He2, Yuxin Niu2, Ling Liu2, Kunyong Zhang *1,3 Institute of Geotechnical Engineering, Hohai University, Nanjing,210098, China

1

China Highway Engineering Consulting Group Company LTD, Beijing, 100097, China

2

Key Laboratory of Geomechanics and Embankment Engineering of Ministry of Education, Nanjing 210098, China

3

zzzhangys@163.com; 2799015556@qq.com; 2 hicky@foxmail.com; 2 llspy‐123@163.com; * 1,3ky_zhang@hhu.edu.cn

1,3

Abstract Inverse computation method was applied in this paper to estimate the in‐situ shear strength parameters of unstable slope including cohesion and fricton angle due to the difficulty of obtaining the insitu parameters through laboratory experiments.

Three typical slope sections were selected and limit equilibrium method was applied to do this job. Strength parameters of the slope on the verge of sliding were caculated, through which further calculation could be carried out to estimate the landslide thrust force and then to the design of supporting structures.

Keywords Inverse Computation; Unstable Slope; Stability Calculation; Slope Reinforcement

Project Overview No.317 state road was located in complicated geological conditions including the developing geological fissures and the widely distributed loose soil. So the reconstruction project of the road faces the great challenge of slope unstability.

The subgrade was originally designed as step‐slope at the rate of 1:0.75 with fixed slope‐height of 8~10 meters. The slope was bare and no reinforcement measures were taken.

The subgrade of the section K71+560～K71+850 on this road was excavated in September 2012, and one month later cracks emerged in the direction of the road line. Finally landslide formulated which caused great disasters. It is necessary that effective treatment scheme be carried out.

Geological Conditions Soil Condition Three kinds of soil are involved in this area according to the geology data.

The first layer consists of silt and soil blocks. The silt is slightly dense and the thickness is around 1 to 2m which mainly lies on the surface of the slope. The soil blocks mainly contains sandstone with the thickness of 5 to 10m.

The second layer mainly contains soil blocks, which is quite dense and has good permeability. Similarly it lies on the surface of the slope and own the thickness of about 26m.

The third layer is gravel and it is moderate dense with good permeability. The thickness is estimated to be 3 to 6m, but almost has no influence on the stability of slope.

The parameters of the soils are not available for reference in consideration of variability of the climate but can be estimated empirically according to the composition.

Adverse Geological Phenomena There is a developed ground faults lying in the area which owns huge risk to the stability of slope.In addition, the potential risk of earthquake should also be taken into account because of the former disasters taking place in

1

www.seipub.org/ce Construction Engineering Volume 3, 2015

Sichuan. Finally, the hydrogeological condition cannot be ignored in the area. The water level alters greatly in different seasons which means seepage constantly occurs and it could take away the loose soil, thus resulting in the unstability of slope.

Mechanism of Landslide As mentioned above, the emergence of cracks occured a month after the excavation was finished. And then the landslide quickly developed so that some parts of the subgrade was buried. The main part of the landslide was located in the section area K71+620～K71+790, and the side part locates in K71+560～K71+620 and K71+790～ K71+850 due to the pulling force of the main part. The landslide in the main part was already developed and began to slide, while the one in the side part was still in the process of developing cracks.As is illustrated in FIG. 1.

FIG. 1 LANDSLIDE FORMULATION

In combination of the geology condition on site, it can be concluded that the reasons of the formulation of landslide are as following: Geology Factors The landslide is loacated in the area of the ground fault. In this area, many crushed gravels and loose soils are covered on the ground surface after the dislocation of the fault. It means that the slope has poor stability and is prone to landslide disasters under the influence of external disturbance. In addition, there is a ditch on the right side of the road line, which may distributes to the unstability of slope because of the unloading effect.

Rainfall After the permeation of rain, the pore water pressure will rise dramatically, thus reducing soil in cohesion drastically and in friction angle at some extent, which contributes to the formulation of the weak interlayer.

Water Erosion There is a ditch located at the foot of the slope, which reduce the stability of slope in rainy season.

Freezing and Thawing The project is located in Sichuan, where temperature changes in a large scale throughout the whole day. So it aggravates the effect of freezing and thawing, especially in winter.

Stability Evaluation The landslide in some areas is quickly developing but can be saved under the circumstances of being enhanced.

Neither insitu test nor indoors experiments can provide reliable parameters because of the complex geology and soil composition. So the method of inverse computation was carried out to get the plausible parameters with the help of the fixed landslide shape observed from the site.

Three typical sections were selected to carry out inverse computation, including Section K71+807.57, Section K71+827.57 and Section K71+852.38. The outline of the landslides can be got from the survey. Thus three basic

2

Construction Engineering Volume 3, 2015 www.seipub.org/ce

models could be set up for calculation respectively and limit equilibrium method was applied to carried the calculation. According to the design code, Bishop method was adopted to calculate the safety factor of slope. The model and the landslide shape are given in FIG. 2 to FIG. 4.

FIG. 2 GEOMETRY OF SECTION K71+807.57

FIG. 4 GEOMETRY OF SECTION K71+852.38

FIG. 3 GEOMETRY OF SECTION K71+827.57

Grading Curve Percentage of not passing (%)

100 90 80 70 60 50 40 30 20 10 0 100

10

1

0.1

0.01

Grain Size (mm)

FIG. 5 GRADING CURVE OF THE SOIL ON SITE

The safety factor was set as 0.98 during the calculation because the landslide was just developed. And the slip surface was also given based on infield survey. As for the other related parameters of soil needed for calculation, unit weight was set as 22 kN/m3 at first which doesn’t matters a lot in calculating the safety factor. Then the soil was considerd mainly composed of gravels and has little cohesive soil based on the laborory grain size analysis as given in FIG. 5.

So it is reasonable to set the range of cohesion as 5 to 10 kPa. And correspondingly the value of friction angle was selected as 28° simultaneously in consideration of the repeated results fed back. With the consideration of the fact that friction angle varies little compared with cohesion, only the value of the latter parameter was changed to cater to the predetermined safety factor. The result is listed in TABLE 1.

After inverse computation of three sections, three groups of strength parameters were caculated. Then, the average value, standard deviation and variable coefficient were given according to the code to estimate the real parameters on site. Range analysis was also carried out to eliminate the data which are far away from the standard rate based on the criteria of the predetermined range valued as ±15%. So, after calculation, the data of section K71+807.57 was abandoned due to the above policy. The modified average value were obtained as following: c=9.2kPa, φ=28°.

Along with the unit weight 22kN/m3 , this group of paramters were applied finally to calculate the landslide thrust force to help to accomplish the reinforcement design.

TABLE 1 THE RESULTS OF INVERSE COMPUTATION

Selected K71+807.57 Sections Cohesion 6.5 c(kPa) Frction Angle 28.0 φ(°)

K71+827.57

K71+852.38

Average Value

Standard Deviation

Variable Coefficient

Range Value

Modified Average Value

9.6

8.7

8.3

1.3

0.16

‐21.0%

9.2

28.0

28.0

28.0

0

0

0

28

Slope Reinforcement After the key parameters were obtained in determining the stability of slope including c and φ through the method

3

www.seipub.org/ce Construction Engineering Volume 3, 2015

of inverse computation, these parameters could be deemed as the real strength parameters on site and then the thrust force of any part of a certain section could be calculated by means of imbalance thrust force method. The slide thrust force could be obtained at any point where reinforcement is needed after calculation. Then, reinforcement structures could be designed at the weak point of slope according to the calculated thrust force as external force of the structures.

Conclusions This article aims to get the real insitu strength parameters of slope on site through the method of inverse computation. After modelling in a usual way, the location and the shape of the slide were fixed according to survey, then a group of initial data was set and finally the real one was obtained to reach the predetermined safety factor by trial, which was set as fundamental to the further reinforcement design.

ACKNOWLEDGMENTS

The work was supported by Construction Research Projects of The Ministry of Communications of China (No. 2013 318 800 020), Research Project of China Communications Construction Company Ltd. (No. 2012‐ZJKJ‐11) REFERENCES

[1]

Liu Wenhong, Zhang Yaqing, Li Tonglu, Xi Yu, Wang Ning (2014). “Analysis on Failure Mechanism of Unsaturated Loess Slope with Inverse Results of Soil Strength Parameters.” Journal of Engineering Geology, 22(5): 915‐920.

[2]

Finno, R. J., and Calvello, M. (2005). ʺSupported Excavations: Observational Method and Inverse Modeling.ʺ Journal of Geotechnical and Geoenvironmental Engineering, 131(7), 826‐836.

[3]

Budhu, M., and Gobin, R. (1996). ʺSlope Instability from Ground‐Water Seepage.ʺ Journal of Hydraulic Engineering, 122(7), 415 ‐ 417.

[4]

Worman, A. (1994). ʺSeepage‐induced mass wasting in coarse soil slopes : A Woman, Journal of Hydraulic Engineering ‐ ASCE, 119(10), 1993, pp 1155–1168.ʺ International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 31(3), 158.

[5]

National Standard of the People’s Republic of China. (2009). “Code for Investigation of Geotechnical Engineering(GB50021‐2001).” The Ministry of Construction of China, In Chinese.

[6]

Industrial Standard of the People’s Republic of China. (2011). “Code for Highway Engineering Geological Investigation (JTG C20‐2011).” The Ministry of Communications of China, In Chinese.

[7]

Industrial Standard of the People’s Republic of China. (2004). “Specification for Design of Highway Subgrades (JTG D30‐ 2004).” The Ministry of Communications of China, In Chinese.

4