Landslides hazards for new rail road in south Uzbekistan

ABIROV, Rustama, and MIRALIMOV, Mirzakhidb

a Soil dynamics, Institute of mechanics and seismic stability of structures, Tashkent, Uzbekistan, e-mail: rustam_abirov@mail.ru b Bridges and tunneling Department, Tashkent automobile and road Institute, Tashkent, Uzbekistan, e-mail: mirzakhid_mirakimov@yahoo.com

Abstract — The stability of escarpment is one of the great problems at construction and exploitation of the transport pathways in mountain regions. Behooving its solution enables to install the rational profile of the road, this provides their condition in all the possible ranges of the loads. The last time landslides disturb the schedule of trains that deliver inconvenience and financial losses. Here the landslides risk on new rail road in south Uzbekistan is investigated and ways for its mitigation are proposed.

Keywords – landslide, slope, instability

1 Introduction or Background

Nowadays the logistics are great problems for Uzbekistan. For solving these problems, the transport communications are developing intensively in our country. It has led to an increase of many tasks including stability problems. Uzbekistan is an earthquake prone region. The rate of destructions in any seismic active regions depends in many cases on soil conditions and morphological factors. This case study describes a mountainous area where a new railway was constructed. For the solving of mechanics problem a finite element approach was used here.

2 The Case Study

The area under study is divided into iso-parametric finite elements. Initial data to solve these problems include:

  • Transversal profiles on unloading of landslide on certain sections of railway line;
  • Properties of elements (density, elasticity modulus, Poisson’s coefficient, cohesion and an angle of internal friction of rock);
  • Surface loads and boundary conditions.

A quality study of stress-strain state and stability of rock slope demands a well substantiated schematization. When building geo-mechanical schemes to study the aspects considered in this work, we bear in mind the following preconditions: rock layers close to each other in engineering-geological properties and parameters of physical-mechanical characteristics were combined into packs with averaged characteristics. In building design scheme, we took into consideration specific features of the worked out method, which made variation of the change of parameters on the basis of complex approach possible.

In the region of area under study, the rock is characterized by extremely complicated conditions of bedding laying. Slope rock consists of sandy soils with layers of argillite and alevrolite and the strata of loam and sandstone. Argillite presents greatly re-compacted soil with a thin-layer and sometimes implicitly expressed texture. It contains mostly (more than 50%) clayey particles (less 0,005mm). When clayey cement prevails it is intensively damaged. Variety with siliceous and loam cement are more resistant to weathering. In natural conditions argillites in most cases are re-layered by lime, sandstone and alevrolites. Alevrolite is greatly re-compacted soil. The number of (alevrolite) particles (0,05—0,005 mm) exceeds 50%. They are intensively damaged when clayey cement prevails. With the presence of siliceous and loamy cement alevrolites are more resistant to erosion and weathering. In natural conditions alevrolites are often re-layered with lime, sandstone and argillites. Alevrolites are mostly weakly-cemented soils and are characterized by plasticity, natural water content from 0.23 to 0.36 at average value 0.29. Their density in natural condition is changing within the limits from 1.71t/m3 to 1.95t/m3, average value 2,15t/m3. Average value of moisture content on yield boundary is 50%, average value of moisture content on smoothing boundary is 34%. The density of soil particles is taken equal to 2.1t/m3. Design values of characteristics of resistance to the cut of alevrolites in natural conditions are recommended as the following: =300, С=42,7КPа, and deformation modulus Е=1300МPа. Design value of density of argillite-like soils is taken equal to 2,4t/m3. Strength characteristics of these soils according to tests data in normal state are equal to: =250, С=71КPа. Recommended values of elasticity modulus (deformation) is E=1000 МPа. Fig.1 shows transversal profile of landslide-risk massive (without unloading and with unloading of earth slide) on the slope side. On the basis of physical and geometrical data design schemes with two variants were built (Fig.1). For both cases design seismicity of the construction-site of road-bed was with magnitude 8 (MSK scale).

Figure 1: Slope’s deformation: a) without unloading, b) with unloading of slope. 1 – boundary conditions, 2 – load on earth foundation

An assessment of slope stability was done on the basis of information on strength safety coefficient in each separate point of the rock massive. A way to check the accuracy of strength and stability of a rock massive, typical for this case, is based on the statement that first the stress state and the pattern of deformation of rock mass were determined. Characteristic data thus obtained is given in Fig. 3. Besides software allows to obtain lines and directions of sliding areas in potential points of loss of slope stability. To determine the stress state, one should calculate and build distribution of normal and principal stresses in the body of a slope. To determine coordinate points of rotation of circular-cylindrical surfaces N.Yanbu’s graph was used. We determine and give minimal safety coefficients of stability for two variants of design. In the first case it is seen that stability of the rock slope is not ensured, as the slope inclination to horizontal axis does not provide the necessary level of stability of stress state; this may lead to slippage of rock mass. In the second variant, slope unloading was proposed; it ensured slope stability. An analysis of the studies of many authors concerning the error of the method of circular-cylindrical surfaces of sliding, when using them to determine safety coefficient of slope stability, revealed that such method always gives a greater result compared to results obtained with the methods, based on solution of the problems of elasticity theory or mechanics of deformable rigid bodies.

So to reveal the most characteristic points or surfaces of potential loss of stability in the body of a slope, prone to sliding, principal stresses were determined with this design and lines of potential sliding in each element of design scheme was built. Calculations show that on the places of slope surface with prevailing processes of rock weathering there are sections with rock movement. In the first case they are more essential and have maximum depth inside the rock approximately down to 4–6 meters. On slopes with unloading of potential landslides’ dangerous sections of sliding are not revealed. With known values of obtained principal stresses and design parameters of resistance to shear in each point (element) limit admissible (critical) for this point tangential stresses were calculated. Safety coefficients of strength in the point were determined as a ratio of critical tangential stress to acting tangential stress. Further average (integral) safety coefficient of slope stability was calculated.

Figure 2: Plastic elements: a) without unloading, b) with unloading of slope

Fig. 2 shows the results of elastic-plastic design. Calculations are done by step method and for each portion of load in the body of a slope potential plastic elements are calculated. Plastic elements are the elements of rock mass, where their damage on the area of sliding occur. Fig. 2 shows the distributions of Mohr-Coulomb’s yielding function (plasticity) in design plane of slope. The values of these functions testify of distribution of unstable values of functions on the surface points of a slope. So the dynamics of changes of plasticity function and safety coefficient of strength from upper points to lower ones of a rock is seen (to their increase); this allows to choose the second variant of design scheme and consider it as the most safe one in further operation in the process of excavation of road-bed.

On fig. 3 the results of Mohr-Coulomb function distribution are shown.

Figure 3: Distribution of Mohr-Coulomb function: a) without unloading, b) with unloading of slope

3 Conclusions

It is easily seen that for obtained design profile of slope the transfer to limit state does not occur. However to ensure general stability of slope sides, the sections of the first crest of slope excavation about 1-2meters should be strengthened; their strength characteristics, such as cohesion coefficient and an angle of internal friction should be strengthened by their compaction or substitution by more solid rock mass.

References

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Citation

Abirov, R. and Miralimov, M. (2015): Landslides hazards for new rail road in south Uzbekistan. In: Planet@Risk, 3(2): 1-4, Davos: Global Risk Forum GRF Davos.