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10th International Conference Interdisciplinarity in Engineering, INTER-ENG 2016
Study on the Stability of a Road Fill Embankment
Dorin-Vasile Moldovan *, Andor-Csongor Nagy, Lavinia-Elena Muntean, Madalina Ciotlaus
Technical University of Cluj-Napoca, 28 Memorandumului Street, Cluj-Napoca, 400114, Romania
University of Agricultural Science and Veterinary Medicine, 3-5 Calea Manastur Street, Cluj-Napoca, 400372, Romania
This paper aims at making an analysis of the stability of a road fill embankment which relates to a green landfill in the immediate vicinity of the Alba County. Stability was analyzed with the Geostru Slope software and the sliding surfaces were calculated considering the Limit Equilibrium Methods (LEM). The characteristics of the foundation ground of the adjacent earth layers were introduced according to the results of the geotechnical drilling. The stability of the slope was modelled in seven proposed scenarios (different inclinations, with and without berm). The results highlight the advantages and disadvantages related to every scenario, and from the viewpoint of the stability factors as well.
@ 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
Peer-review under responsibility of the organizimg committee of INTER-ENG 2016
Kywords: fill embankment; man-made slope; sliding surfaces; stability factor, LEM.
Embankments are used in road construction when the vertical alignment of the road has to be raised above the level of the existing ground to satisfy design standards, or to prevent damage from surface or ground water. Many embankments are only 0.5-I.5m high, but heights of 5 m or more may be used on major highways [I, 2]. Therefore, it is a common challenge for the geotechnical engineers to estimate stability of the embankment and evaluate certainty of the stability calculation results .
Slopes in soils and rocks are found in nature and in man-made structures . For a man-made slope (fills such as embankments, earth dams or cuts etc.) [I] to be stable, both design and building works made properly and professionally are necessary as well as assessment of stability (mainly stability against sliding) regarding the earth bulk.
Improper road construction techniques, including improper selection of equipment, lack of stability related calculi are a common cause of slope instabilities. The embankment or slope loss of stability occurs when an earth bulk detaches from the earth mass and moves towards its lower part. The sliding potential can be met in all soils in slope. The detachment surface is called sliding surface; along the surface, the soil is in shear. Hence, sliding can occur when the tensions in the earth bulk given by loading reach the resistance to shear of the soil and tend to exceed the tension in a certain portion of the bulk surface. The principle is simple, theoretically. However, actually, the slope and embankment stability-related issues are very complicated because of the large number of surfaces where sliding can occur or because of the variation in time of the earth resistance to shear (e.g. by increasing the pressure of the water in the pores, because of having a higher level of underground water sheet). It would be simpler if, in stability tests and analyses, one could use the smaller future value of the resistance to shear of the soil, but sucha magnitude is impossible to know in advance .
The stability to sliding of a slope or embankment is commonly analysed on the basis of the notion of factor of safety against sliding (or stability factor). This indicates a potential slide of the slope in the circumstance at the moment of the investigation. Thus, for a certain work (dam, dike, earthwork) made with local materials (be it adimensioning or checking issue), the stability factor has to meet the condition:
Feffective ≥ Fadmissible
For road earthworks, the standard STAS 2914-84 provides an admissible coefficient of safety of Fadmissible=1.3…1.5.
Factors of safety are generally used in a consistent manner for all types of limit equilibrium analysis and indicate the adequacy of slope stability and play a vital role in the rational design of engineered slopes (e.g. embankments, cut slopes, landfills) [6, 7]. The analysis of slope stability is affected by the material properties of the slope model, the calculation of the stability factor and the definition of the slope failure. In general, the factor of safety against sliding (or stability factor) is noted F and is defined as the ratio between the available resistance to shear (that can be mobilised to the maximum) of the earth and the sher stress called average resistance to shear needed to be mobilised (i.e. the tangent tension produced in the earth by the test) for the earth to reach the limit of equilibrium (F=1) along the failure surface. It is common practice to use tofollowing empirical guidelines for slope stability assessments (Table 1) :
Table 1. Factor of safety .
|Factor of safety||Guidelines for limit equilibrium of a slope|
|1.0 – 1.25||Questionable safety|
|1.25 – 1.4||Satisfactory for routine cuts andfins,questionable for dams, or where failure would be catastrophic|
|> 1.4||Satisfactory for dams|
The shear strength of the slope material is usually calculated through Mohr-Coulomb linear relationship, where τf and τ are defined by:
The available shear strength depends on the type of soil and the effective normal stress, whereas the mobilizedb shear stress depends on the external forces acting on the soil mass. In accordance to the shear failure, the factor of safety against slope failure is simply calculated as:
The values of the factor of safety against sliding (or stability factor) are strongly influenced by a set of factors, such as the geological structure of the slope, the presence of discontinuities (voids, cracks) in that structure, the material properties, the pressure of the water in the pores, the state of stresses and strains in the earth bulk or the initial state of tensions. The factors mentioned are variable along the sliding surface making the factor of safety against sliding variate. It is for this reason that the values of available and mobilised shear strengths should be
interpreted as average values in the sliding surface.
The limit equilibrium method is considered by engineers in practice as a traditional, well established method and one of the most widely used stability analysis due to its reliability for most practical cases [3, 4]. The limit equilibrium method studies the equilibrium of a rigid body, made up of an embankment and a sliding surface of any shape (straight line, arc of a circle, logarithmic spiral). From the equilibrium, the shear tensions (τ) are calculated and compared to the available shear strength (τf), which is calculated according to the Coulomb failure criterion.
Among the limit equilibrium methods, several of them consider the overall equilibrium of the rigid body (Culman), others divide the body in strips as it is nonhomegeneous and discuss the equilibrium of each strip (Fellenius, Bishop,Janbu, Sarma etc..)
All LE methods are based on certain assumptions for the interslice normal forces and interslice shear forces, and the basic difference among the methods is how these forces are determined or assumed [9, 10]. The objective of the present study lies in analysing the stability of a filling with granular material, used as the base of an access road to a green landfill in the commune Galda de jos, Alba County. The analysis of the stability was performed with the Geostru Slope software, which permits the calculation of the sliding surfaces in various
methods and approaches, among which Fellenius, Bishop, simplified Janbu or Morgenstern-Price. The study lies in the geometrical configuration of the embankment in the section under investigation, by alternating 1:3 and 2:3 slopes, with and without berm; an attempt to identify and recommend the optimal slopes relative the stability factor for high level embankments aws also made.
F,effective effective work safety factor, estimated by calculations
F,admissible safety factor, admissible according to standards or good parctice, with more or less conventional character
τf shear strength (available)
τf shear strength (mobilised)
φ maximum internal friction angle
γ bulk density
F factor of stability